STABILITY
Stability is designed into aircraft to reduce pilot workload and increase safety. A stable aircraft, such as a typical general aviation training airplane, requires less attention from the pilot to maintain the desired flight attitude, and will even correct itself if disturbed by a gust of wind or other outside forces. Conversely, an unstable aircraft requires constant attention to maintain control of the aircraft.
Pendular Action—The lateral or longitudinal oscillation of the fuselage due to it being suspended from the rotor system. It is similar to the action of a pendulum. Pendular action is further discussed in Chapter 3— Aerodynamics of Flight.
There are several factors that contribute to the stability of a gyroplane. One is the location of the horizontal stabilizer. Another is the location of the fuselage drag in relation to the center of gravity. A third is the inertia moment around the pitch axis, while a fourth is the relation of the propeller thrust line to the vertical location of the center of gravity (CG). However, the one that is probably the most critical is the relation of the rotor force line to the horizontal location of the center of gravity.
HORIZONTAL STABILIZER
A horizontal stabilizer helps in longitudinal stability, with its efficiency greater the further it is from the center of gravity. It is also more efficient at higher airspeeds because lift is proportional to the square of the airspeed. Since the speed of a gyroplane is not very high, manufacturers can achieve the desired stability by varying the size of the horizontal stabilizer, changing the distance it is from the center of gravity, or by placing it in the propeller slipstream.
FUSELAGE DRAG (CENTER OF PRESSURE)
If the location, where the fuselage drag or center of pressure forces are concentrated, is behind the CG, the gyroplane is considered more stable. This is especially true of yaw stability around the vertical axis. However, to achieve this condition, there must be a sufficient vertical tail surface. In addition, the gyroplane needs to have a balanced longitudinal center of pressure so there is sufficient cyclic movement to prevent the nose from tucking under or lifting, as pressure builds on the frontal area of the gyroplane as airspeed increases.