The Allison V-1710 aircraft engine was the only indigenous US-developed V-12 liquid-cooled engine to see service during WWII. A sturdy and trustworthy design, it unfortunately lacked an advanced and efficient mechanical centrifugal supercharger. Although versions with a turbosupercharger did give excellent performance at high altitude in the Lockheed P-38 Lightning, the turbosupercharger was only fitted to experimental single engined fighters, with the same excellent results. The preference for turbosuperchargers, arguably to the neglect of mechanical supercharger development, reflected US Army philosophy, and not the inherent qualities of the Allison engine.
The Allison Division of General Motors began developing the ethylene glycol-cooled engine in 1929 to meet a US Army need for a modern, 1000 hp (750 kW), engine to fit into a new generation of streamlined bombers and fighters. To ease production, the new design could be equipped with different propeller gearing systems and superchargers, allowing a single production line to build engines for everything from fighters to bombers.
The U.S. Navy purchased the first V-1710s, the B model (the only V-1710 that did not have a gear-driven supercharger) in 1931 and installed them on the airship aircraft carriers the Akron and Macon. The U.S. Army Air Corps purchased its first V-1710 in December 1932. The Great Depression slowed development, and it was not until December 14, 1936 the engine next flew, in the Consolidated XA-11A testbed. The V-1710-C6 successfully completed the Army 150 hour Type Test on April 23, 1937 at 1000 hp (750 kW), the first engine of any type to do so. The engine was then offered to aircraft manufacturers, where it powered the unsuccessful Curtiss X/YP-37. All entrants in the new pursuit competition were designed around it, powering the Lockheed P-38, Bell P-39 and Curtiss P-40. When North American Aviation was asked to build the P-40, they instead responded with an improved design, using the V-1710 in their P-51A.
The Army had earlier decided to concentrate on turbosuperchargers for high altitude boost, believing that further development of mechanical turbochargers would allow their engines to outperform European rivals using superchargers. Turbosuperchargers are powered by the engine exhaust and so do not draw power from the engine, whereas superchargers are connected directly by gears to the engine crankshaft. Superchargers as a result require increasing proportions of engine power as altitude increases (the two-stage supercharger of the Merlin 60 series engines consumed some 230-280 horsepower at 30,000 ft). General Electric was the sole source for research and production of American turbosuperchargers.
Turbosuperchargers were indeed highly successful in U.S. bombers, which were virtually all powered by radial engines. The P-47 fighter had the same combination of radial engine (R-2800) and turbosupercharger and was also quite successful, apart from its large bulk, which was caused by the need for the ductwork for the turbosupercharger.
However, mating the turbosupercharger with the Allison V-1710 proved to be far more problematic. As a result, designers of the fighter planes utilizing 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 V-1710-powered fighters of World War II would thus hinge on that choice.
The original XP-39 was built with a turbosupercharged V-1710. Numerous changes were made to the design of the production version (after a review by aerodynamicists at Langley Field), including a decision to drop the turbosupercharger. This decision came out of a combination of the severe teething problems encountered with the turbosupercharger mated to the V-1710, as well as the belief of the Army at that time that high altitude performance was not necessary in a fighter plane. The 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, used briefly by the U.S. in the early Pacific air war, and then was exiled by the USAAF to the Soviet Union under the Lend Lease program. The Soviets were still 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 largely short ranged, tactical, and conducted at lower altitudes.
The P-40, which also had only the single stage, single speed supercharger V-1710, would have similar problems with high altitude performance.
The P-38 would be the only fighter to make it into combat during World War II with turbosupercharged V-1710 engines. The operating conditions of the Western European air war - flying for long hours in intensely cold weather at 30,000 feet - unmasked several severe problems with the P-38's turbosupercharged V-1710. The V-1710s in these P-38s 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 mid-flight. Specific details of the failure patterns of the P-38's turbosupercharged V-1710 were spelled out in a report by General Doolittle to General Spatz in January 1944. 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 all gone by October 1944. It is not clear if all of the problems of the P-38 were ever fixed, as the Merlin-engined P-51 arrived at this same time and succeeded well enough to redirect attention to the more successful design. The P-38 had fewer engine failures in the Pacific Theater, where the weather was warmer, and the Japanese did not operate at such high altitudes.
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 Packard versions were not mere blueprint copies of the British Merlin. Packard engineers contributed to the success of the Merlin throughout the war by making a number of modifications and design changes that were shared across the ocean with the British Merlins. Packard-Merlins were also manufactured to a far more uniform and rigorous standard, using standard U.S. mass production techniques, unlike the British Merlins which were hand fitted individually by craftsmen.
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 series with slightly improved a single stage, two-speed supercharger, so the P-40F saw 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 of the P-51 to the Merlin thus 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 an aircraft developed for the USAAF, this conversion was allowed to proceed rapidly with no input from the USAAF. A similar attempt to fix the problems of the P-38 by replacing its Allison engines 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 all of 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. P-82 performance never matched the Merlin P-51. 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 AP-10 turbo-superchargers), Curtiss XP-55 Ascender, and Douglas XB-42 Mixmaster.
Not much is known about the fate of the P-63 as almost all of these airplanes were sent to the Soviets.
The F-82 did see brief action in the Korean War, but the type was completely withdrawn from Korea by 1950. It had a very short service life that was probably due to a combination of factors: poor reliability from the G-series V-1710 engines, low numbers of F-82s produced, and the arrival of newer and better jet powered designs. The initial production P-82B had Merlin engines, but North American was forced to use the Allison V-1710 for the E/F/G models. According to Edgar Schmued, the designer of the P-51 and F-82, "The United States Air Force was tired of paying a $6,000 royalty to England for each Merlin engine built in this country by Packard......It was really pathetic to see a good design simply ruined by politics and the lack of cooperation by the Allison people in building a good engine."
In total, over 70,000 V-1710s were built by Allison during the war, all in Indianapolis, Indiana.
The V-1710 has 12 cylinders with a bore and stroke of 5.5 by 6 inches (149.7 by 152.4 mm) in 60° V-format, aggregating to 1,710.6 in³ (28.03 L) total displacement, with a compression ratio of 6.65:1.
The engine design benefited from the General Motors philosophy to build-in production and installation versatility. The engine was constructed around a basic power section from which different installation requirements could be met by fitting the appropriate Accessories Section at the rear and a tailored reduction gear for power output at the front. This approach allowed easy changes of the supercharger(s) and supercharger drive-gear ratio. That gave different critical altitude ratings ranging from 8,000 to 26,000 feet (2,400–7,900 m). It allowed a variety of propeller drives and also remote placement of the reduction gear.
The P-39, P-63, and XB-42 installations used V-1710-E series engines that exchanged the integral reduction gear for an extension shaft that drove a remotely located reduction gear and propeller. Aircraft such as the P-38, P-40, P-51A, and P-82 used close-coupled propeller reduction gears, a feature of the V-1710-F series.
Another feature of the V-1710 design was its ability to turn the output shaft either clockwise or counter-clockwise by assembling the engine with the crankshaft turned end-for-end, by installing an idler gear in the drive train to the supercharger and accessories and by installing a starter turning the proper direction. So, there was no need to re-arrange the ignition wiring and firing order, nor the oil and Glycol circuits to accommodate the direction of rotation.
The V-1710 has often been criticized for not having a "high-altitude" supercharger. The comparison is usually to the later, two-stage, versions of the Rolls-Royce Merlin V-12 engine built by Packard as the V-1650 and used in the P-51B Mustang and subsequent variants. The US Army had specified that the V-1710 was to be a single-stage supercharged engine and, if a higher altitude capability was desired, the aircraft could use their newly developed turbosupercharger as was featured in the P-37, P-38, and XP-39.
The benefits of a two stage supercharger eventually became so clear cut that Allison did make some efforts in this direction. Allison attached an auxiliary supercharger in various configurations to the existing engine mounted supercharger and carburetor. Early versions of these two stage supercharger engines were used on the P-63. No intercooler, aftercooler, or backfire screen were incorporated into these two stage V-1710 engines (except for the V-1710-119 used on the experimental P-51J, which had an aftercooler). The two stage Merlin engines had all of these features, which were designed to prevent detonation from charge heating and backfire into the supercharger. The G-series V-1710s installed on the F-82 E/F/G models had only anti-detonation injection to deal with these problems, and not surprisingly had severe reliability and maintenance problems. In one record, it was stated that the F-82 required 33 hours of maintenance for one hour of flight [Wagner, "Mustang Designer"].
Although the early V-1710 powered P-39, P-40 and P-51A airplanes were limited to combat operations at a maximum of about 15,000 feet (5,000 m) they were available in comparatively large numbers and were the mainstay of some Allied Air Forces in all but the European theater of operations. The engines proved to be robust and little affected by machine-gun fire. In total, over 60 percent of the US Army Pursuit aircraft operated during WWII were powered by the V-1710.
Allison continuously improved the engine during the war. The initial rating of 1,000 hp (750 kW) was increased in stages to where the final V-1710-143/145(G6R/L) was rated for 2,300 hp (1,715 kW). By 1944, the War Emergency Power rating on the P-38L was 1,600 hp (1,200 kW).
The most powerful factory variant was the V-1710-127, designed to produce 2,900 hp at low altitude and 1,550 hp at 29,000 feet. This engine was static tested at 2,800 hp and was planned for installation in an XP-63H aircraft. The end of the war ended this development, so this promising experiment never flew. The extra power of this version was derived from using exhaust turbines, not to drive a turbosupercharger, but to return that energy to turning the crankshaft. This was called a "turbo-compound" arrangement.
Improvements in manufacturing brought the cost to produce each engine from $25,000 down to $8,500 and allowed the installed lifetime of the engine to be increased from 300 hours to as much as 1,000 hours for the less stressed powerplants. Weight increases needed to accomplish this were minimal, with the result that all models were able to produce more than 1 hp/lb (1.6 kW/kg) at their takeoff rating.
Comparisons between Allison engine and the Rolls-Royce Merlin engine are inevitable. What can be said for the Allison is that its part count was nearly half that of the Merlin engine which facilitated mass production greatly. The British-made Merlin engines were to a degree still reliant upon hand crafted and fitted parts from skilled craftsmen. There also was a very high degree of commonality of parts throughout the series. The individual parts of the Allison series were produced to a very high degree of standardization and reliability with the best technology available at the time. The design of the Allison was very modular and it was capable of being mated to many different styles of turbo-superchargers and various other accessories. Although the variety of turbo-superchargers available for installation was limited due to the constraints of single engine fighter design. Since it was produced in mass numbers and highly standardized, many post war racing designs used the Allison engine. Its reliability and well mannered operation allowed it to operate at high rpm for extended periods.
Following the war North American built 250 P-82E/Fs for various air defense roles into the early 1950s. This was the final military role for the V-1710.
The V-1710's useful life continued, as thousands were available on the surplus market. In time, the warbird movement began to restore and return to the air examples of the classic fighters of the war and many V-1710 powered pursuit airplanes are flying again, with freshly overhauled engines. The reliability, maintainability, and availability of the engine has led others to use it to power flying examples of aircraft that people had otherwise been unable to find an airworthy example of the original engine. This includes newly manufactured Russian Yak-3 and Yak-9 airplanes as well as ambitious projects such as a replica Douglas World Cruiser and Focke-Wulf Fw 190D by Flug Werk of Germany.
- Ludwig. "P-51 Mustang: Development of the Long Range Escort Fighter", p.188-189.
- White, "Allied Aircraft Engines of World War II", p.92.
- White, Graham. Allied Aircraft Piston Engines of World War II.
- Wagner, Ray. Mustang Designer: Edgar Schmued and the P-51.
- Ludwig, Paul A. P-51 Mustang: Development of the Long-Range Escort Fighter
- Allison V-1710 engine in hydroplanes
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