Investigation of bandwidth and disturbance rejection properties of a dynamic inversion control law for ship-based rotorcraft

A previous investigation studied the use of advanced response types and non-linear dynamic inversion (NLDI) control to improve handling qualities for shipboard operations from a moving ship deck with unsteady airwake. The results showed potential for ship-relative Translation Rate Command (TRC) control modes to significantly reduce pilot workload, at least in mild sea states. The paper extends the investigation to better understand the bandwidth and the disturbance rejection requirements of the NLDI controller (and for rotorcraft control characteristics in general), when operating in a range of sea states and airwake conditions. The US Army's rotorcraft handling quality specification, ADS-33E-PRF, provides no specific design guidance on bandwidth or disturbance rejection properties for maritime operations. A family of controllers was developed to test varying levels of bandwidth and disturbance rejection properties of Attitude Command / Attitude Hold (ACAH) and TRC control modes. The controllers gain sets were baselined at the minimum Level 1 ADS-33 requirements for response to pilot inputs and the 45° / 6-dB stability margins recommended by standard flight control design specifications. Piloted simulation tests were conducted to evaluate the handling qualities of the family of controllers in the midst of higher sea states and a Computational Fluid Dynamics (CFD) model for airwake turbulence. Simulations used the GENHEL-PSU UH-60 model integrated with the Penn State rotorcraft flight simulator. A maritime mission task element (MTE) was flown to evaluate handling qualities ratings (HQR) using the various response types and gain parameters. Results indicate that ACAH can improve HQRs over the conventional rate command mechanical control. HQRs with ACAH were still Level 2, and results indicate that the required ACAH bandwidth is significantly higher than that currently specified in ADS-33E-PRF. Results indicated that level 1 HQR could be achieved with ship-relative TRC in sea state 5, but that handling qualities were sensitive to rise time, with the required rise time at the low end of the range recommended by ADS-33E-PRF.