Plane on conveyor... Will it ever take off?

[HurricaneHeather]

Originally Posted by askmrjesus

Yeah, if the Air Force doesn't control the laws of physics, who does?

JC

Well up until a few minutes ago I never wondered about these things! Not I'm in a dead panic!

[Lucky3623]

Originally Posted by Tmall

No. Just because he is in the air force does not mean he controls the laws of physics.

No? Really? Damn... I need to do more research then... I also thought that wind is made by trees swaying back and forth in the atomsphere...


When a fighter jet lines up at the end of the runway, it typically runs its engines up to full speed before releasing brakes. The speed of the wheels on the jet is equal to the speed of the runway below it (0 mph). The air coming out of the back of the engine is over 1000 mph, much faster than the aircraft needs to get airborne. If the myth is right, the aircraft should lift off!

And here is the argument you typically hear:
But wait, the brakes are set!!! That is why it doesn’t work!
So the second the pilot lets go of the brakes, the jet gets airborne?
No, it gets airborne when the speed through the air (not over the ground) reaches the point to cause enough lift over the wings to overcome gravity (the weight of the jet.)

The compressibility of air and temperature differences result in vast differences in one if you try to hold the other constant.

[Rider]

Originally Posted by Tmall

In fact, if the plane originally could take off on 100 feet of runway, it would only need 100 feet of conveyor.

In theory yes but because the wheels are a friction point it would take slightly longer to get to Vr(Rotate speed). Your V1, V2, and Vr would all be the same speed as normal takeoff, it would just take a little longer to get there dependent on how much faster the conveyor was running than the take off speed.

[Tmall]

So you agree?

[Tmall]

Rider, I know what you're saying.

But, I honestly think the friction increase woudld be so small that the bearings of the wheels would still be well within their operating range.

[Lucky3623]

Originally Posted by Tmall

So you agree?

I agree that if you add thrust, yes the plane will take off, so long as the pressure around the wings is great enough to sustain lift... but if you do not add thrust, no the plane will not move.

[Rider]

Originally Posted by Lucky3623

I agree that if you add thrust, yes the plane will take off, so long as the pressure around the wings is great enough to sustain lift... but if you do not add thrust, no the plane will not move.

Yes it will, it will move in the direction the conveyor is moving.

[pauldun170]

Originally Posted by Rider

Yes it will, it will move in the direction the conveyor is moving.

[Avatard]

Gentlemen: Since the plane is on a treadmill, it is not being forced through the air, creating an artificial headwind. The only thrust to create speed differential with the surrounding air is the plane's thrust.

If the plane is of a design as to have enough thrust to get airborne essentially with no headwind (at least none induced by moving the plane forward, which the treadmill assures cannot happen), and by its own thrust alone, then it will take off.

The freewheel on the tires is exactly doubled, as has already been said. I believe the frictional losses, if within the spec of the wheels, is probably relatively inconsequential.

[Flexin]

Originally Posted by Lucky3623

I asked this to a buddy whom is a retired AF Maj. Here is his answer:

Despite what Mythbusters says, this is not possible. If the treadmill is traveling at the same speed as the plane, there is no airflow over the wings. Without airflow, there is no lift.

Now, with that said, here is how they get around the issue. By using a propeller to generate the thrust needed to “move” the airplane at the same speed as the treadmill, the propeller is pushing air over the wings, causing some lift. In very light aircraft, you might even create enough lift to get the aircraft airborne, but you have changed the original equation. If you use a jet engine which bypasses the wings, the answer is a little clearer. Better yet, use a glider that matches the speed of the treadmill by having a support that keeps the glider from moving backwards. Unless you can get the treadmill going fast enough to generate airflow over the wings, it’s not gonna happen!

If you stick to the physics involved in the original setup, it is not possible. Mythbusters “disproved” the myth, but in both cases (model and full size) the aircraft was moving forward when it got airborne.

Thats the thing. The glider has no thrust. If the tread mill is doing 300 mph and the gliders wheels are turning at 300 mph because of the tread mill moving then you have no lift. But a glider glides and as someone said uses gravity to pull it through the air which allows the wings to create lift.

Now take a regular plane and do the same thing. Leave the engines off and its like the glider. With the tread mill moving at 300 mph and the wheels turning at 300 mph we have a plane sitting still. BUT...if the tread mill is moving 300 mph and the PLANE itself is moving 300 mph it will fly.

The plane gets trust from engines that move air NOT a transmission connected to wheels.


Try this one. The bottom of a small plane is about 3 feet off the ground or so, right? Lets take a crazy pilot and have them fly over the runway at 3 feet off the ground (in a small plane that can retract its wheels) its over the same height off the ground as the plane on its wheels. If it does this at 150 mph, the ground will be moving under it at 150. The plane flies.

Now take this same crazy pilot and the same plane but have him fly the same way over a big ass tread mill the size of a runway. The ground (or treadmill belt) will be moving at 150 mph, the plane is going in the opposite direction at 150. Because of this it if you took the speed of the belt it should be like its going 300 mph. But the plane will still fly over it. It would have no affect on the plane.

If the plane did this over a tread mill with the belt going in the SAME direction of the plane you could have someone hang from the plane and step off the plane and they would be standing beside the plane moving (well the 150 mph wind should not the guy on his ass but if he could stand in the wind he would be standing by the plane.

The treadmill doesn't stop the air from moving. When the PLANE is doing the speed its needs to create life it will take off. It doesn't care what speed the ground is moving at, it only matters what speed the plane is moving throught the air.

Thats the key there. Air speed.

James