1.     Introduction.

        Comparitive performance trials were required on Spitfire HF. Mk.IX BS.310 with the following combination of propeller and reduction gear.

        (i)      0.477:1 reduction gear, 4 - blade propeller.

        (ii)     0.42:1 reduction gear, 5 - blade propeller.

2.     Condition of aircraft relevant to tests.

        2.1.   General. The principal features of the aircraft were as follow:-

        The difference in weight between the 4 and 5 blade propellers was 65 lb. and the above two loadings should have differed by this amount. In practice, however, some additional ballast was carried in loading (ii). The actual speed results, however, have been corrected to 95% of what the respective weights should be.

        2.3.   Engine numbers and limitations. The numbers of the Merlin 70 engines 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 the 5 blade propeller. The climbing speed for the trials with the 0.477:1 reduction gear and the 4 blade propeller was that used on Spitfire HF. Mk.IX BS.551.

        (ii)     Ceiling climbs were made at combat rating in each case. 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 were measured in each case. The radiator flaps were closed during these tests.

4.     Results of tests.

        All results in this Report have been corrected to I.C.A.N. standard atmospheric conditions by the methods of Report No. A.& A.E.E./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 has 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. A.& A.E.E./RES/147.

        The position error correction used was that measured on Spitfire F. Mk. IX BF.274. The make, position and setting of the pitot-static head were the same on BF.274 and BS.310.

        4.1.   Climbing speeds. The climbing speed used with the 0.477:1 reduction gear and the 4-blade propeller were that deduced from tests on another aircraft as stated in para. 3 (i) above. This speed was 160 mph ASI up to 25,000 ft. (approximately full throttle height in F.S. supercharger gear) decreasing by 3 mph per 1,000 ft. thereafter.

        The partial climbs made with the 0.42:1 reduction gear and the 5 blade propeller showed the best climbing speed to be 160 mph ASI up to 22,000 ft. decreasing by 5 mph per 2,000 feet thereafter.

        4.2.   Climb performance. The results of the climbs are given in full in Tables I and II and Fig. 1 and are summarized in the table below. With the 5 blade propeller the nominal combat boost of +18 lb/sq.in. was slightly exceeded due to inaccurate functioning of the boost control, and accordingly an extra column (B) is given with the results corrected to nominal boost.

        5.1.   Full throttle heights. With the 5-blade propeller fitted the full throttle heights both on the climb and in level flight were noticeably lower. This is thought to be due to the different nature of the airflow at the entry to the air intake with the 5-blade propeller fitted. The tests with the 5-blade propeller were made after the tests with the 4-blade propeller, and to ensure that the loss of full throttle heights was not due to engine deterioration the 4-blade propeller was finally refitted (the reduction gear being 0.42:1) and a few check performance measurments made, when it was found that the full throttle height had increased almost to its original value.

        5.2.   Propeller tip Mach number. The propeller tip Mach numbers in level flight were a follow:

        The differences in air temperature on test from the I.C.A.N. standard values were within the ranges given below:-

        The comparison between the two conditions is best seen from the tables given in paras, 4.2 and 4.3 or Figs. 1 & 2.

        On the climb the difference between the performance under the two conditions was within 200 ft/min. at all heights.

        In level flight the aircraft showed an increase of 3-5 mph with the 5 blade propeller in F.S. gear between 18000 ft. - 24000 ft. but at other heights the difference was small.

        In general, therefore, the performance of the aircraft is similar with either propeller-reduction gear combination and no particular advantage in performance would be obtained by replacing the 4 blade propeller by a 5 blade propeller.