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Kline-Fogleman (KFm) airfoils, despite their seeming contradiction with known principles of aerodynamics, seem to be very popular in RC modelling, and are known to have a peculiar stall behavior, with a gradual transition to high drag without loss of lift, unlike the sharp stall of traditional smooth airfoils.

There's little information I could find about how the stepped design affects the airfoil's aerodynamic efficiency, however. The design seems to be plenty good for powered fun-flyer planes; I've seen at least some KFm slope gliders, too, and a well-circulated picture (provided below) states that the KFm3 variant has "fantastic flight characteristics" and is good for sailplanes... But there are no hard numbers that I could find on just how good these airfoils are (or how good they can be) in terms of lift to drag (L/D) ratio.

Are there any known numbers for the typical and best case L/D values of aircraft with Kline-Fogleman (KFm) airfoils, and if so, what are they? How does this characteristic differ between variants of the airfoil design?

KF variants

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This paper (AE-74-1054-1) from the University of Tennessee has a graph on page 15 showing the best they managed was 1:17 with the step on top and 1:8 with the step on the bottom but these were at a very specific angle of attack, and about half that was more typical.

The wind tunnel results show that for this new airfoil the lift/drag ratio is lower than for the flat plate, and the pressure data show that the airfoil derives its lift the same way as the inclined flat plate. This airfoil offers no discernable advantages over the conventional airfoil.

Anecdotally, that matches my experience with KFm gliders. They work about as well as a flat plate. Ok for a light-wieght park-flier with loads of thrust but no good for a glider.

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    $\begingroup$ Thank you for the answer and the excellent paper! However, skimming through the plots I found that the best they got actually was around 16-17 with the step on top, and the sharp-stepped version more typical for hobbyists got a max L/D value of around 10 (see fig. 11). You might want to add that to your answer as well. $\endgroup$
    – FlashCactus
    Commented Apr 28, 2020 at 23:22
  • $\begingroup$ Oh, I'd missed that! I've updated my answer. $\endgroup$
    – Robin Bennett
    Commented Apr 29, 2020 at 8:04

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