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

[Adeptus_Minor]

Originally Posted by Lucky3623

If the belt and was moving at the exact same speed as the plane, how was the plane able to move forward? If I run at 10 mph on a treadmil that is going 10mph, I don't move forward, I stay stationary... If I speed up my pace to 11 mph while the treadmil is still at the orig 10 mph, I move forward on the belt, if I slow my pace to 9 mph, I fall off the back...

Read back a bit.
The movement of the plane has nothing to do with anything touching the conveyor. The thrust from the jet engines will push it forward while the wheels spin free.
We're sort of pre-wired to think only in terms of movement based on pushing against the ground... that's not how planes operate.

[Lucky3623]

Originally Posted by Adeptus_Minor

Read back a bit.
The movement of the plane has nothing to do with anything touching the conveyor. The thrust from the jet engines will push it forward while the wheels spin free.

Ahhh... I didn't even think about the movable wheels not having any affect on the thrust. If the wheels were locked, and the thust was on, yes... the plane would be standing still...

[HurricaneHeather]

Originally Posted by Adeptus_Minor

We're sort of pre-wired to think only in terms of movement based on pushing against the ground... that's not how planes operate.

That says it right there. Planes don't make sense.

[Avatard]

Originally Posted by Adeptus_Minor

Read back a bit.
The movement of the plane has nothing to do with anything touching the conveyor. The thrust from the jet engines will push it forward while the wheels spin free.
We're sort of pre-wired to think only in terms of movement based on pushing against the ground... that's not how planes operate.

There's one other factor [er, two].

Because the plane (due to its being essentially temporarily "immobilized" on a treadmill) is not really moving against the ambient air (well, no more air than the thrusters will provide in airflow); the plane is essentially taking off with a matching tailwind, as the ambient air is not aided in differential movement across the plane by any takeoff-speed-provided airflow.

I agree that beyond this factor (and the minor issue of increased wheel speed, and a corresponding increase in bearing losses) the wheels play essentially no part in this.

The plane will still fly, because its true interaction (primary exchange of thrust) is via the air, not the ground via the tires, as they free-spin (as you pointed out).

[fasternyou929]

Originally Posted by Avatard

There's one other factor [er, two].

Because the plane (due to its being essentially temporarily "immobilized" on a treadmill) is not really moving against the ambient air (well, no more air than the thrusters will provide in airflow); the plane is essentially taking off with a matching tailwind, as the ambient air is not aided in differential movement across the plane by any takeoff-speed-provided airflow.

I thought that was the point of the riddle, that the conveyor cannot immobilize the plane since the plane propels itself using air, not the wheels. So if the thrusters are set for 240mph and the treadmill is set for 240mph in the other direction, the plane still moves at 240mph but the wheels spin freely against the conveyor at 480mph. Or no?

[Rider]

Originally Posted by fasternyou929

I thought that was the point of the riddle, that the conveyor cannot immobilize the plane since the plane propels itself using air, not the wheels. So if the thrusters are set for 240mph and the treadmill is set for 240mph in the other direction, the plane still moves at 240mph but the wheels spin freely against the conveyor at 480mph. Or no?

Yes

[anthonyk]

The problem with this whole thing is that it's impossible for the magical treadmill to match the wheel speed of the plane. Unless some laws of physics are seriously tweaked, the plane will always move forward, and that means the wheels will always spin faster than the belt on the treadmill. If you made something in the real world that tried to constantly match the speed of the belt to the wheels, the belt would quickly keep going faster until something blew up (a la AMJ). The plane would continue on, blissfully unawares.

LOL, damn those smilies...

[Flexin]

Originally Posted by anthonyk

The problem with this whole thing is that it's impossible for the magical treadmill to match the wheel speed of the plane. Unless some laws of physics are seriously tweaked, the plane will always move forward, and that means the wheels will always spin faster than the belt on the treadmill. If you made something in the real world that tried to constantly match the speed of the belt to the wheels, the belt would quickly keep going faster until something blew up (a la AMJ). The plane would continue on, blissfully unawares.

LOL, damn those smilies...

That wouldn't be impossible. If someone had pockets deep enough that could be built. But they done this on tread mills with remote control planes so its not to hard to believe.

And a float plane can take off on water. I think the floats would cause more drag then the wheels/wheel bearings being over worked and it has no problem taking off.

If you stand on a tread mill with roller skates on you are like a plane before the engines are turned off. Don't try to skate, just stand there because the plane doesn't get forward movement from its wheels. They are just like casters on your office chair. The wheels allow the object they are attached to to move freely across the ground when they pushed.

You push your office chair with your feet to move it. The plane uses and engine that will pull/push it self through the air, Once it is moving fast enough through the are for the wings to work it takes off. The speed of the ground under the wheels has no affect.

The ground is a moving platform on a aircraft carrier.

James

[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.

[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