plane on a conveyor belt?

[APDrone]

Originally posted by 2bighorn
Not quite. If your conveyor is just trying to match the wheel rotation than rolling resistance and static friction forces would never come even close to thrust force.  Rolling resistance coefficient would not significantly change either with increased speed.
At any given time and speed, to match the wheels rotation conveyor would accelerate at about the same rate as at the beginning (assuming that plane's acceleration caused by thrust would be linear).

Where Eskimo fails is that he think acceleration equals speed.

This is the painful part. As overwhelming as this force may be ( especially if you are thinking in terms of jet propulsion ) the fact simply remains that an airplane must roll on its wheels to gain the necessary forward speed ( in relation to the world that is not on the conveyor ) to attain the airspeed necessary to become airborn.  

If a counter force is present that negates the wheels rotation, this speed will never be achieved.  Simple.  Period.    Now.. especially with jets, it's going to be an insane amount of energy necessary to make that happen, but, as long as the conveyor matches the rotation speed of the wheels, the plane isn't going to have any net movement.  I know that seems impossible, but it has to be that way.  The only way it doesn't have to be is if you can devise a way for the plane to move forward without the wheels rotating.

I prefer to think in terms of biplanes or other propeller driven aircraft, as the amount of time it would take for accelleration to overcome the brain-tolerant numbers of such a device are a little easier to grasp.

If the acceleration ( and I do so hope I don't use these wrong ) of an aircraft is 10ft/sec/sec and the conveyor accelerates at the same rate, then the net difference is zero.  Hard to fathom, but I believe the numbers add up.

[eskimo2]

Originally posted by 2bighorn
Nice film but you're not trying to match the rotation of a circular body attached to the large forward moving object.

Your experiment is about creating rotational inertia with sufficient initial acceleration and has little to do with the original question.

Remember, in order to match the wheel RPM you'd never have to accelerate at the greater rate than plane is accelerating down the conveyor no matter the speed.

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!

Bizarre how one word change makes it a completely different question.

[2bighorn]

Wheels on a plane are there to reduce the friction between plane and runway. They do not propel plane forward.


P.S
I have never seen message board where laws of physics would be changed as often as here.

[lukster]

Originally posted by 2bighorn
Wheels on a plane are there to reduce the friction between plane and runway. They do not propel plane forward.


P.S
I have never seen message board where laws of physics would be changed as often as here.

Wheels on the plane are also there to slow it down upon landing. What do you think would happen to a plane that was landing on a runway moving at 10,000 mph in a direction opposite to that of the plane? Same principal, same physics.

[Terror]

Originally posted by eskimo2
Yup!

Hard to imagine, isn’t it?

Imagine that you’ve made a human powered fan car.  You have bicycle type pedals that turn a propeller for a drive system. You’ve made the frame drive, prop, seat, etc. at just under a hundred pounds.  You put four 50 pound wheels on it, however.  Luckily it’s got great bearings though.  If you pedal really hard you can get it up to 10 mph.  

Now you park your contraption on this super treadmill, get on and pedal.  I turn the acceleration control so that your wheels are gaining 1,000 rpm per second (at the end of 1 second they are going 1,000 rpm, at the end of 2 seconds they are going 2,000 rpm, etc).  Do you think that you can move forward?  Or will you go shooting back?

Now just imagine the same concept of a grander scale!

So for this to happen, the conveyor runway would have to be able to accellerate to infinity.  Because the only way to counter the forward rotation of the wheels is to use accelleration.  So according to your theory, if I had extremely light wheels (not massless, just very light ) the belt would accelerate quickly, but if I had very heavy wheels, the belt would accelerate more slowly.  If I leave the throttle set, the belt would continue to accellerate because that is the only way to counter the energy (from thrust) being added to the mix.  

I guess I'm not convinced that:
If the wheel travels 20in in one RPM and the conveyor matches that distance with 20in, and 2 RPM moves the conveyor 40in that it would produce enough counterforce from inertia to keep the plane from moving.  The plane would accelerate, the wheels would rotate at twice the RPM for any given speed, and the conveyor would accelerate to match the wheels RPM.

Terror

[eskimo2]

Just to give us some hypothetical numbers to work with, let’s say that Al’s plane took off in 100 feet, Bob’s plane took off in 99.5 feet, and Chuck’s plane took off in 99 feet.  (If you don’t like those numbers, feel free to imagine 100 feet, 99 feet 11 inches, 99 feet 10 inches.  It doesn’t matter, the idea is what counts.)

Our story continues,

The brothers leave the air show.  On the way home they stop and refuel at Pondersville.  The fine citizens of Pondersville have been debating the “Plane will take off on a conveyor” question so furiously, they decided to actually build a giant conveyor runway.  While refueling the brother’s planes, the citizen’s convince the brothers to test out their new runway.  The brothers, line up and throttle up together.  As they roll down the runway, the conveyor speeds up to 50 mph.  (The Pondersville citizens built their runway to match the plane’s speed, not the wheel’s speed.)  When the planes take off, their wheels are spinning at 100 mph.  That means that compared to the still ground, Al’s plane took off in 101 feet, Bob’s plane took off in 100 feet, and Chuck’s plane took off in 99 feet.

[lukster]

Originally posted by Terror
So for this to happen, the conveyor runway would have to be able to accellerate to infinity.  Because the only way to counter the forward rotation of the wheels is to use accelleration.  So according to your theory, if I had extremely light wheels (not massless, just very light ) the belt would accelerate quickly, but if I had very heavy wheels, the belt would accelerate more slowly.  If I leave the throttle set, the belt would continue to accellerate because that is the only way to counter the energy (from thrust) being added to the mix.  

I guess I'm not convinced that:
If the wheel travels 20in in one RPM and the conveyor matches that distance with 20in, and 2 RPM moves the conveyor 40in that it would produce enough counterforce from inertia to keep the plane from moving.  The plane would accelerate, the wheels would rotate at twice the RPM for any given speed, and the conveyor would accelerate to match the wheels RPM.

Terror

The mass of the wheel does not determine the speed of the belt. Since mass cannot accelerate to the speed of light that is our limiting velocity. If it were possible to build a belt, wheel, and bearing that could withstand lightspeed velocity, and the force applied to the plane by the accelerating belt equaled the thrust of the plane, then the plane would sit stationary at full power until the belt and wheel got really close to lightspeed. Then the plane would begin to roll forward and take off normally.

[APDrone]

Originally posted by lukster
The mass of the wheel does not determine the speed of the belt. Since mass cannot accelerate to the speed of light that is our limiting velocity. If it were possible to build a belt, wheel, and bearing that could withstand lightspeed velocity, and the force applied to the plane by the accelerating belt equaled the thrust of the plane, then the plane would sit stationary at full power until the belt and wheel got really close to lightspeed. Then the plane would begin to roll forward and take off normally.

Actually.. if the speed of the belt and wheel got close to lightspeed.. wouldn't they expand in size as they approached light speed?  Then the gear would probably rip through the wing or fuselage and produce wreckage.

Still won't fly..

[eskimo2]

Originally posted by Terror
So for this to happen, the conveyor runway would have to be able to accellerate to infinity.  Because the only way to counter the forward rotation of the wheels is to use accelleration.  So according to your theory, if I had extremely light wheels (not massless, just very light ) the belt would accelerate quickly, but if I had very heavy wheels, the belt would accelerate more slowly.  If I leave the throttle set, the belt would continue to accellerate because that is the only way to counter the energy (from thrust) being added to the mix.  

I guess I'm not convinced that:
If the wheel travels 20in in one RPM and the conveyor matches that distance with 20in, and 2 RPM moves the conveyor 40in that it would produce enough counterforce from inertia to keep the plane from moving.  The plane would accelerate, the wheels would rotate at twice the RPM for any given speed, and the conveyor would accelerate to match the wheels RPM.

Terror

Not to infinity.

Suppose you ran the plane at full throttle on the conveyor for an hour.

Take the propeller off the plane.  Connect the engine to a perfect transmission capable of any gear ratios.  Let’s say the final output in energy is the same amount that the propeller can deliver.  Hook the other end of the transmission to the wheel(s).  Rev up the engine and let it accelerate the wheels for an hour.  The wheels in both examples will be spinning at the same rate at the end of the hour.  Faster than we could comprehend, but no where near infinity!

[eskimo2]

Originally posted by lukster
The mass of the wheel does not determine the speed of the belt. Since mass cannot accelerate to the speed of light that is our limiting velocity. If it were possible to build a belt, wheel, and bearing that could withstand lightspeed velocity, and the force applied to the plane by the accelerating belt equaled the thrust of the plane, then the plane would sit stationary at full power until the belt and wheel got really close to lightspeed. Then the plane would begin to roll forward and take off normally.

LOL!  The light speed twist!  I love it!

[SteveBailey]

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

Exactly how do you figure this ?


edit:  forgive me, I misread as will not fly. I apologize.

[lukster]

Originally posted by APDrone
Actually.. if the speed of the belt and wheel got close to lightspeed.. wouldn't they expand in size as they approached light speed?  Then the gear would probably rip through the wing or fuselage and produce wreckage.

Still won't fly..

As an object approaches the speed of light it's mass approaches infinity. Don't confuse mass with size (volume) though. Black holes can be relatively small in size but very massive.

[john9001]

people who say the plane will not fly are saying that the drag of the wheels is greater then the thrust of the engines, if that was the case no plane would ever fly.

[2bighorn]

Originally posted by lukster
If it were possible to build a belt, wheel, and bearing that could withstand lightspeed velocity

OK, I'll try one last time.

The only way to apply force from belt to the plane would be through wheels and would never exceed the force of the friction which is considerably smaller than thrust force otherwise no plane would ever take off.

Eskimo is working under the assumption that acceleration differential of the belt transfered through friction force to the plane on order to match the wheel RPM could overcome the inertia of the plane but the difference in rate of acceleration would never be sufficient to work his way.

If you look at the few formulas necessary to calculate the forces and punch in real life numbers like weight of the plane, wheels, thrust, acceleration, rolling resistance coefficient, increase of Crr due to temps and use conveyor speed control system which at least matches that of everyday ABS or traction control system, you'll see that plane would take off normally and that the conveyor final speed would be very close to the Vfto of the plane.

EDIT: John beat me to it

[Terror]

Originally posted by lukster
The mass of the wheel does not determine the speed of the belt. Since mass cannot accelerate to the speed of light that is our limiting velocity. If it were possible to build a belt, wheel, and bearing that could withstand lightspeed velocity, and the force applied to the plane by the accelerating belt equaled the thrust of the plane, then the plane would sit stationary at full power until the belt and wheel got really close to lightspeed. Then the plane would begin to roll forward and take off normally.

I did not mean to say the mass determines the actual speed, but it determines the rate at which the belt will accelerate.  (also, the diameter of the wheel also...)

Limitations of any of the physical parts are not a part of the discussion.  You have to assume the parts (conveyor, plane, engine, wheels, controller) are of limitless capabilities and will not fail under any circumstance.  Energies are tranferred, but the energies cannot cause failures within the systems they are being transfered to/through.  Otherwise you can only speculate as to which system or part will fail first.  

I think eskimo2 may have converted me.  As long as energy can be tranferred from the conveyor to wheel to the plane, then there is the possibility that the plane would sit still on conveyor.  As long as thrust is input via the engine, the conveyor would accelerate at a rate to completely counter the input energy.

The question remains in my head as to whether the above situation matches the original "the controller monitors plane's wheel speed and tunes the speed of the conveyor to be exactly the same but in the opposite direction" would allow for the belt to accelerate at a rate high enough to impart enough counter force....

Terror