A Wednesday conundrum

[Moose.]

A plane is standing on a runway that can move (some sort of giant conveyer belt). The plane moves in one direction, while the conveyer moves in the opposite direction. This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in the opposite direction). Can the plane take off?

[Moose.]

I had no idea this was a repost (saw the thread on pprune a figured the PH collective might enjoy it ) I'm sticking with yes it will take off because the wheels on a plane aren't driven and hence the plane will still go along the runway, abet with the wheels spinning much faster.

[Moose.]

scorp said:

Hmm.. thrust will push the plane (like how you would push a car), which will cause the wheels to turn, the conveyor see's this and moves the plane backward be an equal amount in response. So the plane stays put.

The plane will still continue to move, as the wheels are free-wheeling and hence the conveyor can move as fast as it likes in the other direction. The plane will still take-off.

[Moose.]

Eric Mc said:

The conveyor belt and the rotation of the wheels has no bearing whatsoever on whether the plane is going to take off. ALL theat matters is that there is sufficient airflow over the wings.

Exactly! To turn it round the other way, if you stuck a plane in a wind tunnel and increased the wind speed to greater than rotation speed (takeoff speed), it would fly but not be moving relative to the ground.

[Moose.]

percy flage said:

The thrust pushes against the air behind it, meets resistance from this air and this causes the plane to move forward. This forward motion creates airflow over the wings, yes? If the conveyor negates the forward movement of the plane there is no airflow. So the plane cannot take off. No?

But the point is the conveyor cannot negate the forward movement of the plane becuase the wheels are free-wheeling! Hence the plane takes off.

[Moose.]

greg_D said:

i have just had another idea: bear with me here: let's say a real plane on a real runway sets off and experiences a freak tailwind that exactly mirrors it's takeoff speed (let's say 150mph) Will it take off???????

No, because the airspeed over the wings will be 0kts and hence no lift.

[Moose.]

dougc said:

Aha! Now I understand (I think) If it were a car with wings i.e. drive comes from the wheels in contact with the conveyor then it would be unable to take off - correct??

Correct! Another convert to the realities of physics

[Moose.]

greg_D said:

good, is everyone with me so far reverse the circumstances so that the wind is still and the runway (aka sainsbury's finest) is going at 150 mph and the plane is at full power. will the plane take off????

But that's not the opposite circumstance! Wind is the crutial factor here as it's the wind going over the wings that creates the lift. To reverse your example would mean having a head wind of 150mph which would allow to plane to take off with no runway.

[Moose.]

I think the only way we're going to convince the doubters is to build a giant conveyor belt! I've got an R/C model plane we could use. I just need a convoyer belt about 10m long for the runway. I guarantee it will take off even with the conveyor running at full pelt