STATIC STABILITY AND CONTROL

INTRODUCTION

FAR Part 23 (Ref. 8.1) states that “the airplane must be safely controll­able and maneuverable during—(1) take off; (2) climb; (3) level flight; (4) dive; and (5) landing (power on and off) (with the wing flaps extended and retracted).” Part 23 also says that “The airplane must be longitudinally, directionally, and laterally stable.” This particular FAR, which defines the airworthiness standards for normal, utility, and acrobatic airplanes, then goes into detail to state what requirements an airplane must meet in order to satisfy the foregoing general statements on controllability and stability.

Stability of an airplane refers to its movement in returning, or the tendency to return, to a given state of equilibrium, frequently referred to as trim. More specifically, static stability refers to the tendency of an airplane under steady conditions to return to a trimmed condition when disturbed rather than any actual motion it may undergo following the disturbance. The forces and moments are examined to determine if they are in the direction to force the airplane back to its equilibrium flight conditions. If so, the airplane is statically stable.

Dynamic stability encompasses the unsteady behavior of an airplane responding to time-dependent aerodynamic forces and moments produced by the airplane’s motion. Following a disturbance of the airplane from a trimmed condition, its movement under the influence of the unsteady forces and moments is examined to determine whether or not the airplane ultimately returns to its trimmed condition. To the uninitiated, it comes as a surprise to learn that many airplanes exhibit a dynamic instability known as spiral divergence. Unless counteracted by control input, many airplanes will gradu­ally drop one wing and go into an ever tightening spiral dive. Fortunately, this spiral divergence initially is so gradual that the pilot will compensate for it without even realizing it. However, if a noninstrument-rated pilot is suddenly without a reference to the horizon, the spiral divergence instability can prove disastrous.

An airplane that is statically stable will not necessarily be dynamically stable. However, one that is statically unstable will be dynamically unstable; that is, static stability is necessary but not sufficient for dynamic stability. In this chapter we will consider static stability and control; in other words, we will consider the forces and moments acting on an airplane undergoing steady motion.