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.

Clark YH wingroot of a Yak-18T

The Northrop Tacit Blue stealth technology demonstrator aircraft also used the Clark-Y.^[1] 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.^[2] 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.^[3]

Model aircraft

The Clark Y has found tremendous favour for the construction of model aircraft, thanks to the flight performance that the section offers at medium Reynolds number airflows. Application on model aircraft is very wide, ranging from free-flight gliders through to multi-engined radio control scale models.

The Clark Y is appealing for its near-horizontal lower surface, which aids in the accurate construction of wings on plans mounted on a flat construction board. Young and inexperienced modellers are able to build model aircraft which provide a good flight performance. The benign stalling characteristics of the airfoil are another aid, as this allows an inexperienced radio-control model hobbyist a better possibility of recovering from a stall in flight, to the benefit of the model.^[4]

The depth of the section lends itself to easier wing repair, as the modeller will often then have better access to balsa structural elements. In addition, the Clark Y section's depth also permits the flush installation of what are considered "standard" sized R/C servo motors within a wing for actuation of ailerons, flaps etc., without an excessive reduction in performance.

Applications

Some representative aircraft that use the Clark Y and YH are:

Clark Y[5] Ace Baby Ace Aeronca 50 Chief Avia B.122 Blue Yonder Merlin Curtiss P-6 Hawk Fleet Fawn Hatz Classic Heath Parasol Lockheed Vega Long Henderson Longster Monocoupe 90 Polikarpov R-5 Spirit of St. Louis Stinson Reliant Waco UPF-7

Clark YH[5] Currie Wot Hawker Hurricane Ilyushin Il-2 Mikoyan-Gurevich MiG-3 Miles Magister Nanchang CJ-6 Polikarpov I-153 Stolp SA-900 V-Star Yakovlev Yak-18T

References

1. ↑ Piccirillo, Albert, "The Clark Y Airfoil - A Historical Retrospective," SAE/AIAA paper 2000-01-5517, presented at the World Aviation Congress & Exposition, October 10, 2000, San Diego, California.
2. ↑ The Incomplete Guide to Airfoil Usage Retrieved 27 May 2013.
3. ↑ Шавров В.Б. (1994) История конструкций самолетов в СССР 1938-1950 гг. (3 изд.). Машиностроение (Shavrov V.B. (1994) Istorija konstruktsij samoljotov v SSSR, 1938-1950 gg. (3rd ed.). Mashinostroenie. ISBN 5-217-00477-0) (History of aircraft design in USSR: 1938-1950)
4. ↑ Martin Simons, Model Aeroplane Aerodynamics, Model & Allied Publications 1978. Chapter 7 - 9, Aerofoil Sections.
5. ↑ 5.0 5.1 Lednicer, David, (15 September 2010), The Incomplete Guide to Airfoil Usage, retrieved 28 May 2013