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.
