plane on a conveyor belt?

[Terror]

Originally posted by lukster
We're talking rotation speeds much much great than what the plane's thrust is capable of generating. I'm not an engineer so I can't give you some realistic esitmates off hand but I will dig out my old physics book and get back to you with some numbers.

Why would the rotational speeds be so great?  If the wheel needs to spin at 1000rpm (just a number) to get the plane airborne, then if "the control system that tracks the plane's wheel speed and tunes the speed of the conveyor to be exactly the same but in the opposite direction", wouldn't the wheel be doing 2000rpm at the plane's take off speed?

I submit that to get the wheel spinning 2000rpm, it would take twice the amount of energy than to get it to 1000rpm.....

Terror

PS.  Of course, this is if you translate "opposite direction" to making the wheel spin faster, not sit still....

[lukster]

Originally posted by Terror
Why would the rotational speeds be so great?  If the wheel needs to spin at 1000rpm (just a number) to get the plane airborne, then if "the control system that tracks the plane's wheel speed and tunes the speed of the conveyor to be exactly the same but in the opposite direction", wouldn't the wheel be doing 2000rpm at the plane's take off speed?

I submit that to get the wheel spinning 2000rpm, it would take twice the amount of energy than to get it to 1000rpm.....

Terror

PS.  Of course, this is if you translate "opposite direction" to making the wheel spin faster, not sit still....

You're using the same interpretation as have others and it's not wrong because of the ambiguity in the question. You're assuming that the speed of the belt is intended to counter the forward speed of the airplane whereas others are interpreting it to mean the belt will counter the speed of the wheel rotation which in turn causes the wheel to rotate faster which causes the belt to rotate faster ad nauseum.

I don't want to dig into the numbers, too many cobwebs in my mind and I don't like spiders. Just assume the belt acceleration is capable of sustaining a backwards force on the plane equal to half it's thrust. While the plane would need a much longer rollout it would eventually takeoff. It would move as if half power were applied.

This assumes the wheel bearings won't freeze which is a completely erroneous assumption since if they are made of any material on this earth they most certainly will heat, expand, and freeze or disintegrate. Remember, the belt is accelerating which is steadily increasing the spin rate of the wheel.

[2bighorn]

Originally posted by eskimo2
MiniD and anyone else who does not understand the forces involved in accelerating the rotational velocity of a wheel needs to explain where the energy in Al’s and Bob’s planes’ wheels came from.  This really simplifies the problem.

No, miniD is right. Your experiment is flawed.


If you want to move plane backwards on the conveyor you have to overcome not only the wheel's rotational inertia (which is easy), but also the plane's inertia. The maximum amount of USABLE force to move plane created by conveyor acceleration EQUALS that of friction between wheels and axles (if we assume that grip between conveyor and wheel is optimal). Add the thrust to equation and plane will take off without difficulties.

Keep in mind that the greater its mass, the less a body accelerates under given force.

[lukster]

Originally posted by 2bighorn
No, miniD is right. Your experiment is flawed.


If you want to move plane backwards on the conveyor you have to overcome not only the wheel's rotational inertia (which is easy), but also the plane's inertia. The maximum amount of USABLE force to move plane created by conveyor acceleration EQUALS that of friction between wheels and axles (if we assume that grip between conveyor and wheel is optimal). Add the thrust to equation and plane will take off without difficulties.

Keep in mind that the greater its mass, the less a body accelerates under given force.

It's quite easy to add a belt acceleration that will cause the tire to "skid" due to the tire's inability to match the belt's velocity, even without a plane or rubber band attached. That means the plane's thrust would have to overcome the friction of a tire at least unmoveable relatively to the surface it is sitting on. That would be like taking off with the brakes fully engaged.

[eskimo2]

Originally posted by Terror
Why would the rotational speeds be so great?  If the wheel needs to spin at 1000rpm (just a number) to get the plane airborne, then if "the control system that tracks the plane's wheel speed and tunes the speed of the conveyor to be exactly the same but in the opposite direction", wouldn't the wheel be doing 2000rpm at the plane's take off speed?

I submit that to get the wheel spinning 2000rpm, it would take twice the amount of energy than to get it to 1000rpm.....

Terror

PS.  Of course, this is if you translate "opposite direction" to making the wheel spin faster, not sit still....

Imagine that you are the control system.  

You see the plane’s wheel roll forward a foot so you turn on the speed control for the conveyor; it moves a foot.  But, when the conveyor moved back a foot, the wheel also moved a foot.  So the wheel has moved two feet, but your conveyor has only moved one foot.  The only way that they will ever be equal is if the plane’s wheel moves back a foot.  So, you turn the conveyor acceleration control like mad.  Now the wheel is accelerating at the rate of 100,000 rpm per second.  That increase in acceleration moves the wheel back a foot so you adjust the rpm acceleration rate to 99,000 rpm per second and the plane is still, at full throttle.

[Terror]

Originally posted by lukster
I don't want to dig into the numbers, too many cobwebs in my mind and I don't like spiders. Just assume the belt acceleration is capable of sustaining a backwards force on the plane equal to half it's thrust. While the plane would need a much longer rollout it would eventually takeoff. It would move as if half power were applied.

I also think that the forces the plane are overcoming during a normal take off roll (without the conveyor) are not generated by rotational inertia.  A small percentage is, but not much.  The forces in order being overcome in a normal takeoff roll is:  AirFrame Inertia (total plane mass), Aerodynamic Drag, Wheel Friction, Bearing Friction, and lastly Rotational Inertia from the wheels.

The conveyor will potentially double the Rotational Inertia needed to be overcome, but doubling the Rotational Inertia of the wheels is still a small portion of the overall forces that need to be overcome.

Terror

[Wes14]

y not take a gokart put wings on the sides,and put a jet engine on it..then see if it would take off on this conveyor(of death) belt

[lukster]

Originally posted by Terror
I also think that the forces the plane are overcoming during a normal take off roll (without the conveyor) are not generated by rotational inertia.  A small percentage is, but not much.  The forces in order being overcome in a normal takeoff roll is:  AirFrame Inertia (total plane mass), Aerodynamic Drag, Wheel Friction, Bearing Friction, and lastly Rotational Inertia from the wheels.

The conveyor will potentially double the Rotational Inertia needed to be overcome, but doubling the Rotational Inertia of the wheels is still a small portion of the overall forces that need to be overcome.

Terror

Agreed, normal wheel inertia is a small force compared to the plane's thrust. To increase the wheel's inertia to the point at which the plane struggles to overcome it would require a high belt acceleration indeed. Now we are back to what is the belt trying to counter, the forward airspeed of the aircraft or the rotating wheel speed?


I should say what is the belt trying to do the opposite of rather than counter as that brings in another ambiguity.

[Terror]

Originally posted by eskimo2
Imagine that you are the control system.  

You see the plane’s wheel roll forward a foot so you turn on the speed control for the conveyor; it moves a foot.  But, when the conveyor moved back a foot, the wheel also moved a foot.  So the wheel has moved two feet, but your conveyor has only moved one foot.  The only way that they will ever be equal is if the plane’s wheel moves back a foot.  So, you turn the conveyor acceleration control like mad.  Now the wheel is accelerating at the rate of 100,000 rpm per second.  That increase in acceleration moves the wheel back a foot so you adjust the rpm acceleration rate to 99,000 rpm per second and the plane is still, at full throttle.

So you are saying at full throttle, the plane will just sit at one spot on the conveyor runway with all the energy being generated by the engine being lost to rotational inertia of the wheels?  So the plane would be thrusting against the air sitting still and the wheels whirring and conveyor screaming along?

Terror

[2bighorn]

Originally posted by lukster
It's quite easy to add a belt acceleration that will cause the tire to "skid" due to the tire's inability to match the belt's velocity, even without a plane or rubber band attached. That means the plane's thrust would have to overcome the friction of a tire at least unmoveable relatively to the surface it is sitting on. That would be like taking off with the brakes fully engaged.

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.

[eskimo2]

More and better movies.  

Here is a paper treadmill; the source off acceleration is a falling shoe tied to the paper.  On the paper treadmill are a mouse ball, a copper pipe with a rubber band glued around it for traction, and an acrylic ball that may have skid/slip some.

Picture of the set up:


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

QuickTime
http://hallbuzz.com/movies/paper_treadmill.MOV

[lukster]

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.

It seems you are assuming a low acceleration. I'm talking about the belt rotating in the opposite direction as that of the wheel. This induces a greater speed in the wheel which in turn causes the beltl to turn faster resulting in the wheel spinning up as fast as the speed of light, limited only in time which we control by limiting the acceleration of the belt. Are we agreed on this?

[eskimo2]

Originally posted by Terror
So you are saying at full throttle, the plane will just sit at one spot on the conveyor runway with all the energy being generated by the engine being lost to rotational inertia of the wheels?  So the plane would be thrusting against the air sitting still and the wheels whirring and conveyor screaming along?

Terror

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!

[lukster]

Originally posted by eskimo2
More and better movies.  

Here is a paper treadmill; the source off acceleration is a falling shoe tied to the paper.  On the paper treadmill are a mouse ball, a copper pipe with a rubber band glued around it for traction, and an acrylic ball that may have skid/slip some.

You should go into showbiz.  

[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.