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Old October 24th, 2006, 03:18 PM   #1
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OT: Any mensa members here? ;)

Here is a question that will for sure get you guys thinking.

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 October 24th, 2006, 03:29 PM   #2
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currently i am not sure if the take off speed is measured on wind speed or ground speed.
The device you described create a differential ground speed with wind speed =0 , since the plane itself is not moving at all.
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Old October 24th, 2006, 03:32 PM   #3
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That's a good question. Ground speed.

Just to keep things simple...

If the jet was traveling at 250mph that would mean the conveyor belt would be going in 250mph in the opposite direction.
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Old October 24th, 2006, 03:35 PM   #4
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yes, you described the same device that some people use to run indoor.
they can run as fast as they want, they are not moving.
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Old October 24th, 2006, 03:44 PM   #5
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you can put the plane in front of a big fan and have a wind speed that allow the plane to lift vertically while not moving at all again.
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Old October 24th, 2006, 04:02 PM   #6
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No no... no additional devices. Just givin what I said in the first post....... can the jet take off?
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Old October 24th, 2006, 04:56 PM   #7
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if your plane roll forward on a track that runs backward at same speed, you plane is not moving. There is no chance it is taking off that way.
Actually, you have to know how the device on the plane is recording speed, since speed is recorded by unit of distance divided by unit of time.
if the reference is the surface of the conveyor belt, the speed will be different than if measured from its start position.
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Old October 24th, 2006, 06:00 PM   #8
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It would not. The wheels would be turning like mad but the plane would otherwise be still, so there would be no lift.
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Old October 24th, 2006, 06:24 PM   #9
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Okay, I'll bite

If, as you say, the "runway" is a conveyor belt, then we could assume that the conveyor belt is as long as a runway for a 747, so, more than 10,000 feet. As the plane's thrust increases and the belt begins to move, it will also move the air above it. Not efficiently, but when the belt is moving at high speed, it will generate quite a wind. So, I'll say, given the implausible "ifs," a lightly-loaded 747, and a conveyor capable of keeping up with it, the wind created by the moving belt could eventually let the wing produce enough lift to raise the plane off the belt. At that moment, when contact is lost with the belt, the plane would accelerate rapidly and climb away.
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Old October 24th, 2006, 06:36 PM   #10
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Quote:
Originally Posted by Giroud Francois
Actually, you have to know how the device on the plane is recording speed, since speed is recorded by unit of distance divided by unit of time.
In aircraft, you are only concerned about ground speed in determining how long it will take you to go from point A to point B. For making the aircraft fly, you must have airspeed, measured by the differential of a static source and a pitot tube which faces the oncoming wind. Airplanes fly when air moves over their wings at sufficient speed to cause lower air pressure above the wing and higher pressure on the bottom (Bernoulli's principle). The pressure difference is created by air being forced to travel a greater distance over the curved top of the wing which increases its velocity because it will try to reach the rear of the wing at the same time as the air moving below it. Air moving at a higher velocity will have lower pressure.

A fun way to demonstrate Bernoulli's principle is to take a piece of cardboard or any lightweight flat object and place it with your hand an inch or so above it. Now, take a compressed air gun and place it between your second and third fingers and blow straight down on the paper. It will lift up and stick to your hand because you are creating high velocity air traveling outward over the top of the paper.

As for the original question, it still comes down to airspeed. If the plane was facing into a headwind with sufficient velocity, the plane will fly regardless of its speed on the ground.

-gb-
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Old October 24th, 2006, 06:36 PM   #11
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Yes, the plane will take off.

The aircraft develops thrust by pressure WITHIN the engine. It is independent of contact with the ground.

Unlike people on a treadmill, who must push against the ground to move forward, the airplaine's thrust is generated WITHIN the engine.

It will fly.
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Old October 24th, 2006, 06:50 PM   #12
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Originally Posted by Richard Alvarez
Yes, the plane will take off.

The aircraft develops thrust by pressure WITHIN the engine. It is independent of contact with the ground.

Unlike people on a treadmill, who must push against the ground to move forward, the airplaine's thrust is generated WITHIN the engine.

It will fly.
But the thrust of the engine is used to overcome the friction of the ground and move the plane forward until sufficient airspeed is achieved for flight. If that forward inertia creates airspeed which is then fed to the conveyor to move it backwards at that speed, the wheel speed should increase, but the aircraft will not move forward.

However, if a big gust of headwind came along and got the wheels off the conveyor, the thrust of the engines would then be the sole factor and the aircraft would accelerate above the ground until it could lift off.

Just playing devil's advocate here, Richard.

-gb-
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Old October 24th, 2006, 06:54 PM   #13
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Planes take off from rivers all the time.

Planes take off from frozen lake beds.

The thrust from the engine, pushes the aircraft in the opposite direction THROUGH THE AIR.

What's happening beneath it is irrelevant.
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Old October 24th, 2006, 08:15 PM   #14
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A fixed wing (Non Harrier) plane takes off based on the air flow over and under the wings. Lift off speed is typically measured in terms of speed over the ground but only for convenience. This is why the plane takes off into the wind, and air craft carriers point into the wind, and power head.

If the airflow over/under the wing is not sufficient to create lift, the plane will not take off

Sharyn
Having flown turbo prop/jet aircraft for years ;-)
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Old October 24th, 2006, 08:21 PM   #15
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Another Brain Teaser

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
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