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Old January 31st, 2008, 10:23 AM   #166
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So in our scenario as originally posted, the conveyor sped up every time the plane would speed up, in the Myth Busters version, they just used a constant speed take off speed for the conveyer. Point of us nay sayers has always been that you have to attain air speed. If the conveyor kept speeding up the, downforce of gravity is enought to created the friction necessary to keep the plane from going forward, as the conveyor sped up, the plane would never move forward and wheels would fall off at some speed. I still say not busted, as applied to the scenario posted here.. I also note that both planes were ultra light, certainly not comparable to a 747. :)
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Old January 31st, 2008, 12:29 PM   #167
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Originally Posted by Chris Barcellos View Post
So in our scenario as originally posted, the conveyor sped up every time the plane would speed up, in the Myth Busters version, they just used a constant speed take off speed for the conveyer. Point of us nay sayers has always been that you have to attain air speed. If the conveyor kept speeding up the, downforce of gravity is enought to created the friction necessary to keep the plane from going forward, as the conveyor sped up, the plane would never move forward and wheels would fall off at some speed. I still say not busted, as applied to the scenario posted here.. I also note that both planes were ultra light, certainly not comparable to a 747. :)
If the "downforce of gravity is enough to create the friction necssary to keep the plane from going forward", the plane would never move, even if the ground were still. This is EXACTLY what happens when the breaks are applied. "The force of gravity" never changes as a result of the treadmill speed. "The force of gravity" is the same when the treadmill is not moving as it is when it is moving.

The treadmill speed has no impact on the plane ... it could be double the plane's "speed" and not make a difference.

Think of it this way (again).....
If you have a plane chained to the ground and rev up the engines ... the plane stays still, because the chains counteract the force of the engines attempting to push the plane forward.

Now get rid of the chains.
What is there to prevent the plane from going forward when the engine is throttled up? ... Nothing. It rolls forward.

Finally ... how will the treadmill moving under the spinning wheels ever have the same force pulling backward on the plane as the chains did?
The answer is that it doesn't ... at any speed.
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Old January 31st, 2008, 03:18 PM   #168
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Originally Posted by Chris Barcellos View Post
So in our scenario as originally posted, the conveyor sped up every time the plane would speed up, in the Myth Busters version, they just used a constant speed take off speed for the conveyer. Point of us nay sayers has always been that you have to attain air speed.
But the wheels decouple the airframe from the conveyor. Airspeed is attained. The correct version of the problem is busted.

Quote:
...downforce of gravity is enought to created the friction necessary to keep the plane from going forward
The two are unrelated. On a horizontal surface (runway), gravity acts vertically and friction acts horizontally. The two are completely orthogonal.
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Old January 31st, 2008, 04:02 PM   #169
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Chris,

There were three examples on the show.

The first one consisted of the model on the treadmill. They started the treadmill, and 'held the model in place'. This meant that the wheels and the treadmill were moving AT EXAXCTLY the same speed. THEN the engine was engaged, and the plane accelerated and off the treamill. No, the treadmill did not accelerate, but the engine went from 'O' thrust to positive thrust overcoming the friction on the wheels. (From standpoint of neutral observer - plane goes from "O" to + while conveyor belt is already going 'faster' in the opposite direction)

In the SECOND example the model plane was placed on a longer 'treadmill' tarp. WATCH THE SHOW TAPE - first the tarp accellerated in reverse, THEN THE ENGINE IS ENGAGED - in those shots, the plane is already MOVING BACKWARDS while the tarp is moving - and it still ACCELLERATED IN THE OPPOSITE DIRECTION regardless of the fact that the tarp is already 'faster' than the plane. The motive force once again overcomes the directional speen of the 'conveyor belt' and the plane takes off.
(Plane goes from "-" to "+" against a faster belt starting belt)

in the THIRD example - The ultralight is sitting on the stationary tarp, when the plane and truck pulling the tarp - begin to accelerate. This is as close to the scenario described as can be achieved. Perhaps the plane accelerated faster than the truck towing the tarp, but I doubt it. I'd be willing to guess that the truck's acceleration exceeded the planes, and in fact - from the point of a neutral observer, the 'conveyor belt ' was moving faster than the plane in the beginning of the experiement. It still doesn't matter - the plane accellerates independent of the belts speed and direction, and it takes off.

Myth. Busted. Plane. Flies.


It would make no difference whatsoever if it were a jet or rocket or multi engined aircraft - all of these get their motive propusion FROM THE ENGINES not from their interaction from the ground.

Every Single Plane That Take Off from the EARTH facing WEST takes off from a 'conveyor belt' that is moving MUCH FASTER than the plane is in the opposite direction - and takes off just fine.

You'll just have to accept it. :)
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Old January 31st, 2008, 04:32 PM   #170
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You'll just have to accept it. :)
Thats what the church said to Gallileo.....

I accept the premise as presented in Myth Busters. That makes clear sense. The conveyor belt had a limited speed.

I provided an illustration based on the scenario presented at the beginning of the this game. The scenario was that the conveyor belt sped up every time the plane moved forward. We all wondered how that could be instantaneous, but the point was that it did. So in that situation, I posited that the plane could n't reach air speed, because of the mechanic of friction and gravityh. I think that is different from what was demontrated on Myth Busters.

Simple experiment: Get youself a tread mill with unlimited speed. Tie a rubber band to the nose of you plane, and start the tread mill. Pull forward on the nose using the rubber band, but at the same time speed up the tread mill to equalize the forward thrust caused by your rubber band pull, and the plane will remain stationary, and the rubber band will stretch longer, demontrating the actual forces of friction and gravity that is applied.
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Old January 31st, 2008, 04:42 PM   #171
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http://youtube.com/watch?v=0ul_5DtMLhc

For those who missed it.

You keep talking about the conveyor belt having a limited speed, but the truck and the sedgeway DIDN"T and those are the examples where the plane took off - so just dismiss the one where the belt had a limited speed AND distance.

It doesn't matter how fast the belt is moving. That was proved. In the opening shot of the sequence with the model on the tarp/belt, you can see the tarp start moving in the opposite direction.

ANY conveyor belt you build, at any speed, is already 'moving' because its sitting on the earth.

The speed and direction of the belt won't prevent the plane from moving forward.

In your rubber band experiment, the plane will continue to move forward... even if it stretches.

Youre conflating gravity and friction - they are two seperate forces at work, just as lift and drag are two seperate forces. The gravity acting on the plane is a constant - EVEN WHEN ITS IN THE AIR. Wheel friction is minimal, thats what they are designed to do (Yes, even spinning at many times the aircrafts take off speed - they have to be designed that way to land).

The plane has plenty of power to overcome the gravity and wheel friction.

To quote Gallileo "Nevertheless it moves.." - I think YOU represent the 'churchs' position in this one ;)
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Old January 31st, 2008, 04:48 PM   #172
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For reference, her is the original post language:

"If you have a large jet plane (747) sitting on a runway that was actually a giant conveyor belt (go with it). And there is also a device on the plane that communicates with the conveyor belt to tell it how fast the plane is traveling, which would then make the conveyor belt match the speed IN REVERSE.

Can the jet take off? "
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Old January 31st, 2008, 04:50 PM   #173
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Originally Posted by Chris Barcellos View Post
demontrating the actual forces of friction and gravity that is applied.
Gravity has NOTHING to do with it except increase the contact area between the deformable wheels and the deformable conveyor. If both both had zero compressibility, gravity would do nothing.

BTW, since we are verging on the realms of Flat Earth (the Galileo reference), the Earth can't be flat by virtue of its atmosphere. :)
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Old January 31st, 2008, 04:57 PM   #174
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Originally Posted by Chris Barcellos View Post
For reference, her is the original post language:

"If you have a large jet plane (747) sitting on a runway that was actually a giant conveyor belt (go with it). And there is also a device on the plane that communicates with the conveyor belt to tell it how fast the plane is traveling, which would then make the conveyor belt match the speed IN REVERSE.

Can the jet take off? "
That's the point I made a few posts ago - the problem statement posted on DVInfo is not the same as the one challenged by the rest of the world, including Mythbusters and is ambiguous.
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Old January 31st, 2008, 04:59 PM   #175
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The original post says the device communicates with the belt to tell it how fast the plane is going so that it will match the speed in reverse.

SO the plane is going a hundred miles an hour east, the belt is going a hundred miles an hour west, the wheels rotate at two hundred miles an hour and the plane takes off.

No problems. It doesn't say anything about 'matching the rotational speed of the wheels" - which isn't important anyway.

I can imagine a scenario where the plane will take off - WITHOUT MOVING in relation to the ground.

I can imagine a scenario where the plane will take off - WHILE MOVING BACKWARDS in relation to the ground.

(Both of those depend on headwinds. Indeed, its possible to be flying 'forward' while watching your progress on the ground move backwards... now imagine extending your landing gear down to earth and having the wheels touch.)

It has nothing to do with the wheels turning.
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Old January 31st, 2008, 08:56 PM   #176
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Originally Posted by Chris Barcellos View Post
Simple experiment: Get youself a tread mill with unlimited speed. Tie a rubber band to the nose of you plane, and start the tread mill. Pull forward on the nose using the rubber band, but at the same time speed up the tread mill to equalize the forward thrust caused by your rubber band pull, and the plane will remain stationary, and the rubber band will stretch longer, demontrating the actual forces of friction and gravity that is applied.
Great experiment!
Try it.
The rubber band won't get longer .... it'll just pull the the plane forward. :-)

Another variant would be to simply hold the plane in place with the rubber band, and continually increase the speed of the treadmill. If the faster treadmill exerted more and more force on the plane, the rubber band would get longer and longer until it snapped.

Instead, the rubber band will barely register any speed change in the treadmill .... the treadmill doesn't "pull" on the plane.
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Old January 31st, 2008, 09:26 PM   #177
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Instead, the rubber band will barely register any speed change in the treadmill .... the treadmill doesn't "pull" on the plane.
Exactly! It just "pulls" on those parts of the aeroplane that are in contact with the treadmill due to friction. Those parts are the bottom surfaces of the wheels and the wheels turn. This completely decouples the airframe from the treadmill. The treadmill does nothing more than make the wheels spin and has no bearing (!) on the airframe which is where the thrust is generated. Ergo, wave goodbye as the 'plane ventures forth into the sky.
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Old January 31st, 2008, 09:38 PM   #178
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Exactly! It just "pulls" on those parts of the aeroplane that are in contact with the treadmill due to friction. Those parts are the bottom surfaces of the wheels and the wheels turn. This completely decouples the airframe from the treadmill. The treadmill does nothing more than make the wheels spin and has no bearing (!) on the airframe which is where the thrust is generated. Ergo, wave goodbye as the 'plane ventures forth into the sky.
I guess you are saying though the wheels are attached to the plane, by some magic of physics, they are momentarily diconnected from the airframe....
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Old January 31st, 2008, 10:12 PM   #179
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No magic. Just basic mechanics. The aircraft's wheels rotate freely about an axle unlike the wheels on a car or a train which are fixed to the axle.

The only point of the aircraft having wheels is so that it can move on the ground either by thrust from its engines or from a towing vehicle and so that it can accelerate along the runway with minimum friction between it and the ground.

The purpose of the wheels on a car or a train is to convert the energy from the engine into rotational energy, providing a force to which the friction between the wheel and the ground will react, thereby providing the forward acceleration of the vehicle's mass. (F = ma)
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Old February 1st, 2008, 09:47 AM   #180
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A good comparison is wearing roller blades on a running treadmill, hands on the rails on either side.

To "simulate" the thrust of the jets, simply pull yourself using the handrails. You will have no problem rolling yourself off the treadmill.
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