Directionally Variable Stiffness to Reduce Actuation Requirement in Airfoil Camber Morphing

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. ABAQUSTM 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.