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Old October 26th, 2006, 02:40 PM   #61
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Quote:
Originally Posted by Mike Teutsch
What pushes the plane back? If you are talking about the belt, it is just turning the wheels.

Mike
Hehe Mike, I know all of this. I was quoting someone else and asking Why? :)

Quote:
Originally Posted by Robert Martens
I'm no pilot, but this analogy just popped into my head, thought it might be useful:

I'm standing on a nice, long treadmill. I start to walk, and some sort of fancy sensors watch my speed, turning the treadmill's belt so as to counter me. I, therefore, remain stationary as far as the surrounding environment is concerned. As I begin to jog, the sensors speed up the belt, producing the same result: I go nowhere. Even as I run top speed, I'm accomplishing nothing. A rather depressing metaphor, I must say.

This, I imagine, is what would happen with a car driving along this hypothetical motorized runway of Gene's.

However, let us now assume that the treadmill I'm running on top of is equipped with two waist high handrails, something you might find a gymnast practicing on. If I grab these with my hands, and pull myself forward, I will indeed move forward. The belt may match the speed of my feet, but since my grip is on the handrails, it makes no difference.

This is what I understand to be happening when we're dealing with an aircraft; the propulsion of the vehicle is conceptually disconnected from the machinery used to rest said craft on the ground. The jet's engines are "grabbing" the air, so to speak, and so long as they have a good grip, the thing will want to move.
What everyone can't seem to understand is that a plane isn't driven BY its wheels. A car IS driven by it's wheels. A human is driven by it's LEGS!! Both the car and human are completely different than a Jet which is NOT driven by it's wheels. Sinking in yet? Ok one more... Jet = free spinning wheels. Car = NOT free spinning wheels.

All this question means is that the wheels will be spinning exactly twice as fast when the plane takes off. That's all this means.

If you replaced Jet with Car in the original question then yes, it wouldn't move anywhere. And if that car had wings on it, it would NEVER take off. The difference is that the car is driven by it's wheels. Get it now?

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Old October 26th, 2006, 03:28 PM   #62
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Quote:
Originally Posted by Gene Crucean
What everyone can't seem to understand is that a plane isn't driven BY its wheels. A car IS driven by it's wheels. A human is driven by it's LEGS!! Both the car and human are completely different than a Jet which is NOT driven by it's wheels. Sinking in yet? Ok one more... Jet = free spinning wheels. Car = NOT free spinning wheels.

All this question means is that the wheels will be spinning exactly twice as fast when the plane takes off. That's all this means.

If you replaced Jet with Car in the original question then yes, it wouldn't move anywhere. And if that car had wings on it, it would NEVER take off. The difference is that the car is driven by it's wheels. Get it now?

Cheers
Yes, I got it a long time ago; that was supposed to be the point of my example. Perhaps my choice of a human walking on a treadmill was poor, it may have been better to visualize someone on rollerskates, but the example still works.

The human in my scenario is only "driven by its legs" as long as he's walking or running. As soon as this imaginary person grabs the handrails and pulls himself forward, he's powered by his arms, and the means of forward movement is separated from his connection to the ground.

I agree with you, Gene, and I was trying to provide an analogy to help explain why I believe you're correct. Maybe I wasn't clear about that when typing my last post. It was late and I was tired, what can I say?
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Old October 26th, 2006, 03:33 PM   #63
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Ah sorry Robert I didn't mean to direct my comment at you. I just happened to use the person walking on a treadmill as an example.

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Old October 26th, 2006, 03:33 PM   #64
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Maybe it would be easier for some to picture a skier on skis, on an ice covered belt, and a rocket backpack. He may not fly with out the wings, but he will surely get going fast if he can stay on his feet, er skis!

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Old October 26th, 2006, 06:21 PM   #65
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Try this experiment

Here is my proposed experiment to prove my position:

1. Go to your kids toy box and get a "free wheel" tonka truck out.

2. Get a long rubber band. Tie it to the front bumper.

3. Go to your variable speed tread mill. Point the the truck in the opposite direction of the tread mill direction. Mark the position

4. Pull the rubber band forward enough that the truck starts to pull forward. That should be your independent forward thrust not connected to the free spinning wheels

5. According to the scenario, the conveyor belt instantly senses the forward motion, and adjust by moving opposite direction. So turn on the variable speed until the truck "just due to friction and down ward force of gravity takes the vehicle back to it original position by stretch rubber band back. You are at static position, with no air speed for lift.

6. Pull tighter on the rubber band, and the tonka truck will move forward again, but with "instantaneous" adjust per the scenario, if you adjust the speed of the tread mill, you are again back at static position, still no air speed.

7. Of course if you suddenly shut of tread mill off, the tonka truck should lurch forward with the thrust that is being applied, approaching air speed required.

So that is the way I imagined the the scenario, and that analysis is why I think it can't reach lifting air speed. I think those who are saying other wise are failing to account for 200 tons of weight, inertia, and friction on tires and wheels. I have been accused of adding something to the equation, but I did not. The scenario indicates a 747. I assume those characteristics.

Where I can be wrong is with respect to the jet engine itself. I am acting under the understanding that it does produce its own airs speed for lift. It uses air pulled throught itself to produce propulsion which in turn produces air speed as the jet is propelled down the runway.

Also, I may be misunderstanding the what is proposed in the first place. I think some here are assuming the conveyor belt is free wheeling, and doesn't have its own motive force. That is not what I read into the proposition, and a motorized conveyor belt such as seen on a motorized treadmill.
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Old October 26th, 2006, 07:37 PM   #66
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Quote:
Originally Posted by Chris Barcellos
...the truck "just due to friction and down ward force of gravity takes the vehicle back to it original position by stretch rubber band back.
And that is where I disagree. I understand what you're proposing, but I can't possibly imagine it would be a perfect balance; just enough friction and gravity to pull it back to its starting point, but not enough to snap the rubber band and send the truck flying off of the conveyor belt.

I'm operating under the assumption that jets are not equipped with engines that can produce only the bare minimum thrust required to move the vehicle forward under ideal conditions. It's my understanding that most aircraft, especially commercial airliners, have more than enough power available to move themselves forward (they can lose engines midflight and keep going, after all), and pushed far enough could easily overcome any friction presented by the wheels, no matter how fast this belt is moving.
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Old October 26th, 2006, 07:44 PM   #67
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Old October 26th, 2006, 07:47 PM   #68
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Quote:
Originally Posted by Robert Martens
And that is where I disagree. I understand what you're proposing, but I can't possibly imagine it would be a perfect balance; just enough friction and gravity to pull it back to its starting point, but not enough to snap the rubber band and send the truck flying off of the conveyor belt.

I'm operating under the assumption that jets are not equipped with engines that can produce only the bare minimum thrust required to move the vehicle forward under ideal conditions. It's my understanding that most aircraft, especially commercial airliners, have more than enough power available to move themselves forward (they can lose engines midflight and keep going, after all), and pushed far enough could easily overcome any friction presented by the wheels, no matter how fast this belt is moving.
Nevertheless, the scenario proposed initially was that the conveyor would instantaneously adjust. And its not just friction, its the 90% downward thrust of gravity that has to be over come, even by a car driving down a perfectly flat surface. That why on a perfectly flat surface we can just push a car and expect it will roll forever...
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Old October 26th, 2006, 07:50 PM   #69
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That's something that's bothering me; what does gravity have to do with this? I mean, of course, it's always there, and has to be overcome, but how is gravity any different in this situation than it is on a regular runway? It's just as strong, isn't it? Planes overcome gravity all the time, right? They can go from dead stops, not moving at all, to the required takeoff speed without issue. Why would the force of gravity be harder to overcome here?
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Old October 26th, 2006, 08:06 PM   #70
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Gravity has nothing to do with the wheels. The wheels are simply the interface between the ground and the plane.

The sole purpose of the wheels, is to overcome the force of DRAG.

There are two 'negative' forces working on an aircraft. GRAVITY and DRAG. In order to overcome those forces, the aircraft must generate sufficient LIFT and THRUST.

The sole purpose of the wings is to overcome the force of GRAVITY.

LIFT is provided by the wings. THRUST is provided by the engines.

GRAVITY is provided by the earth. DRAG is provided by the friction with the air (as the plane moves through it) and THE GROUND as the plane moves along it.


In the scenario described, we know that the 747 is designed to provide MORE than enough lift and thrust to overcome GRAVITY, and the DRAG proudced by friction with the air. But what about friction with the ground?

Friction between the ground and the 747 is minimized, by employing very very efficient devices called WHEELS. On a ski-plane, they use teflon coated skis, on a seaplane, it's pontoons. But in the case of our aircraft, its wheels.

The ONLY element of takeoff that is being altered from a normal takeoff in our scenario is an INCREASE IN FRICTION between the interface of the aircraft and the ground. (Someone described and increase in friction between an airplane and a muddy field. In that case MORE power would be needed to overcome the clinging mud).

In OUR situation, there is no 'clinging' property to the moving runway. It is simply 'pushing' back against the surface of the wheel twice as hard as it normally would. Since NOTHING else changes - IE Airpseed, thurst or lift, then the wheels will accomodate the increase in friction with an increase in ROTATIONAL SPEED. (and yes, probably an increase in temperature as well, but certainly nominal.)

There is NOTHING to prevent the wheels from rotating at twice the usual speed of takeoff. (At whatever given altitude and air density our imaginary take off assumes)

So the aircraft uses the same thrust and lift it normally uses to take off, and accomodates the increase 'drag' of the runway interface, by allowing the wheels to turn twice as fast as they normally do.

The. Airplane. Will. Fly.
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Old October 26th, 2006, 08:14 PM   #71
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Quote:
Originally Posted by Robert Martens
That's something that's bothering me; what does gravity have to do with this? I mean, of course, it's always there, and has to be overcome, but how is gravity any different in this situation than it is on a regular runway? It's just as strong, isn't it? Planes overcome gravity all the time, right? They can go from dead stops, not moving at all, to the required takeoff speed without issue. Why would the force of gravity be harder to overcome here?

It isn't. Okay, you know how when you start to push a stalled car when you first push it, it is hard to move it at all. You are overcoming the downward pull of gravity, as well as the friction and flexion of the tires. Then, as you get in rolling it get easier if you roll at a constant speed, but if you want to push it faster to pop the clutch, it gets harder to pick up speed.

Hey Robert, maybe this is one for the the MythBuster to figure out. I love that show...

Okay, with the scenario we have here, the conveyor speeding up instantaneously seems to be bring us back, under my analysis, to that inertial state again, and we really are only standing still--- depending on how instantaneous the conveyor belt adjusts.....

Again, I am not MENSA, and this seems freaky to me, but I also can't figure it any other way... So I am looking for somebody to actually show me where I'm wrong rather than act like that Apple guy on the Apple v. PC commercials.
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Old October 26th, 2006, 09:01 PM   #72
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Okay. Here are the facts.

The 747 has four engines, that generate appx 58,000 llbs of thrust EACH.

There is NOTHING preventing the wheels from turning in our scenario. They are as free as they ever are, to turn at whatever rotational speed is required.

In order to prevent the plane from moving forward, the drag provided by the wheels turning 'twice as fast' as they normally would, would have to EQUAL the thrust of the four engines generating 58,000 lbs of thrust EACH.

You MUST supply equivelant thrust of the engines in the OPPOSITE direction to prevent movement. And the wheels rotate beneath the plane, in order to prevent that.

Not gonna happen.

The plane will take off.
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Old October 26th, 2006, 11:18 PM   #73
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It's pretty simple - the plane takes off as it is airspeed that is required to gain lift, the mass is accelerated by the jet turbines and airspeed increases until takeoff velocity is reached. Velocity differential between the conveyor and plane is largely irrelevant.

It is worth mentioning that the conveyor/runway would need to be just as long as a normal runway.

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Old October 26th, 2006, 11:19 PM   #74
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Anyway, here’s another one:

A boat is floating in a contained, fixed volume of water – eg a pool. At no time does any part of the boat touch the sides or bottom of the pool, it is always floating freely in the water. On the deck of the boat is a large steel girder weighing 5 tons. The water level in the pool is marked. Now the steel girder is pushed off the deck and into the water. Obviously the girder sinks to the bottom of the pool, totally submerged.

Does the water level in the tank rise, fall or stay the same?
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Old October 26th, 2006, 11:39 PM   #75
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Quote:
Originally Posted by Kyle Ringin
Anyway, here’s another one:

A boat is floating in a contained, fixed volume of water – eg a pool. At no time does any part of the boat touch the sides or bottom of the pool, it is always floating freely in the water. On the deck of the boat is a large steel girder weighing 5 tons. The water level in the pool is marked. Now the steel girder is pushed off the deck and into the water. Obviously the girder sinks to the bottom of the pool, totally submerged.

Does the water level in the tank rise, fall or stay the same?
The water level falls. The amount of water volume required to support or bouy the beam, while in the boat, is much more than the water volume the beam itself displaces.

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