The present paper is focused on techniques aimed at including hydraulic servo-actuator dynamics into linear flutter equations. A large aircraft has been taken as test case for setting up the methodology. The hydraulic servo-actuator pertains the elevator. Without losing generality symmetric modal association has been used. The extra degrees of freedom of the control surface and additional terms due to the actuator have been modeled by means of dynamic sub-structuring approach. Starting from a set of flutter analyses performed at different stiffness values of the control surface rotational degree of freedom a minimum requirement for the elevator stiffness is obtained. This value has been used for a preliminary design of the hydraulic servo-actuator, allowing its subsequent dynamic characterization. The next step has been the inclusion of the servo-actuator dynamics into flutter equations (open loop flutter). The equations governing the dynamics of the servo-actuator are added to the aeroelastic stability equations after a suitable manipulation based on a derivative approach. This step is necessary in order not to have a singular mass matrix of the whole aeroelastic system. The interesting aspect of the work is that the actuator can be modeled as six external matrices to be properly assembled into the aeroelastic system. This is advantageous when already written codes for flutter evaluation are available since the requested modifications are minimal. The approach has been verified both for State Space and PK flutter solution methods, showing consistent results.
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