Peformance degradation of natural laminar flow airfoils due to contamination by rain or insects

Abstract

Large L/D laminar flow airfoils, which have become common in competition sailplanes over the past two decades, are extremely sensitive to surface contamination in the form of insects, water or ice. In order for these airfoils to operate within the low drag "laminar bucket", a laminar (smooth) boundary layer must be maintained over a substantial fraction of the wing chord. Any significant irregularity or protuberance from the surface causes the boundary layer to become turbulent in a wedge-shaped region downstream of the irregularity. If the turbulent boundary region becomes substantial, the drag on the airfoil will increase, reducing the L/D. In addition the turbulent boundary layer may be susceptible to premature separation, resulting in an increase in stall speed. The size at which an irregularity or roughness element becomes significant is, at present, not well understood. However, some investigators have suggested that laminar-to-turbulent transition will occur when the Reynolds number, based on the roughness height and the velocity at the top of the roughness, exceeds some critical value. Critical values from 50 to 600 have been recorded. The two primary contamination mechanisms which affect sailplane operations are insect impingement, where the insect residue exceeds the critical roughness height, and water contamination during flight though rain. A brief discussion of each of these mechanisms follows.