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. However, it does hit right in the lower drag region of the Eppler and the FX quite well. 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. Admittedly, this does not translate exactly into an airfoil lift coefficient, but it suits the need for a comparison. Plugging all those in, we get a wing lift coefficient of 0.75. 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.įor 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). If we work some numbers, it starts to become apparent. However, this is not necessarily a good thing. 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! 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). 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. 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. Based on a quick perusal of the "Incomplete Guide to Airfoil Usage", my initial answer is no.
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