A Rolls-Royce S.U. fuel pump has been fitted to Spitfire (F) Mk.VB, W.3322 in place of the normal A.V.T.40 float type carburettor and the performance of the aircraft has been measured as part of a general schedule of tests including fuel consumption, carburation and behaviour under negative "g".

          The principle results at a weight of 6,516 lb. are:-

          The performance of the aircraft with normal carburattor has not yet been measured, but compared with the performance of other Spitfires Mk.V and VI, it is favourable mainly on account of the higher full throttle height obtained.

1.       Introduction.

          1.1   The normal float type carburettor has certain inherent weaknesses, such as insufficiently precise fuel metering and distribution coupled with inability to withstand negative "g" without cutting. A fundamentally better system is to use a mechanically driven pump as the metering device with suitable compensation for altitude and boost, etc.; the metered fuel under pressure may then be injected into the supercharger intake, or direct into each cylinder.

          1.2   The S.U. Company, in conjuction with Messrs. Rolls-Royce Ltd. have designed and developed such a mechanically driven pump injecting the fuel into the supercharger intake and the second of these Mk.I pumps has been fitted to the Merlin 46 in Spitfire (F) Mk.VB, W.3322. Flight trials have been made to ascertain the behaviour of the system and the stability of its metering characteristics during the trials.

          1.3   The test schedule includes climb and level speed performance, carburation, fuel system and general handling tests; the present report deals only with the performance aspects, whilst the remainder will be covered in later reports.

2.       Condition of aircraft relevant to tests.

          2.1   The aircraft was a Spitfire (F) Mk.VB, fitted with a Rolls-Royce Merlin engine, incorporating the Rolls-Royce S.U. fuel injection pump. The armament carried was two 20 mm. Hispano guns and four .303 in. Browning guns. The muzzles of the former were covered with rubber caps, the gun ports in the leading edge of the wing and the ejector openings under the wings were sealed with fabric. External features included an aerial mast and rear view mirror. A stoneguard was fitted on the flange of the air intake and triple ejector fishtail exhausts were used.

          The all-up weight of the aircraft was 6,516 lb. with the C.G. at 8.9 in. aft of the datum, undercarriage up, this corresponding to typical service loading.

          Limitations.

          2.3   Propeller details. De Havilland, variable pitch. Diameter 10 ft. 9 in. Type 5/39. Hub serial No. D.H.519426. 3 duralumin blades, Nos. BAQ.410,404,443.

3.       Brief description of pump.

          The following is a brief description of the operation of the pump, but a more comprehensive description, with a drawing, will be given in a later report.

          3.1   In essence the orthodox carburettor and engine driven pump are replaced by this fuel injection pump which is fitted to the mounting previously utilized for the engine driven pump. A pair of throttle butterfly valves are retained, whilst a Bendix injection unit is fitted at the supercharger intake, these two items being employed as a temporary expedient pending designs of more suitable replacements.

          3.2   The pump body incorporates a gear driven fuel pump which delivers its output via a built-in de-aerator to the inlet chamber of a distributing valve controlling the fuel entering and leaving the injection pump cylinders. The latter consist of 5 cylinders having plungers to which a reciprocating motion is imparted by a swashplate mechanism, gear driven from the engine; the fuel delivery is regulated primarily by the engine r.p.m. whilst boost, atmospheric pressure, and charge temperature compensating devices operate via a series of capsules and an oil servo to alter the stroke of the plungers by altering the angle of the swashplate to the driving axis. By this means the fuel delivered by the pump is very nearly proportional to the power developed, which is nearly the same as the expression for the power developed by the Merlin engine.

4.       Results.

          4.1   The tests consisted of full throttle climbs to 39,000 ft. and level speed measurements between ground level and 38,000 ft. The results were reduced to standard atmospheric conditions by the methods detailed in Report No. A.& A.E.E./Res/170, incorporating A.& A.E.E.Memorandum dated 27th August, 1942. The position error correction was that of Spitfire (F) Mk.VC, A.B.488, corrected for the difference in weight.

          4.2   General. Comparison with other Spitfires, fitted with Merlin 46 or 47 engines, is given in Table II. Suitable comparison with the Spitfire (F) Mk.VB was unfortunately not possible, the only other aircraft available (W.3134) having a very poor performance due to low full throttle height. Table III shows that the aircraft is appreciably better than most other Spitfires, after allowing for differences in weight and wing span, the gain being about 1,500 ft. in ceiling and 8-10 m.p.h. in top speed. Both of these improvments may be attributed to the removal of the carburettor, causing a lower pressure drop of the engine air before entering the supercharger, and thus giving an appreciable rise in boost pressure.

          4.3   Full throttle climb. (Table I, Fig.1). Besides the general comparison given in Table III, results for Spitfire (F) Mk.VC, A.B.488 and Spitfire (F) Mk.VA, N.3053 were plotted in Fig.1, after correction for difference in weight. This confirms that the performance is above average, and up to the standard of N.3053 which had the prototype engine.

          4.4   Full throttle level speeds. (Table II, Fig.2). Comparison at similar weights again shows the superiority of this aircraft, the extra boost giving an increase in full throttle height of 800 ft. and increase in speed of 8-9 m.p.h.

          4.5   No trouble has been experienced with starting and general running. Details of handling will be reported later, but up to the present the only trouble encountered has been an occasional engine cut during slow rolls to the left.

5.       Conclusions.

          5.1   Climb and level speed performance of this aircraft are above average. This is attibuted chiefly to the increase in boost and full throttle height due to the reduction in pressure losses occuring before the air from the intake enters the supercharger.

6.       Further developments.

          Fuel consumption measurements are now being made and if practicable some check performance tests with the A.V.T.40 float type carburettor fitted will be made on the same aircraft.