Adaptive Blade Twist - Calculations and Experimental Results.

Applying adaptronics to helicopters has a high potential to significantly suppress noise, reduce vibration and increase the overall aerodynamic efficiency. This paper presents recent investigations on a very promising specific concept described as Adaptive Blade Twist (ABT). This 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. 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 the structural concept. With respect to these considerations, the ABT concept will be presented. This concept bases on the actively controlled tension-torsion-coupling of the structure. For this, an actuator is integrated within a helicopter blade that is made of anisotropic material based on fiber composites. 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 paper starts at giving a review on conventional concepts and potential adaptive solutions for shape control (2),(3),(9),(13)-(15). Hereafter, some calculations of the adaptive twist control concept are presented. These are based on a representive model in which the active part of the rotor blade is simplified with a thin-walled rectangular beam, that is structurally equivalent to a model rotor blade of the Bo 105 with a scaling factor 2,54. The calculations are performed using an expanded Wlassow Theory. The results are valid for static and dynamic conditions. For the dynamic condition excessive deformations near the blade resonance frequency shall be utilised. Therefore, the actuated blade section has to be properly designed for this preconditions. This has been demonstrated and verified in experiments (7) which will not be discussed in this paper. For experimental investigations on the ABT concept the skin of the outer part of the model rotor blade was manufactured of fibre composite material using the above mentioned tension-torsion-coupling effect with an additional uncoupling layer between skin and spar. The experimental results have shown that near to the resonance frequency dynamic forces of 550+_550 N are required for a deformation of 3 degrees at the blade tip