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Why don't ultralight/LSA aircraft owners put vortex generators (VGs) on the top of their flaps?

I understand that the maximum flap angle for a single lift creating flap is 40°, but 80° with a double flap, if each flap is rotated at 40°.

Why not put VGs on the top of a single flap to increase its maximum lift creation angle, and thus lift, to 45°, 50° or maybe even 60°, which would allow for a slower landing speed, hopefully in the 15-20mph range?

While Vg's might have been designed for wings, they are now used for elevators as well, which is very similar to an upside down wing and flap. See picture below. Source: Zenith

Zenith Elevator VG

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  • $\begingroup$ Assuming they do have significant aerodynamic benefits (I have no grounds for a reasonable opinion on that), I'd suggest that the cost and the lack of STCs is probably the primary factor. $\endgroup$
    – jamesqf
    Commented Aug 27, 2020 at 17:28

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Because the separation bubble, that vortex generators attenuate, occurs in front of the flaps. There's no advantage to putting VGs that far back, on an ultralight or any other fixed wing aircraft.

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  • $\begingroup$ Actually VG's are used for horizontal tails as well and are put on the elevator ( upside down flap), instead of at the front of the stabilizer ( wing portion). See pic above. $\endgroup$
    – Fred
    Commented Aug 27, 2020 at 17:09
  • $\begingroup$ Good luck finding a wind tunnel photo or other evidence of flow separation starting that far aft, on the tailplane or on any airfoil, with or without a deflected TE control surface. Zenith's doodle shows that VGs located there, tucked into the hinge, cause no harm when the elevator is undeflected. But it doesn't prove that, when exposed to airflow at that location, they give any benefit. $\endgroup$
    – Camille Goudeseune
    Commented Aug 27, 2020 at 17:50
  • $\begingroup$ lol. Just a bad doodle. Aircraft like the Savanah come factory equipped with VG's. $\endgroup$
    – Fred
    Commented Aug 27, 2020 at 22:49
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Who says that the maximum flap angle is 40°? There are plenty of planes with flaps that can go beyond that. For instance, the Hawker series has flaps that can go as high as 75°.

But the thing is, having such an extreme flap angle doesn't actually increase lift. In fact, it reduces lift. That's why it's referred to as the "lift dump" setting; it's only used after touchdown, reducing lift to increase braking efficacy while also acting as an air brake.

So, to answer your question, VGs wouldn't enable flap settings over 40° because you can already do that, and even if they did, going that high wouldn't reduce the landing speed.

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    $\begingroup$ I understand some cubs at the Valdez STOL competition even have 90 deg flaps, to increase drag and shorten that landing roll to under 10 ft. I meant flaps for lift creation and have revised my question. $\endgroup$
    – Fred
    Commented Aug 31, 2020 at 15:03
  • $\begingroup$ @Fred The only difference between a flap "for lift creation" and a flap to increase drag is, in fact, the angle. Here's a video of lift dump in action. You will note that the only difference between "extra lift" mode and "extra drag" mode is the angle at which the flaps meet the incoming air. $\endgroup$
    – HiddenWindshield
    Commented Aug 31, 2020 at 15:49
  • $\begingroup$ A tripple slotted flap at 45 deg ( 15 deg each flap ), has a lot less drag and probably infinite more lift, than a single flap at 45 deg ( which probably has no lift at 45 deg). That's my point - how to partially mimic the lift of a double or triple flap, which a single flap with no tracks or linkages. $\endgroup$
    – Fred
    Commented Sep 1, 2020 at 14:31
  • $\begingroup$ @Fred But you didn't ask about a 45° flap. You asked about an 80° flap. If you'll watch the video I linked, you will note that the Jetstream 31 has a double-slotted flap. According to your logic, that must mean that it can increase lift up to 80° of deflection. So why didn't the pilot use that capability when they were in the air? Why didn't they fully deploy the flaps until after touchdown? Because that's not what actually happens. Increasing flaps beyond about 45° (depending on wing design) increases drag with a decrease in lift, regardless of VGs, slots, whatever. $\endgroup$
    – HiddenWindshield
    Commented Sep 1, 2020 at 15:52
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Well, it turns out that the maximum lift for plain, and single/double/tripple slotted flap is surprisingly the same, about 40-45 deg, as shown in this post:

https://aviation.stackexchange.com/a/24652/45806

So, there would be no increased lift by adding vortex generators to deflect the flap past 40-45 deg.

The only "lift benefit" I can think of, is if the wing could generate the required lift at a lower speed, but can't due to flow separation. In this case, as shown in the bottom right diagram of the picture above, vortex generators would be of benefit.

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