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Old November 19th, 2006, 04:40 PM   #106
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For the last time.

The airplanes forward movement is NOT offset by the converyor belt.

There is NOTHING offsetting, or counteracting the thrust of the engines.




AB SO LUTELY NOTHING is offsetting the thrust.

The converyor belt interacts with the wheels, and the wheels ROTATE twice as fast, to offset the conveyor belts backward movement. It's as simple as that.

Simple Math.

As I stated in POST #11 and other posts thereafter. In order for the thrust of the engines to be 'negated' an equal and opposite thrust must be provided. AGAINST THE AIRFRAME. For this is where the thrust is being applied. There is no force being applied against the ground by the wheels in order to get the plane moving, such as there is in a car to get a car moving.

The movement of the conveyor belt applies friction againts the wheels, and the wheels ROTATE in response to this friction.

That is the function of the wheels. It takes comparitively little force to get the aircraft moving, in fact. Anyone recall the truck commercial (Dodge I think) that had a truck towing a 747? They made a big deal about it untill volkswagon towed one. The wheels and bearings are designed to minimize the friction.

If your plane was attached to a rope anchored a the front end of the runway, and the converor belt was started up, with the engines OFF - the wheels would ROTATE under the plane, without the plane moving. THE wheel rotation is INDEPENDENT of the planes 'ground speed' because of the static force of the rope, which acts on the airplanes body independant of the rotation of the wheels and speed of the converyor belt.

Same for a person in rollerskates standing on a treadmill and holding onto the side bars. The wheels rotate at whatever speed the treadmill passes under them. The motion of the person standing is independent of the treadmill, because of the static force between his hands and the rails. He can even PULL himself forward on the treadmill, and the wheels on the rollerskates will rotate FASTER than the treadmill, because of the added forward momentum. Because his arms on the sidebars are INDEPENDANT FORCES acting on his body, not interconnected or related to the treadmill. Just. Like. The. Engines.


The Planes thrust is generated independent of the motion of the wheels. The planes forward motion is NOT generated or determined by the velocity of the wheels rotation. The planes wheels are simply acting as 'bearing's to 'lubricate' the friction between the ground and the plane. The wheels are not turning when the plane lands, and that doesn't stop the plane. They simply accelerate up to speed as the plane lands. That is their function. As the plane applies thrust it will move forward. The converyor belt will move backwards, and the wheels will rotate at twice the speed they normally would in response.

The. Plane. Will. Fly.
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Old November 19th, 2006, 05:49 PM   #107
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One final illustration for those who have difficulty understanding that the plane will fly because its motive force is independent of its relationship of its wheels to the ground.

Assume a HUMMER is parked on our imaginary runway. The runway is rigged to respond to the Hummer’s forward motion, by moving BACKWARDS at the same speed. You get in, and start the engine, and accelerate. As the wheels TURN to move the vehicle forward, the conveyor belt moves BACKWARDS at exactly the same speed. The net forward movement of the Hummer is ZERO. (At least to an independent observer standing off to the side. The speedometer will read acceleration to the driver.)

It is exactly like walking on a treadmill and having the tread mill match your speed. It is just like walking on an escalator, at the same speed, and standing still. Indeed, who among us has not done this? It is such a simple concept, that we are inclined to apply the same physics to our aircraft.

That would be a mistake.

For unlike the Hummer, or a Human on a treadmill or escalator, forward motion is NOT generated by exerting force against the ground, (And having the ground ‘push back’)

THAT IS THE FALACY in assuming it will stand still.

Now, back to the Hummer. It is still sitting on our runway, but this time we have modified it. We have disengaged the wheels from the engine completely. They are free to rotate. We HAVE modified the engine however, as we have connected it to a winch, mounted on the front of the vehicle. The winch is attached to a steel cable, a mile long. The other end of the cable is attached to a post, OUT BEYOND the end of the runway. It is not “connected” in any way to the surface of the runway.

The conveyor belt runway is still ‘rigged’ to move backwards at the same speed that the Hummer moves ‘forward’.

We start the engine, and engage the winch, which starts ‘winding’ the cable onto its drum, PULLING THE HUMMER FORWARD. (We also know that the engine, towline and winch are strong enough to tow the Hummer forward, because we have used them before, just as we know the engines of the aircraft are more than strong enough to generate speed for takeoff)


What does the conveyor belt do? Why, it’s rigged to move in the opposite direction… so it does that.

What do the WHEELS on the hummer do? Why they rotate, because they are free to do so, having been disengage from the drive train in any manner.

What does the HUMMER do? It moves forward, being ‘towed’ along by the cable attached to the drum. IF we ‘accelerate’ the winch, the Hummer’s forward motion will also accelerate.

In response, the conveyor also accelerates.

The wheels turn TWICE AS FAST as they normally would to an observer standing by the side of the conveyor belt runway.

The hummer MOVES FORWARD because the method of impulsion is not coupled to the interface between ground and tires. Even though the ENGINE for it’s movement is still attached to the Hummer’s body, it is applying a force that is NOT generated AGAINST THE GROUND (as it’s wheels were before.) It is independent of this.

Same thing would happen to our person in roller skates on a treadmill, holding a rope, and ‘pulling himself along’ by his arms. The roller skates would simply rotate at a speed that would appear to be ‘greater’ than the normal forward velocity, but there is nothing impeding his forward motion.

In the examples of the Hummer and the Human, the impulsion forces acting on the ‘bodies’ (fuselage, airframe) of both, are independent of the interface of the Hummer and Human with the treadmills. The impulsion force is independent of the speed of the Hummer’s wheels, or those of the Human’s roller skates.

When discussing the four factors affecting the airplane, LIFT, THUST, GRAVITY and DRAG, the only element that is being altered is DRAG. The drag of the runway AGAINST THE WHEELS. But the wheels are designed to rotate at ANY SPEED NECESSARY to facilitate a smooth interaction between the aircraft and the ground., and so eliminate, or minimize such a force. So they will simply rotate twice as fast as normal, and the plane will lift off.

The. Plane. Will. Fly.
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Old November 19th, 2006, 06:19 PM   #108
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Richard, the problem as posed introduces a feedback mechanism that causes the conveyer belt speed to match "the speed" of the airplane. I put the speed in quotes because the problem doesn't state how that speed is measured. If that speed is the forward speed of the aircraft, then my interpretation is correct. Aircraft speed is measure using an airspeed indicator and uses air pressure, not rotational speed of a wheel. If it is the wheel rotation, then you are closer to being correct, and good enough for a thought experiment.
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Old November 19th, 2006, 06:39 PM   #109
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I am a pilot, I know how airspeed is measured in flight, and the difference between airspeed and groundspeed.

The thought experiment postulates that as the aircraft moves forward, the conveyor belt moves 'backwards' matching its speed.

The 'apparent' speed of the aircraft to an observer standing off to the side of the runway/belt is a continual accelleration forward.

The 'apparent' speed of the aircraft in IAS (indicated airspeed) in the cockpit will be EXACTLY the same as if the aircraft were taking off from a stationary runwary. (Assuming still air.)

The 'apparent' ground speed, if reading off of a speedometer attached to the wheels, would indicate TWICE the apparent ground speed to an observer standing off to the side of conveyour belt.

The plane will fly, even if the conveyor belt is moving at twice or three times the speed of the aircraft.

The conveyor belts motion has very little effect on the aircraft's forard motion, it's main effect is the rotational speed of the wheels.

The Planes FORWARD motion is NOT offset by the conveyor belt/runway. The Planes ROTATIONAL WHEEL SPEED is. That is the only affect the belt has on the plane.

The. Plane. Will. Fly.
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Old November 19th, 2006, 07:04 PM   #110
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Quote:
Originally Posted by Richard Alvarez
I am a pilot, I know how airspeed is measured in flight, and the difference between airspeed and groundspeed.
Oh good. Me too. We'll have to go flying sometime.

Quote:
Originally Posted by Richard Alvarez
The thought experiment postulates that as the aircraft moves forward, the conveyor belt moves 'backwards' matching its speed.
Good so far, but to make absolutely certain we agree on this point it is my understanding of the statement of the problem that the conveyer belt moves the AIRCRAFT backwards at a rate that instantaneously compensates for any detected forward movement of the aircraft as measured by the airspeed indicator. This implies that the ASI will always register 0.

Quote:
Originally Posted by Richard Alvarez
The 'apparent' speed of the aircraft to an observer standing off to the side of the runway/belt is a continual accelleration forward.
This is incorrect, but irrelevant. The only thing that matters in this problem is the airspeed registered on the ASI, which you have established as 0.
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Old November 19th, 2006, 07:51 PM   #111
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I rescind my previous statement...at some point the bearings of the wheel would fail, connecting the airframe to the treadmill...then the treadmill would be pushing directly against the airframe and would also fail...with all the available pieces disconnecting the airframe from the ground, the airplane would melt into oblivion and not fly!
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Old November 19th, 2006, 08:08 PM   #112
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Okay, we're going to leave out bearings failing or tires blowing out, for the same reason we're going to leave out the pilot spilling coffee on his lap and hitting reverse thrust, or a flock of geese getting sucked into the engines.

Now, lets go back to the original stipulations of the thought experiement.

"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."

The 'device' communicates with the conveyor belt to tell it how fast the plane is travelling.

Since the airspeed indicator is our 'speedometer' then it would be sending it's apparent airspeed to the conveyor belt.

The engines apply thrust.
The plane moves.
The airspeed indicator advances.
The belt moves backwards.
The wheels ROTATE at twice the speed they normally would.

The plane flies. The airspeed indicator would always read what it normally would at takeoff. (Assuming still air, as no head/tail/or crosswinds were stipulated.)

There is a curious effect, IF the 'device' measuring the planes 'speed' were NOT an airspeed indicator, but rather a speedometer attached to the wheels... such as in an automobile. (I've never flown a 747, only small single engine craft and sailplanes... NO engines! More fun..) then there would be a difference between the speed of the belt and the plane.

At any rate. IF the 'device' sensing the airplanes 'speed' WERE a speedometer sending it's speed to the runway, THEN the speed of the runway would NOT match the forward speed of the aircraft. It would ACCELLERATE at EXPONENTIAL rates. (The sensor in the wheel says it's moving at ten knots, the runway accellerates, the wheel rotates twice as fast, more accelleration, ETC... ALL INDEPENDENT OF THE AIRSPEED of the Jet through the air. As, indeed groundspeed is always independent of IAS or TAS)

At some point, would the bearings fail and the tires blow before the plane rotates? I don't know. But again, I don't know if the geese fly into the engine, the pilot spills his coffee, or the device that powers the runway/belt runs out of fuel... all irrelevant to the thought experiment.

The point is, the movement of the ground beneath the plane has no bearing on the planes forward movement through the air, even if the plane is trailing it's wheels on the ground. (Yes, yes, increased friction in the bearings, wear/tear on the tires aside please... the plane still moves forward.)


(Jim, please note that the thought experiment does not say the belt moves the airplane backwards. It says that the speed of the belt is set by the airplane, and that the belt moves 'in reverse'. I never said the airspeed would indicate 0)
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Old November 20th, 2006, 05:11 AM   #113
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If the conveyer belt does not move the airplane backwards at a rate that equally and instantaneously compensates for its forward motion then I agree with your analysis.
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Old November 20th, 2006, 06:21 AM   #114
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That's one heck of a very long conveyer belt by the way. Assuming it ain't a STOL aircraft...
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Old November 20th, 2006, 06:26 AM   #115
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The conveyor belt only moves 'in reverse' in response to the planes forward motion... being driven by the thrust of the engines. It cannot/does not/ move the plane backwards, or cause it to remain stationary.

And with that,

The earth IS round.

It revolves AROUND the sun.

Elvis is STILL dead.

The. Plane. Will. Fly.


I'm outahere....
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Old November 20th, 2006, 09:58 AM   #116
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Well, actually the earth is very close to, although not perfectly, spherical and it orbits around a mutual center of mass with the sun (and other masses wandering around the neighborhood).
:-)

But I do also really believe that Elvis is still dead...and earnestly hope this riddle and thread will be equally deceased!
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Old November 20th, 2006, 10:56 AM   #117
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I shoulda changed the topic of this thread to "OT: Any pilots here? ;) "
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Old November 20th, 2006, 04:06 PM   #118
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If Elvis is in the plane versus standing on the conver belt, the plane won't fly. It would ruin the thrust/weight ratio versus if he were standing on the belt which would slow the belt down, but have no effect on the friction in the planes wheel bearings thus reducing drag and rotational mass of the wheels because this time of year the earth is farthest from the sun keeping gravitational pull to a minimum allowing the planes wheels to spin at maximum thermal and frictional efficiency and allowing tire expansion to a maximum at higher speeds. Also note that cooler air tempatures produce more dense air and will produce more engine power, allowing the jets engines to overcome the weight of Elvis standing on the conveyer belt.

This Plane Won't Fly.
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Old November 20th, 2006, 04:27 PM   #119
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Adam, Adam , Adam,

If Elvis is on the plane...

It's ALREADY flying....
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Old November 20th, 2006, 04:30 PM   #120
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No way man, even with Elvis on the plane, the plane will fly.

The reason is down in the avionics bay underneath the cockpit is a Rockwell Automation Retro-Encabulator that not only provides inverse reactive current for use in unilateral phase detractors but is also capable of automatically synchronizing cardinal grammeters. Check it out for yourself:

http://video.google.com/videoplay?do...89882805593268

Case closed. This baby is going to fly.
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