Critical Protuberance Height for Transition of Leading Edge Boundary Layers

Abstract

Leading edge contamination on aircraft wings can occur through the accumulation of insect residues, ice, or dirt or through existing surface defects. Such protuberances can increase skin friction drag by causing an earlier transition of the boundary layer from laminar to turbulent flow. To determine the likelihood of this effect, a simple and approximate model was developed for determining the critical height for boundary-layer transition on the airfoil leading edge. The model assumes an outer potential flow based on an effective leading edge radius coupled with an inner flow based on semi-empirical laminar boundary profiles. The effective leading edge radius was based on leading edge pressure distribution as related to flow over a cylinder and tends to the geometric leading edge radius for the limits of no camber and zero angle of attack. The model was compared with available experimental transition results, which showed that the predictions are reasonable but approximate. The potential susceptibility of various airfoils to transition was next considered using sample insect residues heights. The results indicate that laminar flow airfoils for large commercial aircraft would be significantly susceptible to transition due to insect contamination, whereas high-altitude low-speed UAVs would generally not have such susceptibility.