New Curveball Interview Question

[Pipe_Dream]

Ok, we all know about the question:

"why don't saloon cars have a rear wash/wipe?" :rolleyes:

Here's a new one I've just been emailed:

A plane is standing on a runway that is made of a large conveyor belt. The
plane fires up its engines, but as it moves forward, the conveyor belt
senses the speed of the plane's wheels and moves at exactly the same speed
in the opposite direction. Can the plane take off?

Any of you smart enough to come up with a probable answer? I know what I think.....let's see if you agree!! :D

[Craggenmore]

Provided it gets the correct airflow over the wings to the calculated Take Off speed then yes I think that it would because it does not matter if the a/c is moving through the air or that air is moving over the stationary wings!

I hope :ouch:

[5150]

Covered in detail in Jet Blast (of all places!) :8

[Superpilot]

Provided it gets the correct airflow over the wings

Yes, but that is the question! ;)

...Answer is 'No' - I just tried it in the gym :}

[small_dog]

My initial response is to say that the aircraft cannot take off because the net air flow over the wings is always zero regardless of what the undercarriage is doing.
The aircraft is effectively stationary, therefore, in still air conditions, it will not generate any flow over the wing, thus there will be no lift, therefore the aircraft will not take off.

The scenario sounds similar to a car on a set of rollers; the car's wheels turn at various speeds, but the car does not move.

I'll think about it a bit more to see if I've missed anything obvious.

:)

edited because I cannot spell

[deice]

Ofcourse not. Airspeed will be zero and that's what counts.

[EGBKFLYER]

AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAGGGGGGGGGGGHHHHHHHHHHHHHHH!

Is there no forum on PPRuNe where this thread will not appear?:{ :{ :{

[Wazzoo]

Ugh, this thread is gonna be painful to watch as it gets battled out for the umpteenth time!

Ahwell...everyones got to have their say and work it out themselves eventually! :)

[Pipe_Dream]

Calm down lads

If you've seen the question soooooooooo many times before then give us your answer.

If not, then kindly :mad: off!

[benhurr]

YES

The aircraft gets airborne - thrust is provided by the engines has nothing to do with the wheels.

The thread is Jetblast is at least the second which covers the subject.

Do a search using the word "conveyor" but the answer is still YES.

YES the aircraft gets airborne - airspeed is not zero etc etc etc etc

THE AIRCRAFT GETS AIRBORNE.

Of more interest is which company is asking this question at interview.

[Wazzoo]

Ok, ok, easy there. You would understand if you knew how many times it has come up, not only on this forum but others around the net. Not criticising you for not having seen it, just explain the exasperation!

Long and short of it is that it will take off.

Best thing you can do is not think of the aircraft as a car. With a car, the engine is linked to the wheels which are in contact with the road. The force to move the car forward is on the roads surface. Therefore if you stuck a car on a conveyor belt moving in the opposite direction, it wouldn't go anywhere.

With a aircraft, the wheels are free spinning. The aircrafts engine (be it jet/prop etc) acts against the air around it. So if you stuck a aircraft on a moving conveyor belt, the wheels will just spin. The engines will still produce a force against the surrounding air that moves the aircraft forward and thus takes off as normal.

(This negates any ideas of friction on the bearings of the wheels etc and assuming the conveyor belt doesn't move so fast the wheel bearings don't disintegrate etc. The wheels just spin as fast as the conveyor belt goes, plus the speed the aircraft is moving forward at.)

Hope that helps and saves some people some agony!