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

DSTO-TN-02391. IntroductionHelicopter components whose fatigue lives are limited are assigned Component RetirementTimes (CRTs) that define the safe number of flying hours that they can remain in service. Atthe time a fleet of helicopters first enters service with a particular operator the CRTs are setby the manufacturer based on an assumed usage spectrum that has been defined with theagreement of the operator. The Australian Defence Force discards components when theyhave reached their CRT. However, the CRTs are based on a worst-case scenario in terms offatigue damage and do not take into account perturbations from this spectrum.Consequently, most components are, in effect, retired prematurely. The development ofloading histories for critical components has two beneficial possibilities, increasing safetyand reducing operator costs. The perception that these benefits are achievable explains therecent increase of work in this area [1–9].The rotor systems and their attachments contain most of the fatigue-critical structuralcomponents in helicopters. To monitor loads on these components would require slip ringsor rotating telemetry systems that currently lack the reliability and maintainability neededfor everyday fleet usage. Hence, there is a great impetus to develop a load transfer function,which can predict the loads in rotor components using information from flight parametersand loads from static components.In this report, we review published work on the estimation of loads necessary forcalculating fatigue damage to helicopter components. Initially we demonstrate the vastvariability in component loads and hence fatigue damage due to pilot technique.Subsequently we provide an overview of various methodologies for estimating loads instructural component in helicopters.The parameters to be monitored for estimating component loads for helicopter componentfatigue life estimation purposes fall into one of three broad categories: loads on fixedcomponents, flight parameters, or both loads on fixed components and flight parameters.Only one paper was found that implemented the estimation of dynamic component loadbased solely on fixed component load estimation. This approach, which was undertaken inthe frequency domain and termed holometrics, develops a linear relation between thestrain in fixed components and the strain in dynamic components. In contrast, severalpapers estimate dynamic component loads based on flight parameters. These methodsbased on flight parameters were further subdivided into four categories: regression, neuralnetworks, flight condition recognition, and combined regression and neural networks.Using the loading history, determined using the above procedures, several authors went onto estimate component fatigue damage using strength data presented as S-N curves. InSection 5 of this report we review an extension of the holometric method, from just onepaper found on this topic.1