Plane on a coneyor belt
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The glider that is being towed is moving relative to the physical ground and in doing so air is moving across and under it's wings which in turn will generate lift, the plane on the conveyor is not moving relative to the physical ground and no air is moving fast across and under it's wing thus no lift.
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Not like a glider really, no. Lift is gained by air rushing towards the plane's wing, and getting pushed underneath it. Thus creating a bed of air that is so dense it can support the weight of the aircraft essentially. But it is the air rushing towards the wing that is important, not the ground speed or relation to any other object. So the plane could sit perfectly still and have a huge amount of air blown towards it and it could lift off that way. That is a wind tunnel, which works.
Chris
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Just to clarify, its not about the speed relative to the ground at all. As in a wind tunnel, the ground and the aircraft move at the same speed - zero, while the air is what is moving. The plane only gets lift in relation to the wind speed coming towards it.
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Basically for the example of the conveyor, there will be no rushing wind, hence no lift as the plane would in essence be stationary relative to the ground and unless a mass of rushing wind suddenly came at it from the front it would remain grounded.
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Chris, i dont understand why this has nothing to do with ground speed. If we imagine a calm day there will be no wind speed, so the plane must first achieve ground speed to gain wind speed.
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no way is that bad boy taking off.
i'll try it at the gym tomorrow, i'll attach some wings to my sides and jump on the treadmill.if this did work, airports would be a hell of a lot smaller. he he
pav
[EDIT] Just walked into my kitchen about 6 meters away. poured a glass of milk, and walked back to the computer, and my A level Physics kicked in, and i realised what a fool i had been.
the plane does of course take off.
feel like such a fool, thank god for the EDIT function eh!
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If it did indeed work then I believe the next aircraft carriers will have treadmill decks and not catapult launchers.
Could a plane then land on a conveyor too?
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Ah!!
If the question was asked like that in Myth busters:
@unknownuser said:
Can an airplane on a conveyor belt running in opposite motion to the airplane's tires at the same speed keep the plane from taking off?
Tires!
Now that changes everything since the tires are not the propulsion method of a plane, hence the propeller/jets will move the plane forward and lift will be possible. The top question indicates that the plane would be stationary.
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I agree Solo. I was under the impression we were talking about a plane w/o a propulsion method. But thinking of it in terms of a jet sucking in air, plus the conveyor belt underneath it, than that changes everything.
Still though, the speed of the aircraft in relation to the ground is not important, which is precisely why the excercise works. The plane can sit still as long as it has enough lift. So its engines are strong enough to suck in the amount of air required to give it loft. Once its wings have enough air flowing over them (under), the aircraft will lift up off the conveyor belt and should then move forward.
So if you've ever seen a bird soar, but in place, its the same idea. I saw a crow floating perfectly still with its wings out, not moving because it found a spot where the wind was flowing steadily at the same speed. It was hitting a freeway berm and the wind headed upwards. So the crow could float on the air w/o having to flap its wings or move forward. So the crow and the ground were moving the same speed, it was the wind that was moving.
Its an interesting question for sure. More complex than I had understood the first time I read it (obviously).
I saw the 2 mythbusters guys on TV last night on Dave Letterman (late night TV show). It was interesting. They also have an open call for volunteers to help hold mirrors in September if you are in the San Francisco area.
Chris
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if we asume that the conveyor belt always keeps the same speed as the airplane's forward motion (which is, what they want, if I understood correctly), thus keeping the jet at zero speed relative to it's surroundings, it can not take off, no matter if it is an aircraft with a propeller, jet engine or even rocket propulsion.
the only thing that matters is, like explained several times already, the relative speed of air rushing past the planes' wings.
If we use a jet fighter instead of a glider it only means, that we need even more speed to take off, because the wings provide a smaller surface.
and because the formula isair-pressure x wing-surface
(at least I think it is like this
) the variable air-pressure needed to increase if the wing-surface is reduced.you can think it the other way arround. if a plane flew happily through the sky and suddenly a big wall (lets say a ufo) would decide to fly directly in front of it with the same speed, thus providing slipstream, the poor plane would simply fall out of the sky...
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@plot-paris said:
...thus keeping the jet at zero speed relative to it's surroundings, it can not take off, no matter if it is an aircraft with a propeller, jet engine or even rocket propulsion.
the only thing that matters is, like explained several times already, the relative speed of air rushing past the planes' wings...
Yes, but your thinking of it how I was too. Its important to note that the question doesn't say that the engines are off, it just says that a conveyor belt keeps it from moveing forward. So the engines are on full blast, thereby sucking the air towards them. So the plane is not standing still relative to its surroundings, because its surroundings are rushing towards it through its propellers/jet engines at a high enough velocity to achive lift, even while sitting still on a conveyor.
Chris
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If it's a Harrier then it WILL take off!
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The problem is the instinctive analogy with a car...which would remain motionless in such a situation. In fact, because the plane gets its propulsion from its props/jet turbines, as soon as the engine is fired the plane will move forward through the air, regardless of what the conveyor belt is doing. The conveyor could be moving at light speed, all it will do is make the plane's wheels spin faster than they normally need to.
A much closer analogy would be if you were wearing roller skates and attached to a winch which pulled you forward at 30 mph, while the conveyor was rolling backwards at 30 mph. The result would be that you would still move forward at 30 mph...but the skate wheels would be doing 60. -
just had a full blown office argument about this, for about an hour on an off, (and this is no exageration).
only one other person in my office agreed with me on the plane being able to fly, and together, we had to convince the other 6 in the office they were wrong.
not an easy task, but one i enjoyed none the less.
at one point, someone went to their car and got out a toy car ( i presume their kid left it in there!) and did a demonstration as to why it would not work.after it was pointed out to him that a cars propulsion was very different from a planes he totally understood. and also cherried up like i have never seen before, it was hilarious!
thanks remus, hours of fun!
pav
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I am afraid I have to disagree with you, Alan.
important to understand is, that if the plane's engines were turned off and the conveyor would start to move, the plane would be moving backwards.
only because it is pushing itself forward with it's engines, it is able to prevent from moving backward, but standing still, relative to the spectators (because, if the plane speeds up, so does the conveyor belt - that is the whole idea after all).
Chris, you are right that the plane is sucking air through it's engines and therefore creating a stream towards itself. but essential is, that it is sucking the air through the engines. therefore no stream of air hits the wings, that would give the plane upwarts thrust.
the only way to move upward is to direct the stream of air towards the ground (which solo's harrier does). but that would mean cheating in the case of this experiment
to make it more clearly, take pav's idea of attatching wings to your arms and running on a treadmill. you can run as fast as you can, you will most certainly not be able to take off.
if you now jump from the treadmill to the side on the ground, you will discover, that your relative speed to your surroundings (and therefore to the air surrounding you) was zero! -
@plot-paris said:
to make it more clearly, take pav's idea of attatching wings to your arms and running on a treadmill. you can run as fast as you can, you will most certainly not be able to take off.
if you now jump from the treadmill to the side on the ground, you will discover, that your relative speed to your surroundings (and therefore to the air surrounding you) was zero!hold on a sec, i added an edit to my post, i believe that the plane WILL take off.
running on a treadmill is very different as the propulsion is from the legs.
the propulsion for the plane is not from the wheels, but the jet, or prop.the air would pass over and under the wings and create lift, all that the conveyor belt does is negate the planes forward movement realtive to the ground, which makes no difference anyway. you need to look at the planes movement relative to the air around it.
the plane will attain lift.
pav
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unfortunately you are wrong in this case, pav.
the air is NOT flowing past the wings. the air is flowing through the jet engine.
you need a big fan in front of the plane to create an air flow past the wings...otherwise you would not be able to stand on a plane's wing (beside the engine) when it is accellerating (well, if there were no conveyor belt, you would obviously fall down from the accelleration
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I have to draw things in order to understand them . . ..I think I solved this one. . . .
No lift.
note. . . this cartoon does not necessarily reflect the views of the this website, its management, or even the guy that drew it. . . .it was just too irresistable.
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beautyful sketch, David!
I like the political criticism
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