Decision that the fixed radial air-cooled type of aeronautic engine offers the most possibilities in light weight and maximum dependability was arrived at by the company with which the author is connected after a careful and comprehensive engineering analysis of many types of both water-cooled and air-cooled engine and therefore it undertook the development of this type. Because the Navy was desirous of obtaining a 400-hp. direct-drive engine and a 500-hp. engine to drive either direct or geared and because a commercial air-cooled engine to replace the Liberty-12 water-cooled engine was desirable owing to the increased pay-load it would make possible, it was decided to undertake first the development of a 400-hp. engine of this type.
Work on the design was started Aug. 1, 1925, and the first engine was finished on Dec. 24, or approximately 5 months later. This engine was named the Wasp. In the winter of 1925 and 1926 work was started on a 500-hp. engine of almost identical design, and the Hornet engine was ready to run early the following June. The design of these engines is characterized by high cooling-efficiency, high mean effective pressure, low fuel consumption per horsepower-hour, light weight, high power-output, rigidity and strength of crankcase, maximum accessibility of parts and accessories, easy demountability of crankcase and cylinders from the accessories assembly, and complete protection of the accessories between the crankcase and the fire-walls of the fuselage.
Unique features include a solid one-piece master connecting-rod without split big-end, a two-piece crankshaft, a vertically divided crankcase formed of two large identical aluminum forgings, assembly of the power end and accessory end in two units, driving all the accessories from the main shaft by three lay-shafts through large spur-gears, distribution of the fuel mixture from a double carbureter by a high-speed impeller through separate tangential pipes to the individual cylinders, the casting of boxes enclosing the valve rocker-arms integrally with the cylinder-heads, large size of the valves and ports, and hemispherical combustion-chambers and concave piston-heads.
Bore of the cylinders and stroke of the pistons are equal. The required guaranteed power is calculated on a basis of 1344-cu. in. displacement and 125-lb. mean effective pressure at 1900 r.p.m. Provision was made for an over-speed of 2400 r.p.m. to be attained in a dive. On standard straight fuel a mean effective pressure of 130 lb. is obtained with zero pressure in the intake system, and 140 lb. has been developed consistently with 30 per cent of benzol or its equivalent. To operate the engines on straight aviation-gasoline at their rated power with a fuel consumption of 0.48 lb. per hp-hr. has been found possible.
Test flights of the Wasp in fighting airplanes designed for water-cooled engines developed high speeds equal to those made with the original engines and demonstrated a materially better rate of climb and lower landing-speeds. The air resistance seems less than that of a water-cooled engine with its radiator. As experience with air-cooled powerplants accumulates their superiority should be even more evident.