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Old October 25th, 2006, 12:41 PM   #31
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Quote:
Originally Posted by Mike Teutsch
Chris my man,

There is no counter force applied to the wheels, they just roll. Thrust of the engines will move the plane. Read my full post for the explanation.

And as far the laser goes, I already answered it! :)

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You must be Mensa, I'm not, but there has to be a counter force being applied to make the plane remain stationary. You have jet engines asserting force that will make the plane move forward by overcoming inertia from the plane's weight, friction from tires on ground, friction in the ball bearings, and, then you have conveyor belt running in an opposite direction overcoming the force of the get engines, to extent that it is not allowing the jet to move forward. That would have to be a counter force with respect to the planes mass being moved forward. In other words to drive that belt fast enough to offset the speed the plane would normally attain on stationary ground, energy is applied to creat a static stationary plane.....
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Old October 25th, 2006, 12:56 PM   #32
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So then the bearings would fail? ;)
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Old October 25th, 2006, 01:11 PM   #33
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Quote:
Originally Posted by Chris Barcellos
You must be Mensa, I'm not, but there has to be a counter force being applied to make the plane remain stationary. You have jet engines asserting force that will make the plane move forward by overcoming inertia from the plane's weight, friction from tires on ground, friction in the ball bearings, and, then you have conveyor belt running in an opposite direction overcoming the force of the get engines, to extent that it is not allowing the jet to move forward. That would have to be a counter force with respect to the planes mass being moved forward. In other words to drive that belt fast enough to offset the speed the plane would normally attain on stationary ground, energy is applied to creat a static stationary plane.....
The belt speed is inverse of the planes speed not the wheel speed. In order for the belt to move, the plane has to move and move it will.

The wheels, belt direction like I said, will either go twice as fast as normal takeoff speed or not move at all.

The belt has no real effect on the plane speed, just the wheel speed.

Mike
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Old October 25th, 2006, 01:27 PM   #34
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The initial post:

"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?"

Reach air speed to create lift to overcome gravity is the issue, not the speed of the ground underneath the plane. I still say in the scenario presented that every time the jets rolling speed was increased, there is a counter reaction with respect to the belt underneath, there would be virtually no change in speed of air over the wing. Therefore gravity can't be overcome, and flight not attained.

That is why planes take off into the wind. IF you take of with the wind behind you, you have to attain a higher ground speed to reach air speed, and vice versa.
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Old October 25th, 2006, 01:36 PM   #35
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Cole summed it up perfectly:
Quote:
Originally Posted by Cole McDonald
yes, the plane could take off as it doesn't exert force against the ground to gain momentum...making the treadmill irrelevant.
Yes the plane will take off.
Thanks to the wheels, the motion of the belt does not apply force/drag on the plane, so it continues to move forward, build up speed and fly.


Here's two more .... (I was actually asked these once as part of a job interview)

A) You have nine cubes that look exactly alike. Eight have exactly the same weight, one weighs more than the others. The only tool you have to measure the weight of the cubes is a balance (like the Justice statue), but you are only permitted to use it two times.

How do you find the one cube that has a different weight from the rest?

B) You're in a room with no clock, but you can hear a bell clock in the next room that you know rings once every 15 minutes, and rings out the hour at each o'clock (rings 5 times at 5 o'clock, 6 times at 6 o'clock, etc.).

You just woke up and hear the clock ring once. What's the longest amount of time you may have to wait before being absolutely certain about what time it is? (AM/PM not a factor)
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Old October 25th, 2006, 01:59 PM   #36
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Hehe, here we go. :D

Chris, the difference is that a plane isn't propelled by it's wheels. It's propelled by the jets which suck air in and blow it out... thus propelling the jet forward. The runway/conveyor belt has absolutely no effect. At all.

If you switched the jet with a car then yes. It wouldn't move anywhere because it IS propelled by it's wheels. Unlike a plane which has free spinning wheels.

Btw, in my initial post I didn't say anything about friction, ropes or any of the other things you guys are trying to introduce to the question. It's a set of widgets you have to problem solve with. And no you can't phone a friend either. ;)
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Old October 25th, 2006, 02:12 PM   #37
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Quote:
Originally Posted by Gene Crucean
Hehe, here we go. :D

Chris, the difference is that a plane isn't propelled by it's wheels. It's propelled by the jets which suck air in and blow it out... thus propelling the jet forward. The runway/conveyor belt has absolutely no effect. At all.

If you switched the jet with a car then yes. It wouldn't move anywhere because it IS propelled by it's wheels. Unlike a plane which has free spinning wheels.

Btw, in my initial post I didn't say anything about friction, ropes or any of the other things you guys are trying to introduce to the question. It's a set of widgets you have to problem solve with. And no you can't phone a friend either. ;)
I denied being mensa, but my common sense says:

Yes but initially, before flight is achieved, in order to fly, the mass of the plane must be moved through the air in order to gain speed. This is achieved by rolling against the ground to gain the air speed necessary. In this hypothetical, every time the plane is inched forward, a signal is sent to conveyor to speedup, and that pushs the plane back. The wheels are not without some friction to enable that to happen. As power is applied, it continues the forward thrust continues to match the reverse thrust of the conveyor belt. So plane is not moving with relation to surrounding air, so no lift-- no flight. I guess I need a MENSA guy to pound something into my head...
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Old October 25th, 2006, 02:37 PM   #38
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Chris,

Think of it this way. If that plane is sitting on the runway and all of a sudden the belt takes off at 200 mph, is the plane going to immediatly start moving at 200 mph, no. It has wheels and only a gradual increase in speed will occur, in proportion to the friction in the wheel bearings. But, the planes engines will move the plane no matter what direction the belt is going.

Mike
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Old October 25th, 2006, 02:37 PM   #39
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I was initially confused, thinking about the Bernoulli effect as it was explained to me some years ago; "The engines don't lift the plane, the wings do. If the air isn't moving over the wings, the thing won't lift off." But then I saw the explanations about a jet's propulsion, and kicked myself for being so dumb (of course the wheels don't move it forward, how could I forget a thing like that?).

To make it simpler, can't we ignore the conveyor belt? During a regular takeoff, the aircraft is being propelled down the runway with its engines. The wheels are rolling forward, and the ground is moving backward at the same speed (in a sense...it's not, really, but we can look at it that way, can't we?). There is friction between the ground and the tires, the tires and the wheels, the wheels and the axles, and even the surface of the vehicle and the air, but the thing still gets itself airborne, correct? Friction doesn't seem to matter, unless it's so strong as to prevent the craft from moving through the air, which in this case it's not.

Those engines are strong.
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Old October 25th, 2006, 02:50 PM   #40
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-and that pushs the plane back.

Why?
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Old October 25th, 2006, 03:18 PM   #41
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I have to admit when I first read this I put my pilot hat on and quickly thought it would not fly because as a pilot the first thing you think is that to generate lift you need relative airspeed over the wings and if the plane is not moving there will be no relative airspeed (assuming no wind).

But then when I put on my engineer's hat, I realized the plane's thrust is being reacted by the air, not the conveyor, so it will move forward through the air no matter how fast the conveyor is moving (and eventually take off). The key is that the wheels on a plane are free spinning so it doesn't matter what the conveyor is doing as long as the jet engines' thrust is much greater than the friction in the wheels. The plane's jet engines are pushing against the air so it will move forward relative to the air and fly.

Another way to think about this is to ask yourself, would a submarine move through a tank of water even if it was connected (through free spinning wheels) to a track underneath it that was moving in the opposite direction? The answer is yes because the submarine's propeller is pushing against the water which is independent of the track. This pushing will propel the submarine forward through the water no matter what the track is doing. Now replace the water with air and you have the same thing with the airplane.
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Old October 25th, 2006, 03:22 PM   #42
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Quote:
Originally Posted by Gene Crucean
-and that pushs the plane back.

Why?
What pushes the plane back? If you are talking about the belt, it is just turning the wheels.

Mike
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Old October 25th, 2006, 03:38 PM   #43
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It makes for a fun discussion

IF the pilot locked the brakes, then it would be interesting, since outside of military aircraft which can climb straight up, the thrust of the engines is not equal to the weight of the aircraft. This is why you can stand on the brakes and rev the engines and not move. IF the wheels are not locked, then as stated, they have no effect since they are there simply to reduce friction and the plane would take off.

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Old October 25th, 2006, 03:54 PM   #44
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Quote:
Originally Posted by Tim N Le
I have to admit when I first read this I put my pilot hat on and quickly thought it would not fly because as a pilot the first thing you think is that to generate lift you need relative airspeed over the wings and if the plane is not moving there will be no relative airspeed (assuming no wind).

But then when I put on my engineer's hat, I realized the plane's thrust is being reacted by the air, not the conveyor, so it will move forward through the air no matter how fast the conveyor is moving (and eventually take off). The key is that the wheels on a plane are free spinning so it doesn't matter what the conveyor is doing as long as the jet engines' thrust is much greater than the friction in the wheels. The plane's jet engines are pushing against the air so it will move forward relative to the air and fly.

Another way to think about this is to ask yourself, would a submarine move through a tank of water even if it was connected (through free spinning wheels) to a track underneath it that was moving in the opposite direction? The answer is yes because the submarine's propeller is pushing against the water which is independent of the track. This pushing will propel the submarine forward through the water no matter what the track is doing. Now replace the water with air and you have the same thing with the airplane.

So why don't we just build conveyor belts on carriers and at airports ? And if I have my 747 sittign with lock brakes at full throttled, assuming planed doesn't rip apart, it should fly lift straight up because of the air flowing through the engines ?. I air speed over the wings it what gives lift, not air being sucked through engines. That provides the speed to overcome inertia to get the speed needed.
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Old October 25th, 2006, 03:58 PM   #45
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Quote:
Originally Posted by Sharyn Ferrick
It makes for a fun discussion

IF the pilot locked the brakes, then it would be interesting, since outside of military aircraft which can climb straight up, the thrust of the engines is not equal to the weight of the aircraft. This is why you can stand on the brakes and rev the engines and not move. IF the wheels are not locked, then as stated, they have no effect since they are there simply to reduce friction and the plane would take off.

Sharyn
And to take that concept one further, suppose you are on a sheet of ice and stand on the brakes. The wheels won't turn, but with a lack of friction between wheels and the ice, the plane could conceivably slip forward and get enough airspeed to fly.

As to the water analogy made earlier, if you had an air boat like they use in the swamps, you could face the boat into the current then apply just enough throttle to hold your position relative to the stationary landmarks on either side of you on dry land (zero groundspeed). Or, put yourself in the water and swim. If the water current is moving against you beyond what thrust you can generate with your body, you will be swimming like mad but getting swept downstream none the less. If you were in still water and started to swim forward and someone could measure your progress and apply a current opposite to your efforts, you will swim ever faster trying to move forward, but you wil go nowhere.

Hehe, we can have a lot of fun with vector math. Just as every pilot knows how to calculate a wind correction angle to maintain a straight path over the earth. Or like when I use the trolling motor on my bass boat to face into the current and turn the prop just ehough so that I don't move relative to the landmarks (again, zero ground speed).

I kind of agree with Chris in that as thrust tries to push the aircraft forward, the conveyor belt will increase its opposite direction to keep the aircraft from moving relative to a stationary landmark (zero ground speed). Or, let's say the engines on this jet are shut off. What would happen to the aircraft if you turned on the belt?

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