The present study proposes and explores a new autonomous morphing concept, where a 12–13° increase in camber is realized over a spanwise section of a helicopter rotor blade with increase in ambient temperature. The camber change is achieved through integration of Shape Memory Alloys (SMAs) on the lower surface of the blade, aft of the leading-edge spar. For a reference rotor of a utility-class helicopter generating 21,000 lbs thrust, a loss in lift of 2590 lb was observed due to operation in hot conditions. With the SMA camber morphing section extending from the blade root to 25%, 50%, and 75% span, the rotor recovered up to 11%, 43%, and 82% of the lift loss at high temperature (compared to a no-SMA blade). If the morphing section instead spans the outboard 25% of the blade (from 75% span to the blade tip), up to a 66% lift recovery is achieved due to the higher dynamic pressures over this region. While these results are achieved with existing SMA properties, idealized target values are also presented. For the SMA considered in the study, while a 40–115°F temperature change was required to achieve the full 12–13° design camber change, partial camber is achieved over a smaller temperature range. The paper identifies desired SMA properties that would produce a 12–13° camber change over an 80–100°F temperature change.

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This paper explores a new design for an airfoil whose chordwise bending stiffness depends on the direction of load. By designing the region aft of the spar to be very stiff under upward load, uncommanded camber deformation under aerodynamic pressure can be minimized. At the same time, a much higher compliance under a reversed load reduces actuation requirement to achieve a desired downward camber deformation. A rigid cantilever extending from the rear of the spar toward the trailing-edge, and flush with the lower skin, is used to realize this goal. Under upward (aerodynamic pressure) load the rigid cantilever engages and its added stiffness minimizes deformation. But under downward (actuation) load, the lower skin breaks contact with the cantilever, and camber deformation can be achieved at low actuation effort. ABAQUS TM finite element simulations were conducted for a variable camber NACA 0012 airfoil. For a cantilever extending over the entire length of the conformable section (between the leading-edge D-spar and the trailing-edge section), the effective stiffness under upward loading was calculated to be 15.43 times the stiffness under downward loading, but the maximum downward camber deformation was limited to 10 deg due to contact between the cantilever and the upper skin. Reducing the cantilever length increased the maximum downward camber deformation achieved but with a smaller increase in stiffness under upward load.

This paper explores a new design for an airfoil whose chordwise bending stiffness varies with direction of applied load. By designing the region aft of the spar to be very stiff under upward load, uncommanded camber deformation under aerodynamic pressure can be minimized. At the same time, lower stiffness under reversed load reduces actuation requirement to achieve a desired downward camber deformation. Rigid cantilevers extending from the rear of the spar toward the trailing edge, and flush with the lower skin are used to realize this goal. Under upward load the rigid cantilever engages and supplements the chordwise bending stiffness. But under downward load the lower skin breaks contact with the cantilever, and camber deformation can be achieved at low actuation effort. From twodimensional ABAQUS™ finite element simulations an upward-to-downward stiffness ratio of 13.82 was obtained with a cantilever extending over the entire length of the conformable section, but the maximum downward camber deflection was limited to 10 deg. Reducing the cantilever length reduced the stiffness ratio but allowed higher maximum camber deformations. A three-dimensional prototype was fabricated using "moderate-length" cantilevers and the measured stiffness ratio (under upward-to-downward loading) was determined to be 5.12. The corresponding stiffness ratio from a three-dimensional ABAQUS finite-element simulation was found to be within 6% of experimental results. Nomenclature EI = flexural stiffness, N ⋅ m 2 F, P = applied tip load, N L = length, m w = upward tip displacement, m

The present study proposes and explores a new autonomous morphing concept, whereby an increase in helicopter rotor blade camber of the order of 12-13° is realized over the inboard section of the blade with increase in ambient temperature. The camber change is achieved through a proper integration of Shape Memory Alloys (SMAs) on the lower surface of the blade aft of the leading-edge spar. For a reference rotor (no-SMA) generating 21,000 lbs thrust, operation in hot conditions resulted in a 2,590lb loss in lift. When the SMA camber morphing section extends from the blade root to 50% span, the rotor recovered up to 43% of the lift loss at high temperature. If the camber-morphing section is further extended to 75% span, up to 82% of the lost lift can be recovered.