Clark Y airfoil
A flat-bottomed airfoil known for benign stall and easy construction.
The Clark Y is a widely used airfoil profile, designed in 1922 by Virginius E. Clark. It is notable for its flat lower surface aft of 30 percent of chord, which simplifies construction and angle measurements, and for its gentle stall characteristics.
- Designer
- Virginius E. Clark
- Year designed
- 1922
- Thickness percent
- 11.7
- Based on
- Goettingen 398 airfoil
- Flat lower surface aft of chord percent
- 30
Lore & Background
The Clark Y airfoil was designed in 1922 by Virginius E. Clark using the thickness distribution of the German-developed Goettingen 398 airfoil. It has a thickness of 11.7 percent and is flat on the lower surface aft of 30 percent of chord. This flat bottom simplifies angle measurements on propellers and makes for easy construction of wings. For many applications the Clark Y has been an adequate airfoil section, giving reasonable overall performance in respect of its lift-to-drag ratio, with gentle and relatively benign stall characteristics. The flat lower surface is not optimal from an aerodynamic perspective, and it is rarely used in modern designs. A variant, the Clark YH, is similar but with a reflexed trailing edge producing a more positive pitching moment, reducing the horizontal tail load required to trim an aircraft.
Reader's Guide
The Clark Y airfoil has been used on several notable aircraft, including the Lockheed Vega and Spirit of St. Louis. The Clark YH variant was used on mass-produced aircraft such as the Ilyushin Il-2 and Hawker Hurricane. The Northrop Tacit Blue stealth technology demonstrator also used a Clark Y, chosen because its flat bottom worked well with the design goal of a low radar cross-section. In model aircraft, the Clark Y has found favor for its flight performance at medium Reynolds number airflows, with applications ranging from free-flight gliders to multi-engined radio control scale models. Its near-flat lower surface aids construction on flat boards, making it appealing for inexperienced modellers who can achieve good flight performance with benign stalling characteristics. Additionally, an inverted Clark Y airfoil was used on the spoilers of the Dodge Charger Daytona and Plymouth Superbird. Despite its historical popularity, the flat lower surface is not optimal aerodynamically, and the profile is rarely used in modern designs.
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