Development of EC 135 turbulence models via system identification

The paper focuses on the development of empirical hover and low-speed turbulence models in particular for the EC 135 helicopter. The approach used is the Control Equivalent Turbulence Input (CETI) method, i.e. the determination of control inputs required to generate aircraft angular and vertical rates in calm conditions that are consistent with rates observed when flying in atmospheric turbulence. For the extraction process, this method uses a mathematical model of the aircraft dynamics and aircraft angular and vertical rates measured during flight tests in turbulence. Analyzing and modeling the power spectral densities of the extracted control disturbances allows the generation of low order equivalent turbulence models that can be used for control system optimization, handling qualities investigations, and pilot training. The paper describes the two applications of system identification within the development of the turbulence model: First a high fidelity state space model of the EC 135 helicopter model in hover has to be developed as a prerequisite for the extraction of the equivalent control input traces from flights tests in turbulence. Once the control equivalent turbulence input traces have been extracted and their power spectral densities determined, a second identification step yields the desired turbulence models. The results show that the effects of atmospheric turbulence on a hovering rotorcraft can be effectively modeled and simulated using the CETI method. The identified turbulence model has received favorable pilot comments during handling qualities trials.