Ajay Singh
Published: 2018
Total Pages:
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Helicopters must carry out a variety of missions, ranging from military to civilian uses. Missions mayinvolve delivery of a payload from one location to another. Some loads are externally attached to thehelicopter by cables. In this configuration, the loads are referred to as slung loads. Due to the couplingbetween the slung load aerodynamics and inertial forces, loads dynamics can become unstable whenairspeed increases. Slung load instabilities limit the flight operations of a rotorcraft. Because limitingflight speeds reduce the operational efficiency of the rotorcraft, methods for stabilizing external loadsin forward flight are the subject for research. In recent years, the dynamics and control of slung loadswere studied using analysis, dynamic wind tunnel tests, and flight tests.The research presented in this thesis investigated a control design methodology and its feasibilityto stabilize an external load across the flight envelope, including high speed flight. The capability of anactive cargo hook (ACH) to provide external load stabilization in high speed flight is studied. The ACHis an electromechanical device that can slide longitudinally and laterally along the base of the fuselage.Previous work used the ACH to directly control the loads roll and pitch but only during hover and lowspeed flight. Previous studies results proved promising.The test load and helicopter simulated in this thesis is a CONEX cargo container and a UH-60 BlackHawk helicopter, respectively. During high speed flight, the load can become unstable, exhibiting sustainedperiodic motion, or limit cycle oscillations, which can degrade helicopter handling qualities. Previousstudies observed the load dynamics in a wind tunnel. The findings showed the excessive swingingand 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 positionsare used as inputs to the controller with the commanded cargo hook longitudinal and lateral positionsas outputs. Results showed high damping in the loads attitude response with little saturation in theACH stroke and stroke rate. The full-state LQR controller demonstrated success in stabilizing the slungload.The FSF controller, however, requires sensors to measure the loads states real-time. A more practicalapproach 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. Thereduced order model is used to design an LQR controller for the ACH. The inputs for the controllerare the relative cable angles, relative cable angular rates, and ACH positions. The results demonstratedbetter performance than the FSF LQR controller, stabilizing the load approximately 20 percent quicker.The loads damping of the RCAF controller is higher than the FSFs and the ACH does not saturate instroke or stroke rate. The settling time of the load was also improved significantly. Furthermore, thecontrollers robustness was tested through applying a Dryden Turbulence model in the simulations. TheRCAF was able to appropriately stabilize the load through low, mild, and severe turbulence levels.