Large experimental W-configuration piston aircraft engine Allison V-3420
The Allison V-3420 was a significant development in the realm of liquid-cooled aircraft engines during the mid to late 1930s, representing a major effort by Allison and the U.S. Army to produce a high-power engine suitable for future military aircraft.
Design and Configuration
Origins and Initial Plans:
- In 1936, the U.S. Army sought a 2300-hp class engine, favoring liquid cooling as the best method to achieve high power outputs. Despite other companies like Wright and Pratt & Whitney exploring similar concepts, Allison was initially tasked with developing a new multi-bank engine using a single crankshaft.
Design Decisions and Configuration:
- Conservative Approach: Allison opted for a more conservative approach by leveraging the design of the existing V-1710 engine, creating the V-3420 essentially as two V-1710 engines mounted on a common crankcase. This decision was motivated by the ongoing development challenges with the V-1710.
- Engine Layout: The V-3420 featured a 60-degree V-12 configuration for each pair of engines, arranged at 90 degrees to each other, resulting in a total configuration of 24 cylinders. The outer cylinder blocks were set at 150 degrees from each other, with the inner blocks at 30 degrees, ensuring even power pulses with a cylinder firing every 30 degrees of crankshaft rotation in the A-series engines.
Crankshaft and Propeller Drive:
- A-Series Engines: These engines had both crankshafts rotating in the same direction, driving a common propeller reduction gear, which effectively phased the crankshafts together.
- B-Series Engines: These featured counter-rotating crankshafts, requiring crankshaft timing idler gears to synchronize the firing sequences. This configuration allowed for the use of contra-rotating propellers, which balanced the load on each propeller by splitting the power output equally.
Accessories and Supercharging
Accessories Section:
- The accessories were designed to be interchangeable with future configurations, similar to the V-1710. The right-hand V housed a single-stage, single-speed supercharger, while other accessories like the generator, starter, and pumps were mounted on the left-hand V.
Supercharging:
- The engine incorporated a 10-inch diameter impeller supercharger, and some later models included an auxiliary-stage supercharger. This auxiliary supercharger used a variable-speed drive via hydraulic coupling, which optimized supercharger efficiency across different power and altitude conditions, avoiding the “sawtooth” performance curve typical of multi-speed superchargers.
Performance and Legacy
The V-3420 was an innovative design that managed to integrate two V-1710 engines into a cohesive and powerful unit. Despite being one of the few “doubled” engines to function reliably, the V-3420 faced competition from other engine designs and technologies emerging during the period, such as jet engines. Nevertheless, its development was a notable achievement in the evolution of aircraft piston engines.