A control design method to address stability and handling qualities issues associated with helicopter slung load operations in hover and low speed is presented. A low-order model is developed using first principle physics, with application of basic control techniques. Subsequently, a nonlinear slung load model is developed and integrated with the GENHEL-PSU simulation of the UH-60. Linear model frequency responses are verified against Aeroflightdynamics Directorate (AFDD) OVERCAST models and flight data, showing good correlation in the relevant frequency ranges. A control architecture based on dynamic inversion is developed, combining fuselage and load state feedback. Slung load states are added in feedback linearization, and lagged cable angle feedback is introduced. The controller is shown to reduce load oscillations with trade-offs in pilot response. A controller that uses only lagged cable angle feedback (and no cable state feedback)is also investigated and found to provide good load stabilization without the use of noisy cable angle and rate sensor signals. Sensitivity to parameter variations and optimization methodologies are considered in aiding the design process. Nonlinear batch and real-time simulations are conducted to evaluate performance of the controller.
This study examines the effectiveness of an on-blade extendable tab mechanism for rotor track-and-balance. The tab is essentially a chord-extension morphing mechanism, implemented as an extendable trailing-edge-plate over a spanwise section of each blade. A simulation model of the UH-60 Black Hawk rotor with seeded imbalance is developed, with the extendable tabs minimizing the 1/rev (1P) vibratory loads using a weighted least-squares optimization method. The extendable tab has the ability to reduce the 1P in-plane forces over the entire airspeed range with very large reductions observed in hover. The tabs are unable to reduce 1P vertical forces in hover, but are very effective in reducing these vibratory loads in cruise. The extendable tab is moderately effective in reducing 1P in-plane moments over the range of airspeeds. Best reductions in 1P loads are achieved by employing an active tab mechanism (adjusted at different airspeeds) over a passive mechanism (with constant setting over the airpseed range), with the active tab yielding additional gains in 1P in-plane forces and moments in hover and in 1P vertical forces in cruise. In hover, an examination of the load reduction mechanism indicates that a seeded radial shear and root torsional moment imbalance is cancelled by the generation of net blade root chordwise shear and root flap bending moments, respectively, on orthogonal blades. In cruise, similar mechanisms were observed, but the generation of net radial shears and root torsional moments on parallel blades were also contributors to total reduction in 1P hub in-plane and pitching moment vibrations.
This study focuses on the flight simulation and control of a helicopter undergoing rotor span morphing. A model-following dynamic inversion controller with inner and outer loop Control Laws (CLAWS) is implemented, and radius change is introduced as a feedforward component to the inner loop CLAWS. Closed-loop poles associated with the low-frequency aircraft modes are observed to be robust to change in rotor span, eliminating the need for model updates due to span morphing during the dynamic inversion process. The error compensators in the CLAWS use PID control for roll and pitch attitude, PI control for yaw rate and lateral and longitudinal ground speed, but require PII control for vertical speed to avoid altitude loss observed with only PI control, during span morphing. Simulations are based on a span-morphing variant of a UH-60A Black Hawk helicopter at 18,300 lbs gross-weight and 40 kts cruise. From a baseline rotor radius of 26.8 ft, retraction to 22.8 ft, as well as extension to 31.5 ft is considered, nominally over a 60 sec duration. The controller is observed to regulate the longitudinal, lateral and vertical ground speeds well over the duration of the span morphing. Further, the controller is observed to maintain its effectivess in regulating the ground speeds when the span morphing duration is reduced to 30 sec.
A flight simulation model for the UH-60 Black Hawk, based on Sikorsky's GenHel model, is modified to simulate a locked failure of a main rotor swashplate servo actuator, which is compensated by using the stabilator as a redundant control effector. Steady-state trim analysis is performed to demonstrate feasibility of trimmed flight in various conditions with different locked servo actuator positions for the forward, aft, and lateral actuators. A model-following, linear dynamic inversion controller is implemented and modified to account for locked actuator position. Postfailure, the controls are reconfigured to partially reallocate the control authority in the longitudinal axis from the main rotor longitudinal cyclic to the stabilator. This is done by manipulation of only the control allocation relating pilot stick inputs to servo actuator positions, whereas the feedback control gains and mechanical rigging between servo actuators and rotor pitch controls remain identical to the baseline. Flight simulation results demonstrate the ability of this reallocation to compensate for locked failure of the forward main rotor swashplate servo actuator, as well as the ability of the aircraft to decelerate from cruise at 120 kt to 50 kt, slower than the published safe rolling landing speed of 60 kt. A similar range of locked positions of the forward and aft actuators is demonstrated to be feasible for aircraft recovery using control of the stabilator. Feasibility of aircraft recovery for locked positions of the lateral servo actuator is also considered, as well as the effect of variation in gross weight, speed of actuator locking, and delays in fault detection and identification.
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