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The Allison V-3420 was a large experimental American piston aircraft engine, designed in 1937.

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Allison V-3420 Engine

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The V-3420

Propeller shaft and Gearbox

The V-3420 is a 24-cylinder double-V, twin crankshaft, liquid-cooled engine derived from the V-1710, a 12-cylinder engine that powered such WWII aircraft as the Lockheed P-38, Bell P-39, and the Curtiss P-40. Essentially, the V-3420 is made up of two V-1710 engines mounted on a single crankcase with the two crankshafts geared together. It was manufactured from 1941 until 1945 and used in several experimental AAF aircraft including the XB-19, XP-58, XB-39, XP-75, and the P-75A. None of these aircraft were produced in quantity, therefore only about 150 V-3420s were built. This display illustrates how the V-3420 is mounted amidships in the P-75A, connected by dual driveshaft running under the cockpit to a gearbox that drives two coaxial contra-rotating propellers.

Air Force Museum

History

In 1937, at the behest of the United States Army Air Corps, the Allison Engine Company agreed to design and build a large-displacement high power aircraft engine. The resulting V-3420 was essentially a pair of 12 cylinder Allison V-1710 engines mated to a common crankcase with a 30° angle between the inner cylinder banks. The crankshafts of the two V-1710 engines were geared together to drive a common propeller shaft. Most V-3420 parts were interchangeable with those for V-1710-E and -F engines.

The V-3420 had a power-to-weight ratio of 1.6 kW/kg or 1 hp/lb, excellent for its time. It was envisioned as a powerful yet compact engine for several advanced Air Force projects of the day, including the Douglas XB-19, the Boeing XB-39 Superfortress, the Lockheed XP-58 Chain Lightning, and the General Motors P-75 Eagle. As none of these designs reached full-scale production, only about 150 V-3420s were built.

 

Specifications (V-3420)

General characteristics

• Type: 24-cylinder piston engine. liquid-cooled, double-vee, with gear-driven supercharger and exhaust-driven turbo-supercharger 

• Bore: 5.5 in (140 mm)

• Stroke: 6 in (152 mm)

• Displacement: 3,420 in³ (56 L)

• Length: 100.0 in (2,540 mm)

• Width: 56.0 in (1,422 mm)

• Height: 34.0 in (864 mm)

• Dry weight: 2,600 lb (1,180 kg)

Components

• Valve train: Single overhead camshaft per 6-cylinder block, two intake and two exhaust valves per cylinder, sodium-cooled exhaust valves.

• Supercharger: Single-speed one-stage gear-driven supercharger

• Turbocharger: General Electric

• Fuel system: Bendix-Stromberg PT-12E1 three-barrel injection-type downdraft with automatic mixture control.

• Cooling system: Liquid-cooled with a mixture of 70% water and 30% ethylene glycol, pressurized.

Performance

• Power output:
   

  • 2,600 hp (1,940 kW) at 3,000 rpm at 25,000 ft (7,625 m) for 15 minutes

  • 2,100 hp (1,565 kW) at 2,600 rpm at 25,000 ft (7,625 m) maximum continuous power

• Specific power: 0.76 hp/in³ (34.6 kW/L)

• Compression ratio: 6.65:1

• Power-to-weight ratio: 1.0 hp/lb (1.64 kW/kg)

References

• Bridgman, L, (ed.) (1998) Jane's fighting aircraft of World War II. Crescent. ISBN 978-0-517-67964-7

Wikipedia

 

Note

An interesting episode in the development of the Allison V-3420-B10 engine (which powered the Fisher P-75) was its initial inability to meet the contract guarantee for rated power and how it was resolved.

Performance guarantees had been based on testing using an engine configured as a V-3420A to enable its running on the available Allison altitude test stand. So configured, the test engine used the standard V-3420A firing order with each of the 24 cylinders firing 30 degrees apart.

As designed and delivered the B-10 engine used 60 degree firing, i.e. simultaneous firing of two cylinders (one in each vee), referred to as “zero” phasing by Allison. When the engines were run in the altitude dynamometer it was found that the B-10 was about 3 percent low on power. In an effort to restore this power Allison adopted an alternate firing scheme known as 150 degree phasing and utilized a 30 degree interval. Historically, the details of how they accomplished this have been a mystery. Allison Competition Engines recently had an opportunity to teardown a B-10 known to have been modified with the 150 degree phasing. By taking careful notes of the as-found configuration we are now able to provide the details.

 

 

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