Aerodynamic control surfaces efficiency is among the major parameters defining the performance of generic aircraft and is
strongly affected by geometric and stiffness characteristics. A target of the '3AS' European Project is to estimate the eventual
benefits coming from the adaptive control of the torque rigidity of the vertical tail of the EuRAM wind tunnel model. The specific role of CIRA inside the Project is the design of a device based on the “Smart Structures and Materials” concept, able to produce required stiffness variations. Numerical and experimental investigations pointed out that wide excursions of the tail torque rigidity may assure higher efficiency, for several flight regimes. Stiffness variations may be obtained through both classical mechanic-hydraulic and smart systems. In this case, the attainable weight and reliability level may be the significant parameters to drive the choice. For this reason, CIRA focused its efforts also on the design of devices without heavy mechanical parts. The device described in this work is schematically constituted by linear springs linked in a suitably way to the tail shaft. Required stiffness variations are achieved by selectively locking one or more springs, through a hydraulic system, MRF-based. An optimisation process was performed to find the spring features maximising the achievable stiffness
range. Then, the hydraulic MRF design was dealt with. Finally, basing on numerical predictions, a prototype was manufactured and an experimental campaign was performed to estimate the device static and dynamic behaviour.