YakYak

This appears to be a popular subject for discussion at the moment.

In brief, an aeroplane is put on a magical conveyor belt that automatically matches the plane's speed and keeps it stationary. The question is, can the aeroplane take off?

To which, as far as I can see, the answer is a resounding no, probably followed by a "What kind of stupid fucking question is that? Of course it won't take off. The plane's stationary so there's no airflow over the wings so there's no lift." And then maybe some Joey Deacon noises just to ram the point home.

And yet people are arguing quite vehemently that of course it'll take off, you idiots.

Am I missing something?

I haven't read the details but based on your description, yes it'll take off.

Aircraft are pushed forwards by thrust from the engines, not the wheels. If the plane is sitting on a rolling road, it doesn't matter how fast that goes, all it will succeed in doing is making the wheels turn faster - the plane will still move forward at roughly the same speed, so will still have lift etc.

So it's a trick question really, feel free to make "Joey" noises at the questioner.

What if it was a Harrier?

But the Plane is sat still (relative to the earth) on the Treadmill and thus no air flowing over the wings to produce lift, so I'm going with no it won't take off.

kalmar wrote:Aircraft are pushed forwards by thrust from the engines, not the wheels. If the plane is sitting on a rolling road, it doesn't matter how fast that goes, all it will succeed in doing is making the wheels turn faster - the plane will still move forward at roughly the same speed, so will still have lift etc.

If the wheel bearing were perfect, the speed of the rolling road wouldn't matter at all. If there is some friction between the wheels and the body of the plane, as you said - the plane will move forward at "roughly" the same speed.

Does the magical conveyer belt perfectly match the aircraft's landspeed or airspeed?

The question is flawed. Mnnng.

Ely wrote:But the Plane is sat still (relative to the earth) on the Treadmill and thus no air flowing over the wings to produce lift, so I'm going with no it won't take off.

Nope, that's the trick part of the question. They don't say that the plane is sat still, only that the conveyor belt adjusts its speed to the speed of the wheels.

*adjusts glasses*

I think the point is that the engines do not drive the wheels, thus the treadmill is redundant. The engines would still move the plane forward through the air and thus there is flow over the wings and thus take off.

If it wasn't a big treadmill but rather some strong ropes that kept the plan stationary then it would not take off.

I think the question is fundamentally flawed. As kalmar says, the plane would take off unless the conveyor belt magically fixed the plane to the spot. The question *wants* you to believe that the plane will remain stationary, and we all make the common connection to cars where of course the power is delivered through the wheels. However, given that the wheels turn freely, the thrust of the engines will push the plane forward, and this movement will cause it to take off.

If the magical conveyor belt keeps the plane exactly stationary then no take off.

In short: need more details

Cheers

Bubba

Ely wrote:But the Plane is sat still (relative to the earth) on the Treadmill and thus no air flowing over the wings to produce lift, so I'm going with no it won't take off.

Yes, but there's no way that the treadmill could make it "stand still" by opposing the direction of travel - if the plane is trying to propel itself in the normal way, it will do so regardless.

It's a stupid question posed by someone with a stone-age understanding of physics and aerodynamics (or who's just trying to confuse and wind people up)

fred_zen wrote:

kalmar wrote:Aircraft are pushed forwards by thrust from the engines, not the wheels. If the plane is sitting on a rolling road, it doesn't matter how fast that goes, all it will succeed in doing is making the wheels turn faster - the plane will still move forward at roughly the same speed, so will still have lift etc.

If the wheel bearing were perfect, the speed of the rolling road wouldn't matter at all. If there is some friction between the wheels and the body of the plane, as you said - the plane will move forward at "roughly" the same speed.

Yes, there would be some rolling resistance, probably insignificant. Especially since the plane would have moved forwards off the treadmill in a the first few seconds.

Jedburgh wrote:

keeps it stationary.

That's the important bit. If that works it can't take off. However, for the rest I agree with kalmar, it would take more than a conveyer belt under the wheels to stop a plane moving. Though that would produce some extra friction in the wheels, and might well make things unsafe. Blocks under the wheels might stop a plane (they used to use them, up to WW2 I think, called "chocks", but I suspect a jet would roll up over them), and taking the wheels off might produce enough friction to prevent it reaching takeoff speed.

Well, this thread really backfired, eh?

kalmar wrote:

fred_zen wrote:

kalmar wrote:Aircraft are pushed forwards by thrust from the engines, not the wheels. If the plane is sitting on a rolling road, it doesn't matter how fast that goes, all it will succeed in doing is making the wheels turn faster - the plane will still move forward at roughly the same speed, so will still have lift etc.

If the wheel bearing were perfect, the speed of the rolling road wouldn't matter at all. If there is some friction between the wheels and the body of the plane, as you said - the plane will move forward at "roughly" the same speed.

Yes, there would be some rolling resistance, probably insignificant. Especially since the plane would have moved forwards off the treadmill in a the first few seconds.

But it's a 'magical' treadmill, so I think we can assume it will perfectly match the plane's wheel rotation at every stage of the process?