Recent IBC flight test results from the CH-53G helicopter

Since December 2001, ZFL has conducted Individual Blade Control (IBC) flight tests with a CH-53G testbed of the German Federal Armed Forces Engineering Center for Aircraft. The experimental IBC system used has been designed, manufactured, installed, certified, and tested by ZFL. This paper shows a comprehensive overview of the results from the concluded open loop and the just started closed loop campaigns. Vibration reduction turned out to be highly successful, especially with respect to the small required IBC authority. Without being explicitly optimized with respect to amplitude or phase, IBC was able to reduce the vibrations by more than 90% in a single axis or by more than 60% in all spatial directions in certain cases. The noise experiments were focused on the reduction of BVI in descent flight conditions. With 2/rev IBC of 0.67 deg amplitude, noise reductions of up to 3 dB have been recorded. Following the trends known from prior wind tunnel experiments, the increased amplitudes, which are used during the closed loop campaign, should allow for even higher reductions. The positive impact of IBC on rotor performance at high forward speed was also successfully demonstrated. The simultaneous effect of IBC on both the rotor power and the flight condition corresponds to net power savings of up to 6% at 130kts forward speed. This means the application of optimum IBC not only reduces the rotor torque but also tends to increase forward speed and let the rotorcraft climb. The lower frequencies primarily used to reduce noise and power required also have considerable effect on the control system loads. In the optimum case encountered during the open loop tests, 2/rev IBC of the right phase was able to reduce the vibratory pitch link loads by more than 30%. Fortunately, it turned out that at high speed, the IBC input for optimum rotor performance also decreases the pitch link loads. The last part of the paper gives an overview of the system modifications which had to be implemented for the closed-loop flight tests. It will be shown what control system architecture is available and what algorithms have been tested so far. Notation AFCS Automatic Flight Control System AccHGx, -y, -z g acceleration at main 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 IBC amplitude BVI Blade Vortex Interaction FMEA Failure Mode Effect Analysis G(...) = (x+y+z) unweighted cost function g m/s 9.81, gravity constant HHC Higher Harmonic Control IBC Individual Blade Control J Cost function to be minimized N 6, number of blades T1, T2 g/deg g/deg IBC to vibration response transfer matrix (linear and nonlinear) (M)TOW (Maximum) Take-off Weight P kW Power Q Nm Rotor Torque VIAS kts Indicated Air Speed u deg vector of higher harmonic control inputs (cos, sin, compon.) Wi Weighting matrix with respect to i z g vector of vibrations and control loads (cos, sin compon.) ∆θIBC=Σ Ancos(nψ−φn) nominal pitch angle due to IBC γ deg Flight path angle φn deg n/rev IBC control phase angle ψ deg rotor azimuth angle Ω rad/s Rotor Speed