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The present paper is focused on techniques aimed at including control law dynamics into linear flutter equations. An UAV in an unconventional joined wing configuration has been taken as test case for setting up the methodology. For this test case the pitch control law is included in the flutter equations, obtaining a model to perform closed loop flutter calculations. The extra degrees of freedom of control surfaces and additional terms due to the control law have been modeled by means of dynamic sub-structuring approach. The equations governing the dynamics of the control law are added to the aeroelastic stability equations after a suitable manipulation based on a derivative approach. The interesting aspect of the work is that the control law 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 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).

Aim of this study is the development of an integrated methodology able to support the structural design of an UAV demonstrator, having a joined-wing configuration and a high structural flexibility. HAPD (High Altitude Performance Demonstrator) is an UAV multi-mission, very light vehicle under development at CIRA, the Italian Aerospace Research Centre. HAPD structure is redundant as regards constraints, so the internal forces depend upon the stiffness distribution. In addition the evaluation of flight loads has to be performed without neglecting the flexibility of the structure. Because of the extreme unconventionality of the configuration the design of the primary structures has been widely affected by aeroelastic analyses.