re: Myth Busters/Can a plane take off on a conveyor belt

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well you would be wrong

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How so, kind sir?

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If I understand the question correctly now, then JustMe is correct.

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Like I was trying to explain earlier... but it's not simple to comprehend.


ETA: or to explain for that matter.

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Guess I'll turn in my degree now.

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Nah, I'm a pretty smart guy and this one had me arguing the plane wouldn't take off the first time I read through it.

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If they are talking about getting the plane up to takeoff spead relative to the surrounding air, then lift.

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YES.

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If they are talking about getting the plane up to takeoff spead relative to the treadmill, then no lift.

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But the treadmill does not move the air. The engines (prop or jet) are pushing against stationary air, or better yet, air that is piling up.



Separate air speed for thrust from air speed for flight. As the plane moves backwards, the thrust is increasing because the thrust is pushing against air that is pushing against the thrust. The reverse motion of the plane cause the air to move towards the back of the engine. Thrust increases. This is good for getting the plane to start moving forward. The engine is not just pushing against a stationary object; it's pushing against something that is pushing back.

The plane cannot take off yet. Because the wings need positive speed. However, the reversd motion of the plane, or negative speed of the plane, will become zero and then become forward motion or positive speed relative to the earth until it reaches ALMOST 100 knots, at which point the plane flies.

You guys are acting like the plane has an infinite top speed. The thrust of the plane allows the plane to move forward but it will top out just like a car will. It doesn't matter where the thrust or propulsion comes from. The wheels facilitate the forward motion until the plane has enough speed to create lift and become airborne. If the conveyor is counteracting the forward motion then their will be none.

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ETA: does this qualify for the most intellectual post on the OT ever?

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Maybe... this same exact discussion went a lot longer one time... probably shorter this time around because we've seen this before.

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It's about relative motion not what's being pushed against what. I see what your saying with the car analogy but if the wind is strong enough the wheels will begin to spin out and the car will remain stationary.

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It's about force.

Let's take your analogy. A car on a road in a hurricane with 100 knot wind. The wind is the conveyor belt. The wind starts pushing that car backward until the velocity = -100 knots.

Then you turn on the engine. Once you put the car in gear (forget about dropping the transmission for a second), the wind won't have enough linkage on the car to negate the thrust of the tires.

In this analogy, linkage is easy to explain. If the car is really a camper, there's a lot of linkage, kinda like friction. If the car is a Ferrari, there's very little linkage.

The linkage is what allows the force of the wind to negate the force of the car. The greater the linkage, the greater the reduction.

The plane's free-spinning wheels simply don't allow the conveyor belt to achieve much linkage with the plane. Thus, the force of the conveyor belt cannot negate the foce of the engines.


Prop planes do the same thing. Props push air back. As the prop pushes the air back, the air pushes the prop (and the plane to which it connected = perfect linkage) forward with an equal and opposite force. Thus, it doesn't matter if it's a prop or jet engine.

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You guys are acting like the plane has an infinite top speed. The thrust of the plane allows the plane to move forward but it will top out just like a car will. It doesn't matter where the thrust or propulsion comes from. The wheels facilitate the forward motion until the plane has enough speed to create lift and become airborne. If the conveyor is counteracting the forward motion then their will be none.

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Take it one step at a time.

Q: A plane is sitting stationary on a treadmill that is not moving and the engines are not running. Start the treadmill up. What happens?
A: Let's assume that there is no friction in the bearings. Unlike a car, the wheels are not connected to a drive train. The wheels of a plane are free to rotate without any extra help from the engines. The plane stays stationary.

Q: You now have a stationary plane with a treadmill running under it. Fire up the engines. What happens?
A: The plane itself (cockpit, fuselage, everything) will start to move forward. NOW AND ONLY NOW does the plane start moving relative to the air arround it. So now we have the engine pushing the plane up to takeoff speed. The only forces the engines have to overcome are the drag from the air (which as you know happens in a normal takeoff) plus the friction in the bearings. Any jet engine better be capable of doing this or crash, boom.

I hope this explanation helps.

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Well in your scenario the treadmill could shorten the distance required for take off but it couldn't take off on a treadmill that isn't several thousand feet long.

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Right. The treadmill would have to be long enough to allow the plane to get up to spead.

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If the conveyor is counteracting the forward motion then their will be none.

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Bingo. But the conveyor does not counteract the forward motion of the plane because the only linkage between the conveyor belt and the plane is the friction of the wheels.

Last attempt. (I think the insubstantiality of the air that the engines are pushing against is causing the problem.)

Put the plane on superslick skates on a frictionless conveyor belt. Put a person sitting on the trailing edge of each wing facing towards the rear. As they face the rear, there is a tractor trailer to the rear of them. They can sit on the trailing edge of the wing and push off the tractor trailer to make the plane go forward.

Start the conveyor belt. Because there is no friction, the plane doesn't move. The conveyor belt just slides underneath its skates.

Now the two people push of the tractor trailer. The only force they need to make the plane go forward is enough force to overcome the inertia of the plane. The tractor trailers follow them, and they push off again, and the plane is moving forward relative to the earth. Theoretically they could continue this until the plane is traveling at 100 knots. That plane is flying.



There are only two differences between that example and the one we've been discussing throughout this thread. First, wheels have some friction, but that is merely a matter of degree. Just imagine frictionless wheels. Second, the people are clearly more substantial than the air, but that also is merely a matter of degree.