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Old October 24th, 2006, 08:34 PM   #16
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Quote:
Originally Posted by Richard Alvarez
What's happening beneath it is irrelevant.
Have to disagree Richard (respectfully of course). Say my aircraft is sitting in a muddy field and the tires are sinking in. I put the throttle to the wall. The excess ground friction will not allow the aircraft to move forward and it will never get off the ground.

Or, I pull into position on the runway and stand on the brakes while going to full throttle. The aircraft will just sit there as the engine produces full thrust. This is in fact the procedure for short-field take-off.

What's happening beneath an aircraft in contact with the earth is indeed relevant.

The question here is does the counter movement of the conveyor belt create the effect of zero forward movement upon the surface of the earth, like being stuck in the mud.

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Old October 24th, 2006, 08:44 PM   #17
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Quote:
Originally Posted by Sharyn Ferrick
Lift off speed is typically measured in terms of speed over the ground but only for convenience.
But it's always the airspeed indicator you reference, correct? (It has been for me). If you have a 30kt headwind, you take-off roll is over the ground at 30kts less than your airspeed indicator. But as we know, the airplane only cares how fast the air moves over its wings.

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Old October 24th, 2006, 08:56 PM   #18
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Greg.

Irrelevant was a poor choice of words.

We have determined that the mud and brakes PREVENT THE WHEELS FROM TURNING.

The conveyor belt will do no such thing. There is no more friction than when the plane is rolling along at any speed. The engines have the power to move the plane through the air, while the wheels are rolling along the ground.

The 'ground speed' indicator of the wheels on such a takeoff would indicate a nominal double of airspeed, assuming still air.
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Old October 24th, 2006, 08:59 PM   #19
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Quote:
Originally Posted by Sharyn Ferrick
A jet flying at mach 3 crosses over laser tower on the ground. At the same precise moment a laser beam is flashed from the Jet and the tower on a target the same distance ahead. Which laser beam hits the target first ?

Sharyn

They both hit at the same time, as the speed of light is always the same, which I think is 186,000 miles/sec.

Use a bullet and you have a different story though!

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Old October 24th, 2006, 09:05 PM   #20
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Quote:
Originally Posted by Richard Alvarez
Greg.

Irrelevant was a poor choice of words.

We have determined that the mud and brakes PREVENT THE WHEELS FROM TURNING.

The conveyor belt will do no such thing. There is no more friction than when the plane is rolling along at any speed. The engines have the power to move the plane through the air, while the wheels are rolling along the ground.
I wish I still had my RC planes. I'd throw one on a treadmill and try it. And of course I would videotape it all for the amusement of everyone here. (-:

I just like to throw out the various thought processes to have a good discussion so don't take me too seriously on this.

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Old October 24th, 2006, 09:08 PM   #21
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As far as the plane question______

It indeed would take off and fly and here is why.

The conveyor sensing or communicating device you said is measuring "how fast the plane is moving" not how fast the wheels are turning, so if the plane doesn't move the belt doesn't move.

Obviously the plane will move when the thrust is applied and when the belt moves all it will do is change the speed at which the aircrafts wheels are turning.

I'm assumming that you mean that the conveyor belt is moving forward to match the planes speed, so all that would happen is the the plane would take off without the wheels ever making a single rotation. If the belt is going in the other direction, the wheels would simply turn twice as fast until lift off, if the Goodyears hold up! :)

It would save a bunch of tire wear if they go in the same direction!

Mike
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Last edited by Mike Teutsch; October 24th, 2006 at 10:04 PM.
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Old October 24th, 2006, 09:18 PM   #22
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All in good fun Greg. I'm crunching renders, hence my terse replies.

Basically, the thrust from the engines is generated against the wings, transmitted into forward motion. It is incorrect, but often stated, that the thrust from the engines pushes 'against the air'... but for our purposes, that's close enough. The hot air is expelled, pushing the airplane forward. The wheels beneath the plane are there to ease the friction against the ground. They are still free to roll in this example. Hence, the thrust moves the plane forward, the wheels rotate twice as fast, and the plane gets enough airspeed to lift off.

A closer analogy, but still flawed, would be to imagine your person on a treadmill, wearing rollerskates. He holds in his hands, a rope connected to the far wall. The faster he pulls himself forward, the faster the treadmill rotates beneath him. BUT HIS PROPULSION comes from the interaction of his hands and the rope, not the wheels.

Make sense?

Last edited by Richard Alvarez; October 24th, 2006 at 09:54 PM.
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Old October 25th, 2006, 07:01 AM   #23
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Quote:
Originally Posted by Richard Alvarez
A closer analogy, but still flawed, would be to imagine your person on a treadmill, wearing rollerskates. He holds in his hands, a rope connected to the far wall. The faster he pulls himself forward, the faster the treadmill rotates beneath him. BUT HIS PROPULSION comes from the interaction of his hands and the rope, not the wheels.
That would also make for a funny video. (hehe)

The demonstration of Bernoulli's principle I outlined in an earlier post always baffles the uneducated. They just can't believe you can lift something up by blowing DOWN on it.

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Old October 25th, 2006, 09:01 AM   #24
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Nice call Richard. He nailed it from the get go... and with really good examples.

You guys wouldn't believe me if I told you how crazy convorsations get with this question. On another forum I saw one guy who claimed to be an engineer and refused to believe it would take off. That thread was like 20 pages long at least.

And for the record, in my experience probably 95% of people get this question wrong. It just feels so wrong to answer correctly. :)
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Old October 25th, 2006, 09:11 AM   #25
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I'm just curious... how many pilots answered incorrectly?
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Old October 25th, 2006, 10:59 AM   #26
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I'm not sure any pilots were on the forum I referred to. It was a motocross website.

I'll try and dig up the thread. It's worth a read just based on the fact that it will make people not be so hard headed when they are 'positive' they are right. :)

I got this question correct initially but once I thought about it I thought my answer was way to simple. So I second guessed myself thinking there was actually something more to it. So my official answer was wrong :) Next time I'm gonna stick to my instinct.
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Old October 25th, 2006, 11:16 AM   #27
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yes, the plane could take off as it doesn't exert force against the ground to gain momentum...making the treadmill irrelevant.
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Old October 25th, 2006, 11:59 AM   #28
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Quote:
Originally Posted by Sharyn Ferrick
A jet flying at mach 3 crosses over laser tower on the ground. At the same precise moment a laser beam is flashed from the Jet and the tower on a target the same distance ahead. Which laser beam hits the target first ?

Sharyn
Not mensa but:

Plane: Unless conveyor belt is creating airspeed to flow across the wing, it will not fly. What is happening in the scenario provided is that the thrust from the engines trying to get plane up to airs speed, is being offset by an equal counter force, offsetting it.


Laser question: They tie. The speed of light is a constant. Laser is at light speed. Light wave emitted from a single point in time and space will reach a distant object at the same time. Now there may be some red shift, which would allow measurement of relative speeds of the sources, but both beams should arrive at same time.
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Old October 25th, 2006, 12:02 PM   #29
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Quote:
Originally Posted by Greg Boston
The question here is does the counter movement of the conveyor belt create the effect of zero forward movement upon the surface of the earth, like being stuck in the mud.

-gb-
Or you could have a rope tied to the back of the plane with engines at full thrust, still wouldn't lift off. The wings might rip off the plane, though :)
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Old October 25th, 2006, 12:13 PM   #30
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Quote:
Originally Posted by Chris Barcellos
Not mensa but:

Plane: Unless conveyor belt is creating airspeed to flow across the wing, it will not fly. What is happening in the scenario provided is that the thrust from the engines trying to get plane up to airs speed, is being offset by an equal counter force, offsetting it.


Laser question: They tie. The speed of light is a constant. Laser is at light speed. Light wave emitted from a single point in time and space will reach a distant object at the same time. Now there may be some red shift, which would allow measurement of relative speeds of the sources, but both beams should arrive at same time.

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! :)

Mike
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