Why the plane takes off in laymans (my) terms

Originally posted by likeforest
suqiezd please give up you are only making matters worse. Marsan i like your analogy and was hoping it would finnaly put an end to this question. Goodluck everyone!

In your dreams.

Originally posted by Marsan
My point was that the airstream the propeller pulls over the wings gives an inconsequential amount of lift.

The only job a propeller has is to pull the plane forward, as has been phrased, rephrased and paraphrased many times in this and other threads.

Propellers pull plane forward

Forward motion of plane makes air move over wing

Air moving over wi ...[text shortened]... ng and betrays a fundamental misunderstanding of why flying works.

The plane will take off.

The airstream from the prop is not sufficient to lift the plane - it passes down the side of the fuselage.

Originally posted by Mat Kelley
OK I have read all three posts and have swung with whom I agree. The reason for my confusion is in understanding the original question and the concepts of flying.

My initial understanding of the question was that if you were to place a flag beside the aeroplane and the "treadmill" runway was turning and the engines work just enough to keep the flag in ...[text shortened]... ly believe the answer to be yes - and for my realistion I have Marsan to thank - good anology.

How do you arrive at the conclusion that the plane is not stationary relative to the ground?

If the surface on which it is moving is moving itself in the opposite direction at the same speed- net progress is zero.

Originally posted by Marsan
Moment of inertia refers to object which are being made to rotate you are using this term out of context.

Yes i know the wheels rotate but that is completely irrelevant, they are their own little closed system, a horizontal force cannot be transmitted from the conveyor belt to the plane via the wheels because they spin freely. They decouple the plane from ...[text shortened]... ect the motion of the car? It is exactly the same as with the plane.

Surely you see it now.

I doubt that you have actually tried that!

Originally posted by sugiezd
How do you arrive at the conclusion that the plane is not stationary relative to the ground?

If the surface on which it is moving is moving itself in the opposite direction at the same speed- net progress is zero.

Because he understands what's happening?

The propeller will provide a forward thrust. The belt will not be able to counteract this thrust sufficiently (for the reasons explained several times in the threads). So the plane will accelerate forwards,

So even if it starts with zero velocity with respect to the ground, this will not continue. The plane will move forwards, getting faster. Eventually, it takes off.

Originally posted by mtthw
Because he understands what's happening?

The propeller will provide a forward thrust. The belt will not be able to counteract this thrust sufficiently (for the reasons explained several times in the threads). So the plane will accelerate forwards,

So even if it starts with zero velocity with respect to the ground, this will not continue. The plane will move forwards, getting faster. Eventually, it takes off.

All I've seen is a lot of erroneous logic.

Lets try looking at it another way.

Both are stationary and a marker is placed in the ground opposite the plane.

The belt is set to move at 5 kph and the plane goes with it (wheels locked).

Start engine and release brakes run to an indicated ground-speed of 5kph.

You're effectively telling me that the plane will come back to the marker and keep going.

In my opinion, it will come to equilibrium with the belt and remain stationary with repect to the marker downstream from its starting point.

Originally posted by sugiezd
All I've seen is a lot of erroneous logic.

Lets try looking at it another way.

Both are stationary and a marker is placed in the ground opposite the plane.

The belt is set to move at 5 kph and the plane goes with it (wheels locked).

Start engine and release brakes run to an indicated ground-speed of 5kph.

At this point the plane will have zero ground-speed.

But at this point the propeller is still producing thrust (unless you turn it off), but the belt is not producing a significant force. So the plane will continue to accelerate. A few seconds later it will have positive ground-speed.

Originally posted by mtthw
At this point the plane will have zero ground-speed.

But at this point the propeller is still producing thrust (unless you turn it off), but the belt is not producing a significant force. So the plane will continue to accelerate. A few seconds later it will have positive ground-speed.

If the belt speed increases to match, it will not.

The planes ground-speed is its speed relative to the ground (pretty obvious) but here, the ground is the belt - if the belt speed is the same as the plane's ground speed then the plane CANNOT move forward.

Originally posted by sugiezd
All I've seen is a lot of erroneous logic.

Lets try looking at it another way.

Both are stationary and a marker is placed in the ground opposite the plane.

The belt is set to move at 5 kph and the plane goes with it (wheels locked).

Start engine and release brakes run to an indicated ground-speed of 5kph.

You're effectively telling me that the ...[text shortened]... h the belt and remain stationary with repect to the marker downstream from its starting point.

Why would it come to equilibrium? Are you suggesting that the plane engine is applying power to the wheels to move the plane forward?

Originally posted by mwmiller
Why would it come to equilibrium? Are you suggesting that the plane engine is applying power to the wheels to move the plane forward?

No.

Originally posted by sugiezd
No.

In your example, why did you start with the wheels locked? Would it make any difference? I think the plane would initially move with the belt regardless of whether the wheels are locked or not.

(added) This is before the plane engine is started, as in your example.

Originally posted by sugiezd
If the belt speed increases to match, it will not.

The planes ground-speed is its speed relative to the ground (pretty obvious) but here, the ground is the belt - if the belt speed is the same as the plane's ground speed then the plane CANNOT move forward.

The belt can't prevent it! It can't apply enough force. You could keep accelerating the belt in order to keep the belt moving at the same speed as the plane if you wanted, but it still needs to apply a force equal to the thrust in order to counteract the thrust. And it can't.

Try this one. Put a plane on a stationary belt, with the brakes off. Assume for simplicity that there is zero friction in the wheels. Now, turn the belt on.

- Will the plane start moving along with the belt, or will it stay where is was (with respect to everything else)?

- If the former, where does the force come from to move it? Can't be friction.

So the plane doesn't move. It's like pulling a tablecloth out from under a bunch of plates. Moving belt, but zero airspeed for the plane. Now, turn the engines on. It has to move forwards!

Originally posted by sugiezd
How do you arrive at the conclusion that the plane is not stationary relative to the ground?

If the surface on which it is moving is moving itself in the opposite direction at the same speed- net progress is zero.

The plane is NOT moving on the surface its wheels are in contact with. Planes DO NOT move along surfaces they move through the air. How many more times does this need to be explained?

Originally posted by mwmiller
In your example, why did you start with the wheels locked? Would it make any difference? I think the plane would initially move with the belt regardless of whether the wheels are locked or not.

No real difference except that without a lock the plane would lurch forward in relation to the belt as it's moment of inertia was overcome.

Originally posted by Ian68
The plane is NOT moving on the surface its wheels are in contact with. Planes DO NOT move along surfaces they move through the air. How many more times does this need to be explained?

Of course it moves along the surface, through the interface of its wheels.

OK, the wheels are not powered - ok, they have low friction bearings but they do exist.

How do you measure ground-speed when the plane is on the ground?