No. Due to the severe lack of wind speed, (as opposed to treadmill speed). Therefor ... no lift!

Having left school much longer ago and never taken A levels my physics memory is virtually nil.

But I'm not saying the aircraft is going to lift off vertically it requires air speed not ground speed.

If you took a remote controlled car and placed it on a treadmill and set the treadmill so the car remained still when you used the controller, would the car move forward if you placed you hand behind it and pushed it, provided some thrust as it were?

http://pogue.blogs.nytimes.com/2006/...ill-conundrum/

Exactly.

When taking off in a hang glider you need to have the wind hit the front of the wings at a certain airspeed and angle to provide lift. You have to face the wind and often run into it so the air passes over the wings - how fast you need to run is determined by the speed of the wind coming at you. You can't stand in a field, in still air and take-off. You have to wait for a strong wind or run like hell. Or you can run off a cliff.

If the plane is still, how can lift be achieved?

Do you have to run up the treadmill to board the plane?. If so I think I'll go with Air Canada

The thing is though, the plane isn't still (unless I'm missing something).

If the treadmill is moving backward at the same speed as the plane is moving forward, then the ground speed (relative to the treadmill) is twice the ground speed relative to the actual ground. The air speed will still be the same and the plane can fly.

Unless its some kind of magic treadmill that somehow makes the plan motionless to the actual ground.

The planes motion comes from the engines pushing against the air. Not the wheels driving it on the ground. If the treadmill somehow also made the air move backward at the same speed as the treadmill then we'd have a different situation.



I think.

Thanks Greenhill. This had got me really intrigued and I've just wasted a lunch hour browsing the web and discussing with colleagues. The post from the blog you link to that sums it up best for me is this one:

I will now stop tearing out what little hair remains to me.

I mean forward.

Think of it like a bicycle on rollers... the engines push the planes wheels round at 50mph, and the forward force of this immediately rolls the treadmill backwards at 50mph. The pilot floors it to 200mph, and the treadmill goes backwards at 200mph - the plane goes nowhere, a bit like spinning your wheels on ice. The ground speed relative to the plane is 200mph. The air speed is zero - no air is moving over the plane's wings.

No air movement = no lift.


I think!

But the problem is that you can't think of it like that.... the plane's engines aren't pushing its wheels round, they're pushing it forward through the air, never mind whether it's on wheels, skids, or a whole runway full of spilt marbles. If the conveyor starts to move backwards, even at 1,000 miles an hour, the only effect that will have is to make the plane's wheels go round really, really fast, but the plane itself will still move forward due to the thrust of the engines.

This scenario is simply not possible, even as a thought experiment, because the interaction between the wheels and the conveyor is irrelevant to how fast the plane moves forward.

I think!

I thought I agreed with that but now I'm wondering ... according to: http://www.grc.nasa.gov/WWW/K-12/airplane/lift1.html

(air = the fluid)

"NO MOTION, NO LIFT

Lift is generated by the difference in velocity between the solid object and the fluid. There must be motion between the object and the fluid: no motion, no lift. It makes no difference whether the object moves through a static fluid, or the fluid moves past a static solid object. Lift acts perpendicular to the motion. Drag acts in the direction opposed to the motion."

I'm wondering ... if the plane is essentially still (like a person walking on a treadmill would be) can the plane's engines create enough motion in 'the fluid'/air by 'sucking' the surrounding air around the wings? I just asssumed by the placement of the engines that this would not create that airfoil effect... but maybe it does? I thought that the point of engines was to push the object/the plane through the air and the air would hit the airfoil wings and create lift.

The treadmill will not keep the aircraft motionless. That is what the "no takeoff" voters are missing. Automobiles and bicycles and the like move because of friction between the tires and the road surface. If you put a car on a frictionless surface, it cannot be driven by the wheels. Aircraft do not require friction with the ground to move; place an aircraft on a frictionless runway and it will take off just fine. The motive force of an aircraft is thrust, not friction. The treadmill under the wheels is a red herring.

I'm beginning to doubt myself now too!!

This will require more thought and pencil-scratching!

Ah... you've explained that in a way that helps me make better sense of some earlier posts.

So, why are there runways then? There is obviously a need for them (I'm assuming)... what is it that runways do in the equation?

Absolutely. The scenario cannot be constructed, because it would not be possible for the conveyor to stop the aircraft from moving forward.

The only way for the conveyor belt to apply a force to the aircraft is through friction in the wheel bearings. If we first look at a simplified case, and ignore that friction in the wheel bearings, then there is no way for the belt to apply a force of any sort to the plane and thus keep it stationary. It will move forward under the thrust generated by the engines and take off with its perfect, frictionless wheels turning very very fast on a very very fast-moving conveyor.

In the real world, of course, there is friction. The engines start to deliver thrust. The only force the belt can impose on the jet is through friction. At very low speeds the friction in the wheel assembly is very much less than the thrust of the jet and it will not stop its forward progress. So the increased speed of the belt only serves to increase the rolling speed of the wheels which requires a further increase in the speed of the belt and so on ad (nearly) infinitum. The belt will continue to accelerate, always trying, but always failing, to catch the speed of the wheels (which will always be the speed of the plane plus the speed of the conveyor). Eventually the wheels will spin so fast that the friction is enough to counter the thrust of the engines or, more likely, the wheels will fail and come off the plane with a big bang. But in reality (ha!) the belt will simply not be able to keep the plane stationary relative to the surroundings.

Actually, I think the magic conveyor would be likely to fail a long time before the wheels, so the ground would suddenly become stationary again and the plane woudl be able ot take off as normal.

It's all about forces. The thrust of the engine acts on the plane. Without another force to keep it in place it will move. The only force trying to stop it moving is the friction in the wheel bearings. If the plane moves fast enough before the wheels fall to pieces it will take off.


I think....