External Load Stabilization Across the Flight Envelope Using an Active Cargo Hook
Open Access
- Author:
- Singh, Ajay
- Graduate Program:
- Aerospace Engineering
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- January 19, 2018
- Committee Members:
- Joseph Francis Horn, Thesis Advisor/Co-Advisor
- Keywords:
- Slung
External Load
Active Cargo Hook
LQR
Full-State Feedback
Reduced Order Model
Relative Cable Angle
Cable Angle Feedback
CONEX
Dryden
Bifurcation
Sling - Abstract:
- Helicopters must carry out a variety of missions, ranging from military to civilian uses. Missions may involve delivery of a payload from one location to another. Some loads are externally attached to the helicopter by cables. In this configuration, the loads are referred to as slung loads. Due to the coupling between the slung load aerodynamics and inertial forces, loads dynamics can become unstable when airspeed increases. Slung load instabilities limit the flight operations of a rotorcraft. Because limiting flight speeds reduce the operational efficiency of the rotorcraft, methods for stabilizing external loads in forward flight are the subject for research. In recent years, the dynamics and control of slung loads were studied using analysis, dynamic wind tunnel tests, and flight tests. The research presented in this thesis investigated a control design methodology and its feasibility to stabilize an external load across the flight envelope, including high speed flight. The capability of an active cargo hook (ACH) to provide external load stabilization in high speed flight is studied. The ACH is an electromechanical device that can slide longitudinally and laterally along the base of the fuselage. Previous work used the ACH to directly control the load’s roll and pitch but only during hover and low speed flight. Previous studies’ results proved promising. The test load and helicopter simulated in this thesis is a CONEX cargo container and a UH-60 Black Hawk helicopter, respectively. During high speed flight, the load can become unstable, exhibiting sustained periodic motion, or limit cycle oscillations, which can degrade helicopter handling qualities. Previous studies observed the load dynamics in a wind tunnel. The findings showed the excessive swinging and rotation in the slung load are due to its nonlinear dynamics. The control methodology first examined designed a full-state feedback (FSF) linear-quadratic regulator (LQR) controller. In this controller, the load states and cargo hook longitudinal and lateral positions are used as inputs to the controller with the commanded cargo hook longitudinal and lateral positions as outputs. Results showed high damping in the load’s attitude response with little saturation in the ACH stroke and stroke rate. The full-state LQR controller demonstrated success in stabilizing the slung load. The FSF controller, however, requires sensors to measure the load’s states real-time. A more practical approach is using a reduced order model (ROM) using relative cable angle feedback (RCAF). With RCAF, the relative cable angles can be measured real-time, requiring less sensors and measurements. The reduced order model is used to design an LQR controller for the ACH. The inputs for the controller are the relative cable angles, relative cable angular rates, and ACH positions. The results demonstrated better performance than the FSF LQR controller, stabilizing the load approximately 20 percent quicker. The load’s damping of the RCAF controller is higher than the FSF’s and the ACH does not saturate in stroke or stroke rate. The settling time of the load was also improved significantly. Furthermore, the controller’s robustness was tested through applying a Dryden Turbulence model in the simulations. The RCAF was able to appropriately stabilize the load through low, mild, and severe turbulence levels.