Canard Configuration Stall Characteristics
Canard aircraft are characteristically difficult to stall at all. The canard surface is generally designed to stall before the main, or aft, wing does, when the angle of attack is increased at a normal, gradual rate. When the canard does stall, with the main wing still unstalled, the airplane tends to pitch down, recovering normal flight. However, William H. Phillips comments that in the airplane pitch-down following canard stall, the canard surface’s angle of attack is increased again by the airplane’s angular velocity. This could delay recovery from an unstalled condition until the airplane has reached a steep nose-down attitude. It can be argued that an aft-tailed airplane also tends to recover automatically from a stall. On aft-tailed airplanes the horizontal tail, operating in the wing’s downwash, experiences a relative upload when the wing stalls. This is because the wing downwash drops off when the wing stalls.
The main concern in canard airplane stalls is the dynamic stall, entered at a high rate of angle of attack increase. Pitching momentum could carry the angle of attack up to the point where the main wing stalls, as well as the canard. In combination with unstable pitching moments from the fuselage, this could produce a total nose-up pitching moment that cannot be overcome by available canard loads. Wing trailing-edge surfaces that augment canard pitching moment control would be ineffective with the main wing stalled. Thus, a canard airplane’s main wing stall could produce deep stall conditions, in which a recovery to unstalled flight cannot be made by any forward controller motion (see Chapter 14). Deep stall at aft center-of-gravity positions and high power settings was identified in NASA tests of a tractor propeller canard configuration (Chambers, 1948).
The possibility of dynamic stalling on canard airplanes is minimized if the configuration is actually a three-surface case: main wing, canard, and aft horizontal tail. Examples of three – surface configurations are the Piaggio P180, the Sukhoi Su-27K, the DARPA/Grumman X-29A forward-swept research airplane, and the many three-surface airplanes designed by G. Lozino-Lozinsky, of MiG-25 fame. Even at extreme angles of attack that stall the main wing, the aft horizontal tail may be in a strong enough downwash field to remain unstalled, or it may be unstalled by nose-down incidence. With an unstalled aft horizontal tail, longitudinal control can be maintained.
Another way to minimize the possibility of dynamic stalling of canard airplanes is to operate them at centers of gravity far forward enough so that elevator power cannot produce high nose-up rotation rates. This amounts to restriction of the available center-of-gravity range and a reduction in the airplane’s utility.