Allison V-3420

Allison V-3420. The Allison V-3420 was an American 24-cylinder liquid-cooled W engine developed during World War II by the Allison Engine Company. It was essentially two Allison V-1710 V-12 engines coupled together on a common crankshaft to create a powerful powerplant intended for large aircraft. Despite its significant power output, the engine saw limited production and operational use, being installed in only a handful of experimental and prototype aircraft before the end of the war.

Development and Design

The development of the V-3420 was initiated by the United States Army Air Corps in the late 1930s to meet a demand for a high-power engine in the 2,000 to 3,000 horsepower class for emerging heavy bombers and long-range fighters. Engineers at the Allison Engine Company, a division of General Motors, based the design on their proven Allison V-1710, joining two of these engines at a 60-degree angle to form a "double V" or W-24 configuration. This approach aimed to expedite development by utilizing existing components and tooling. The design team, led by key figures from the V-1710 program, faced significant challenges in managing the engine's complexity, cooling, and the synchronization of its two crankshaft sections. The project received support from the United States Department of War and was seen as a potential competitor to other advanced engines like the Pratt & Whitney R-4360 Wasp Major.

Technical Specifications

The V-3420 was a four-stroke, fuel-injected, supercharged W engine with 24 cylinders arranged in four banks of six. It had a displacement of 3,420 cubic inches (56.0 L), derived directly from two Allison V-1710 units. The engine typically produced between 2,600 and 3,000 horsepower, depending on the specific variant and supercharger configuration. It featured a single-stage, two-speed supercharger and required a sophisticated liquid-cooling system to manage the significant heat generated by its cylinders. Key components included a forged crankshaft, aluminum alloy pistons, and sodium-cooled poppet valves. Its physical dimensions and substantial weight made it suitable only for the largest airframes of the era.

Operational History

The operational history of the V-3420 was brief and confined almost entirely to experimental programs. The engine first ran in 1940 and underwent extensive testing at the Allison Engine Company facilities and by the United States Army Air Forces. It was flight-tested in several aircraft, but persistent issues with reliability, cooling, and complexity prevented its adoption for mass production. The shift in military priority towards jet engine development in the later stages of World War II, exemplified by programs like the Bell P-59 Airacomet, further diminished the need for such a large, complex piston engine. Consequently, no V-3420-powered aircraft entered squadron service or saw combat during the war.

Variants

Several variants of the V-3420 were developed or proposed, primarily differing in their supercharging and accessory arrangements. The V-3420-11 was an early production model intended for the Fisher P-75 Eagle fighter. The V-3420-13 and V-3420-19 variants were developed with different supercharger gear ratios for improved high-altitude performance in aircraft like the Lockheed XP-58 Chain Lightning. A post-war variant, the V-3420-T, was proposed as a turbocompound engine, using exhaust-driven turbines to boost power, but this remained a paper project. Other experimental versions explored different propeller reduction gearing and mounting configurations.

Applications

The V-3420 was installed in a small number of experimental military aircraft. Its primary application was in the Fisher P-75 Eagle, a high-altitude interceptor developed for the United States Army Air Forces, though the engine's troubles contributed to the project's cancellation. It was also fitted to the heavy fighter prototype, the Lockheed XP-58 Chain Lightning. Another significant installation was in the Boeing XB-39 Superfortress, a testbed version of the Boeing B-29 Superfortress re-engined with V-3420s to evaluate alternative powerplants. Proposed applications included the Douglas XB-19 bomber and various naval aircraft, but none progressed beyond initial studies.

Category:Aircraft piston engines Category:Allison engines Category:World War II aircraft engines of the United States