CONTROL POWER

The term control power is used to describe the efficacy of a control in producing a range of steady equilibrium or maneuvering states. For example, an elevator control which by taking positions between full up and full down can hold the airplane in equilibrium at all speeds in its speed range, for all configurations and C. G. positions, is a powerful control. On the other hand a rudder that is not capable at full deflection of maintaining equilibrium of yawing moments in a condition of one engine out and negligible sideslip is not powerful enough. The flying qualities requirements normally specify the specific speed ranges that must be achievable with full elevator deflection in the various important configurations, and the asymmetric power condition that the rudder must balance. They may also contain references to the elevator angles required to achieve positive load factors, as in steady turns and pull-up maneuvers (“elevator angle per g,” Sec. 6.10).

CONTROL FORCES

The requirements invariably specify limits on the control forces that must be exerted by the pilot in order to effect specific changes from a given trimmed condition, or to maintain the trim speed following a sudden change in configuration or throttle setting. They frequently also include requirements on the control forces in pull-up maneuvers (“stick force per g,” Sec. 6.10).

STATIC STABILITY

The requirement for static longitudinal stability (see Chapter 6) is usually stated in terms of the neutral point (defined in Sec. 6.3). It is usually required that the relevant neutral point (stick-free or stick-fixed) shall lie some distance (e. g. 5 % of the mean aerodynamic chord) behind the most aft position of the C. G. This ensures that the airplane will tend to fly at a constant speed and angle of attack as long as the controls are not moved.

The requirement on static lateral stability is usually mild. It is simply that the spiral mode (see Chapter 9) if divergent shall have a time to double greater than some stated minimum (e. g. 4 see).

DYNAMIC STABILITY

Generally the requirement on dynamic stability takes the form of a specifi­cation on the time to damp to half amplitude. The damping required for good flying qualities varies with the period.

STALLING AND SPINNING

Finally, most requirements specify that the airplane’s behavior following a stall or in a spin shall not include any dangerous characteristics, and that the controls must retain enough effectiveness to ensure a safe recovery to normal flight