Rotorcraft Precision Hover Control in Atmospheric Turbulence via Eigenstructure Assignment

The rotorcraft Aeronautical Design Standard (ADS-33E-PRF) specifies a frequency domain criterion to define handling qualities rating for pitch–roll angular rate cross-axis coupling in the "bandwidth" frequency range. The specification is derived based on flight tests in forward flight conditions. For rotorcraft operating near "hover," cross-axis coupling in the pitch–roll–yaw axes originates from both angular rate and low-frequency translational rate interaction. It is shown that this multiaxis cross-coupling including the entire rigid body mode spectrum is a dominant factor for handling qualities deterioration. Hover operations under turbulent atmospheric disturbances further degrade handling qualities, leading to aggravated pilot workload and cause safety of flight concerns. Control augmentation schemes, based on an eigenstructure assignment technique, are proposed to mitigate cross-axis rate coupling and improve disturbance rejection to enable precision "hover" control in turbulent atmospheric conditions.