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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?

John Wiseman
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lWindy
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  • 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. – sophit Feb 22 '23 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 – lWindy Feb 22 '23 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. – sophit Feb 22 '23 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? – lWindy Feb 22 '23 at 14:44
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    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. – sophit Feb 22 '23 at 14:55
  • Some airfoils, especially supercritical ones that develop separation bubbles at high AOA, can barely tolerate ANY contam anywhere above the stagnation point. The non-slat CRJs and Challengers have sep bubbles, and light morning frost on the LE is enough to bring one down on takeoff as stall AOA is reduced to about 9 deg, and the bubble results in instantaneous stall of the entire wing at once when it does let go. There was a CRJ crash in China from taking off on a bright clear morning with just some frost on the LEs that the crew thought was not a big deal. – John K Feb 22 '23 at 18:39
  • @sophit, are you sure? This implies that at a.o.a. = 10 deg the effect of the very small laminar BL (recall up to x/c = 0.02) becoming turbulent (say at x/c = 0.0) has such immense effect? – lWindy Feb 23 '23 at 09:04
  • This implies that the boundary layer transitions to fully turbulent at say 6° instead of 10° and the airfoil stalls at 14° instead of 16° and with a $C_d$ of 0.06 instead of 0.03 – sophit Feb 23 '23 at 10:24
  • The canard on Rutan Long EZs, as originally designed, is infamous for this. On almost any amount of precipitation, it would rapidly experience a drop in lift coefficient. Because the elevator is on a canard, and the lift generated there controls the pitch trim force, the aircraft would immediately pitch down, and, (if sufficiently above canard stall speed), the pilot would have to apply additional elevator (back stick), to reestablish pitch attitude. – Charles Bretana Feb 23 '23 at 13:30
  • If close to canard stall, the change would result in canard stall, but the resulting nose drop would lower canard AOA and it would begin flying again. If the pilot held full aft stick pressure, it would stall again, and the aircraft would enter a continuous cycling nose bobble, up and down, as the canard would stall, fly again, stall, and fly again, over and over in a 2-3 second oscillation. This was not dangerous, unless you were in the flare close to the ground, as the aircraft would lose 8-10 feet with each bobble. – Charles Bretana Feb 23 '23 at 13:36

1 Answers1

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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.

Thomas Perry
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