Vortex generators are little fans that generate vortex to attach flow to an airfoil and delay stall. If frost or ice are formed on an airfoil, technically they also generate turbulence and vortex,I am not sure if frost and ice also works like vortex generators on the wing.
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4$\begingroup$ Little fans? Every VG I've ever seen is fixed to the wing, with no moving parts. $\endgroup$– Camille GoudeseuneSep 6, 2022 at 2:01
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$\begingroup$ IF the ice were to form in the shape of a VG, then it might stand a chance of being useful, but that's highly unlikely. $\endgroup$– FreeManSep 8, 2022 at 17:41
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2 Answers
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No. A vortex generator protrudes up into the free stream, and the horizontal tornado it creates circulates partly in the free stream and partly in the turbulent boundary layer. Spiralling free stream air is mixed into the turbulent boundary layer, adding more energy into it and helping to delay separation
A VG will raise stalling AOA by several degrees, say 4-6. A leading edge slat achieves about double that, up to about 10 degrees. VGs are sometimes called "the poor man's slat" when installed to improve takeoff performance and low speed behaviour.
Frost does the opposite. It just disrupts the flow by tripping the laminar portion into turbulent at a much lower angle of attack. Frost or light rime ice on the leading edges will lower the stalling AOA from, say, 15 degrees, to 9 or 10 degrees.
It's not just high performance wings. People have crashed Piper Cubs and Cessna 150s with a little bit of morning frost on the wings, that the pilot couldn't be bothered to clean off.
There have been aircraft with leading edges with a very rough texture designed to work like turbulators, that seem to be successful at it, but in the case of frost or ice with any kind of roughness, expect it to be bad news if you allow AOA to get very high at all on any normal airplane with a smooth, non-slatted leading edge.
You would be out of your mind to take off in a jet with no leading edge devices and frost on the wings (slats when extended make the wing pretty much immune to light frost); I know of several Challenger and CRJ crashes from doing this.
If, say, a hijacker put a gun to your head and forced you to take off with frost on the wings, and you had no anti-ice to melt it off, you would want to accelerate well above normal rotation speed, rotate very slowly, and let it accelerate right away to climb out at a higher than normal climb speed. Whatever it takes to keep angle of attack as low as possible.
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1$\begingroup$ achieving some of the effect of having the boundary layer laminar. - really? Isn't it more like a super-turbulent boundary layer? $\endgroup$ Sep 6, 2022 at 5:16
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$\begingroup$ A spiraling stream that is moving is preferable to a completely chaotic layer that is semi-stagnant no? I understood there was a drag reduction from this effect that compensates for the energy consumed to start the air spinning, which is why VGs have a more or less neutral effect on cruise speeds. $\endgroup$– John KSep 6, 2022 at 13:01
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$\begingroup$ You can hardly call a turbulent BL stagnant. The important part is the speed gradient at the wall, which increases from laminar to turbulent and then again with VGs. In the end, with VGs you take energy from the free stream and bring it close to the wall. Hardly something you do to avoid drag. $\endgroup$ Sep 6, 2022 at 17:20
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The first question is less "will it?" and more "can it?" delay stall, where the answer to the latter question is yes! There is research abound studying roughness effects like this. Generally the results depend on roughness size and location but results like the following are common: 1) The zero-lift drag coefficient will increase due to increased skin friction, and 2) at high angle-of-attack lift increases and drag decreases due to delayed separation.
The transition from laminar to turbulence is not uniquely accomplished by the "normal" vortex generator geometry as the other answer suggests, and all kinds of research has been done to investigate the effects of various other roughness and element geometries that accomplish this.
The answer to the question of "will it?" though of course is: it depends. But if were to analyze the characteristics and distribution of frost on an aerodynamic surface, you should be able to apply existing results such as [1] to determine what effect it will have. And in some cases it may result in desirable changes, and other cases not.
In the real world, however, no pilot is going to look at an icy, frosty wing, characterize it, and make an accurate determination as to how the aircraft aerodynamics and flight characteristics will change, and whether theses changed characteristics were advantageous for the flight operation you were conducting.
And even if you did determine the changes were desirable, you're still required to comply with (in the case if Part 91 operations, although I'll let you dig up the reference for other parts) 14 CFR 91.9 which states:
Except as provided in paragraph (d) of this section, no person may operate a civil aircraft without complying with the operating limitations specified in the approved Airplane or Rotorcraft Flight Manual, markings, and placards, or as otherwise prescribed by the certificating authority of the country of registry.
And I'd expect in your aircrafts POH/AFM you will find language stating that all frost, ice, and snow be removed from the aircraft. Hence, you've got to remove even advantageous frost from your aircraft to be legal.
