AIRCRAFT PROPULSION — TAYLOR 271 



16 and plate 17, figure 2, show the outstanding modern air-cooled ra- 

 dial engines which are basically descendants of the Gnome, but with 

 greatly imjDroved detail design, including the composite steel and 

 aluminum cylinder construction pioneered by Gibson, Pleron, and 

 Lawrance. Figure 12 shows the evolution of Wright cylinders from 

 1920 to about 1930, and plate 17, figure 1, shows the cylinder used on the 

 Wright turbo-compound engine, the most highly developed air-cooled 

 radial. 



The following comparison illustrates the development of air-cooled 

 engines from 1922 to the present time. 



1922 195S 



Engine Lawrance Wright 



J-1 Turbo- 

 Compound 



Maximum horsepower 200 3,700 



R.p.m 1,800 2,900 



Brake mean effective pressure, psi 112 302 



Mean piston speed ft. /min 1,650 3,070 



Horsepower per sq. in. piston area 1.4 7. 



Weight per hp. "dry" 2.38 0.96 



As in the case of liquid-cooled engines, developments in fuels, super- 

 charging, and cooling systems were important factors in this develop- 

 ment, as well as great improvements in detail design. 



The subject of the air-cooled engine should not be left without men- 

 tion of the remarkable development of the light-airplane engine begin- 

 ning with the small British 4-cylinder vertical air-cooled "Cirrus" 

 (1927) followed 5 years later by the 4-cylinder Continental A-10 

 which set the modern style of horizontal-opposed light-airplane en- 

 gines. These have developed to a remarkable degree of reliability and 

 (unsupercharged) performance. Recently, a license to build Con- 

 tinental engines of this type was acquired by Rolls-Royce, a real 

 compliment to the high quality of these small powerplants. Plate 18, 

 figure 1, shows the Continental A-65 4-cylinder engine brought out in 

 1938. This engine used composite steel and cast-aluminum cylinders, 

 which are now standard for engines of this category. 



Another category of air-cooled engines comprises those built for 

 installation in model airplanes. These are usually single-cylinder 

 engines of less than 1 inch bore and stroke (pi. 18, fig. 2) . Some are 

 rated up to 1 hp. at speeds of 15,000 r.p.m. or more. Originating in 

 the U.S. about 1930, these engines were produced in very large quan- 

 tities between 1945 and 1950. It is claimed that there were 180 manu- 

 facturers of model engines in the U.S. during this period. Their 

 total production, in number of engines, probably exceeded that of all 

 other aircraft engines combined. The popularity of engine-powered 

 model airplanes fell off about 1950, and fewer are now manufactured. 



