The Straight Dope Tackles How Planes Fly

[mikea]

The Straight Dope is taking on Bournoulli vs. Langewiesche:

Dear Straight Dope:

I'm a pilot and I work for an aircraft manufacturing
company. Just like every other pilot and many non-pilots,
I've learned that aircraft fly because of the low
pressure created on the top of an airfoil. However, I'm
not sure that's the whole story. I recently read a book
called Stick and Rudder by Wolfgang Langewiesche, and he
argues that the primary thing that keeps planes in the
air is the downward force created by the wing--that the
aircraft mostly pushes itself into the sky instead of
pulling itself by the top of the wings. I've been
thinking (perhaps too much) that he may be right. Perhaps
lift is created by both effects. Maybe one is stronger
than the other but I think that most people haven't been
told the real story about what keeps an airplane in the
air. It's got to be more than the low pressure on top of
the wing. I want to know the real story. --Bill Rehm

Guest contributor aerodave replies:

You'd think that after a century of powered flight we'd
have this lift thing figured out. Unfortunately, it's not
as clear as we'd like. A lot of half-baked theories
attempt to explain why airplanes fly. All try to take the
mysterious world of aerodynamics and distill it into
something comprehensible to the non-technical
audience--not an easy task. Nearly all of the common
"theories" are misleading at best, and usually flat-out
wrong.

A couple of the ideas you mention provide part of the
answer, but are incomplete in fundamental ways. To hear
some tell it . . .

For more, see:
http://www.straightdope.com/mailbag/mairplanesfly.html

 

[Dave Krall CFII]

The Straight Dope is taking on Bournoulli vs. Langewiesche:

There's no way the lower surface of a wing can't supply significant lift via the air molecules force against it (Newton's II ?). My HS physics teacher & I got into a big argument about that cause he said it was all Bournulli's effect from the top wing surface !

Just hold your fist or hand in any shape, in front of a fan blast.

 

[Let'sgoflying!]

Just hold your fist or hand in any shape, in front of a fan blast.

or fly a non-symmetrical airfoil upside down.

 

[tom.]

I put this question to my son, who builds and flies models and is currently taking flight lessons. He says,


"This is a very common misconseption.

In many books, it says that lift is created by diverting air down. This is commonly considered to mean that the bottom of the wing forces air down. However, with a little research, it is easy to see that this is not true.

There is a force called the Coanda Effect that is very important in this discussion. It is the force that causes water to bend around a glass when held in a stream of water. The reason is because, due to friction, the speed of the water decreases as it approaches the surface of the glass, causing the stream to bend.

When air flows over a wing, the air slows down as it approaches the surface, just like the water did with the glass. This causes the airstream to bend over the wing. This is like the wing "pulling" the air towards itself. Due to Newton's first (or second? I forget) law of motion, any action has an equal and opposite reaction. When the wing "pulls" the air towards itself, the air "pulls" the wing towards itself. Hence, we have lift.

This is just another way to look at it, and I'm not saying it's the right way, but it seems to be the way of science."

You will have to decide...

 

[PoAdeleted5]

I use an electric leaf blower to dry my cars after washing them. It works pretty good, especially for blasting the water out from all the crevices and from under the badges.

But what does this have to do with this thread?

Well, there's an interesting thing that happens that is counterintuitive. As the blower gets closer to the car's surface, the effect of the wind intensifies. But once you get to a certain point and get too close, the opposite happens and the end of the blower will suck up to the car, like it was magnetised. Bernoulli in action.