Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: moses lawn on 12/10/2004 21:44:59
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When an aeroplane passes low over your head, why doesn't the weight of the aircraft crush you to death?
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Maurice....I think the plane has to literally land on your head to crush you !!!!! ....if it just passes low OVER your head then it hasn't even touched you and that is why you're not crushed to death.
'Men are the same as women...just inside out !' (https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fwww.world-of-smilies.de%2Fhtml%2Fimages%2Fsmilies%2FSchilder2%2Finsanes.gif&hash=4f18432872d0188852a6f4a3170ec758)
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The airplane is primarily held up in the sky by the weight of the atmosphere, which is already weighing down on you. The airplane wing is shaped to force the air traveling over the top of the wing to travel a longer path than that flowing over the bottom. The air has to speed up, and its pressure on the top of the wing is reduced, compared to that on the bottom. This is known as Bernoulli's principal. (Daniel Bernoulli, 1738). The airplane flies on this pressure difference, pushing the wings upwards, supporting the airplane's weight.
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So what happens when an aeroplane flies upside down ? The wing shape is reversed isn't it, so why doesn't the plane fall ?
Chris
"I never forget a face, but in your case I'll make an exception"
- Groucho Marx
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There's always a heckler...
As a matter of fact some airplanes DO fall out of the sky if they fly inverted. And I don't recommend you try it in a helicopter.
The answer is that some airfoils are compromised to allow it. The bottom of the airfoil has curvature as well as the top, but generally less. The rest of the trick is to control the angle-of-attack between the airfoil and the slipstream. With the airfoil pointed somewhat above the slipstream, the air still flows a longer distance over the top of the wing than below it. Hence, lift. The inverted lift will be less than the normal lift, and with the aircraft above a critical weight it becomes "category 3", which means inverted flight makes it stall.
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Moses does have a point in that the weight of the plane must in some way be transferred to the ground eventually. Think about a helicopter to stay up it must be pushing air downwards, this is the downdraft you feel. This will push you downwards, but the weight is spread over such a large area that the force is not dangerous an eh101 has a rotor area of about 270m2 and it weighs about 14tonnes or about 50kg/m2 which is about 5mBar which will make your ear's pop but not crush you.
A plane will behave similarly - the maths would be more complex as you would have to take into account the speed of the aircraft, but basically planes are much bigger than they are heavy so the air pressure below them isn't very high, so they aren't going to crush you unless they actualyl hit you.
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A plane does not stay aloft by "pushing" down on the air under it but rather by way of the increased curvature of the topside of the wing, by being pulled into the partial vacuum created by the Bernoulli effect of the airfoil - essentially, the air molecules are slightly more separated from one another above the wing than below it.
The only place where "pushing" really becomes a factor is at the leading edges of surfaces and there you can reach the limits of compression of air and get a strong shock wave - commonly called a "sonic boom".
Those can be felt quite easily, and would be extremely powerful if felt by someone in close proximity to an airplane doing this at low level.
Thanks,
L. Lisov
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There's an old joke about a motivational speaker who was walking into the auditorium and saw the "Push" sign on the door. He got an idea for a speech. When he was giving his lecture, he told the audience, "If you want to get ahead in life, look at the sign on the door in the back of the auditorium for how you do it." Everyone turned around, and saw the sign, which said "Pull".
The airplane is pushed up by the atmospheric pressure on the bottom of the wing. That pressure is always present in the atmosphere, whether or not an airplane is passing overhead. We do not feel the force of the atmosphere on the airplane, because that force is already on us. The surface of the earth supports the weight of the atmosphere against the force of gravity. Since the atmosphere is a gas, it has no tensile strength, so it cannot "pull" anything.
Inside the wing of the airplane, there is quite a bit of pushing and pulling going on. The top and bottom of the wing are joined with forged aluminum structures so the atmospheric forces providing lift do not pull the wing apart. However, we can't feel any of that on the outside, and we don't feel the weight of the airplane going overhead.
It is true that the situation would become more complex if we were close enough to the airplane to feel the boundary-layer airflow on the wings, but I am assuming that we are talking about an airplane going overhead at a large distance.
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I believe it was newton who said something about actions and reactions, so if the plane is being pushed up something must be being pushed down. Now as there isn't a whole lot else up there this has pretty much got to be the air, so there has got to be air being pushed down somewhere.
If a plane flew close overhead you would feel a bit of a downdraft, like you do from a helicopter. If a plane flies really low you also get groud effect coming into play, where the air gets trapped between the wings and the ground and produces extra lift, if this is happening the air pressure where you are would go up a bit, which you may be able to feel on your eardrums.
Just being picky, but the bernoulli principle will produce about 42 pounds of the lift of a cessna light aircraft, most of the lift is due to the air being forced round a corner by the fact that both sides of the wing are curved, and the air tends to stick to the top side as well as being forced round by the bottom side. read http://www.aa.washington.edu/faculty/eberhardt/lift.htm
for an explantion.
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the aircraft is being sucked up due to the presure difference above and below therefore no weight is transfered down threw the air
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quote:
the aircraft is being sucked up due to the presure difference above and below therefore no weight is transfered down threw the air
Firstly if the plane was being sucked up by the air, then the air would have to be sucked down (Newtons ? law), and this could be felt as wind blowing downwards - a helicopter rotor is esentially several aeroplane wings flying in circles, and I felt a pretty strong downdraft last time I stood under one!
Secondly, the bernouli effect is a pretty small part of what holds a plane up, if you have time read the link I posted, it is interesting.