The airplane is very maneuverable with good controllability at indicated speeds to 400 MPH. The stability about all axes is good and the rate of roll is excellent, however, the radius of turn is fairly large for a fighter. The cockpit layout is excellent, but visibility is poor on the ground and only fair in level flight.

        High speed and climb performances have been completed on this airplane at a take-off weight of 9205 lbs. This loading corresponds to the average P-51B combat weight with full oil, 180 gallons of fuel and specified armament and ammunition.

        The principal results are as follows:

      2. All tests were flown with the airplanes loaded to their maximum combat gross weight. The P-38J airplane tested was P-38J-15, AAF No. 43-28392, equipped with Allison V-1710-89 and 91 engines with Curtiss electric three blade propellers. Gross weight at take-off was 17,360 lbs. with the c.g. at 26.72%. The P-47D tested was AAF No. 42-26167 and was equipped with Pratt & Whitney R-2800-63 engine and an A-23 turbo regulator. Gross weight at take-off was 13,320 lbs. with the c.g. at 29.5%, gear up. The P-51B tested was the P-51B-15, AAF No. 43-24777 and was equipped with a Packard V-1650-7 engine with a 11 ft. 2 in., four blade constant speed propeller. Gross weight at take-off was approximately 9680 lbs. The weight included 265 gal. of fuel, full oil, and no ammunition (85 gal. in auxiliary tank instead of ballast for ammunition).

      3. There was no noticeable change in handling characteristics of any of the airplanes tested when operating at the higher powers which were obtainable with the 44-1 fuel. Only a slight increase in vibration was noted at the higher powers. On one long range test made with the P-51B, there was no apparent trouble due to the 44-1 fuel.

      4. All performance data obtained on the P-51B is included in the attached curves. It will be noted that all tests were run with the wing racks installed. Speeds would be approximately 12 mph faster with the wing racks removed as shown by the dash line curve on the Speed vs Altitude Curve. Approximately 16 MPH increases in speed below critical altitude and approximately 600 ft. per minute increase in rate of climb below critical altitude was obtained by using the 75” Hg. Manifold pressure allowed by 44-1 fuel. No tests were made on this airplane with standard fuel.

       The airplane is very maneuverable with excellent controllability at all speeds up to slightly over 400 MPH indicated, the highest speeds attained in level flight. Stability is good about all axes, and recovery from stalls is normal. The airplane has a fairly short radius of turn and an excellent rate of roll. Visability is poor in taxing and only fair in level flight. The cockpit layout in general is good.

       All tests were made at a gross weight at takeoff of 9335 lbs. which included full oil and 265 gallon of fuel. Level flight and climb performance were completed with wing racks. In addition level flight speeds were obtained at the critical heights without wing racks.

       In level flight the high blower critical altitude for 75 in. Hg. manifold pressure and 3000 RPM was 20,800 ft. At this altitude the airplane attained a maximum true speed of 444.0 MPH at 75 in. HG. manifold pressure without wing racks, a high speed of 431 MPH at 75 in. Hg. manifold pressure with wing racks, and a high speed of 417 MPH at 67 in. Hg. manifold pressure with wing racks. At 24,000 ft., high blower critical altitude for 67 In. Hg. manifold pressure and 3000 RPM, a maximum true speed of 426 MPH was attained.

       In climb the low blower critical altitude for 75 in. HG. manifold presure and 3000 RPM was 2,200 ft. At this altitude the airplane attained a maximum rate of climb of 4380 ft/min. at 75 in. Hg. manifold pressure and 3,820 ft/min. at 67 in. Hg. manifold pressure. The service ceiling of the airplane was 40,500 ft. and the absolute ceiling was 41,200 ft.

       High speed in level flight at 3000 RPM, mixture auto rich, and oil and coolant shutters in automatic.

P-51-B-15 Airplane.

Increase of power from the standard war emergency rating of sixty-seven inches Hg. to the test rating of seventy-five inches Hg. resulted in an average true air speed increase of fifteen m.p.h. from sea level to the seventy-five inches Hg. low blower critical altitude (about 8000 feet). Speed increase was also approximately fifteen m.p.h. from fourteen thousand feet to the high blower seventy-five inches for critical of about twenty-one thousand feet. No measurable difference was found between airplanes. The aneroid controlling supercharger shifting point was reset at the begining of the test to shift from low to high blower at sixty-two inches Hg. in a war emergency climb. This change resulted in a blower shift altitude of approximately seventy-five hundred feet, so that it was necessary to select low blower manually for cruise at medium altitudes where the desired power was available in low blower.

Climbs were made to thirty thousand feet at the standard, and at the test war emergency ratings. Climbs at seventy-five inches Hg. required about one minute less than was required when climbing at sixty seven inches Hg. All engine temperatures were normal during climb at the increased power.

       High speed and climb performance have been obtained on this airplane at a take-off gross weight of 9760 pounds. Performance was obtained up to an altitude of 35,000 feet in increments of 5000 feet in a clean configuration. The clean configuration included one external bomb rack on each wing. Additional configurations were flown at 5000 and 15,000 feet including two 110 gallon tanks, two 500 pound bombs, and two 250 pound bombs. The principal results are as follows:

       A.  Clean Configuration (with bomb racks).

Data at war emergency rating with water injection at 3000 rpm.
 

Climb data for war emergency power rating (3000 rpm with water injection).

                  Preliminary flight performance data obtained on a Merlin 65 engine re-rated to +23 lb. boost prior to tests on R.M. 14 S.M. engine.

                  This engine was fitted with an S.U. injection pump to give the increased fuel flow requirments necessitated by the higher boost.

      The principal results are as follows :-

1.   Climb

      The maximum true airpseed 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 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 25 mph.

On 29 March 1944 the Commanding General, Army Air Forces authorized the procurement of the necessary parts to modify all P-38, P-47 and P-51 airplanes in the United Kingdom for the use of Grade 150 fuel, subject to the relevant engines being cleared to use the fuel. ^{1 2}   During March & April 1944 flight tests were conducted at Wright Field on the P-51B-15 airplane, AAF No. 43-24777, using 44-1 fuel, at the request of the Power Plant Laboratory, Engineering Division. ³   These tests were made to determine the performance of the airplane at the higher powers allowable with 44-1 fuel as compared with the performance at powers allowable for standard aviation fuel. Parallel tests were conducted by Proving Ground Command at Eglin Field, Florida on P-51B airplanes, AAF Nos. 43-24755, 43-24757, and 43-24775. The Power Plant Laboratory concluded in a 19 April 1944 memorandum report that the "Packard built Rolls-Royce V-1650-7 engine will satisfactorily comply with a 75 In. Hg manifold pressure war emergency rating with Grade 44-1 fuel". ⁴   As a result of the engine clearance and airplane trials the P-51-B airplane was cleared for operation at 75" Hg by late April. ⁵   The modifications required to the P-51 to use the 150 grade fuel were: modify manifold pressure regulator, modify supercharger volute drain valve, install new type induction center manifold extension gland seals, use of Lodge RS5/5 or KLG RC5/3 spark plugs, installation of bulged exhaust stacks, and reset supercharger aneroid switch. ⁶   By June 1944, final release on Project P.P.F. had been made approving 75" manifold pressure for the P-51 (both the 1650-3 and 1650-7 engines), as well as increased powers for the P-38 and P-47. ⁷

Deliveries of Grade 100/150 aviation fuel to Eighth Air Force fighter airfields commenced in June 1944. ^{8 9 10}  This coincidentally occured about the same time as the introduction of the P-51D into service.   Even though the USAAF had cleared the P-51 for 75" Hg., the Eighth Air Force chose 72" Hg as the P-51's War Emergency Rating. ^{11 12}   Apparently there is more to the story, however, as Encounter Reports demonstrate that 75" Hg was used operationally. ^{13 14}

By January 1945, fourteen of the Eighth Air Force's fifteen Fighter Groups were operating Mustangs, the sole holdout being the 56th FG in P-47's. Maintenance difficulties with spark plug fouling led to the decision to convert all fighter groups to 100/150 grade fuel reformulated with increased levels of ethylene dibromide (1.5T). Deliveries of PEP, as the new 100/150 blend was called, began to be issued to all fighter groups in February 1945. The use of PEP, however, cooroded the valve seats of the V-1650 at an unacceptable level. Consequently, the standard 100/150 (1T) grade fuel was reverted to by the end of March 1945. ^{15 16}   The Eighth Air Force also had hoped to supply the 352nd and 361st Fighter Groups based on the continent with 100/150 grade fuel. This was deemed impractical from a logistical viewpoint, although admittedly such difficulties did not prevent the RAF's 2nd TAF from being supplied with 100/150 grade fuel. ¹⁷

Technical Operations, Eighth Air Force issued a 4 April 1945 Memorandum in which 100/150 grade fuel experience in the Eighth Air Force was summarized. It is reproduced in full below:

            1.   The following is a summary of 100/150 grade fuel experience in Eighth Air Force.

            2.   a.   This fuel was first service tested by Technical Operations Section, this headquarters, in October 1943, said service test lasting through until March 1944, at which time it was recommended that if extra performance from P-38, P-47 and P-51 aircraft was desired it could be secured by the use of this fuel. It was pointed out at that time that the only apparent deleterious effect of this fuel on any one of the three types was the extra lead fouling of spark plugs.

                  b.   A decision was made in May 1944 to have all fighter units supplied with this fuel no later than 1 June. As of that date operations with this fuel continued until approximately 1 February 1945 when all fighter units switched to “Pep” (100/150 plus 1.5 T’s ethylene dibromide). As of 1 April 1945 all units switched back to 100/150 fuel containing 1.0 T ethylene dibromide.

            3.   At the time the 150 grade fuel was first used all three fighter types listed above were in operational use by this Air Force. Shortly after June 1 P-38 units were re-equipped with P-51 type aircraft so that experience with 150 grade fuel in P-38 aircraft is limited. Gradually, conversion of P-47 outfits to P-51’s took place during the Summer and Fall of 1944, and as of approximately 1 November only one P-47 group remained in this Air Force.

            4.   Maintenance difficulties can be summarized as follows:

                  a.   P-38 (V-1710 Engine).

                       Spark plug leading was increased. The extent of this leading was such that plug change was required after approximately 15 hours flying. This conditions was aggravated considerably by low cruising powers used to and from target areas, while trying to get the maximum range possible. It was found, however, that regular periods of high power running for a minute of two in most cases smoothed out any rough running engines unless the cause was other than leading.

                 b.    P-47 (R-2800 Engine).

                       Spark plug fouling was the only maintenance difficulty encountered during the period in which 150 grade fuel was used. Spark plug life was reduced by about 50%, the same low power cruising as described above being the principle cause for the extra fouling. No deleterious effects on diaphragms, fuel hose or any other rubber of synthetic rubber materials were noted.

                  c.    P-51 (V-1650 Engines).

                       The same type of lead fouling as described in a and b above happened in the case of the P-51 except that is was probably more serious than in either of the other two types. Using 130 grade fuel with 4½ cc. of lead, the average operational P-51 could last 5 missions (roughly 25 hours) before the fouling required plug change. With 150 grade fuel containing 6 cc. of lead, 10 to 12 hours, or normally 2 missions, was the average length of time between spark plug changes or cleaning. At various times in the six months of operation of P-51 aircraft on 150 grade fuel many other maintenance difficulties were attributed to the fuel, but final analysis proved that the only real effect of the fuel was the lead fouling. Some units maintained that they had some deteriorations of seals, but this was not borne our throughout the command, nor was there any concrete evidence that it existed in the units.

                       The excessive fouling of spark plugs usually exhibited itself in roughing up of engines after a couple of hours of low power cruising. Periodic bursts of high power in most cases smoothed the engine out. However, if the engine was allowed to go too long a period without being cleaned out, the accumulation of lead bromide globules successfully withstood any attempts to blow them out. In some instances, long periods of idling while waiting for take-off and a failure to use high power on take off resulted in loss of power during take-off run and in some cases caused complete cutting out with subsequent belly landing. The cases of cutting-out on take-off definitely attributed to excessive fouling were comparatively few, although numerous enough to list it as an effect of the extra lead.

                       As a result of several months operational use with the fuel, an SOP – designed to reduce power failures on take-off, leading troubles in flight, and other things which were causing early returns and abortive aircraft – was published. This is inclosure no. 1. Almost immediately after this section published this SOP practically all of the troubles then existing ceased, although it was necessary to change plugs after each two missions or thereabouts.

                      In an effort to reduce the lead fouling, tests were conducted by this section with 150 grade fuel containing 1.5 T’s of ethylene dibromide. A total of about 120 hours was run by this section and the three squadrons given the “Pep” fuel for accelerated service tests. The results of these service tests showed a considerable reduction in lead fouling with no apparent effects otherwise. As a results, all fighter units of the Air Force were put on Pep fuel late in January 1945. About thirty days thereafter a sharp increase in valve trouble was experienced with the V-1650 engine. Inspection of engines at overhaul revealed that the hydrobromic acid was eroding the silchrome valve seat inserts to such an extent that after approximately 100 hours of operation all the valve clearance was gone. This 100-hours is the minimum life some engines going 170 to 180 hours before this condition prevailed. There are no other deleterious effects of this fuel noted. As of 1 April 1945 fighter units of the Air Force returned to the use of 100/150 grade fuel containing 1.0 T of ethylene dibromide. ¹⁸