omgwtfbbq said:
it's clearly not a no-brainer since you got it wrong =) no forward velocity won't happen, no treadmill's going to stop a plane taking off |
It's really a tricky situation. A specially built propeller plane would actually be able to take off (or a model plane on a real life conveyor belt) - and probably result in a spectacular crash due to uncontrollable sudden forward movement. A jet engine plane will not take off.
Now we all agree that planes fly because the forward movement of the plane creates an airflow on the wings that pushes air down and momentum conservation requires something else and of equal magnitude pushing up (yes I know that the demo in the Smithsonian museum tells you something different..)
Now if the plane rolls forward and the conveyor belt spins at the exact opposite speed, the plane has a net forward speed of zero (we can actually enter and leave the plane if we dare) - and there is now airflow over the wings since the plane is not moving in reference to us or the air around us. Imagine these tests they do with your car in a garage where the conveyor belt is two rolls and the wheels of your axis sit on the wheels. You can accelerate to any speed and still be on the same spot.
Now the tricky part: Let's make a propeller plane that blows the propelled air entirely over its wings (jet engines don't blow air over their wings). This plane is still stationary on the belt rolling in the opposite direction. Now the situation is different, we actually have airflow over the wings and therefore generate momentum. If we accelerate the plane and the conveyor belt to takeoff speed, we actually take off and hover shortly above the belt - and then rapidly accelerate forward after loosing contact with the belt which eould make this test rather uncomfortable to the pilot.
(Greeting from Dieter, who has taught physics to students for some years, so I hope I am not too wrong or none of my former students reads this...)
