Jetrader - September/October 2008 - 30

aviation history The Piston Engine – Part 2 By Bill Bath Photo courtesy of the United States Air Force These Boeing P-26s, known as “Peashooters,” were parked on the ramp at Hickam Field, Hawaii, in January, 1940. The Fairey Swordﬁsh I t was the emergence of more powerful liquid-cooled engines that doomed the rotary radial design by the end of 1917. The gyroscopic forces imposed on the ﬂimsy airframes by the heavy rotating mass and controllability by the pilot limited the size and hence power of the rotary—plus high fuel and oil consumption sharply curtailed its endurance. Much thought and research on both sides of the Atlantic was being expended to solve the problem of cooling the non-rotary radial engine to produce the increased power needed to meet the demand for performance and greater payload. In 1927, Dr. Hubert Townend, of the U.K. National Physical Laboratory, developed the Townend Ring, which surrounded the cylinders of a radial engine and had an airfoil cross section to reduce the drag generated by formerly uncowled cylinders. It also contributed some forward thrust due to the divergent shape of the inner lining at the ring’s aft end. Boeing used it on its all-metal heavily braced monoplane P-26 ﬁghter in 1931, but by then years of research work at the National Advisory Committee for Aeronautics (NACA) in the U.S. on fully streamlined cowlings, resulted in a major improvement in drag reduction and cylinder cooling that eventually made the narrow Townend Ring obsolescent. Using an empirical method of research from 1926 though 1936, utilizing the wind tunnel and a series of cowlings, showed that the tenth design in November 1928 had one third the drag of design #1. Flight tests of the new cowl on a Curtiss Hawk AT-5A increased its speed from 118 mph to 137 mph. The 1929 Lockheed 5-C Vega had the NACA cowling; both Wiley Post and Amelia Earhart ﬂew this model in various recordbreaking ﬂights. By 1934 it was decided that the experimentalists had run their course, so Henry Reid, the engineer-in-charge at the NACA tunnel research center in Langley Virginia, transferred the project to Theodore Theodorsen, the pre-eminent theoretician of his day. (In 1945, he was to refer to German and NACA swept-wing research by Robert Jones as “hocus pocus!”) Theodore’s team developed the aerodynamic analysis, which by 1939 would give an aircraft designer the empirical formulae for attaining minimum external form drag and minimum internal cooling drag. By then the test ﬂights of the Boeing B-247 of 1932 and the Douglas DC-1 in 1933 introduced a new standard of commercial ﬂight to an eager public, with the NACA cowls, ﬂ aps, retractable landing gear and having two engines with the capability to continue climb out after an engine failure during takeoff. Hitting the Highlights Although space issues prevent me from discussing in detail the many innovations and successful and failed engine designs leading up to the jet age, the following are a few picked from the many notable air-cooled radial designs by British and American 30 The ofﬁcial publication of the International Society of Transport Aircraft Trading

Jetrader - September/October 2008

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