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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).