Engineer Boards

# Riddle me this batman

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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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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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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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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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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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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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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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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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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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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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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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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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you get a dirty sheet.

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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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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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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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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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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!

:rotfl:

:thumbsup:

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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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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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Great pic mizzou!

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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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

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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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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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.