Flight Dynamics and Control Modeling with System Identification Validation of the Sikorsky X2 Technology Demonstrator

Physics-based flight dynamic models are vital to the aircraft design process and are used for design loads prediction, flight control development, handling qualities evaluation, and in support of design and vehicle layout trade studies. These models must be validated with flight-data whenever available to increase confidence in model fidelity and to help determine model deficiencies. This paper presents the validation of coaxial compound helicopter flight dynamics models developed in GenHel and HeliUM by correlating with the X2TD flight test data. GenHel is the flight dynamics model used by Sikorsky and HeliUM is used at the U.S. Army ADD. Both modeling tools are modular in architecture, include flexible rotor blade models, and allow for complete free-flight analysis. Both tools have been extensively correlated against a variety of single-main-rotor flight test data. When applied to the X2TD configuration, both models show good correlation to flight data for steady-state and dynamic responses. The models accurately predict the bare-airframe X2TD frequency response from 0.2 to 1.0 Hz. Key modeling parameters are determined using an analytical model of the coupled rotor and fuselage, and then identified from flight data using system identification. When propagated back into the HeliUM model, the fidelity was increased to a larger frequency range and approached the response obtained from GenHel. The broken-loop and closed-loop behavior of the X2TD are also analyzed and match well with flight data.