The Napier Javelin was a British six-cylinder air-cooled aero engine designed by Frank Halford and built by Napier & Son. The six cylinders were arranged inverted and inline. Unlike the similar, the poppet valves operated by a single gear driven overhead camshaft.
First run in 1932 and flown in March 1934 in the prototype of the Percival Mew Gull racing aircraft, the engine was also used in the Spartan Arrow biplane and the Percival Gull.
Initial version producing 150 hp.
A longer stroke and increased capacity version of 160 hp.
A Mark III with a starter motor, generator and other improvements for the Martin Baker MB.1.
Percival Gull Four Mk.IIA
Percival Mew Gull Mk.I
Type: 6-cylinder inverted inline air-cooled engine.
Bore: 4.5 in (114.3 mm)
Stroke: 5.25 in (133.35 mm)
Displacement: 501 cu in (8.21 lt)
Length: 58.5 in (1486 mm)
Width: 26 in (660.4 mm)
Height: 31.5 in (800 mm)
Dry weight: 410 lb (186 kg)
Valvetrain: Overhead cam poppet-valves
Fuel system: Twin updraught carburettors
Fuel type: Petrol
Cooling system: Air-cooled
Reduction gear: Direct drive, left hand tractor
Power output: 150 hp (112 kW) at 2,000 rpm
Compression ratio: 5.3:1