Flight Dynamic Simulation with Refined Aerodynamics and Flexible Blade Modeling

The paper describes a coupled rotor-fuselage flight dynamic model that includes a maneuvering free wake model and a coupled flap-lag-torsion flexible blade model, and the results of an investigation on some effects of inflow and blade modeling on the free flight response of a hingeless rotor helicopter to pilot inputs. The wake model is a relaxation type free wake, developed by Bagai and Leishman, and capable of modeling the wake geometry changes due to maneuvers; no assumptions are made on the wake geometry, which is free to evolve based on the maneuver. Theoretical predictions are compared with flight test data. The results show that the free flight, on-axis response to pilot pitch and roll inputs can be predicted with good accuracy with a relatively unsophisticated model. It is 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 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 using a dynamic inflow theory extended to account for maneuver-induced wake distortions, 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 calculations.