1. Introduction:

...........Climb and level speed performance was required on a Mustang fitted with sealed radiator intake scoop and an Allison V.1710 F.21.R engine. The tests were repeated with the F.21.R engine converted to the normal F.3.R version.

...........The F.21.R engine is similar to the F.3.R except for the supercharger gear ratio which has been lowered to 7.48:1 from the normal 8.8:1 of the F.3.R.

...........The level speed measurements with the F.21.R engine installed were first made using a boost limitation of 51" Hg. Measurements were made subsequently using full throttle down to ground level to determine the maximum boost and airspeeds obtainable. With the supercharger gear changed to that of the F.3.R, level speeds were measured using various boost pressures up to a maximum of 56" Hg.

...........For simplicity the notation used in this report for the engine fitted in turn with the 8.8:1 and 7.48:1 supercharger gear ratios, will be F.3.R. and F.21.R respectively.

2. Condition of aircraft relevant to tests.

....2.1 General. The aircraft was flown throughout the tests with the following items fitted:

...........A Curtiss electric 11'9" diameter propeller
...........Individual fishtail exhausts
...........Radiator intake scoop sealed closed
...........Stone guard over radiator and oil cooler
...........F.24 camera panel behind pilot covered with fabric
...........4 x .3" and 2 x .5" guns in the wings with the ports and ejection chutes sealed
...........2 x .5" guns in the nose with the muzzles sealed with tape
...........Aerial mast behind cockpit with W/T aerial to fin tip
...........I.F.F. aerials from tailplane tips to sides of fuselage

...........The pressure head installation was identical to that on AG.351.

....2.2 Engine details and limitations. An Allison V.1710 F.21.R engine Nos. 9115/A.208067 was installed in the aircraft on arrival at this Establishment. The F.21.R is similar in all respects to the F.3.R which is normally fitted to Mustang aircraft except for the supercharger gear ratio which is lowered form 8.8:1 to 7.48:1 in the F.21.R, giving increased power at low attitude. During the tests a standard induction manifold was fitted.

...........The operational limitations at the time of tests for the above engine are given below:

...........Special tests were however called for and made, in which level speeds were measured with boost up to 56" Hg. on the F.3.R and at full throttle on the F.21.R.

....2.3 Loading. The tests were made at a take-off weight of 8,955 lb. representing the typical service loads for the type.

3. Scope of tests.

....(i) Level speed measurments were made between 500 ft. and 15,000 ft. with the radiator exit duct closed (the intake scoop being sealed shut) under the following conditions and sequence.

...........(a) With the F.21.R (supercharger gear ratio 7.48:1), using a maximum boost of 51" Hg.
...........(b) With the F.3.R (supercharger gear ratio 8.8:1), using maximum boosts of 42, 44.5, and 56" Hg.
...........(c) With the F.21.R (supercharger gear ratio 7.48:1), using full throttle at all heights.

...........Tests (c) were made last so as to avoid possible damage to the engine, due to the high powers involved, occuring before the other series of tests could be completed.

....(ii) The performance on climb with the F.3.R and F.21.R versions was measured with the radiator exit duct flap half open. The normal climbing speed for Mustang aircraft of 160 m.p.h. A.S.I. up to full throttle height reducing 2 m.p.h. per 1000 ft. thereafter was used.

....(iii) The position error was measured on this aircraft by the aneroid method over a speed range of 400 to 180 m.p.h. A.S.I. with flaps and undercarriage up.

4. Results of tests:

....The performance measurements have been corrected to standard atmospheric conditions and the level speeds to 95% of the take-off weight viz. 8510 lb., by the methods given in Report No. A.& A.E.E./Res/170. A "strut correction" has been applied for the Kollsman type under wing strut mounted pressure head, the source of this correction being ARC report 6420.

....4.1 Level speeds. The level speed results are given in detail in Tables I to V and in Figure 1, and are summarized below:-

....4.2 Climb. The climb results are given in detail in Tables VI and VII and in Figure 2, and are summarized below:

....4.3 Position error. Curves showing the position error correction and the correction to altimeter when connected to the static of the airspeed indicator are given in Figures 3 and 4 respectively.

5. Discussion of results:

...........The main performance results under the various conditions tested are obvious from paragraph 4 and the figures, and do not require comment.

...........An estimate of the effect on performance of sealing shut the radiator intake scoop can be made by comparision with the results obtained on AG.351 on which the intake was not sealed. This estimate cannot be taken as in any way exact since the drags and powers of the individual aircraft and engines may vary slightly.

...........The difference in speed between the two aircraft with the F.3.R engine at 42 ins. Hg. boost is 11 m.p.h., taking the figures given in the reports. About 3 m.p.h. of this difference is due to the different methods of reduction now in use at this Establishment, and the remaining 8 m.p.h. can be attributed to sealing the radiator intake scoop.

...........Allowing for the fact that AG.351 was 330 lb. lighter than AP.222 the climb performance of the two aircraft with the F.3.R engine is similar. This indicates that sealing the radiator intake scoop has no measurable effect on climb performance.

...........As already stated neither of these results can be taken as exact due to possible differences in the indiviual aircraft and engines.

6. Further developments:

...........Level speed tests were in progress on AP.222 using the F.21.R engine with a modified induction manifold designed to obtain higher boosts at full throttle. The engine failed before any evidence could be obtained and the tests will be continued when a new engine becomes available.