Design for Flying Qualities; Stability and Control Augmentation

In the helicopter design trade-off, flying qualities have often had to take a low prior­ity. In the early days of rotorcraft, just as with fixed-wing aircraft, solving the basic control problem was the breakthrough required for the development to progress with pace, driven largely by performance considerations. The basic layout of the single rotor helicopter has remained the same since the early Sikorsky machines. What char­acterizes a modern helicopter is its higher performance (speed, payload), much im­proved reliability – hence greater safety, smoother ride and a suite of mission avionic systems that enable civil operations in poorer weather and military operations as an autonomous weapon system. Performance, reliability, comfort and functionality have been the drivers in helicopter development, and for many years flying quality was a by-product of the design, with deficiencies compensated for by highly trained pilots with a can-do attitude. As we have seen from our discussions earlier on this tour, heli­copter flying characteristics are typically much poorer than fixed-wing aircraft in the same ‘class’. In some cases, helicopters fall in the Level 3 quality area when built. The principal flying qualities deficiencies in the helicopter can be summarized as follows:

(1) impurity of the primary response in all axes, i. e., typically a mix of attitude or rate and varying significantly from hover to high speed;

(2) strong cross-couplings in all axes;

(3) the degradation of response quality at flight envelope limits and the lack of any natural carefree handling functions, e. g., the aerodynamic capability of the rotor typically exceeds the structural capability;

(4) the stability of a helicopter is characterized by a number of modes with low damping and frequency at low speed; as forward speed is increased, both longitudinal and lateral modes increase in frequency, as the tail surfaces contribute aerodynamic stiffness, but the modal damping can reduce and stability can often worsen, particularly with highly responsive hingeless rotors;

(5) the rotor presents a significant filter to high bandwidth control.

The combination of the above has always demanded great skill from helicopter pilots and coupled with today’s requirements for extended operations in poor weather and visibility, and the need to relieve the piloting task in threat-intensive operations, led to the essential requirement for stability and control augmentation. Before discussing artificial stability, one first needs to look more closely at the key natural design fea­tures that contribute to flying qualities. The discussion will map directly onto the five headings in the above list and an attempt is made to illustrate how, even within the flying qualities discipline, compromises need to be made usually to satisfy both high – and low-speed requirements simultaneously.