Napier Javelin


The Napier Javelin was a British six-cylinder inline air-cooled engine designed by Frank Halford and built by D. Napier & Son.[1] First 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.[2]

Type Piston aero-engine
Manufacturer D. Napier & Son
First run 1932
Major applications Martin-Baker M.B.1 (one aircraft only)
Percival Gull Four Mk IIA

Development and designEdit

In 1930 D. Napier & Son identified a market for light aircraft engines, noting that light aircraft cost much the same as their cars used to and were being purchased by much the same class of gentleman. Frank Halford had been engaged as consultant designer and now began work on an air-cooled inverted six-cylinder design. Following a change of personnel, the engine was given the go-ahead in 1932, with aircraft manufacturer de Havilland identified as a likely customer.[3]

Unlike the later de Havilland Gipsy Six the poppet valves were operated by a single gear driven overhead camshaft.

The engine was first named the E97 and introduced to the market in 1932.[4] However none were sold and, after slight modifications, it was reintroduced the next year as the Javelin.[3]

Development continued but only a few were sold for experimental and prototype aircraft.

By 1935 Napier's needed to refocus their efforts and, with Halford's broadly similar but more powerful de Havilland Gipsy Six now on the market, the Javelin was withdrawn.[3]



Initial version offered in 1932.

Javelin IEdit

First version bearing the Javelin name, producing 150 hp (110 kW).

Javelin IIIEdit

A longer stroke and increased capacity version of 160 hp (120 kW).

Javelin IIIAEdit

A Mark III with a starter motor, generator and other improvements for the Martin Baker MB.1.


Specifications (Javelin I)Edit

Data from [1]

General characteristics

  • 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 L)
  • 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


See alsoEdit

Comparable engines

Related lists



  1. ^ a b Lumsden 2003, p.173.
  2. ^ Lumsden 2003, p.174.
  3. ^ a b c Charles Wilson and William Reader; Men and Machines: D. Napier & Son 1808-1958, Weidenfeld & Nicolson, 1958. pp. 132-3, 141, 145-6.
  4. ^ "Messrs. Napier Inaugurate a New Policy with a Six‐Cylinder Air‐Cooled Type", Aircraft Engineering and Aerospace Technology, Vol.4, No.8, 1 August 1932. pp. 210-212.


  • Lumsden, Alec. British Piston Engines and their Aircraft. Marlborough, Wiltshire: Airlife Publishing, 2003. ISBN 1-85310-294-6.