Cloosed loop IBC results from recent CH-53G flight tests

Since December 2001, ZF Luftfahrttechnik GmbH (ZFL) has conducted open and closed loop IBC (Individual Blade Control) flight tests with the CH-53G IBC testbed of the German Federal Armed Forces Engineering Center for Aircraft. Over 25 flight hours had been spent in an “open loop” campaign to investigate the positive IBC effects. Early during the flight tests, it became clear that the response of the aircraft to the IBC inputs exceeded the initial expectations. During the past two years, ZFL has expanded the open loop IBC system to enable a closed loop operation for automatic online optimization of the required IBC inputs. The controller design was performed under Matlab/Simulink and automatically implemented on a dSPACE real-time system. The closed loop control system added to the open loop core system extends the existing cascade control structure by an outer control loop. This outer control loop was used to implement different control tasks which were then investigated during the closed loop campaign. The main focus was put on vibration reduction in the fuselage. Further tests have addressed the load reduction potential of IBC. The capabilities of IBC were demonstrated for different singleand multiple-harmonic inputs in different steady and maneuvering flight conditions at different forward speeds. The predicted improvements through the application of more than one frequency could clearly be shown. Different cost functions have been investigated with the controller using single or multiple sensor signals. It turned out that even simple cost functions based on only few sensor signals were practical since often also the not included sensor locations clearly benefited from that particular input. The amplitudes applied by the controller were in the order of 0.1 to 0.5deg which has underlined the effectiveness of even small blade pitch inputs. The paper gives a brief overview of the architecture of the closed loop system and the implemented control strategies. The main focus is put on the test results gathered during the different closed loop flights. Beside the beneficial vibration reduction also some other IBC effects are discussed. Presented at the 30 European Rotorcraft Forum, 14 – 16 September 2004, Marseilles, France Notation AccHGx, -y, -z g acceleration at main gearbox in x-, y-, z-direction AccHeckx, -y, -z g acceleration at tail rotor gearbox in x-, y-, z-direction AccLadx, -y, -z g acceleration at cargo compartment in x-, y-, z-direction AccPilx, -y, -z g acceleration close to pilot seat in x-, y-, z-direction An deg n/rev component IBC amplitude (AMPLn in figures) CLCS Closed Loop Control System g m/s 9.81, gravity constant HHC Higher Harmonic Control IBC Individual Blade Control Jctrl cost function to be minimized J -/g/N square root of cost function J without weighting of IBCinputs and IBC-input changes MTOW Max. Take-Off Weight N 6, number of blades PLL Pitch link load (=actuator axial force) T g/deg N/deg IBC to output response transfer matrix (linear and quasi-static) Ts,ctrl s controller sample time VIAS kts Indicated Air Speed Wi weighting matrix w. respect to i z g vector of vibrations and control loads (cos, sin component) ex,cos, ex,sin g estimation error of cosine and sine component of x θIBC=Σ Ancos(nψ−φn) nominal pitch angle due to IBC θ deg vector of higher harmonic IBC control inputs (cos, sin, compon.) φn deg n/rev IBC control phase angle (PHASEn in figures) Ω rad/s rotor rotational frequency