NACA cowling
Aerodynamic fairing that reduced drag and improved radial engine cooling.
The NACA cowling is an aerodynamic fairing designed to smooth airflow around radial engines on aircraft. Fred Weick of the National Advisory Committee for Aeronautics (NACA) created it in 1927. This innovation significantly cut aerodynamic drag, and the resulting fuel efficiency savings repaid its development and installation costs many times over. NACA received the 1929 Collier Trophy for this work.
The cowling is a symmetric, ring-shaped airfoil wrapped around the engine. On a standard wing airfoil, pressure differences between top and bottom surfaces generate lift. With the NACA cowl, this ring-shaped airfoil is oriented so that the lift effect pulls forward, producing a small thrust that partially offsets the cowling’s own drag. The airflow difference across the cowl’s surfaces comes from both the airfoil shape and the engine cylinders inside, which further slow the air. Despite this, total airflow through the cowl usually exceeds that of an uncowled engine because air flowing around the outside sucks air through the interior. This directs fast-moving air mainly over the cylinder heads—where cooling is most needed—rather than between cylinders and the crankcase, where it does little good. Turbulence behind the formerly exposed cylinders is also greatly reduced. Together, these effects cut drag by up to 60%. Test results led to nearly all radial-engined aircraft adopting this cowling starting in 1932.
The test aircraft, a Curtiss AT-5A Hawk biplane with a Wright Whirlwind J-5 radial engine, flew at 137 miles per hour (220 km/h) with the cowling, versus 118 miles per hour (190 km/h) without it. Thomas Carroll served as chief test pilot at Langley during the 1929 tests. The notion that the NACA cowling produced thrust via the Meredith effect is incorrect—while heated air expanding from the engine could theoretically create some thrust if expelled at high speed, this required a specially shaped cowling (which the NACA cowling was not), and at 1930s airspeeds the effect was negligible.
- Developer
- Fred Weick of the National Advisory Committee for Aeronautics (NACA)
- Year developed
- 1927
- Award
- 1929 Collier Trophy
- Test aircraft
- Curtiss AT-5A Hawk biplane with Wright Whirlwind J-5 radial engine
- Speed with cowling
- 137 miles per hour (220 km/h)
- Speed without cowling
- 118 miles per hour (190 km/h)
- Drag reduction
- as much as 60%
Lore & Background
The NACA cowling consists of a symmetric, circular airfoil wrapped around the engine. In a normal planar airfoil, the difference in airspeeds on top and bottom surfaces enhances lift; with the NACA cowl, the ring-shaped airfoil is positioned so this lift effect is forward, producing thrust that does not fully counter the cowl's drag but greatly mitigates it. The difference in airspeed on the two sides is due not only to the shape of the airfoil but also the presence of the cylinders on the inside surface, which further slows the airflow. Total airflow through the cowl is generally greater than without it, as air is sucked through by the air flowing around it, keeping fast-moving air primarily on the cylinder heads where it is most needed and reducing turbulence after the air passes the free-standing cylinders.
The test aircraft, a Curtiss AT-5A Hawk biplane with a Wright Whirlwind J-5 radial engine, reached 137 miles per hour (220 km/h) with the NACA cowling compared to 118 miles per hour (190 km/h) without it. Thomas Carroll tested the engine cowling as chief test pilot at Langley in 1929. The idea that the NACA cowling produced thrust through the Meredith effect is fallacious—although in theory heated air expansion could create some thrust, in practice this required a cowling designed for high-speed air exit (which the NACA cowling was not), and at 1930s airspeeds the effect was negligible.
Reader's Guide
The NACA cowling was a major advancement in aerodynamic drag reduction, and paid for its development and installation costs many times over due to the gains in fuel efficiency that it enabled. NACA won the 1929 Collier Trophy for its development. The cowling enhanced speed through drag reduction while improving both engine cooling and fuel consumption. The sum of all its effects reduced drag by as much as 60%. The test conclusions resulted in almost every radial-engined aircraft being equipped with this cowling, starting in 1932. The cowling's significance lies in its widespread adoption and the substantial fuel efficiency and speed improvements it provided, making it a standard feature on radial-engined aircraft for decades.
Did You Know?
- The NACA cowling was developed by Fred Weick in 1927.
- The test aircraft, a Curtiss AT-5A Hawk, achieved 137 mph with the cowling versus 118 mph without.
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