Author Topic: Airplane on a Conveyor Belt... (Read 30966 times)

eskimo2 · « **Reply #105 on:** January 31, 2008, 07:20:55 PM »

I explained it here:

Chuck’s wheels have no mass, and no rotational energy. Bob’s wheels have half the mass of Al’s wheels. Therefore, at 50 mph Chuck’s wheels are storing 0 energy, Bob’s wheels are storing ½ X energy and Al’s wheels are storing X energy. Chuck’s plane must take off first, then Bob’s, then Al’s. It doesn’t matter if Chuck’s plane takes off 4 feet before Al’s or 4 mm; the concept shows that getting the wheels up to speed consumes energy. A conveyor belt of unlimited speed and power can constantly load enough energy into the wheels to counter-act the plane’s thrust.

A wheel that has a mass of X and has an outside speed of 50 mph will have twice the rotational energy of a wheel that has a mass of 1/2 X and has an outside speed of 50 mph. The one with no mass, has no energy.

Does that make sense?

BlkKnit · « **Reply #106 on:** January 31, 2008, 09:03:53 PM »

Quote

Originally posted by eskimo2
Imagine that “exactly matches” really means that YOU have a big acceleration control dial for the conveyor and your job is to keep it in place. The plane fires up and begins to inch forward; you see this and crank the acceleration dial. You go too far, however, so the plane drifts back behind its starting point. Seeing this you back off on the acceleration dial and adjust it to keep the plane pretty much where it started.

Look at the concept; don’t get caught up in the semantics of the word “exactly”.

I still dont see it. Matching the tires rotational speed in this way causes the tires to spin. Thats creating a false result using outside influence. There is no way the belt affects anything without those outside influences.

I think this myth has degenerated to this level and probably was not the original question. It must have started as a simple trick question of logic.

But, of course, if you can keep the aircraft stationary you can keep it from lifting. I just think my understanding of the question may be incorrect and if so, then the question is incorrect. It must be, I cant be wrong!

2bighorn · « **Reply #107 on:** January 31, 2008, 09:16:39 PM »

Quote

Originally posted by AKIron
Energy transferred to the wheels is energy not used to move the plane forward.

That is true.

But in his story (apart from the wheels), all planes are identical. The difference in wheel mass is compensated with airframe ballast.

So, size, rolling friction, drag, and total weight is the same, therefore thrust needed (Force) to move the plane would be the same. (at least according to the Newton).

It does not matter if mass of some parts vary slightly (ie wheels). The total inertial mass to be accounted remains equal among all three systems (planes), so does sum of all forces.

That said, take off distance would still vary for about 0.08%, not due to Eskimo's theory, but because the plane with lightest wheel would have the heaviest frame and therefore exercise slightly larger force on the wheel bearings hence bearing friction would increased slightly...

eskimo2 · « **Reply #108 on:** January 31, 2008, 09:23:35 PM »

2bighorn,

Watch the AVI's:

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

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

2bighorn · « **Reply #109 on:** January 31, 2008, 09:42:15 PM »

Quote

Originally posted by eskimo2
2bighorn,
Watch the AVI's:

Film 1:
Find two more wheels of equal size but different weight, compensate that with different strength of attachment (dont know if it's spring or rubber) for each wheel and repeat.

Film 2:
Attach objects of different mass to each of your rotating body, so that all three groups have the same mass and repeat the experiment.

That's what you did with your planes, you compensated difference in wheel mass with ballast mass.

If total mass does not change, nor forces applied, the distance of movement should neither.

AKIron · « **Reply #110 on:** January 31, 2008, 09:44:24 PM »

Assume 2bighorn that each of eskimo's planes must reach exactly the same airspeed to lift off. To make things simple, since I don't recall if eskimo specified, also assume that each of the 3 wheel sets all have the same diameter.

Since they are all spinning at the same rate on each plane for a given speed the wheels with more mass will have more energy. That energy didn't mystically appear, it was created by the plane's engine. Let's also assume that each of the three planes were running at full power and they were all three putting out the same amount of power. All other things being equal, the plane(s) that put the least amount of it's energy into the wheels will accelerate and takeoff faster.

2bighorn · « **Reply #111 on:** January 31, 2008, 11:55:15 PM »

Quote

Originally posted by AKIron
Since they are all spinning at the same rate on each plane for a given speed the wheels with more mass will have more energy. That energy didn't mystically appear, it was created by the plane's engine. Let's also assume that each of the three planes were running at full power and they were all three putting out the same amount of power. All other things being equal, the plane(s) that put the least amount of it's energy into the wheels will accelerate and takeoff faster.

Yes, but he specified all planes have same weight. Therefore the energy you save with lighter wheels goes into moving heavier frame.
Heavy airframe + light wheels = light airframe + heavy wheels
Same mass, same force needed...

SD67 · « **Reply #112 on:** February 01, 2008, 03:30:12 AM »

I remember reading an piece about this on AVweb by the CEO of the Cockpit. Funny guy, always a good read.
http://www.avweb.com/news/pilotlounge/191034-1.html

AKIron · « **Reply #113 on:** February 01, 2008, 07:45:38 AM »

Quote

Originally posted by 2bighorn
Yes, but he specified all planes have same weight. Therefore the energy you save with lighter wheels goes into moving heavier frame.
Heavy airframe + light wheels = light airframe + heavy wheels
Same mass, same force needed...

Okay but think about this. Assume the tire riding in the back seat has the same mass as the tire rolling under the plane. Which has more energy at takeoff? They are both moving forward with the same amount of energy relative to the fixed starting point but the one rolling has more energy because it is both moving forward and spinning. That energy could have been used to accelerate the plane but wasn't.

Yknurd · « **Reply #114 on:** February 01, 2008, 08:05:55 AM »

The fat pilot who just ate the bean burrito at Taco Bell has the most kinetic energy in the plane.

john9001 · « **Reply #115 on:** February 01, 2008, 08:14:51 AM »

it takes very little energy to spin a wheel, and that energy is coming from the belt, not the planes thrust.

the belt is spinning the wheel. all the energy put into the wheel has come from the belt.

AKIron · « **Reply #116 on:** February 01, 2008, 08:21:55 AM »

Quote

Originally posted by john9001
it takes very little energy to spin a wheel, and that energy is coming from the belt, not the planes thrust.

the belt is spinning the wheel. all the energy put into the wheel has come from the belt.

It takes energy from the plane's engine to keep the wheel from rolling backwards with the belt. Amounts are relative to the speed of the belt. With a belt moving at only 100mph that amount of energy is insignificant relative to what's used to get the plane off the ground.

Accelerate that belt to say 10,000 mph and the plane's engine will be transferring a lot more of it's power to keep that wheel from rolling backwards with the belt. It may not have enough left to move the plane forward.

Chairboy · « **Reply #117 on:** February 01, 2008, 08:44:53 AM »

That's just about entirely inaccurate.

Frictional force between two surfaces is u*N (coefficient of rolling friction * Normal Force). The treadmill applies EXACTLY the same force of friction, without regard to whether it's moving at zero, 100, 1,000, or 50,000 mph.

AKIron · « **Reply #118 on:** February 01, 2008, 08:47:58 AM »

Quote

Originally posted by Chairboy
That's just about entirely inaccurate.

Frictional force between two surfaces is u*N (coefficient of rolling friction * Normal Force). The treadmill applies EXACTLY the same force of friction, without regard to whether it's moving at zero, 100, 1,000, or 50,000 mph.

That's not entirely accurate. The coefficient of friction will most certainly change as the tire heats up due to increasing speed.

But I wasn't including friction as a factor (though of course it is).

AKIron · « **Reply #119 on:** February 01, 2008, 08:58:28 AM »

There are a lot variables in these scenarios and several scenarios have been supposed to illustrate the forces involved. It's easy to mix these up which is probably why this discussion/argument has been so long lived.