The Allison V-1710 was an American V12 aircraft engine designed and produced by the Allison Engine Company. Liquid-cooled, it was the most common US-developed engine of its type in service during World War II. Versions equipped with turbochargers (called "turbo-superchargers" at the time) gave excellent performance at high altitude in the twin-engined Lockheed P-38 Lightning, and turbochargers were also fitted to experimental single-engined fighters with similar results.
The United States Army Air Corps (USAAC) preference for turbochargers early in the V-1710's development program meant that less effort was spent on developing suitable mechanically driven centrifugal superchargers for the Allison V12 design, as other V12 designs from friendly nations like the British Rolls-Royce Merlin were already using.
When smaller-dimensioned or lower-cost versions of the V-1710 were desired, they generally had poor performance at higher altitudes.
Design and development The Allison Division of General Motors began developing an ethylene glycol-cooled engine in 1929 to meet a USAAC need for a modern, , 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 various fighters and bombers.
The United States Navy (USN) hoped to use the V-1710 in its rigid airships Akron and Macon, but both were equipped with German-built Maybach VL II enginesSmith, 1965 p. 182 as the V-1710 was still in testing when the Macon was lost in February 1935 (the Akron having been lost in April 1933).Smith 1965, p. 178 The USAAC purchased its first V-1710 in December 1932. The Great Depression slowed development, and it was not until December 14, 1936, that the engine next flew in the Consolidated XA-11A testbed. The V-1710-C6 successfully completed the USAAC 150 hour Type Test on April 23, 1937, at , the first engine of any type to do so. The engine was then offered to aircraft manufacturers, where it powered the prototype Curtiss XP-37s. All entrants in the new pursuit competition were designed around it, powering the Lockheed P-38, Bell P-39 and Curtiss P-40. When war material procurement agents from the United Kingdom asked North American Aviation to build the P-40 under license, NAA instead proposed their own improved aircraft design, using the V-1710 in their NA-u73, which became the first generation of the famed P-51 Mustang.
Technical description The V-1710 has 12 cylinders with a bore and stroke of in 60° V format, for a displacement of , with a compression ratio of 6.65:1. The valvetrain has a single overhead camshaft per bank of cylinders and four valves per cylinder.
Flexibility and reversibility of rotation The engine design benefited from General Motors’ adoption of modular design that provided built-in aviation powerplant production and installation versatility. The engine was constructed around a long block, which could meet various design requirements by fitting the appropriate accessories section at the rear, and an appropriate power output drive at the front. A turbocharger could be used, if desired.Whitney 1998, p 49
The P-39, P-63, and experimental Douglas XB-42 used V-1710-Es, exchanging the integral reduction gear for an extension shaft driving a remotely located reduction gear and propeller. Aircraft such as the P-38, P-40, P-51A, and North American P-82E used close-coupled propeller reduction gears, a feature of the V-1710-F series.
The accessory end had a one- or two-speed engine-driven supercharger that might have a second stage with or without an intercooler, the ignition magnetos and the customary assortment of oil and fuel pumps, all dictated by the application requirements. The front of the engine could have one of a number of different output drives. The drive might be a "long-nose" or close coupled propeller reduction gear, an extension drive to a remote gearbox, or a gearbox that could drive two wing-mounted propellers from a fuselage-mounted engine.
Another key feature of the V-1710 design was its ability to turn the output shaft 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, camshafts, and accessories, installing a starter turning the proper direction, and re-arranging the ignition wiring on the right side to accommodate a changed firing order. No change to the oil pump nor coolant pump circuits was needed. The ability to reverse the direction of rotation with a minimum of extra parts allowed the use of either a "tractor" or "pusher" propeller. This approach allowed easy changes of the supercharger(s) and supercharger drive-gear ratio. That gave different critical altitude (the maximum altitude at which the engine could produce full power) ratings ranging from .
Supercharger 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 60-series engines also built by Packard as the V-1650 and used in the P-51B Mustang and subsequent variants. The USAAC 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 turbocharger as was featured in the XP-37(YP-37), P-38, and XP-39.
The benefits of a two-stage supercharger eventually became so clear 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 (flame trap) 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 and Flame Traps). 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 (ADI) 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 each hour of flight.Wagner, Ray. Mustang Designer: Edgar Schmued and the P-51
Although the early V-1710 powered P-39, P-40 and P-51A were limited to combat operations at a maximum of about they were available in comparatively large numbers and were the mainstay of some Allied Air Forces in all but the European theater of war. The engines proved to be robust and little affected by machine-gun fire. In total, over 60 percent of the post-June 1941 USAAF's pursuit aircraft operated during WWII were powered by the V-1710.
Allison slowly but continuously improved the engine during the war. The initial rating of was incrementally increased. By 1944, the War Emergency Power rating on the P-38L was . The final V-1710-143/145(G6R/L) was rated for .
The most powerful factory variant was the V-1710-127, designed to produce dry at low altitude and at . This engine was static tested at wet War Emergency Power 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 to drive the crankshaft, called a turbo-compound engine.According to Aerospace Engineer Jim Leonard's Monograph dated 2010.
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 power-plants. 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.
There was also a high degree of commonality of parts throughout the series. The individual parts of the Allison series were produced to a high degree of standardization and reliability, using the best technology available at the time. Even after the war, racing Merlins used Allison connecting rods. As stated previously, General Motors' policies regarding versatility meant that their Allison division would also employ modular design features on the V-1710 from its "long block" core V12 unit outwards, so that it was capable of being mated to many different styles of turbochargers and various other accessories, although the variety of turbochargers available for installation was limited due to the constraints of single-engine fighter design. Since it was produced in large numbers and was highly standardized, the engine has been used in many postwar racing designs. 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/F for air defense roles into the early 1950s. This was the final military role for the V-1710.
Turbocharger The USAAC had earlier decided to concentrate on turbochargers for high altitude boost, believing that further development of turbochargers would allow their engines to outperform European rivals using crankshaft driven superchargers. Turbochargers are powered by the engine exhaust and so do not draw much power from the engine crankshaft, whereas displacement superchargers are coupled directly by shafts and gears to the engine crankshaft. Turbochargers do increase the exhaust back-pressure and thus do cause a small decrease in engine power, but the power increase due to increased induction pressures more than makes up for that decrease. Crankshaft-driven superchargers require an increase in directly driven percentage of engine power as altitude increases (the two-stage supercharger of the Merlin 60 series engines consumed some at . General Electric was the sole source for research and production of American turbochargers during this period, from its four decades worth of steam turbine engineering experience.
Turbochargers were highly successful in U.S. bombers, which were exclusively powered by radial engines. The P-47 Thunderbolt fighter had the same combination of radial engine (R-2800) and turbocharger and was also successful, apart from its large bulk, which was caused by the need for the ductwork for the aft-mounted turbocharger.
However, mating the turbocharger with the Allison V-1710 proved to be problematic. As a result, designers of the fighter planes that used 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 turbocharger. The fates of all of the V-1710 powered fighters of World War II would thus hinge on that choice.