In Italy, air-cooling is now used extensively for high-powered military aircraft engines. Although water-cooled engines of the V and W types with twelve and eighteen cylinders were much in evidence two or three years ago the present trend is toward air-cooled engines such as one-row and two-row radials and inverted-vee type engines. Undoubtedly this trend has been influenced to a great extent by climatic conditions in the Mediterranean and North Africa where difficulties often are encountered in obtaining cooling water.
Four well-known aircraft engine manufacturers are building air-cooled engines for military purposes. Perhaps the most striking development is the new Isotta Fraschini Delta RC 35 IDS engine which has twelve cylinders arranged in two inverted banks below the crankcase. The angle between the cylinder banks is 60° and this reduces the frontal area to 7.3 sq ft and permits excellent streamlining for high-performance aircraft. This particular model is equipped with four Isotta Fraschini carburetors and a one-speed gear-driven supercharger. It can be fitted with a hollow propeller shaft to accommodate a 20-mm shell gun.
The performance of the Delta is good and it develops 770 hp at 2,600 rpm for takeoff. It is rated at 750 hp at the same crankshaft speed at an altitude of 11,500 ft when running on 87-octane gasoline. As it weighs 1,120 lb, its specific weight is 1.45 lb/hp. A more powerful engine of the same type is now under development and is equipped with a two-speed supercharger and has a rating of approximately 1,200 hp at high altitudes. The type of warplane in which these engines are used is a military secret.
In the field of the air-cooled radial, three manufacturers produce engines of approximately 1,500 hp Alfa Romeo acquired licenses for the Bristol Mercury and Pegasus engines from Great Britain in 1931 and after gaining experience with them has developed some excellent engines of its own. The most powerful engine in production is the Alfa 135 RC 34 model which has eighteen cylinders in two banks and a one-speed gear-driven supercharger. This engine is rated at 1,500 hp at 2,400 rpm for takeoff and delivers 1,400 hp at 2,400 rpm at an altitude of 11,200 ft. It weighs 2,105 lb, or 1.40 lb/hp.
Equipment on the Alfa 135 RC 34 engine comprises a Zenith downdraft carburetor with automatic boost control and mixture control, two Marelli eighteen-cylinder magnetos and a Garelli compressed gas starter with engine-driven distributor. A smaller engine known as the 126 RC 32 model and developing 750 hp is used in Savoia-Marchetti SM 79 three-engined bombers.
Fiat radials are manufactured in various sizes and power outputs. The Fiat A 82 RC 42 is the largest production engine and has eighteen cylinders and runs on 87-octane gasoline. The displacement of this engine is 2,873 cu in and it is rated at 1,400 hp at 2,400 rpm for takeoff and 1,250 hp at 2,300 rpm at an altitude of 13,800 ft. Its one-speed supercharger is geared at 7 : 1 and its propeller drive is geared at 1.6 : 1. It weighs 1,910 lb which is equivalent to a specific weight of 1.36 lb/hp. Like other Italian aircraft engines, it is equipped with a Garelli compressed gas starter.
A slightly smaller model known as the Fiat A 80 RC 41 develops 1,000 hp. This engine is used in Breda 65 two-place fighters, Breda 82 two-engined bombers and Fiat BR 20 two-engined bombers. The 850 hp Fiat A 74 RC 38 model is used in Fiat G 50 one-place interceptors which have a ceiling of 35,000 ft, and Macchi C 200 one-place interceptors which can climb to an altitude of 20,000 ft in 65 minutes and have a ceiling of 34,000 ft. Fiat air-cooled radials have an excellent reputation and have established numerous aviation records.
The most powerful radial manufactured by Piaggio is a two-row engine with eighteen cylinders. This engine is known as the Piaggio P XII RC 35 model and is rated at 1,500 hp at 2,100 rpm for takeoff and delivers 1,300 hp at 2,000 rpm at an altitude of 11,500 ft. When running on 87-octane gasoline it has a consumption of 0.46 lb/hp/hour. Its weight is 1,875 lb, or 1.25 lb/hp. Its compression ratio is 6.5 : 1.
The 1,500 hp Piaggio P XII RC 35 engine powers the new Piaggio P 108 four-engined long-range bomber which is now in production. A slightly smaller engine known as the P XI RC 40 model and developing 1,000 hp is used in the Caproni Ca 135 bis two-engined bomber, the Piaggio P 32 two-engined bomber, and the Breda S8 two-engined bomber which has a maximum speed of 321 mph and a ceiling of 28,000 ft. In addition to the engines listed, the firms of Alfa Romeo, Fiat and Piaggio are developing two-row radials of approximately 2,000 hp. which should be in production at an early date.
This article was originally published in the November, 1940, issue of Aviation magazine, vol 39, no 11, pp 72-73.
The original article includes 2 photos, of the Isotta Fraschini Delta RC 35 IDS and the Fiat A 82 RC 42.
Photo of Isotta credited to Aerosphere; photo of Fiat not credited.