AUTOMATIC FLIGHT CONTROL SYSTEMS
Helicopter automatic flight control systems (AFCS) are many and varied. In some cases they provide slight compensation for the ‘raw’ aircraft characteristics whereas in others these characteristics are completely masked (so-called superaugmentation) and the pilot’s perception of the aircraft is solely based on the nature and performance of the AFCS. This section is intended to give the reader an understanding of rotorcraft AFCS without describing in great detail all the various systems currently available or envisaged for the future. After a brief review of the important stability characteristics of helicopters, and a discussion of the levels of augmentation available, the major system components are described. The section concludes with descriptions of generic systems.
6.5.1 Helicopter stability deficiencies
Before discussing AFCS in detail it is instructive to review the important stability characteristics of helicopters. The inherent stability problems associated with generating lift and thrust from an ‘edgewise’ rotor mean that only the lightest and cheapest helicopters on the market are unaugmented.
220.127.116.11 Longitudinal long-term mode
The longitudinal long-term mode contributes to the multi-axis oscillation, the ‘falling leaf mode’, found in the hover. This mode is usually unstable and will only be satisfactory if the time to double amplitude (T2) and period are sufficiently long that mission workload is tolerable. In forward flight the long-term mode is similar to the classic phugoid found in conventional fixed wing aircraft, although with greater changes in pitch attitude. Depending on the characteristics of the horizontal stabilizer and, therefore, the degree of speed stability, the mode can be either convergent or divergent. Once again the rate of divergence, the period of the motion and the intended role of the helicopter will determine if this mode can be tolerated without augmentation. In some extreme cases, such as a semi-rigid rotor at high speed, this mode may become aperi – odically divergent, in which case augmentation will be imperative.
18.104.22.168 Manoeuvre stability
The manoeuvre stability of all helicopters will degrade to instability at high speed and high load factor due to the progressively greater destabilizing effect of the main rotor. Therefore, for helicopters required to operate at high load factors, some form of augmentation will normally be fitted. The greater pitch damping provided by rate stabilization is a possible solution, as is the use of a programmable stabilizer.