Design of Extendable Chord Sections for Morphing Helicopter Rotor Blades

Chord extension in helicopter rotors allows for expansion of the flight envelope, with the helicopter capable of flying at higher gross weights, altitudes and maximum speeds. A fixed large chord, however, results in a penalty when the helicopter is well within the envelope (for example, at low to moderate gross weight, sea level, and at moderate speed cruise). Chord morphing allows the helicopter to perform optimally in these diverse conditions. In this paper, the authors present a morphing mechanism to extend the chord of a section of the helicopter rotor blade. The region aft of the leading-edge spar contains a morphing cellular structure. In the “compact” state the edge of the cellular structure aligns with the trailing-edge of the rest of the blade. When the morphing cellular structure is in the “extended” state the chord of that section of the blade is increased by close to 30% (with the trailing-edge extending beyond that of the rest of the blade). In transitioning from compact to extended states, the cellular structure slides along ribs which define the boundaries of the morphing section in the span-wise direction. The cellular section has mini-spars running along the span-wise direction to attach the flexible skin and provide stiffness against camber-like deformations due to aerodynamic loads. The paper presents a finite element analysis and a design study of the morphing cellular structure, ensuring that the local strains in the cellular structure do not exceed maximum allowables even as the section undergoes large global strain. On the other hand, the morphing cellular structure is required to be stiff enough so that the pre-stretched skin that is attached to the surfaces does not result in deformation. Another question that is considered in detail is various methods of attachment of the flexible skin to the morphing substructure, the levels of pre-strain required, and their ramifications. A model of a blade section is fabricated and shown to undergo chord morphing, as designed.