Aeroplane Wing

[lafatthis]

We all know how a wing provides lift for an aircraft;- The experts tell us it because of the curve along the top of the wing, creating a larger surface area - less pressure results along the top, so the air below pushes the wing up. My question is; Doesn't the lack of pressure above the wing create a vacuum which in my mind pulls the wing up instead of the pressure below pushing it. ?... Think of it in a similar way to when you suck the air out of a plastic bag, you're pulling the bag in, it's not the air around the bag that's compressing it.

[Ewood27]

In a word, yes. The thing that matters is that the air pressure is higher below the wing and lower above the wing. It is the pressure difference that causes lift. It is the lower pressure above the wing that contributes by far the most to the total lift.
In your plastic bag analogy, the outside air is at atmospheric pressure, about 14.7 lbs. p.s.i. When you suck, you induce a lower pressure than that inside the bag. The pressure difference causes the bag to collapse until the pressures inside and outside are equal.
For a fuller explanation of lift and other forces acting on an aeroplane, have a look at http://travel.howstuffworks.com/airplane.htm

[djg23]

I would say no it isn't a vacuum. Think of it like this. There are lots of air molecules moving around and they collide with the wing. When they collide with the wing they exert a force on it pushing it in one direction. When the wing is flat you have just as many from the top colliding and pushing down as you do colliding from the bottom and pushing up. So top and bottom push against each other but by the same amount so you stay level. If you curve the top of the wing so that the air molecules on top are moved away from the wing they don't get chance to push down on it as much as the ones on the bottom are pushing up, so the air molecules pushing up from below 'win' and so you go up. OR think of a rugby scrum that is equally matched and so it isn't moving. If you took away half the players from one team, the scrum would move in the direction of the team with fewer players on it.

[djg23]

Just to add to that a little and maybe raise another question - there is really no such thing as a vacuum. A 'vacuum' doesn't exist as a thing, it is the ABSENCE of a thing. You only know a vacuum is there if there is air to rush into it. Air moves from high to low pressure and so if you have air next to a vacuum it will rush in to fill the low pressure but it is the air doing the work not the 'vacuum' which can't be doing any work as it isn't there. The vacuum in space doesn't suck the planets closer together afterall.

[trilobite]

"when you suck the air out of a plastic bag, you're pulling the bag in, it's not the air around the bag that's compressing it." Errr.... I'm afraid it is. By the same token, in an old fashioned water pump, it is atmospheric pressure that PUSHES water up the pipe to compensate for the partial vacuum you create in the pump cylinder by movement of the handle. Incidentally, this means that for an open well system (ie not an artesian system) there is a maximum vertical distance that you can actually pump a fluid which, simply put, occurs when the force of the column of fluid acting downwards equals that of atmospheric pressure pushing it up. In real terms, for a water pump, this is around 11 metres.

[Pinotage]

How come then that aeroplanes can fly upside down?

[bernardo]

On the contrary - when you suck air out of a plastic bag, it is the air outside which is compressing the air. At school my physics teacher always told us that there is no such thing as "suck". When you "suck" the air out of a bag, what you are actually doing is joining up the pocket of air in the bag with the air in your lungs; "sucking" takes place when you lower the diaphragm in your chest to make your lungs expand. The combined pocket of air in your lungs and bag expands to a greater volume, therefore it goes to a lower pressure. The air outside then squashes the bag in order to equalise the pressure. It is the same idea when you breathe normally - the lowering of the diaphragm and the expansion of the lungs only enable you to breathe air in if there are lots of air molecules whizzing around outside, waiting to go in.

[sddsddean]

To answer Pinotage: When the aircraft is flying inverted it will tend to head for the ground, both due to gravity and the Bernoulli effect (the pressure difference that makes a wing work). All the pilot has to do is push the joystick away from him just enough to make the aircraft 'dive' up into the sky (if you see what I mean).

[PaulB]

I will agree with Ewood27 as he is a pilot :o)

[Ewood27]

Thank you, PaulB. In fact I don't totally disagree with any of the answers. Most of the lift force is generated by the reduced pressure induced on top of the wing, and only a small amount by the increased pressure beneath the wing. That's a simple matter of measurements in a wind tunnel.
In order to generate lift, the wing must be at a slight angle to the airflow (the "angle of attack"). When flying upside down, the pilot pushes forward on the controls, pushing the nose what would normally be down, but is now up because he's upside down. Thus the wing still has an angle of attack and can still generate lift. It's a lot easier to explain with a few pictures!
The other theory is that all flight is an illusion. All it takes is for enough people to start believing that aeroplanes can't fly, and they will immediately stop doing so. Though, of course, experts* have proved conclusively that by the laws of aerodynamics a bumble-bee cannot fly!
*Expert: x is an unknown quantity, and a spurt is a drip under pressure.

[norfolk boy]

Hmm, so where does gravity fit in to this !

[Ewood27]

If there weren't no gravity, you wouldn't need no lift!
To be a bit more serious, there are four forces acting on an aircraft: lift vs. weight (gravity) and thrust vs. drag. If each pair are in balance you have balanced flight; if not, you have a change taking place as the stronger force overcomes the weaker one. For example, open the throttle = more thrust. If more thrust than drag, then the aircraft accelerates. Pull the control column back = raise the nose = give the wing a greater angle of attack = more lift (provided going fast enough to not be in the stall range). More lift than weight, the aircraft climbs.
There are a lot more possible combinations and considerations, but I hope you get the basic idea.

[Hamish]

Ewood27: Absolutely, airplanes dont fly! They have cleaver wind devices that move the universe about them to make it look like they are flying. Hamish

[tomd]

You can illustrate this by hanging two apples in a doorway at the same vertical level and about 3cm apart - blow between them and rather than moving apart, they will move together.

[lafatthis]

Thank you everybody for your constructive answers. All very interesting and informative. I just wish I'd have gone and done Physics. I'm happy now - it's all down to molecules and a bit of pressure. Cheers Ian