Wee brain teaser

[pedrobedro]

Turn the wheels backward while flying along to slow down for landing as well. ;-)

The wheels don't drive so the conveyor belt is not going to make any difference, the plane will still go forwards and run off it.

 

[Cobbler]

The plane would travel forward at normal speed minus the slight resistance of the tyres & wheel bearings turning at twice the air speed. The jets power the plane forward irrespective of what the wheels are doing, so the plane would take off in the normal way.

 

[66-V8]

The plane would take off vertically, like a helicopter, in fact if the belt went fast enough the plane would fly backwards, back to the airport from which it took off.

 

[The_Yellow_Ardvark]

Yes it can. Mythbuster proved it.

 

[atomant48]

Seriously people?

No, I mean, really?

The wheel bearings provide negligible drag compared to the few hundred tonnes of the plane, and the wheels aren't driven anyway.

Take the wheels off, the plane will still slide along the floor when the engines are fired up.

In fact, the conveyer wouldn't turn because the wheels only move as a product of shoving the plane along the runway...

But this is the point. The original guy who asked the question mentions the conveyor turning at exactly the same speed as the wheels. So as soon as the plane started to move the conveyor presumably starts up and the plane remains stationary. You then end up in a delicate balance where the thrust can only be just enough to overcome the rolling friction otherwise the plane will move forward (relative to a stationary observer).

 

[bigegg]

The question doesnt say the engines are running.

 

[rtbcomp]

.... So as soon as the plane started to move the conveyor presumably starts up and the plane remains stationary. ...

If the wheels were driven by an engine it would , but the force on the plane that makes it move is due to thrust of the engines, this acts on the mass of the plane and reacts against the atmosphere, not the runway. As long as the wheels were free to rotate what happens on the ground/belt is irrelevant.

If the handbrake was applied :-

1) If the thrust from the engines was greater than the force of the belt motor the aircraft would move forward but more slowly
2) If the thrust from the engines was less than the force of the belt motor the aircraft would move backwards

 

[WorkshopChris]

All we can accurately say is, the question does not provide enough data for us to attempt to accurately answer the theoretical question.

 

[atomant48]

If the wheels were driven by an engine it would , but the force on the plane that makes it move is due to thrust of the engines, this acts on the mass of the plane and reacts against the atmosphere, not the runway. As long as the wheels were free to rotate what happens on the ground/belt is irrelevant.

Mmmm.

Let's agree it's a stupid question.

 

[rtbcomp]

All we can accurately say is, the question does not provide enough data for us to attempt to accurately answer the theoretical question.

The question says "can the plane take off", not "will the plane take off" Yes it can but it won't unless you start the engines.

 

[wookie]

The plane would take off vertically, like a helicopter, in fact if the belt went fast enough the plane would fly backwards, back to the airport from which it took off.

I really really really hope you are joking, because if not you must have blonde hair and have to sit down to pee.

If you were on a gym treadmill running at the same speed the belt is travelling in the opposite direction do you move?
No air flow over wings, no lift, no take off.

If, however the wind was blowing at around 250 knotts (unlikely!) from the front of the air craft - we have lift off.

The difference is you are propelled by your legs, the plane is not propelled by it's wheels but it's engines acting on the air.

Steam launch - its done on aircraft carriers(though engines still on full chat)...though i think the energy required to haul a 747 into the air......

They also usually sail into the wind at maximum speed to increase the airflow over the plane wings as it launches to increase the available lift.

The problem is constrained by the question itself. Because they have put a constraint that the treadmill always moves at the same speed as the wheels, they are constraining the maximum power that can be used. So the engines will likely never get above idle no matter how fast the treadmill runs, and it cannot takeoff with such low power set.

The engines can run at maximum power even with the plane stationary, that's how they usually take off, they just hold it on the brakes, run the engines up to max power and then release the brakes, Jet engines take a while to spool up to maximum power and you don't want to be trundling down the runway wasting the limited length while the engines spool up.

 

[bump]

Danger Science Content