[Geezer (OP)]

Wings do not support an aircraft primarily by compressing the air beneath it.  Were it to be so then the pressure exerted beneath the aircraft would be equal to its weight and so any one disembarking from a hovering helicopter would be crushed: as

OK. So, if there is no force acting on the lower surface of the wings, why does a plane fly?  I always thought the wings were more than just decorative []

[graham.d]

Wings produce most lift from the low pressure above rather than high pressure below. I expect that is what Lee is referring to.

[Geezer (OP)]

I agree. But even if there was a total vacuum above the wing, the wing will only lift by virtue of the air pressure on the bottom side of the wing, and if there is pressure exerted by air on the bottom side of the wing, that air is being compressed.

[LeeE]

Geezer; you seem to be thinking that if you reduce the pressure on one side of something it automatically results in an increase of pressure on the other side; this is not the case.

Consider a sealed box incorporating an air-tight but freely moving wall that divides the box into two compartments.  Start with both compartments at the same air pressure and then start evacuating the air from just one of the compartments, let's say the left one, lowering its pressure, whilst the other compartment on the right remains totally sealed.  As you start to remove the air from the left compartment, the air-tight wall will move towards left, pushed by the expanding air in the sealed right hand compartment.  However, far from being compressed above its original starting pressure, the air in the sealed right hand compartment will have dropped in pressure.

An even simpler example is drinking something through a straw; when you suck on the end of the straw to draw the drink up it, you are not increasing the pressure on the surrounding air to force the drink up the straw.

[Geezer (OP)]

Lee: I understand your analogy, but I'm not sure it is good anaolgy for a wing.

By point is that the top of the wing is not "sucked", so the bottom of the wing must be "blown". Therefore, the air under the wing is supporting the weight of the aircraft as opposed to the absence of air above the wing. And since the wing moves rapidly into a volume that was previously occupied by air, the air has to be compressed in the process of getting out of the way of the wing.

[LeeE]

If you simply winch an aircraft up beneath a crane and then drop it, the air beneath it will be compressed "in the process of getting out of the way of the wing".  It will not support the aircraft though, despite compressing a far greater area of air than when its flying; the cross section of an aircraft flying forward is much less than its plan area when being simply dropped.

Refer back to that youtube vid of that very slow GA aircraft landing; an aircraft that is simply dropped will soon be moving faster than that, and compressing considerably more air, yet it still won't be flying.

[Geezer (OP)]

I can see where this is going  [] You're trying to get me to calculate the volume of air displaced by a wing as it flies through the air!

OK. So do we agree that the air underneath the wings supports the aircraft? If that's not the case, I'm at a complete loss and cannot understand why they stay in the air.

You don't suppose we've established they can't actually fly? I'm always suspicious when I get into one that it's more a matter of willpower on the part of the passengers than anything else. They really don't look as if they should fly.

BTW, you didn't happen to notice a black helicopter by any chance? There's been one hanging around all night. Funny thing is, its rotor does not seem to be, er, rotating. You don't suppose it's using flubber or something, or did I say too much?