What airfoils are used for VTOL ultralights at 60-70 deg AOA without flaps or slats?

An example of this would be the Blackfly with a takeoff distance of 3', about 25% of it's own length....!!!!

I understand most airfoils stall at about 15deg, slats and flaps allowing up to 30 deg AOA such as with the Zenith 701.

Yet the Blackfly has no slats or flaps and has controlled flight even at at 60,70,80,90 deg AOA.

I assume the distributed propulsion reduces the LOCAL AOA significantly, so that lift is only via the props from say 40-90 deg AOA, and then wings start to generate lift at about 40 deg AOA even without slats or flaps.

Yet, I was told distributed propulsion only works for low AOA. Very confused....

What airfoil type is used for this?

Would an airfoil like the Selig 1233 or Eppler 399 be appropriate for this? I understand these are low Re airfoils, but these usually stall at a lower AOA than a classic airfoil like the NACA65018 used in the Zenith701.

Or do you need something very fat that will stall at a much higher AOA, like an airfoil with 25%-30% thickness?

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    $\begingroup$ I am at a loss about what you are asking. If you are wondering how does the Backfly fly at very high angles of attack, it is because it has a thrust to weight ratio higher than 1, clearly seen here $\endgroup$ Oct 8, 2019 at 17:37
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    $\begingroup$ At that angle of attack a flat plate will work just as well as any airfoil. $\endgroup$ Oct 8, 2019 at 18:30

1 Answer 1


Those airfoils never see angles of attack of more than a few degrees. Since both wings of the Blackfly carry propellers along their full span, their prop wash ensures that airflow is mostly along the wing's chord. Slow flight is not much different from the hover of quadcopters, in both cases the propellers create most if not all of the lift.

Blackfly static exhibition

Blackfly ultralight (picture source)

So to answer your question directly: There are no VTOL ultralights where the wing is exposed to flow at 60-70° AoA. The airfoils used are most likely about 15% thick with moderate camber because they operate at single-digit angles of attack.

  • $\begingroup$ Oh. So that means that these wings will never stall as long as the props are spinning, and they will create lift at 40,50,60,70 deg wing AOA ( but under 10 deg local AOA). Nice. $\endgroup$
    – Fred
    Oct 9, 2019 at 19:10

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