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Armstrong Siddeley Deerhound


The Deerhound I was a triple-row, 21-cylinder, air-cooled radial engine design with the unusual feature of inline cylinder banks. Unlike earlier Armstrong Siddeley engines the Deerhound featured overhead camshafts to operate its poppet valves, using one camshaft for each row of three cylinders.

Flight testing began in 1938 using an Armstrong Whitworth Whitley II, serial number K7243, during which cooling problems were encountered with the rear row of cylinders. This problem was solved by engineering a 'reversed-flow' cooling system with a large air duct at the rear of the cowling taking in air and directing it forward to exit behind the propeller. The project suffered a severe setback when the Whitley crashed on takeoff in March 1940, fatally injuring its crew. The accident was attributed to an incorrect elevator trim setting and was not related to the engines. A single prototype Deerhound III was built and ran, and survived until the late-1970s before being scrapped. Development work on the early engines was cancelled by the Air Ministry on 23 April 1941 (when the company factory was bombed) with running of the Mk III being allowed to continue until 3 October 1941, at which point all records were ordered to be handed over to Rolls-Royce.

A projected increased capacity variant known as the Boarhound was planned but never built, and a related much larger design, the Wolfhound, existed on paper only. The latter engine featuring six banks of four cylinders, would have had a displacement of around 61 litres (3,733 cu in) and a projected takeoff power rating of 2,600–2,800 horsepower (1,900–2,100 kW).

In all 11 engines were built.


Deerhound I
1,115 hp (831 kW): four built.

Deerhound II
1,500 hp (1,118 kW), capacity enlarged to 41 L (2,509 cu in) by increasing bore and stroke: six built.

Deerhound III
1,800 hp (1,342 kW), major redesign by Stewart Tresilian: one engine built.


This engine's sole aircraft application was in a modified Armstrong Whitworth Whitley which was used as a testbed.


Deerhound I
Type: 21-cylinder, 3-row air-cooled radial engine with 7 inline banks
Bore: 5.26 in (135 mm)
Stroke: 4.95 in (127 mm)
Displacement: 2,259.75 cu in (38.19 L)
Valvetrain: Overhead camshaft
Supercharger: Fully supercharged
Fuel type: 87 Octane Avgas
Cooling system: Air-cooled
Power output: 1,115 hp (831 kW) at 1,500 rpm


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