1.0     Introduction.

                  Two low altitude versions of the intercooled Merlin engine were made which were designated the RM-9 SM and RM-10 SM. In addition an improved type of high altitude engine designated the RM-11 SM was made. The RM-10 SM and the RM-11 SM were subsequently given mark numbers and are now known as the Merlin 66 and Merlin 70 respectively.

                  Three Spitfire IX aircraft were allocated to this establishment to determine the performance of the type fitted with each of these engines. The aircraft tested were BS.354 (RM-9SM engine, no Mark number), BS. 543 (RM-10 SM or Merlin 66) and BS. 551 (RM-11 SM or Merlin 70).

2.0     Condition of aircraft relevant to tests.

            2.1   General. Except for the propellers the condition of the three aircraft was identical. The propeller details are summarised below:-

                  Standard Mark Vc wings were fitted, and the armament consisted of two 20 mm. guns and four 0.303 in. machine guns. The two remaining gun stubs were fitted with hemispherical blanks. All gun ports were sealed, but the ejection chutes beneath the wings relevant to the guns fitted were open.

                  Internal bullet-proof winscreens were fitted and rectangular rear view mirrors without fairings. Multi-ejector exhaust manifolds were fitted. There were no ice guards on the air intakes.

                  Since Bendix-Stromberg carburettors were fitted no fuel cooler is necessary and none fitted, the ducts in the port wing root being blanked off.

            2.2   Engines. An improved type of supercharger was fitted to each of the three engines. The diameter of the first stage rotor was increased from 11.5" to 12.0", various general improvements were made, whilst a Bendix-Stromberg carburettor, injecting petrol into the eye of the supercharger, was fitted. These modifications had the effect of raising the supercharger top speed, thus improving the efficiency. In addition, all engines were cleared for operation at +18 lb. per square inch boost pressure for all out level flight and for five minute climbs at combat rating.

                  All engines were fitted with 0.477:1 reduction gears.

            2.3   Loading. All three aircraft were flying with C.G. 5.0 in. aft of the datum, BS. 354 and BS. 543 weighing 7485 lb., and BS. 551 weighing 7470 lb.

            2.4   Engine limitations and numbers. The following limitations were applicable to the tests made:-

3.     Tests made.

                  The following tests are dealt with in this report:-

4.0     Results of tests.

                  All results given in this report have been corrected to standard atmospheric conditions by the methods of report No. A.& A.A.E.E./Res/170 (which incorporates A.& A.E.E. Memorandum dated 27.8.42). The level and speed results have been corrected to 95% of the take-off weights by the method of the same report.

                  The position error correction used was that measured on Spitfire IX BF. 274 which was issued in the 3rd Part of No, A.& A.E.E./692,m. The make, position and setting of the pressure head on all three aircraft dealt with in this report was the same as that on BF. 274.

            4.1   Climbing speeds. The partial climbs made on BS.354(RM-9SM) showed the best climbing speed to be 170 m.p.h. A.S.I. up to full throttle height in F.S. gear (about 17,000 ft.) decreasing speed by 2 1/2 m.p.h. per 1000 ft. thereafter.

                  Since the powers of the RM-9SM and RM-10SM (Merlin 66) in F.S. supercharger gear are nominally the same, the best climbing speed found for BS.354 was also used on BS.543 (Merlin 66)

                  The climbing speed used on BS.551 (Merlin 70) was that found to be the best on a standard Spitfire IX with the Merlin 61, since the powers in F.S. supercharger gear are similar. (This admittedly ignores possible differences in propeller characteristics). This speed was 160 m.p.h. A.S.I. up to full throttle height in F.S. supercharger gear (about 25,000 ft.) decreasing speed by 3 m.p.h. per 1000 ft. therafter.

            4.2   Climbs. The climb results obtained are detailed fully in Tables I, II, and III. Due to the fact that the boost gauges on the RM-9SM and the Merlin 70 engine had rather high negative corrections, these engines were running above their rated powers. In order to compare the performances of the three aircraft it is necessary to correct the measured results by reducing both the rate of climb and the level speed below the full throttle heights. The original and the corrected climb results will be seen in Figure 1, the corrected results being the lower in each case. A summary of the results, corrected to 18.0 lb/sq.in. boost follows:-

            4.4   Level speeds. The maximum level speed results obtained are detailed fully in Tables IV, V, and VI. The remarks above in para.4.2 also applied to the speed results which have been similarly corrected. The original and corrected speeds are given in Fig.2, and a summary of the corrected results follows:-

5.     Conclusions.

                  Reference to figures 1 and 2 will give a complete picture of the performance test made on the three aircraft. It will be seen that considering both the level speed and the rate of climb, the RM-9SM engined aircraft is inferior to the other two, and in fact it is understood that this version will not be produced.

                  Another point of note is that the performance of Spitfire IX BS.551, fitted with the improved high altitude Merlin 70 engine, has a lower ceiling by about 3000 feet than the standard Merlin 61 engined aircraft and subsequently the fall off in rate of climb and level speed is greater than would be anticipated. This rapid fall off is attributed to carburettor richness at altitude, and it is hoped that at a later date further tests with an adjusted carburettor may be made.

                  It is a matter of interest that at sea level, Spitfire BS.354 with the RM-9SM engine, although nominally develping 50 B.H.P. more power than the Merlin 66 engines BS.543, is 7 m.p.h. slower, indicating either that the nominal powers were not realized and/or that there was a considerable difference between the drags of the two aircraft.

                  The powers of the RM-9SM and the Merlin 66 engines in F.S. gear should be identical, since the high speed supercharger gear ratio is the same. It will be seen that on the climb, the performance and boost pressures were similar, within the limits of experimental error, but in level flight above the full throttle height the Merlin 66 engine was developing about 1 lb/sq.inch higher boost pressure than the 9 SM engine, with a consequent higher full throttle height and improved performance. This discrepancy must be due to variations in the manufacture of the engines and possibly of the air intakes, but it should be borne in mind that any small differences in performance of a high compression ratio supercharger or of the intake will be more noticeable at high speeds because of the increase in the dynamic head.

6.     Further developments.

                  An accident to Spitfire BS.551 interrupted a series of tests with a modified carburettor setting. The engine has been returned to the makers for installation in another aircraft, and it is hoped to continue the flight tests with this engine in due course.