# Airplane wing .

So I know that the faster the fluid moves the lower the pressure . But what I don’t get about an airplane wing producing lift , so the plane is cutting through the air , and the air molecules are still . Why does the air molecule on the top part magically just speed up ?.

berkeman
Mentor
So I know that the faster the fluid moves the lower the pressure . But what I don’t get about an airplane wing producing lift , so the plane is cutting through the air , and the air molecules are still . Why does the air molecule on the top part magically just speed up ?.

It's being pushed up and pulled back down, by the curve of the upper surface of the wing.

ok i see

rcgldr
Homework Helper
So I know that the faster the fluid moves the lower the pressure.
Only in the situation where no work is done on the fluid, such as the idealized case of zero viscosity fluid in a pipe of varying diameters.

It's easier to understand this with a propeller. The propeller does work, so Bernoulli relationship doesn't hold in the immediate vicinity of the propeller, but Bernoulli relationship holds in the air flow away from the prop where no work is being done. From NASA link:

We can apply Bernoulli'sequation to the air in front of the propeller and to the air behind the propeller. But we cannot apply Bernoulli's equation across the propeller disk because the work performed by the engine (propeller on air) violates an assumption used to derive the equation.

http://www.grc.nasa.gov/WWW/K-12/airplane/propanl.html

In the case of wings, the goal isn't maximum thrust, but just enough to support the plane in flight and handle higher load factors in turns. So the lift and drag aspects are less than a prop, and the losses are much less, less work is done on the air, and Bernoulli based math can be used to model lift at moderate wing loadings, but it doesn't take into account the small amount of work done on the air.

Why does the air molecule on the top part magically just speed up ?.
The wing has an effective angle of attack, one or both surfaces of the wing are angled downwards. It the bottom surface is angled downwards, it simply deflects the air flow downwards. If the upper surface is angled downwards, then it introduces a "void" as the the wing passes through the air and the air has to fill in this void (else a vacuum would be created). I've been credited and/or accused of inventing the term "void theory" or "void abhorence theory" for wings, but it's commonly used to explain why drag on a bus is mostly due factors at the back of the bus and not the front. Also the Wiki article on wings mentions "void" (so there's at least one other advocate of "void theory"):

In that case a low pressure region is generated on the upper surface of the wing which draws the air above the wing downwards towards what would otherwise be a void after the wing had passed.

http://en.wikipedia.org/wiki/Wing

The void concept seems obvious, but it's seldom mentioned in articles on lift. The amount of lift that occurs above a wing depends on how the air fills that "void". If the factors, speed, angle of attack, sharpness of leading edge, ... aren't excessive, then the air mostly accelerates downwards (lift) and a bit forwards (drag) to fill the void. If the factors get excessive, the air circulates in small vortices or in a severe case one large vortice to fill in the void. The small vortices don't hurt too much and most wings get this king of turblent flow over most of the air foil. The very large vortice still lowers pressure, but not very much and results in a lot of forwards acceleration of air, so less lift and more drag. Delta wing airfoils are designed to take advantage of small vortice flow near the angled leading edge, allowing for high angles of attack (20 degrees or so).

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We get these airplane wing questions wayyyyyyyyy too often. Someone should just stick one of the answers and delete all new threads on airplane wings.

russ_watters
Mentor
So I know that the faster the fluid moves the lower the pressure . But what I don’t get about an airplane wing producing lift , so the plane is cutting through the air , and the air molecules are still . Why does the air molecule on the top part magically just speed up ?.
Via the principle of relativity, there is no difference between a still wing with a 100 mph wind (ie, in a wind tunnel) and still air and a wing going through it at 100 mph.

there IS a void above the wing. there is low pressure above the wing and high pressure below the wing. the low pressure pulls the air into itself thereby speeding up the air above the wing. as the air leaves the low pressure area it slows down again.

the back of the bus is where most of the turbulence is (because the back of the bus isnt shaped like the back of a wing. it doesnt come to a point)

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QuantumPion
Gold Member
It's being pushed up and pulled back down, by the curve of the upper surface of the wing.

The air flow separation is created by the angle of attack of the wing, not its shape.

russ_watters
Mentor
The air flow separation is created by the angle of attack of the wing, not its shape.

FredGarvin
There's a lot of that going on in this thread.

rcgldr
Homework Helper
The air flow separation is created by the angle of attack of the wing, not its shape.
?
I think he means the lift is related to the effective angle of attack (which is zero when there is zero lift). Seperation is going to occur with any solid object. The shape of airfoil mostly determines the amount of drag for a given lift, and targets a specific speed range.

We get these complaints of duplicate threads wayyyyyyyyy too often. Someone should just stick one of the answers and delete all new complaints of duplicate threads.