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From this answer, “Of course, these also add drag, even when the wing is not stalling, so they are only added where needed”. Are there any aircraft that have vortex generators that are retractable so that if the flight computers determine that the wing is stalling, the vortex generators are automatically deployed without them causing drag all the time?

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    $\begingroup$ I'm curious if they would begin working during a stall, or if they only work when in place during stable flight. $\endgroup$ – zymhan Aug 15 '19 at 17:51
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From my research on them, when used as a "poor man's slat" along the top of the leading edge, that is, to increase stalling AOA (about 1/3 to 1/2 the effect of a slat) and make the stall more gentle, as opposed to reattaching flow somewhere, the drag penalty of the VGs at cruise speed is too small to be worth the complexity and weight of the sort of under-skin gang-bar device that you would need to design that could allow the VGs to be extended and retracted all at once along the LE.

Even on faster airplanes like larger twins, the speed penalty of the VGs is only a few knots if at all, not enough to justify that kind of system.

The reason the drag penalty is so small appears to be that at low AOA/high speed, there is actually some some recovery of the energy used to create the vortices in the form of a drag reduction farther aft where the boundary layer starts to get thicker, because the boundary layer is better managed and is less turbulent due to the vortices. Enough to negate a significant part of the drag penalty of generating the vortices in the first place.

Some background on the use of VGs as a stall behaviour modification device here .

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    $\begingroup$ Not doubting you at all, but if you could quote/link some of your research that lead to this answer, it would make it even better. $\endgroup$ – FreeMan Aug 15 '19 at 18:46
  • $\begingroup$ These are things I learned many years ago, so it's really just typing off the top of my head, but I'll put in a link to a VG maker. $\endgroup$ – John K Aug 15 '19 at 20:17
  • $\begingroup$ microaero.com "We have flown a Baron with Dick Rutan at Edwards Air Force Base and used radar tracking to measure speed at different altitudes. The only conditions where we found a speed reduction was a 1 knot reduction in top speed at 12,000 ft. at full power, and no change in top speed at 75% power. There were no speed reductions at any other altitude. The small amount of drag created by the vortex generators is offset by a cleaner flow of air from the trailing edge of the wing." $\endgroup$ – CrossRoads Aug 16 '19 at 12:02
  • $\begingroup$ VGs are a 'hack' that is added to a design during testing. Retractable VGs would be such a major redesign that there are easier ways to soften the stall. $\endgroup$ – Robin Bennett Aug 16 '19 at 15:15
  • $\begingroup$ @RobinBennettYes in general they are a band-aid to correct a flow separation problem or tame some behaviour found in testing. However, in the context of the kits put out by companies like Microaero, they are used to improve Clmax and soften stall behaviour on numerous designs that are technically fine the way they are. A "poor man's slat" as I said, insofar as they let a wing that stalls at 14 or 15 deg to operate up to 18 or 19 deg, with an accompanying Clmax increase, + gentler stall behaviour. Main strike against them is the need to care for them. Knock too many off and you are grounded. $\endgroup$ – John K Aug 16 '19 at 15:42
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Bell Textron filed a patent for retractable vortex generators in 2006. The patent is here.

Boeing also filed a patent for vortex generators that extend simultaneously with a leading edge slat here.

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