Rotorcraft Operating Envelope Expansion Using Extendable Chord Sections

Alleviating retreating blade stall is of much interest for helicopter flight envelope expansion. This study examines the benefits of quasi-statically increasing the chord through the extension of a flat plate through a slit trailing edge over a section of the blade, for increase in maximum speed, altitude and gross-weight, and reduction in main rotor power near the envelope boundaries. The concept, referred to as the Static Extended Trailing Edge (SETE) appears to be a better high-lift device than trailing-edge flaps or even Gurney flaps in that it results in higher lift-to-drag coefficients at high CL. Simulation results, based on a UH-60 helicopter, indicate that in stall-dominant conditions (high grossweigh, altitude, sufficient available power) increases of up to 3,000 ft in the maximum altitude, 2,400 lbs in the maximum gross-weight, 26 knots in the maximum speed, and reductions up to 33.4% in the rotor power can be obtained. Practical realization of the concept appears quite feasible, and a morphing X-truss mechanism was designed to actuate the extendable plate through the trailing-edge. The blade section with the SETE mechanism was fabricated and appeared to operate well.