North American P-51By Joe Baugher
Testing of the two was rather slow at first, almost as if the Army didn't really want to bother with these airplanes and that they were some sort of nuisance. Some authors have suggested that there were dark and evil motives behind the Army's reluctance to test the ships; however, the slow pace can probably be blamed more on bureaucratic inertia than on anything all that sinister.
At that time, Wright Field was overloaded with test programs, with the Lockheed's P-38 Lightning, Bell's P-39 Airacobra, and Republic's P-47 Thunderbolt being thought to meet all the Army's requirements for fighter aircraft. Nevertheless, once testing of the XP-51s did get under way, the Army's test pilots reported very favorably on their performances.
Despite high scores in the tests, strangely no Army orders were forthcoming. Much later, a Senate Special Committee to Investigate the National Defense Program (the "Truman Committee") looked into the system under which military production contracts were awarded during wartime conditions. They sought specifically for the reason why the Army had sat on its hands for so long before ordering any examples of the P-51, an airplane with demonstrably superior performance.
A-36 Apache, Invader, Mustang
The A-36 seems to have been known by several different namesit was initially called Apache, which was the AAC name initially assigned to the P-51, but there was an effort to change the name to Invader. However, the name Mustang was generally applied by most people to the A-36.
The A-36A differed from previous Mustang versions in having a set of hydraulically-operated perforated door-type dive brakes mounted at approximately mid-chord on both the upper and lower wing surfaces outboard of the wing guns. The brakes were normally recessed into the wings, but were opened to 90&37; by a hydraulic jack to hold diving speeds down to 250 mph. A rack was fitted under each wing for a 500# bombs, a 75-gallon drop tank, or smoke-curtain equipment. It carried six .50 machine guns (two in lower fuselage nose, four in wings); however, the two nose guns were often omitted in service. Wing guns were closer to the main landing gear strut in order to minimize stress during taxi and take-off. Engine was the 1325hp Allison V-1710-87 (F21R). With the bombs, range and service ceiling were 550 miles and 25,100'.
The first A-36A flew on Sep 21, 1942 and deliveries were completed by the following March to the 27th and 86th Fighter Bomber Groups (FBG) based in Sicily and Italy. They initially were painted in olive-drab and light-gray finish with yellow wing bands and yellow circles around the national insignia. Both Groups arrived in North Africa in Apr 1943 just after the end of the Tunisian campaign. They saw their first action during aerial attacks on the island of Pantelleria, with the first sortie being flown on June 6, 1943. The A-36A was involved in the taking of Monte Cassino and participated in the sinking of the Italian liner Conte di Savoia. The only other A-36 user was the 311th FBG based in India and saw extensive use in the CBI theatre
The A-36 did not see much air-to-air combat since it was optimized for low-altitude operations and lost its effectiveness above 10,000'. It was generally believed that an A-36 Invader was no match for a Messerschmitt Bf.109 at high altitudes, and that it was therefore best for A-36 pilots to avoid such encounters or, if combat was unavoidable, to force the battle down to altitudes below 8000', where advantage could be taken of the A-36A's excellent low-altitude performance. Although not a fighter, the Invader claimed 101 enemy aircraft destroyed in aerial combat. One pilot of the 27th FBG, Lt Michael Russo, became the only ace in the Allison-powered Mustang, although several other of his colleagues scored victories, as well.
F-6A (P-51) Mustang
The F-6As went to Peterson Field in Colorado where they were assigned to the newly-established aerial recon school. In Mar 1943 a batch of 25 F-6As were assigned to the 154th Observation Sqdn at Oujda in French Morocco as the first US Mustang unit. Their first mission there was a photo coverage of Kairouan airfield in Tunisia on Apr 10, 1943, which was also the first AAF Mustang mission of WW2. RAF 225 Sqdn frequently borrowed Mustangs from the 154th to augment its shorter-range Spitfires. The F-6A was quite successful in operation, but it did have one important defectits shape was similar to that of the Bf.109. The 154th's first combat loss was from friendly in which Allied anti-aircraft teams failed to recognize the differences, with fatal results.
Two F-6A/P-51 airframes were diverted to the XP-78 project, about which more will be said.
These aircraft had the same external stores capability as the A-36A Invader, but had no dive brakes or fuselage guns, armament being limited to four .50 machine guns in the wings. The inboard pair had 350 rpg and the outboard pair had 280 rpg. An underwing load consisted of two 250#, 325#, or 500# bombs. Maximum take-off weight rose to 10,600#, with maximum ferry range at 2350 miles. The P-51A had the 1200hp Allison V-1710-81 (F20R), with significantly better high-altitude performance than the V-1710-39, with a new supercharger that further enhanced low-altitude performance. In addition a larger-diameter propeller was fitted. Maximum airspeed rose to 409 mph at 11,000 feet, faster at medium altitudes than any other fighter then in service.
Because of the thin wing cross-section, wing guns lay almost on their sides and ammo belt feeds had to be built with some rather sharp kinks in them in order to feed the bullets. This awkward arrangement resulted in many gun jams, particularly after high-G maneuvers.
Fifty P-51As went to the RAF late in 1942 as Mustang IIs (they replaced the NA-91s that had been diverted from Mustang IA orders for conversion as AAF F-6As) [FR890/FR939]. [FR901] was fitted with special deep-section fuel tanks beneath the wings for ultra-long-range flying.[FR893] was tested and demonstrated a best rate-of-climb of 3800 fpm with an altitude of 20,000' reached in 6.9 minutes and 34,000 feet in 24 minutes. Mustang I, IA, and II had impressively long service with the RAF, with the last front-line RAF Allison-powered ships being phased out in early 1945.
Management was intrigued by the idea and requested three Mustangs to test with Merlins. Various Merlins were studied, including the single-stage Mk XX and the two-stage Mk 61the two-stage Merlin was the better choice because of its superior high-altitude performance. Its crankshaft was geared to two supercharger blowers stacked in series. Because of rapid compression, temperature of the air after it passed through both stages increased by 200° C. To lower this temperature and thus increase the air mass flow to the engine, an intercooler was added, requiring an extra radiator underneath the nose, in the same duct as the ram inlet for the updraft carburetor.
Initially, three Mustang Is were allocated to the program on Aug 12, 1942 and two more later [AL963, AL975, AM121, AM203, AM208]. As Mustang X, no two were exactly alike, but all had small chin-type radiators under the engine and four-bladed props to absorb the extra power of a Merlin 65, which in comparison with the Merlin 66 had a lower full-throttle height but gave higher power at lower altitudes. Compared the Allison V-1710, it was 205hp more powerful at 20,000'and 490hp more powerful at 25,000'.
The first Mustang X [AL975] took wing on Oct 12, 1942, piloted by Capt R T Shepherd, [AL963] followed on Nov 13, with [AM121] on Dec 13 going to Duxford for service evaluation. The remaining two were evaluated by the AAF in US markings. The Mustang Xs were busy the rest of the war testing various later marks of the Merlin engine.
The performance of those aircraft was excellent, with speeds of 433 mph at 22,000'; however, yaw stability was degraded by the increased side area of the nose. But success of the tests led Rolls-Royce to propose production of 500 Merlin 65s to bring most of the RAF's Mustang fleet up to Mark X standards, but there was no place where such conversions could be done and plans were dropped.
NAA selected two P-51s from the batch of RAF Mustang IAs repossessed by AAF [41-37350] and [41-37421] and gave the project company designation NA-101. That designation was changed to XP-51B while work progressed. Although the early work of Rolls-Royce of Mustangs and Merlins provided valuable insight to North American engineers, the British manufacturer did not directly participate in the project any further.
Carburetor air intake was moved to below the nose in order to accommodate the Merlin's updraft induction system. The intercooler radiator was added to the radiator group already located inside the scoop under the rear fuselage, and the ventral radiator group was made noticeably deeper than before and had a sharp-angled inlet standing more than two inches away from the underside of the fuselage. Instead of the oil cooler being situated in the center of a circular coolant radiator, it was relocated to the front of the duct and provided with its own ventral exit door. Further downstream, in a greatly enlarged duct, was the huge rectangular coolant matrix with a much bigger exit door at the rear.
Airframes were strengthened to make full use of the increased power available. New ailerons were fitted and the underwing racks were increased in capacity to take two 1000# bombs or their equivalent weight in drop tanks. A new four-blade Hamilton Standard hydromatic paddle-blade propeller was fitted. Fuselage-mounted guns were eliminated in favor of four 0.50s exclusively in the wings.
The first XP-51B was flown by Bob Chilton on Nov 30, 1942, initially without armament. Performance improvement was nothing short of astoundinga level speed of 441 mph at 29,800' was more than 100 mph faster than the Allison P-51 at that altitude, and at all heights the rate of climb about doubled!
P-51B/C (F-6B/C) Mustang
In late 1942 a deal between Britain and the USA was made in which Spitfire VBs would be transferred to the 8th AF in England, mainly for use as fighter-trainers. This cleared the way for Lend-Lease supplies with the USAAF P-51B/C as Mustang III. The RAF ultimately received 274 P-51Bs and 626 P-51Cs, and 59 Mustang IIIs were diverted to the Royal Australian AF and other Allied air arms.
As 1943 dawned, the Mustang program suddenly expanded. Massive production of Merlin engines was to take place at both Packard at Detroit and Continental at Muskegon IL. The huge Inglewood factory was expanded and dedicated solely to P-51 production, with the B-25 program transferred to Kansas City. Production of AT-6 trainers had earlier been transferred from Inglewood to a new plant built in great haste at Dallas TX. NAA even expand the Dallas plant further as a second source for Mustangs. Inglewood-built planes were designated P-51B, Dallas-built Mustangs were P-51C. The aircraft were almost identical and generally distinguished only by serial number.
By the end of Jan 1943 production standard for the P-51B/C had been decided. To take full advantage of the additional power, the airframe was re-stressed in detail and the aircraft was made capable of operating at considerably greater weights than was previously possible. Wing racks were modified to carry bombs of 1000# each or a range of other stores, including drop tanks or triple rocket tubes.
Engine installation was further refined with a rectangular filtered-air inlet added in each side of the carburetor duct, and the exhaust expelled through individual ejector stubs projecting through a slim fairing. Ailerons were modified aerodynamically and structurally, although the changes were visible externally only by the fact that the tabs were made of plastic. Armament was four .50 Browning MG53-2 guns in the wings, with 350 rounds for each inner gun and 280 rounds for each outer gun. Fuselage nose guns were deleted.
The first P-51B flew on May 5, 1943 and the first P-51C on Aug 5. Inglewood built 1,988 P-51Bs and Dallas built 1,750 P-51Cs. P-51Cs on the 1942-43 budgets were given the company designation NA-1031,350 NA-103s were built. Texas-built aircraft in the 1944 budget were designated NA-111.
During P-51B/C production it was also decided to omit the olive drab camouflage and to deliver aircraft in their natural metal finish. The objective was not only to save extra cost, weight, and drag, but to try and coax the Luftwaffe into battle, not hide from it.
The first combat unit equipped with Merlin-powered Mustangs was the 354th Fighter Group (FG), which reached England in Oct 1943. The 354th consisted of the 353rd, 355th and 356th Fighter Sqdns and was part of the 9th AF, which had the responsibility of air-to-ground attacks in support of the upcoming invasion. However, they were instead ordered to support bomber operations of the 8th AF. The 354th flew their first cross-Channel sweep mission on Dec 1, 1943 and scored a first victory Dec 16 during a Bremen mission. However, inexperienced pilots and ground crews and numerous technical problems limited operations with the P-51B/C until about eight weeks into 1944, when from early spring of that year the Merlin Mustang became an important fighter in the ETO.
The 15th AF was formed in Nov 1943 with three P-38 groups based in the Mediterranean theatre to escort Allied bombers. During April 1944 Merlin Mustangs began replacing Spitfires of the 31st and 52nd FGs, which transferred from the 12th to the 15th AF. The 31st flew its first mission on Apr 21, 1944 when its Mustangs escorted B-24s in an attacking Romanian oil refineries at Ploesti.
Merlin-powered Mustangs also entered service in the CBI theatre in Sep 1943, assigned to the 23rd and 51st FGs of the 5th AF. Early in 1944 the 311th FG of the 10th AF saw action in Burma with its Mustangs supporting airborne troops attacking Japanese lines of communication. The top Mustang ace of the CBI theatre was Maj John C "Pappy" Herbst, with 18 victories. About 100 P-51B/Cs were also supplied to the Chinese AF in 1943-44.
Continue to XP-51D, P-51D, P-51K, XP-51F, XP-51G, XP-51J, P-51H, P-51M