[2.0] The Merlin Mustangs

v1.2.8 / chapter 2 of 3 / 01 feb 17 / greg goebel

* The Allison-powered Mustangs were good aircraft, and would have an honorable place in the books even if the design had gone no further. However, mated with the British Rolls-Royce Merlin engine, the Mustang became a great aircraft.

North American P-51D Mustang

[2.2] P-51B, P-51C (MUSTANG III)
[2.3] P-51D, P-51K (MUSTANG IV)
[2.4] P-51D: SUMMING UP
[2.5] XP-51F, XP-51G, XP-51J, P-51H, P-51L, P-51M


* The poor high-altitude performance of Allison Mustang was a concern from the start, but nobody was sure what to do about it. The Allison was about all that was available to NAA for the first production batches of the Mustang. NAA engineers considered moving to a turbocharged version of the Allison, which was used successfully on the Lockheed P-38 Lightning twin-engined fighter, but decided against it.

The American engine manufacturer Packard had come to an agreement with Rolls-Royce in October 1942 to build the Rolls-Royce "Merlin XX" engine, which was used in the Supermarine Spitfire V. The name "Merlin", by the way, was not in honor of the magician, but was the English name for a species of hawk. Like the Allison, the Merlin was a 12-cylinder liquid-cooled inline vee engine, but had a better power-to-weight ratio.

By 1942, Packard was producing Merlin XX engines in quantity as the Packard V-1650-1, but supplies were limited, and there were valid doubts that the Merlin XX was a big enough improvement over the Allison to make converting to the new engine worthwhile. The Merlin didn't offer much more power and only had a two-speed supercharger; in fact, the P-40 would be converted from Allison to Merlin power, to show no great improvement in performance.

Rolls-Royce had been continuously improving the Merlin, so much so that the company began to regard further work on the engine as yielding diminishing returns, with the future lying in the more formidable next-generation "Griffon" engine. Despite that, in mid-1941 the RAF issued a requirement for a high-altitude bomber, the "Wellington VI", and so Rolls-Royce began work on a Merlin variant optimized for high-altitude operation.

Rolls-Royce engineers rejected a turbocharger as too big and bulky -- that was the main problem in fitting a turbocharged Allison to the Mustang -- and experimented with a two-stage supercharger scheme, with the two superchargers mounted on a common shaft and driven off the engine crankshaft. The compressed inlet air was heated to such a degree that the engine required an intermediate intercooler system to cool it off. The Wellington VI was never built in large quantity or saw wide service, but the new Merlin engine showed obvious promise for the Spitfire, and was developed for that aircraft as the "Merlin 61".

A new Spitfire variant, the "Spitfire Mark VIII", was designed for the two-stage engine. However, the RAF was confronted with rising numbers of Luftwaffe Focke-Wulf Fw 190s that outclassed the current Spitfire V, and needed a better Spitfire in a hurry. As an interim measure, Supermarine quickly modified the Spitfire V to accept the Merlin 61, resulting in the "Spitfire IX". The Spitfire IX would prove so successful that it would be produced in greater numbers than the Spitfire VIII.

* In April 1942, Rolls-Royce test pilot Ronald Harker visited the RAF Air Fighting Development Unit field at Duxford, and was invited to try out a Mustang. He flew the aircraft for a half-hour and was extremely impressed with it, until he ran into its poor high-altitude performance. He realized that a two-stage Merlin engine was exactly what was needed, and consulted with the Rolls-Royce chief engineer to see what the British engine would do for the American fighter. Studies for a Mustang with a Merlin engine suggested a top speed of 710 KPH (440 MPH) at an altitude of 7.8 kilometers (26,000 feet), well in excess of the capabilities of an Allison Mustang, and possibly even superior to the Spitfire IX in some respects.

Harker pressed his case through the air establishment, and within days Rolls-Royce's facility at Hucknall began conversion of three Mustangs to two-stage Merlin power, with two more conversions put in the queue later. These aircraft were fitted with the Merlin 65 engine subvariant and a four-bladed propeller. The Merlin 65 was optimized for better performance at lower altitude, though unlike the Allison its performance did not drastically fall off at high altitude.

The Merlin conversion was very promising, but the director of the Hucknall establishment, Ray Dorey, had an even more ambitious idea. He wanted to mate the Mustang to a Griffon engine, mounted behind the pilot as was the Allison in the Bell P-39 Airacobra. Rolls-Royce engineers believed this aircraft would be capable of a top speed of 800 KPH (500 MPH), but it never progressed beyond the mock-up stage.

* Back in California, while these events were taking place in England, NAA engineers were working along a similar path. They had known about the two-stage Merlin engine since late 1941, and had been considering how to use it with the Mustang.

Packard had negotiated with Rolls-Royce to build a licensed version of a two-stage Merlins, and was moving into production of the Packard V-1650-3, equivalent to the Merlin 68 sub-variant. The first V-1650-3 engines were to roll off Packard production lines in mid-December 1942, with production ramping up to full volume in the following months. If NAA engineers wanted to use Merlin power, the engines would be available when they were needed.

NAA didn't start work on the concept in earnest until late July 1942, when the company received the two cannon-armed NA-91s reserved for experimental purposes by the USAAF, mentioned earlier, and Merlin 65 engines sent from England. This step-up in activity appears to have been partly the work of Lieutenant Colonel Tommy Hitchcock, the US Assistant Military Attache at the American embassy in London. He had flown the Mustang, knew about the Rolls-Royce effort at Hucknall, and was enthusiastically promoting the idea through American channels. NAA engineers threw themselves into the project with the same energy that they had shown in rolling out the initial NA-73X. The new variant was given the company designation of "NA-101" and military designation of "XP-78". By the summer of 1942, work on the Merlin Mustang was in full swing on both sides of the Atlantic.

The two groups were aware of each other's activities, and the spirit was competitive but friendly. The Rolls-Royce group had a head start and got into the air first with their "Mustang X", as the conversions were known, with the initial aircraft flying on 13 October 1942. The next day, Packard officials sent a letter to Rolls-Royce congratulating them on "beaten us to it on the flight of the [Merlin] Mustang. Hope performance is up to expectations."

The Rolls-Royce conversions were experimental improvisations. The conversions featured a deep chin scoop underneath the prop spinner that was faintly reminiscent of late-model P-40s, and gave them a unique appearance among the Mustang family. The first Mustang X was originally fitted with a standard Rotol (Rolls-Bristol) four-bladed propeller, as used by the Spitfire IX. This was later upgraded to a custom-built propeller, though it proved to have little influence on performance.

The performance was nonetheless very satisfactory. The Merlin 65 could provide more horsepower at altitude than the Allison V-1710 did at takeoff, and the initial Mustang X conversion achieved 700 KPH (433 MPH) at an altitude of 6.7 kilometers (22,000 feet). The aircraft could reach an altitude of 6.1 kilometers (20,000 feet) in 6.3 minutes, about two-thirds the time required by an Allison Mustang. Greater power and torque resulted in a degree of lateral instability in the Mustang X. Various fixes for this problem were considered during the evaluation, including a bigger tailfin, but the problem would get worse before it got better.

The second Mustang X flew on 13 November 1942, and the third flew a month later. Rolls-Royce kept NAA informed of the results of the tests while NAA refined their own conversion, which had been redesignated "XP-51B" in the meantime. The first XP-51B flew on 30 November 1942. As with the Mustang X, performance of the XP-51B demonstrated that faith in the Merlin conversion was justified. NAA test pilot Bob Chilton achieved a level speed of 710 KPH (441 MPH) at an altitude of 9 kilometers (29,800 feet), and the XP-51B could climb almost twice as fast as an Allison Mustang.

This first XP-51B was roughly an "80% conversion", but the second, which flew soon afterward, was close to a production design. The Merlin 65 had a similar physical "envelope" to the Allison V-1710, but weighed about 136 kilograms (300 pounds) more, and required fitting the intercooler someplace in the fuselage.

The Merlin Xs had the intercooler under the nose, but that arrangement was cluttered, and after a six-week bout of headaches NAA engineers managed to accommodate it in the radiator system under the cockpit. They also managed to obtain a small amount of thrust out of the radiator exhaust. In any case, the modification resulted in the belly airscoop hanging a bit lower than in the Allison Mustang. This was a slight visible change, as was the slightly fatter nose; more noticeable was the switch of the carburetor intake from above to below the prop spinner, giving the new Mustang version sleeker looks than its predecessor. The new design also featured a four-bladed Hamilton Standard propeller with a span of 3.4 meters (11 feet 2 inches), replacing the 3-bladed propeller used on the Allison Mustangs.

Back in Britain, the two final Mustang Xs would be flown in early 1943 and evaluated by the USAAF, but by that time they were of no critical importance. Even before the first Merlin Mustangs flew, back in August 1942, the USAAF had ordered 400 P-51Bs, gambling that they would prove everything that was hoped for them. Rolls-Royce wanted to convert 500 RAF Allison Mustangs to Merlin power, but the resources were simply not available. The Americans were not under such constraints, and so in early 1943 the RAF ordered 1,000 P-51Bs with the designation of "Mustang III".


[2.2] P-51B, P-51C (MUSTANG III)

* Moving from test to production of the P-51B was a massive job, involving a big expansion of NAA's Inglewood plant. Another huge plant that NAA had recently established in Forth Worth, Texas, to manufacture trainers was scaled up to build even more Merlin Mustangs. Fort Worth aircraft were to be designated "P-51Cs", though they were essentially identical to P-51Bs. Following comments about the P-51B apply to the P-51C, unless otherwise noted. Note that in some documents, Mustang designations may have a "-NA" suffix, indicating that the aircraft was built in Inglewood, or a "-NT" suffix, indicating that it was built in Forth Worth.

The first production P-51B flew on 5 May 1943, with the first production P-51C flying on 5 August 1943. Production P-51Bs were reinforced and could handle a 450-kilogram (1,000-pound) bomb on each wing rack, or equivalent stores such as drop tanks or triple-tube M-8 rocket launchers. There were minor aerodynamic improvements relative to the XP-51B prototypes as well.

Gun armament on most production P-51Bs was four 12.7-millimeter (0.50-caliber) Browning machine guns, with two guns in each wing. The ammunition box for each inboard gun had a capacity of 350 rounds of ammunition, while the box for each outboard gun had a capacity of 280 rounds. The Merlin redesign had crowded out the two nose guns fitted in some versions of the Allison Mustang, and the Mustang would never carry them again.

Initial production of the P-51B used the Packard V-1650-3 engine, with 1,045 kW (1,400 HP) for take-off, while later production was uprated to the V-1650-7 engine, with 1,080 kW (1,450 HP) for take-off. These Merlins used carburetors instead of fuel injection, and of course had a two-stage supercharger. The supercharger operated at two speeds and engaged automatically at high altitude. The engine was limited from redlining with a safety wire, but in an emergency a pilot could push the throttle past the stop, breaking the wire. The engine could be redlined for five minutes without damage.

   _____________________   _________________   _______________________
   spec                    metric              english
   _____________________   _________________   _______________________

   wingspan                11.28 meters        37 feet
   wing area               21.65 sq_meters     233 sq_feet
   length                  9.83 meters         32 feet 3 inches
   height                  4.16 meters         13 feet 8 inches

   empty weight            3,380 kilograms     7,450 pounds
   max loaded weight       5,080 kilograms     11,200 pounds

   maximum speed           708 KPH             440 MPH / 380 KT
   service ceiling         12,770 meters       41,900 feet
   range with drop tanks   2,575 KM            1,600 MI / 1,390 NMI
   _____________________   _________________   _______________________

* P-51Bs finally reached squadron service in October 1943, when the USAAF 354th Fighter Group in England was equipped with the variant. This assignment was something of a "snafu", however, since the 354th was part of the Ninth Air Force, which was focused on ground attack. Apparently the brass failed to get the word that the Merlin Mustangs were a different breed from the ground-attack Allison Mustangs. The USAAF Eighth Air Force had begun daylight raids into Germany in early 1943, but by the fall of that year had been forced by murderous combat losses to give up such missions until long-range escort fighters became available. The Eighth desperately needed the new Mustangs, and so the 354th was immediately ordered to operate in support of Eighth Air Force long-range bomber missions, even though the group remained in the Ninth's chain of command.

P-51Bs began flying fighter sweeps over Europe in early December 1943, and were escorting bombers on raids by the middle of the month. On 13 December, Lieutenant Glenn Eagleston drew first blood with the P-51B, damaging a Messerschmitt Bf 110 during a raid on Kiel. Eagleston would eventually become the top ace of the Ninth Air Force, claiming a total of 10.5 victories.

On 16 December, Lieutenant Charles Gumm claimed the P-51B's first actual kill, knocking a Messerschmitt Bf 110 out of the sky over Bremen. The door was being closed on the days of the Luftwaffe savaging unescorted bomber formations. That same month, the RAF formally received its first Mustang III, flying with RAF Number 65 Squadron, and would quickly equip other RAF squadrons with the new aircraft. RAF Mustang IIIs would be in principle dedicated to ground attack, but they participated in escort duties while the USAAF built up strength in the new fighter.

On 11 January 1944, Major James H. Howard of the 354th was on an escort mission over Germany in his P-51B, named "Ding Hao". Howard was a mild-mannered but highly experienced pilot who had scored 6.5 victories against the Japanese with the American Volunteer Group in China. Howard had become separated from his flight when he saw a group of German fighters attacking a formation of B-17s. Although he was alone, Howard apparently regarded this as a "target-rich environment" instead of a threat, and dived into the fight without hesitation. He quickly shot down three German fighters, and then for the following half hour engaged in duels with the remaining Luftwaffe pilots. By the time the fight was over all his guns had jammed but one. With these kills, he became the first Mustang ace, and was awarded the Congressional Medal of Honor.

By early 1944, other USAAF fighter groups were forming up in England. Two Ninth AF fighter groups, the 357th and 363rd, were equipped with the type, though the 357th FG was traded to the Eighth AF in exchange for the 358th FG, which flew the Republic P-47 Thunderbolt. This exchange was apparently mutually satisfactory, since the Eighth AF needed P-51Bs for long-range fighter escorts, while the Ninth AF could make good use of the big, tough, and more heavily armed P-47 for ground attack.

Eighth AF fighter groups were also receiving new P-51Bs directly, beginning with the elite 4th FG. The 4th was the oldest and most experienced fighter group in the Eighth AF, having been formed out of three RAF "Eagle" squadrons, composed of American pilots in British service. The commander of 4th FG, Colonel Donald J.M. Blakeslee, had been leading 354th FG P-51Bs since their arrival. On his return to his own group, he promised his superiors that his pilots would be operational with the new fighter within 24 hours. They were all flying with the type by the next day, 25 February 1944.

The new Mustang quickly began to appear on other fronts. In February 1944, P-51Bs began flowing into USAAF 14th Air Force units in Burma, and in April, P-51Bs and Mustang IIIs began to arrive at USAAF and RAF squadrons in Italy.

* As P-51Bs flew into combat, the deficiencies of the type were identified and gradually dealt with. There were engine problems, of course. The Merlin 61-series engines were pushing limits and had been pressed into combat as quickly as possible, as had the Mustang itself. Even when the engine worked fine, it took some getting used to for pilots who had flown Allison Mustangs, since the Merlin ran louder and rougher than the Allison, and in particular produced an unnerving THUMP! when the supercharger cut in or out. Working out the bugs and learning the right ways to fly with and maintain the engine took time.

One significant problem was the tendency of the 12.7-millimeter guns to jam. As in earlier Mustangs, the wing machine guns in the P-51B were canted onto their sides, and the sharp angle at which the ammunition fed into the guns could lead to jamming under high-gee maneuvers. This problem would require rethinking the armament scheme.

A particularly alarming problem, to say the least, was the occasional tendency of the wings to fall off during a high-speed dive. Ironically, Tommy Hitchcock, who had done much to promote the Merlin Mustang, was killed over England in a P-51B in this way in April 1944. These structural failures proved to be due to the tendency of wing ammunition doors or the main landing gear to open under high-gee conditions, producing excessive forces on the wing. These problems were eventually resolved.

One long-standing annoyance was the canopy scheme, which gave a poor view to the rear; prevented the pilot from getting a clear view ahead over the long nose in takeoff; could not be opened in normal flight; and was difficult to get out of in an emergency. The RAF came up with their own solution by replacing the three NAA canopy panels with a one-piece bubble "hood", similar to that used on the Spitfire, that slid back to the rear. The new canopy design was implemented by the British firm of R. Malcolm Limited, and consisted of one-piece blown Perspex bubble that could be easily refitted to the Mustang by field maintenance personnel. The "Malcolm hood" was refitted to most RAF Mustang IIIs, and was apparently fitted to some USAAF P-51Bs and even some of the old Allison Mustangs.

Mustang X, P-51B, P-51B with Malcolm Hood

Another problem was that, even with drop tanks, the Mustang still didn't have enough range to escort Eighth AF bombers all the way to Berlin and back. The USAAF had been testing a potential long-range fighter, the Fisher XP-75, that had proven unsatisfactory, and needed something immediately that could do the job. USAAF Colonel Mark Bradley, who had been in charge of testing the XP-75, told NAA's Dutch Kindelberger that there was some empty space in the rear fuselage of the Mustang, and that another fuel tank should to be put there.

NAA engineers devised a 322-liter (85 US gallon) tank that fit between the pilot's seat and the radio. The new tank gave the Mustang the necessary range, solving one problem, if at the expense of creating another. The new fuel tank was added without concern for its effect on the Mustang's center of gravity. With a full fuel load, getting the fighter off the runway was downright dangerous, and the aircraft was only marginally controllable for the first hour or so that it took to drain the tank.

That had to do, there was a war on and something was needed right away, so the third tank was fitted to late-production P-51Bs. The British also made their own contribution to the range problem in the form of a new drop tank that accommodated 409 liters (108 US gallons) and which was made of plastic-impregnated paper. That sounds a little crazy, particularly since the fuel would rot the tank if left in it for more than eight hours. However, the paper drop tanks were perfectly effective, and were lighter and cheaper than metal drop tanks. Dropping them over German territory also did not provide the enemy with aluminum they could scavenge for their own war effort. Production rates of the paper drop tanks eventually reached 24,000 a month.

A 416-liter (110 US gallon) drop tank was also built, plus a 568-liter (150 US gallon) ferry tank. Information on these tanks is sketchy, but the ferry tank looked like a tub whose upper rim fit up against the wing, and so it appears it could not be dropped in flight.

The problems with misidentification with the Messerschmitt Bf 109 continued. White stripes were painted on the nose, wings, and tail, but with only limited effectiveness. On 10 February 1944, for example, Glenn Eagleston was shot up by a P-47 over Germany and only barely managed to make it back to England to bail out. Before the year was out, the problem would be reduced by the simple measure of delivering Mustangs to England in natural metal finish. The USAAF wanted to take on the Luftwaffe, not hide from them, and eliminating the paint actually reduced aircraft weight and slightly improved performance. RAF Mustangs would continue to be painted in camouflage until near the end of the war.

North American P-51C Mustang

* While the bugs were worked out of the P-51B and its range extended, the USAAF and RAF built up new squadrons with the aircraft and put them to use. Using the new third fuel tank and drop tanks, on 6 March 1944, P-51Bs escorted Eighth AF B-17 Flying Fortresses all the way to Berlin for the first time.

This is a somewhat simple-minded description of the action that gives the impression that the Mustangs joined the bomber formations after takeoff from England and stayed with them through the entire raid. In fact, for escort missions, a number of fighter formations were sent to staged rendezvous points to join the bombers at specific intervals, with shorter-range aircraft like P-47s providing escort cover at relatively short ranges and the long-range P-51Bs handling the most remote legs of the mission.

On the 6 March Berlin raid, the P-51Bs got into a massed dogfight that a Fortress pilot called "one of the damnedest dogfights of the war. It was off to the right of us as we headed east and several thousand feet higher -- about one and a half miles away. It looked like a giant swarm of bees -- P-51s, Me 109s, Fw 190s." This mission was not a complete success, since B-17 losses were heavy, but the USAAF would be back and would learn. Early on, bomber crews sometimes accused the fighter jocks of wanting to dogfight for the glory instead of protecting the bombers, and it took time for the bombers and fighters to learn to work in better coordination.

The logistics of long-range escort missions were difficult in any case. Getting a Mustang with drop tanks and a full fuselage tank off the runway was fearsome, particularly since the fighter groups didn't have the luxury of waiting for one aircraft to clear the runway before sending another out. The result was sometimes a hideous fireball on the runway, followed by an even more disastrous pile-up that incinerated men and machines. Once airborne, P-51B pilots had to perform a tricky exercise in fuel management before they entered combat. Initially, USAAF procedure was to drain the fuselage tank before switching to the drop tanks, but this led to Mustang pilots discarding full drop tanks when they were forced to combat prematurely. The USAAF then reversed the procedure, instructing pilots to drain the drop tanks first. That was not entirely satisfactory either, since the Mustang was very hard to fly with a loaded fuselage tank, and ultimately to get out of this dilemma the USAAF was forced to limit the amount of fuel pumped into the fuselage tank to 246 liters (65 US gallons).

Despite these problems, the P-51B was proving extremely effective in air combat. One of the early stars was Don S. Gentile (pronounced "Jentilly") of the 4th FG's 336 Fighter Squadron. Flying his P-51B, "Shangri-La", Gentile was able to claim a total of 16 victories in March and early April 1944, all but one of them against Luftwaffe single-engine fighters. On 11 April 1944, Gentile was awarded the Distinguished Service Cross by General Dwight D. Eisenhower for his actions. Two days later, Gentile flew his Mustang into the ground while buzzing his home base. Gentile walked away unharmed, but "Shangri-La" was a total loss.

The RAF continued to use their new Mustang IIIs in the close-support role. Racks for eight RPs were evaluated again, and while they once more imposed a performance penalty, the Merlin Mustang's higher performance made it more tolerable. Such rocket-firing Mustang IIIs were used in Italy.

* By late spring 1944, NAA was moving P-51 production to the next variant, the "P-51D", which would become the most heavily produced and best known of all the Mustangs. It was still the P-51B that actually led the USAAF challenge the Luftwaffe on its home ground. The P-51B was the most numerous Mustang in service well after D-Day, and by VE day was still fighting in large numbers. Some pilots who flew the P-51B regarded it as the best of the Mustangs.

A total of 3,738 P-51Bs and P-51Cs was built. The RAF received 910 Mustang IIIs under Lend-Lease. 91 of the USAAF total were fitted as reconnaissance aircraft, with two cameras installed, one ahead of the tailwheel and the other staring out the left side of the aircraft. This variant was designated "F-6C". The F-6C retained the four machine guns of the standard P-51B and often engaged in air combat. One USAAF F-6 pilot, Captain Clyde East, would become an ace, with 15 confirmed kills.

two-seat P-51B conversions

Some war-weary P-51Bs were converted by ground crews into two-seaters, for use as trainers or "hacks". These conversions could be extremely clean, with a second "razorback" canopy inserted into the rear fuselage immediately behind the first.


[2.3] P-51D, P-51K (MUSTANG IV)

* NAA was quick to recognize the deficiencies of the P-51B and come up with a new version to fix most of them.

Although the RAF remedied the poor rear view of the razorback Mustang with the excellent Malcolm hood, NAA decided on a different approach, though it was derived from British work as well. Colonel Mark Bradley had visited England in early 1943, and had seen the "bubble" canopies that the British had developed for the Spitfire and Typhoon.

The bubble canopies gave a pilot unobstructed all-round vision. Bradley returned to the US and pushed to get bubble canopies fitted onto American fighters. Republic quickly put one on a P-47, and Bradley flew the modified Thunderbolt to California to show it to NAA's Dutch Kindelberger. NAA very quickly came up with a plan to cut back the rear fuselage of the Mustang and mount a large bubble canopy over the cockpit. The canopy would slide open back over the rear fuselage. Two P-51Bs were pulled from the assembly line as proof-of-concept demonstrators, and the first bubble-top "XP-51D" (company designation "NA-106") took to the air on 17 November 1943.

The four-gun armament of the P-51B was also unsatisfactory -- not merely because of its light firepower, but because of the previously-mentioned tendency to jam due to the guns' installation at an angle. NAA engineers took a good look at the problem, and came up with a much better solution for the P-51D. The new armament fit provided three 12.7-millimeter machine guns in each wing, and allowed them to stand upright. The two outer machine guns were mounted side by side. The ammunition boxes for these two weapons were stacked in the wing outboard of the weapons, with the box for the outer gun on the bottom and the box for the middle gun on the top. The ammunition feed for the middle gun arched over the outer gun. Each of these boxes could store 270 rounds of ammunition.

The inner machine gun was set back of the two other guns, and was fed from an ammunition box stowed directly behind the boxes for the outboard guns. This box could store 400 rounds of ammunition. The middle gun could be removed to allow the ammunition store for the outer gun to be increased to 400 rounds. This new armament fit helped greatly reduce jamming. Eventually, production P-51Ds would also be fitted with an ammunition belt power feed system that reduced the incidence of jamming still further.

Another improvement did much to increase the P-51D's lethality. Once again British engineering made a considerable contribution to the Mustang, this time in the form of the British Ferranti "GGS Mark IID" computing gyro sight, replacing the relatively simple Bell & Howell gunsights used to this time. The Ferranti sight, manufactured under license in the US as the "K-14", allowed the pilot to dial in the target wingspan and target range, and then told the pilot when he had a good shot at the target. The K-14 was regarded as a marvel of technology and greatly increased the pilot's accuracy in deflection shots. Later P-51D production would also feature AN/APS-13 tail warning radar, with rod antennas sprouting from the tailfin, to give warning of attack from the rear.

Traditionally, Mustangs had two pylons for bombs or drop tanks, but late production P-51Ds also had streamlined stubs for potent 12.7-centimeter (5-inch) "High Velocity Air Rockets (HVAR)". These rocket-firing Mustangs could carry ten HVARs, or six HVARs plus two bombs or two drop tanks.

Initial production P-51Ds used the Packard V-1650-3 engine, but late production would be uprated to the improved V-1650-7 engine. The P-51D used a four-bladed Hamilton Standard propeller, with some variation in propeller design in the course of production.

   _____________________   _________________   _______________________
   spec                    metric              english
   _____________________   _________________   _______________________

   wingspan                11.28 meters        37 feet
   wing area               21.65 sq_meters     233 sq_feet
   length                  9.83 meters         32 feet 3 inches
   height                  4.16 meters         13 feet 8 inches

   empty weight            3,466 kilograms     7,635 pounds
   max loaded weight       5,493 kilograms     12,100 pounds

   maximum speed           700 KPH             435 MPH / 380 KT
   service ceiling         12,770 meters       41,900 feet
   range with drop tanks   2,655 KM            1,650 MI / 1,435 NMI
   _____________________   _________________   _______________________

Other changes included a wing with slightly greater chord at the root, plus stronger main gear to handle the aircraft's increased weight. One significant change was the addition of a dorsal fin fillet to the tailfin, added after initial P-51D production. The fillet compensated for yaw instability problems that had become increasingly troublesome with the introduction of the Merlin engine, the addition of the fuselage fuel tank, and the cut-back of the rear fuselage.

By May 1944, P-51Ds were flowing from the production line in Inglewood to combat units in England, starting with the 4th FG. NAA's Fort Worth plant was also producing the version in quantity, though that production also included a variant named the "P-51K" whose only significant difference from the P-51D was a slightly smaller Aeroproducts propeller. This change was stipulated by the USAAF, which was concerned about the supply of the Hamilton Standard propeller. The Aeroproducts propeller was also lighter, but apparently was not completely satisfactory and production reverted to the Hamilton Standard propeller. Unless otherwise noted, all following comments about the P-51D also apply to the P-51K.

The P-51D and P-51K were provided to the RAF, which named them the "Mustang IV" and "Mustang IVA" respectively. However, the RAF had no desperate need for the new Mustang and didn't begin to put it into service until September 1944. The USAAF also flew reconnaissance versions of the P-51D and P-51K, named "F-6D" and "F-6K" respectively. The reconnaissance versions featured a camera fit much like that used in the F-6A, with two cameras just ahead of the tailwheel, and retained armament. Some P-51Ds would be converted to the reconnaissance configuration, and were known as "FP-51Ds".

* The P-51D still had its problems. In fact, as its increased armament and other changes made it somewhat heavier than the P-51B, it had slightly inferior climb performance relative to the P-51B and was slightly slower as well -- which is likely why some pilots thought the P-51B was the better aircraft. Like the P-51B, the P-51D was difficult to handle with a full fuselage tank, and it had high stick forces under combat maneuvers. Worst of all, under such maneuvers it gave no warning of stall and could fly abruptly and wildly out of control. It was also difficult to bail out of, since air pressure tended to trap the pilot in his seat, and Mustang pilots had to learn the Luftwaffe trick of turning the aircraft over and falling out. However, the aircraft's virtues were such that these vices were forgiveable, though not forgettable.

With a better field of view, greater firepower, and an improved gunsight the P-51D was a match for the Luftwaffe's piston-engined fighters. USAAF pilots were also assisted in air combat by another innovation, the "G-suit", which guarded against pilot "blackout" during high-speed turns. The G-suit worked by applying pressure when needed to the pilot's abdomen and legs, preventing blood from pooling in the lower body. The British developed a hydraulic G-suit, known as the "Frank", but the Americans preferred their own, less cumbersome pneumatic G-suit, known as the "Berger". The Berger G-suit was available in quantity by June 1944, and put to good use by Mustang pilots.

* With the P-51D, the USAAF achieved the decisive air superiority over the Reich that the P-51B had promised. Pressured by a war on two fronts and harassed from the air, Luftwaffe resistance began to crumble, and the USAAF cruised through the sky against fading opposition. Mustangs escorted the bombers to their targets and then strafed airfields on the way home, coming in at high speed just above the grass and in single line abreast to take the defenders by surprise.

Many USAAF pilots became "aces in a day" in the P-51D. On 12 October 1944, Lieutenant "Chuck" Yeager, at the start of a famous career, shot down five Messerschmitt Bf 109s on a single mission. Similarly, Captain Bill Wisner destroyed six Focke-Wulf Fw 190s on an escort mission on 21 November 1944, while Lieutenant Claude Crenshaw shot down five Fw 190s during the same escort mission. On 30 August 1944, Major George Preddy was leading his flight to escort a bomber raid over Berlin when he shot down six Bf 109s. Preddy would become the highest-scoring Mustang ace, with a total of 24 victories. He was killed by "friendly" anti-aircraft fire on Christmas Day, 1944.

The Germans were running out of fuel, aircraft, and experienced pilots. Even the new jet fighters like the Messerschmitt Me 262 did them little good, since Allied fighters like the Mustang seemed to be everywhere all the time. While the Me 262 could outpace the Mustang in high-altitude combat, the Mustang could out-turn the jet easily, making it no disastrous threat to an alert pilot, and could outlast the Messerschmitt, which had limited endurance. When a Me 262 came in for a landing, it was often bounced by lurking Mustangs. On 7 October 1944, USAAF Lieutenant Urban Drew dived on two indistinct German fighters and shot them both down before he realized he had taken on a pair of Me 262s. On 4 November 1944, Walter Nowotny, the fifth-ranking German ace, with 258 kills, was attacked by P-51Ds as he approached the runway, and flew into the ground while taking evasive action. Mustangs destroyed three other Me 262s that day.

* In the meantime, the RAF had been using their Mustang IVs to combat a different type of jet aircraft, the German V-1 "flying bomb", or "Doodlebug". After the V-1 attacks began in early summer, the RAF set up patrols of fast aircraft like the Typhoon, Spitfire, and Mustang to intercept them. These fighters would circle near the coast and were directed to a V-1 by a radar ground-control station. Intercepting the Doodlebug bombs could be tricky, since they were small and fast. Mustangs still accounted for hundreds of these weapons before the missile launch sites were overrun by the Allies in the early fall of 1944.

* The P-51Ds saw extensive service outside of Northern Europe as well. The Fifteenth Air Force in Italy featured several fighter groups flying the Mustang, in particular the 332nd FG. This fighter group consisted entirely of black American pilots, known as the "Tuskegee Airmen", or "Red-Tails" for the paint job on their fighters.

Fighter groups of the Fourteenth Air Force in the CBI had been flying P-51Bs but upgraded in part to P-51Ds in 1945. CBI P-51Ds were often field-modified for unusual stores configurations, such as two drop tanks and six 45-kilogram (100-pound) bombs; or four drop tanks; or 45-kilogram bombs and 36 small fragmentation bombs. They were also usually fitted with direction-finding loop antennas ahead of the tailfin for navigation on long missions.

Fifth Air Force Mustangs began to operate out of the Philippines in December 1944. One of the Fifth's pilots, Major William A. Shomo, was flying an F-6D over northern Luzon on 11 January 1945, when he ran into a swarm of Japanese fighters, including the Ki-61 "Tony" and formidable Ki-44 "Tojo". Shomo shot down seven of them, to be awarded the Congressional Medal of Honor for this feat.

The Seventh Air Force began to obtain P-51Ds on Iwo Jima beginning in March 1944, to be used to fly escort for Boeing B-29 Superfortress raids over Japan. The Mustangs flew their first mission over Japan on 1 April 1945. These missions were very difficult, with a round-trip distance of 2,400 kilometers (1,500 miles) over trackless ocean. A B-29 was assigned to each fighter group to provide navigation.

Iwo Jima's airstrips were very dusty and caused engine failures that could be deadly. Tropical storms were also very violent and hazardous. On 1 June 1945, a group of 148 Mustangs left Iwo Jima to rendezvous with B-29s for a raid on Honshu. The fighters ran into a weather front and lost 27 of their number in mid-air collisions and other accidents. 22 pilots died.


[2.4] P-51D: SUMMING UP

* 7,956 P-51Ds and 1,500 P-51Ks were built by North American, more than all other models of Mustangs combined. Of these aircraft, 875 ended up as RAF Mustang IVs and Mustang IVAs. The USAAF would credit about 5,000 air combat kills to Merlin Mustangs operating in Europe, a little less than half the total claimed, and these aircraft were also credited with destroying over 4,000 enemy aircraft on the ground, a little more than half the total claimed.

As with the P-51B, there were a number of field-modified two-seat conversions. The second seat was installed under the bubble canopy in place of normal radio gear. North American also designed a "formal" two-seat conversion with dual controls, and this variant was known as the "TP-51D". One such two-seater was used by General Dwight D. Eisenhower as a fast observation aircraft to allow him to personally inspect the Normandy beachhead in June 1944. In the 1950s, the Temco company would also convert 15 F-51Ds (as the P-51D was designated by that time) to a two-seat trainer configuration designated the "TF-51D", featuring twin tandem cockpits.

Of course, given the great numbers and wide use of the P-51D, there were a few oddball variants. One was fitted with an arresting-hook "stinger" and used for carrier trials on the USS SHANGRI-LA in November and December 1944, with Navy Lieutenant Robert Elder at the controls. This aircraft was designated the "ETF-51D". The US Navy was very interested in a long-range carrier-based fighter, but although the trials went well, the capture of Iwo Jima and American air superiority over the Pacific eliminated the need for a naval Mustang.

P-51D with Marquardt ramjets

There was actually a jet-powered version of the P-51D, sort of. After the war a P-51D was modified as a ramjet test platform, with one Marquardt ramjet attached to each wingtip. The piston engine was retained, of course, since a ramjet has to be moving at relatively high speed before it can be started. The ramjet-boosted Mustang flew extensive tests until August 1948, when one of the ramjets flamed out. The test pilot tried to relight it, and ramjet exploded in a fireball; the pilot managed to turn the aircraft over and bail out, leaving the Mustang to fall to earth.


[2.5] XP-51F, XP-51G, XP-51J, P-51H, P-51L, P-51M

* The P-51 was a relatively heavy aircraft even for its size, mostly due to the fact that it had been designed in a hurry, and with an emphasis on ease of manufacture instead of airframe optimization. Edgar Schmued had traveled to England to inspect Supermarine's factories and study captured Luftwaffe aircraft, and felt that much could be done to reduce the Mustang's weight.

In early 1943 NAA began a project to develop a "lightweight" Mustang, with the company designation "NA-105". The new Mustang that resulted had few parts in common with earlier versions, featuring an even more streamlined wing, a redesigned cooling system, a longer but more aerodynamically efficient bubble canopy, plus extensive minor changes to cockpit layout, control surfaces, landing gear, and so on. The troublesome fuselage fuel tank was deleted. In compensation, the integral wing tanks were increased to a capacity of 386 liters (102 US gallons) each. Machine-gun armament was reduced from three to two 12.7-millimeter Browning guns in each wing, with each gun provided with 440 rounds of ammunition. Interestingly, the lightweight design reverted to a three-bladed propeller.

Two lightweights were initially built as "XP-51Fs", followed by a third produced in RAF colors and shipped to Britain as a "Mustang V". Bob Chilton made the first flight in an XP-51F on 14 February 1944. The aircraft was very fast, with a top speed of 750 KPH (466 MPH) at altitude, but its handling left something to be desired.

Two more lightweights were built using the experimental Rolls-Royce Merlin 145M engine, driving a five-bladed Rotol propeller. These two were known as "XP-51Gs", and the first one flew in early August 1944. The five-bladed propeller was apparently unsatisfactory, since it was immediately replaced with an Aeroproducts four-bladed propeller. One of the XP-51Gs achieved a top speed of 796 KPH (495 MPH) some months later, making it the fastest of all the Mustangs. An XP-51G was provided to the RAF for flight tests. It was not produced due to limited availability of the Merlin 145M.

NAA also developed a lightweight Mustang named the "XP-51J" that featured (for comparative studies) an Allison V-1710-119 engine, which had a two-stage supercharger to improve high altitude performance and a water-injection system for boost power. This aircraft had the carburetor intake moved under the fuselage to draw air from the belly airscoop, and as a result the XP-51J had a cleanly streamlined nose, unbroken by any significant openings. It gave the aircraft the look of a custom-built air racer. Two XP-51Js were built, but only one actually flew, in April 1945. Performance did not meet expectations due to engine problems, and so the second aircraft was shipped to Allison, where the engine was scavenged for parts for engineering studies.

Interestingly, one of the lightweights, an XP-51G, has survived to the present. It is unlikely that it will ever fly, since many of its parts are unique and unavailable.

* Although the various NA-105 lightweights were not regarded as entirely practical for field use, the ideas incorporated in them were used to enhance the P-51D design, leading to the final production version of the Mustang, the "P-51H".

This aircraft was powered by an uprated V-1650-9 engine, featuring water-methanol injection for power boost and driving a four-bladed propeller. It had a lengthened fuselage similar to that of the XP-51F, a taller tailfin to deal with lateral stability problems, and a cockpit that was slightly raised to provide a better forward field of view. The P-51H reverted to six wing guns. The fuselage fuel tank made a comeback as well, though it was sensibly smaller than that of the P-51B and P-51D, with a capacity of 191 liters (5O US gallons).

North American P-51H, XP-51J

The P-51H was one of the fastest piston fighters ever to reach full operational service, with a level speed of 784 KPH (487 MPH). Thousands of P-51Hs were ordered, with the company designation "NA-126", but the end of the war and the rise of the jet fighter reduced the actual quantity produced to 555.

The NAA plant in Fort Worth was to produce the P-51H with a different model number, "P-51M" (or "NA-124"), though only one such aircraft was actually built. A minor variation of the P-51H designated "P-51L" (or "NA-129") was considered, which was to feature an improved V-1650-11 engine with fuel injection, but none were actually built.

   _____________________   _________________   _______________________
   spec                    metric              english
   _____________________   _________________   _______________________

   wingspan                11.28 meters        37 feet
   wing area               21.65 sq_meters     233 sq_feet
   length                  10.16 meters        33 feet 4 inches
   height                  3.37 meters         11 feet 1 inch

   empty weight            3,193 kilograms     7,040 pounds
   max loaded weight       5,221 kilograms     11,500 pounds

   maximum speed           785 KPH             485 MPH / 425 KT
   service ceiling         12,770 meters       41,900 feet
   range with drop tanks   1,866 KM            1,160 MI / 1,010 NMI
   _____________________   _________________   _______________________

The P-51H saw little or no combat. It was quickly replaced by the Lockheed P-80 Shooting Star in the air superiority role. In the years immediately following WW II, the USAAF standardized on the P-51D for close support, since the P-51H was not regarded as being as robust.

* A total of about 15,600 Mustangs of all types was built by North American, not including the aircraft built in Australia by Commonwealth Aircraft Corporation and discussed in the next chapter. Almost two-thirds of these were P-51Ds, and that type survives today as the most common Mustang.

A P-51 cost about $51,000 USD in WW II dollars. Even allowing for inflation of about a factor of ten to the present day, it is an indication of the evolution to greater sophistication and expense of modern fighter aircraft that a half-million dollars wouldn't even buy a light attack aircraft, much less a first line fighter.