The de Havilland Gipsy Six is a British six-cylinder, air-cooled, inverted inline piston engine developed by the de Havilland Engine Company for aircraft use in the 1930s. It was based on the cylinders of the four-cylinder Gipsy Major and was developed into a series of similar aero engines which were still in common use until the 1980s.
The engines were of particular note for their exceptionally low cross-sectional area, a drag-reducing feature which made them ideal for the many racing aircraft of that period. In 1934, the basic bronze-headed Gipsy Six, rated at 185 horsepower (138 kW) at 2,100 rpm was modified for use in the DH.88 Comet air racer as the Gipsy Six "R" which produced 223 horsepower (166 kW) at 2,400 rpm for takeoff. Many Gipsy Six engines remain in service powering vintage aircraft types today.
The De Havilland Engine Company had hoped to produce a version of the basic engine capable of utilising a hydraulically actuated variable-pitch (VP) airscrew based on the American Hamilton "Bracket-Type" in time for the 1934 MacRobertson Race. Since there was so little time to perfect this installation, as a compromise a FrenchRatier variable pitch airscrew was fitted to the Comets' "R" engines; this utilised a simple air-filled bladder for a once-per-flight automatic pitch change. Later on, Comets were fitted with the Series II engine, which eliminated this crude system. Power for the "R" was increased to a takeoff rating of 223hp by increasing the compression ratios from 5.25:1 to 6.5:1, while fitting aluminium alloy cylinder heads, domed-pistons and modified valve gear. Production of the basic fixed-pitch Gipsy Six unit began in 1934, with the engines rated at 200hp at 2,400 rpm for takeoff. The DH86 Express airliner prototype which first flew on 14 January 1934 was fitted with four Gipsy Six engines. By the end of 1934 a total of 14 production examples had been built, flown and delivered to various customers including Holymans and Qantas in Australia, each fitted with four Gipsy Six engines, requiring engine production to have reached at least 60 by that time.
This was quickly followed by production of the 205hp Gipsy Six Series II for use with the hydraulically actuated airscrews that de Havilland were by then producing under a licence acquired from Hamilton Standard in 1934. This was effectively the engine that the company did not have time to develop for the 1934 MacRobertson race, but without the significant increase in compression-ratio needed for the Comet - being 6:1 - compared to the Six-R's 6.5:1. The Series II unit, while superficially similar to its predecessor, had a hollow, splined crankshaft to permit the fitting of the VP airscrew using pressurised oil from the engine's dry-sump lubrication system via a pilot-operated control valve. As well as differences to the crankcase and crankshaft, different cylinder barrels, pistons and aluminium cylinder heads were fitted. There was also an added, very limited ability provided on the Series II to run accessories. The "pilot-friendly" constant speed units (CSUs) were not available for these engines until 1939. The Series II was also equipped with automatic mixture control, a barometric device, without which it was exceedingly difficult to adjust for a correct mixture with a CSU fitted. This feature was also carried over onto the similar Queen II. Pilots were then able to simply select the required RPM and boost pressure required. This reduction in over-leaning also tended to extend the time between overhauls.Twin down-draught, Claudel-Hobson A.I.48 carburettors were common to all these engines. The induction system was designed to automatically obviate the effects of carburettor icing.
Alfa Romeo built two engine types based on the Gipsy Six design, the Alfa Romeo 110 and Alfa Romeo 115.
The series of engines proved to be reliable, and the time between overhaul was more than doubled within a few years. The Achilles' heel of the engine, its lack of facilities for running ancillary items, an increasing requirement during the 1930s, remained so. It was subsequently addressed on the similar Gypsy Queen III and Queen II engines developed for military applications.
Later developments, such as the postwar Queen 30 and Queen 70 series engines took the rated power from 185hp up to nearly 500hp. While new designs, these still used all of the same basic configurations of the original Gipsy Six of the mid-1930s and were in use by operators such as the Royal Air Force (RAF) and Royal Navy (RN) until the 1980s.
Today, with de Havilland having long disappeared, very few of these engines remain in use and only a few specialist facilities can carry out overhauls.
Gipsy Six engines remain in service worldwide as of April 2010. Twelve Gipsy Six-powered de Havilland Dragon Rapide aircraft are on the Civil Aviation Authority register although not all are currently airworthy.[1]
A preserved Gipsy Six engine is on public display at the Shuttleworth Collection, Old Warden, Bedfordshire, another is on display at the Science Museum, London. The National Military Museum, Romania displays an engine cut open.