Turbocharger
Turbosuperchargers were indeed highly successful in U.S. bombers, which were exclusively powered by radial engines. The P-47 fighter had the same combination of radial engine (R-2800) and turbosupercharger and was also successful, apart from its large bulk, which was caused by the need for the ductwork for the aft-mounted turbosupercharger.
However, mating the turbocharger with the Allison V-1710 proved to be problematic. As a result, designers of the fighter planes that utilized the V-1710 were invariably forced to choose between the poor high-altitude performance of the V-1710 versus the increased problems brought on by addition of the turbosupercharger. The fates of all of the V-1710 powered fighters of World War II would thus hinge on that choice.
The original XP-39 was built with a V-1710 augmented by a Type B-5 turbosupercharger as specified by Fighter Projects Officer Lieutenant Benjamin S. Kelsey and his colleague Gordon P. Saville. Numerous changes were made to the design during a period of time when Kelsey's attention was focused elsewhere, and Bell engineers, NACA aerodynamicists and the substitute fighter project officer determined that dropping the turbocharger would be among the drag reduction measures indicated by borderline wind tunnel test results; an unnecessary step, according to aviation engineer and historian Warren M. Bodie. The production P-39 was thus stuck with poor high-altitude performance and proved unsuitable for the air war in Western Europe which was largely conducted at high altitudes. The P-39 was rejected by the British, but used by the U.S. in the Mediterranean and the early Pacific air war, as well as shipped to the Soviet Union in large numbers under the Lend Lease program. The Soviets wer e able to make good use of P-39s because of its excellent maneuverability and because the air war on the Eastern Front in Europe was primarily short ranged, tactical, and conducted at lower altitudes. In the P-39, Soviet pilots scored the highest number of individual kills made on any American, or British fighter type.
The P-40, which also had only the single-stage, single-speed-supercharged V-1710, had similar problems with high-altitude performance.
The P-38 was the only fighter to make it into combat during World War II with turbosupercharged V-1710s. The operating conditions of the Western European air war – flying for long hours in intensely cold weather at 30,000 feet (9,100 m) – revealed several problems with the turbosupercharged V-1710. These had a poor manifold fuel-air distribution and poor temperature regulation of the turbosupercharger air, which resulted in frequent engine failures (detonation occurred in certain cylinders as the result of persistent uneven fuel-air mixture across the cylinders caused by the poor manifold design). The turbosupercharger had additional problems with getting stuck in the freezing air in either high or low boost mode; the high boost mode could cause detonation in the engine, while the low boost mode would be manifested as power loss in one engine, resulting in sudden fishtailing in flight. These problems were aggravated by suboptimal engine management techniques taught to many pilots duri ng the first part of WWII, including a cruise setting that involves running the engine at a high RPM and low manifold pressure with a rich mixture. These settings can contribute to overcooling of the engine, fuel condensation problems, accelerated mechanical wear, and the likelihood of components binding or "freezing up." Details of the failure patterns were described in a report by General Doolittle to General Spatz in January 1944. In March 1944, the first Allison engines appearing over Berlin belonged to a group of P-38H pilots of 55FG, engine troubles contributing to a reduction of the force to half strength over the target. It was too late to correct these problems in the production lines of Allison or GE, and so the P-38s were steadily withdrawn from Europe until they were no longer used for bomber escort duty with the Eighth Air Force by October 1944. A few P-38s would remain in the European theater as the F-5 for photo reconnaissance.
The P-38 had fewer engine failures in the Pacific Theater, where operating techniques were better developed (such as those recommended by Charles Lindbergh during his P-38 flight testing in the PTO,) the fuel quality was consistently superior and the Japanese did not operate at such high altitudes. Using the same P-38Gs which were proving difficult to maintain in England, Pacific-based pilots were able to use the aircraft to good advantage including, in April 1943, Operation Vengeance, the interception and downing of the Japanese bomber that was carrying Admiral Isoroku Yamamoto. New P-38 models with ever-increasing power from more advanced Allisons were eagerly accepted by Pacific air groups.
When Packard started building Merlin V-1650 engines in America, certain American fighter designs using the Allison V-1710 were changed to use the Merlin. The P-40F, a Lend Lease export to Britain, was one of the first American fighters to be converted to a Packard-Merlin engine. However, the installed engine was the V-1650-1 with a slightly improved single-stage, two-speed supercharger, yielding only modest gains in performance.
The first production P-51A had the Allison V-1710 without turbosupercharger and thus, poor high altitude performance. At low altitudes, the P-51A was substantially faster than the Spitfire, which very much impressed the British when they first received the plane; they quickly realized the P-51 had an outstanding low-drag airframe and the airplane could become one of the best of the war if the Allison V-1710 engine were replaced by the two-stage-supercharged Merlin. Conversion proceeded on both sides of the Atlantic, with North American Aviation engineers making the definitive changes to the airframe to fully integrate the Packard-Merlin V-1650-3 into the P-51B. Ironically, because the P-51 was not originally developed for the USAAF, this was allowed to proceed rapidly with no Army input (or interference). A similar attempt to cure the problems of the P-38 by replacing its Allisons with Merlins was quashed by the USAAF, after protests from Allison.
Starting with the V-1710-45 around 1943, Allison attached an auxiliary supercharger to some of its engines in an effort to improve high-altitude performance, with limited success. Although described as a two-stage supercharger, it was essentially an afterthought and did not have the full refinements of the two-stage Merlin, such as the pressure-altitude governed two-speed gearbox and the intercooling system. Various configurations of this auxiliary supercharger were used in production versions of the V-1710 that powered aircraft such as the Bell P-63 and North American P-82E/F/G series. In addition, it was tried or studied as the powerplant for many experimental and test aircraft such as variants of the Boeing XB-38, Republic XP-47A, both with turbo-superchargers (AP-10), Curtiss XP-55 Ascender, and Douglas XB-42 Mixmaster.
[Source - Wikipedia]
