By: Norm Goyer
Cooling radial or round engines has always been a problem for aircraft designers. The first round engines appeared during World War I as rotary engines which had no cooling problem because the cylinders rotated along with the prop and were constantly in their own slipstream. Early designers were able to produce simple cowlings which did reduce the drag but did not impede the airflow over the cylinders. Soon after the war ended so did the use of rotary engines. Radial engines made their debut in the 1920s and a new cooling ballgame was in progress. Most early aircraft with radial engines simply left them out in the open air but this left the huge engine producing immense drag as the air encountered the wide relatively flat surface of the engine. In the later 1920s, the National Advisory Committee for Aeronautics (NACA) developed a covering, or cowling, for the engine that not only reduced drag but also improved cooling of the engine. It was a major technological breakthrough achieved through careful, systematic research and testing. When the numbers were analyzed it was found that a properly designed radial cowling could produced added speed and well as increased cooling in very large numbers. The idea for the NACA low-drag cowling originated in 1926 during a demonstration of the new Propeller Research Tunnel (PRT) at Langley Memorial Aeronautical Laboratory (LMAL) in Virginia near the Atlantic coast. The PRT was then the largest wind tunnel in the world and capable of holding a full-size airplane engine and propeller and the fuselage behind it (but not the wings). Several people witnessed a test of a Sperry Messenger airplane and immediately wondered if it was possible to cover the cylinder heads in order to reduce drag. The problem was that covering the cylinder heads was likely to reduce the cooling of the engine, so any engine covering would have to be carefully designed.
Researchers at Langley started an effort to explore engine cowlings under the direction of Fred Weick, a young Chicago-born engineer. Weick designed ten different cowlings, from a partial covering to a complete covering of the engine. These were fitted to a Wright Apache biplane with a J-5 Whirlwind air-cooled engine. The goal was to produce a cowling that reduced drag but still cooled the engine as much as an uncovered engine. Weick later became well known as the designer of the Ercoupe two-control, safe and easy to fly aircraft.
NACA inadvertently stumbled upon the bump cowl which managed to turn a quite ugly round plain cowling into a well sculpted quite pretty nose on many aircraft. The Monocoupe and Cessna 195 had bump cowls which greatly added to their looks and reduced the diameter of the cowling giving a sleeker look. History tells us that “Langley engineers then mounted cowling their Number 10 design onto a borrowed Curtiss Hawk AT-5A biplane that used the same Wright Whirlwind J-5 engine that had been tested in the PRT. The tests showed that the airplane’s maximum speed increased from 118 to 137 miles per hour with the cowling. On February 4-5, 1929, Frank Hawks, a barnstorming pilot, flew a Lockheed Air Express equipped with a NACA low-drag cowling from Los Angeles to New York nonstop and established a new record. Hawks flew this distance in 18 hours and 13 minutes in an airplane whose top speed had been increased from 157 to 177 miles per hour. Gerry Vultee, of Lockheed, sent the NACA a telegram that stated, “Cooling carefully checked and OK. Record impossible without new cowling. All credit due NACA for painstaking and accurate research.” Soon many new speed records were being established with aircraft equipped with versions of the NACA low-drag cowling. In January 1930, the National Aeronautic Association awarded the NACA the Collier Trophy for the greatest achievement in American aviation in 1929.”
NACA then tackled the proper placement and cooling of propeller engines, particularly on large multi-engine craft, such as bombers. “For instance, most multi-engine craft before the mid-1930s, such as the Ford Trimotor, had the engines mounted below the wings in pods (or “nacelles”). But further research at the NACA, using the same methods developed for the low-drag cowling development, demonstrated that the best place to mount the engines was directly in front of the wing, blended into it. Thereafter, planes like the B-17 bomber had engines that were fitted into the wing itself and achieved greater efficiency.”
One of the biggest challenges that NASA engineers had was the interior design of baffling which needed to be designed and installed on the Pratt & Whitney four row “corncob” engine which powered many large aircraft and fighters during the waning days of World War II and Korea. This huge radial engine was the last of the big radial engines. Jet engine cowlings were then the subject of NASA engineers.