On the Flight of Airplanes

October 18, 2000

At the second half of class last night in the open forum section that preceded and followed the breaking of bread, a careful discussion arose concerning the flight of airplanes. Since several of you were elsewhere gainfully employed during that discussion and in the interest of education, let me summarize for you.

At the outset of our journey behind the dark side, I offered a conjecture about the flight of airplanes. The statement went somewhat loosely along the following lines (and I am paraphrasing here):

"It is widely held that airplanes fly because of the Bernoulli Principle, that a fast moving jet of air creates a partial vacuum, and that the top side of an airplane wing is curved up, the bottom is flat and hence the air must move faster as it flows over the longer, upper surface area, thereby giving lift. The statement usually goes something like, "Airplanes fly because the top surface of the wing is curved." To the extent that that statement implies a necessary condition, as most any reasonable listener would have to say that it does, it is simply wrong. It is not a necessary condition of flight that the upper edge of the wing be longer than the bottom edge. I went on to say that flight occurs because of wing angle and Newton's Law of action/reaction, not because of wing asymmetrical curvature. I went on to add that if the Bernoulli argument were correct then planes could not logically fly inverted. By observation then, we can dismiss the Bernoulli explanation as the reason that planes fly."

Now that soliloquy was not universally accepted by the learned members of the class. Indeed some of them rejected that statement outright. Let me clarify and amplify.

Airplanes need lift to fly. Lift is obtainable from many different forms. One of these is an asymmetrically shaped wing with the upper edge curved and the bottom edge flat. Importantly however, there are many other ways to get lift. One of these is a symmetrical wing which is tilted upwards on the leading edge. That is the point I was (poorly perhaps) trying to make. The Bernoulli Principle is not the reason planes fly. Planes can fly that way, but they don't have to take advantage of the Principle to do so.

Some additional references on the matter include:

http://www.geocities.com/proppitch/Newton.htm which discusses Newton's Third Law as the basic principle of lift.

http://www.geocities.com/proppitch/Airfoils.htm talks about the combination of Bernoulli and Newton.

http://beadec1.ea.bs.dlr.de/Airfoils/flywing1.htm incorporates a technical discussion and includes several diagrams that clearly demonstrate the absence of any necessity for the bottom edge of a wing to be flat or shorter than the top edge.

http://www.eskimo.com/~billb/wing/airfoil.html is absolutely the best and perhaps a must read. Let me summarize. There is a big debate on this topic about flight and most arguments get it wrong. Read this and you will get the point that I was attempting to make in class many days ago.