What's Your Angle?

摘要

In our April issue, we explored the theories of generating lift from Bernoulli and Newton, and how neither of them alone fully explains how airfoils work. Bernoulli's basic understanding of low pressure above an airfoil is correct, but the math is wrong and his assumptions are faulty. He also doesn't explain inverted flight. Meanwhile, Newton has a decent handle on using angle of attack to generate lift and his Third Law-every action has an equal and opposite reaction-tidily handles inverted flight, but he has little to offer us in explaining low pressure above the airfoil. The closest aerodynamicists reportedly have gotten to a unified theory of how lift is generated involves combining the two, and adding an understanding of how air is deflected when an airfoil passes by. Debating these theories is great for the late-night dorm room session, but once we have an understanding of their strengths and shortcomings, how do we put them into practice? And can we leverage the differences between them to more precisely fly our airplanes? The sidebar at the top of the opposite page is adopted from our April issue as a refresher and a way to drag us closer to that elusive unified theory. The punchline is that both theories are correct, as far as they go, but sometimes aren't correct at the same time. This mostly has implications when we're nibbling at the edges of a theory's operating envelope. The diagram over there nicely combines the two theories.