Investigation of Bandwidth and Disturbance Rejection Properties of a Dynamic Inversion Control Law for Ship-Based Rotorcraft

Open Access
Author:
Zheng, Albert
Graduate Program:
Aerospace Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
None
Committee Members:
  • Joseph Francis Horn, Thesis Advisor
Keywords:
  • Helicopter
  • Rotorcraft
  • Flight Controls
  • Simulation
  • Handling Qualities
  • Shipboard
  • Bandwidth
  • Disturbance Rejection
  • Pilot
Abstract:
This thesis is an investigation to better understand the bandwidth and the disturbance rejection requirements of a non-linear dynamic inversion (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 qualities 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 for bandwidth were baselined at the minimum Level 1 ADS-33 requirements for response to pilot inputs. Two gain sets for disturbance rejection were developed with the first set of gains baselined at the 45$^{\circ}$ / 6-dB stability margins recommended by standard flight control design specifications. The second gain set was baselined at the proposed disturbance rejection criteria developed by the Aeroflightdynamics Directorate (AFDD). Piloted simulation tests were conducted to evaluate the handling qualities of the family of controllers in the midst of high sea states and a turbulent airwake. A Computational Fluid Dynamics (CFD) airwake model was used for testing the disturbance rejection gain set baselined at the standard stability margins. The second gain set using the AFDD baseline was assessed with a Control Equivalent Turbulence Equivalent (CETI) model. 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. Time-frequency metrics were used to supplement the pilot ratings in order to assess the impact of pilot workload and strategy on the handling qualities in the shipboard environment. Results indicate that ACAH can improve HQRs over the conventional rate command mechanical control system of the UH-60A. HQRs with ACAH were still Level 2, and the required bandwidth in order to improve HQRs is significantly higher than that currently specified in ADS-33E-PRF. Results also demonstrated that HQRs can be improved by increasing the disturbance rejection bandwidth (DRB) in ACAH in mild sea states or without ship motion. Pilot ratings indicated ACAH can achieve Level 1 HQRs in sea state 3, but the required DRB is much greater than the proposed minimum criteria recommended by the AFDD. The HQRs also show that sea state 4 diminishes the benefit of DRB where the ratings become strictly Level 2. Results indicated that a 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. Initial results also show that HQRs can be improved by increasing the DRB in ship-relative TRC in sea state 4. Pilot ratings indicated ship-relative TRC can achieve Level 1 HQRs, but the required DRB is again significantly higher than AFDD's minimum criteria. The results also demonstrate that utilizing time-frequency metrics for pilot workload provide useful indicators on the impact of gain selection to the handling qualities in shipboard operation. The metrics tended to correlate with pilot comments and ratings for the family of controllers.