AN INVESTIGATION OF THE IMPACT OF AUTOMATIC FLIGHT CONTROL SYSTEM SATURATION ON HANDLING QUALITIES IN HOVER/LOW SPEED MANOEUVRES

This paper describes a ground-based piloted simulation study of Partial Authority Flight Control Augmentation concepts. PAFCA seeks to achieve the functionality of highly augmented FBW/ACT systems within the constraints of current generation control system architectures with a view to the potential for in-service upgrades. The objective of the study was to investigate the impact of AFCS saturation on handling qualities in hover/low speed manoeuvres. It was found that: • To avoid AFCS saturation, a ±35% pitch AFCS limit and a ±25% roll AFCS limit were required. Note that these data relate to the maximum manoeuvre capability of a ACAH response type, with no auto-trim follow-up, in good visual conditions and with no prevailing atmospheric disturbances. • Saturation was not always detrimental and borderline Level 1 handling qualities ratings (HQR ≤ 4) were achieved, even at maximum aggression, with a ±25% pitch AFCS limit and a ±15% roll AFCS limit. • Data would suggest that pilots were not perceiving saturation per se, but rather the magnitude and/or phase of the model-following error. • Matching augmented and unaugmented dynamics in the frequency range of the task bandwidth resulted in more benign and predictable saturation characteristics. • Frequency-matching also increased Agility Factor by 10% in a quickhop manoeuvre and by 20% in a sidestep manoeuvre as well as improving HQRs. Further research is required to generalise these data to different rotor systems, response types, series/parallel actuation architectures and environmental conditions, particularly DVE operations. It is hypothesised, however, that independent of the PFCS - i.e. mechanical or FBW - the AFCS need only be limited authority to achieve Level 1 handling qualities, although pitch axis limits may need to be increased with respect to currently fielded systems.