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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?

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  • $\begingroup$ 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. $\endgroup$
    – sophit
    Commented Feb 22, 2023 at 13:28
  • $\begingroup$ 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 $\endgroup$
    – lWindy
    Commented Feb 22, 2023 at 13:44
  • $\begingroup$ 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. $\endgroup$
    – sophit
    Commented Feb 22, 2023 at 14:22
  • $\begingroup$ As I mentioned, the transition happens almost instantly. Therefore, how can dirt 'turbulate' the already turbulent boundary layer and thereby change the stall behaviour? $\endgroup$
    – lWindy
    Commented Feb 22, 2023 at 14:44
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    $\begingroup$ 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. $\endgroup$
    – sophit
    Commented Feb 22, 2023 at 14:55

1 Answer 1

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

But they are not the same! This problem had already been studied to death by at least 50 years ago, and now there has been nearly a century of study on exactly this problem. F.X. Wortmann has commented on this before saying that the issue lies with the fact that the laminar boundary layer downstream of the instability point is a strong amplifier for incoming perturbations. Wortmann makes note of a rather startling fact: 'If we compare the roughness height, which will not shift the transition and not increase skin friction, we find for higher Reynolds numbers that the turbulent boundary layer requires usually a smoother surface than the laminar flow…

In particular, if the laminar boundary layer is prematurely tripped to become turbulent, the relationship between momentum thickness and displacement thickness becomes altered and the turbulent boundary layer becomes loaded by the intense pressure gradient. Consequently, the turbulent boundary layer can no longer remain attached against the increasing pressure gradient and therefore becomes abruptly detached. As Wortmann notes, ‘[t]his… example illuminates the well-known fact that changes in surface condition and hence boundary layer condition at the nose [of the airfoil] can have drastic effects near maximum lift. Sometimes one [wishes the] designer[s], builders and users of aircraft would be more aware of this fact.’

Reference: Wortmann, FX, 1976. Airfoil Synthesis Techniques. Institut fur Aerodynamik und Gasdynamik, Stuttgart, in cooperation with Department of Aerospace Engineering, University of Texas, Arlington. A scanned third-order copy with Wortmann’s hand-written margin notes, is available on the internet. Read carefully and understand the math. Richard Eppler has also exhaustively investigated turbulent separation.

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