(February 2012) Books about airplane design often mention wing incidence as if it were a parameter of some importance. It isn’t. In fact, “wing incidence” is a misnomer. I propose — and fully expect my proposal to have no effect — that the term be abandoned, and that we speak of “fuselage incidence” instead.
The so-called angle of incidence is the angle between the chord line of the wing at the fuselage and the fuselage’s longitudinal axis. Now, this sounds as if it ought to matter, because after all, if the wing is not at the correct angle it will not have enough lift, or will have too much, and the airplane will mush, or plow, or otherwise behave inappropriately. But to think that way is to slip into the mindset of the lubber who says, “The airplane flew into a storm and the wings fell off.” Wings never fall off. It is the fuselage that falls off the wing.
When you fly, what you are flying is not a fuselage; it is a wing. To fly straight and level, you adjust the angle of attack of the wing so that its lift (actually, the lift of the entire airplane taken together, but the wing is by far the dominant contributor) is exactly equal to the weight of the airplane. Most likely, you change the adjustment in the course of the flight. You might consume 10 percent of the weight of the airplane in fuel in the course of a long flight; if you never retrimmed, you could find yourself climbing a couple of hundred feet a minute at the end of the trip.
None of these trim adjustments has anything to do with the fuselage, however. Aerodynamically, the fuselage is a minor player.
Somewhere between full and empty tanks, between full and empty seats and between economy and high cruise power is the airplane’s “design point” — an average cruising case used by the designer to establish the positions of various parts of the airplane and their angles with respect to one another for the purpose of minimizing drag. The designer begins by determining the angle of attack required for the lift to equal the weight at the design point; that will be the position of the wing in level cruising flight. The fuselage is then placed on the wing in the position in which its drag is at a minimum.
You might think that, as a streamlined body, the fuselage should be pointing exactly the way it’s going.
Actually, however, the drag of fuselages, especially tubular ones, is not very sensitive to the angle at which they meet the air. Besides, the wing and empennage themselves distort the airflow around the fuselage so that the meaning of “straight ahead” is no longer obvious.