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.[dubious– discuss] 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.