Appraisal of handling qualities standards for rotorcraft lateral-directional dynamics

The coupled vehicle roll/yaw/sway motion of lateral-directional oscillations is often a contributor to rotorcraft handling qualities deficiencies. The extent of the deficiencies, and the required pilot control compensation to mitigate their effects, depends critically on the damping and frequency of the lateral-directional oscillatory mode. Current rotorcraft performance/certification standards (e.g. ADS-33E-PRF/CS-29) for mode stability have been developed from standards that date from the 1950s or from fixed-wing requirements; there has been limited flight testing to support their validation. This paper examines the suitability of these dynamic stability criteria to modern rotorcraft using ground-based simulation, by assessing simulation model configurations covering a range of handling qualities, selected based on frequency and damping. The underlying simulation model is a FLIGHTLAB Bell 412 model, augmented using a model renovation technique, to ensure that the baseline configuration resembles National Research Council of Canada’s test aircraft, and the non-lateral-directional oscillation handling qualities are Level 1. The test configurations have been developed with delta-derivatives added to the nonlinear model to change the mode frequency and damping, whilst preserving yaw sensitivity and the magnitude ratio of the roll/ yaw motion. The preliminary results demonstrate that the handling qualities improve with increasing damping, giving a reasonable match with the military standards. However, Level 2 deficiencies generally prevented achievement of desired performance at low workload. Further work is needed examine the suitability of current lateral-directional oscillation civil certification criteria.