Napier Sabre
The Napier Sabre engine was one of the most viewed sections of our old website and the engine where the Trust gets the most enquiries. As such we include more detail about this engine, and our grateful thanks go to NPHT President Alan Vessey for this section of the website. The content is based on articles first published in the Trust’s “Napier Heritage News” Issues 88, 92-94.
There has been an awful lot written about the Napier Sabre aero engine. Many ‘experts’ criticise the engine for its unreliability and help to embellish its apparent yet undeserved notoriety. The engine’s teething problems were no different to other aero engines including the illustrious Rolls-Royce Merlin which was similarly plagued with development problems. These included coolant leaks, cylinder head cracks, excessive wear to camshafts and crankshaft bearings as well as failure of the accessory gear trains and coolant jackets. Why then did the Sabre receive such bad press compared to the Merlin? Probably the biggest reason was that, whilst the R-R Merlin was developed in peacetime, Napier Sabre development work was more stressful since it was wartime and the engines urgently required.
Whilst the Sabre did have development problems, especially relating to the sleeve valve, these were cured over time and the later Marks became both reliable and incredibly powerful engines.
It is hoped that the following will help dispel a lot of the myths about the Napier Sabre engine that have grown over the years. Part of this information comes direct from a DNS report written in 1947. It has been edited for reasons of space but can be regarded as factual and accurate.
Origins
It has been generally believed, that all of the ‘H’ configured engines manufactured by D Napier & Son Ltd were attributable to their Consultant Designer Major Frank Halford. First there was the small 16-cylinder air-cooled Rapier being referred to by Montague Napier as “my little “H” engine”. This was followed by the much larger 24-cylinder air-cooled Napier-Halford Dagger well after Napier’s death in January 1931, and undeniably a Frank Halford ‘H’ production lasting until 1942 with the RAF.
However the flat ‘H’ of the massive 24-cylinder water-cooled Napier Sabre engine with sleeve valve cylinders was a quite different creature. Its origin dates from the 1929/30 period and came from a designer then residing in Cannes, France. It was in fact Montague Napier himself who designed his E96 ‘H’ Type four-stroke 24-cylinder compression ignition (diesel) engine with a 5.3/4“ bore x 5.1/2“ stroke. This was at a time when the Halford/Napier contractual design agreement specifically limited all Halford engines to a maximum cubic capacity of just 11.8 litres. Indeed when Montague started developing his “H” engine there was discussion with Halford about this resulting in him being paid £4,000 to amend the contract so NAPIER could develop this engine.
Very little information exists about the E96 engine design however the Napier Company minutes between 1929 and 1930 reveal that it was intended for marine or stationary use. Drawing office and development expenditure was authorised under Sanction No. 2230 eventually amounting to £5,500 and Sanction No. 2240 £500 towards developing a two cylinder test engine (around £500,000 in today’s money!). This development was shelved in August 1930 when the Napier Board took the decision to concentrate on developing the E97 straight 6-cylinder Napier Javelin engine for civil aviation purposes. . . for the time being.
Following the unexpected death of Montague Napier on 22nd January 1931 the new Napier Board ensured the E96 development continued by reducing the E96 capacity whilst retaining the Napier patented worm sleeve drive mechanism. The new E101 24-cylinder C.I. engine now had the identical 5” bore and 4.3/4” stroke to that which also formed the petrol fuelled E107 Napier Sabre engine of 1935 onwards.
Two and six cylinder test units (E101T and E101/6T) were built followed by a full 24-cylinder version. In keeping with the company’s tradition (since the Napier Rapier) of naming its aero engines after swords this new engine was named the Napier Sabre. Development work was slow and the design and the Air-Ministry realised that it was a long way behind the Junkers Jumo 204 & 205 6-cylinder opposed piston engine designs. They ‘persuaded’ the DNS Board to abandon the E101 project in favour of building the Jumo 204 engine under licence (Napier Culverin).
Development
As well as a long range C.I. engine the Air Ministry had a requirement for a new high-powered petrol aero engine so in 1935 DNS revived the E101 and commenced work on a new design designated E107. The conversion work was headed up by Napier designers such as Capt. George Wilkinson, Ernest Chatterton and R. W. “Ben” Barlow – who turned the “H” layout on its side – PLUS the input of Major Halford who had now joined the Napier Board. Certainly the considerable experience gained with his successful 1,000 BHP Napier Dagger ‘H’ engine greatly helped design work on the new engine and led DNS in 1935 to evolve an engine of 2,000 BHP.
Development was slow and hindered by an ageing Company Board Company so the War government stepped in and D. Napier & Son Ltd was absorbed into the English Electric Group of companies in December 1942. The new Napier Board under the chairmanship of Sir George Horatio Nelson included Halford’s departure. From 1943 development work was continued solely by Napier engineers and saw Sabre engine power increasing to just over 3,000 bhp by 1945. This repeated increase in power output from the Napier Sabre engine was attributable to the considerable amount of study carried out in framing the basic design, and to the requirements during the war.
The first Sabre engine was completed and successfully run and flown within two years. Much of the flight testing was completed using various single engined Folland Fo.108 aircraft developed as a flying testbed and as such gained the nickname ‘Frightful’ or ‘Frightener’ in view of the fact that not all development aero engines quite work as expected!!
The basic features of the Sabre, namely, 24-cylinders, liquid cooling, sleeve valves, and compact design have remained throughout the Series. By 1941 the 2,090 BHP Series II engine had overcome most of the teething troubles and was chosen to power the Hawker Typhoon, then Britain’s leading fighter aircraft. Other problems such as engines ingesting sand from the Normandy beaches were solved remarkably quickly! Subsequent design developments gave a progressive increase in power so that in 1945 the VA, which incorporated a boost corrected, servo controlled, ignition unit and single lever cockpit control, gave a maximum output of 2,600 BHP. The latest developments which include water / methanol injection have brought the maximum power output of the Series VII up to 3,055 BHP.
Noteworthy features which mark the successive stages of development are the strengthening of certain components to stand up to greater loads, the increased capacity of the two-speed turbocharger, the introduction of the Hobson-R.A.E. injector and single-lever cockpit control, and finally the introduction of water / methanol metering equipment.
For convenience we have split the development of the Napier Sabre engine into those with suction-type carburettors and those utilising bulk injection carburettors.
