Simulation and Control of a Helicopter Operating in a Ship Airwake

Open Access
Lee, Dooyong
Graduate Program:
Aerospace Engineering
Doctor of Philosophy
Document Type:
Date of Defense:
June 28, 2005
Committee Members:
  • Joseph Francis Horn, Committee Chair
  • Lyle Norman Long, Committee Member
  • Edward C Smith, Committee Member
  • Qian Wang, Committee Member
  • helicopter simulation
  • stochastic airwake
  • pilot model
  • validation
  • SAS optimization
This thesis describes a study in simulation and control of a helicopter operating in proximity to a ship. The helicopter/ship combination used in the study is a UH-60A helicopter operating off an LHA class ship. This represents the same aircraft ship combination used in the JSHIP program. The flight dynamics model is based on the GENHEL software and this flight dynamics model has been updated to include high-order dynamic inflow model and gust penetration effects of the ship airwake. To simulate the pilot control inputs for typical shipboard operations, an optimal control model of the human pilot is developed. The pilot model can be tuned to achieve different tracking performances based on a desired crossover frequency in each control axis and is designed to operate over a range of airspeeds using a simple gain scheduling algorithm. The pilot model is then used to predict pilot workload for shipboard operations in two different wind-over-deck conditions. Validation studies are conducted using both time and frequency domain analyses to understand the impact of a time-varying ship airwake on the pilot control activity for the approach and departure operations. The pilot control input autospectra predicted from the simulation model are compared to those of flight test data from the JSHIP program. It is found that the control activities are similar in low frequency range but underestimate in magnitude in the high frequency range (over 1.5 Hz). There is clear evidence that the human pilot is continually moving cyclic stick in the maneuver. At this stage of the study no attempt has been made to optimize the parameters of the human pilot model. The paper also discusses the application of a stochastic airwake model for more efficient simulation. This new airwake model is derived from the simulation with the full CFD airwake by extracting an equivalent six-dimensional gust vector. The spectral properties of the gust components are then analyzed, and shaping filters are designed to simulate the gusts when driven by white noise. It is proposed that the stochastic gust model can be used to optimize the automatic flight control system in order to improve disturbance rejection properties of the aircraft. A stability augmentation system (SAS) is optimized for a UH-60 helicopter operating in the turbulent ship airwake. For disturbance rejection, a new performance specification is designed based on the power spectral density of the transfer function from the gust inputs to aircraft rate responses. The baseline limited authority SAS is modified and optimized using CONDUIT (Control Designer's Unified Interface) in order to improve handling-qualities and stability, and to minimize a weighted objective of gust responses. In addition, a {$H_{infty}$} controller is designed to provide an alternative SAS configuration. The optimized SAS and {$H_{infty}$} SAS are then tested using the non-linear simulation model with time-varying airwake. Time domain and frequency domain analyses of the simulation show that the modified SAS results in significant reduction of pilot workload.