Airplane on a Conveyor Belt...

[eskimo2]

Originally posted by WWhiskey
Again you are working on a false premiss!(spell check)

In order to have a test you must first find a base in reality, the second question does not have a basis in reality therefore cannot be tested!
 the 2nd question was not posed by an expert in the feild but was an adaptation of the original!  he has not come back in this thread and said otherwise that i have seen! if he does then i will stand corrected.
theory can be debated all day long but the facts can only be found in reality!

 The v-22 will fly in either test, it exists in this world but your magic belt does not! i would like to see your explanation if you think it wont!

It is found in reality.  Did you watch the movie?  How do you explain what happened in the movie?

Like I said, both questions are theoretical, but can be proven on a smaller scale.

Look at the simpler version of the question:

“A plane is standing on a runway that can move like a giant conveyor belt. The plane applies full forward power and attempts to take off. This conveyor has a control system that tracks the plane's speed and tunes the speed of the conveyor to be exactly the same but in the opposite direction, similar to a treadmill.

The question is:

Will the plane take off or not?”

What’s the answer to this one (wheel removed)?

[VERTEX]

Originally posted by eskimo2
Please explain this then:


http://hallbuzz.com/movies/wheel_on_sander_250th.AVI


http://hallbuzz.com/movies/paper_treadmill.AVI

From what you are saying, the wheel should remain stationary as the sander accelerates.  At a constant speed, the speed of the wheels does not consume/absorb energy.  Acceleration is a different matter.

I'm not sure what you think those vids prove, or how they relate to the airplane on conveyor question.

ask me a question and I will try to answer it.

[hitech]

Vertex: The video shows how acceleration of the belt and hence acceleration of the rotating wheel, generates a force on the plane. The belt sander shows a plane setup with the prop being replaced by a spring.

In the 3 different piece film shows how a larger moment generates a larger force.


So the basic question is , do you agree that a constant acceleration of the belt can generate a force holding an airplane back.

If you do not agree, then please explain why the wheel moves when the sander is being accelerated, and then the wheel moves back when at a steady state speed.

[VERTEX]

Two factors make the wheel move back as the sander belt accelerates.

1 - friction between the axle and wheel

2 - inertia of the wheel


It takes more energy to overcome friction when an object is at rest than when it is in motion

the mass of the wheel resists acceleration as it spins up.

Once the rest friction is overcome, and the wheel has spun up to match the sander, the distance the wheel has been defected returns to zero.

The amount of wheel deflection that occurs can be reduced by,

1 - placing a large mass on the axle

2 - reducing the friction between the axle and the wheel,

3 - reducing the mass of the wheel

4 - reducing the rate of acceleration of the conveyor/belt sander

an airplane is a large mass, relative to its wheels
airplane wheels have pretty good bearings.
the airplane accelerates slowly as compared to the belt sander.


The airplane flies.

[john9001]

Originally posted by hitech
Vertex: The video shows how acceleration of the belt and hence acceleration of the rotating wheel, generates a force on the plane. The belt sander shows a plane setup with the prop being replaced by a spring.

 

the plane needs a stronger spring, more thrust.

to replace the prop with a spring  you cannot start with the spring in a de-tensioned state, to simulate the thrust you need to stretch the spring out to max and then turn on the belt and let the plane go.

the belt cannot move until the plane moves. And the belt will not equal the pull of the spring.

[VERTEX]

To specifically answer your question, a constant acceleration will not hold the airplane back because the forces acting on deflecting the wheel are smaller during a constant acceleration, than during the initial acceleration from rest.

In practice, the force the conveyor belt would impart by constantly accelerating to match the planes forward motion would be too small to be a factor large enough to keep the plane grounded.

Unless the airplane had really crappy bearings, or hugely massive wheels, both of which would keep the plane grounded without a conveyor.

[VERTEX]

john is on the right track.

He points out some non parallels between the sander experiment and the airplane on the conveyor experiment.

[hitech]

Originally posted by VERTEX
To specifically answer your question, a constant acceleration will not hold the airplane back because the forces acting on deflecting the wheel are smaller during a constant acceleration, than during the initial acceleration from rest.

Vertex: We can choose the Acceleration. The force will vary linearly with the Acceleration. So if we double the Acceleration we double the force.

Hence with enough acceleration we equal the force of the Propeller.

I did real calculations on My RV8. My plane would require a belt acceleration of 910 Feet per sec per sec to hold the plane.

HiTech

[VERTEX]

you realize thats 620 mph right.

(910 x 60 x 60) / 5280

Any way, do you disagree with anything i said?

[2bighorn]

Originally posted by hitech
So the basic question is , do you agree that a constant acceleration of the belt can generate a force holding an airplane back.

Your usable acceleration rate, or better said torque exercised on the wheel by belt, can be no greater than traction (friction) allows it, otherwise slippage occurs.
By your own calculation you got the rate of about 910 ft per s^ which is equivalent of 0.17 mps^ or after only one second of acceleration, you hit velocity of 620 mph. (you'd reach speed of light in just over 1,081,639 seconds)
No aircraft tire in this world has that amount of traction.

Without doing any calculation I'd say tires could hold up to no more than 100 fps^ of constant acceleration before considerable slippage occurs, hence most of your torque would go into the air (heat).

[VERTEX]

In order to hold you plane back the belt would have to accelerate at 620mph/sec/sec

That means 620 mph in the first second, 1240 in the second second, 1860mph in the 3rd second, 2480mph in the 4th second etc etc.

One must conclude that a high rate of acceleration is required to hold back your airplane.

Have you worked out what the actual speed of the conveyor would be for your airplane at take off airspeed?

I'll bet the conveyor does impart a force to increase the take off roll of you airplane but it would be small to insignificant.

[hitech]

Yes Vertex it is 610 MPHPS, but it does not change the fact that with that acceleration the plane stands still.

And yes I disagree with your statement about the plane takes off, the question is generic , we can set our numbers as we wish.

Your # items I agree with.

[john9001]

Originally posted by hitech
I did real calculations on My RV8. My plane would require a belt acceleration of 910 Feet per sec per sec to hold the plane.

HiTech

910 ft /sec is about 680 mph, i wonder what the boundary layer air flow over the belt ( and wing) would be?

[VERTEX]

1 - If we put your airplane with you in it on the conveyor as specified in the original question will you be able to get to rotation speed and fly away?

2 - You are correct in theory that a conveyor could be set up to accelerate at a rate that would prevent take off, but you could not overcome the practical engineering challenges to make it real.

3 - When I say the plane flies I am referring to the original question.

[VERTEX]

Let me ask another question, what do you think would happen if the bearings were frictionless, and the wheels had zero mass?