Comparitive performance trials have been carried out on Spitfire HF. Mk. IX EN.524 with the following combinations of propeller and reduction gear:-

         2.3.   Engine numbers and limitations. The numbers of the Merlin 70 engine fitted were:-

                  The following tests are dealt with in this report:-

         (i)     Partial climbs were made at combat rating to determine the best climbing speeds with the 0.42:1 reduction gear and a 10'9" propeller and with a 11'1" propeller. The climbing speed for the trials with the 0.477:1 reduction gear and 10'9" propeller was deduced from tests on other aircraft as explained in para. 4.1.

         (ii)    Ceiling climbs were made at combat rating with each combination of reduction gear and propeller. Combat rating is normally permitted for periods of five minutes only, but a concession for test purposes allowed complete climbs at this rating to be made. The radiator flaps were fully open for climbing.

         (iii)   Maximum level speeds in both supercharger gears with each combination of reduction gear and propeller were measured. The radiator flaps were closed during these tests.

4.     Results of tests.

                  All results in this Report have been corrected to standard atmospheric conditions by the methods of Report No. AAEE/Res/170. The level speed results have been corrected to 95% of the take-off weight by the methods of the same Report.

                  In addition a "strut correction" had been applied to the level speed results, the source of this correction being report No. ARC.6420; the compressibility correction has been calculated by the methods of the Addendum to Report No. AAEE/RES/147.

                  The position error correction used was that measured on Spitfire F. Mk. IX BF.274. The make, position and setting of the pressure head was the same as on BF.274.

         4.1.   Climbing speeds. The climbing speed used with the 0.477:1 reduction gear and 10'9" diameter propeller was that deduced for and used on an identical aircraft, Spitfire H.F. Mk. IX BS.551. This speed was 160 mph ASI up to 25,000 ft. (approximate full throttle height in F.S. gear), decreasing by 3 mph per 1,000 ft. thereafter.

                  The partial climbs made with the 0.42:1 reduction gear and either propeller fitted showed the best climbing speed to be 175 mph up to 25,000 ft., decreasing by 4 mph per 1,000 ft. thereafter.

         4.2.   Climbs. The results of the climbs are given in full in Tables I, II and III and Fig. 1.

                  Briefly they are as follow:-

                  Briefly they are as follow:-

                  Previous performance tests have been carried out on Spitfire H.F. Mk IX B.S.551, fitted with a 0.477:1 reduction gear and a 10'9" propeller. The performance of E.N.524 is, on the whole, slightly worse than that obtained on B.S.551 due probably to small differences between the individual aircraft, but the results nevertheless give a comparison between the three conditions tested.

                  It was stated in the report mentioned above that further tests were to be carried out with a modified carburettor setting as it was hoped to improve the performance of the Merlin 70 at altitude. It is understood that EN.524 incorporated this modification and a comparison of the two sets of results shows a considerable improvement has been made. The absolute ceiling is now about 2,500 feet higher than that obtained on BS.551, although the rate of climb below 37,000 ft. is less.

6.     Conclusions

                  The comparison between the three conditions tested is best seen from the tables given in paras. 4.2 and 4.3 or figs. 1 and 2.

                  The differences obtained are small. On the climb 200 ft/min. covers all three conditions up to 24,000 ft. above which height the difference lessens considerably. Similarly the differences in speed are within 3 mph up to 27,000 ft.; above this the differences increase to a maximum of 18 mph at 39,000 ft.

                  The combination giving the best compromise between climb and level speed performance appears to be the 10'9" diameter propeller with 0.477:1 reduction gear up to about 28,000 feet. Above this height the best combination appears to be the 11'1" propeller with 0.42:1 reduction gear.

                  However, the differences as a whole are so small that a change from any combination already produced and in Service (0.477:1/10'9") would not appear to be worth while.