Toward a Better Understanding of Helicopter Stability Derivatives

The influence of higher-order rotor-flapping dynamics on six-degree-of-freedom stability and control derivatives was investigated. It was shown that six-degree-of-freedom helicopter derivatives are time-varying, being similar, in principle, to aerodynamic indicial functions. An amended six-degree-of-freedom helicopter stability derivative model was then developed in which body acceleration and control-rate derivatives were included in the Taylor series expansion. These additional derivatives were derived from consideration of the effects of the higher-order dynamics. The amended model was found to be a substantial improvement over the conventional model, effectively doubling the relative bandwidth and providing a more accurate representation of the short-period and cross-axis characteristics. Further investigations assessed the applicability of the two stability derivative model structures for flight-test parameter identification. Three lower-order models were identified from, and compared to, a baseline fourteenth-order model. Both the amended six-degree-of-freedom model structure and an eight-degree-of-freedom model structure more faithfully represented the characteristics of the baseline model than did the conventional model structure.