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Are US ultralights allowed to have vortex generators?

I understand US ultralights are required to have a landing speed of 28mph without the use of high lift devices. I understand this is so that the aircraft can land at 28mph no matter what, even if the flaps jam.

Hight lift devices are usually moveable flaps which makes aircraft operation more complex.

Would a vortex generator be considered a high lift device, and therefore not allowed to be used to land at 28mph? As they are non moveable, it does not make the aircraft more difficult to fly.

As they also allow an aircraft to land slower with the same wing area, I would argue that they make an aircraft safer to land.

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  • $\begingroup$ I don’t have knowledge about US regs about ultralight aircraft, but here’s an interesting article you can check out: Investigating active vortex generators as a novel high lift device. $\endgroup$
    – PapaMike99
    Jun 11, 2021 at 21:26
  • $\begingroup$ The definition of an ultralight is very simple and doesn't mention either landing speed or high lift devices. If you have a source that says there are other requirements that aren't in the regulations, could you share a link to it? What makes you think that high lift devices wouldn't be allowed? $\endgroup$
    – Pondlife
    Jun 11, 2021 at 22:30
  • $\begingroup$ @ Pondlife: Thank you for the link and clarification. $\endgroup$
    – Fred
    Jun 12, 2021 at 0:38
  • $\begingroup$ Apparently the question was based on a misconception. But even when you held that misconception, I don't think you really meant to ask "Are US ultralights allowed to have vortex generators?". I think you really meant to ask whether a design that could not meet the 28 mph landing speed requirement without vortex generators, would be permissible if it could meet the 28 mph landing speed requirement with vortex generators. $\endgroup$ Jun 12, 2021 at 12:16

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The title question is moot as "vortex generator" does not appear anywhere in the FARs or AIM— the FAA does not appear to define it or a high-lift device. From an engineering perspective high-lift devices are temporarily deployable when needed, whereas the vortex generators are an embarrassment, a lifelong drag penalty to fix an unexpected late-stage aerodynamic anomaly.

Yes, they are implicitly allowed on ultralights. Per 14 CFR §103.1 an ultralight aircraft has performance requirement(s) but the FARs generally do NOT specify or prohibit a particular design solution to achieve compliance with the regulations—that could stifle innovation. So the performance requirements can be achieved by any means and since an ultralight has no airworthiness certification, the means can be changed at will!

Thus vortex generators may be used if necessary to achieve the max. power-off stall speed of 24 knots CAS. Likewise high-lift devices. Or anything, really.

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