# Effect of leading edge contamination wing performance

When a wing accumulates irregularities such as insects or dirt on the leading edge, its performance decreases. There are two main effects of this which are explained in https://aviation.stackexchange.com/a/16956/63452.

However, I do not understand is why stall angle and lift coefficient are reduced. That is, at sufficiently high Re (3e6, very common in aerospace applications) the boundary layer becomes turbulent, regardless the presence of contamination, almost directly at the leading edge at moderately high angles of attack (10 degrees).

What is the reason that a contaminated airfoil stalls earlier given that the boundary layer characteristics are the same?

• At those Reynolds numbers and AoA a modern airplane should have some 20 to 50% of the chord with laminar boundary layer, so dirt can make a difference. Commented Feb 22, 2023 at 13:28
• Oke, suppose we increase the aoa to 10 degrees. XFOIL shows transitioning at 0.02c for NACA4412 at Re = 3e6. This is also the region where stall is starting for contaminated leading edges and not for clean leading edges. PS I made an edit to the question for this case Commented Feb 22, 2023 at 13:44
• Well, I think that the answer you linked answers in a very good way your question. Turbolent boundary layer implies higher quantity of air's speed which is eaten up by viscous friction. This gives both higher drag and less inertia against rise of pressure in the aft part of the airfoil which is what cause stall. Dirt, promoting an earlier transition from laminar to turbolent, makes these effects worse. Commented Feb 22, 2023 at 14:22
• As I mentioned, the transition happens almost instantly. Therefore, how can dirt 'turbulate' the already turbulent boundary layer and thereby change the stall behaviour? Commented Feb 22, 2023 at 14:44
• Dirt accumulates on the leading edge and therefore it "turbulates" the laminar part of the boundary layer, not the already turbolent one. This makes the airfoil stall a couple of degrees in advance and at a lower lift coefficient and higher drag coefficient. Commented Feb 22, 2023 at 14:55