Napier Dagger

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The Napier Dagger was a 24-cylinder H-pattern air-cooled engine designed by Frank Halford and built by Napier before WWII. It was an outgrowth of the earlier Napier Rapier. The rationale for the H is fairly straightforward. Rather than having an engine with fewer large cylinders, why not more small cylinders? It was believed that an H pattern engine would provide substantially more power and higher RPM's for the same frontal area as a large liquid-cooled V engine.

The H-block has a compact layout, as it essentially consists of two vertically opposed inline engines lying side by side, driving side by side crankshafts. Another advantage is that since the cylinders are opposed, the motion in one is balanced by the opposite motion in the one on the opposite side, leading to very smooth running. The Dagger was remarkable for its fast rotation, running at up to 4000 rpm, but unlike the later Napier Sabre, it had a conventional poppet valve system.

Although considered a masterpiece of engine design there were problems with cooling, maintenance, manufacturing and weight. These problems weren't solved during the Dagger's lifetime and went unresolved well into its successor the Napier Sabre's lifetime. The Dagger powered the Hawker Hector army co-operation aircraft and the Handley Page Hereford bomber. The operational usefulness of the Hector was restricted by engine cooling problems, which made it unsuitable for operations in the tropics, and the Hereford was found to be unsuitable for combat because its Dagger VIII engines were noisy and unreliable. The Dagger also found an application in the experimental Martin-Baker MB 2 fighter.

[edit] Specifications (Napier Dagger II)

General characteristics

• Type: 24-cylinder air cooled H engine
• Bore: 97mm (3.8 in)
• Stroke: 95mm (3.75 in)
• Displacement: 16.85 Liters (1,027.5 cu in)
• Length: 2,032mm (80 in)
• Width: 584mm (23 in)
• Height: 1,146mm (45.12 in)
• Dry weight: 580kg (1,280 lbs)

Components

• Valvetrain: 1 inlet and 1 exhaust valve per cylinder.
• Supercharger: yes
• Fuel system: 1 Claudel Carburetor
• Fuel type: 87 octane
• Cooling system: air

Performance

• Power output: 890 hp @ 4,000 RPM @ 12,500 ft
• Compression ratio: 7.75:1
• Power-to-weight ratio: 2.27lb/hp @ cruise speed

[edit] References

• http://www.oldengine.org/members/diesel/Duxford/ukaero3.htm
• Gunston, Bill (1986). World Encyclopedia of Aero Engines. Wellingborough: Patrick Stephens, 121. 

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