Ultralight Airplane Pitch Stability
Conventional ideas about the need for longitudinal static stability are misleading in the case of ultralight airplanes. The reason is that, instead of the normal short – and long – period, or phugoid, modes of motion, four unfamiliar first-order modes may appear. For example, the Gossamer Condor’s center of gravity is aft of the neutral point, in order to unload somewhat the canard surface. This produces a positive or unstable value for the CMa derivative. As a result, one of the four first-order modes is unstable. However, the corresponding divergence has a time constant of about 1,000 seconds, making it imperceptible to pilots.
Another way to explain the benign pitch behavior of ultralight airplanes flying at centers of gravity behind the neutral point is to consider their maneuvering stability. Maneuvering stability disappears at the maneuver point. The maneuver point of ultralight airplanes tends to be far aft of the neutral point because of high pitch and heave damping levels. For flight at centers of gravity behind the neutral point but ahead of the maneuver point, the machine would have no tendency to diverge unstably in pitch attitude at constant airspeed. Its unstable behavior would require a simultaneous loss of airspeed and nose-up pitch change in level flight, a process that is very slow.
To illustrate the concept of the maneuver point or maneuvering stability, consider an airplane with an unstable gradient in pitching moment with angle of attack, and suppose it to be disturbed nose-up with respect to its flight path. The unstable pitching moment gradient would tend to increase the size of the disturbance, but at the same time the increase of angle of attack would cause the flight path to curve upward if the speed is constant. The upward curvature of the flight path implies an angular velocity in pitch, which is resisted by the aerodynamic damping in pitch.
In the case of the Gossamer aircraft, the stabilizing effect of the pitch damping due to flight path curvature overwhelms the destabilizing gradient of pitching moment with angle of attack. The neutral point is 5 percent ahead of the center of gravity, but the maneuver point is four chord lengths behind the center of gravity, due to the large path curvature for a given angle of attack.