Theoretical question about airplanes

[Reload]

Okay.

Imagine a plane is on a runway that moves. Kinda like a treadmill.

So as the plane accerates forward, the treadmill runway matches the same speed in the opposite direction.

Would the plane be able to takeoff?

 

[rb420]

no it wouldn't

 

[stroker888]

nope, I'm pretty sure lift is dependent on the air movement over/under the wings not how fast the wheels are going

 

[Gentle Ben]

nope, no lift from the wings

 

[bangcamaro]

No, no lift is being produced because the plane is not moving.

 

[cancowboy2001]

What if the airplane was a turboprop?
The propeller(s) would be pushing air over the wings.

 

[cypherpunk]

The answer is of course yes. If the ground is moving to counteract the motion of the plane, the wheels will simply spin twice as fast because the plane is not pushing against the Earth like a car.

EDIT: This is actually supposed to be on Mythbusters tomorrow if any of you want to be schooled or just geek out. I can't wait.

 

[Aardvark154]

The answer is of course yes. If the ground is moving to counteract the motion of the plane, the wheels will simply spin twice as fast because the plane is not pushing against the Earth like a car.

Are you sure about that? I'm not an engineer and it's been years since Secondary School physics.

But, if the aircraft is attempting to accelerate forward and the "treadmill" is accelerating in the opposite direction at the same speed isn't the forward speed of the aircraft over the ground going to be zero? If so there will be no airflow over the wings and therefore no lift.

 

[Moraff]

Are you sure about that? I'm not an engineer and it's been years since Secondary School physics.

But, if the aircraft is attempting to accelerate forward and the "treadmill" is accelerating in the opposite direction at the same speed isn't the forward speed of the aircraft over the ground going to be zero? If so there will be no airflow over the wings and therefore no lift.

If the aircraft accelerated via the wheels (like a car) you would be 100% correct. However an aircraft accelerates by taking the air in front of it and pushing it out behind. The wheels only serve to reduce the friction effect of the ground.

So the plane would accelerate relatively normally while the wheels spun faster and faster until the plane left the ground.

Of course one could argue about whether the wheels would survive a rotation speed of 2x what they normally experience on takeoff.

 

[Aardvark154]

What if the airplane was a turboprop?
The propeller(s) would be pushing air over the wings.

But not enough air to generate the needed lift.

 

[hairyfucker]

The aircraft will not takeoff since there is no fluid force (air) on the airfoil (wings) to generate a lifting force. The wheels of aircraft are just spinning in place. Nothing more. The key to this argument is reference points. i.e. wing to fluid flow

 

[hairyfucker]

If the aircraft accelerated via the wheels (like a car) you would be 100% correct. However an aircraft accelerates by taking the air in front of it and pushing it out behind. The wheels only serve to reduce the friction effect of the ground.

So the plane would accelerate relatively normally while the wheels spun faster and faster until the plane left the ground.

Of course one could argue about whether the wheels would survive a rotation speed of 2x what they normally experience on takeoff.

You are speaking of vectored thrust. With vectored thrust wheels are almost irrelevant since a vertical takeoff is possible. My favorite aircraft utilizing vectored thrust is the AV-8B. http://www.fas.org/man/dod-101/sys/ac/av-8.htm

 

[Aardvark154]

If the aircraft accelerated via the wheels (like a car) you would be 100% correct. However an aircraft accelerates by taking the air in front of it and pushing it out behind. The wheels only serve to reduce the friction effect of the ground.

So the plane would accelerate relatively normally while the wheels spun faster and faster until the plane left the ground.

But wouldn't this be like taking a model putting it on a treadmill and pushing it? If the force exerted either by my hand or by the aircraft's engines is counterbalanced by the opposite speed of the treadmill wouldn't the airspeed be zero? I agree that if the aircraft had a thrust ratio greater than the opposite force exerted by the treadmill so that it had forward airspeed and could accelerate to v1 than the above would be more or less correct.

 

[Moraff]

You are speaking of vectored thrust. With vectored thrust wheels are almost irrelevant since a vertical takeoff is possible. My favorite aircraft utilizing vectored thrust is the AV-8B. http://www.fas.org/man/dod-101/sys/ac/av-8.htm

I don't think so, vectored thrust is no more than aim-able engine thrust. The wheels on the plane are not driven by the engines and therefore do not contribute to acceleration.

Ignoring the treadmill part of the scenario, why does a plane start moving on a normal runway? Because the engines are pushing the air out the back, not because the wheels are pulling the plane up the runway.

 

[Moraff]

But wouldn't this be like taking a model putting it on a treadmill and pushing it? If the force exerted either by my hand or by the aircraft's engines is counterbalanced by the opposite speed of the treadmill wouldn't the airspeed be zero? I agree that if the aircraft had a thrust ratio greater than the opposite force exerted by the treadmill so that it had forward airspeed and could accelerate to v1 than the above would be correct.

The plane's wheels are free-turning, therefore the treadmill doesn't add any acceleration in the direction opposite to the engines.

Okay there would be a little force imparted due to friction in the wheel bearings if you wanna get picky, but that would be negligible compared to the thrust of the engines.

 

[cypherpunk]

Are you sure about that? I'm not an engineer and it's been years since Secondary School physics.

But, if the aircraft is attempting to accelerate forward and the "treadmill" is accelerating in the opposite direction at the same speed isn't the forward speed of the aircraft over the ground going to be zero? If so there will be no airflow over the wings and therefore no lift.

I'm very sure. Imagine that it's being pulled by a rope by something that's off of the treadmill if that helps. Were the ground to match the FORCE of the engine, it would be another story entirely.

 

[Aardvark154]

Were the ground to match the FORCE of the engine, it would be another story entirely.

Ok, I think I see how I’m misunderstanding the problem. I was thinking of the treadmill applying an opposite force to the thrust of the aircraft's engines and it is not. So basically the aircraft is going to move forward, however the wheels are going to spin at logarithmically increasing speeds until takeoff. (The crash from tire failure should be spectacular).

 

[Reload]

Okay. Lets throw another wrench into the equation here.

Let us assume that we've got frictionless wheels. So the tires can spin as fast as they want to with no wear and tear on any of the components in the landing gear or any other part of the plane for that matter.

Also, there's no wear and tear on the treadmill runway either.

 

[Moraff]

Okay. Lets throw another wrench into the equation here.

Let us assume that we've got frictionless wheels. So the tires can spin as fast as they want to with no wear and tear on any of the components in the landing gear or any other part of the plane for that matter.

Also, there's no wear and tear on the treadmill runway either.

That makes it simpler. Frictionless wheels -> absolutely no effect by the treadmill -> plane flies.

 

[Aardvark154]

Okay. Lets throw another wrench into the equation here.

Let us assume that we've got frictionless wheels. So the tires can spin as fast as they want to with no wear and tear on any of the components in the landing gear or any other part of the plane for that matter.

Also, there's no wear and tear on the treadmill runway either.

Having, I believe gotten my head around this, how would that change the problem other than the rather practical elimination of tire failure during takeoff?