AIRCRAFT PROPULSION — TAYLOR 273 



AIR VS. LIQUID COOLING 



The classic and often emotionally charged argument over the rela- 

 tive merits of liquid and air cooling started with the early days of fly- 

 ing ("Antoinette" vs. "Gnome," for example) and persisted to the end 

 of World War II, when the advent of jets and turbo-props diverted 

 attention elsewhere. 



As we have seen, water cooling was dominant through World War 

 I, except for the rotaries, which were obsolescent at its end. European 

 military aviation remained generally committed to water cooling up 

 to and through World War II, although some air-cooled engines were 

 used in bombers and transports, and there was one excellent air-cooled 

 European fighter, the Focke-Wulf with the B.M.W. two-row radial, 

 developed from a Pratt and Whitney license. Japanese fighter air- 

 craft also used air-cooled radials copied from Wright and Pratt and 

 Wliitney designs. Their other military aircraft used these and copies 

 of the German Daimler-Benz liquid-cooled engine. 



In the United States, the Navy made a commitment to air cooling 

 in 1921 which has held for reciprocating engines to this day. It was 

 chiefly Navy support that underwrote early Pratt and Whitney and 

 Wright air-cooled developments. The reason for this choice lay in the 

 limitations of the airplane carrier, which required short takeoff, com- 

 pact size, and minimum maintenance. Commander Bruce Leighton 

 was probably the one most responsible for this well-considered 

 decision. 



The most intense controversy on this subject took place in the U.S. 

 Army Air Service, whose support for air-cooled engine development 

 in the 1920's and 1930's was never as enthusiastic as that of the Navy, 

 because of the assumed "larger frontal area" and greater "drag" of 

 air-cooled radials, especially for use in fighter airplanes. That cool- 

 ing drag was a real problem in the early days is illustrated by plate 

 19, figure 1, and plate 20, figure 1, showing typical installations of the 

 1920's. 



The "drag" of air-cooled engines was greatly reduced by the advent 

 of the very effective cowling and cylinder baffling developed by the 

 (U.S.) N.A.C.A. starting in 1929 (pi. 20, fig. 2, and text fig. 13). 

 Further reductions in cooling drag were achieved by increased cooling- 

 fin area, which reduced the air velocity required for cooling (com- 

 pare fig. 12 with pi. 17, fig. 1 ) . These developments put the air-cooled 

 radial virtually on a par with the water-cooled engines with regard 

 to cooling drag, until the advent of high-temperature liquid cooling 

 with glycol- water mixtures. Plate 19, figure 2, shows modem cowling 

 for the air-cooled radial engine. 



The use of high-boiling-point liquids (mixtures of water and ethyl- 

 ene glycol) for engines formerly water-cooled was introduced in 1932 



