Improving Helicopter Flight Mechanics Models with Laser Measurements of Blade Flapping

A high fidelity linear model of the UH-60 helicopter flight dynamics at hover is developed using a combination of system identification and component-type simulation modeling techniques. The results are enhanced through the use of a new flight test database generated with the NASA/Army RASCAL JUH-60A helicopter which includes dynamic response data for the main-rotor blade motions in addition to those of the aircraft fuselage and drive train. Very high quality measurements of the main rotor blade motions are obtained using a new measurement method which employs hub-mounted laser distance transducers. The flight test data are first used to optimize a component-type flight mechanics simulation model of the UH-60 for hover. Correlation of the off-axis responses of the model with the flight data is shown to be greatly improved when an empirical aerodynamic phase lag correction is included in the main rotor aerodynamic model formulation. The updated simulation model is then used to refine the model structure used for a parametric system identification study. A 16 degree-of-freedom linear model is identified using the refined model structure and the laser flapping data. The model is valid over a frequency range of 0.5 to 40 rad/sec for the hover flight condition. Previously unidentifiable parameters are determined through use of the new flight data and refined model structure, and excellent identification of the off-axis response characteristics of the aircraft is achieved. The identified primary rotor flapping parameters agree well with theoretical values and provide a high level of confidence in the newly identified model.