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TRIM AND CONTROLLABILITY
An aircraft is said to be trimmed if all moments in pitch, roll, and yaw are equal to zero. The establishment of equilibrium at various conditions of flight is the function of the controls and may be accomplished by pilot effort, trim tabs, or bias of a surface actuator.
The term “controllability" refers to the ability of the aircraft to respond to control surface displacement and achieve the desired condition of flight. Adequate controllability must be available to perform takeoff and landing and accomplish the various maneuvers in flight. An important contradiction exists between stability and controllability since adequate controllability does not necessarily exist with adequate stability. In fact, a high degree of stability tends to reduce the controllability of the aircraft. The general relationship between static stability and controllability is illustrated by figure 4-3-
Figure 4-3 illustrates various degrees of static stability by a ball placed on various surfaces. Positive static stability is shown by the ball in a trough; if the ball is displaced from equilibrium at the bottom of the trough, there is an initial tendency to return to equilibrium. If it is desired to “control" the ball
and maintain it in the displaced position, a force must be supplied in the direction of displacement to balance the inherent tendency to return to equilibrium. This same stable tendency in an aircraft resists displacement from trim by pilot effort on the controls or atmospheric disturbances.
The effect of increased stability on controllability is illustrated by the ball in a steeper trough. A greater force is required to “control” the ball to the same lateral displacement when the stability is increased. In this manner, a large degree of stability tends to make the aircraft less controllable. It is necessary to achieve the proper balance between stability and controllability during the design of an aircraft because the upper limits of stability are set by the lower limits of controllability.
The effect of reduced stability on controllability is illustrated by the ball on a flat surface. When neutral static stability exists, the ball may be displaced from equilibrium and there is no stable tendency to return. A new point of equilibrium is obtained and no force is required to maintain the displacement. As the static stability approaches zero, controllability increases to infinity and the only resistance to displacement is a resistance to the motion of displacement—damping. For this reason, the lower limits of stability may be set by the upper limits of controllability. If the stability of the aircraft is too low, control deflections may create exaggerated displacements of the aircraft.
The effect of static instability, on controllability is illustrated by the ball on a hill. If the ball is displaced from equilibrium at the top of the hill, the initial tendency is for the ball to continue in the displaced direction. In order to “control” the ball to some lateral displacement, a force must be applied opposite to the direction of displacement. This effect would be appreciated during flight of an unstable aircraft by an unstable “feel” of the aircraft. If the controls were deflected to increase the angle of attack, the aircraft would be trimmed at the higher angle of attack by a push force to keep the aircraft from continuing in the displacement direction. Such control force reversal would evidence the airplane instability; the pilot would be supplying the stability by his attempt to maintain the equilibrium. An unstable aircraft can be flown if the instability is slight with a low rate of divergence. Quick reactions coupled with effective controls can allow the pilot to cope with some degree of static instability. Since such flight would require constant attention by the pilot, slight instability can be tolerated only in airships, helicopters, and certain minor motions of the airplane. However, the airplane in high speed flight will react rapidly to any disturbances and any instability would create unsafe conditions. Thus, it is necessary to provide some positive static stability to the major aircraft degrees of freedom.
