From: island Subject: Re: Then, how DO airplanes fly? Date: 11 Jun 2001 21:18:03 GMT Newsgroups: sci.physics.research Summary: What keeps airplanes aloft? Matt McIrvin wrote: > > In article <9flcl601b1s@edrn.newsguy.com>, daryl@cogentex.com (Daryl > McCullough) wrote: > > >That is, the wing is tilted, and the rushing wind caused > >by the forward motion of the plane is forced downwards by > >the wing. The resulting upwards thrust raises the plane. > > > >Then I had to learn that this intuitive picture was all > >wrong, that the true explanation is the Bernouilli effect > >(which I could never understand intuitively, although I > >could derive it). > > > >Now, they're telling me that my first understanding > >was correct, and that the second explanation is a > >misconception. [...] > There's no way a plane can stay up without exerting a downward force on > something. Newton's Third Law has to apply, and in that sense the > "Newton" explanation *must* be a complete explanation. The plane stays up > by forcing air downward. (Well, if it's very low to the ground, I suppose > the ground effect of the air might be important, but that's another > special case of the "Newton" explanation.) > > The question, then, is *how* the air gets pushed downward. The advocates > of the "Newton" explanation sometimes go on to imply that the effect comes > solely from the angle of attack of the wing pushing down air at the > leading edge (as your picture implied), which is wrong-- there's stuff > going on above the wing too. And all of this can probably be described > well using Bernoulli's principle to some approximation. > > In fact, I wouldn't be surprised if a sufficiently sophisticated > "Bernoulli's principle" explanation could be applied to inverted flight. > It's just the "unequal distances/equal times" version of the Bernoulli > explanation that makes no sense at all. > > -- > Matt McIrvin I would agree that the explanation makes no sense at all. If you hydroplane an object, like a flat board, a water ski, or even skip a rock over the surface of water, then you can certainly argue that it is only the dynamically increased pressure against the bottom surface which keeps the 'wing' aloft. Air pressure against the top surface is reduced as the air moves over it, but the vacuum effect is not increased in proportion to the increase in pressure against the bottom surface, and so the Bernoulli effect alone doesn't get it done in this case example. You can hydroplane over the surface of water regardless of whatever... "stuff is going on above the wing too", as you can still achieve 'lift' even if the wind is moving the air along behind you just as fast as you are moving forward. > A lot of the confusion seems to > come from the fact that the debate gets framed as a dispute between > two opposing "teams," each of which has an explanation that is true when > properly formulated but is often stated in incorrect caricature form. That's probably because the chambered air foil wing design has been shown to work to achieve lift via the Bernoulli effect, but Bernoulli's principle probably doesn't apply so much as dynamic pressure on the lower surface does, in situations where you have to uh, bend... the rules of distance/time ;-)