The operational limitations of the Merlin 45 have been increased, the use of 3000 R.P.M. and +16 lb/sq.in. boost being now permitted for periods not exceeding 3 minutes during combat. Tests have been made to determine the performance of the aeroplane at this new rating, and also whether the oil and radiator cooling are adequate. The performance fiqures obtained at the normal engine rating are included for purposes of comparison.

          The results show that below 8,800 ft. the rate of climb is increased by about 1,050 ft/min. by the use of combat rating. The maximum rate of climb is 3,710 ft.min.

          In level flight below 13,000 ft. the maximum level true airspeed is higher by about 35 m.p.h. using combat rating than at the same height using normal rating. The maximum level speed is 369 m.p.h. at 13,000 ft. Above this height the increase in speed is less and falls to zero at 19,900 ft., the full throttle height at normal rating, where the speed in either rating is 360 m.p.h.

          A.A.878 is fitted with a temperate type cooling system. The engine oil inlet and coolant outlet temperatures were measured on two climbs to 24,000 ft. This takes considerably longer than the 3 minutes for which the combat power may be used, a concession for test purposes. The observed temperatures are therefore likely to be higher than would normally be obtained in a 3 minute period at the same outside temperature. Even so, the oil inlet temperature is within requirements (100 deg C) for temperate summer conditions and does not exceed the emergency maximum temperature of 105 deg.C when corrected to tropical summer conditions.

          The radiator suitability ratio for temperate summer conditions is below unity for less than 4 minutes on the climb and reaches a minimum of 0.985. For tropical summer conditions, the minimum radiator suitability is 0.865.

          Oil inlet temperatures and coolant outlet temperatures were measured in level flight at the end of a 3 minute combat period at 12,000 ft. (approx. full throttle height). The results show that under these conditions the oil inlet temperature is within requirements for both temperate and tropical summer conditions, whilst the radiator temperature is within requirements for temperate but not for tropical summer conditions.

          Further cooling tests at combat rating on a Spitfire V with tropical modifications are being made.

1. Introduction.

          The operational limitations of the Merlin 45 have been increased, and the use of 3,000 R.P.M. and +16 lb/sq.in. boost is now permitted for periods not exceeding 3 minutes during combat. Tests were required to determine the performance of the Spitfire Vc and also whether the oil and radiator coolings were adequate at the new engine rating. Performance fiqures obtained on this aeroplane at the normal engine rating are included for comparison.

          The performance results at normal rating have been forwarded to M.A.P. (D.D.T.D.) by letter ref: AAEE/4493/-A.S. 56/18 and A.S.56/42 dated 13th August, 1942. The results given in this report have been reduced by the method of A.& A.E.E. Memorandum dated 27th August, 1942 and hence differ slightly from those given by letter.

2.       Condition of aeroplane relevant to tests made.

          A.A.878 was a Spitfire Vc fitted with the universal type wing at the time the tests at combat were made. Two 20 mm. guns were fitted, one in each wing, with the muzzles but not the ejection chutes sealed. The stubs to take the other two 20 mm. guns were fitted with hemispherical blanks. The ports for the 4 x .303" guns were sealed, but the ejection chutes were open.

          An aerial mast was fitted behind the pilot's hood, with an aerial between the mast and the tailplane. An I.F.F. aerial was fitted between the side of the fuselage and the tailplane on the starboard side only.

          The bullit-proof windscreen was fitted internally. An external rear view mirror was fitted, without a fairing on its forward side. The air intake had neither stonegaurd or snowgaurd. Triple ejector exhausts with fishtails were fitted on each side of the engine.

          The radiator, of Morris manufacture, type QVC was in a duct under the starboard wing. The flow through the duct was controlled by a shutter at the rear. With the shutter full open the exit area was 1.50 sq.ft., and with it the minimum drag position, 0.74 sq.ft. The oil cooler, of Serck manufacture, type S.799-3c-524R was in a duct under the port wing. The exit duct was not flared out, the exit area being .15 sq.ft.

          The thermostats were removed from the coolant system before the tests were made.

          Before the combat rating was used, Spitfire modification 650 was incorporated, strengthening the port side engine bearers.

          The propeller was a de Havilland 20 ° constant speed type 5/39 of 10'-9" diameter.

          2.1   Loading.   The tests were made at a take-off weight of 6,965 lb. with the centre of gravity 7.3 inches aft of the datum. This is the loading of the aeroplane when fitted with 4 x 20 mm. guns and full service equipment. Although at the time of test 2 x 20 mm. and 4 x .303" guns were fitted, the aeroplane had been ballasted internally the give this weight in connection with the other tests being made to obtain comparitive performance figures with the two types of armament.

          2.2   Engine limitations relevant to tests.   The engine operational limitations for the Merlin 45 applicable to these tests were:

          1.   Engine revolutions (R.P.M.)

4.   Oil pressure (lb/sq.in.)

3.       Scope of tests.

          The performance on climb with the radiator shutter fully open was measured at combat rating from ground up to 24,500 ft. Above 25,000 ft. the use of 3000 R.P.M. is permitted at normal rating, so that combat and normal ratings are identical.

          Readings of the temperature of the oil inlet to the engine and of the coolant from the engine were taken on these climbs.

          The maximum level speed of the aeroplane at combat rating with radiator shutter in minimum drag position was measured between 8,000 ft. and 20,000 ft. The oil and coolant temperatures were noted at the end of the three minutes at full throttle.

          The climb performance was measured at normal rating up to 36,000 ft., and the maximum level speed between 16,000 and 26,000 ft.

          The position error correction was measured by the aneroid method.

4.       Results of tests.

          The results of the performance measurements reduced by the methods of A.& A.E.E. Memorandum dated 27th August, 1942 are given in Tables I-IV and in Figs.1 and 2, and of the engine cooling tests in Tables V and VI and in Fig.6. The position of the pressure head is given in Fig.3, the position correction in Fig.4 and the correction to altimeter when connected to the static of the airspeed system in Fig.5.

          The following is a summary of the performance and cooling results obtained.

          (a) Climb performance.

          The results were:-

          In level flight at 12,000 ft. (approx. full throttle height) with the radiator shutter in the minimum drag position, the following results were obtained at the end of a 3 minute period at combat rating:-

5.       Discussion of tests.

          The results show that the rate of climb is increased by about 1,100 ft/min. below 8,800 ft. by the use of combat rating. Above this height the increase is less, falling to 300 ft/min. at 15,000 ft.

          It will be noted that in the tables and figures, the time to height is given as if combat power was used throughout the climb. These figures, of course, cannot be obtained in practice without exceeding the permitted time limit. The curve is useful, however, as it gives the height that can be covered in the permitted 3 minutes at combat power.

          In level flight the maximum level true airspeed is higher by about 35 m.p.h. below 13,000 ft.; above this height the increase in speed is less and falls to zero at 19,900 ft., the full throttle height at normal rating. The speed difference below 16,000 ft. is obtained by extrapolation of the normal rating curve.

          The engine oil inlet and coolant temperatures were measured on two climbs to 24,000 ft. This takes considerably longer than the 3 minutes for which the combat power may be used. The observed temperatures are therefore likely to be higher than would normally be obtained on a 3 minute period at the same outside air temperature. Even so, the oil inlet temperature is within requirements for temperate summer conditions and does not exceed the emergency maximum temperature of 105 deg.C when corrected to tropical summer condition.

          The radiator suitability ratio for summer conditions is below unity for less than 4 minutes of the climb and reaches a minimum of 0.985. For tropical summer conditions, the minimum radiator suitability is 0.865. These figures would be improved if Tn could assume emergency maximum figure of 135 deg.C for combat periods.

          In level flight at the end of a 3 minute period at combat rating with the radiator shutter in minimum drag position the oil temperature is within requirements when corrected to temperate or tropical summer conditions, whilst the radiator suitability is within requirements for temperate but not for tropical summer conditions.

6.       Further tests.

          Cooling tests on the climb using combat rating are being made on another Spitfire V, B.R.288, which is fitted with tropical modifications. These will be reported separately.