Concept of a variable chord extension

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Balzarek, C.
Riemenschneider, J.
Majeti, R.K.
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Among all kinds of aircraft, helicopters are standing out due to their capability for both forward flight and hovering. This range of flight conditions requires a special blade design, resulting in a rotor blade geometry, which represents a compromise between the different flight conditions. Morphing rotor blades could address this issue, by changing the rotor blade shape according to the demands of the current flight state. From rotor dynamic calculations it is known, that longer chord length at the blade root and increased pre-twist would increase the performance in hover [1,3], whereas shorter chord and less twist are beneficial for fast forward flight. Based on this assumption, a structural concept has been worked out for a blade design with a variable chord length in the rotor root region. The model, which belongs to that concept forms the basis of a performance calculation. The structural concept consists of a pivot point at around 60% R and the chord extension is linearly increasing all the way to the blade root at 22% R. In this region an auxiliary spar is dividing the blade into a conventional rigid front part and a morphing rear part, whose structural design is the main focus of this paper. It consists of two components: a flexible skin made of rubber-like EPDM material, which is reinforced in the spanwise direction by fibers, as well as an inner support structure. The design drivers for the skin thickness are shape accuracy on one end and actuator force to extend the mechanism and stretch the skin on the other end. The underlying support structure consists of vertical GFRP extending in span wise direction. Design parameters for those webs are the distance between webs, which relates to the skin design, as well as the thickness of the members, which influences the overall stiffness of the design. The publication will present a workflow, in which the rotor is being structurally designed coming all the way from a generic CAD model, considering cg location as well as elastic deformation of the elastic skin and calculating the cross section wise stiffness of structure. This section wise approach is followed by the setup of a beam model of the blade for dynamic analysis as well as the setup of a 3D model for strength analysis as well as aeroelastic simulation by fluid structure interaction (FSI), the first results of which are shown as an example. This is followed by a performance calculation in order to evaluate the efficiency of the concept and to provide input for further iteration steps. This includes trim analyzes for various blade loading conditions in hover as well as various forward flight velocities using DLR comprehensive analysis code S4. Chord-extension of up to 100% and chord-extension-deflection of up to 15deg are considered. Results show that the linearly variable chord-extension concept is effective in reducing power requirement in both hover and forward flight. The chord-extension-deflection helps reduce power requirement in hover, especially at higher blade loadings.