Bi-Directional Stiffness for Airfoil Camber Morphing

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 two-dimensional 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.