EFFECT OF AFCS ON HANDLING QUALITIES TESTING

In Chapter 6 the hardware associated with typical automatic flight control systems and the functioning of generic systems was described briefly. Although it is always important to distinguish between the functionality of stability augmentation and outer-loop or autopilot modes, a possible blurring of the definitions was pointed to when the hardware implementations of typical heading and height holds were considered. In terms of system architecture these channels tend to mimic basic pitch/roll attitude stabilization. In each case a primary sensor (compass or altimeter) is used to generate an error signal with the tightness of the hold being enhanced by rate feedback obtained from a gyro (for yaw) or integrated acceleration (for height). To provide a useful shorthand for the following discussion system based definitions of two generic forms of automatic flight control systems will be made. Thus a SAS is defined as an AFCS that provides pitch, roll and yaw rate stabilization (enhanced rate damping) and short-term attitude hold. An ASE or ATT mode AFCS is a system that provides pitch/roll attitude stabilization for enhanced stability augmentation and gust rejection via long-term attitude hold with pitch, roll and yaw rate feedback active at all times (unless disabled by the action of a trim release switch). The ASE will also provide heading hold at all speeds and possibly turn co-ordination in forward flight. It is interesting to note that these two definitions are sometimes used to describe modes provided by a dual functioning AFCS that overcomes the reduced agility commonly associated with ACAH by providing a SAS/ ASE or SAS/ATT switch to restore rate command.

7.5.1 Stability augmentation systems (SAS)

7.5.1.1 Longitudinal long-term, LDO and falling leaf modes

The additional rate damping associated with a SAS will help to suppress the nuisance modes. The action of the SAS in applying a control input in opposition to any measured rate of change of attitude will reduce the ease with which any long-term mode is excited. In addition the pseudo-attitude hold available by integrating the rate signal will provide a short-term datum to which the aircraft can be returned following minor excursions arising from atmospheric disturbances. Due to the limited authority of the series actuators employed and the low quality attitude datum, it is unlikely that these long-term modes will be completely suppressed in all levels of turbulence although an acceptable reduction in pilot workload is usually provided.

7.5.1.2 Control response

In addition to providing a reduction in pilot workload, during straight and level flight and hovering, a SAS can be employed to improve the control response. As mentioned earlier the increase in rate damping resulting from rate feedback will tend to increase control predictability by reducing the time constant (at the expense of steady state rate). Thus for any given discrete attitude change although the control will need to be displaced further the time taken for the rate of change of attitude to achieve a steady value will be reduced.