Napier Sabre

In the late 1920’s Montague Napier designed his E96 ‘H’ Type 24-cylinder compression ignition (diesel) engine with a 5.75″ bore x 5.5″ stroke.  After his death the design was resurrected and, as a direct result of the considerable experience gained with their successful 1,000 B.H.P. H-type Dagger engine, D. Napier & Son Ltd commenced design work on their design E101 engine.  This was a horizontal ‘H’ water-cooled 24-cylinder C.I. engine.  Two, six and twenty-four cylinder versions of this engine were built and it was named the Napier Sabre.  Development work was slow and the design and was a long way behind two 6-cylinder opposed piston engines designed by Hugo Junkers, the Jumo 204 & 205 engines.  The British Air Ministry persuaded DNS to abandon their E101 project in favour of building the Jumo 204 engine under licence.  This became the Napier Culverin.

The Air Ministry however had a requirement for a new high-powered petrol aero engine and in 1935  DNS revived the E101 and redesigned it as the E107.  The basic features of the Sabre engine, namely twenty-four cylinders, liquid cooling, sleeve valves, and compact design remained throughout the Series.

Within two years the first petrol Sabre engine was completed and successfully run.  By 1941 the 2,090 B.H.P. Series II engine had overcome most of its initial teething troubles, and was chosen to power the Hawker Typhoon, then Britain’s leading fighter aircraft.  Subsequent developments in design provided a progressive increase in power output so that in 1945 the Series VA, which incorporated a boost corrected servo-controlled, ignition unit and single-lever cockpit control, gave a maximum of 2,600 B.H.P.  The Series VII (E121 design) included water / ethanol injection brought the maximum output to 3,055 B.H.P.

Noteworthy features marking the successive development stages of development include, the strengthening of certain components to enable them to stand up to greater loads, the increased capacity of-the two-speed supercharger, the introduction of the Hobson-R. A.E. injector and single-lever cockpit control, and finally, the inclusion of water/methanol metering equipment.

Development in power output of ‘Sabre engines

The “prototype” Sabre engine ran for the first time on 23rd November 1937, and completed its initial acceptance tests by 17th January 1938.  By March the power delivered had already risen from its acceptance test figure of 1,350 B.H.P. to 2,050 B.H.P.  Gradually the Series I (E114) engine was evolved type tested in June 1940 at a maximum power rating of 2,060 B.H.P.

In early 1939 tests were undertaken with a special, ground-boosted version of the Sabre to be used in a Heston aircraft for an attack on the world speed record.  An air blast was applied to the air intake on the test bed to simulate a forward speed of 400 M.P.H. and under these conditions a maximum power reading of 2,560 B.H.P. was achieved. This was obtained at 1,000 C.R.P.M. and 9.2 psi boost pressure, which represented a 5 minute limit at full throttle.  Unfortunately, this aircraft, through no fault of the engine, crashed during its initial flight trials and, owing to the war, further tests were abandoned.

The Sabre Series I engine was fitted with a supercharger with an 11 inch diameter impeller and a clutch which was capable of gear changing at maximum cruising speed only (designated Mark I clutch).  After around 60 Series I engines had been produced the design was changed to include:

(i) a supercharger with a 1,2 in. dia. impeller

(ii) a supercharger clutch capable of gear changing at maximum

climbing speed, (Mark III)

(iii) larger carburettor chokes.

This standard of engine was designated Series II.  The Sabre Series I and II engines were rated at the same take-off and combat boost pressure but because of the larger supercharger impeller the maximum power altitude of the Series II  was raised by 1,500 feet in “M” gear and 2000 feet in “S” gear.

During 1941 and 1942 Service experience in the “Typhoon” had faults which had not been apparent during the test bed development of the engine.  A further modified version of the engine, designated Series IIA, (E115) was produced in May 1943 which increased the combat boost to + 9 psi and raised the maximum power output further to 2,220 B.H.P.  This engine included further improvements such as a strengthened starter gear train, fully nitrided sleeves, and strengthened sleeve drives.  The Series IIA engine was eventually superseded in 1944 by the Series IIB (E107A) engine.  The advent of the V1 flying bomb in June 1944 called for an Improved performance at low altitudes and this was achieved by increasing the combat rating boost to +11 psi.  This raised the maximum power output to 2,420 B.H.P. enabling the Hawker Tempest fighter to catch and destroy large numbers of flying bombs.

The Series IIC design E107B was evolved in 1945 with an improved type of carburettor with a weaker mixture setting.  This marked the peak in the design and performance of Sabre engines fitted with suction type carburettors.  The two-speed supercharger drive was altered to give a closer ratio; this being achieved by raising the ratio of the “M” gear whilst leaving the “S” gear unchanged,

A modified version of the Series II engine incorporating balanced crankshafts and strengthened sleeve drives was evolved in 1941 to meet the special requirements of a Fleet Air Arm Fighter.  This engine, designated the Series III, had a maximum power rating of 2,305 B.H.P., and a take-off power of 2,250 B.H.P. at 4,000 C.R.P.M. the high take-off R.P.M. being in the interests of propeller thrust.  About 30 of these engines were built between 1942 and 1943 were fitted (as a complete power unit) to the Blackburn “Firebrand” aircraft.  This aircraft never became fully operational, but was used extensively for training purposes.

A  further range of Sabre engines employing bulk injection carburettors commenced with the Series IV.  They had a single entry supercharger as well as strengthened supercharger and reduction gear drives to withstand the higher gross powers which resulted from the use of higher boost pressures coupled with higher ratio supercharger drives.  They additionally incorporated sundry technical improvements including boost and R.P.M. corrected ignition, as well as an improved supercharger clutch.

The Series IV engine first utilised a S.U. twin-choke suction carburettor but this was soon changed to a Hobson-R.A.E. Injection Carburettor.  Intensive development of this unit followed.  In 1943 the Series V (E107C) engine superseded the Series IV.  Both series of engines commenced life with fully balanced crankshafts and developed 2,180 B.H.P. at 8,500 foot (at 4,000 C.R.P.M. with + 9 psi boost).  As a result of torsiograph tests it was decided to remove the balance weights from the crankshafts and reduce the maximum R.P.M. slightly owing to the presence of a certain amount of torsional vibration at the top end of the speed range.

Gradually the power output of the Sabre V was stepped up until, in November 1944, a development type-test was passed at a combat power rating, of 2,600 B.H.P.  Technical improvements to the engine were continually added and ultimately the the Series VA production engine passed an official type-test at the N. S.81.S.M combat power rating of 2,600 B.H.P. in June 1945

The Sabre VA incorporated the following features;

(i) Single lever control

(ii) Improved oil sump

(iii) Improved auxiliary drive casing

(iv) Increased coolant flow through cylinder blocks

(v)  Strengthened reduction gears

(vi) Single piece sleeve drive idler’ gears

Concurrently with the development of the Sabre V, a modified version of the same engine, but incorporating a gear driven cooling fan in the nose for use with nose radiators, was developed and designated the Series VI engine.  A small number of engines were built for flight trials in Hawker Tempest and Vickers Warwick aircraft at Napier’s Flight Development Establishment at Luton Airport.

In the summer of 1945 tests commenced using water/methanol injection to increase the power rating with standard fuel.  These tests were entirely successful and the Series VII (E121) engine was evolved.  This strengthened and improved engine, incorporated many refinements including an improved axial-flow air inlet bend.  Later in 1945 it also completed a type-test at a take-off rating of 3,000 B.H.P. and a combat rating of 3,055 B.H.P, at 2,250 ft. altitude.

The Hawker Fury was the next development after the Hawker Typhoon and Tempest.  Hawker Fury Prototype LA610 was fitted with a Napier Sabre VII to become the fastest piston-engined Hawker aircraft after reaching a speed of around 485 mph (780 km/h).

The Sabre VIII (E122) Series engine of higher boost with 2 stage 3 speed Supercharger, contra rotating propellers and fan drive engine capable of 3,350 B.H.P.

Testimony is given to the development in the Sabre design by the power/weight ratio figures. The Series I had a ratio of 1.155 lbs. per B.H.P.  This was improved to 0.992 lbs. per B.H.P. for the Series IIC and, with the advent of the Series VII the ratio was further reduced to only 0.831 lbs. per B.H.P.  The maximum permissible boost figures also rose by 10¼ lbs. from the original value of 7 psi to 17¼ psi.  The rise in power output and performance is the more remarkable when it is remembered that the development covered a period of only 10 years, 5 of which the engines were in full production in wartime conditions.