NACA cowling

The NACA cowling is a type of aerodynamic fairing used to streamline radial engines installed on airplanes.

It 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.

[3] The NACA cowling enhanced speed through drag reduction while improving engine cooling.

In a normal planar airfoil, like a wing, the difference in airspeeds, and their associated changes in pressure, on the top and bottom surfaces, enhances lift.

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 serves to further slow the airflow.