# Riddle me this batman



## Road Guy (Jul 25, 2006)

The question is:

A plane is standing on a runway that can move (some sort of band conveyer). The plane moves in one direction, while the conveyer moves in the opposite direction. This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in opposite direction).

Does the plane ever take off, and why or why not?


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## civengPE (Jul 25, 2006)

I almost agree with Sapper.

Since the thrust created is directed to the air and not the ground, the plane will accelerate at the same rate as if it were on a non moveable surface.

If you were talking about a car that imputs its power through the wheels to the ground, then I would agree that it's position would not change.

So the bottom line is, as long as the conveyor is long enough, the plane would take off normally.


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## Ferg_AR (Jul 25, 2006)

Yes, the plane takes off. The conveyor has no effect on the plane other than causing the wheels to spin twice the speed that the airplane's airspeed is. The plane will move exactly as it would move if it were on a stationary runway.


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## redrum (Jul 25, 2006)

planes take off due to thrust under there wings(or some shit like that), so it wouldnt matter what the speed of the wheels or what the conveyor belt is doing, so long as the turbines kick in it should take right off?


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## MA_PE (Jul 25, 2006)

No.

Planes take-oof by pushing the plane through "static" air. If the thrust is the subject plane is used simply to move the plane forward at the rate of the conveyor then the remaining air (outisde of the influence of the jet intake) will still be "static" and as soon as the plane lifts of the convyor it will not have any forward push.

Doppler effect.

For the plane to fly your assuming that the engines move enough air to lift the plane. That's a hovercraft or a helicopter. Planes push the body and wings through the air that already there.

That's my 0.02


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## MA_PE (Jul 25, 2006)

No.

Planes take-off by pushing the plane through "static" air. The thrust generated by the subject plane is used simply to move the plane "forward" in the same air at the rate of the conveyor. Air outside of the influence of the jet intake will still be "static" and as soon as the plane lifts off the conveyor it will not have any forward push.

Doppler effect.

For the plane to fly, you're assuming that the engines move enough air over the wings to lift the plane. That's a hovercraft or a helicopter. Planes push the body and wings through the air that already there.

That's my 0.02

Wow. do I need typing lessons.


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## civengPE (Jul 25, 2006)

The plane would in fact take off as normal. the wheelas have no (minimal) effect on take off. The thrust is imparted to the air and not the ground. As an earlier poster stated, the wheels would just turn twice as fast.

Imagine you have your roller blades on and are on the conveyor. You are holding on to a rope attached to a stationary car. as long as the car is stationary, you will remain in the exact same place no matter how fast the conveyor is moving. Now accelerate the car, you will move at the same rate as the car.

The car is the thrust supplied by the prop. it is absolutly independant of the ground ( for the most part).

It is how fast you move through the air and not the ground.

If you had a 80 mph head wind you could take off with zero ground speed.

Pilot in Training.

PIT


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## EdinNO (Jul 25, 2006)

Maybe I'm not thinking this through, BUT, wouldn't the plane essentially be sitting still and the wheels would be spinning?

If this is true, the wings can't be moving through the air, in which case it woudln't fly.

Ed


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## civengPE (Jul 25, 2006)

Ed that would be the case if the engine was not running.

Since the engine is running it supplying thrust independant of the ground.

the engine is pulling on the air and not hte ground.


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## civengPE (Jul 25, 2006)

I did some searching and found this explanation that is much clearer than mine. I found dozens of variations to this explanation, but this one seems clear enough even someone from Mandeville might get it!

:rotfl:

http://www.michaelbuffington.com/archives/...eyor_belts.html

Picture this - you're standing on a skateboard that is riding on a treadmill. One person is standing in front of the skateboard on firm ground, and the two of you are holding a rope. This person pulls on the rope that you're holding so that the rope moves exactly an inch per second, advancing you forward. No matter what speed the treadmill is going, as long as that person maintains the same rate of pull, you'll advance forward an inch per second. Your skateboard wheels might go faster or slower in relation to the speed of the belt, but you'll pretty easily advance forward. Change the rope to a stick, and the conveyor belt can travel in either direction at either speed and be just as irrelevant.

The airplane's engines provide the forward force, pushing against the air behind their outputs. The air is like the person standing firm (as firm as air can be) and the engines pushing against that air provide the same kind of force that someone pulling on the rope provides. In both cases, the speed of the conveyor belt has no correlation with the force that the rope or engines produce against the air or the person standing firm.

Granted, in both cases, wheel friction will come into play. With a skateboard and treadmill, friction might be noticeable. With the kind of forces a jet turbine can produce the wheels would probably melt off before the engines noticed anything.

:rotfl:


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## cement (Jul 25, 2006)

It's going to fly. There is no drive in the wheels of an airplane, they are free turning. No matter how fast that conveyor turns, that bad boy is moving forward because the wheels neither contribute or detract from the trust of the engines (neglecting friction on the bearings).

Fun Q RG!

edit - doh! I did not read civengPE's copious link, does that say the same?


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## civengPE (Jul 25, 2006)

More.... I want more questions.

I like these kind in stead of the stupid riddles. THis one is actually physics (engineering) based.

what happens if you tug on a toilet paper roll parallel to the ground???


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## cement (Jul 25, 2006)

you get a dirty sheet.


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## civengPE (Jul 25, 2006)

I mised that one in college. We had a prof that would always put one bonus question on the tests. He used to have some good ones!


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## Road Guy (Jul 25, 2006)

sorry i watched this question go on for about 10 pages on a jeepforum (shade tree mechanics) and figured it could get answered?

I have no clue :lol:


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## mizzoueng (Jul 25, 2006)

The plane will go nowhere. For the plane to take off there needs to be lift forces present. Lift force is directly related to the velocity of the fluid around the wing. As the wing does not move there is no velocity, unless there is a hurricane in front of the wing.....

So as a plane moves foreward, lift is created by the "wind". But if the plane does not move then there is no lift. You can test this theory by putting a treadmill in neutral and strapping wings and a rocket to your butt. The only thing you will get is a few stares at the gym.

edit: now that I think about it, I do think it would work, just thinking of the forces in action helped


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## Kipper (Jul 25, 2006)

> Yes, the plane takes off. The conveyor has no effect on the plane other than causing the wheels to spin twice the speed that the airplane's airspeed is. The plane will move exactly as it would move if it were on a stationary runway.


Way to go!

Someone also stated it was like a rocket. That is pretty much what a plane engine is. Rockets move relative to the air that is around them. Ferq AR nailed it, the wheels will rotate 2X their normal. The plane will accelerate at its normal speed and as the air moves over and under the wing and accumulates sufficient lift, it will take off.

:thumbsup:


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## Kipper (Jul 25, 2006)

> The plane will go nowhere. For the plane to take off there needs to be lift forces present. Lift force is directly related to the velocity of the fluid around the wing. As the wing does not move there is no velocity, unless there is a hurricane in front of the wing.....
> So as a plane moves foreward, lift is created by the "wind". But if the plane does not move then there is no lift. You can test this theory by putting a treadmill in neutral and strapping wings and a rocket to your butt. The only thing you will get is a few stares at the gym.


:rotfl:

It is worth a try! :GotPics:

:rotfl:


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## mizzoueng (Jul 25, 2006)

:GotPics: :thumbsup:


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## civengPE (Jul 26, 2006)

Miz,

It would fly because it is moving. if you were standing to the side of the conveyor the plane would accelerate and takeoff normally. the wheels would just be spinning twice as fast. once again, the engine imparts it's energy on the air not the ground. if it did planes couldn't fly anytime.


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## Hill William (Jul 26, 2006)

> Miz,It would fly because it is moving. if you were standing to the side of the conveyor the plane would accelerate and takeoff normally. the wheels would just be spinning twice as fast. once again, the engine imparts it's energy on the air not the ground. if it did planes couldn't fly anytime.


Agree 100% the wheels having nothing to do with making a plane fly.


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## Kipper (Jul 26, 2006)

Great pic mizzou! :lmao:

Is that one of the stones from the strong man competition in your avatar?

I've heard of guys with a chip on their shoulder, but not a freakin boulder! :lol:

I think I saw daylight underneath the treadmill.


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## MA_PE (Jul 26, 2006)

> It is how fast you move through the air and not the ground.
> If you had a 80 mph head wind you could take off with zero ground speed.
> 
> Pilot in Training.
> ...


I agree with this statement. However, in the example problem you're creating a "local headwind" by moving air past the plane using the jets. I am not convinced that a jet will move enough air past it to cause lift. It will cause enough local pressure behind the jet on the surrounding to motivate the aircraft through the atmosphere enough to cause lift but I don't believe it could do this while staying stationary relative to the overall atmosphere. As soon as the plane left the conveyor I believe it would drop. It depends on the where the boundary transitions from the moving air mass created by the planes engines to the actual ambient wind speed. If you were in a closed boundary where the jet could in fact move ALL of the air in the atmosphere then the plane would fly. However, there will be a boundary plane.

Think of a big fan in front of a stationary tethered glider on a conveyor, as soon as the glider rose above the wake of the fan, it wouldn't fly anymore.


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## civengPE (Jul 26, 2006)

Ma,

It's not the localized wind caused by the engines that is causing the lift.

The plane will actually move! the thrust is imparted on the air not the ground. it accelerates exactly the same way it would off of the conveyor.

if you had two planes side by side with one on a conveyor and the other not, they would remain side by side the whole time.


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## civengPE (Jul 26, 2006)

would everyone agree that if the plane was tethered by a rope and the conveyor turned on, at say 100 mph, that the tension on the rope would only be cause by the friction of the wheel hub?

replace that tether with a small amount of thrust from the engine.

now, add full power, you have a net forward force that causes acceleration.


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## EdinNO (Jul 26, 2006)

I'm picturing a commercial jet. Isn't it just pumping air through the jet turbine and not over the wings? ISn't it just sitting stationary with air pumping through the turbine? AIr is not moving across the wings. If these thought are right and ir doesn't move across the wings, how can Bernoulli's equations come into play and lift the plane? We need air across the wings and the way I see it, its not moving acorss the wings because they are stationairy relative to the air. gRanted, air does funnel through the turbines, but I don't think its hitting the wings.

Ed


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## civengPE (Jul 26, 2006)

Ed,

see above.


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## civengPE (Jul 26, 2006)

??FX = F(thrust) - F(Friction) - F(Drag)

If F(thrust) is substantially larger than both F(Friction) and F(Drag) Then the net force is positive. Therefore the plane accelerates.

The only way for the plane not to move would be the following:

F(thrust) = F(Friction) +F(Drag)

The only difference between the conveyor and non conveyor scenarios is that the F(Friction) is larger in the conveyor case.

:MIG:


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## DVINNY (Jul 26, 2006)

WOW, how did I just miss this thread until now?

Good stuff. The plane will take off just fine.

Screw the conveyor, it could be solid ice on the runway for all I care, it will take off. However, landing on solid ice will suck.


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## DVINNY (Jul 26, 2006)

> More.... I want more questions.
> I like these kind in stead of the stupid riddles.  THis one is actually physics (engineering) based.
> 
> what happens if you tug on a toilet paper roll parallel to the ground???


You didn't like my 'stupid' riddles before?

geesh.

I was trying to spur this type of thing, but nobody would help out.

now my feelings are hurt.


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## Hill William (Jul 26, 2006)

I like hangin out with other engineers. My wife would say "Who cares". We will argue for hours to prove our points and have guys putting equations and stuff on the board to solve a riddle. GOOD STUFF :claps:


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## civengPE (Jul 26, 2006)

DVINNY,

I only think the other ones are stupid, because I can never get them without google.

Thank God for Google.

With the engineering (physics) ones, I feel I at least have a fighting chance without having to cheat.


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## MA_PE (Jul 26, 2006)

> Ma,It's not the localized wind caused by the engines that is causing the lift.
> 
> The plane will actually move! the thrust is imparted on the air not the ground. it accelerates exactly the same way it would off of the conveyor.
> 
> if you had two planes side by side with one on a conveyor and the other not, they would remain side by side the whole time.


sorry civengPE, your reply makes no sense to me. You need to re-read the problem statement.



> The question is:
> A plane is standing on a runway that can move (some sort of band conveyer). The plane moves in one direction, while the conveyer moves in the opposite direction. This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in opposite direction).
> 
> Does the plane ever take off, and why or why not?


the conveyor MOVES. I assume under its own power like a treadmill and counteracts the "forward" motion of the plane. It is not accelerating into the atmosphere like a plane on a runway, it is a feedback system where the ground below keeps pace with it so that the net movement forward is zero. It is "accelerating" relative to the grouns but the net airspeed is zero (except for the air moving through the turbines.

Same comment applies to DVINNY's thoery. Putiing ice underneath is not equivalent to the problem statement the plane will move out of position on ice, same as if it took off on asphalt. This is completely different from the stated problem.

Think of two runners side by side. One on a treadmill one on the ground. If they run in unison with the same stride length. Ambient air is static. The guy on ground will be movig forward and feel a breeze in his face. The guy on the treadmill won't feel any breeze because his body is stationary relative to the ambient air.


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## Hill William (Jul 26, 2006)

Yeah but they are moving from interaction with the ground. The plane is not.


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## civengPE (Jul 26, 2006)

MaPE,

You almosty there. The runner is pushing off of a moving surface, the conveyor,.

The airplane is not. It is pushing aginst the air.

it wouldn't matter, except for friction in the wheels, if the conveyor was traveing a million miles per hour.


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## Hill William (Jul 26, 2006)

> You almosty there. The runner is pushing off of a moving surface, the conveyor,.
> The airplane is not. It is pushing aginst the air.
> 
> it wouldn't matter, except for friction in the wheels, if the conveyor was traveing a million miles per hour.
> ...


Agreed


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## civengPE (Jul 26, 2006)

Very well put Sap.

Class Dismissed!

:claps:


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## TouchDown (Jul 26, 2006)

> I'm picturing a commercial jet. Isn't it just pumping air through the jet turbine and not over the wings? ISn't it just sitting stationary with air pumping through the turbine? AIr is not moving across the wings.


Air is moving across the wings, and could come into play, but the engine supplies enough power to push the plane forward getting much more thrust due to a significantly higher volume of air moving around wings (just not enough to pump through the engines)...

However, when the jets sit on runways, they will most often be at low throttle and when you are sitting on the runway just ready to take off, you hear the engines power up, but you don't move until they remove the brakes and you feel the sudden acceleration. The jets also use the redirection of their exhause to push the plane backward from the terminals, just by putting plates behind the jet exhaust it'll force the jet backward.

I agree with those who said the plane will take off. I'd like to see who can build a conveyor fast enough and wheels strong enough to handle moving it to the point of causing enough drag on the jets to prevent lift off. In fact - REMOVE the wheels and see if the jet can take off. That'd be an interesting experiment. :hung:


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## MA_PE (Jul 26, 2006)

Still haven't convinced me and bman didn't come to a conclusion.

I'm agreeing with everthing he writes except his assumption of a "stationary air mass". The plane has velocity relative to what? In a nomal take-off the ground and air mass are stationary and the jet propulsion causes a relative velocity between the plane and the stationary air mass (and ground) causing lift. In the problem the relative velocity between the plane and the air mass is created by the jet propulsion inducing flow of the stationary air mass across the plane. I'm not convinced a jet create a flow large enough to maintain flight. I believe that the problem will reach equilibrium before the plane is airborne. Equilibrium being the conveyor will keep pace with the jet until it can push no more.

If the air mass moved by the jet had infinite boundaries then I believe it would take-off but practically speaking, I don't believe it would happen.

Time to write to mythbusters.


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## DVINNY (Jul 26, 2006)

I still stand by my statement, ice is the same as a conveyor.

Fact is, an airplane moves because the jet engine thrusts it. Not because the wheels on the ground are moving it.

Those wheels are just along for the ride. They have no influence other than frictional loses.

Hell, I say cut the damn wheels off, lay that airplane on its belly on the conveyor belt, fire that belt up, and the thrust from a jet engine will STILL propel that sucka up the road a piece. It will light that conveyor on fire, but that plane will still move forward.


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## DVINNY (Jul 26, 2006)

> Time to write to mythbusters.


That's the best idea that I've heard yet.

And I'm serious. we should.


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## civengPE (Jul 26, 2006)

DVINNY,

I went to mythbusters site and they have a 9 page forum discussion on the exact same topic.

It looks like this debate has made the rounds before.


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## civengPE (Jul 26, 2006)

Ok. This is my last attempt to convince the NON-BELIEVERS! rayers: :brick:

Assume that the wheel hubs are frictionless just for argument sake.

When you turn on the conveyor, the plane, with the engine off, does not move.

now provide thrust from the engine. Viola! The damn thing starts to move.

since the wheel hubs have very little friction compared to the thrust the plane will take off!!!

Just draw a Free Body Diagram.

Rant Over!!!


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## civengPE (Jul 26, 2006)

Check it out.

http://community.discovery.com/eve/forums/...76/m/5471975538


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## MA_PE (Jul 26, 2006)

I'll try it a again. Planes are not rockets. they don't fly from thrust alone.

They NEED the lift from the air traveling over/under the wings. Whena a plane is stationary the whhels support it. At the end of the runway thrust is applied against the static air mass behind them. Note the thrust nneds to react against something because in a vaccuum you wouldn't get any tendency for forward motion. As the plane starts to move forward it pushes through the atmosphere causing lift. The amount of lift is proportional to the velocity of the plane relative to the atmosphere. In the subject problem you don't allow the plane to got forward. Instead, the ground moves back so the plane stays in its initial location. However, the air mass moved by the turbines needs to go somewhere right? Air is compressible so it can displace rather readily allowing the plane to slip back with the conveyor. At the initial start up, I believe this scenario will happen because the thrust is still ramping up. Each time the plane tries to move forward the conveyor restores it ot the initial location. Work by the turbine is used to move air from in fornt of the plane to the rear. This will continue until one of the following conditions controls:

1) Resistance to the thrust behind the plane (I believe this is controlled by air density which is a function of temperature and velocity) remains sufficient to increase the velocity of the moving air mass around the plane to a critical velocity where the plane will rise due to the lift provided by the air traversing the wings.

2) There is not sufficient resistance to the thrust to increase velocity and there is not enough lift to raise the plane off of the conveyor so the plane continues to "roll" in place ad infinitum.

3) the air mass moved by the jets is sufficient to saturate the area behind the jet such that air behind the jet is no longer compressible, a net forward thrust develops which overcomes the normal force of the plane, the jet breaks free and launches up or forward off of the conveyor with the wheels spinning at inifinite RPM's and crashes to a violent death because ti still hasn't reached critical velocity relative to the local ambient air to fly.


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## civengPE (Jul 26, 2006)

"In the subject problem you don't allow the plane to got forward. "

Thats it! The plane does go forward. There is nothing holding it back!


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## EdinNO (Jul 26, 2006)

MA'a theory sounds the most correct to me. It MAY rise into the air theoretically, but I ain't buying no ticket for that flight!

Ed


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## civengPE (Jul 26, 2006)

Ed, It won't rise into the air. It will take off normally.

There is no force to counteract the thrust. Therfore the plane accelerates.


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## EdinNO (Jul 26, 2006)

Again, I'm still not 100% sure on this one. MA's latest 3 scenarios seem the most logical to me. Maybe no one else has broken it down explicitly enough for me to see it any other way.

Theoretically, MA's post covers three possibilities. Perhaps the jets do induce movement of the overall surrounding air mass across the wings. That could, possibly happen I suppose. I'm not arguing that EVENTUALLY, this could occur. Would it be enough movement, enough velocity to lift the plane? I don't know. I wouldn't want to be on it.

I would like to see a concise, scientific arguement on both sides so I can make a better juudgement.

Either way, great discussion from both camps.

Ed


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## EdinNO (Jul 26, 2006)

I think that in MA's first scenario, he is agreeing that it will fly and take off.

Not sure though.


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## MA_PE (Jul 26, 2006)

> If a plane required the ground to be stationary in order to fly, sea planes wouldn't be able to take off.


OK we'll use your example. the sea plane is on a river the river is running wildy. The seaplane goes to take off by running up river. If the wind over the river and the river flow are traveling at a fixed velocity wich is equal to his critical velocity, I predict the following will happen. As he points up river and throttles up the relative poisiotn of the plane will be moving downstream. The rate he moves downstream will decrease as he throttles higher because his upstream velocity is increasing. (Remember kids, change in velocity with time is acceleration). He continues to accelerate until he reaches critcial velocity at which point he is now stationary with respect to his position over the river and he is airborne because he has enough lift to raise the plane.

The key is that the air around the plane needs to move at the critical velocity to lift the plane.

I don't believe that a jet, allowed to slip downstream would be able to take off because I don't think thrust alone is sufficient to over come the loss of ground during the start up.

Come to think of it. Are there any jet sea-planes? That might answer the question.

FWIW: I'm having a good time.


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## petergibbons (Jul 26, 2006)

The plane takes off. The belt can't counteract the acceleration of the plane. The belt REACTS to the plane so the belt acceleration would always occur after the acceleration of the plane thus the plane moves forward. It might take a long time for the plane to reach the required speed for lift-off though.  But what do I know?

BTW: My head hurts!


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## civengPE (Jul 26, 2006)

NEED BEER HEAD HURT.

Beer was always one of my best design aids in college. We had a fridge in the concrete lab that was supposed to be for samples etc. We kept it full of beer.

Our profs would come down and have a few with us. it was always a good time.


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## MA_PE (Jul 26, 2006)

> MA_PE, you are thinking about a sail boat, which requires the wind to move. A plane does not need moving air, it needs to move against air by way of thrust. The air could be moving or stationary and a plane would fly from solid ground or a river. I am having a good time as well. However, the truth of the matter is that as a damn civil engineer, I would just design a better runway, and not fool around with the damn thrust and all that freaking mechacal BS.


sapper:

do you read my posts at all? FWIW: I'm an ME with a structural PE.

Planes don't need thrust to fly they need a relative velocity between the plane and atmosphere sufficient to produce enough lift under the wings to make the plane fly (ever heard of a glider?)

It has nothing to do with sailboat. Of course that powered by the realtive velocity between the air and the ground.

Using the same example if you had the wind blowing upstream at the same speed that the river is flowing down stream, you could theoretically have a sailboat that is filled with wind but making no headway upstream.

Just like uncles and cousins, it's all relative.


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## DVINNY (Jul 26, 2006)

> I'll try it a again. Planes are not rockets. they don't fly from thrust alone.


No,

They don't fly from thrust alone. BUT THEY DO MOVE FROM THRUST ALONE.

they fly because the wind gets under the wings


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## EdinNO (Jul 26, 2006)

Peter's theory makes sense. The conveyor would react to the palne's motion and eentually, before the conveyor could react and "catch up" to the mothion of the plane, the plane could pop up into the air from that second or two of forward motion relative to the air. It would be sketchy in my mind, but I suppose it could happen. Its all theoretical and I woudl still think the plane could crash pretty quickly if things weren't just right.

But then again, what if we took the original statement to mean that the conveyor was perfect and there were no lag in the conveyor's speed to the plane's speed.

I still see the best scenarios so far as being the three by MA just a few posts up.

Ed


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## DVINNY (Jul 26, 2006)

You guys are thinking WAY too much into this.

The plane would not lag or anything. The wheels would be spinning twice as fast, but the plane would take off just as it would on an asphalt runway.


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## civengPE (Jul 26, 2006)

Or concrete. Don't forget about concrete. everyone always forgets about concrete.

I think I'm losing it.

Is it just me or does it annoy you when peole refer to concrete as cement? :brick:


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## EdinNO (Jul 26, 2006)

What do you mean by the wheels would spin TWICE as fast? Not following you on this one?

Ed


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## DVINNY (Jul 26, 2006)

> Is it just me or does it annoy you when peole refer to concrete as cement? :brick:


My wife was doing it last night.

Said her parents need to cement their whole walk.

Go figure.


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## Kipper (Jul 26, 2006)

> MA'a theory sounds the most correct to me. It MAY rise into the air theoretically, but I ain't buying no ticket for that flight!
> Ed


Ed, if it has sufficient lift to rise into the air at all then that means the plane is moving forward. The only thing the conveyor affects is the wheels, not the fuselage, wings, or the air around it. Book the flight! You only live once.


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## DVINNY (Jul 26, 2006)

I'm being serious.

The plane would take off at the exact speed and acceleration it normally would.

However, the wheels would spin twice as fast on the conveyor as what they would on non-moving pavement.


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## EdinNO (Jul 26, 2006)

The plane is moving opposite the conveyor, but at the same speed. How do the wheels spin twice as fast?

Ed


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## DVINNY (Jul 26, 2006)

> The plane is moving opposite the conveyor, but at the same speed. How do the wheels spin twice as fast?
> Ed


Are you messing with me?

or do you really not get what I'm trying to say?

You're just eggin' me on aren't ya?


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## Kipper (Jul 26, 2006)

sapper:

do you read my posts at all? FWIW: I'm an ME with a structural PE.

Planes don't need thrust to fly they need a relative velocity between the plane and atmosphere sufficient to produce enough lift under the wings to make the plane fly (ever heard of a glider?)

It has nothing to do with sailboat. Of course that powered by the realtive velocity between the air and the ground.

Using the same example if you had the wind blowing upstream at the same speed that the river is flowing down stream, you could theoretically have a sailboat that is filled with wind but making no headway upstream.

Just like uncles and cousins, it's all relative.


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## DVINNY (Jul 26, 2006)




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## DVINNY (Jul 26, 2006)

> Essentially, the plane is moving twice as far relative to the conveyor, but not twice as far relative to the earth, wind, atmosphere whatever. The plane still takes off in the same distance as it would relative to the air. But because the conveyor is moving the same speed as the plane, the plane is twice as far from the starting point on the conveyor than it is releative to the air, and the wheels have to travel twice the distance in the same amount of time, causing them to spin twice as fast.


You're dead on brother.


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## civengPE (Jul 26, 2006)

with our technical skills we should be able to do just that DVINNY.

I am thinking we could build a conveyor belt powered by a lawn mower engine. It should be around 50 feet long. We could then get one of those cheap battery powered R/C planes and away we go.

What do you think?

Do you think it would finally convince the non believers?


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## DVINNY (Jul 26, 2006)

I'm just thinking that it would be impossible to have the speed of the conveyor match that of the plane's thrust.

Or I'd be game.


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## civengPE (Jul 26, 2006)

I agree on matching the speed, but we could set it at the maximum speed of the plane.

ie. the worst case scenario.

we could just use some sort of backstop to hold the plane in place prior to hitting the gas. After that the plane should just pull away from the backstop.

One problem we have not discussed is controlability. I know I have enough trouble keeping the plane straight down the runway on a stationary surface. all control inputs on the conveyor will be multiplied until the control surfaces take ove and you are not steering by the nosewheel anylonger.


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## Hill William (Jul 26, 2006)

If we are talking frictionless hubs, once the conveyor started, the plane would sit still (relative to the ground) unless you pushed it, right? No matter how fast the conveyor. Crank up the engines and it would apply a force using the air, right? Plane moves forward relative to the ground and takes off. Just my $0.02


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## MA_PE (Jul 26, 2006)

You guys are killing me.

I interpret the problem statement as the conveyor belt is in fact a driven treadmill.

Draw dotted lines indicating a controlled volume (CV) around an image of the plane and the conveyor. Consider ythe conveyor as an infinte movable horizontal surface, one line entering the CV in the front and exiting in the back.

The jets start , they move air from the fron of the CV compress it and exhaust it out the back of the CV. They hit cold stagnant air and exert thrust that wants to push the body of the plane forward. If it were on firm ground it would translate in the x-axis. As soon as the jet overcomes static friction and starts to roll forward (on its wheels) the conveyor moves to the rear so as a point on the wheel rotates forward the contact points on the conveyor roll backward. Net is that there is no translation of the plane body. Resistance to the thrust is not constant because you have a compressible gas behind the jet that may or may not be able to resist the weight of the plane being pushed back by the conveyor. Da plane does not advance on the x-axis.

The exhaust gasses have displaced the static air immediately behind the jet (how far???) we don't know. Air sucked in by the jets is replaced by static air in front of the CV due to atmopsheric pressure.

As the thrust increases the plane body wants to move forward faster. The counter action of the conveyor keeps the plane body stationary. Simultaneously air is entering and exiting the CV at a faster rate.

My previous post lists the three outcome scenarios for this model.

the rotating speed of the wheels is completely irrelavent. What is important is 1) Can the air behind the CV keep exerting resistance to the thrust? If it doesn't then the whole thing reaches equilibrium and the flow throught he CV is not enough tot lift the plane so the plane and conveyor roll merrily along until you trun it off. 2) Will the air passing through the CV reach critcal velocity and provide lift? Note: the CV has a top boundary, too. Once the plane is outside of its own little "wind tunnel" what happens? dunno either it flys or see 3. 3)Can the air behind the CV keep exerting resistance to the thrust? If it does then it should lift off of the conveyor What happens? dunno see 2.

Kipper:

what the H-E-double hockey sticks are you talking about? The sailboat was for slapper's benfit that you can have a sailboat working it's ass off and go nowhere too.

DVINNY sorry dude, I don't know where your going with wheel speed but it is completely irrelavent to this problem in my mind. FWIW: planes need wheels to reduce friction so that they can attain thier critical velocity with minimal thrust. Plus it makes for nice landings.


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## MA_PE (Jul 26, 2006)

> Crank up the engines and it would apply a force using the air, right? Plane moves forward relative to the ground and takes off.


Air is a gas, it flows. think of a boat propeller in a running stream. not to mention gasses are compressible.

jet thrust is in thousands of pounds atmospheric pressure is 14.7 psi (2116.8 psf) you push more than that and you need to resistance from somewhere.


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## Kipper (Jul 26, 2006)

> The sailboat was for slapper's benfit that you can have a sailboat working it's ass off and go nowhere too.


:beerchug

But the plane isn't working its ass off, it is taking its ass off.

:beerchug


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## civengPE (Jul 26, 2006)

:rotfl:


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## DVINNY (Jul 26, 2006)

> DVINNY sorry dude, I don't know where your going with wheel speed but it is completely irrelavent to this problem in my mind.  FWIW: planes need wheels to reduce friction so that they can attain thier critical velocity with minimal thrust.  Plus it makes for nice landings.


The wheels are VERY relevant because they are what makes the whole problem.

The planes thrust (acceleration) is relative to the air. The conveyor is relative to the planes speed. As that plane goes faster in the positive direction, the conveyor moves at the equal rate in the negative direction.

the (very important) wheels in between spins between the two, at the combined speeds.

If the wheels were fixed to the plane, and DID NOT spin, then they would create a force in the negative direction for the plane that would counter the thrust, but that is not the case. The plane is not fixed to the conveyor, it has wheels in between.

BIG DIFFERENCE


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## DVINNY (Jul 26, 2006)

FWIW,

I'm having fun too. :thumbsup:


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## MA_PE (Jul 26, 2006)

> If the air moved at a rate of speed equal and oppostite the plane, then the plane wouldn't move forward because there would be a net force of zero of thrust against the air.


That is exactly where you and DVINNY go off track. Fluids CAN move past objects. Ever hear of a wind tunnel? How about the new craze of indoor sky diving.

The plane CAN fly without moving forward as long as the windspeed relative to the plane is greater than the critical velocity needed to provide enough lift.

How a bout a bird hovering over the ocean in the same spot or a kite flying?

It has nothing to do with forward motion.

DVINNY:

Don't be pullin' this stuff on me.



> Hell, I say cut the damn wheels off, lay that airplane on its belly on the conveyor belt, fire that belt up, and the thrust from a jet engine will STILL propel that sucka up the road a piece. It will light that conveyor on fire, but that plane will still move forward.


then you go agreeing with me and trying to make it look like I'm inconsistent.



> The wheels are VERY relevant because they are what makes the whole problem.
> The planes thrust (acceleration) is relative to the air. The conveyor is relative to the planes speed. As that plane goes faster in the positive direction, the conveyor moves at the equal rate in the negative direction.


AAAAAH :suicide:

I believe that we're all converging on this one. good time, but I really should get some work done.


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## Hill William (Jul 26, 2006)

Somebody should post this on "the other board". Im sure they would know whats up. 

:"the other board":


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## petergibbons (Jul 26, 2006)

> If you were sitting in a wheel chair and had big wings strapped to your back, and the wheel chair were on a conveyor belt, And I was walking along side the belt on solid ground and pushing you, then you would move. If I pushed you fast enough the wings would provide lift and you would take off.


Sapper,

Are you saying this is equivalent to what we have in the problem statement?


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## Hill William (Jul 26, 2006)

If I ever get hurt real bad, I want a wheelchair with wings.


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## MA_PE (Jul 26, 2006)

sapper:

I feel a strong wind coming on now.

Quick I should grab the covers and pull them over my wife's head.

I think this horse is dead. I not going to beat it anymore.

Next!!!


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## Kipper (Jul 26, 2006)

> sapper:I feel a strong wind coming on now.
> 
> Quick I should grab the covers and pull them over my wife's head.
> 
> ...









Here you go MA.


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## DVINNY (Jul 26, 2006)

But the plane will still take off just fine. Even with the covers over your head.

LOL.


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## DVINNY (Jul 26, 2006)

> Ever hear of a wind tunnel? How about the new craze of indoor sky diving.


OK,

if the guy that is doing the indoor skydiving was wearing a pair of roller blades, and they were on a vertical conveyor that was moving in the opposite direction, would he now no longer be able to skydive? would the roller blades scooting along that conveyor make him fall on his face?

I guess so with your theory.


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## MA_PE (Jul 26, 2006)

> But the plane will still take off just fine. Even with the covers over your head.



The question is whether I can sustain the velocity needed to initiate flight 



> if the guy that is doing the indoor skydiving was wearing a pair of roller blades, and they were on a vertical conveyor that was moving in the opposite direction, would he now no longer be able to skydive? would the roller blades scooting along that conveyor make him fall on his face?


If he got his lift from a jet pack, needed wings to fly laterally away from the wall, and gravity acted horizontally, then ...yes I believe so. :blink:


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## DVINNY (Jul 26, 2006)

*FROM A GOOGLE SEARCH*

Airplane on a Conveyor Belt

A riddle was proposed on the Neal Boortz show today:

If an airplane is on a large conveyor belt and is trying to take off by exerting the thrust needed to move it forward at 100 knots, and the conveyor belt starts moving backwards at 100 knots, will the plane be able to take off, or will it just sit stationary relative to the ground, with the backwards speed of the conveyor belt counteracting the forward thrust of the plane?

Astoundingly, Neal and the rest of his crew took the position that the plane would sit there stationary! Good God? this man is a pilot and has a law degree! I could understand a random high school dropout being fooled by this, but a pilot?

Then I googled the riddle, and found a thread on Airliners.net that has been raging on, with the vast majority of people taking Neal?s position? that the plane would not be able to take off.

Their argument is this, to quote one poster:

Thrust acts accordingly to Newtons Third Law of Motion - every action has an equal and opposite reaction. In the case of an aircraft, the reaction of the engines is that of forward motion, against whatever medium it is stationary. But the ground the aircraft is sitting on in this case is NOT stationary, its providing an exactly CANCELLING force pushing the aircraft back.

The problem here, of course, is that the poster (and Neal) cannot disengage themselves from seeing the airplane as a car. The difference between a car and a grounded airplane is that a car uses its wheels to propel itself forward, and an airplane moves itself forward by moving air. They assume that the runway moving backwards would move the plane backwards. This is what would happen with a car (that is in gear), so why not for an airplane? Well, because an airplane?s wheels are free rolling. There is obviously some friction, so there would be some small backwards force, but it would be infinitely small as compared to the forward thrust of the airplane.

You can test this with a piece of paper and a matchbox car (which has free rolling wheels like an airplane? or like a car in neutral.) Place the paper on a table, and place the matchbox car on the paper. Take your hand, and hold the car still with a lightly placed finger on top of the car. At this point you are providing no forward thrust, and the ?conveyor belt? is not moving. The car remains stationary. Now, continuing to hold the airplane with a lightly placed finger, and start to pull the paper out from under the car, in the backwards direction. According to Neal?s logic, the car should push back on your finger with the same force that you are exerting on the paper? but this is not what will happen. You will find that your lightly placed finger is not stressed to any noticeable extent. The paper will slide out, and the wheels will spin, but the car will not be propelled backwards. The reason for this is is that the rotation of the wheels is not related to the movement of the matchbox car except by the very small friction component of the axle, which your lightly placed finger can easily control.

So now we have established that movement of the surface beneath a free wheeling object does not exert a noticeable force on the object. Next, we?ll see what happens when the object is trying to move forward. Attach a string to the matchbox car. Place the car at one end of the paper, and use the string to start pulling the car forward with a steady force. As the car moves forward, start pulling the paper out from under the car, backwards. Do you feel increased resistance as you pull the string? Of course not. The wheels are free rolling! Spinning the wheels does not make the object move!

When an airplane takes off, there is one major forward force? the forward thrust. The main rearward force is air resistance. The turning of the wheels provides a small frictional force, but because the wheels are free-rolling, this friction is very small. Unless the wheels are locked, the friction is always going to be less than the thrust, which means that the overall force is still forward, and the plane will still move


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## DVINNY (Jul 26, 2006)

> and gravity acted horizontally, then ...yes I believe so. :blink:


Dude,

if I could do horizontal gravity, we'd all be RICH.


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## MA_PE (Jul 26, 2006)

DVINNY:

Are you trying start from scratch again????

The key to the puzzle is how consistent the air resistance immediately behind the plane is at providing a reaction for the forward thrust. I'm not convinced it's a fixed value because the plane is not allowed to move forward into "fresh" air. answer "dunno" see scenarios 1 through 3 above.

time for a little horizontal bop.


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## EdinNO (Jul 26, 2006)

I think this is it:

The two sides are looking at the definition of the situation differently.

1. Some look at it as the plane is totally stationary in that however it tries to move, the conveyor counteracts it. That in mind, the plane wouldn't move or fly.

2. Others look at it as the conveyor moves backward say 150 MPH relative to the ground, the plane moves forward 150 MPH relative to the ground and the wheels move forward 300 MPH relative to the ground. In this case, the plane will fly as normal.

Its all in the way question is perceived. I have been on both sides of the fence. Now I can see the second scenario as being more correct. But if the givens of the question were as assumed in scenario 1, it might not move or fly. This would assume that something (a cable, a large arm, etc...) were holding back the plane.

I think it will fly. Now quit boggling my feeble mind with this kind of stuff. Jessh! I'm just recovering from the April exam! 

Ed


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## DVINNY (Jul 26, 2006)

For what it's worth. The others in my office all disagree with me and think that it won't move.

Sometimes its lonely being correct. (just kiddin)


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## cement (Jul 26, 2006)

> For the record. I found this same problem and subsequent discussions on another invision board. The thread is 341 pages long. We all need to just stop it right now.


at 110 posts, this rivals the +1 fest stickied above. what is the next question, oh riddler?


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## DVINNY (Jul 26, 2006)

Regardless. You're screwed.


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## EdinNO (Jul 27, 2006)

I would say yes to the last question- you would add the amount of PE (potential energy) you had stored in you.

Maybe also look at the impact loading equation.

Ed


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## civengPE (Jul 27, 2006)

Ok Guys. I emailed my friend who is an aerospace engineer that designs and tests missiles for the air force. Here is what he says. (it wil take off normally)

I've never heard of this riddle, but it's interesting, and after some thought...I've decided it's a trick question and depends on your assumptions. I'll TRY to give you a quick explanation but I don't think I can. Forget about the conveyor belt for a second and let's establish the physics. If you sum vertical forces, for an airplane to takeoff, the lift generated just has to barely overcome the weight. We know that lift is proportional to dynamic pressure which is equal to 1/2*density*V^2. That velocity is the velocity of the median or in our case air particles (could be any gas) relative to the wing. Period end of story, if air particles don't reach a certain speed relative to the wing then no lift. So reading the problem again, if you assume the airplane is not moving in some inertial reference frame (the ground) then the airplane won't do shit. Propeller airplanes will pull some air over the wings but not nearly enough to counteract weight and we're assuming there's not a 300 knot headwind.

But now lets look at the horizontal components, thrust and drag. As any good PE and pilot should know, airplanes create thrust from Newton's 2nd law of equal and opposite forces. Propeller airplanes take a whole lot of air (big mass) and push it back just a little bit (small velocity), while jet aircraft take a little bit of air (small mass) and push it back really fast (big velocity) so that pushes the airplane forward. So our airplane can create thrust but now drag. If we assume we're not moving in the ground frame then there is zero aerodynamic drag, but even if we are moving it's not a large number. And now we come to our conveyor belt, the only other horizontal forces would be drag forces on the wheel, which I'll put all into rolling friction (axle, wheel deformation,etc) which once again ain't much. So we've established that we can create a shit load of thrust and the only force to counteract it comes from rolling friction. So if you assume rolling friction is constant vs rolling speed it would be impossible to speed the conveyor up enough to give it a ground speed of zero, but if you assumed rolling friction increases with speed then it would be "theoretically" possible to move the conveyor fast enough to zero out it's speed, but obviously you'd blow out any real world tire due to heat well before that happens. And that speed is certainly faster than an equal and opposite speed of the airplane.

So again, if I take the question literally, because it states that a plane is moving in one direction while the conveyor is moving in the equal and opposite and it states that this is accomplished by measuring the plane's speed and matching it with the conveyor, then if you assume the velocities are measured in the ground frame, then simply matching the plane's ground speed wouldn't do much. Therefore, *yes it would take off* (assuming your tires stay intact). Man, I hope that was clear.


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## MA_PE (Jul 27, 2006)

All these aerospace guys and noone has yet described the ambient air pressure in the immediate vicinity of the plane.

civengPE: I follow with what your friend is saying and agree. However, I see two keys to the problem that no one has answered and I have said repeatedly I don't know the answer.

1) How is the thrust of a commercial jet engine affected by ramping up to speed at a fixed location in the atmosphere? The answer would be a comparison between the thrust curve for an engine mounted in a stationary test fixture comapared to the thrust curve for a moving plane taking-off. the difference is that the test scenario has to draw its intake air from the reservoir in front of it while the moving plane draws from an undisturbed reservoir (it is constantly moving into a "new" section).

2) Assuming the thrust is unaffected. Will the jet engines draw enough air flow over the wings to initiate flight.

Think about this. As long as the plane is on the ground, the forward thrust of the plane will cause it to roll (it ain't airborne).

As long as its rolling the wings aren't cutting through the atmosphere because the plane is not advancing forward because its on the conveyor.

The only air passing over the wings is air flow past the plane caused by the engines. This airflow has to reach critical velocity over the wings to cause the plane to lift off of the conveyor and advance forward in the air. Once this happens then the plane flys normally.

If there is not enough airflow over the wings then the plane won't take off and it will try to move forward off of the ground and sit and spin its wheels on the conveyor until it runs out of gas.

If I had to pick I'd say: I don't think the engines can do number 2 so I don't think the plane will take off. This is especially ttrue for a plane like a jet fighter where the intake is at the nose and the exhaust is at the rear with no path through the turbines over the wings.

I can't see any other scenario happening.

It's all about the airflow over the wings caused by the engines on a stationary plane and no one has addressed that.


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## MA_PE (Jul 27, 2006)

Another view is that the problem would be the same if you tethered the plane so it couldn't move forward until it lifted off of the ground, and didn't block air flow fore and aft. (take the wheels and conveyor right out of the picture. Would it take off?

I really need to do some work today.


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## civengPE (Jul 27, 2006)

I have an easeir one. This was one of those bonus questions on an exam in college.

You have a helium filled baloon in your car while traveling down the highway. What happens to the baloon if you suddenly slam on your brakes?

:MIG:


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## EdinNO (Jul 27, 2006)

HEre is one way civeng123 put it that I think aids in the discussion:

On the plane issue. Since your a boater, let me try this with you. 

You're in your boat with an icechest full of beer and three or four half naked girls. Below your boat (under water) is a long conveyor belt.

If you were to drop a wheel attached to your boat to the conveyor and turn on the conveyor would anything happen?

If you hit the gas, would you go. Of course. you see your boat is imparting it's thrust to the water not the air.

I think this will become clearer after many beers. That always helps me! 

MA, I was with you for a while, but now have corssed over to the dark side! 

Forget about lift for a minute. You know when a plane is taxiing, you feel the quick shots of thrust that get ti rolling- especially from a stop? Well this thrust will get it trying to roll. The conveyor will try to conteract this. Ultimatey, the thrust against the air by the jets will get the plane going forward, say up to 150 mph. It just has to. That's what jets do. By problem definition, the conveyor is hauling butt backwards, at 150 MPH. Now the plane is moving 150 MPH relative to the ground and 300 MPH relative to the conveyor. Granted, it may take a little longer to accomplish this because now you have more friction in the wheels pushing backwards.

Another quandry is wht happens if the plane is sitting still on the conveyor and then the conveyors starts rolling backwards at 150 MPH. THEN, the plane starts up its jets? Would it ever reach take off? I am thinking that the thrust of the jets would eventually negate its negative 150 MPH speed relative to the ground, get it to 0 MPH relative to the ground, then as the air stabilizes around the plane it would evnetually make it up to 150 MPH relative to the ground (and ambient air).

I am assuming 150 MPH is the magic number to get it to fly. Just pulled it out of the air- no pun intended! 

Ed


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## MA_PE (Jul 27, 2006)

Oh boy:

Ed using your boat example. The connection between the wheel and the boat is rigid and can take vertical load. The water is low enough so that there is vertical load on the wheel. The boat is only partially floating and is ultimately supported on the wheel. Turn on you motor. The boat wants to mnove formard but the conveor is moving so the wheel (and support and your boat) aren't moving up stream either. The water is moving under your boat and your speed realtive to it increases as the motor goes faster. as you go faster the boat starts to come up on plane (actual boating term). As it goes up on plane the wheel lifts off the conveyor and away you go up stream.

The question is: can you get the water under the boat moving fast enough to get the boat on plane?

If not then you won't go anywhere. Which you really don't care about with beer and naked girls.

Same problem, same answer.


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## Seajay (Jul 27, 2006)

I'm kinda stuck on the requirements to build a giant high speed conveyor belt that can keep speed with a Boeing or Airbus type aircraft. Given the loading it would have to be a series of gigantic rollers with some type of composite-flexo fabric-belt. I would hate to see the lubrication order on that site. Anywise, I don't think the plane will take off because its not moving forward to cut through the air and reduce the air pressure on the top of the airfoils in order to produce lift .


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## MA_PE (Jul 27, 2006)

and in a tug of war.

One team pulls with a 1000lbs in one direction and the other team pulls with a 100lbs in the other direction. What's the tension in the rope?


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## DVINNY (Jul 27, 2006)

Sapper,

You're post sums it best. And I agree 100%.

The wheels are the whole factor.


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## civengPE (Jul 27, 2006)

I just posted this in "the other board"'s forum. With their arcane structure, it aught to be funny to watch this thread completely take over the forum!!!

:bad:

You guys remember the engineering equations thread over there?


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## DVINNY (Jul 27, 2006)

> and in a tug of war.
> One team pulls with a 1000lbs in one direction and the other team pulls with a 100lbs in the other direction. What's the tension in the rope?


There is 1100 lbs of tension, but that rope is still gonna move in the direction of the 1000

:thumbsup:


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## DVINNY (Jul 27, 2006)

> I just posted this in PPI's forum. With their arcane structure, it aught to be funny to watch this thread completely take over the forum!!!
> :bad:


Why would you do that?

They are going to ban you now. I hope you didn't say the pilot was eating a sandwich, cause you'll get burned at the stake.


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## civengPE (Jul 27, 2006)

I don't care if they ban me. I never post over there. This place is much much better.

:"the other board":


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## cement (Jul 27, 2006)

what is the link to that other forum? :dunno:


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## benbo (Jul 27, 2006)

I've been reading this post and have to say my vote is with MA-PE. But then I'm an electrical engineer so I would just cut the power to the conveyor belt, or interfere with the control system. I do have a question though-

I am assuming we are taking a fixed reference point that is independent of hte conveyor and the plane to describe relative speed (or velocity). This reference point is the same as the air, assuming no wind. so the questionis the motion of the plane, or more specifically the wings, with respect to this fixed reference point (or the air). My question is, if my assumptions are correct, how can the plane and the wheels travel at different speeds? (I'm not talking rotation speed of the wheels, I'm talking forward motion of the wheel assembly). Wouldn't the wheels come off?


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## MA_PE (Jul 27, 2006)

> There is 1100 lbs of tension, but that rope is still gonna move in the direction of the 1000


very good, but I intended to post.

One team pulls with a 1000lbs in one direction and the other team pulls with a &gt;1000lbs&lt; in the other direction. What's the tension in the rope?

following this logic:



> 1. The conveyor moves as fast as the plane, not the wheels. The plane moves 300 feet forward relative to the ground, the conveyor move 300 feet backwards relative to the ground, the plane moves 600 feet forward relative to it's initial point on the belt. The wheels spin enough times to cover 600 feet, the conveyor does not move 600 feet. In this case the plane will take off.


there's 2000 lbs in the rope. :wtf:


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## DVINNY (Jul 27, 2006)

A rope is fixed in tension, a wheel isn't. It rotates as a result of the forces put on it.

If the plane were roped to the conveyor belt, we'd have no flight.


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## MA_PE (Jul 27, 2006)

I believe that the intent of the the problem statement means that the conveyor, acting as the ground that the plane is sitting on will effectively cancel any forward displacement that the plane attempts to make resulting in a net displacement of zero. This is the same as the rope problem. If the conveyor was frozen and the plane were traveling at 10 ft/sec then making the conveyor travel ay 10 /sec in the opposite direction would make the plane stationary again.

Reading anything further into the wording of the problem statement changes the problem dramatically.


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## benbo (Jul 27, 2006)

I'm reading the problem the same as MA_PE. Maybe that's why we agree. I see a treadmill with a pole sticking out of the ground next to it. You are running on the treadmill with your arms out like wings. Your fingers point directly at the pole, and never move relative to the pole. That way, I don't see how you could take off.


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## civengPE (Jul 27, 2006)

http://www.straightdope.com/columns/060203.html

On first encounter this question, which has been showing up all over the Net, seems inane because the answer seems so obvious. However, as with the infamous Monty-Hall-three-doors-and-one-prize-problem (see The Straight Dope: "On Let's Make a Deal" you pick Door #1, 02-Nov-1990), the obvious answer is wrong, and you, Berj, are right--the plane takes off normally, with no need to specify frictionless wheels or any other such foolishness. You're also right that the question is often worded badly, leading to confusion, arguments, etc. In short, we've got a topic screaming for the Straight Dope.

First the obvious-but-wrong answer. The unwary tend to reason by analogy to a car on a conveyor belt--if the conveyor moves backward at the same rate that the car's wheels rotate forward, the net result is that the car remains stationary. An aircraft in the same situation, they figure, would stay planted on the ground, since there'd be no air rushing over the wings to give it lift. But of course cars and planes don't work the same way. A car's wheels are its means of propulsion--they push the road backwards (relatively speaking), and the car moves forward. In contrast, a plane's wheels aren't motorized; their purpose is to reduce friction during takeoff (and add it, by braking, when landing). What gets a plane moving are its propellers or jet turbines, which shove the air backward and thereby impel the plane forward. What the wheels, conveyor belt, etc, are up to is largely irrelevant. Let me repeat: Once the pilot fires up the engines, the plane moves forward at pretty much the usual speed relative to the ground--and more importantly the air--regardless of how fast the conveyor belt is moving backward. This generates lift on the wings, and the plane takes off. All the conveyor belt does is, as you correctly conclude, make the plane's wheels spin madly.

A thought experiment commonly cited in discussions of this question is to imagine you're standing on a health-club treadmill in rollerblades while holding a rope attached to the wall in front of you. The treadmill starts; simultaneously you begin to haul in the rope. Although you'll have to overcome some initial friction tugging you backward, in short order you'll be able to pull yourself forward easily.

As you point out, one problem here is the wording of the question. Your version straightforwardly states that the conveyor moves backward at the same rate that the plane moves forward. If the plane's forward speed is 100 miles per hour, the conveyor rolls 100 MPH backward, and the wheels rotate at 200 MPH. Assuming you've got Indy-car-quality tires and wheel bearings, no problem. However, some versions put matters this way: "The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation." This language leads to a paradox: If the plane moves forward at 5 MPH, then its wheels will do likewise, and the treadmill will go 5 MPH backward. But if the treadmill is going 5 MPH backward, then the wheels are really turning 10 MPH forward. But if the wheels are going 10 MPH forward . . . Soon the foolish have persuaded themselves that the treadmill must operate at infinite speed. Nonsense. The question thus stated asks the impossible -- simply put, that A = A + 5 -- and so cannot be framed in this way. Everything clear now? Maybe not. But believe this: The plane takes off.


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## civengPE (Jul 27, 2006)

That article had this engineering diagram.

View attachment 135


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## civengPE (Jul 27, 2006)

http://www.avweb.com/news/columns/191034-1.html

What I learned from Old Hack was that an updated version of a question aimed at confusing folks over relative measurements of airplane motion and the medium in which it operates had shown up on the Internet, and it was causing the fracas in the Lounge. The question that has been going around is not particularly artfully worded, and I think that has caused some of the disagreements, but I'll repeat it as it is shown: "On a day with absolutely calm wind, a plane is standing on a runway that can move (some sort of band conveyor). The plane moves in one direction, while the conveyor moves in the opposite direction. The conveyor has a control system that tracks the plane speed and tunes the speed of the conveyor to be exactly the same (but in the opposite direction). Can the airplane ever take off?" My comment: Notice that the question does not state that the conveyor's movement keeps the airplane over the starting position relative to the ground, just that it moves in the direction opposite to any movement of the airplane. Initially, about a third of the folks here said that the airplane could not ever takeoff, because the conveyor would overcome the speed of the airplane and it could never get any airspeed. The rest said the airplane would fly. The "It won't fly, Rocky" group said that the conveyor would hold back the airplane. They asked us to imagine a person running on a treadmill. As he or she sped up, the treadmill would be programmed to speed up, just as the conveyor in the problem, and the person would remain over the same locus on the earth, while running as fast as possible. The argument was that if the airplane started to move forward, the conveyor program was set up to move the conveyor at exactly that speed, in the opposite direction, thus, the airplane would never move relative to the ground, and, because the air was calm, it could never get any wind over its wings. One of the analogies presented was the person rowing at three mph upstream in a river on a calm day. However, the current was flowing downstream at three mph, so the resultant speed with reference to the stream bank and air was zero, and thus there was no wind on the rowboat. I watched and listened to the disagreement for a while and was fascinated to see that the argument seemed to split between those who had some engineering or math background, all of whom said the airplane would takeoff and fly without any problem; and those with some other background, who visualized the airplane as having to push against the conveyor in order to gain speed. Because the conveyor equaled the airplane's push against the conveyor, the airplane stayed in one place over the ground and in the calm air could not get any airspeed and fly. It was an interesting argument, but as things progressed, more rational heads prevailed, pointing out that the airplanes do not apply their thrust via their wheels, so the conveyor belt is irrelevant to whether the airplane will takeoff. One guy even got one of those rubber band powered wood and plastic airplane that sell for about a buck, put it on the treadmill someone foolishly donated to the Lounge years ago, thinking that pilots might actually exercise. He wound up the rubber band, set the treadmill to be level, and at its highest speed. Then he simultaneously set the airplane on the treadmill and let the prop start to turn. It took off without moving the slightest bit backwards.


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## civengPE (Jul 27, 2006)

Those are two explanations from very informed sources. If you still can't see that the plane will take off, there is no hope. :suicide:

I think this horse is dead. Anyone want to venture a guess on the baloon in the car question?


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## benbo (Jul 27, 2006)

Now I see it! There is something in that question still drives me nuts. By the way- I do have an engineering and math background, including a BS, 800 on my math SATs and a PE, but it didn't seem to make any difference. :dunno:


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## civengPE (Jul 27, 2006)

Damn Sap. Did you write that with your feet?

Good illistration.

I don't think any amount of LOGIC will sway some.

At least benbo has finally seen the light.

Welcome to the logic side of the force!!!!!

I absolutely love this thread!! It should get stickied.


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## benbo (Jul 27, 2006)

If I didn't come around before I certainly will after that drawing. I always agree with the first person to whip out a drawing accompanied by some equations.


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## DVINNY (Jul 27, 2006)

I love the drawing. That's great.


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## MA_PE (Jul 27, 2006)

civengPE:

Thanks, those were the best explanations that I have seen yet, especially the first guy's.

You've convinced me and I have this to say to you, DVINNY, and slapper:

I was wrong and I admit it.

Now, can I get some work done, please?


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## MA_PE (Jul 27, 2006)

I just noticed that I wrote "slapper" not "sapper"

my bad, it was not intentional.

Thanks for a fun discussion!

FWIW: sap nice drawing, but it didn't help me a bit. "A" for effort, though.


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## civengPE (Jul 27, 2006)

Glad to have ya! I really enjoyed (for the most part) this debate. Especially since it never degraded into name calling and or insults. This is what being an engineer is all about in my eyes. Professionals rationally debating the merits of his / her argument.

Best Thread Ever!!! IMHO, but I'm a nerd.


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## DVINNY (Jul 27, 2006)

This in my opinion has been the best thread on this board yet.

MA_PE, why you gotta end it? I don't think there are any other's left out there?

I still got a PE here in the office that believes it won't move. I'm gonna go stir the pot with him. LOL.


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## DVINNY (Jul 27, 2006)

Thanks civengPE, now I chimmed in over there, and I need to refuse to go back to that site. I can't get drawn into there.

Heck, it would take 3 days to click on each individual message to see who is saying what.

I can't waste THAT much time. LOL.


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## MA_PE (Jul 27, 2006)

I'm not entering anything on this topic over there.

You guys can feel free to cut and paste my musings if you want to boost the responses. 

civengPE:

don't give away the riddle right away. Make them work for it.

I can't imagine taking 314 pages to reach a final concensus.


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## civengPE (Jul 27, 2006)

I am really hoping it will pick up some steam and completely take over the other forum. I wasn't trying to give the riddle away just yet, I was trying to stir the pot.

Feel free to throw some garbage around over in that thread to get em goin!!!!

:"the other board":


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## cement (Jul 27, 2006)

I already had lunch.


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## civengPE (Jul 27, 2006)

It only took about 8 responses before the insults started!

How F'ing funny!


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## civengPE (Jul 27, 2006)

Sap,

Don't give it away over there! I'm trying to stir the pot and see how bad it gets.

Throw some nonlogic into the mix to stir it up, but don't let them know the answer.


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## Road Guy (Jul 27, 2006)

Sorry guys, I watched this question go a about 30 pages on the jeep forum, hated to do it to you. I always felt the plane would take off, but had no clue on how it would work.

The thread ended over there when a navy pilot chimed in with a bunch of "physics-garbage" about how it would take off ... so that ended it...

be curious to see how it "takes off" on ppi


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## DVINNY (Jul 27, 2006)

Wait till some of the ones over there get involved. Like JoshTheToad, and others.

I can't even remember who all the trouble makers are over there, and I don't care. LOL.

I know that FudgePump is somebody here, it's got to be. Who knows.


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## civengPE (Jul 27, 2006)

Cool. I just wanted to make sure everyone here knew the purpose for my post over there. I really hope it pans out like I think it will.

H20 is a fucktard. (I Love That Word)

;guns;

:"the other board":


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## DVINNY (Jul 27, 2006)

I 'coined' fucktard a long time ago. LOL. (not really, but pretty much within my circle I did)

It is when someone doesn't deserve the full ennounciation of being called a "fu$*ing retard".


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## civengPE (Jul 27, 2006)

The first time I saw that word on this forum, I almost shot diet coke out of my nose. :blink:


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## cement (Jul 27, 2006)

> I 'coined' fucktard a long time ago.  LOL.  (not really, but pretty much within my circle I did)
> It is when someone doesn't deserve the full ennounciation of being called a "fu$*ing retard".


are you kidding? the first time i heard that I fell off my dinosaur. we were cruel kids in Jersey :bad:


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## civengPE (Jul 27, 2006)

Not only that, but he thinks everyone else is an idiot.

I Love It!

:???:


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## DVINNY (Jul 27, 2006)

> > I 'coined' fucktard a long time ago.? LOL.? (not really, but pretty much within my circle I did)
> > It is when someone doesn't deserve the full ennounciation of being called a "fu$*ing retard".
> 
> 
> are you kidding? the first time i heard that I fell off my dinosaur. we were cruel kids in Jersey :bad:


We're backwoods here. So I started it up in this area, (circle)

I also started the phrase of "dirty who'er".

It's a not so nice term for a lady of the night.

Pronounced (who er) and sometimes with the Budweiser frogs tone.

It caught on nicely.


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## redrum (Oct 6, 2006)

who bumped this thread :angry:


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## RIP - VTEnviro (Oct 6, 2006)

Looks like you did, butt blaster.


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## redrum (Oct 6, 2006)

???

View attachment 136


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## benbo (Oct 6, 2006)

Warning to anyone actually studying for the October test - RUN! RUN! RUN FROM THIS TOPIC IF YOU NEED TO STUDY!


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## EdinNO (Oct 6, 2006)

Good one benb!

Ed


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## RIP - VTEnviro (Oct 6, 2006)

> ???


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## redrum (Oct 6, 2006)

old


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## DVINNY (Oct 7, 2006)

Now that is ghey


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## tmckeon_PE (Nov 4, 2006)

Are you guys talking about the "Little Sheet Heads" again?

Me thinks someone might be.


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## DVINNY (Nov 4, 2006)

eh?


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## Art (Nov 5, 2006)

> If a plane required the ground to be stationary in order to fly, sea planes wouldn't be able to take off.


what if the plane is trying to take off UP a fast flowing river, one just fast enough to keep the plane stationary relative to shore?


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## cement (Nov 5, 2006)

that is a different question because an airplane on a conveyor is a frictionless system with the landing gear turning. a seaplane in a river would have to work harder aginst the current, and less hard with the current to get suffifient air speed to ake off.


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## FlyPaper (May 27, 2007)

What is this "the other board" that everybody keeps talking about? I know about "the other board" for their books and stuff, but do they have a forum?


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## FlyPaper (May 27, 2007)

I don't think the plane would fly, because I think it would just get tired and go have a beer and watch tv, that is what I do after I work out on a conveyor belt.


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## Guest (May 27, 2007)

FlyPaper said:


> What is this "the other board" that everybody keeps talking about? I know about "the other board" for their books and stuff, but do they have a forum?


:15: :"the other board":

JR


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## RIP - VTEnviro (May 28, 2007)

This guy a friend of Fudgey's or something?


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## Dark Knight (May 28, 2007)

VTEnviro said:


> This guy a friend of Fudgey's or something?


You did read my mind. I think he is Fudgey and if he is not I don't mind. He is funny. This is not the place to do that but I will do it anyways; Welcome FlyPaper. This place was fun to be yesterday with you and VT going toe to toe. :15: my friend...


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## Bigwolf (May 28, 2007)

This thread = :15: :16:  :wacko: :hung-037: :deadhorse: :deadhorse: :deadhorse: :deadhorse:

:eyebrows:


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## FlyPaper (May 28, 2007)

I don't know fudgey.


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## kevo_55 (May 28, 2007)

Did someone find a way to fold space?

I'm just surprised to see this topic here on this board.


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## RIP - VTEnviro (May 28, 2007)

^ Does anyone read when the original topic was posted? :reading:


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## kevo_55 (May 29, 2007)

Oops!

Still, I thought that EB was better than that other board. :"the other board": This sort of topic belongs there along with all of the cracked software topics.


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## Guest (May 29, 2007)

kevo_55 said:


> Oops!
> Still, I thought that EB was better than that other board. :"the other board": This sort of topic belongs there along with all of the cracked software topics.


I was under the impression that the software topics weren't the only things that were cracked at :"the other board":

:reading:

JR


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## Guest (May 30, 2007)

I was watching the movie Independence Day last night on AMC. I noticed that as the alien ships were hovering above the cities, that the underlying landscape, structures, people, etc. were not being crushed underneath the force that was supporting the ships.

Perhaps there were alien conveyor belts that I somehow missed ?? :reading:

JR


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## DVINNY (Jun 1, 2007)

They use different alien technology for their hovering abilities, they don't use a resistive force pushing against the earth.

It's like beaming up to the starship. It just happens.


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## RIP - VTEnviro (Oct 24, 2007)

Well my friends, it looks like Mythbusters is actually going to take this one on...



> Airplane Hour(Weds., December 12 at 9 PM ET/PT)
> 
> Adam and Jamie find out if either of them can safely land a Boeing 747-400 on a runway in varying weather conditions. Meanwhile, Kari, Tory and Grant risk life and limb to investigate skydiving myths regularly featured in Hollywood action films. Is it possible to catch up with someone in freefall if that person jumps out a plane before you do? Can you really hold a conversation during freefall? And would you survive if you opened your parachute only a few feet off the ground? *Finally, Adam and Jamie carefully navigate their way through a myth that has baffled everyone from web bloggers to pilots. If a plane is traveling at takeoff speed on a conveyor belt, and the belt is matching that speed in the opposite direction, can the plane take off? Extensive small-scale testing with a super treadmill and a nearly uncontrollable model airplane don't completely resolve the myth, so our flight cadets supersize the myth with help from a willing pilot and his Ultralight flying machine.*


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## benbo (Oct 24, 2007)

I wish they'd do it for the Monty Hall problem. Although I suspect these things are sort of like the OJ trial. People have their opinion, and it doesn't matter what they are shown.


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## RIP - VTEnviro (Oct 24, 2007)

Isn't that why it's in the Hall of Fame?


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## frazil (Oct 24, 2007)

wow...I just wasted a LOT of time reading all this. Great thread! Why didn't I see this before??

FWIW: The plane won't fly.

:lmao:


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## frazil (Oct 24, 2007)

Can you watch Mythbusters episodes online?


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## RIP - VTEnviro (Oct 24, 2007)

^ hasn't aired yet.


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## MA_PE (Oct 24, 2007)

frazil said:


> wow...I just wasted a LOT of time reading all this. Great thread! Why didn't I see this before??
> FWIW: The plane won't fly.
> 
> :lmao:


I'm with Fraz, the plane will never get enough lift to fly. :dunno:

Welcome to ....The Twilight Zone.


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## DVINNY (Oct 24, 2007)

Please not again, I think you guys (gal) are just trying to bait me again.

Of course the thing will fly. With no issues either.

I'm gonna have to watch that episode. Let's remind everyone on here the day before and day of.


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## Guest (Oct 24, 2007)

^^^ Would that make MA_PE the master baiter then ?? :Locolaugh: :Locolaugh:

JR


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## frazil (Oct 24, 2007)

VTEnviro said:


> ^ hasn't aired yet.


I know, but I don't get the channel anyway. Some of the networks post their episodes online - so I was wondering if this show was on there too. If not I'm sure it'll be on youtube soon after.


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## frazil (Oct 24, 2007)

DVINNY said:


> I still stand by my statement, ice is the same as a conveyor.
> Fact is, an airplane moves because the jet engine thrusts it. Not because the wheels on the ground are moving it.


well if that's true it should have no problem...but it might need studded tires


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## MA_PE (Oct 25, 2007)

jregieng said:


> ^^^ Would that make MA_PE the master baiter then ?? :Locolaugh: :Locolaugh:
> JR


I guess I must be then :dunno:


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## civengPE (Oct 25, 2007)

frazil said:


> wow...I just wasted a LOT of time reading all this. Great thread! Why didn't I see this before??
> FWIW: The plane won't fly.
> 
> :lmao:


Oh Lord here we go again!


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## Road Guy (Oct 25, 2007)

THE plane does fly!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!


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## frazil (Oct 25, 2007)

SapperPE said:


> Frazil.... You really must be kidding me, you haven't seen this thread before? Holy cow, how did you understand all of the airplane / conveyor jokes then?


There are a lot of Fudgey lunch jokes that I don't get either (before my time), but I just smile and laugh so I look cool.


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## Dleg (Oct 25, 2007)

This thread was on its dying breaths when I joined, so I never participated.

Until now. THE PLANE WILL FLY!!!!! It's all about thrust, people! There's no power to the wheels, so they don't matter!


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## Flyer_PE (Oct 25, 2007)

I can't leave an aviation thread alone.

The plane will fly. Just make sure to tap the breaks before retracting the gear or you'll get black marks in the wheel wells.

As a side note, my stupid pilot trick of the day was taxiing half-way across the Greater Rockford Airport with the parking brake partially engaged. Burned some extra gas and heated up the brakes a little. This is the crap that happens when you're trying to configure the new radio instead of concentrating on operating the aircraft.

Jim


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## Road Guy (Oct 25, 2007)

we may have to put this thread back in general population after the exam!


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## PE-ness (Oct 25, 2007)

Well, I'm not an aeronautical engineer, but as long as the conveyor belt matches the speed of the airplane down the runway, how could it ever get anough lift to take off?

Maybe I should be studying for the exam tomorrow...


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## mudpuppy (Dec 12, 2007)

DVINNY said:


> Please not again, I think you guys (gal) are just trying to bait me again.
> Of course the thing will fly. With no issues either.
> 
> I'm gonna have to watch that episode. Let's remind everyone on here the day before and day of.


Reminder: it's on tonight.


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## RIP - VTEnviro (Dec 12, 2007)

Thanks dude!


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## Dark Knight (Dec 12, 2007)

mudpuppy said:


> Reminder: it's on tonight.


Time?...Where?


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## RIP - VTEnviro (Dec 12, 2007)

Dec 12, 9:00 pm

(60 minutes) MythBusters

Air Plane Hour

TV-PG

Jamie and Adam take wing to test if a person with no flight training can safely land an airplane and if a plane can take off from a conveyor belt speeding in the opposite direction. Tory, Grant, and Kari jump on some Hollywood-inspired skydiving myths.

Replay at 1 AM.

(I may need to start a running commentary thread while this is on.)


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## frazil (Dec 12, 2007)

^ Please do. I don't get that channel.


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## roadwreck (Dec 12, 2007)

frazil said:


> ^ Please do. I don't get that channel.


WHA???? hmy:

How can you survive without that channel? I don't think I'd bother owning a TV if it weren't for the discovery channel and history channel (and college football).


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## cement (Dec 12, 2007)

VTEnviro said:


> Dec 12, 9:00 pm
> (60 minutes) MythBusters
> 
> Air Plane Hour
> ...


^^ is that Kari? :wub:


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## roadwreck (Dec 12, 2007)

VTEnviro said:


>


That picture reminded me of this unaired myth involving Kari.


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## RIP - VTEnviro (Dec 12, 2007)

frazil said:


> ^ Please do. I don't get that channel.


If you want I will tape it.

How do you live in a world without Discovery Channel?

Deadliest Catch, Dirty Jobs, and Mythbusters are required watching at my house.


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## frazil (Dec 12, 2007)

Do they post the episodes online?


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## roadwreck (Dec 12, 2007)

Cement said:


> ^^ is that Kari? :wub:


Yes that is. A few more from that FHM photo shoot/interview can be seen here

http://www.fhmonline.com/articles-1276.asp


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## RIP - VTEnviro (Dec 12, 2007)

> is that Kari?


Yes it is.


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## Dleg (Dec 12, 2007)

I hardly watch any TV aside from childrens shows that I am forced to watch, but most of what I do watch is on History and Discovery. I'm glad VT turned me on to Dirty Jobs. I am still not sure when it airs, but I have caught it a few times recently and it was cool. Deadliest catch was awesome, but it changed times and I can't find it now. (No TV guide out here). I probably will not get to see the Mythbusters episode, so I would appreciate the commentary too.


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## RIP - VTEnviro (Dec 12, 2007)

> if a plane can take off from a conveyor belt speeding in the opposite direction.


What the fuck? I missed the first 10 minutes of this one, but unless it happened then, we got pwn3d.

All I saw was skydiving myths with the B-team, and Adam and Jamie trying to land a flight simulator. Damn.


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## civengPE (Dec 12, 2007)

Did I miss something? I didn't see anything on the conveyor belt. I checked the website and this episode was supposed to have it and I didn't see it even mentioned.


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## rdbse (Dec 13, 2007)

They did not air the episode, but I believe it will be on next month.

By the way, without wind passing over the wings, the plane will not fly. The jets might give is some lift initially, but it will crash moments later.


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## Hill William (Dec 13, 2007)

rdbse said:


> By the way, without wind passing over the wings, the plane will not fly. The jets might give is some lift initially, but it will crash moments later.



Here we go again.


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## FLBuff PE (Dec 13, 2007)

:deadhorse:


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## IlPadrino (Dec 13, 2007)

Hill William said:


> Here we go again.


This would be easy to resolve if we just did a breakdown of the problem... The only ground rule is that you have to address *ONLY* the question being asked or reference already agreed upon answers. If anyone's game, I'll start:

Fact 1: Any flying (not falling!) airplane requires sufficient air velocity over its wings to generate enough upward lift which counteracts the airplanes mass (which given gravity creates a downward force).

Anyone disagree or want to qualify the fact to make it more precise? But keep it simple.


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## Hill William (Dec 13, 2007)

Read the whole thread. Don't make Sapper bust out his Free Body Diagram.


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## IlPadrino (Dec 13, 2007)

Hill William said:


> Read the whole thread. Don't make Sapper bust out his Free Body Diagram.


I did read the whole thread... this ain't rocket science. But in my experience, when a problem is broken up into small enough pieces, it's easy enough to see where opinions diverge. Then it's a simple matter of debating a small piece.

Or not...


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## roadwreck (Dec 13, 2007)

Hill William said:


> Read the whole thread. Don't make Sapper bust out his Free Body Diagram.


I want to see the Free Body Diagram!!


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## rdbse (Dec 13, 2007)

The problem statement needs to address the amount of force generated by the jet engines, or simply what type of plane is this? I am sure there would be different results from military jets vs. cargo planes vs. small prop planes. In theory a light plane with large jet engines could achieve flight, but I have serious doubts about a fully loaded passenger plane staying up for more than five seconds. These planes need much more air velocity than the jets alone can generate, they need wind from the plane actually moving at a high rate of speed.

Again, Myth Busters will air there show in January.


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## benbo (Dec 13, 2007)

IlPadrino said:


> This would be easy to resolve if we just did a breakdown of the problem... The only ground rule is that you have to address *ONLY* the question being asked or reference already agreed upon answers. If anyone's game, I'll start:
> Fact 1: Any flying (not falling!) airplane requires sufficient air velocity over its wings to generate enough upward lift which counteracts the airplanes mass (which given gravity creates a downward force).
> 
> Anyone disagree or want to qualify the fact to make it more precise? But keep it simple.


I already know the answer. I was wrong at first, but they convinced me.

But I'm curious where you're going with this. I'll agree just to see the next step. Sorry folks.


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## Dleg (Dec 13, 2007)

It doesn't matter how much thrust it has, or even if it's a jet/prop engine. All that matters is that the engine is acting against the air, not the conveyor belt. There is no drive shaft to the wheels, they are free spinning. Therefore, the conveyor could be travelling backwards at 500 miles per hour, and the plane would still move forward and take off at it's normal takeoff velocity. A small amount of additional thrust to overcome the friction forces on the free-spinning wheels would be all that would be required.

Duh!


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## rdbse (Dec 13, 2007)

Since I never got in on the "original" debate. An airplane achieves lift due to the wind velocity over the wing.

See link

http://www.allstar.fiu.edu/AERO/airflylvl3.htm


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## DVINNY (Dec 13, 2007)

oh man. :brickwall: :brickwall: :brickwall:

OK, as much as I can't believe that I'm about to get into this again, I just can't help it.

The result will be the same with a jet plane, prop plane, as Dleg said. The wheels would never offer enough friction to slow the plane down enough for it to not take off. They are free spinning wheels for crying out loud.

Will a plane in the air still fly with the landing gear down? YES.

Then same plane will take off with the landing gear down.

conveyor belt means nothing in the equation. But most people think of how a car thrusts through the wheels, and from the wheels to the ground, but planes use thrust against the air, not the ground.


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## rdbse (Dec 13, 2007)

I did not say anything about the wheels or friction. I agree the plane will move forward by the jets affect on the air, but not fast enough (nor far enough) to take flight. The only possibility I see is a military jet that can take off from an aircraft carrier. The others will crash off the end of the conveyor. And, this goes back to the type of plane.


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## benbo (Dec 13, 2007)

rdbse said:


> I did not say anything about the wheels or friction. I agree the plane will move forward by the jets affect on the air, but not fast enough (nor far enough) to take flight. The only possibility I see is a military jet that can take off from an aircraft carrier. The others will crash off the end of the conveyor. And, this goes back to the type of plane.


You are the first person Ive seen agree the plane's wings move forward through the air, but that it will not take off.

I'm curious what you mean that the plane will crash off the end of the conveyor and will not travel far enough forward.

I have two questions -

How far forward does it have to travel, and how long do you think the conveyor belt is supposed to be? Because I don't see any length given for the conveyor. I just assumed it was inifinite in length. And if it does crash off, what exactly is there after the conveyor ends? I can't see the picture you have in your head, and where it comes from.

Let's put some numbers in and let's pick some ridiculous ones - lets say it takes one mile traveling forward for the plane to take off. THen lets say it's on a 50 mile treadmill. Now will it take off?


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## roadwreck (Dec 13, 2007)

^^

it would seem that rdbse sees the conveyor as having a finite length. A length that is not long enough for a plane to take off. reasonable assumption I suppose. I mean it would be quite an achievement to make a conveyor long enough for a real plane to take off on. Not to mention one that "exactly matches the speed of the plane". And even if you could construct such a conveyor, it really would beg the question, why would you want to try and make a plane take off from a conveyor?


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## Dleg (Dec 14, 2007)

WOW. I just now, for the first time ever, read this entire thread (it pre-dated me by a few months). Now I see where civingPE's 500 posts came from.

Good to see that it all worked out, and that MA and benbo saw the light!

:laugh:

BTW, a jet engine does kind of act like a rocket engine - it does not pull air in and push it back out to create its thrust, like a propeller inside a tube or something. Nor does it draw air across the wing surfaces like some sort of giant vacuum machine or fan. The air drawn into the front of the engine is simply the oxygen needed to support the combustion of fuel, compressed by the fans to allow the burning of even more fuel, which creates thrust by expelling the expanding exhaust gases out the rear. Just like a rocket. And yes, I know that modern jet engines also harvest some of that exhaust gas power to turn larger "fans" in the front to also provide some thrust through the propeller-in-the-tube method. (around the outside of the primary jet combustion chamber, though).


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## IlPadrino (Dec 14, 2007)

IlPadrino said:


> This would be easy to resolve if we just did a breakdown of the problem... The only ground rule is that you have to address *ONLY* the question being asked or reference already agreed upon answers. If anyone's game, I'll start:
> Fact 1: Any flying (not falling!) airplane requires sufficient air velocity over its wings to generate enough upward lift which counteracts the airplanes mass (which given gravity creates a downward force).
> 
> Anyone disagree or want to qualify the fact to make it more precise? But keep it simple.


You guys are fucking this up... it only works well if we stick to small facts taken one at a time. Does anyone disagree with Fact 1?


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## Hill William (Dec 14, 2007)

Would a plane take off of a sheet of ice???

Same concept.


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## ODB_PE (Dec 14, 2007)

I'm on board with you, sray - hopefully there's nobody that disagrees with that.

taking it to the next step - I think the question is flawed. There are two ways to look at the "conveyor matches speed of airplane" aspect of the problem statement:

1: conveyor matches speed of the plane: plane gets up to 100, conveyor is going 100 the other way, and wheels are going 200. Plane takes off normally***. Would require a sufficiently long conveyor, though.

2: conveyor matches speed of wheels: obviously in this situation, the plane has no relative forward movement, and could not take off.

so which camp are you in?

IMHO, situation #1 is what will happen, but situation #2 is what the problem implies will happen. Since the plane provides thrust to the air, however, #2 is impossible-until, of course, you consider friction. Imagine the plane just sitting there with the conveyor going 100 the other way. In a frictionless world, the plane would remain at rest with the wheels spinning away but in reality the plane would need a little bit of thrust to keep it stationary relative to the ground.

*** Normal is not quite accurate, because a little extra thrust would be required to offset the added rolling resistance

I think of the problem this way: imagine walking through an airport at 5mph pulling a bag with wheels. You walk adjacent to a conveyor going 5mph the other way, and put your bag on the conveyor while you continue to walk on the stationary ground at 5mph. You and the bag continue to move forward at your walking speed, although you will have to use a little more effort to pull the bag due to the added rolling resistance.


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## Capt Worley PE (Dec 14, 2007)

DVINNY said:


> oh man. :brickwall: :brickwall: :brickwall:
> OK, as much as I can't believe that I'm about to get into this again, I just can't help it.
> 
> The result will be the same with a jet plane, prop plane, as Dleg said. The wheels would never offer enough friction to slow the plane down enough for it to not take off. They are free spinning wheels for crying out loud.
> ...


Oh! I see it now! I feel like a real dope!

:brickwall: :brickwall: :brickwall: :brickwall:


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## civengPE (Dec 14, 2007)

Dleg said:


> Now I see where civingPE's 500 posts came from.


What are you trying to say Dleg? ldman:


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## benbo (Dec 14, 2007)

ol said:


> I'm on board with you, sray - hopefully there's nobody that disagrees with that.
> 
> taking it to the next step - I think the question is flawed. There are two ways to look at the "conveyor matches speed of airplane" aspect of the problem statement:
> 
> ...


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## frazil (Dec 14, 2007)

opcorn:


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## civengPE (Dec 14, 2007)

From what I have read, MBs constructed a quarter mile conveyor belt and used an ultralight to test it out.

I was extremely disappointed by them not airing this in the airplane episode. :deadhorse:


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## roadwreck (Dec 14, 2007)

civengPE said:


> From what I have read, MBs constructed a quarter mile conveyor belt and used an ultralight to test it out.
> I was extremely disappointed by them not airing this in the airplane episode. :deadhorse:


I wonder how they achieved the "match speed" of the conveyor. That seems to be a difficult thing to do at full scale.


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## civengPE (Dec 14, 2007)

roadwreck said:


> I wonder how they achieved the "match speed" of the conveyor. That seems to be a difficult thing to do at full scale.


I don't know if matching speed is necessary to debunk the myth. If I was going to do it. Here is how I would set it up.

1. Determine the maximum airspeed of the aircraft.

2. Build a conveyor belt capable of achieving this (hardest part)

3. Place the aircraft on the conveyor belt and fasten it so it will not move.

4. turn the conveyor belt on full speed.

5. Apply full power to the engine and release the restraints.

6. If it can overcome this situation (and it will), then the other is, by default, debunked.

what happens if you are in a spaceship traveling near the speed of light and you turn your headlights on?


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## Flyer_PE (Dec 14, 2007)

^Do you need to used max speed for the aircraft or just the rotation speed? Typical GA aircraft will lift off at less than 60kts. If you use a motor-glider or an ultralight, you can get the required treadmill speed down below 30kts.

Jim


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## IlPadrino (Dec 14, 2007)

ol said:


> I'm on board with you, sray - hopefully there's nobody that disagrees with that.
> taking it to the next step - I think the question is flawed. There are two ways to look at the "conveyor matches speed of airplane" aspect of the problem statement:
> 
> 1: conveyor matches speed of the plane: plane gets up to 100, conveyor is going 100 the other way, and wheels are going 200. Plane takes off normally***. Would require a sufficiently long conveyor, though.
> ...


OK...

Fact 1: Any flying (not falling!) airplane requires sufficient air velocity over its wings to generate enough upward lift which counteracts the airplanes mass (which given gravity creates a downward force).

Fact 2: An airplane creates thrust independent of its wheels.

Ok... Fact 2 is so easy, let's move on! Any suggestions on how to phrase a conveyor fact? The moral is that the conveyor can impart very little force on the airplane.


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## rdbse (Dec 14, 2007)

I see the conveyor as platform just under the plane. After reviewing the thread, I appears others see an infinite conveyor.

Also, I have excluded the AV-8B Harrier from problem.


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## Guest (Dec 14, 2007)

^^^ I really haven't weighed in on this argument because I will be honest there is one point that I am not clear on.

I agree with your statement - the lift is generated by air flowing over the wing. So my follow-on thought is redirected to the issue of frame of reference, meaning that everyone has fixated on plane's position (and velocity) relative to the conveyor belt but wouldn't one need to think about the plane's position (and velocity) relative to the surrounding air mass?

My confusion arises because I am not sure how to draw the control volume for the air mass to represent the forces (pressures) that arise from the flow. The volume (and area) are not fixed so I am not sure how to represent it.

I hope you don't take this as a hijack or a jump beyond the simple first principles because I think this concept really is the critical linkage from your first point to follow through towards the conclusion. IMHO.

JR


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## roadwreck (Dec 14, 2007)

rdbse said:


> I see the conveyor as platform just under the plane. After reviewing the thread, I appears others see an infinite conveyor.
> Also, I have excluded the AV-8B Harrier from problem.


If the conveyor were a platform just under the plane (and once it moved off the conveyor it would fall?) then no plane could take off other the something with vertical lift capability. But the same could be said for a plane not on a conveyor. If the plane were only given a length of runway equal to the length of the plane itself then that plane could also not take off under it's own power (unless it again has vertical take of capabilities).


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## cement (Dec 14, 2007)

the only way a plane on a conveyor could take off would be with a 100 mph headwind.


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## frazil (Dec 14, 2007)

^ I agree!


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## IlPadrino (Dec 14, 2007)

Fact 1: Any flying (not falling!) airplane requires sufficient air velocity over its wings to generate enough upward lift which counteracts the airplanes mass (which given gravity creates a downward force).

Fact 2: An airplane creates thrust independent of its wheels.

Now let's get to the interesting part...

Fact 3: Given an airplane sitting stationary on any movable surface (e.g. a conveyor belt, a trailer, etc.), a force equal but opposite to the rolling resistance (i.e. rolling friction) of the airplane wheels will keep the airplane stationary despite the moving surface.

We're getting close if no one disagrees with Fact 3.


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## benbo (Dec 14, 2007)

IlPadrino said:


> Fact 1: Any flying (not falling!) airplane requires sufficient air velocity over its wings to generate enough upward lift which counteracts the airplanes mass (which given gravity creates a downward force).Fact 2: An airplane creates thrust independent of its wheels.
> 
> 
> Now let's get to the interesting part...
> ...


I don't know if I disagree or not. I don't really understand what your saying. I mean, obvviously, if we tie the plane in a stationary position it won't move regardless of what the treadmill does. But that's irrelevant to the problem. It is impossible to hold the plane stationary with just the thrust of the jets or prop, and the treadmill, because the treadmill has zero effect on the motion of the plane. I would restate the Fact -

Given enough thrust and a sufficiently long treadmill the plane's wings will move forward through the air regardless of what the treadmill does.

This is the critical part of the problem.

I assume you believe the plane will take off.


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## roadwreck (Dec 14, 2007)

benbo said:


> I don't know if I disagree or not. I don't really understand what your saying. I mean, obvviously, if we tie the plane in a stationary position it won't move regardless of what the treadmill does. But that's irrelevant to the problem. It is impossible to hold the plane stationary with just the thrust of the jets or prop, and the treadmill, because the treadmill has zero effect on the motion of the plane. I would restate the Fact -
> Given enough thrust and a sufficiently long treadmill the plane's wings will move forward through the air regardless of what the treadmill does.
> 
> This is the critical part of the problem.
> ...


I think his point is that if the wheels are allowed to move freely and the conveyor starts to move but no force is applied to the plane via the engines the wheels should turn, but the plane should remain stationary despite what the conveyor is doing. Newtons first law of motion, a body will stay at rest unless a force acts upon it. Since the wheels rotate freely, no force should be transfered to the body of the plane, the plane shouldn't move and the conveyor will just cause the wheels to role.


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## benbo (Dec 14, 2007)

roadwreck said:


> I think his point is that if the wheels are allowed to move freely and the conveyor starts to move but no force is applied to the plane via the engines the wheels should turn, but the plane should remain stationary despite what the conveyor is doing. Newtons first law of motion, a body will stay at rest unless a force acts upon it. Since the wheels rotate freely, no force should be transfered to the body of the plane, the plane shouldn't move and the conveyor will just cause the wheels to role.


Oh - in that case I agree.

Carry on!


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## civengPE (Dec 14, 2007)

First the obvious-but-wrong answer. The unwary tend to reason by analogy to a car on a conveyor belt--if the conveyor moves backward at the same rate that the car's wheels rotate forward, the net result is that the car remains stationary. An aircraft in the same situation, they figure, would stay planted on the ground, since there'd be no air rushing over the wings to give it lift. But of course cars and planes don't work the same way. A car's wheels are its means of propulsion--they push the road backwards (relatively speaking), and the car moves forward. In contrast, a plane's wheels aren't motorized; their purpose is to reduce friction during takeoff (and add it, by braking, when landing). What gets a plane moving are its propellers or jet turbines, which shove the air backward and thereby impel the plane forward. What the wheels, conveyor belt, etc, are up to is largely irrelevant. Let me repeat: Once the pilot fires up the engines, the plane moves forward at pretty much the usual speed relative to the ground--and more importantly the air--regardless of how fast the conveyor belt is moving backward. This generates lift on the wings, and the plane takes off. All the conveyor belt does is make the plane's wheels spin madly.

A thought experiment commonly cited in discussions of this question is to imagine you're standing on a health-club treadmill in rollerblades while holding a rope attached to the wall in front of you. The treadmill starts; simultaneously you begin to haul in the rope. Although you'll have to overcome some initial friction tugging you backward, in short order you'll be able to pull yourself forward easily.

As you point out, one problem here is the wording of the question. Your version straightforwardly states that the conveyor moves backward at the same rate that the plane moves forward. If the plane's forward speed is 100 miles per hour, the conveyor rolls 100 MPH backward, and the wheels rotate at 200 MPH. Assuming you've got Indy-car-quality tires and wheel bearings, no problem. However, some versions put matters this way: "The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation." This language leads to a paradox: If the plane moves forward at 5 MPH, then its wheels will do likewise, and the treadmill will go 5 MPH backward. But if the treadmill is going 5 MPH backward, then the wheels are really turning 10 MPH forward. But if the wheels are going 10 MPH forward . . . Soon the foolish have persuaded themselves that the treadmill must operate at infinite speed. Nonsense. The question thus stated asks the impossible -- simply put, that A = A + 5 -- and so cannot be framed in this way. Everything clear now? Maybe not. But believe this: The plane takes off.


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## civengPE (Dec 14, 2007)

I found this explanation on another website. I hope it clears things up for you non believers!!

:holyness:


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## civengPE (Dec 14, 2007)




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## civengPE (Dec 14, 2007)

FBD


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## roadwreck (Dec 14, 2007)

civengPE said:


> The plane takes off.


You jumped ahead of us by a few steps, but we were getting there.


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## MA_PE (Dec 14, 2007)

So really what you're saying is that your mother and my mother are both mothers therefore we could be brothers, correct? :dunno:


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## civengPE (Dec 14, 2007)

MA_PE said:


> So really what you're saying is that your mother and my mother are both mothers therefore we could be brothers, correct? :dunno:


What would we be if we were both mothers also? :wacko:


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## DVINNY (Dec 14, 2007)

still, the best thread ever.

still, the plane takes off just fine.


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## IlPadrino (Dec 14, 2007)

roadwreck said:


> I think his point is that if the wheels are allowed to move freely and the conveyor starts to move but no force is applied to the plane via the engines the wheels should turn, but the plane should remain stationary despite what the conveyor is doing. Newtons first law of motion, a body will stay at rest unless a force acts upon it. Since the wheels rotate freely, no force should be transfered to the body of the plane, the plane shouldn't move and the conveyor will just cause the wheels to role.


We've got to be careful to define "stationary". If the airplane moves at the same speed of what it's sitting on, this is not stationary, right? Let's call this "staying still on the moving surface".

I almost agree with what you wrote above... except the airplane won't remain stationary (it will stay still on the moving surface) as the moving surface first begins to move because there is rolling resistance in the wheels. At some velocity, though, the relative wind pressure will provide sufficient force to counter the rolling resistance and the airplane will stop moving at the same speed as the moving surface.

How can we rewrite Fact 3 to me more clear? Maybe:

Fact 3: Given an airplane sitting freely on any movable surface (e.g. a conveyor belt, a trailer, etc.), a force equal but opposite to the rolling resistance (i.e. rolling friction) of the airplane wheels will keep the airplane stationary despite the moving surface.


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## benbo (Dec 14, 2007)

Wait a second. Where does Monty Hall fit into all this? Now there's a problem on which you will never get agreement.


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## civengPE (Dec 14, 2007)

benbo said:


> Wait a second. Where does Monty Hall fit into all this? Now there's a problem on which you will never get agreement.


Sorry about that. i did a quick cut/paste. it had to do with some earlier comments the OP had made.


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## benbo (Dec 14, 2007)

civengPE said:


> Sorry about that. i did a quick cut/paste. it had to do with some earlier comments the OP had made.


There's nothing wrong with what you posted. My comment was supposed to be an unrelated joke. As it turns out, it was unrelated but not much of a joke. It refers to another old riddle that had a rather lengthy argument thread. Lucky I have my day job for now.


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## Guest (Dec 14, 2007)

^^^ Wait a min ... I thought we all agreed (even I rescinded previous statements) that it was better to *SWITCH* than stay.

Right?

JR


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## Dleg (Dec 14, 2007)

Good God.


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## rdbse (Dec 14, 2007)

Ha! Is the model airplane on a treadmill the basis for the problem statement?

We can see it does not take off from the treadmill, but it only moves forward with no flight.

Can someone post the original riddle, so we can clarify the infinite conveyor.


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## civengPE (Dec 14, 2007)

rdbse said:


> Ha! Is the model airplane on a treadmill the basis for the problem statement?
> We can see it does not take off from the treadmill, but it only moves forward with no flight.
> 
> Can someone post the original riddle, so we can clarify the infinite conveyor.



:brickwall: :brickwall: :brickwall: :brickwall:


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## rdbse (Dec 14, 2007)

Well, did it take flight? I have agreed all along it would move forward.

Now, let's seperate theory from reality.


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## benbo (Dec 14, 2007)

rdbse said:


> Well, did it take flight? I have agreed all along it would move forward.
> Now, let's seperate theory from reality.


Yes. It took flight. Satisfied?

The infinite conveyor is not in the original problem statement (which is in the in the thread). There was no length given, so I assumed it was a long conveyor, you assumed it was a very short conveyor. I already agreed that if the conveyor is short like you think then you are right - no plane could take off. But, given a long enough conveyor, any plane would take off.

Agreed? I mean, you already agreed it moved forward, so if it moves forward far enough it will take off.

Then we have no difference of opinion.

I don't know what your issue is.


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## rdbse (Dec 14, 2007)

What video did you watch? The model plane was supported on a string.

I just want a clarification on the original problem, or maybe I should just wait on the MB episode.

You better just hold the controls straight and steady.

http://meignorant.com/2posts/Fly-NotFly.gif


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## benbo (Dec 14, 2007)

rdbse said:


> What video did you watch? The model plane was supported on a string.
> I just want a clarification on the original problem, or maybe I should just wait on the MB episode.
> 
> You better just hold the controls straight and steady.
> ...


I don't know what you're talking about with a video. Where do I say I watched a video? Or are you talking to someboy else. I actually put a model plane with a rubber band prop on an exercise treadmill and guess what, it took off. And it was going a hell of a lot slower than the treadmill.

Here's the original question. It is from the first post in this thread. As I said, I assumed that the conveyor was very very long. You still refuse to answer the question, if the conveyor is infinitely long, or lets say ten miles long - will the plane take off? Is that hard to answer? By the way, hundreds of other people don't seem to have a problem. I'm done beating this horse.

The question is:

A plane is standing on a runway that can move (some sort of band conveyer). The plane moves in one direction, while the conveyer moves in the opposite direction. This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in opposite direction).

Does the plane ever take off, and why or why not?


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## DVINNY (Dec 14, 2007)

Sapper, so you're saying it will fly?

LMAO


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## rdbse (Dec 14, 2007)

Hey Sapper,

I did not force you to look at this thread again, so no reason to get ill. You contradict yourself by saying it does not matter how short the conveyor is, but add that it needs to be long enough to obtain critical velocity. If we can agree that the length does matter, I will agree that the plane will fly and let it rest.


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## MA_PE (Dec 15, 2007)

jregieng said:


> ^^^ Wait a min ... I thought we all agreed (even I rescinded previous statements) that it was better to *SWITCH* than stay.
> Right?
> 
> JR


Despite the lengthy statistical arguments (which are likely valid) I still believe it's a 50/50 chance. :smileyballs:


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## IlPadrino (Dec 15, 2007)

rdbse said:


> Hey Sapper,I did not force you to look at this thread again, so no reason to get ill. You contradict yourself by saying it does not matter how short the conveyor is, but add that it needs to be long enough to obtain critical velocity. If we can agree that the length does matter, I will agree that the plane will fly and let it rest.


You're not following the rules - only one new fact at a time.

I left off with



Fact 1: Any flying (not falling!) airplane requires sufficient air velocity over its wings to generate enough upward lift which counteracts the airplanes mass (which given gravity creates a downward force).

Fact 2: An airplane creates thrust independent of its wheels.

I think I need to change Fact 3 to include axle friction because it'll come to play in the next Fact. So:

Fact 3: Given an airplane sitting freely on any movable surface (e.g. a conveyor belt, a trailer, etc.), a force equal but opposite to the rolling resistance (i.e. rolling friction) and axle friction of the airplane wheels will keep the airplane stationary despite the moving surface.

then we can go to

Fact 4: Rolling resistance is a function of [what?] and axle friction is a function of [what]?

I don't know... a quick Google makes me think I just need to ask the Pine Car Derby enthusiasts.

I think we'll quickly get to the end after this... because no matter how fast the conveyor moves (which causes the wheels to spin even fast that they would if the conveyor wasn't moving), the extra rolling resistance and axle friction is quite small compared to the thrust of the airplane.

Sapper: I only go through this "debate" because I think there's still something to be learned (other than who was right or wrong). I thought it strange there wasn't a lot of discussion about rolling resistance and axle friction.


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## BluSkyy (Dec 15, 2007)

To me, its a relatively simple dynamics problem. The sum of the forces acting on the plane is nonzero, therefore there is an acceleration. I don't see why its so hard.


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## IlPadrino (Dec 15, 2007)

BluSkyy said:


> To me, its a relatively simple dynamics problem. The sum of the forces acting on the plane is nonzero, therefore there is an acceleration. I don't see why its so hard.


I think some might consider the moving conveyor providing a force exactly counteracting the thrust... or else moving the airplane back an inch every time it moves forward an inch. I'm sure both are wrong and the explanation is fairly simple, but it gets confounded by a lot of ancillary (and useless) information (e.g. how long is the conveyor? - it doesn't matter!)


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## benbo (Dec 16, 2007)

IlPadrino said:


> I think some might consider the moving conveyor proving a force exactly counteracting the thrust... or else moving the airplane back an inch every time it moves forward an inch. I'm sure both are wrong and the explanation is fairly simple, but it gets confounded by a lot of ancillary (and useless) information (e.g. how long is the conveyor? - it doesn't matter!)


The reason we don't discuss the rolling resistance is because it wasn't given in the problem. If it is not given we either have to assume that it is zero or neglible, or we can't solve the problem. Because it is possible for rolling friction or static friction to be so great the plane doesn't move. It is caled a "brake" I believe. Since it isn't given we have to neglect it. Any number we put in is just an assumption on our part. What friction force would you put in the problem?

As far as the length of the conveyor, I never thought about it but rdbse did. And he is right in that there is no drawing given for the problem. He pictured a short conveyor that ended in a cliff or a wall or something. I think the point he was trying to make was that even though the conveyor made no real difference in the motion of the plane, it didn't help it take off either. I assume you don't think the plane can take off from a ten foot conveyor? Because that would sure make airport design a lot simpler, at least it would require a lot less real estate. Again, I didn't think of this, but I don't think you can just declare perfunctorily that it doesn't matter.

Sorry folks, I just can't help myself form arguing about this. Maybe it's because I was wrong at first. I'm like a person who has a religious conversion and can't stop talking about it. I need to go to conveyor anonymous. I promise this is my last post,


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## benbo (Dec 16, 2007)

rdbse said:


> Hey Sapper,I did not force you to look at this thread again, so no reason to get ill. You contradict yourself by saying it does not matter how short the conveyor is, but add that it needs to be long enough to obtain critical velocity. If we can agree that the length does matter, I will agree that the plane will fly and let it rest.


Sounds like an argument my wife was making. You mean length does matter?! So PE-Ness was right. Now I'm really done reading this thread. I'm too depressed.


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## IlPadrino (Dec 16, 2007)

benbo said:


> The reason we don't discuss the rolling resistance is because it wasn't given in the problem. If it is not given we either have to assume that it is zero or neglible, or we can't solve the problem. Because it is possible for rolling friction or static friction to be so great the plane doesn't move. It is caled a "brake" I believe. Since it isn't given we have to neglect it. Any number we put in is just an assumption on our part. What friction force would you put in the problem?


Fact 1: Any flying (not falling!) airplane requires sufficient air velocity over its wings to generate enough upward lift which counteracts the airplanes mass (which given gravity creates a downward force).

Fact 2: An airplane creates thrust independent of its wheels.

Fact 3: Given an airplane sitting freely on any movable surface (e.g. a conveyor belt, a trailer, etc.), a force equal but opposite to the rolling resistance (i.e. rolling friction) and axle friction of the airplane wheels will keep the airplane stationary despite the moving surface.

Fact 4: Rolling resistance is a function of [what?] and axle friction is a function of [what]?

Just because it wasn't given doesn't mean we have to neglect it... we live in the real world so we can assume real-world conditions. If you want to jump to

"Fact 5: The rolling resistance and axle friction of an airplane is orders and orders of magnitude less than the thrust provided."

then we're done...

Fact 6: The conveyor makes no difference in the ability of the airplane to take off.



benbo said:


> As far as the length of the conveyor, I never thought about it but rdbse did. And he is right in that there is no drawing given for the problem. He pictured a short conveyor that ended in a cliff or a wall or something. I think the point he was trying to make was that even though the conveyor made no real difference in the motion of the plane, it didn't help it take off either. I assume you don't think the plane can take off from a ten foot conveyor? Because that would sure make airport design a lot simpler, at least it would require a lot less real estate. Again, I didn't think of this, but I don't think you can just declare perfunctorily that it doesn't matter.


If the conveyor is on a runway (as specified in the problem), it doesn't matter how short it is... after the conveyor ends, the runway continues and the airplane is just fine making the transition from the conveyor to the runway. Nor can the conveyor be too long... the airplane will take off at the normal distance. If there's a cliff or a wall, it's not like any runway in the world except an Aircraft Carrier.



benbo said:


> Sorry folks, I just can't help myself form arguing about this. Maybe it's because I was wrong at first. I'm like a person who has a religious conversion and can't stop talking about it. I need to go to conveyor anonymous. I promise this is my last post,


Spoil sport.


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## FLBuff PE (Dec 21, 2007)

benbo said:


> What friction force would you put in the problem?


_f_ = 64/Re from my Environmental Engineering Reference Manual for Laminar Flow, which I would consider a smootly rolling airplane on a conveyor belt. If we assume that the Reynold's # is the maximum for Laminar flow, 2099, then _f_ = 0.0305

On a seprate note, what if the conveyor runs across the runway? Will the plane cartwheel (as my RC plane did once when a particularly nasty crosswind caught the wings during takeoff)? Let the debate begin! opcorn:


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## Dleg (Dec 22, 2007)

^^Now THAT would probably derail the takeoff, and could easily lead to cartwheeling.


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## IlPadrino (Dec 24, 2007)

FLBuff said:


> _f_ = 64/Re from my Environmental Engineering Reference Manual for Laminar Flow, which I would consider a smootly rolling airplane on a conveyor belt. If we assume that the Reynold's # is the maximum for Laminar flow, 2099, then _f_ = 0.0305


I was talking about rolling friction and axle friction... the whole world isn't about Environmental Engineering (despite what you guys would have me believe) - every now and again the mechanical nature of the world makes them MechEs useful. But, hey, if you don't know the answer, don't worry - I'll just find the local Boy Scout troop and ask the 9-year-old Pine Car Derby expert. Yeah, they're not PEs, but they do have practical experience on their side.


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## FLBuff PE (Dec 27, 2007)

IlPadrino said:


> Yeah, they're not PEs, but they do have practical experience on their side.


I have neither, so I shall bow out of this discussion...


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## cement (Dec 27, 2007)

FLBuff said:


> _f_ = 64/Re from my Environmental Engineering Reference Manual for Laminar Flow, which I would consider a smootly rolling airplane on a conveyor belt. If we assume that the Reynold's # is the maximum for Laminar flow, 2099, then _f_ = 0.0305On a seprate note, what if the conveyor runs across the runway? Will the plane cartwheel (as my RC plane did once when a particularly nasty crosswind caught the wings during takeoff)? Let the debate begin! opcorn:


I visited an old B-52 base and the the landing gear would turn about 15 degrees or so to help compensate for partial crosswind. The potential calculations are making my head spin! :blink:


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## Dleg (Dec 27, 2007)

Having no references readily available, all I will say (and I think, all that we need to know) is that the rolling resistance of a rubber aircraft tire on a ball-bearing axle is very small compared with the other horizontal forces involved in the problem (thrust and aerodynamic drag).


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## IlPadrino (Dec 28, 2007)

Dleg said:


> Having no references readily available, all I will say (and I think, all that we need to know) is that the rolling resistance of a rubber aircraft tire on a ball-bearing axle is very small compared with the other horizontal forces involved in the problem (thrust and aerodynamic drag).


I sure wouldn't disagree with that. So even though we don't have the answer to Fact 4, we can accept Fact 5 and move to the end:

Fact 1: Any flying (not falling!) airplane requires sufficient air velocity over its wings to generate enough upward lift which counteracts the airplanes mass (which given gravity creates a downward force).

Fact 2: An airplane creates thrust independent of its wheels.

Fact 3: Given an airplane sitting freely on any movable surface (e.g. a conveyor belt, a trailer, etc.), a force equal but opposite to the rolling resistance (i.e. rolling friction) and axle friction of the airplane wheels will keep the airplane stationary despite the moving surface.

Fact 4: Rolling resistance is a function of [what?] and axle friction is a function of [what]?"

Fact 5: The rolling resistance and axle friction of an airplane is orders and orders of magnitude less than the thrust provided.

Fact 6: The conveyor makes no difference in the ability of the airplane to take off.

I'm sure an aeronautical engineer could fine-tune the facts to make it more precise, but I don't think there's anything here that they'd disagree with. If anyone disagreed with Fact 6, I'd just ask them to point out which previous fact is wrong or which fact is missing.


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## Dleg (Dec 28, 2007)

I don't disagree, but, much like _I am Legend_, I think you jumped to the ending too quickly. Try this:

Fact 6: Given the problem statement, the speed of the conveyor belt will be constantly and instantly changed to match the speed of the aircraft, but in an opposite direction. This means that if at any given instant the aircraft is moving forward (in a positive direction) at speed x, the conveyor will be moving backward (a negative direction) at speed -x. The wheels of the aircraft, therefore, will always be rotating at a speed of (x - -x) = 2x, or twice the speed they would rotate if rolling across a stationary surface.

Fact 7: The difference in the rolling resistance experienced by a free-spinning airplane wheel between speeds x and 2x is very small at ordinary takeoff velocities, and remains several orders of magnitude less than the thrust provided by the airplane's engine(s).

Fact 8: The other important horizontal force that opposes thrust is aerodynamic drag. If the airplane is an ordinary airplane that is capable flying under non-conveyor belt conditions, we can assume that the aircraft designers have taken aerodynamic drag into consideration and provided the aircraft with an engine(s) capable of overcoming this force and accelerating the aircraft to necessary takeoff and flying speeds.

Fact 9: Because the thrust generated by the engines is a horizontal force that greatly exceeds the opposing horizontal forces of aerodynamic drag and the rolling resistance caused by the conveyor acting against the free-spinning wheels, the superior force of thrust will cause the airplane to accelerate forward.

Fact 10: Because the airplane accelerates forward under the influence of the thrust from its engines, it will reach the forward velocity necessary for takeof.

Fact 11 (or perhaps this only belongs as a comment) Due to the small increase in rolling resistance caused by the airplane's wheels spinning at twice their normal rotational velocity, the distance required to achieve liftoff speed will be slightly longer than ordinary.


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## IlPadrino (Dec 28, 2007)

Dleg said:


> I don't disagree, but, much like _I am Legend_, I think you jumped to the ending too quickly. Try this:
> Fact 6: Given the problem statement, the speed of the conveyor belt will be constantly and instantly changed to match the speed of the aircraft, but in an opposite direction. This means that if at any given instant the aircraft is moving forward (in a positive direction) at speed x, the conveyor will be moving backward (a negative direction) at speed -x. The wheels of the aircraft, therefore, will always be rotating at a speed of (x - -x) = 2x, or twice the speed they would rotate if rolling across a stationary surface.
> 
> Fact 7: The difference in the rolling resistance experienced by a free-spinning airplane wheel between speeds x and 2x is very small at ordinary takeoff velocities, and remains several orders of magnitude less than the thrust provided by the airplane's engine(s).
> ...


Holy Shit, Batman!

Fact 6 and 7 are unimportant given Fact 5: If the rolling resistance and axle friction are orders and orders less than the thrust provided, it doesn't matter if the wheels are spinning at 1x or 2x normal (i.e. non-conveyored) speed.

I thought Fact 1 covered Fact 8, but I guess I forgot that different airplanes of the same weight can require different take-off speeds. But, still, we could rewrite Fact 1 to account for that.

Facts 9, 10, and 11 seem unneeded, given airplanes NORMALLY take-off and Fact 5 tells us the conveyor doesn't significantly change normal.

So... if you agree with the conclusion, what are the minimum facts needed? I'm sure we can do less than 11 and maybe 6 wasn't enough.


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## DVINNY (Dec 29, 2007)

You can do 347 facts.

some people still will not believe it.


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## IlPadrino (Dec 29, 2007)

DVINNY said:


> You can do 347 facts.some people still will not believe it.


Less is more. With 347 facts, non-believers are sure to ground themselves on a bunch of unnecessary facts (that although perhaps correct still cloud the picture).

I've made a few changes based on Dleg's input but still haven't included a fact about aerodynamic drag. I challenge any non-believer to address Fact 5 by pointing out which previous fact is wrong or which fact is missing. I promise it will be a short discussion.

Fact 1: Any flying (not falling!) airplane requires sufficient air velocity over its wings to generate enough upward lift which counteracts the airplanes mass (which given gravity creates a downward force); air velocity is created by thrust.

Fact 2: An airplane creates thrust independent of its wheels.

Fact 3: Given an airplane sitting freely on any movable surface (e.g. a conveyor belt, a trailer, etc.), a force equal but opposite to the rolling resistance (i.e. rolling friction) and axle friction of the airplane wheels will keep the airplane stationary despite the moving surface; any force greater will move the airplane in the direction opposite of the movable surface.

Fact 4: Rolling resistance is a function of [what?] and axle friction is a function of [what]?"; the rolling resistance and axle friction of an airplane is orders and orders of magnitude less than the thrust provided.

Fact 5: The conveyor makes no difference in the ability of the airplane to take off.


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## Guest (Dec 29, 2007)

I think there is something missing and in fact I will re-iterate what I believe it is at the crux of the problem.

What is the interaction of the air (fluid) medium?

If we remove the treadmill, everyone will agree that the lift that produces flight is due to the differential pressure across the span of the wing. Right? That pressure differential is a result of the plane air flowing over the wing from forward propulsion (thrust) resulting in the so-called Bernoulli Effect. I think we can all agree upon that scenario.

Going back to the treadmill, in my mind, the interceding fact between #4 and #5 to be confirmed/disputed:

Fact #4.5: "The flow of the air over the wing span and its' effect on the lift of the plane is INDEPENDENT of the treadmill"

:2cents:

JR


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## Dark Knight (Dec 29, 2007)

:deadhorse: :deadhorse: :deadhorse:


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## Guest (Dec 29, 2007)

Whatever happened to Sappers' FBD ?? :dunno:

JR


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## IlPadrino (Dec 29, 2007)

jregieng said:


> I think there is something missing and in fact I will re-iterate what I believe it is at the crux of the problem.
> What is the interaction of the air (fluid) medium?
> 
> If we remove the treadmill, everyone will agree that the lift that produces flight is due to the differential pressure across the span of the wing. Right? That pressure differential is a result of the plane air flowing over the wing from forward propulsion (thrust) resulting in the so-called Bernoulli Effect. I think we can all agree upon that scenario.
> ...


OK, but why not include Fact 4.5 in Fact 1 (which I had wanted to use to set the stage for "normal" but then had to add Fact 2)? I'd also pedantically point out that the flow of air is not INDEPENDENT of the treadmill; Fact 4 tells us the friction is small (orders and orders less than the thrust) but it will still slightly slow down the relative airspeed of the airplane (and therefor the flow of the air over the wings.)

Sapper's free-body diagram won't win any artistic awards, but it's certainly complete and demonstrates Facts 3 and 4. I'd only nitpick that "no new forces" overlooks the fact that the magnitude of the friction forces will certainly change with the doubled rotational speed of the tires.


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## IlPadrino (Dec 29, 2007)

BringItOn said:


> :deadhorse: :deadhorse: :deadhorse:


What, and you're against increasing the post-count of the board? If nothing else, we're a few posts further ahead of "the other board".


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## Dark Knight (Dec 29, 2007)

IlPadrino said:


> What, and you're against increasing the post-count of the board? If nothing else, we're a few posts further ahead of "the other board".


Said what????

:dunno: what you are talking about.

Ah...I am not against :deadhorse: :deadhorse: :deadhorse:


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## IlPadrino (Dec 30, 2007)

BringItOn said:


> Ah...I am not against :deadhorse: :deadhorse: :deadhorse:


I misunderstood... but you know there are laws against that in most States!


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## IlPadrino (Dec 30, 2007)

SapperPE said:


> Also, my handwriting hasn't improved.


Are you left handed?


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## mudpuppy (Dec 30, 2007)

Update on the Mythbusters episode. Apparently there were a lot of people really upset about the "snafu". There is a preview of the actual PoCB episode on the Mythbuster's website now that says the air date is going to be *January 30*. So mark your calendars for the day we will all see the plane take off with no problem.

BTW Frazil, there are select full episodes of Mythbusters on the Discovery website so hopefully this episode will end up on there.


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## IlPadrino (Dec 31, 2007)

SapperPE said:


> Also, when I lift weights, my left arm is stronger than my right, just a tad.


There might be some other explanation for that, but that's a topic for another forum.


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## frazil (Dec 31, 2007)

mudpuppy said:


> Update on the Mythbusters episode. Apparently there were a lot of people really upset about the "snafu". There is a preview of the actual PoCB episode on the Mythbuster's website now that says the air date is going to be *January 30*. So mark your calendars for the day we will all see the plane take off with no problem.
> BTW Frazil, there are select full episodes of Mythbusters on the Discovery website so hopefully this episode will end up on there.


Thanks mudpuppy!


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## PEsoon2B (Jan 16, 2008)

http://dsc.discovery.com/video/?playerId=2...leId=1344511100


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## RIP - VTEnviro (Jan 31, 2008)

It flies!!


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## FLBuff PE (Jun 20, 2008)

No f'in way the thing flies! :deadhorse: :deadhorse: :deadhorse:

Oh wait, I saw that episode...yes it does. :deadhorse: :deadhorse: :deadhorse:


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## Sschell (Jun 20, 2008)

it flies it flies

ground speed...

air speed...

force generated by plane engine acts on air not ground....

wheel friction negligible...

blah blah blah....

dude!


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## FLBuff PE (Jun 20, 2008)

^^^Nice summary!


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## Dark Knight (Jun 20, 2008)

Oh heck no please.

Not this sheat again....


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## Sschell (Jun 20, 2008)

Dark Knight said:


> Oh heck no please.
> Not this sheat again....


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## Flyer_PE (Jun 20, 2008)

HWY PE said:


> hmmm, I know it flies, but I'm gonna play devils advocate and say it doesn't just to keep this thread alive.
> I say it can't fly, the wheels aren't pushing it forward.


Sez the man who went to all the trouble of creating the FBD for the problem.

:lmao:


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## Flyer_PE (Jun 20, 2008)

I remember. I joined after that thread was well on it's way to HOF though. That was a scream.

Bye the way: Welcome Back.


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## MA_PE (Jun 21, 2008)

now let me see if I'm understanding the problem correctly. The plane is on a moving conveyor, like a treadmill, right...and then as the plane starts moving forward.....


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## cement (Jun 22, 2008)

I just called in about this to the "stump the professor" show on the radio here and totally ruined his day. LMFAO!


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## mudpuppy (Jun 22, 2008)

You're wicked!

:appl:


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## DVINNY (Jun 26, 2008)

Cement said:


> I just called in about this to the "stump the professor" show on the radio here and totally ruined his day. LMFAO!


EXCELLENT!!!!!!!

I use it too, whenever someone is too smart for their own good.


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## FLBuff PE (Jul 3, 2008)

Wait, wait...you mean the plane flies???!!!?????


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## maryannette (Jul 3, 2008)

It was rigged.


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## Sschell (Jul 3, 2008)

not this thread again!


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## FLBuff PE (Jul 3, 2008)

Can't help it, sorry. :deadhorse:


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## Sschell (Jul 3, 2008)

dude.


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## sehad (Jul 10, 2008)

Ok, I know everyone is tired of this threat. BUT, explain to me please how the plane flies? As stated earlier, the wings are what cause uplift on the plan and the air has to be going a certain velocity for this to happen. So in my mind(understanding I've never designed an airplane), significant forward movement of the wings must happen. How does the plane get lift?

edit: I'm not reading the entire pages but I do have one question. Someone on the first page was saying that the thrust of the engine acted on the air not the ground so it would still move. Didn't is say that the belt was a reaction to the forward movement of the plane? If so, as the plane accelerated by force on the air, the belt would increase in speed to match that.

DID THE THING ACTUALL FLY?


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## mudpuppy (Jul 10, 2008)

Does this clear things up for you at all?



mudpuppy said:


> Ok, I haven't weighed in on this. So let me try (yet) another approach at explaining this in the off chance that DV is not correct in his conspiracy theory.
> Let's say we have the plane sitting motionless on the conveyor. Let's say it's a small plane; small enough for a couple of people to easily push around. Now say a couple people off to the side of the conveyor (feet on solid ground) approach the plane, one on either side, and push forward equally on each wing with enough force to overcome friction. Does the plane move relative to the ground?
> 
> If you think it does not, please explain what the conveyor is doing to prevent the plane from moving when an external force is applied to it. In order to keep the plane from moving, an opposite force of equal magnitude must be applied. As far as I can see, there is no way for the conveyor to apply this force--if the conveyor moves forward, the plane moves forward with it. If the conveyor does not move, the plane rolls forward. If the conveyor moves backward (as the riddle states), the plane rolls forward with its wheels spinning faster.
> ...


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## IlPadrino (Jul 10, 2008)

sehad said:


> Ok, I know everyone is tired of this threat. BUT, explain to me please how the plane flies? As stated earlier, the wings are what cause uplift on the plan and the air has to be going a certain velocity for this to happen. So in my mind(understanding I've never designed an airplane), significant forward movement of the wings must happen. How does the plane get lift?
> edit: I'm not reading the entire pages but I do have one question. Someone on the first page was saying that the thrust of the engine acted on the air not the ground so it would still move. Didn't is say that the belt was a reaction to the forward movement of the plane? If so, as the plane accelerated by force on the air, the belt would increase in speed to match that.
> 
> DID THE THING ACTUALL FLY?


You have to read every post on this thread and build your own model (to scale!) before I'll entertain (key word: entertain!) any more discussion on this. I do understand why many get this wrong before they really think it through, but for the life of me, I'll never understand how a competent engineer can't understand this after MUCH discussion.


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## sehad (Jul 10, 2008)

IlPadrino said:


> You have to read every post on this thread and build your own model (to scale!) before I'll entertain (key word: entertain!) any more discussion on this. I do understand why many get this wrong before they really think it through, but for the life of me, I'll never understand how a competent engineer can't understand this after MUCH discussion.


Sorry that I haven't been involved in the MUCH discussion and I didn't read all 7 pages due to the time it would take and I'm sure some of the discussion gets pretty technical as I can tell by the tone in your post. BUT, I will be sitting down tomorrow to think this through.


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## benbo (Jul 10, 2008)

sehad said:


> Sorry that I haven't been involved in the MUCH discussion and I didn't read all 7 pages due to the time it would take and I'm sure some of the discussion gets pretty technical as I can tell by the tone in your post. BUT, I will be sitting down tomorrow to think this through.


It took me a little while to get it. THe main thing you have to get past is the notion that the treadmill can exert any real effect on the plane. Except for some minimal friction, the wheels of the plane just roll freely.

I actually had to get a little matchbox car and push it on my exercise treadmill. No matter how fast the treadmill turns it can't do anythiing to the car. If the car goes forward at one foot per second and the treadmill goes backwards at one foot per second, the wheels just spin faster.


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## sehad (Jul 10, 2008)

I conceed. She flies. I usually have to draw things out so I did. Right now my wife is going through some rant about how big of a nerd that I am!

The only force resisting the thrust of the plane is drag and friction of the wheel. The plan will take off slightly slower than usual becaue there is some force there, even though it may be minimal. It still seems to go agains logic and common sense. I'll have to try my brother in law with this one. He's in school for engineering now.

I like these brain teasers as well!


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## IlPadrino (Jul 10, 2008)

sehad said:


> I conceed. She flies. I usually have to draw things out so I did. Right now my wife is going through some rant about how big of a nerd that I am!
> The only force resisting the thrust of the plane is drag and friction of the wheel. The plan will take off slightly slower than usual becaue there is some force there, even though it may be minimal. It still seems to go agains logic and common sense. I'll have to try my brother in law with this one. He's in school for engineering now.
> 
> I like these brain teasers as well!


Oohrah! You're restored my faith in humanity!

I, too, really enjoy brain exercises. I'll start another one we were just discussing the other day.


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## FLBuff PE (Jul 11, 2008)

Sehad, Fudgey posted this. IMHO, it is the best reponse to this thread ever posted on this board...



Fudgey said:


>


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## sehad (Jul 11, 2008)

^^^Can't see whatever it is


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## FLBuff PE (Jul 11, 2008)

Funny pic thread, page 5. You may have to view it at home (damn internet firewalls/filters).


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## engineergurl (Jul 11, 2008)

so... if i flap my arms while running on a treadmill I will take off flying forward instead of tripping and flying off the back like I normally do... :Locolaugh:


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## csb (Jul 11, 2008)

only if your arms agree with Bernoulli...


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## engineergurl (Jul 11, 2008)

By the way... there are too many posts for me to read all the MUCH debate... but this was on mythbusters.... so I say

THE PLANE WILL TAKE OFF!

(Right now I don't think my arms would agree even if my mind does!)


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## sehad (Jul 13, 2008)

It'll takeoff. Other thanthat, who cares!


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## FLBuff PE (Jul 15, 2008)

You guys just don't understand that one of the points of this thread is to :deadhorse: and incite riots!


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## sehad (Jul 15, 2008)

yea yea yea!


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## Guest (Aug 5, 2008)

+1 for the ....

:deadhorse: :deadhorse: :deadhorse: :deadhorse: :deadhorse: :deadhorse: :deadhorse:

and a little bit for the ......





JR


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## Supe (Aug 5, 2008)

I HAVE to print that picture out and hang it up in my cubicle, no ifs, ands, or buts about it. And every time I am on the phone with one of our "construction interface" managers, I will look at it and sigh.


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## Dleg (Aug 5, 2008)

Treadmill technology has so many promising applications in the transportation and military industries:


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## FLBuff PE (Jul 20, 2009)

bump for the n00bs


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## Sschell (Jul 20, 2009)

oh no FL what did you do?

don't you know the thing will never take off?!?


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## FLBuff PE (Jul 20, 2009)

sschell_PE said:


> oh no FL what did you do?
> 
> don't you know the thing will never take off?!?


Well played.


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## Sschell (Jul 20, 2009)

The Dude said:


> dude.


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## engineergurl (Mar 9, 2011)

bump just cause I'm having fun on EB tonight...


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## pbrme (Aug 25, 2011)

*Twist...*

How about the same Jet/Treadmill but add frictionless casters to the treadmill's base. Huh?

Would a Fat Naked old man in a lawn chair petting a persian cat:

a.) Shoot nerf darts at it

b.) Release the gimp

c.) Pick taco bell out of kitty's fur, maintaing visual w/ experiment

d.) Not care, 180 chair and surf internet chat rooms


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## mevans154 (Sep 4, 2012)

MA_PE said:


> Time to write to mythbusters.


Mythbusters did an episode on this exact theory.

The gang put a small single prop plane on a long tarp attached to a pickup truck. The plane's take off speed was determined to be 25mph. The truck pulled the tarp at a rate of 25mph, and the plane accelerated to 25mph. The plane was basically not moving in reference to the ground.

The plane did in fact take off!!!


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## NYMechEng (Nov 19, 2012)

civengPE said:


> MaPE,
> 
> You almosty there. The runner is pushing off of a moving surface, the conveyor,.
> 
> ...


Except this thing called heat, the tires would explode from the increased pressure from the increased heat from the increased friction energy imparted from the wheels traveling a million miles per hour. Eventually a tire will blow apart. Chances are so slim they'd blow at the exact time. Once a tire blows, if the proper lift was not acheived, then this thing called gravity forces the planes landing gear on the belt, and has a higher coefficient of friction, and creates an off balanced friction lateral force, which creates an unblanced moment about the plane to the right or left, and then the plane crashes on takeoff.

Well, that is, if the plane did not have automatic thrust vector control capability to automatically compensate. :bag:


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## NJmike PE (May 12, 2014)

bump


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## Supe (May 13, 2014)

Ah, good times...


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## Troll (Mar 10, 2017)

nice question for HS science students, not professionals who should have some knowledge of aerodynamics. 

assuming 0mph wind speed, if the plane is stationary relative to the surrounding air there will be no lift.

how in the world did i get here?


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## frazil (Mar 10, 2017)

You took a plane on a conveyor belt.


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## NY-Computer-Engineer (Jul 3, 2018)

All I would 'really' care about is that the snacks on the plane were abundant and free of charge.

However, isn't this all up to the *Bernoulli *principle?  As I recall from Physics, it only needs enough air passing over and under its wings to get me to Disney World this summer.


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