I’ve been reading Martin Simons’ wonderful Model Aircraft Aerodynamics (3rd ed.) recently. I was hoping to find out, among other things, where lift really comes from. Simons’ book has lots of fascinating detail about aircraft design (not just for models), but didn’t answer that question.

But I have finally figured it out, and thought I’d share it with you–

First, lift has nothing to do with the curvature of wings. Airplanes with completely flat wings can fly (although not as efficiently as with proper airfoils). And airplanes with curved wings can and do fly upside down.

Wing curvature and airfoil shapes are for the purpose of reducing drag – valuable, but not necessary for lift. Because most aircraft wing shapes are optimized for efficient (low-drag) right-side-up flight, inverted flight is less efficient. But possible. (Aerobatic aircraft that spend lots of time upside down often have symmetrical wings.)

Second, lift has nothing to do with “equal transit time” for molecules on the top and bottom of the wing surface – this is simply a myth (well debunked both at Wikipedia and in a nice article at Plane & Pilot magazine). While it’s true that air flows faster on top of a wing, it does not rejoin the flow on the bottom at the trailing edge of the wing.

So what is it, then?

There are two popular explanations, both of which are correct, because they are two ways of describing the same thing. One, Bernoulli’s Principle, is easily misunderstood and leads to lots of confusion. The other, Newton’s Third Law of Motion, is simple to understand.

Wings generate lift by deflecting incoming air downwards.

Therefore, lift comes from Newton’s Third Law – the action of pushing air down results in a reaction of the wing being pushed up.

As an aircraft flies, the wing presents some angle of attack to the incoming air – a wing that is generating lift isn’t aimed straight forward; instead the leading edge is a little higher than the trailing edge; this is the angle of attack.

To just slightly oversimplify – air molecules come at the wing and whack into the bottom side, bouncing off downwards and pushing the wing up. This transfers momentum from the molecule into the wing, generating lift. The overall stream of air that goes past the wing is deflected downward, resulting in an upward reaction on the wing. This is the Newtonian explanation of lift.

In Bernoulli terms (again, slightly oversimplified), because of the angle of attack, lots of air molecules are swept down under the bottom of the wing, and less go over the top. Since there are now more molecules on the bottom and less on the top, there is more pressure on the bottom and less pressure on the top. So this pressure differential pushes the wing up. This is the Bernoulli explanation of lift, and it is equally correct – Bernoulli’s equations are derived from Newton’s – it’s just another way of saying the same thing.

And, yes, air on top of the wing does move faster than on the bottom, just as Bernoulli says. In Newtonian terms, because the leading edge has swept lots of air molecules under the wing, there are fewer molecules on top. All the molecules in front of the wing (before the sweeping-away) are still pushing on those few remaining molecules on top of the wing, but there’s not much pushing back – so they accelerate. And vice-versa on the bottom of the wing.

If you find the above confusing, I can’t recommend a better introduction to the basic behavior of molecules and matter than the first chapter of Richard Feynman’s Six Easy Pieces – it should be mandatory reading for high school graduation.