Author Topic: plane on a conveyor belt? (Read 26202 times)

2bighorn · « **Reply #585 on:** January 30, 2007, 10:54:01 PM »

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Originally posted by lukster
You do realize that in almost every (probably all of them) plane built, bearing friction is quite capable of locking up the wheel, preventing it from rotating about the axle, right?

I was thinking that same is valid for conveyor bearings

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Originally posted by lukster
Let me anticipate your argument. The bearing is destroyed and the wheel then continues to rotate, rather roughly, in it's absence.

Before that happens, conveyor breaks into pieces

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Originally posted by lukster
My answer to that is that the wheel itself will be ground away in short order as was the bearing. As I said many posts ago you are then left with the plane dragging itself on the wheeless gear.

and you'd be fixing the damn conveyor, if I remember right, many posts ago....

takeda · « **Reply #586 on:** January 31, 2007, 04:22:17 AM »

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Originally posted by hitech

Hitech, the lower arrow in your diagram is static friction, or rolling friction? Because if it's static friction, I think it's pointing the wrong way and if it is rolling friction, aren't you missing another arrow for the static friction pointing to the right?.

Holden McGroin · « **Reply #587 on:** January 31, 2007, 04:43:35 AM »

If static friction were vectored opposite of rolling friction, static friction would be a net thrust.

Static and rolling friction are both drag forces and acting in the opposite direction of thrust. They do not occur simultaneously. As soon as rolling starts, static friction vanishes and rolling friction appears.

takeda · « **Reply #588 on:** January 31, 2007, 05:05:58 AM »

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Originally posted by Holden McGroin
If static friction were vectored opposite of rolling friction, static friction would be a net thrust.

Static and rolling friction are both drag forces and acting in the opposite direction of thrust. They do not occur simultaneously. As soon as rolling starts, static friction vanishes and rolling friction appears.

Not quite, I'm not talking about dynamic friction, which would appear if the wheel was sliding and yes, would drag the movement, but we can agree that is absent from our problem as long as the wheels dont slide.
The friction in the contact patch between wheel and belt is static friction even if the wheel and belt move as long as there isn't any sliding. And it must oppose "impending movement", i.e. the belt sliding under the wheels, so it points forward.

Rolling friction is different from dynamic friction and yes it points backwards and would be due mostly to deformation of the wheel and belt.

Holden McGroin · « **Reply #589 on:** January 31, 2007, 05:28:53 AM »

However you figure it, friction in whatever form is a drag force, a reaction to thrust. If there is no thrust, the friction reaction vector has a amplitude of zero. Any friction vector must be pointed opposite the thrust vector to which it is reacting.

This whole thread is amazing to me, as the thought that a set of rollers to counter the spin of tires could hold back a thrusting 747 is rediculous.

takeda · « **Reply #590 on:** January 31, 2007, 05:36:47 AM »

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Originally posted by Holden McGroin

This whole thread is amazing to me, as the thought that a set of rollers to counter the spin of tires could hold back a thrusting 747 is rediculous.

That I can agree with

eskimo2 · « **Reply #591 on:** January 31, 2007, 06:00:46 AM »

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Originally posted by JCLerch
This is the crux of the problem. Your question, and the original thought experiment, mix "real world" physical items with intellectual constructs. (Non-existent hardware).

For instance, You put a "real Cessna 172" on a intellectually constructed, artificially constrained, treadmill. Aka a "Super Treadmill" Then to answer your question to the finish, we have to add some additional constraints.
Are the Cessna wheel assemblies indestructible? Yes/No
Are the Cessna wheel bearings and axles indestructible? Yes/no
Is the super tread mill indestructible? yes/no
Is the super tread mill capable of continually accelerating at 910ft/sec/sec? Yes/no
shall we ignore the effects of Relativistic Quantum Field theory? Yes/no
IF you answer yes to all the above, then the airplane will not move forward (with respect to the ground the treadmill is attached to) while producing thrust. Of course, this will be an interesting experiment to an outside observer, as the surface of the treadmill will go from 0 to 700mph in about 1 second. 11 seconds later (12 seconds after opening the throttle on the airplane engine) the treadmill surface speed will be traveling at Mach 10 and still accelerating. In a little over 2 years, the linear surface speed of the super tread mill will exceed the speed light. As I said, answering YES to all the thought constraints will be very interesting to observe from a distance!

IF you decide to answer yes to everything except the last question, and include Einesten's theory of relativity, then the end result is sufficient energy in the system to be equivalent to enough mass to create a Black Hole.

IF you answer yes to everything except the last two questions, then the treadmill will eventually stop accelerating, and the airplane WILL take off.

IF you answer no to ANY of the first three questions, you will roll the Cessna up into a little metal ball.

IF we constrain the experiment to things that CAN BE FABRICATED, then I predict it is impossible to build a treadmill assembly that can accelerate at 910ft/sec/sec for anything longer than a few fractions of a second. Once the treadmill stops accelerating, the airplane will move forward relative to the ground the treadmill is attached to, eventually gain enough airspeed and takeoff. How unusual the takeoff appears depends on the rate of acceleration and the top speed the treadmill is capable of. [/B]

This is a great response!

I’d like to add something though: If we were to build one it might:
A.    Hit a top speed, allowing the plane to take off.
B.    Self destruct, which may or may not damage/destroy the plane; which may or may not take off.
C.    Conveyor hits a speed which causes pilot to enter a worm hole and talk to her dad (who is really an alien) but no one believes her because the 14 hour event took place instantly…

eskimo2 · « **Reply #592 on:** January 31, 2007, 06:05:11 AM »

Guys, the main force at work here is NOT friction! The plane is held in place by rotational acceleration. It need friction/traction to work, but that is not the only work.

Holden McGroin · « **Reply #593 on:** January 31, 2007, 06:17:32 AM »

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Originally posted by eskimo2
Golfer,

How would you answer this question (and simply, why?):

“A plane is standing on runway that can move (some sort of band conveyer). The plane moves in one direction, while the conveyer moves in the opposite direction. This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in opposite direction).
The question is:
Will the plane take off or not? Will it be able to run up and take off?”

Note: “Wheel” has been removed.

the question is flawed as posted....

Ponder this: If there is no forward speed, there is no need for the conveyor to move. 0 = 0

Now the plane begins to creep forward, but it does not because the conveyor pulls it back. But that means it hasn't moved, so 0 = 0 and therefore the conveyor is not moving.

What causes a plane to move forward? Thrust that is independant of the wheels, air cushion, skiis, or whatever else it has to keep the paint from being scratched on the tarmac. Thrust is independant of the landing gear.

Net thrust causes acceleration, regardless of whatever rotational acceleration does to the wheel.....

End this thread now!

hitech · « **Reply #594 on:** January 31, 2007, 09:04:44 AM »

takeda: If we lock the brakes which way does the friction vector point?

I think you might be viewing the friction force relative to the belt vs relative to the wheel. But when you do a drawing of body forces you make them all relative to the body.

And my arrow is static friction.

HiTech

hitech · « **Reply #595 on:** January 31, 2007, 09:11:26 AM »

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Originally posted by eskimo2
Guys, the main force at work here is NOT friction! The plane is held in place by rotational acceleration. It need friction/traction to work, but that is not the only work.

Eskimo: I think it is really just a perspective thing, weather you view it as friction or rotational acceleration. The force outside the plane is based on static friction tire to the ground. What creates that friction is the rotational acceleration.

Captain Virgil Hilts · « **Reply #596 on:** January 31, 2007, 09:36:56 AM »

Damn it man, shouldn't you be working on something REAL, like maybe CT?

mietla · « **Reply #597 on:** January 31, 2007, 11:16:43 AM »

Eskimo, hitech

The problem as stated is illdefined and ambiguous. I dont want to go back and read this entire thread again, so can you please state how do you understand the problem setup, and where do you guys get your infinities.

Case 1.
To me, "the belt matches the wheel's speed" simply means that the wheel does not skid on the belt. The belt is not powered, it simply moves along dragged freely by the wheel. The wheel is the driving element. The wheel can translate left or right, and/or it can rotate clock, or counterclockwise. The belt does not care. The belt is a passive element and just follows.

Case 2.
"the belt matches the wheel's speed but in the opposite direction" is only possible if the wheel's rotational speed is 0. The wheel can not rotate clockwise while the belt is moving to the right (or counterclockwise while the belt is moving to the left. The friction prevents that. If the forces exceed the friction, the wheel simply separates and both the belt and the wheel move independently.

The plane moving to the left is pushing the bottom of the wheel to the right. Just because the wheel is pushed does not mean that it starts to rotate. The belt senses the force pushing it to the right (by the wheel), and compensates by applying an equal force ( the belt can not apply "a movement", it can only apply a force which may or may not result in movemnet) to the left. Both forces cancel each other and the result is that the wheel does not rotate at all. It translates to the left along with the plane. An outside observer sees the plane taking off to the left with wheels that do no spin. You could get an analog situation if you put a plane on teflon. The friction between the teflon tarmac and the wheel is miniscule in comparison with a friction in the axle bearing so the wheel does not spin and the wheels skid on teflon.

What is your understanding of the problem statement?

Why does "your" belt need to be powered is case 1.

Where do the infinities come from?

john9001 · « **Reply #598 on:** January 31, 2007, 11:41:12 AM »

need to define "wheel speed" is it the rotational speed of the wheel or the forward speed of the wheel in relation to the earth which would be the ground speed of the plane.

mietla · « **Reply #599 on:** January 31, 2007, 11:46:02 AM »

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Originally posted by john9001
need to define "wheel speed" is it the rotational speed of the wheel or the forward speed of the wheel in relation to the earth which would be the ground speed of the plane.

I've dismissed the later since it would be called the velocity of the plane. But if this is the case, the solution is trivial.

If the belt matches the speed of the plane, the wheels do not spin, plane takes off.

If the belt matches the speed of the plane but in the opposite direction, the wheels spin twice as fast as in a normal take off, the plane takes off.