Design of an MR Based on Device for the Adaptive Stiffness Control of Tail Shafts

Vertical tail design is driven by many elements concerning directional stability and directional control. The overall size of a vertical tail is mainly determined by the stability requirements, while its design is guided by structural constraints, and the rudder configuration and size is mainly linked to the established control characteristics. From the statements above, it follows that reducing the size of the tail but ensuring the same directionality levels means somehow increasing its aerodynamic capability. This can be attained by properly exploiting the elastic (aeroelastic) effects. A possible, and perhaps the most immediate, way to reduce the rudder size is instead to imagine a completely `All Movable Vertical Tail' AMVT. The aforementioned effectiveness should be ensured at high and low speed. The velocity influences this problem for at least two reasons: the first concerns the link between the force amplitude and the velocity itself; the second is related to the translation of the resultant along the chord as the velocity varies. This problem is particularly relevant for supersonic fighters. In fact, structural considerations, aimed at minimizing the structural solicitations, usually direct the designer to place the shaft axis between the extreme values of the aerodynamic forces resultant application point in the supersonic and subsonic region. Strategies based on the adaptation of aerodynamic surfaces could bring further improvements in the manoeuvre and control but are not herein referred to. The possibility of changing one or more design parameters can fit variable specifications for a wide working range. Among the possibilities related to the implementation of adaptive stiffness elements, an interesting solution is the one involving the use of smart materials. Magneto-rheological fluids MRFs modify their viscosity as a function of an external magnetic field, until they reach a semi-solid state. MRFs are basically made of iron dust in a fluid suspension, oil or water-based. Different variants are available in commerce and new products are continuously put on the market. With the target of controlling the stiffness torsion characteristics of the AMVT shaft, the use of hydraulic cylinders hosting an MR fluid may be considered in order to ensure the desired adjustable characteristics. In this article, the working principle, the design, the manufacture, and the tests of such a device, carried out inside the `Active Aeroelastic Aircraft Structures' (3AS) European Project, are illustrated. In more detail, the idea of connecting in a serial way the aforementioned cylinders to form a mechanical chain between the AMVT shaft and the related actuator is presented. By suitably activating the MR fluid in the different cylinder unities, the overall tail torque rigidity may be controlled to provide the best mechanical response in the different flight regimes of a typical fighter aircraft: supersonic, transonic, and subsonic. 3AS consortium specialists issued the aerodynamic specifications. Based on those, the main design parameters (MRF typology, valve dimensions, cylinder pistons features, etc.) were defined. An optimization process, aimed at achieving a stiffness variation law as wide and continuous as possible (despite the finite and small number of cylinders), has been addressed. After assessing the design phase and producing executive drawings, the manufacture task has been dealt with. Finally, the tests characterization activities have been performed on the assembled prototype. The correlation with the numerical predictions and a comparison with another original but classical architecture-based devices concludes the article.