A Control Allocation Method for a Helicopter with On-Blade Control

Open Access
Author:
Wilson, Andrew David
Area of Honors:
Aerospace Engineering
Degree:
Bachelor of Science
Document Type:
Thesis
Thesis Supervisors:
  • Joseph Francis Horn, Thesis Supervisor
  • Robert Graham Melton, Honors Advisor
Keywords:
  • helicopter flight control
  • control allocation
  • trailing edge flaps
  • frequency splitting
  • GENHEL simulation
  • model following control
  • UH-60A
Abstract:
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Helicopter flight during shipboard operations or in turbulent wind conditions can result in very high pilot workload due to the unsteady air flow. Improvements in control methods or expanded control authority of the aircraft could allow the flight envelope to be expanded while decreasing the amount of compensation the pilot must put into the system to maintain stability. This thesis presents the results of a control allocation method with shared control between the traditional swashplate control surface and on-blade trailing edge flaps. Prior research has examined the use of trailing edge flaps solely for wind gust rejection; however, the present research will look at the feasibility of using the flaps for primary flight control as well. <br><br> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;The control architecture is based on a model-following controller which uses model inversion to derive ideal inputs from pilot stick inputs to yield more desirable response and handling from the helicopter. These ideal commands are passed through high and low pass filters to split the control signal based on frequency. The lower frequency control is passed to the swashplate while higher frequency control is passed to the trailing edge flaps. This could enable the helicopter to maintain more desirable handling qualities, even with degradation in swashplate rate actuation. Simulations of the system are performed with both a linear model using Simulink and a full flight simulation using the GENHEL flight simulation code. <br><br> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Results from a linear Simulink model demonstrate the operation of the frequency split allocation controller with degradation in swashplate rate actuation by using trailing edge flaps to handle high frequency control response. However, full flight simulation produces problems with saturation of the trailing edge flap actuators, even under normal flight conditions. With too much control being allocated to the flaps, the travel limits are quickly exceeded with the high frequency commands. To alleviate the issue, lower gains are implemented as well as reduced allocation to the flap actuators. While this allows the system to perform with normal handling quality, there is little qualitative difference in handling between the pure swashplate system and the shared control, even under degradation of the swashplate actuators. The results suggest that while primary control can be allocated to the flaps, there are serious limitations in the current travel limits of the flaps and the ability to handle the control input without reaching saturation limits. Under controlled circumstances, the benefit of the added control may be realized; however, in normal flight simulation, minimal qualitative difference is recorded.