Author Topic: plane on a conveyor belt? (Read 19862 times)

hitech · « **Reply #75 on:** January 20, 2007, 01:59:41 PM »

APDrone: Your argument is missing one large piece.

No mater how fast you run the conveyor under the wheels, it will not transmit any more force to the plane. The force will be the same at 1 MPH or 200 MPH. The rolling friction force of the wheel will be the same as long as the same weight stays on the wheel.

Hence there is no way to stop the acceleration of the plane do to thrust.

2nd The question quite frankly sucks, and is totally none solvilble from the wheel speed statment. The problem is the equation can not be written to be solved based on the wheel speed. (unless you define wheel speed not in rpm, but axle moving accross the ground)

The only solution to the equation is 0 rpm on the wheel. At any other speed, it will instantaneously solve to infinity.

HiTech

eagl · « **Reply #76 on:** January 20, 2007, 02:17:50 PM »

HT,

The wheel speed going to infinity is why it's useful to approach the question as if the plane were on skids... Even though it would be impossible for the situation in the question to occur, equating a rotational speed of infinity to a skid with a fixed drag can help get past the conceptual error that drone is making.

By drone's argument, a plane on skids sitting on a treadmill going any arbitrary speed would never be able to fly even if the engine's thrust was greater than the drag of the skids against the treadmill.

APDrone · « **Reply #77 on:** January 20, 2007, 02:53:28 PM »

Quote

Originally posted by hitech
APDrone: Your argument is missing one large piece.

No mater how fast you run the conveyor under the wheels, it will not transmit any more force to the plane. The force will be the same at 1 MPH or 200 MPH. The rolling friction force of the wheel will be the same as long as the same weight stays on the wheel.

Hence there is no way to stop the acceleration of the plane do to thrust.

2nd The question quite frankly sucks, and is totally none solvilble from the wheel speed statment. The problem is the equation can not be written to be solved based on the wheel speed. (unless you define wheel speed not in rpm, but axle moving accross the ground)

The only solution to the equation is 0 rpm on the wheel. At any other speed, it will instantaneously solve to infinity.

HiTech

Dagnabbit! I want to go take a nap..

Ok Boss..

Please explain how this acceleration can be translated into forward motion without causing the wheels to turn at a different rpm than they were before the acceleration was applied.

The wheels are keeping the propeller clear of the runway. They are a necessary evil. To make them move in relation to the rest of the world, they must roll. If they roll, their rpms will be matched by this fantasy device, thus preventing them from actually moving forward in relation to the rest of the world.

In order for wheels to move forward, the rpms must increase in relation to the surface it is on. Our fantasy surface adjusts itself to match this change.

Sure the plane has acceleration.. it's pushing ( or pulling ) an aweful lot of air. But, if that acceleration cannot be translated into forward motion, then it will never be able to take off. All that energy is being countered by the effort the fantasy device is using to spin faster. Maybe that's an argument people are overlooking.. The fantasy device is powered by itself, not feeding off the inertia of the aircraft's wheels. So, the acceleration effort is being countered by the fantasy engine's effort. This way we keep Newton happy.

Right?

Tuomio · « **Reply #78 on:** January 20, 2007, 02:57:00 PM »

Quote

Originally posted by hitech
APDrone: Your argument is missing one large piece.

No mater how fast you run the conveyor under the wheels, it will not transmit any more force to the plane. The force will be the same at 1 MPH or 200 MPH. The rolling friction force of the wheel will be the same as long as the same weight stays on the wheel.
HiTech

One word: Acceleration.

Edit: Friction might stay the same per revolution, but it does not stay the same per second.

JB88 · « **Reply #79 on:** January 20, 2007, 03:01:28 PM »

Quote

Originally posted by hitech
Boy I'm glad some of you were not involved in the conservation of energy conversations we had concerning the torque of jet engines a few years back.

HiTech

perhaps if you modeled this people might understand better.

(though i am almost certain that some would still argue its validity)

Golfer · « **Reply #80 on:** January 20, 2007, 03:04:22 PM »

Drone,

Take for instance a dyno machine with a motorcycle on it. You can run that sucker up to "100mph" and it's not actually going anywhere.

Take an airplane and put it on that same machine...what you'd like to think is that the same thing will happen. It won't move...

This is wrong.

Because the motorcycle transfers its energy through the wheels in order to move forward...it works. The thrust of the bike relies strictly on the friction between the wheels and the ground to make it go forward.

An airplane isn't the same. The wheels of an airplane are simply a lubricant between a hunk of metal slamming down and the runway.

Because the power of an airplane is generated and transferred through the air whether or not the ground beneath the wheels is moving has no effect.

By the logic you've presented there's a direct correlation between wheel speed and airspeed. If you were to takeoff into a headwind when it came time to rotate into the air and break free of the ground what will happen? Our airspeed will be higher than our wheelspeed/groundspeed because there is that headwind blowing into us.

Same goes with a tailwind. Even though we'd be traveling faster over the ground than our indicated airspeed would say...our airplane still wouldn't fly because there's not enough air moving over the wings.

eskimo2 · « **Reply #81 on:** January 20, 2007, 03:11:26 PM »

I'm going to quote myself here; this is the 5th responce of this thread:

Quote

Originally posted by eskimo2
The plane will take off, but what you described is impossible. The conveyer can’t match the wheel speed in the opposite direction; the plane will move forward because the engines are pushing and pulling air, not the conveyer. The conveyer can speed up all it wants to try and stop the plane but the wheels will just spin as fast as the conveyer PLUS the speed of the accelerating plane.

The original question is impsssible. No conveyer could match the wheel speed if there is an external force.

Kurt · « **Reply #82 on:** January 20, 2007, 03:14:08 PM »

Quote

Originally posted by APDrone
Dagnabbit! I want to go take a nap..

Ok Boss..

Please explain how this acceleration can be translated into forward motion without causing the wheels to turn at a different rpm than they were before the acceleration was applied.

The wheel speed does change drone, nobody is arguing that it doesn't... Its just that it doesn't matter because the wheels aren't a significant source of thrust or drag, they are simply spinning.

eagl · « **Reply #83 on:** January 20, 2007, 03:16:14 PM »

Drone,

Your argument is valid if you take Hitech's observation that the instant an external force is applied to the aircraft, the wheel speed would instantaneously jump from zero to infinity, and that the all energy used to further accelerate the aircraft would simply be pumped into the theoretical infinite-speed treadmill.

The problem with that solution is that the treadmill and wheels would have to have zero mass, zero inertia, and infinite strength to be able to instantly accelerate like that. But if they have zero mass and inertia, then they would not actually absorb the energy as you say they should. In that case, they would simply not factor into the force equations and the plane would still be able to move forward through the reference plane in which the engine thrust is being applied (through the air mass).

Once the wheel speed goes to infinity, the relative motion of the aircraft through the air is not significantly in the equation unless the mass and inertia of the wheels is either zero or so close to zero as to still be impossible in practice.

This is why all theoretical faster than light drives use some sort of field to either reduce the effective mass of the ship to zero, or create some sort of bubble around the ship so that the ship is surrounded by sub-light velocity space while the bubble, which is effectively massless, moves through the rest of the universe. Star Trek uses the second trick, creating a "sub-space" field around the ship so the ship never gets close to the speed of light. You are postulating the first trick, where your wheels and treadmill are capable of moving at an infinite speed. To be able to do so, they would need to have zero mass or infinite strength (or possibly a diameter of zero... hmmm)

AWMac · « **Reply #84 on:** January 20, 2007, 03:19:45 PM »

If and when this said plane does get off the ground HT would patch it into gameplay, perk the hell out of it and have it over modeled. I like it already.

Of course I presented once into a science exibition a dancing Duck synopsis.. Can Man teach a Duck to Tap Dance?

The answer is YES!!!

Then someone stole my hotplate and called PETA.

The Duck is still missing.

Mac

lukster · « **Reply #85 on:** January 20, 2007, 03:24:18 PM »

I didn't read all of the responses and someone probably said this but if the treadmill was capable of producing a speed of say 1,000 miles an hour (no need to get infinite on anyone) the wheel bearing friction would overcome the plane's thrust probably before it built up enough airspeed to lift off.

Tuomio · « **Reply #86 on:** January 20, 2007, 03:45:53 PM »

Quote

Originally posted by eagl
Drone,

Your argument is valid if you take Hitech's observation that the instant an external force is applied to the aircraft, the wheel speed would instantaneously jump from zero to infinity, and that the all energy used to further accelerate the aircraft would simply be pumped into the theoretical infinite-speed treadmill.

Define "Instant". Belts design might incorporate device, which predicts the future movement of a plane sitting on it and the time when the movement is going to happen. Being active instead of reactive.

eagl · « **Reply #87 on:** January 20, 2007, 03:51:53 PM »

It can't be predictive because if the belt starts moving before the engine thrust starts, then the plane would shoot backwards.

The entire question is based around the idea that any force on the aircraft would be translated without loss into wheel spin. That's why the wheel speed goes to infinity the moment a force is applied that would otherwise accelerate the plane.

HT's right, it's really an invalid question because the assumptions required for it to work are not possible.

Tuomio · « **Reply #88 on:** January 20, 2007, 04:09:44 PM »

Quote

Originally posted by eagl
It can't be predictive because if the belt starts moving before the engine thrust starts, then the plane would shoot backwards.

But if they start moving at the SAME time? If you have two lamps attached behind one switch, which one of them theoretically lights up first when the switch is pressed?

john9001 · « **Reply #89 on:** January 20, 2007, 04:13:01 PM »

Quote

Originally posted by eagl
The entire question is based around the idea that any force on the aircraft would be translated without loss into wheel spin. That's why the wheel speed goes to infinity the moment a force is applied that would otherwise accelerate the plane.

the wheels have nothing to do with it, a force (thrust) on the aircraft is to the aircraft, not to the wheels. It's so simple a caveman could figure it out.