Plane on a Treadmill

[TACP]

Imagine a plane sitting on a giant conveyor belt, as wide and long as a runway. The conveyor belt is designed to exactly match the speed of the wheels, moving in the opposite direction. Can the plane take off?

[helper]

Yes, it will. Ex-plane-ation can be given after other comments posted. 

[lordmonar]

lift has nothing to do with the speed of the wheels.

Nor does thrust come from the wheels rolling on the ground.

[CadetProgramGuy]

No it will not, unless I am missing the fact that there is no relative wind alowing over the wings......

From the scenario you listed the aircraft is not moving through the air, or the air is not flowing over the wings.  Just the wheels are moving.

[Al Sayre]

Yes, it will take off.  Here is a demonstration:  http://mythbustersresults.com/episode97

As was said previously the speed of the wheels has nothing to do with the lift.

[DG]

Yes, it will take off.  Here is a demonstration:  http://mythbustersresults.com/episode97

As was said previously the speed of the wheels has nothing to do with the lift.

Huh?

You can see the plane is moving, when you look at the background passing by.

The text suggests that the aircraft will fly from a still position because of engine thrust.  That may be true in very limited conditions / scenarios.

But the video shows the airplane moving well before takeoff.

[JoeTomasone]

What's missing from the OP is the fact that the engines are actually running and producing the normal thrust.   It's not claiming that the treadmill/belt/whatever is generating thrust or lift.

[Spike]

This is a tough one.  If there is an increase in air past the wing it will take off, but if it is moving with no air increase initially it will not??

I must break out the Aerospace book for this one.

[Thrashed]

Engine thrust does not create lift, it creates the movement to make relative wind over the wing that creates lift.  If the belt increases speed so the aircraft remains stationary on it, then there will be no relative wind or lift from the wings.  The aircraft cannot fly no matter what the thrust. Mythbusters did not replicate this. If the aircraft is allowed forward progress by the belt, then eventually it will fly depending on the lift needed on that aircraft. But really, who cares? 

[lordmonar]

Yes, it will take off.  Here is a demonstration:  http://mythbustersresults.com/episode97

As was said previously the speed of the wheels has nothing to do with the lift.

The myth busters demonstartated that he treadmill has nothing to do with it...because the thrust comes from the engine/prop not from the wheels pushing against the tarmac.

The only affect the runway has on the aircraft is drag (the friction of the wheels on the ground) and Ground Effect on lift.

Put the plane on a tread mill and it has almost NO effect on the planes ability to fly. (it may change the lenght of the take off roll and that is it.)

[lordmonar]

Engine thrust does not create lift, it creates the movement to make relative wind over the wing that creates lift.  If the belt increases speed so the aircraft remains stationary on it, then there will be no relative wind or lift from the wings.  The aircraft cannot fly no matter what the thrust. Mythbusters did not replicate this. If the aircraft is allowed forward progress by the belt, then eventually it will fly depending on the lift needed on that aircraft. But really, who cares? 

That would be true....if the trust was developed by the wheels.  A car on a tread mill goes nowhere because the wheels are giving the car thrust by friction with the ground.  If the ground is moveing at the same rate of the wheels then no trust.

But an airplane's trust is deveoped by the prop or the jet.  It does not push against the ground.  So it does not matter what the tread mill is doing... the plane will fly.

[raivo]

That would be true....if the trust was developed by the wheels.  A car on a tread mill goes nowhere because the wheels are giving the car thrust by friction with the ground.  If the ground is moveing at the same rate of the wheels then no trust.

But an airplane's trust is deveoped by the prop or the jet.  It does not push against the ground.  So it does not matter what the tread mill is doing... the plane will fly.

I am not an engineer, but...

It seems to me that the conditions you describe would only apply to an aircraft already in flight - ie, one that already has the weight force canceled out by lift. If an aircraft is flying, and comes into contact with the treadmill, then the wheels will "spin" (assuming that the plane is flying perfectly level and just barely touching the treadmill) and the plane will continue to fly.

When the plane is at rest, the prop has to generate thrust in order to move forward and begin generating lift. But the plane can't simply move forward by pushing air backwards, because it's still being pulled towards the ground, so it has to use the friction of the wheels against the ground in order to generate forward motion. If the plane is already in flight, this isn't an issue, because the weight is already canceled out by lift.

If you put the plane on a treadmill, you remove the friction of the wheels against the ground, and take away the grounded plane's ability to generate forward motion.

[lordmonar]

No....trust in a prop aircraft is generated by the forward screw of the Prop.  It pushes (and pulls) on the air around the prop and pull the aircraft forward.  The wheels on the ground do nothing for thrust (except resist it through friction).  If the tread mill started moving at the same rate the aircraft moved forward then the wheels would just turn twice as fast...but because the wheels turn freely it would not impeed (much) the thrust (forward motion) of the aircraft.

The trust of the aircraft must overcome the drag induced by the wheels in contact with the ground.....but that is not enough to stop the forward motion of the aircraft.

Once again...unlike a car (whose motion is created by the friction of the wheels pushing against the ground) an air plane's forward motion is competely generated by the air screw or jet thrust of the power plant.  Ground friction is something that must be over come (and inertia) but it is not not enough to keep an air plane from becoming airborn.

[Short Field]

Thrust is fine but if the air is not moving over the wings, no lift is generated.  Prop wash is inadequate to generate enough lift for your average airplane to overcome its weight.  Thrust will move the airplane from one end of the airstrip to the other.  If lift is not generated by the movement of the airfoil through the air, the airplane will stay on the ground.   If the airplane is stationary due to a treadmill moving the airplane backwards as fast as the thrust moves the airplane forward - no lift is generated.

[DG]

Music to my ears.

//s//
DG
Philadelphia Liar

[N Harmon]

The trust of the aircraft must overcome the drag induced by the wheels in contact with the ground.....but that is not enough to stop the forward motion of the aircraft.

Given the problem, it actually would be. Remember, we have a conveyor belt that "is designed to exactly match the speed of the wheels, moving in the opposite direction".

I submit to you, that in order for the airplane to have any forward motion relative to the wind (and thus take off), its wheels must spin faster in the positive than the conveyor belt spins in the negative.

A simple mathematical simulation would bear out that a system including an airplane and the type of conveyor belt theorized in the problem, the speed of both the wheels and the conveyor belt will increase over time until an equilibrium is reached whereby the maximum thrust of the airplane's engine will match the kinetic force of friction on the wheels, resulting in zero forward movement.

And the aircraft will never take off.

[Al Sayre]

Thrust is fine but if the air is not moving over the wings, no lift is generated.  Prop wash is inadequate to generate enough lift for your average airplane to overcome its weight.  Thrust will move the airplane from one end of the airstrip to the other.  If lift is not generated by the movement of the airfoil through the air, the airplane will stay on the ground.   If the airplane is stationary due to a treadmill moving the airplane backwards as fast as the thrust moves the airplane forward - no lift is generated.

Unless you chain the wings to the ground, the airplane will move forward and fly.  The speed of the treadmill will only affect the speed at which the wheels turn, which the airfoil and engine could not care less about.  You could run the treadmill backwards at speeds approaching c, and the the airplane would still move forward as soon as prop thrust overcomes the wheel bearing friction.

[N Harmon]

Unless you chain the wings to the ground, the airplane will move forward and fly.  The speed of the treadmill will only affect the speed at which the wheels turn, which the airfoil and engine could not care less about.  You could run the treadmill backwards at speeds approaching c, and the the airplane would still move forward as soon as prop thrust overcomes the wheel bearing friction.

I think you're introducing factors that aren't part of the original problem. Namely, lag between an increase in thrust (and thus increase in wheel spin) and increase in conveyor belt speed. However, the theoretical conveyor belt machine defined in the problem has no such lag. It matches the speed of the wheels exactly.

[FW]

We need to perform the test.  What squadron commander reading this would volunteer to have their cadets do this experiment?  I'd love to see some real data.....

[N Harmon]

We need to perform the test.  What squadron commander reading this would volunteer to have their cadets do this experiment?  I'd love to see some real data.....

And you hope to get real data on a theoretical problem involving machines that can not exist how?