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

DSTO-TN-0239Estimation of Structural Component Loads inHelicopters: A Review of Current MethodologiesExecutive SummaryHelicopter components whose fatigue lives are limited have prescribed Component RetirementTimes (CRTs) that define the safe number of flying hours that they can remain in service. At thetime a fleet of helicopters first enters service with a particular operator the CRTs are set by themanufacturer based on an assumed usage spectrum that has been defined with the agreementof the operator. The relationship between component loads and the various flight regimes thatmake up the usage spectrum is usually obtained via a flight load survey on the prototypehelicopter. The Australian Defence Force discards components when they have reached theirCRT. However, most components are, in effect, retired prematurely, due to the conservativenature of an assumed usage spectrum. At the other extreme, it is possible that somecomponents could exceed their safe fatigue limit before reaching their CRT. The accuratedetermination of a loading history for dynamic components would have the potential to meetthe two major objectives of reducing costs and improving safety.We review the literature for models that use fixed-component loads and flight parameters tomodel the loads in dynamics components. The reviewed papers naturally belong in one ofthree categories depending on model inputs: fixed-component loads, flight parameters, or acombination of fixed-component loads and flight parameters.A review of load variability reveals that even for the same aircraft under the same flightcondition, the loading can vary dramatically due to pilot technique, altitude, and weight toname a few variables. For this reason, although the use of regime recognition models fleet-wideto obtain an estimate of the life fractions expended during flight on individual helicopters canproduce results that are superior to the adoption of fixed CRTs, simply recognising the regimeand its duration is insufficient to determine the true fraction of life expended for a component.A technique that develops a calibration matrix in the frequency domain, termed holometrics,appears promising. The holometrics technique was initially developed using only fixedcomponentloads, but was later extended to include flight parameters. Statistical methods suchas significance tests and confidence intervals allow different modelling options to beprioritised. Neural network (NN) and multiple regression models were also extensivelyutilised by researchers. NNs were more computationally intensive than the regression models,but the NNs had better generalisation capabilities. Most studies focused on high loadmanoeuvres (since these produce the majority of the fatigue damage), and thus used filteringor data weighting to produce a high load bias. Adding the product of certain input parameters,such as swashplate servo position and accelerations also improved results significantly. ExpertNN systems appear to be a promising area for further research.Fundamental loads modelling effects such as noise, rank deficiency, stability, andgeneralisation received no attention by most researchers. These essential questions need to beaddressed if robust and accurate load estimation models are to be developed and implemented.