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

DSTO-TN-0239The spectrum recognition method worked well (resulting in a small amount of scatter)despite consistently over-estimating the pitch link fatigue life usage. (This over-estimationwas expected since the spectra used in the analysis were the upper bounds of theclassification spectra). The on-line method resulted in a larger scatter than the spectrumrecognition method, but this time the fatigue damage was under-estimated for somecomponents and over-estimated for others, the method performing worst for low loads. Acumulative fatigue damage comparison exhibited better results for both the spectrumrecognition and on-line methods.Barndt and Moon [13] developed a flight condition recognition monitoring system for theAH-1W helicopter. Flight data were post-processed to determine fatigue damage incomponents for each aircraft. The monitoring system enabled identification of all flightconditions, and determination of individual aircraft usage and component fatigue damage.Each input parameter was binned (for example the range of vertical accelerations waspartitioned into six bins). Using bin combinations for defining flight conditions, a flightcondition recognition system was developed for the AH-1W helicopter. An example of aspeed-altitude bin combination is speed in the range 10–20 kn and altitude in the range1000–1500 ft. Using bin combinations alone was insufficient to distinguish certain flightconditions, and so the timing and sequence in which bin combinations occurred were alsotaken into account. Taking into account subdivision of various parameters (twin or singleenginemodes, airspeed, load factor, bank angle, rate of climb, gross weight and altitude)310 different flight conditions were defined. Flight trials permitted the use of an iterativeprocess to manually correct for any unrecognised flight conditions, thus improving futureflight condition recognition.Post-processing the duration and sequence of flight conditions allows the determination ofcomponent damage. As already discussed, Figure 2.1 shows the variation in usage across afleet of AH-1W helicopters. The fatigue life impact on three components (main rotor pitchlink, swashplate anti-drive bellcrank, and swashplate inner ring) was considered for eachmonitored aircraft. Differences in component retirement times exceeding an order ofmagnitude were detected across the fleet.4.2 RegressionUsing multiple regression Haas [23] determined rotating component loads (main rotorblade bending and pushrod load) from fixed system measurements on a U.S. Navy SH-60Bhelicopter. Fixed measurements included pedal, collective, lateral, and longitudinal stickpositions, roll, yaw, pitch, and climb rates, load factor at CG, and velocity. The coefficientsof the regression were obtained using a least squares fit of the data. The F statistic was usedto determine the significance of each input on the unknown load, and a tolerance limit wasused to control the collinearity of different inputs. Loads data were filtered into a min/maxformat over each rotor revolution, from which steady (min+max)/2 and alternating(min-max)/2 components were derived. This study focused on high-G and high-loadmanoeuvres, which are responsible for a major proportion of the fatigue damage. These9