| A Rolls Royce S.U. fuel injection pump, Mark II has been fitted to Spitfire LF Mk.IX MA.648 in place of the normal Bendix carburettor and level speed performance has been measured as part of the general schedule of tests including fuel consumption and behaviour under negative 'g'.
........The principal results at combat conditions (i.e. 3000 rpm and +18 lb/sq.in. boost) are as follows:-
|Max. speed at F.T. height (9,400 ft.) in M.S. gear||= 379 mph|
|Max. speed at F.T. height (21,000 ft.) in F.S. gear||= 411 mph|
| These results compare favorably with those of other Spitfire LF Mk. IX aircraft, which fact is attributed cheifly to the higher full throttle height obtained with the S.U. pump.
1.1 The S.U. Company, in conjunction with Messrs. Rolls Royce Ltd, have designed and developed a fuel injection pump which has certain advantages over the normal float type carburettor, such as better metering of the fuel and ability to withstand negative 'g' without engine cutting. The pump is driven by the engine, utilising the normal engine driven fuel pump drive and includes suitable compensation for altitude, boost pressure and charge temperature. It injects the fuel into the eye of the supercharger.
1.2 One of the first Mk.I pumps was fitted to a Spitfire F Mk. VB, W3322, and flight trials of this installation were reported. The control system of the Mk.I pump has been completely redesigned in the Mk.II version, which has several improvements including means of adjustment to slow running and high power metering. One of these Mk.II pumps was fitted to the Merlin 66 engine in Spitfire LF.MK.IX MA.648. This particular pump incorporates a rubber diaphragm which will probably be replaced by an improved metal one in later pumps.
1.3 The complete flight test schedule includes level speed performance, carburation and general handling and aerobatic tests. This report deals with level speed measurements only; the remainder of the tests will be reported later together with a more detailed description of the pump.
2. Condition of aircraft relevant to tests.
2.1 General. The aircraft was a normal Spitfire LF. Mk.IX, fitted with a Rolls Royce Merlin 66 engine incorporating the Mk.II S.U. injection pump which replaces the Bendix carburettor, and engine driven fuel pump. The following were the chief external features of the aircraft.
|Two 20 mm Hispano guns, muzzels sealed.
Two stubs for Hispano guns, with hemispherical fairings. Ejector chutes sealed.
Four sealed ports in the wing leading edge for .303 machine guns. Two ejector chutes sealed, two unsealed.
External circular rear-view mirror with hemispherical fairing.
Multi-stub ejector exhausts.
Internal bullet-proof windscreen.
Aerial mast, no external aerials.
| The air intake was of a new type, designed to combine the temperate and tropical versions.
In tropical conditions the normal air scoop beneath the engine cowling is blanked off by a movable shutter and the air is drawn through a filter from the engine bay.
For these tests a blanking plate was fitted flush with the internal wall of the air intake, i.e. in the normal position for temperate use and the pilot's control was locked in the filter out position.
No snowguard was fitted.
The dimensions of the air intake scoop, compared with that normally fitted to the Spitfire IX are:-
|Frontal area||39 sq.ins.||35 sq.ins.|
|Distance from nose of intake to back of elbow|
(measured along upper surface).
|45 ins.||11 ins.|
| 2.2 Loading. The all-up weight was 7,380 lb. with the CG at 5.8 ins. aft of datum, undercarriage down, this being T.S.L. for the type.
2.3 Engine details and limitations.
|Rolls Royce Merlin 66|
|R.A.F. No. A/445175|
|Makers No. 156729|
|The relevant limitations were:-|
|All out level||3000||+18|
|2.4 Propeller details.|
|Rotol hydraulic variable pitch, 4 hydulignum blades, diameter 10'9"|
|Hub serial No. H.2983.|
|3. Tests made.
3.1 All-out level speed measurments were made between ground level and 13,000 feet in MS gear and between 13,000 ft. and 40,000 ft. in FS gear.
4.1 These were reduced to ICAN standard atmospheric conditions by the methods of Report No. A.& A.E.E./Res/170. The P.E.C. used was that measured on Spitfire IX BF.274 and the CEC was based on the methods of the addendum to Report No. A.& A.E.E./Res/147. A "strut" correction applicable to underwing pressure heads and based on A.R.C. 6420 was also applied to the ASI reading.
Fig 1 and Table I give the results.
4.2 Comparison of results with other Spitfire LF.Mk.IX aircraft. Figure 1 gives comparitive curves and the results are summarised in the table below:
|Report reference||This report||Messrs. Super-|
|A.& A.E.E.||A.& A.E.E.|
|Engine||Merlin 66||Merlin 66||Merlin 66||Merlin 66|
|Propeller||Rotol 10'9" diam.|
|Rotol 10'9" diam.|
|Rotol 10'9" diam.|
|Rotol 10'9" diam.|
|Weight||7000 lb.||7260 lb.||7000 lb.||7100 lb.|
|MS gear full throttle ht.||9400 ft.||8000 ft.||8600 ft.||10900 ft.|
|MS gear top speed, mph||379||372||364||384|
|FS gear full throttle ht.||21100 ft.||19500 ft.||19300 ft.||22100 ft.|
|FS gear top speed, mph||411||404||388||407|
| External equipment of the above four aircraft was similar with the exception that MA.648 had the new pattern of air intake.
It will seen that the above full throttle heights vary considerably. Differences in ram effect due to speed variations account to some extent for this; for instance if the speed of JL.165 were increased to that of MA.468 the full throttle height would be increased roughly 500 ft. The effect of difference in ram effect on the other aircraft would not exceed 150 feet.
The mean full throttle heights for BS.310, JL.165 and BS.543 are 9,200 ft. in MS gear and 20,300 ft. in FS gear, but the engine of BS.543 has full throttle heights far above average. Even then, the full throttle heights of MA.648 with the injection pump are above average by 200 ft. in MS gear and 800 ft. in FS gear. The new type intake on MA.648 is expected to reduce the full throttle heights by a small amount, so these figures may be rather pessimistic.
The improvement is attributed chiefly to the reduction in pressure losses occuring before the air enters the supercharger by the elimination of the choke and other obstructions of the normal carburettor.
Owing to large variations in the performance of the four aircraft tested it is not possible to obtain an accurate value for the improvement in performance but the above quoted figures for increase in full throttle height give an average increase of about 3 mph. around the full throttle height.
The only satisfactory way to obtain exact figures would be to repeat the tests with the normal carburettor substituted for the injection pump. When this was done on Spitfire VB W.3322 (Merln 46 S.U. Mark I injection pump), the increase in speed at full throttle was about 5 mph, and the improvement in full throttle height 1300 ft. The increase in full throttle height indicated by the present report therefore seems to be pessimistic.
5.1 Level speed performance of this aircraft is slightly above average. This is attributed chiefly to the increase in full throttle height due to the substitution of the S.U. pump for the normal carburettor.
|RPM||TAS mph||IAS mph||PEC||CEC||Strut||S/C|