(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.
The fuselages of most airliners ride at a small positive angle in cruise. Two to 2½ degrees seems to optimize the interaction between wing and fuselage. What is optimal for air may not be optimal for flight attendants, who have to push food service carts up and down the aisles. I seem to remember the DC-10 as egregious in this respect, perhaps because, late in its career, rising fuel costs led to lower indicated airspeeds, higher altitudes and larger angles of attack than it had been designed for. The way to the front was distinctly uphill, and looked it. The fuselage incidence of its successor, the MD-11, was reduced by around three degrees.
This may have been done mainly to keep the tail from scraping on landing — the MD-11 was longer — but it also had the effect of making the food carts a lot easier to manage.
Seeing an airplane nose-up in cruise, many people would say that it is “tail-heavy.” But an airplane is not a boat. If the back end of a boat is low in the water, you get people to move forward. But the attitude of an airplane is not controlled by its CG position; it is controlled by speed. In level flight, the fuselage angle is a visual indication of the angle of attack required for the wing to produce lift equal to the airplane’s weight; it has almost nothing to do with where that weight is located. An airplane that is “mushing along” is just flying slowly, nothing more.
Since fuselage incidence is aerodynamically unimportant, what is conventionally called “wing incidence” — that is, the angle at which the wing is attached to the fuselage — is likewise. That is why I suggest that we shelve the term. But if the incidence isn’t important, then what about the decalage?
Decalage — a French word meaning “shift” or “offset” — is, broadly speaking, a difference between the incidences of any two lifting surfaces. It was originally applied to the two wings of a biplane: In the usual arrangement, the upper wing was farther forward than the lower and had a larger (can’t get away from that term!) angle of incidence — called positive decalage — so that it stalled first, shifting the center of lift aft and providing an automatic nose-down moment for recovery.
In a monoplane, the term refers to the angles at which the wing and the stabilizer (or canard) are attached to the fuselage. For an airplane to be longitudinally stable, it must have positive aerodynamic decalage; roughly speaking, the forward surface must be at a greater angle of attack than the aft one. This principle applies to conventional airplanes and to canards alike.
The notion of wing-stabilizer decalage — also sometimes called “longitudinal dihedral” — is a slippery one, however, because of a wonky-sounding term: “zero lift.” The zero-lift angle is the angle of attack at which an airfoil has no lift. For symmetrical airfoils it is the same as the chord line, which runs straight from the leading edge to the trailing edge; but for cambered airfoils, which most airfoils are, there is lift even when the angle of attack of the chord line is zero.
The difference between chord-line angle of attack and zero-lift-line angle of attack depends on the airfoil’s camber; and the camber of a horizontal tail surface changes every time you move the elevator. The angle of attack of the empennage is also affected by the downwash of the wing, which varies with speed. So a monoplane really doesn’t have a fixed decalage, except in the trivial sense that the stabilizer is attached to the fuselage at one angle and the wing at another.
Stabilizer incidence, like fuselage incidence, has a small but detectable effect on drag. If you have a fixed stabilizer set at the wrong angle for cruising, you have to correct by trimming in some elevator angle. As you might guess, a surface like a horizontal tail, composed of two elements hinged together, has the least drag when the movable surface, the elevator, is “in trail,” that is, aligned with the fixed surface.
Many airplanes — Cessna 180s, Mooneys, most jets, all airplanes with stabilators — have adjustable stabilizers, and so never pay a drag penalty for a kink in the horizontal-tail airfoil. For them, the decalage has no fixed value.
Fixed stabilizers, however, sometimes present designers with a problem related to control. The correct stabilizer angle for cruise may not provide enough elevator authority to rotate for landing with flaps down and a forward CG. So it’s necessary to set the stabilizer more nose-down than the ideal, and then to trim the elevator downward for cruise. This is the arrangement on Cessna singles, from the Skylane up. You can see it if you look at the horn balances at the outboard ends of the elevators. They are angled downward in order to line up properly with the stabilizer when the elevator is trimmed downward for cruise.
The fact that incidence and decalage don’t have fixed, ideal values has an important corollary. It implies that you can get things wrong and still be safe. The deck angle may be funny and the elevator may not line up nicely with the stabilizer, but the airplane will still fly more or less correctly.
This is true of conventionally configured airplanes, but not of canards. An argument often made for canards is that the stabilizing surface contributes to lift, whereas in conventional airplanes it usually (though not always) produces a downward force that makes the wing work a little harder. There is a collateral benefit: Since stability requires that the canard produce more lift per unit of area than the wing, the canard will normally stall first, and thereby protect the wing from stalling at all. But if you get the decalage wrong — and a number of designers have — the wing may, under some circumstances, stall before or at the same time as the canard does. Usually, such a stall is unrecoverable. That’s the reason that amateur designers, however great the attraction of the canard, are well advised to stick with the conventional aft-stabilizer arrangement.