Measurment of the position error of the static vent was reguired on this aircraft, one of the first of its type to arrive in this country. Although measurement of level speed performance was not asked for by M.A.P. on this aricraft, an opportunity arose to make such measurements at low altitude in M.S. gear. These results, together with the measurement of the P.E.C. of the static vent form the subject of the present part of the report. Handling tests, also asked for will be dealt with in a subsequent part of this report.

      A suumary of the results was sent to M.A.P. by letter dated 19th August, 1944, and no significant amendments have been made to those.

2.   Condition of aircraft relevant to tests.

      2.1  General. Photographs of the aircraft are attached to the present part of the report.

      The following were the chief external features:-

      Six .5" machine guns in the wings, ports and ejection chutes sealed with fabric.
      Camera gun port in port wing, sealed with fabric.
      Multi ejector exhaust manifolds with shroud-plates.
      Air intake in nose with internal ice-guard.
      Aerial mast without external aerials.
      Bead sight in front of windscreen.
      Air filter intake in lower left cowling.
      Faired bomb racks under the wings.

      The aircraft was not painted. The under surface of the wings back to the main spar and the whole of the top surface had been coated with a smooth composition, the joints being filled and the remainder being bare metal. The fuselage was left with the bare metal except for a matt anti-glare finish on the top engine cowling. All other parts of the aircraft were also bare metal, except the elevator and rudder which were fabric-covered and doped.

      In order to obtain adequate cooling, level speeds were done with the radiator duct flap set to a gap of 8½ inches, as coolant temperatures were excessively high with the normal setting of 7¼" gap.

      2.2  Pressure head and static vent details. The pressure head, details of which are given in Fig.1, was installed under the starboard wing, and interconnected static vents were fitted on opposite sides of the aft part of the fuselage, as shown in Fig.2. This aircraft was the first Mustang received at this Establishment to be fitted with the pressure head-static vent combination instead of the pitot-static head. Use of the two interconnected vents is presumably for equalising the static pressure in asymmetric flight.

      2.3  Loading. The tests were made at a take-off weight of 9480 lb. with the C. of G. at 100.2 ins. aft of the reference axis. This corresponded to a typical loading with no fuel in the auxiliary fuselage tank and no external tanks or bombs.

      2.4  Engine details and relevant limitations. The engine was a Packard Merlin V.1650-7, serial numbers: V.320680/A.572421, and was fitted with a Bendix Stromberg carburettor, type PD.16-Al, modified to have automatic mixture control. The reduction gear ratio was 0.479.

          The following were the relevant limitations at time of test:-

      3.1  The static vent position error was determined by the aneroid method in level flight with flaps and undercarriage up, over a speed range from 120 to 340 mph, ASI.

      3.2  Level speed trials were made from ground level to 12,000 ft. in M.S. gear, at combat conditions using 100 octane fuel and at increased boost war emergency limitations using 150 grade fuel.

4.  Results.

      4.1  Static vent position error correction. Figure 3 gives the P.E.C. of the static vent, and Figure 4 the correction to the altimeter when connected to the vent. The P.E.C. varies linearly from +2.5 mph at 120 mph ASI to +4.8 mph at 340 mph ASI.

      4.2  Level speeds. These were corrected to ICAN standard atmospheric conditions and to 95% of the take-off weight by the methods of Report No.A.A.E.E./Res/170, using a supercharger constant of 0.002. The compressibility correction applied to the ASI was calculated by the methods of Report No.A.A.E.E./Res/208.

              Table I gives the results and the values of boost and true airspeed are plotted against height in Fig.5.

              The salient results were as follows:-

Maximum speed in M.S. Gear at 3000 R.P.M., +25 lb/sq.in. boost = 398 mph at 4,300 ft. (FTH)

Maximum speed in M.S. Gear at 3000 R.P.M., +18 lb/sq.in. boost = 396 mph at 10,300 ft. (FTH)

Figure 5 shows that at 3000 RPM, the use of +25 lb/sq.in. boost instead of +18 lb/sq.in. gave an increase in level speed below full throttle of 25 mph TAS. The full throttle height at 25 lb/sq.in was 6000 ft. lower than that at +18 lb/sq.in.

      4.3  Effect of high propeller-tip Mach numbers.. Propeller-tip speed Mach numbers exceeded 0.9 over most of the hight range tested. No correction has been made for any effect of the high propeller tip speed on change of performance with air temperature. The air temperature ranges on test were:-

5.  Conclusions.

      5.1  The maximum true airspeed of this aircraft, using 3,000 RPM in M.S. gear was 396 mph at 10,300 feet, with +18 lb/sq.in boost, and 398 mph at 4,300 ft. with +25 lb/sq.in. boost.

              Use of +25 lb/sq.in. instead of +18 lb/sq.in boost increased the True Air Speed below full throttle height by 25 mph.