NACA cowling

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NACA cowling on a Curtis AT-5A at the Langley Memorial Aeronautical Laboratory, October 1928

The NACA cowling is a type of aerodynamic fairing used to streamline radial engines for use on airplanes. 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 economy that it enabled.

NACA cowling was more than just streamlining: it was an airfoil for which the force vector was forward. The cowling enhanced speed through drag reduction. The cowling constitutes a symmetric, circular airfoil -- in contrast to the planar airfoil of wings.

The test aircraft, a Curtiss Hawk AT-5A biplane, featuring a Wright Whirlwind J-5 radial engine, reached an airspeed of 220km/h equipped with the NACA cowling compared to 190km/h without it.

The NACA cowling directs cool air to flow onto the center of the motor, 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.

This is also known as a ring cowl.