Polish sports plane PZL Ł.2
with a Townend ring
A Townend Ring is a narrow-chord cowling ring fitted around the cylinders of an aircraft radial engine to reduce drag and improve cooling.
The Townend ring was the invention of Dr. Hubert Townend of the British National Physical Laboratory1 in 1929. Patents were supported by Boulton & Paul Ltd in 1929.2 In the United States it was often called a "drag ring". It caused a reduction in the drag of radial engines and was widely used in high-speed designs of 1930-1935 before the long-chord NACA cowling came into general use. It was also said to generate forward thrust from the expansion of the air as it passed over the engine, adding 10 to 15 mph to the aircraft's top speed.3
Examples of aeroplanes with Townend rings were the Douglas O-38, Vickers Wellesley, the Westland Wallace and the Gloster Gauntlet. Early claims portrayed it as a superior design to the NACA cowling, but later comparisons proved aircraft performance worse when using a Townend ring at airspeeds above 250 mph.4