ENGINE SUPERCHARGING
THE NON-SUPERCHARGED ENGINES used in fighters in the late ’20s had induction air flowing directly from a duct in the nose of the aircraft into the carburetor. When an aircraft gained altitude, the thinner air was insufficient to maintain the sea-level-rated horsepower of these engines, so performance decreased as the altitude increased. The aircraft had a service ceiling of 14,000 to 18,000 feet (depending on the air density), and level-flight high speeds decreased proportionally with altitude.
Because the attainment of a high altitude is so important to fighter aircraft, superchargers were added to engine induction systems in the mid- to late ’30s. These were single-stage compressors geared to the engine crankshaft; they rotated at high rpm and provided the engine carburetor with greater than sea-level pressures. This produced a marked increase in performance (compared with standard aircraft engines of the day). These installations were dependent on the careful control of the allowable carburetor air temperatures, which were higher because of the heated compressed air. They also depended on increases in fueloctane levels. Superchargers allowed military aircraft of the late ’30s to have a service ceiling of around 23,000 feet and commensurate increases in level-flight speed (compared with non-supercharged aircraft).
As supercharging came of age, engineers decided to put a gear-shift arrangement on them; it was actuated similarly to a gear shift in an automobile. This gave two increases in compressor speeds to the supercharger and thereby increased aircraft operating service-ceiling altitudes to 20,000 to 24,000 feet. Level-flight speeds increased as altitude increased. This became known as the “two-speed” supercharger. Its handle was next to the throttle and was inscribed with its two positions: “Low” and “High.” The pilot shifted the handle at the altitude suggested in the operating handbook to obtain a surge of manifold pressure that indicated increased power. The new superchargers also required more carburetor air cooling and higher fuel-octane ratings to make them compatible with the increased power. At the beginning of WW II, the Japanese Zero, the Curtiss P-40, and the Bell P-39 were the only production fighters equipped with two-speed superchargers.
The Grumman F4F-3 Wildcat was one of the first production military aircraft to have the new two-stage supercharger, which was considerably more complicated than the earlier version. The two-stage supercharger was configured with a very large compressor that was mechanically geared to the crankshaft. This gave the engine its rated power at sea level. The two-speed supercharger was geared to the crankshaft and attached to the rear of the engine; it augmented the main stage and boosted service-ceiling altitudes to over 30,000 feet while increasing level-flight performance to 325mph. Because the fuel/air mixture was so highly compressed when the two stages
(high and low) were added to the main stage, large radiators/ intercoolers were needed to cool the air going into the carburetor; this prevented detonation and pre-ignition, which would ruin an engine rapidly. Octane limits were now required to be as high as 145. Fortunately, the United States was way ahead of its enemies in developing octane ratings over 100. This new supercharger now required a three-position handle next to the throttle. The handle had “Main,” “Low,” and “High” inscribed on it and was shifted as the altitude requirements dictated.
The Hellcat and Corsair were both equipped with the same R-2800 engines. Their level-flight performance now exceeded 400mph, and they had service ceilings of 38,000 feet. This gave them a considerable advantage over the Zero, which did not have octane ratings available beyond 90 to
95. By the end of the war, Japanese engine manufacturers had higher-octane fuel and better superchargers in their development fighter aircraft.
The P-47 Thunderbolt and the Lockheed P-38 had turbo supercharging that gave them service ceilings of 40,000 feet and a 460mph level performance. These aircraft and engine combinations did, however, take much longer to develop; they were much heavier; and they required long, complicated ducting to maintain proper weight and balance. An airplane had to be designed especially for such installations. The P-38, for instance, took five years to reach combat. The Grumman F6F-3 only took two years and one month to become combat ready.
In squadron service near the end of the air war, the Grumman F8F-1 Bearcat, because of its 3,000-pounds-lighter structure, reverted to the simple two-speed supercharger. It had a 38,000-foot service ceiling and a level-flight speed of 460mph. Aircraft design had grown along with engine supercharging development.