Characteristics of typical stability augmentation systems
Before describing the assessment of AFCS equipped aircraft in detail (see Section 7.5) it is worth discussing the characteristics of typical systems that arise from their hardware implementation. In our experience we have found such a discussion to be very useful as the precise design of an AFCS not only affects the conduct of stability and control flight testing but often affects the interpretation of the results obtained from standard test techniques and manoeuvres.
Stability augmentation systems are designed to suppress the longitudinal long-term mode and the LDO. The SAS, in its basic form, consists of a device sensitive to the rate of change of attitude, a rate gyro, which feeds a series actuator placed in the control run. Corrective cyclic pitch inputs are thus made by the SAS in opposition to and proportional to, the rate of change of pitch or roll (see Fig. 6.18). A similar system operates in the yaw channel by feeding corrective inputs to the tail rotor control. An airspeed switch may, however, disable it as the fin tends to provide an increased contribution to yaw damping at high airspeed.
As discussed briefly in a previous chapter increased rate damping (angular rate feedback) can be used by the control law designer to enhance the control response characteristics of the helicopter by making the time to steady rate shorter (improved predictability) at the expense of a reduced output. Thus there is a strong argument for retaining the rate feedback path whilst the pilot is manoeuvring the helicopter.
Fig. 6.18 Block diagram of a typical rate-based stability augmentation system.
Unfortunately it may be difficult to find an optimal gain that provides adequate gust rejection as well as appropriate control response shaping.
In order to improve the handling qualities of the helicopter, beyond the application of rate feedback, it is usually necessary to apply corrective cyclic inputs in response to attitude changes (integral feedback). This can be achieved either by integrating the signal from the rate gyro (pseudo-attitude) or by direct measurement of the aircraft’s true attitude. In each case the signal is fed back to the series actuator to provide a command that opposes deviations from some attitude datum. Note that integrated rate is not incorporated into the yaw channel, as it does not relate well to aircraft heading. The key issue arising from this control law improvement concerns how the attitude signal is disabled during manoeuvres and what response type results: Rate Command/Attitude Hold (RCAH) or Attitude Command/Attitude Hold (ACAH).