Riddle me this batman

[Hill William]

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.

 

[Kipper]

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.

 

[MA_PE]

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.

 

[civengPE]

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.

 

[civengPE]

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.

 

[EdinNO]

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

 

[civengPE]

Ed,

see above.

 

[civengPE]

??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:

 

[DVINNY]

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.

 

[DVINNY]

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.

 

[Hill William]

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:

 

[civengPE]

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.

 

[MA_PE]

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.

 

[Hill William]

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

 

[civengPE]

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.

 

[Hill William]

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.

I give up.

Agreed

 

[civengPE]

Very well put Sap.

Class Dismissed!

:claps:

 

[TouchDown]

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:

 

[MA_PE]

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.

 

[DVINNY]

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 **** 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.