Are there any full-size airplanes flying with a Selig S1223 airfoil?

  • $\begingroup$ Not that I can see. Lots of talk about its suitability for heavy lift cargo aircraft but no actual flying examples. $\endgroup$ – Juan Jimenez May 28 '19 at 14:24

Based on a quick perusal of the "Incomplete Guide to Airfoil Usage", my initial answer is no. If you look up any of the big name manufacturers that are present on that list (Schempp-Hirth, Schleicher, Grob, Schweizer were my picks offhand), they use a mixture of Wortmann FX, NACA, and Eppler airfoils. That said, the truer rationale comes a look at a Cl vs Cd curve, or how the lift and drag of an airfoil change with respect to one another.

I have a representative Cl vs Cd curve below (Cl on the vertical axis and Cd on the horizontal axis), where the purple line is a Selig S1223 airfoil, the yellow is a Wortmann FX 61-163 (used on a number of Schleicher sailplanes), and the green is the Eppler 603 (used on a number of Grob sailplanes). Obviously, the S1223 produces far more lift than any of the other airfoils, and hits its most efficient point (maximum lift for minimum drag) at a very high lift coefficient -- just a shade over 1!

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However, this is not necessarily a good thing. If we work some numbers, it starts to become apparent. Using a the definition of lift coefficient, we can estimate the typical lift coefficient () of a Grob 103 in trimmed flight something like this, where is the density of air, is the aircraft's wing area, and is it's flight speed.

For a Grob 103, let's assume that it's flying at maximum gross weight (so maximize the necessary lift), so 5978 N, at sea level the density of air is 1.225 kg/m^3, wing area is 17.9 m^2, and its best L/D speed is 52.5 knots (27 m/s). Plugging all those in, we get a wing lift coefficient of 0.75. Admittedly, this does not translate exactly into an airfoil lift coefficient, but it suits the need for a comparison.

Looking on the Cl vs Cd chart, this value is below the S1223's minimum drag point, and is actually where the S1223's drag is starting to spike. However, it does hit right in the lower drag region of the Eppler and the FX quite well. Furthermore, gliders aren't flying straight and level, which is what this calculation implies, but are, actually, descending (when outside a thermal/ridge lift/etc.), so the actual lift coefficient we need is probably somewhat lower, leading to even higher drag production from the S1223 but falling right into the "sweet spot" of the Eppler and FX airfoils' lift/drag characteristics.

In short, while the S1223 does have amazing lift characteristics (at lower Reynolds numbers -- these trends may not hold to a Reynolds number of 3,000,000, which would be closer to an operating sailplane), it is actually not the right airfoil for a regular sailplane. For model-scale aircraft, however, it does have application, especially when wingspan is limited.

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  • $\begingroup$ Thanks for all the info! Looking at a tandem biplane/triplane with a span of 7' and $\endgroup$ – Fred May 28 '19 at 20:21

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