Estimation of Structural Component Loads in Helicopters: A Review ...

DSTO-TN-02395. Deducing Loads on Dynamic Components fromFlight Parameter and Fixed System Load MeasurementsThe method of holometrics [19, 21, 22] was further developed by Gustavson, Pellum, andRobeson [42]. They used existing flight load data to determine the most cost-effectivecombinations of fixed system parameters and data sample rates to be used to accuratelysynthesise rotating component loads for the AH-64A helicopter. These parameters includedboth currently monitored and newly added parameters.The use of Fourier coefficients in the original holometrics model was modified to reduce theamount of computation. Gustavson et al. now develop a system of equations for a fewparameter-azimuth combinations (thus variations in rotor speed need to be taken intoaccount). The rows of the input matrices (fixed and rotor data matrices) now contain timehistory information instead of Fourier coefficients. The minimisation of a cost-functiondetermined the best combination of parameters and associated sampling frequencies thatwould still achieve a required error tolerance in the rotating component load estimation. Ahigh cost was assigned to both high frequency sampling and the need to measure extraparameters, while a low cost was assigned to low frequency sampling and the inclusion ofcurrently sampled parameters only. Other costs assigned to the cost-function includedengineering development, equipment acquisition, installation, field support, powerrequired, weight, volume, and amount of processing required.A parallel-orthogonal vector reduction algorithm was used. This algorithm selected the bestpotential starting parameter and then chose a minimal set of subsequently less effectiveparameters that would achieve the desired error tolerance. The first parameter was chosenbased on parallelism (or similarity) to the rotating parameter. Since the dot product of twovectors is a measure of parallelism, the angle derived from the dot product was used as ameasure of similarity.Gunstavson et al. state that ‘[s]ynthesis using these parameters did not yield satisfactory results.’Instead, they implemented a modified method where the first (most similar) parameter waschosen as described above, and then they chose the next seven parameters closest to beingorthogonal to the first parameter. This led to a dramatic improvement and thus thisvariation to the original algorithm was adopted.The loads that need to be identified for component fatigue life damage estimation purposescomprised chordwise bending moment, torsion, pitch link force, leading edge lead-lagdamper force (all for the main rotor blade), and vertical bending moment at one of thefuselage stringers. The full set of parameters initially monitored comprised collectivecontrol position, lateral control actuator force, pitch and roll attitudes, lateral acceleration ofthe centre of gravity, yaw rate, longitudinal control actuator force, and longitudinalbending moment of the main rotor mast tube root. (Note that in the AH-64A helicopter themain rotor mast tube is a static member that takes the rotor bending moments, and acts like23