Author Topic: plane on a conveyor belt? (Read 19894 times)

lukster · « **Reply #405 on:** January 26, 2007, 07:15:13 PM »

Eventually we will have an unstoppable force vs an unmovable object. Which is greater?

2bighorn · « **Reply #406 on:** January 26, 2007, 07:22:14 PM »

Quote

Originally posted by lukster
You do the math and see if your engine can overcome the coefficient of friction of your skidding tires. If it can I just increase the acceleration to reduce your tires to smoke in a microsecond. Then you have to figure the drag of the gear. I already addressed this.

I did the math.

You assume that wheels will blow due to speed but not the conveyor. For fairness assume that if conveyor is indestructible so are the wheels.

To eskimo.
Repeat your experiment with little wheel cart or car model instead of wheel alone. Put some weight on it so that ratio between wheels weight and total weight of your cart will match that of airplane/wheels ratio.
Since your model won't have sophisticated axle/bearings and sander has sand paper on it, your Crr (Coefficient of rolling friction) will be few times bigger. Even so, your rubber band will be stretched considerably less than if you're using wheel alone.

lukster · « **Reply #407 on:** January 26, 2007, 07:23:41 PM »

Quote

Originally posted by 2bighorn
I did the math.

You assume that wheels will blow due to speed but not the conveyor. For fairness assume that if conveyor is indestructible so are the wheels.

To eskimo.
Repeat your experiment with little wheel cart or car model instead of wheel alone. Put some weight on it so that ratio between wheels weight and total weight of your cart will match that of airplane/wheels ratio.
Since your model won't have sophisticated axle/bearings and sander has sand paper on it, your Crr (Coefficient of rolling friction) will be few times bigger. Even so, your rubber band will be stretched considerably less than if you're using wheel alone.

Very good. We have an indestructable belt and wheels. Give me a coefficient of friction and I'll give you an acceleration.

As I said many posts ago, this is calcuable. Given the weight of the plane, coefficient of friction, and thrust of the plane we can determine what acceleration of the belt is required to counter the planes max thrust.

eskimo2 · « **Reply #408 on:** January 26, 2007, 07:37:15 PM »

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Originally posted by Golfer
Eskimo in all of your experiments the conveyor is initiating the action onto the wheel. In the question its the wheel that initiates the conveyor.

If your points regarding this are valid I have just one more question...

Why can't you dyno an airplane?

That right there blows any argument could make right out of the water. You dyno an airplane in a still-air environment then I might listen to what you have to say. Other than that...can't be done.

get an airplane to dyno on one of these...your arguments are all valid. You won't...

That's like saying light bulbs don't generate light because they can't generate electricity.
Think about it, in the plane on the conveyor the wheel’s absorb and gain energy from the belt, they are not driving it.

eskimo2 · « **Reply #409 on:** January 26, 2007, 07:43:58 PM »

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Originally posted by SteveBailey
No, i'm saying that the wheels can only begin moving by the plane rolling forward. If the hweels of the plane stop moving forward, as you said they would,(you said the plane would stop) then since the wheels were no longer acceolerating, the conveyer would catch up. If the wheels are no longer moving forward, and the conveyer does not spin the wheels, but merely paces it, the wheels will slow down and stop since there is nothing causing the wheels to spin. Again, in youir model, the plane stops, the wheels stop, the conveyer stops yet the plane goes nowhere.

No,
As long as the plane’s engine is generating thrust the conveyor and wheels must be accelerating! Even at a power setting that would normally make the plane taxi slowly, the conveyor and wheels must be accelerate by hundreds or maybe thousands of rpm per second! The rpm must always be increasing until the pilot shuts off the motor or the plane runs out of gas.

2bighorn · « **Reply #410 on:** January 26, 2007, 07:51:57 PM »

Quote

Originally posted by lukster
Very good. We have an indestructable belt and wheels. Give me a coefficient of friction and I'll give you an acceleration.

As I said many posts ago, this is calcuable. Given the weight of the plane, coefficient of friction, and thrust of the plane we can determine what acceleration of the belt is required to counter the planes max thrust.

That's the point, you can exercise force no greater than rolling resistance force and given the indestructible gear, rolling resistance is constant no matter how fast would you accelerate the conveyor.

Let's see how F-14 would do
Crr = 0.01 (that's very generous)
Loaded weight = 28,000kg
Dry thrust = 2 x 72,000N
F-14 rolling resistance = ((28000 X 9.81) / 4) X 0.01 = 6860N

as you see the thrust force is 21 times greater than rolling resistance force

SteveBailey · « **Reply #411 on:** January 26, 2007, 07:56:08 PM »

Quote

Originally posted by eskimo2
No,
As long as the plane’s engine is generating thrust the conveyor and wheels must be accelerating! Even at a power setting that would normally make the plane taxi slowly, the conveyor and wheels must be accelerate by hundreds or maybe thousands of rpm per second! The rpm must always be increasing until the pilot shuts off the motor or the plane runs out of gas.

Why must they be accelerating? The conveyer is designed to match the speed of the wheels. It does not accelerate the wheels. the wheels have no thrust on their own. They can only spin when the plane moves forward. In your model the plane doesn't move. yet we know that the wheels can only start to spin if the plane moves. How do you explain this?

The conveyer is not designed to offset the thrust of the motor, but only to offset the speed of the wheels. Tell me again how the conveyer went from matching the speed of the wheels to causing the wheels to spin faster?

quig · « **Reply #412 on:** January 26, 2007, 07:57:51 PM »

We have 11 pages, an email from a Prof. and a movie.

So will the damn plane take off or not?

WhiteHawk · « **Reply #413 on:** January 26, 2007, 08:04:34 PM »

So its settled, the plane will take off, if the wheels and conveyor dont fail? Or is there more?

SteveBailey · « **Reply #414 on:** January 26, 2007, 08:06:57 PM »

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Originally posted by WhiteHawk
So its settled, the plane will take off, Or is there more?

Nope.

eskimo2 · « **Reply #415 on:** January 26, 2007, 08:11:01 PM »

Quote

Originally posted by 2bighorn
True, what you don't realize is that maximum amount of force you can transfer from conveyor to the airframe is equal that of rolling resistance which is defined as following:

Rolling resistance = ((Aircraft weight X gravitational acceleration) / number of wheels) X coefficient of rolling friction

Coefficient of rolling friction used in aircraft industry is about 0.0035 in average.

Now do some math and you'll see that amount of your usable force is nothing in comparison to the thrust.

2bighorn,

You’re not recognizing the real force at work. This is not about friction, it’s about rotational inertia. Watch the video; see the force.

Go jack up your car, grab a wheel and spin it; now reach out and try to stop it. Why couldn’t you start it spinning or stop it from spinning instantly? It took force to do both.

eskimo2 · « **Reply #416 on:** January 26, 2007, 08:21:24 PM »

Quote

Originally posted by 2bighorn
I did the math.

You assume that wheels will blow due to speed but not the conveyor. For fairness assume that if conveyor is indestructible so are the wheels.

To eskimo.
Repeat your experiment with little wheel cart or car model instead of wheel alone. Put some weight on it so that ratio between wheels weight and total weight of your cart will match that of airplane/wheels ratio.
Since your model won't have sophisticated axle/bearings and sander has sand paper on it, your Crr (Coefficient of rolling friction) will be few times bigger. Even so, your rubber band will be stretched considerably less than if you're using wheel alone.

Did you see the cylinder move rapidly in the last video? If it had a cart hooked up to it that weighed the same as the cylinder, it would have accelerated at half the rate, right? But what if the shoe didn’t drop at 1 Gee? What if the paper conveyor was accelerated at 2 Gees? It would be pulled to the left at the same rate as the cylinder accelerated by the 1 Gee conveyor. Numbers still too small for you? Fine, hook a cart up to it that weighs 99 times as much, accelerate the conveyor at 100 Gees and the 100x cart would be pulled to the left at the same rate as the cylinder accelerated by the 1 Gee conveyor. See a pattern here?

eskimo2 · « **Reply #417 on:** January 26, 2007, 08:30:45 PM »

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Originally posted by WhiteHawk
So its settled, the plane will take off, if the wheels and conveyor dont fail? Or is there more?

Depends on the question:

If the question were that the conveyor were matching the plane’s speed in the opposite direction, the conveyor would be traveling at the rate of 50 mph at take off and so would the plane. The plane would take off normally except its wheels would be rolling at twice their normal take off rpm. That consumes a tiny bit of energy that really can’t be felt by the pilot.

Conveyor matches the plane’s speed: plane will fly!

Our question, however, states that the conveyor must match the plane’s wheel’s speed.
In this case if the plane moves forward, its wheel has gone further, and therefore faster than the conveyor. The only thing that the conveyor can do about this is to speed up like mad!

Conveyor matches the plane’s wheel speed: plane will NOT fly!

eskimo2 · « **Reply #418 on:** January 26, 2007, 08:33:27 PM »

Quote

Originally posted by SteveBailey
Why must they be accelerating? The conveyer is designed to match the speed of the wheels. It does not accelerate the wheels. the wheels have no thrust on their own. They can only spin when the plane moves forward. In your model the plane doesn't move. yet we know that the wheels can only start to spin if the plane moves. How do you explain this?

The conveyer is not designed to offset the thrust of the motor, but only to offset the speed of the wheels. Tell me again how the conveyer went from matching the speed of the wheels to causing the wheels to spin faster?

What speed wiil match the speed of the wheels?

2bighorn · « **Reply #419 on:** January 26, 2007, 08:34:05 PM »

Quote

Originally posted by eskimo2
You’re not recognizing the real force at work. This is not about friction, it’s about rotational inertia. Watch the video; see the force.

For Christ's sake Eskimo, you're a teacher. Try to include some deductive reasoning in here. For once try to understand that there's a linkage between the wheels and plane which can transfer only limited amount of force.
No matter the force applied to the wheel, there's a limited amount transfered to the plane and that amount can not be greater then force of rolling resistance. In our case rolling resistance of ball bearings. Ball bearings are your force transfer bridge and it can transfer finite amount of force only. This amount can be easily calculated.

In your wheel movie you're transferring force from the sander to the wheel but everything else is missing. If you want your experiment to be valid, you need to TWO force bridges. One between belt and wheel and one between wheel and the airframe.