Something to think about

[mgh 89]

I'm afraid you are missing the key point, which is that all that matters is the net force on the aircraft. What the treadmill is doing is irelevant except as far as the force it imposes on the aircraft.Initially there can only be two forces. - the thrust and the rolling resistance at the wheels. (Obviously as the aircraft gains speed there will be an aerodynamic resistance.) Only if these were equal would the aircraft not move. However, as the thrust is much greater than the rolling resistance the aircraft must accelerate and gain speed. Consider the following situation where the aircraft is moving forward at a speed V and and the belt is moving backwards at the same speed, both relative to the ground. If the net thrust T (thrust minus aerodynamic drag) is greater than the rolling resistance R the aircraft will accelerate and continue to increase speed.http://forums.avsim.net/user_files/135983.jpgFor the aircraft to remain still it's necessary to prove that R will always be exactly equal to T. But it obviouly isn't. T depends on the thrust setting and speed, R only depends on speed. There's no physical mechanism for R to change in response to a change in T.

[kerosene31 349]

>For the aircraft to remain still it's necessary to prove that>R will always be exactly equal to T. But it obviouly isn't. T>depends on the thrust setting and speed, R only depends on>speed. There's no physical mechanism for R to change in>response to a change in T.>Are you saying the plane would fly or no? Or are you just saying that the situation would not be possible? The only force that matters on the aircraft is lift, and for an airplane to generate lift, air needs to flow over the wings.

[mgh 89]

Please explain how the treadmill generates a force that is exactly equal to the thrust?We are not making it too complicated, we're trying to explain it. The important variable is ground speed, which in still air is also the airspeed.

[mgh 89]

I am saying the arircfat will fly.It will have a speed relative to the ground and hence the air (assumed still) and so will generate lift.

[kerosene31 349]

>Please explain how the treadmill generates a force that is>exactly equal to the thrust?>>We are not making it too complicated, we're trying to explain>it. The important variable is ground speed, which in still air>is also the airspeed.It probably couldn't, it is just a hypothetical example. :)Still, groundspeed is not important to the original question which is lift. Airspeed generates lift. Point a Cessna into a 200 mph headwind, and the plane will leave the ground (much to the dismay of the owners). Land a Cessna 152 into a strong tailwind, and the experienced pilot will not care about groundspeed, but airspeed.

[Guest Adverse Yawn]

>>Land a Cessna 152 into a strong tailwind, and the experienced pilot >>will not care about groundspeed, but airspeed. Wrong. A digression from the problem at hand and Mgh's correct assertions, but I suspect the root of the misunderstanding. Here's a couple of questions for you (answers way down below):Q1 What is the relationship between Ground Speed and Air Speed with a tailwind? Q2 What are the three main things about this that would concern a pilot?A1 The Ground Speed will be higher than the Air Speed.A2 Rates tyre speed, brake capability and the ability to stop before the end of the runway.

[michal 44]

>The treadmill moving backward CAN and DOES do SOMETHING to>impede the aircraft

[kerosene31 349]

>Wrong. A digression from the problem at hand and Mgh's correctYes, I was oversimplifying the issue. I was just looking at lift and stalling. Of course, a high groundspeed would concern the pilot on the runway. I should have clarified that. Getting back to the original point, will a plane fly or no? That is all I am saying, that this is only determined by airspeed.

[Guest Adverse Yawn]

>>Getting back to the original point, will a plane fly or no? YES!! :)>>That is all I am saying, that this is only determined by airspeed. Yes, so what is the force that is equal and opposite to propeller thrust that stops the aircraft from gaining airspeed (and of course ground speed)?

[mgh 89]

Yes it will fly as explained. The original question didn't refer to wind speed. It still air ground speed and air speed are the same.

[kerosene31 349]

Ok, lets take the plane off the treadmill and put a racecar instead. Same situation, race car is driving fast, and the treadmill is magically keeping pace. The car has lots of speed relative to the treadmill. Will the race car fly? No, of course not. Why? What makes the car different from the plane? Wings. What makes the wings fly? Lift. What makes lift? Air flowing over the wings. In the treadmill example, air is not moving over the wings, unless you are talking about a prop plane with props on the wings which pushes air over the wings while the engine is running. Even this factor shouldn't be enough to generate lift, otherwise prop planes would be able to take off straight up.

[kerosene31 349]

>I am saying the arircfat will fly.>>It will have a speed relative to the ground and hence the air>(assumed still) and so will generate lift.What ground? Sure the groundspeed on the plane will read something, as the wheels are spinning, but the plane is not moving relative to the atmosphere or the "Earth" ground. The treadmill does not have its own atmosphere moving the air.

[Guest oyvindhansen]

You obviously have no idea about how a plane flies.-

[kerosene31 349]

>You obviously have no idea about how a plane flies.>>-

[Guest oyvindhansen]

For one you fail to identify that the main difference between a car and a plane in our example is not wings, but the fact that the plane does not have wheel drive and it is therefore indifferent about the speed of the threadmill. That is why a car will stand still while a plane will accelerate.-