Closed loop IBC results from CH-53G flight tests

Abstract

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. 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 single- and 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 a few sensor signals were practical since often the not included sensor locations clearly benefited from that particular input. The amplitudes applied by the controller were on the order of 0.1 to 0.5 deg 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.