Design of Extendable Chord Sections for Morphing Helicopter Rotor Blades

Chord extension morphing of helicopter rotors has recently been shown to be highly beneficial for stall alleviation, with the ability to reduce power near the envelope boundaries and increase maximum gross weight, altitude, and speed capability of the aircraft. This article presents 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 30%. In transitioning from compact to extended states, the cellular structure slides along the ribs which define the boundaries of the morphing section in the spanwise direction. The cellular section has mini-spars running along the spanwise direction to attach the flexible skin and provide stiffness against camber-like deformations due to aerodynamic loads. This article presents a finite element analysis and design of the morphing cellular structure, ensuring that the local strains in the elastic ligaments of the cellular structure do not exceed the maximum allowable, even as the section undergoes a large global strain. The morphing cellular structure itself is designed to be stiff enough to support the pre-stretched skin attached to its surfaces. Various methods of flexible skin attachment to the morphing substructure, and their ramifications, are considered. A model of a blade section is fabricated and shown to undergo chord morphing, as designed.