An experimental investigation of the sensitivity of flap hinge moment to airfoil surface contamination was conducted at the University of Illinois at Urbana-Champaign Aerodynamics Research Lab. Tests were conducted on two airfoil models, an NACA 3415 and an NACA 23012, at Reynolds numbers of 1.8 × 10⁶ and 1.0 × 10⁶. The effects of six different simulated contamination configurations on the performance characteristics of both airfoils were tested. These configurations consisted of glaze ice, rime ice, two severities of distributed leading-edge roughness, three-dimensional leading-edge damage, and three-dimensional upper-surface damage. Additionally, the effects of flap deflection and trim tab deflection on the unsteady hinge moment were studied.

Results from this study found that large increases in C_h,StDev often occurred at the same angle of attack as C_l,max. By correlating regions of separated flow observed in C_p distributions and fluorescent-oil flow visualizations to C_h,StDev at discrete angles of attack, it was determined that regions of boundary-layer separation were the primary driver for large increases in unsteadiness in the hinge moment. It was also found that the unsteady hinge moment had negligible dependence on trim tab deflection. The response of C_h,StDev was dependent on the stalling characteristics of the airfoil model.

Of all of the contamination configurations tested, the two simulated ice cases had the largest effect on the performance of the airfoils. For the distributed leading-edge roughness cases, the larger roughness elements had a larger effect on the performance than the smaller roughness elements, but the C_h,StDev response of both roughness cases were comparable. While the 3D simulated damage cases did not significantly affect the lifting characteristics of either model, the magnitude of the C_h,StDev response of the 3D simulated damage case was comparable to the 2D contamination cases. Additionally, the large increase in C_h,StDev occurred prior to stall due to localized regions of separated flow that resulted from the simulated damage.