Helicopter flight dynamics simulation with refined aerodynamic modeling.

摘要

This dissertation describes the development of a coupled rotor-fuselage flight dynamic simulation that includes a maneuvering free wake model and a coupled flap-lag-torsion flexible blade representation. This mathematical model is used to investigate effects of main rotor inflow and blade modeling on various flight dynamics characteristics for both articulated and hingeless rotor helicopters. The inclusion of the free wake model requires the development of new numerical procedures for the calculation of trim equilibrium positions, for the extraction of high-order, constant coefficient linearized models, and for the calculation of the free flight responses to arbitrary pilot inputs.; The free wake model, previously developed by other investigators at the University of Maryland, is capable of modeling the changes in rotor wake geometry resulting from maneuvers, and the effects of such changes on the main rotor inflow. The overall flight dynamic model is capable of simulating the helicopter behavior during maneuvers that can be arbitrarily large. The combination of sophisticated models of rotor wake and blade flexibility enables the flight dynamics model to capture the effects of maneuvers with unprecedented accuracy for simulations based on first principles: this is the main contribution of the research presented in this dissertation.; The increased accuracy brought about by the free wake model significantly improves the predictions of the helicopter trim state for both helicopter configurations considered in this study. This is especially true in low speed flight and hover. The most significant improvements are seen in the predictions of the main rotor collective and power required by the rotor, which can be significantly underpredicted using traditional linear inflow models.; Results show that the free-flight on-axis responses to pilot inputs can be predicted with good accuracy with a relatively unsophisticated models that do not include either a free wake nor a refined flexible blade model. It is also possible to predict the off-axis response from first principles, that is, without empirically derived correction factors and without assumptions on the wake geometry. To do so, however, requires much more sophisticated modeling. Both a free wake model that includes the wake distortions caused by the maneuver and a refined flexible blade model must be used. Most features of the off-axis response can be captured by using a simpler dynamic inflow theory extended to account for maneuver-induced wake distortions, and for a fraction of the cost of using a free wake model. The most cost-effective strategy, for typical flight dynamic analyses, and if vibratory loads are not required, is probably to calibrate such a theory using the more accurate free wake-based model, and then use it in all further calculations.