Airplane on a conveyor belt

**BonnieT100** · 9th November 07, 04:01 AM

Ok, this question has been floating around the internet for a while and since we have a couple of pilots here as well as some very educated gents, I'll pose it here. If an airplane is placed on a conveyor belt that is moving in the opposite direction of the airplane's thrust, but at the exact same speed as that thrust would generate - i.e. holding the aircraft in place on the belt, would the plane take off once its speed reached the appropriate level? I say no because simply put, thrust serves to move the airplane forward through the air which provides lift. if the plane isn't moving through the air, there is no lift. What say yee?

**McGurk** · 9th November 07, 04:13 AM

Theoreticly, no, it would not take off.

However, this theory seems a little flawed. The undercarrage of the aircraft would not provide enough resistence to stop the aircraft moving forward, even though the conveyor was moving backwards, after all, in an aircraft, the engines push against the air, not turn the wheels.

So, where you to calculate the accelleration of an aircraft on a normal runway, and program a conveyor belt to replecate the acceleration in the opposite direction, the aircraft would still move forward. It would not accellerate as quickly, but it would still move forward. The main danger would be that the wheels may pass their rated speed limit and suffer a failure

Ofcourse, I'm also assuming that the breaks would not be applied, which would change the situation again.

I think this question must have been asked by someone with little knolage of aircraft, and unconciously expects the wheels to be providing the forward force, as with a car.

Aircraft fly due to aerodynamic lift, the word itself gives you the clue, aero dynamic = moving air.
There are exceptions that break the rule, for instance, the Harrier and the LightningII whcih vector the engine thrust to counter weight rather then drag, and then accelerate once airborn to begine to generate aerodynamic lift.

**Frank McGrath** · 9th November 07, 04:55 AM

Bernouli's Principle. No lift, No fly.

**Robinhood** · 9th November 07, 08:04 AM

The airplane's thrust pushes against the
air, not the conveyor. The airplane would
begin moving through the air, regardless
of the motion of the surface below the
wheels. When the plane is moving through
the air fast enough, it may take off if
given the appropriate control input by
the pilot.

**cessna152towser** · 9th November 07, 08:14 AM

The thrust pushes the aeroplane though the air so it would accelerate just as fast on the conveyor belt as it would on a runway. Only difference is the wheels would turn faster. I think the added friction from the belt trying to move the wheels the other way would be marginal, so if there was any difference in the aroplane's acceleration it would only be very slightly slower.

**Yaish** · 9th November 07, 08:37 AM

Why do people always think airplanes gain speed on the ground through their wheels?

The motive power comes through the air, not the wheels. Whatever happens to the wheels is irrelevant. The plane would take off and never notice the conveyor beneath it, though the wheels would spin faster.

Think of it this way, on take off, when the wheels first leave the ground does the plane suddenly come crashing down because it lost it's motive power? Of course not, it continues to accelerate and gain altitude.

Now, in the real world the moving conveyor would provide some rolling resistance which would need to be overcome, but at most it would require a bit more thrust to simply achieve take off speed, and not even much more at that.

For those interested, I believe the Mythbusters are testing this myth soon. Actually they already tested it, and are airing the episode soon.

**Cage** · 9th November 07, 08:41 AM