Adaptive Rotor Blade Concepts -Direct Twist and Camber Variation-

Kurzfassung

Applying adaptronics to helicopters has a high potential to significantly suppress noise, reduce vibration and increase the overall aerodynamic efficiency. Since the interaction of non-stationary helicopter aerodynamics and elastomechanical structural characteristics of the helicopter blades causes flight envelope limitations, vibration and noise, a good comprehension of the aerodynamics is essential for the development of structural solutions to effectively influence the local airflow conditions and finally develop a structural concept. With respect to these considerations, this paper presents recent investigations on two different structural concepts: the direct twist and the camber variation concept. The direct twist concept allows to directly control the twist of the helicopter blades by smart adaptive elements and through this to positively influence the main rotor area which is the primary source for helicopter noise and vibration. The concept is based upon the actively controlled tension-torsion-coupling of the structure. For this, an actuator is integrated within a helicopter blade that is made of anisotropic fibre composite material. Driving the actuator results in a local twist of the blade tip, in such a way that the blade can be considered as a torsional actuator. Influencing the blade twist distribution finally results in a higher aerodynamic efficiency. The direct twist concept was analytically modelled using an expanded Vlassov Theory before a proof-of-principle demonstration structure was manufactured. Subsequently, a Mach-scaled Bo105 model rotor blade with an integrated piezoelectric actuator was designed and successfully tested. Next, small scale rotor tests and investigation of thermal loads are planned. The camber variation concept uses the experiences gained in the design of the direct twist concept to create a rotor blade, that will be able to change the shape of its cross-section in operation. This shape control approach uses material anisotropy (e.g. tension-torsion-coupling) to create a smooth aerodynamic surface and to avoid the airflow disturbances created by the leading or trailing edge flaps, that have already been investigated. First, a structural model was numerically investigated to identify the most influential parameters of this concept. From this model, the two-dimensional surface quality of the deformed rotor blade was extracted as a basis for aerodynamic calculations that are necessary to derive the quantity of deformation needed to successfully delay aerodynamic stall onset. As a next step, a proof-of-principle structural demonstrator is presently being designed. Both concepts were designed to be activated using a piezoelectric stack-actuator integrated at the blade tip. Since continiuously integrated piezo sheets promise a potential to increase the concepts’ performance, thin actuator modules are currently under investigation.