A popular misconception is that the Wright brothers, in addition to all of their other achievements, invented the airfoil. They didn’t. Sir John Cayley, an English engineer who also first identified the four forces of flight—lift, drag, thrust and weight—developed the cambered airfoil through detailed experimentation. His three-part work, On Aerial Navigation, published in 1809 and 1810, is often cited as the first description of what we today call an airplane. Also today, we teach that the theories of Sir Isaac Newton (1642-1726) and Swiss mathematician Daniel Bernoulli (1700-1782) provide the detailed science that explains lift. They don’t, at least not fully.
The basic problem is that neither theory completely explains real-world observations. Bernoulli’s principle—that the faster air on top of the wing experiences reduced pressure—is correct but doesn’t explain why it’s correct. It also doesn’t explain inverted flight. That’s where Newton’s second and third laws (see the sidebar on the opposite page for details) come into play. Taken together, Newton’s laws describe how we can fly inverted and how angle of attack works. But they don’t have the details we need from Bernoulli. Still, once we put Bernoulli and Newton in the same room, then sprinkle some Cayley throughout, we have a working idea of how to build and fly an airplane. But we still don’t know exactly why the air on top of the wing is at a lower pressure than the air underneath it.
