In 1927 Montague Napier reached an agreement with Major Frank B. Halford to act as a consultant designer for D Napier & Son. Napier was determined to enter into the the technology of air-cooled engines and Halford was a natural choice. Both Montague Napier and Halford believed that this was the way ahead due to its lightness and having push rod operated overhead valves. Napier’s remit to Halford was for an air-cooled engine between 6.6 litres and 11.8 litres.
The Napier Javelin was the first engine Halford designed for the Company. This was similar to the ‘Cirrus Hermes’ and the ‘De Havilland Gypsy’, having an inverted six cylinder 8.2 litre giving 170bhp at 2,100rpm.
The engine was a marginal success powering a number of Percival light aircraft such as the Gull IV and a racing Mew Gull. It was also installed in the Sparten Arrow, Martin Baker MB1 and the Commonwealth Wackett twin engine aircraft. Regrettably during the 1930’s this engine had earned a reputation for cutting out on takeoff, a problem that always remain unsolved.