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Old October 25th, 2006, 04:00 PM   #46
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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
If you assume the perfect wheel and tire with no friction and perfect conveyor, then you might have a point, but science tells us there is no such perfect machine...
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Old October 25th, 2006, 04:30 PM   #47
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Originally Posted by Chris Barcellos
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.
Because the conveyor is not reducing the takeoff roll. The plane still needs to move forward through the air to generate lift. It is doing this and it is moving forward relative to the air and the earth (i.e. the airport) because the jet engines are reacting its thrust against the air. The wheels are just free spinning. They could be spinning Mach 2 and the airplane would still move forward because the thrust of the engine is way more the friction in the free spinning wheels. As long as the force trying to make the plane go forward is more than the force trying to hold it back (friction and drag), it will go forward.

If you locked the brakes and assuming the brakes were stronger than the thrust of the engine and the plane does not skid, then the plane wouldn't move at all and neither would the conveyor (the forces holding it back are now equal to or greater than the force trying to make it go forward). But that's not what the riddle says.
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Old October 25th, 2006, 06:40 PM   #48
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Originally Posted by Tim N Le
Because the conveyor is not reducing the takeoff roll. The plane still needs to move forward through the air to generate lift. It is doing this and it is moving forward relative to the air and the earth (i.e. the airport) because the jet engines are reacting its thrust against the air. The wheels are just free spinning. They could be spinning Mach 2 and the airplane would still move forward because the thrust of the engine is way more the friction in the free spinning wheels. As long as the force trying to make the plane go forward is more than the force trying to hold it back (friction and drag), it will go forward.

If you locked the brakes and assuming the brakes were stronger than the thrust of the engine and the plane does not skid, then the plane wouldn't move at all and neither would the conveyor (the forces holding it back are now equal to or greater than the force trying to make it go forward). But that's not what the riddle says.
The riddle says that the conveyor speeds up in opposite direction, every time the plane does speeds forward. I assume a balancing of forward motion against the reversing motion. The plane stay in one place relative to the air around it, therefore no air speed for lift.

A plane going down a run way, just the same as one on the conveyors is subject to many variable impeding its progess including downward force of gravity, and its affects on bearing, tire flexion, etc.
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Old October 25th, 2006, 07:51 PM   #49
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Chris, in accordance with Newton's third law, what propels an airplane? Is it the:

a) wheels passively spinning against the ground
or
b) engine thrust against the air?

Answer is b. You don't get to add your own set of variables to a thought experiment, so considering wheel friction, etc. is a foul. Try thinking about it this way...what if instead of a giant treadmill it was wet ice and instead of wheels the plane had giant skates. (Sorry, I tried really hard to resist posting to this thread, but I obviously I succumbed. Aerodynamics 101 for non-aerodynamicist: the four primary forces acting on an airplane are thrust, drag, lift, and of course, gravity). Jet propulsion 101 for non-mechanics: all you need to know is "suck, squeeze, bang, blow."
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Old October 25th, 2006, 09:03 PM   #50
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Chris.

The Plane DOES react to the conveyor belt moving backwards. It reacts by rotating it's wheels TWICE AS FAST as it's forward momentum.

Another thought.

A plane is landing at 175 miles per hour. It lowers it's landing gear. The wheels are not rotating. (Or possibly even free rotating in the OPPOSITE direction of travel)

The plane touches down. The wheels do not STOP the plane. Because they are standing still. Yes, there is a great deal of friction, take a look at the end of the runway sometime.

In the scenario described, the wheels will spin TWICE AS FAST as they normally do on takeoff. That is the sum total of the effect of the conveyor belt runway on the aircraft. More wear and tear on the tires and bearings? Sure, but not enough to prevent takeoff.

And for the record, the engines do not "push against the air" the thrust within the engines pushes against the engines, escaping out the back. The total force vector in the engine translates to forward thrust. The engine pushes against the wing/fuselage. The plane moves forward. When it moves fast enough, lift is generated.

LIFT + THRUST must overcome GRAVITY + DRAG for ANYTHING to fly. We know from experience that the engine and wings of the 747 will easily overcome the drag of it's wheels spinning. THAT is the only change in the equation.

Now can I take off my P.I.C. hat and get back to editing?
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Old October 25th, 2006, 10:03 PM   #51
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Quote:
Originally Posted by Richard Alvarez
Now can I take off my P.I.C. hat and get back to editing?
Oh alright, I suppose so. I had my PIC hat on also, but chose to wear it backwards. (grin)

At least we finally pulled Bauer into the thread. And for all the pilots, what are the four left turning tendencies of a single engine, propeller driven aircraft?

-gb-
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Old October 25th, 2006, 10:10 PM   #52
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Quote:
Originally Posted by Pete Bauer
Chris, in accordance with Newton's third law, what propels an airplane? Is it the:

a) wheels passively spinning against the ground
or
b) engine thrust against the air?

Answer is b.
This propulsion will be countered by the conveyer belt.

The plane will not be moving through the air, no lift will be generated.

As the free spinning wheels are countered by the spinning conveyer we can remove this factor from our calculations, the plane would effectively be sat still with its brakes on - it would not lift into the air, it would not fly.
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Old October 25th, 2006, 10:20 PM   #53
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Lee.
The wheels, are in effect, bearings between the plane and the ground. They have NOTHING to do with the propulsion of the aircraft.

The aircraft gets it's thrust and momentum from it's engines.

The aircraft will move in response to the thrust, just as suredly as if there were a giant, NOT STANDING ON THE RUNWAY, pushing the jet along with his hands. He cares not if the wheels are spinning twice as fast as they normally would.

You must resist the assumption that the wheels add to, or subtract from the speed of the aircraft. They are not the drive force, like in an automobile.

(I swear this will be my last post. I can't think of any more analogies to make it plain.)
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Old October 25th, 2006, 10:25 PM   #54
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Quote:
Originally Posted by Nick Jushchyshyn
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.
First pick any six cubes and put three on each side of the scale, this shows you which of the three groups the heavy cube is in.

Repeat the process with the chosen three cubes, one on each side of the scale one stays on the table.
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Old October 25th, 2006, 10:30 PM   #55
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Quote:
Originally Posted by Richard Alvarez
The aircraft will move in response to the thrust
From the original post: 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.


I am presuming that if the plane is moving at 286.3 mph the conveyer belt is spinning at 286.3 mph in the opposite direction and therefore, relative to the surrounding air and ground the plane is static.

In this scenario the plane will not lift into the air.

Like this >> http://img223.imageshack.us/img223/6784/fffsg3.jpg

Quote:
Originally Posted by Richard Alvarez
You must resist the assumption that the wheels add to, or subtract from the speed of the aircraft. They are not the drive force, like in an automobile.
I understand this and make no claims that the wheels would add or subtract from the speed of the aircraft.
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Old October 25th, 2006, 10:34 PM   #56
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Quote:
Originally Posted by Richard Alvarez
And for the record, the engines do not "push against the air" the thrust within the engines pushes against the engines, escaping out the back. The total force vector in the engine translates to forward thrust. The engine pushes against the wing/fuselage.
Richard, I agree the engine pushes against the wing/fuselage and this creates the forward momentum, but the engine is pushing against the air to create this forward reaction thrust. Each blade on the engine is sucking in all the little air molecules and basically pushing it back (as it compresses the air, combusts it and propels it out the back). Because of Newton's third law, as you push on each air molecule, the air molecule pushes back (the reaction force) on the blades and this force then goes to the engine which then goes to the wings and fuselage. The sum of all the reaction forces is the forward thrust.

It would be different if this was a rocket engine. A rocket engine pushes propellent out the back. The propellent pushes back and creates forward thrust. Thrust is not created by the propellant pushing on the air (when the rocket is still in the atmosphere).

But a jet engine take air and pushes it out the back. The air pushes back and creates forward thrust.

Quote:
Originally Posted by Greg Boston
And for all the pilots, what are the four left turning tendencies of a single engine, propeller driven aircraft?
Torque, P-Factor from the blades, Spiral Slipstream...and I forget the last one....dangit.
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Old October 25th, 2006, 11:52 PM   #57
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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.
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Old October 26th, 2006, 12:10 AM   #58
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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.
That is my point, as long as the plane is in touch with the ground it doesn't seem to matter what propulsion is being used. It still has to overcome the inertia and gravational forces that connect it to the conveyor belt. That connection of gravity and inertia is all centered in the wheels. And with the 200 tons a 747, that is a lot to overcome. Once it reaches a ground speed/air speed, it can transition to flight. But the way I reed the scenario, with the belt in reverse running to counter act the forward motion of the plane, it never can get there.
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Old October 26th, 2006, 02:39 AM   #59
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When the plane's engines are fired up, it will move forward on the treadmill, with the only impediment being the relatively small amount of extra drag on the wheels from the treadmill's backward motion. If this is only a theoretical proposal and the treadmill is as long as a full runway, the plane would be able to reach flying airspeed in just a bit longer distance than it would normally. If this is a real experiment, then the treadmill would probably be very short and the plane would quickly roll past its front end and that little problem would be over. The tires and wheel bearings on this aircraft would be able to withstand a doubling of the normal takeoff speed of about 142 knots for the short time involved.

In addition to all the previous remarks about aerodynamic lift and the Bernouli Effect that produces it, don't overlook the major aspect of "ground-effect" lift that occurs when the plane is within a few hundred feet of the surface. This contributes a large portion of the total lift during landing and takeoff. On landing, the deployment of large slats and flaps on aircraft, creates a cushioning "ram-wing" effect, which is an extreme type of lift that is similar to that of ground-effect.

The Russians have developed large air tranport planes that fly across the Black Sea at a typical altitude of about 50 feet, primarily using ground-effect lift. This results from a compression of the air between the aircraft and the surface. Boats have a similar effect on them in shallow water, that is caused by increased pressure between their hulls and the bottom. By flying in the ground-effect zone, these Russian air transports get more than twice as much fuel-efficiency than those flying higher with aerodynamic lift. Their ratio of lift to drag is much higher with ground-effect. However, with boats, the "bottom-effect" slows them down, as the increased pressure speeds up the flow of the non-compressible water around them, raising the drag.

The Human-powered aircraft, such as the Gossamer Condor and others, flew almost entirely on ground-effect lift, staying within a few feet of the surface. Their airspeed was not much more than about 12-15 knots, which wouldn't generate much aerodynamic lift. For a Human to power an aircraft at an altitude that used only aerodynamic lift, it would require a significant extra amount of thrust.
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Old October 26th, 2006, 06:22 AM   #60
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The principle of thrust is the same between a jet engine and a rocket engine. The thrust is genrerated through different MEANS, but the thrust vector in the compression chamber is the net same as the thrust vector in the ignition chamber.

As you stated the 'air' or 'compressed gasses' are PUSHING AGAINST the engine, and escaping out the back.

Independent of what is happening to the wheels.

I've used the example of someone standing on a treadmill, with the rope in his hands, attached to the far wall. As he pulls himself forward, the treadmill spins backwards,at ever increasing speeds, but he's STILL MOVING forward. Why? because his method of propolusion is independent of the connection to the treadmill.

You can also imagine the person standing on the tread mill, and with super human strength, PUSHING AGAINST THE WALL behind him. He would move forward, even as the treadmill moves backwards. (At least untill the force of the intitial push was overcome.) But the engines continue to push as long as you power them.

The airplane will fly.

(It would fly even if it were a prop driven aircraft as well.)
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