The NACA cowling is a type of aerodynamic fairing used to streamline radial engines for use on airplanes and developed by the National Advisory Committee for Aeronautics in 1927. It was a major advancement in drag reduction, and paid for its development and installation costs many times over due to the gains in fuel efficiency that it enabled.
History and design 
The NACA cowling was more than just streamlining: it improved engine cooling. The cowling enhanced speed through drag reduction and utilising the heat of the engine to generate thrust. The cowling constitutes a symmetric, circular airfoil, in contrast to the planar airfoil of wings.
The test aircraft, a Curtiss AT-5A Hawk biplane, featuring a Wright Whirlwind J-5 radial engine, reached an airspeed of 137 miles per hour (220 km/h) equipped with the NACA cowling compared to 118 miles per hour (190 km/h) without it.
The NACA cowling directs cool air to flow through the engine where it is routed across the motor's hottest parts, i.e. the cylinders and even more importantly, the cylinder heads. Furthermore, turbulence after the air passes the free-standing cylinders is greatly reduced. The sum of all these effects reduces drag by as much as 60%. The test conclusions resulted in almost every radial-engined aircraft being equipped with this cowling, starting in 1932.
See also 
- White, Graham. Allied Aircraft Piston Engines of World War II. pp. Figures 2.2 & 2.3. ISBN 1-56091-655-9.
- White, Graham. Allied Aircraft Piston Engines of World War II. pp. 7–8. ISBN 1-56091-655-9.
- James R. Hansen (1998). "Engineering Science and the Development of the NACA Low-Drag Engine Cowling". History.nasa.gov. Retrieved 2010-07-30.
- Full-Scale Testing of N.A.C.A. Cowlings (Theodore Theodorsen, M. J. Brevoort, and George W. Stickle, NACA Report # 592. Langley Memorial Aeronautical Laboratory: 1937)
Other source 
- Aeronautic exhibit in Smithsonian Institution, Washington, D.C.
|Wikimedia Commons has media related to: NACA cowlings|
- NACA Low-Drag Engine Cowling Essay - details on development of the cowling.
- Abstract of NACA TN 301 report and .pdf file
- Archive of NACA reports 1917-1958