Clark Y

Clark Y is the name of a particular aerofoil profile, widely used in general purpose aircraft designs, and much studied in aerodynamics over the years. The profile was designed in 1922 by Virginius E. Clark. The airfoil has a thickness of 11.7 percent and is flat on the lower surface from 30 percent of chord back. The flat bottom simplifies angle measurements on propellers, and makes for easy construction of wings on a flat surface.

The Lockheed Vega is one example of the Clark Y used in practice. For many applications the Clark Y has been adequate; it gives reasonable overall performance in respect of its lift-to-drag ratio, and has gentle and relatively benign stall characteristics. But the flat lower surface is sub-optimal from an aerodynamic perspective, and it is rarely used in new designs. The Spirit of St. Louis used the Clark Y; the Piper Cub used the quite similar USA 35B.

The Northrop Tacit Blue stealth technology demonstrator aircraft also used the Clark-Y. The Clark-Y was chosen for this application, as its flat bottom worked well with the design goal of a low radar cross-section.

The Clark YH airfoil is the similar but with a reflexed (turned up) trailing edge producing a more positive pitching moment reducing the horizontal tail load required to trim an aircraft. The Hawker Hurricane, Curtiss F9C Sparrowhawk and Nanchang CJ-6 used this profile. Мany Soviet aircraft of the 1930s and 1940s including all of the World War II Yakovlev fighters and MiG-3 used the Clark YH airfoil.

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