Evolution of the breed

When the P-51 first took to the air, no one could know that it would have such far-reaching effects in nearly every theater of combat. Simultaneously, nobody foresaw that the initial disappointment in its performance, when measured against its peers, would turn into near-elation as the concept became a world beater. The Mustang’s evolution began with a cutting-edge airframe mated to a marginal engine, which was then replaced with one of the finest powerplants ever. The original design’s limited range was expanded with more internal fuel and drop tanks that made all the difference as a bomber escort. Armament also evolved, including significant air-to-ground ordnance-plus cameras. Climb aboard for a galloping ride aboard a star player on a global stage.

Allison to Merlin: The most important decision in fighter history?

The legend is well known: how the Allison-powered North American NA-73—named Apache by the Americans and Mustang by the British— evolved from a capable low-level reconnaissance/ ground attack aircraft into a world-beating, high-altitude, long-range escort. But the evolution required time, patience and persistence.

The early Mustangs were powered by Allison’s 12-cylinder, 1,100-horsepower V1710-39 liquid-cooled engine, the same general engine in the P-38 Lightning, P-39 Airacobra and P-40 War-hawk series. Top speeds for the XP-51, Mustang 1 and P-51 were in the 380mph range at 12,000 feet while the P-51 A gained about 380mph with the 1,200 hp «dash 81» Allison.

The prototype fighter first flew in October 1940 and logged its first RAF combat missions in July

1942. Squadron reports were positive, as the low-level recon mission usually kept operating altitudes below 15,000 feet. With its single-stage supercharger, the Allison performed reasonably well in that environment.

Enter Major Thomas Hitchcock, Jr. and Ronald Harker

In the fall of 1942, shortly after the British announced introduction of the Mustang I, an exceptional American officer entered the picture. Maj. Thomas Hitchcock, Jr., assistant air attache in London, saw potential in the North American’s lean airframe. A noted polo player and World War I pilot, he conceived the idea of mating the Rolls-Royce Merlin to the Mustang with exceptional results.

Though Hitchcock usually receives most of the credit, Rolls-Royce test pilot Ronald Harker played a major role. After flying a Mustang I in early 1942, he realized that a Merlin could eliminate most of the fighter’s drawbacks. He lobbied first at Rolls-Royce, then in the Air Ministry, where he convinced Sir Wilfrid Freeman, a power in the aircraft production office. Freeman picked winners: he had already supported acquisition of the Hurricane, Spitfire, Mosquito, Lancaster and Halifax.

Harker had his numerical ducks in order. The engineers had already run their slide rules and concluded that a Merlin Mustang might achieve 440mph at 26,000 feet. Consequently, Ronnie Harker became known in Britain as «the man who put the Merlin in the Mustang.»

Steps toward creating the legend Four Mustangs were sent to Rolls-Royce for adapting the British powerplant to the American airframe. Merlin 61s and 65s were installed, with four-bladed props in place of the three-bladed variety on standard Mustangs. With two-speed, two-stage superchargers, the altitude performance looked promising. Dubbed «Mustang X,» the UK conversion began evaluation in October.

Meanwhile, North American also had measured the British engine, and was well along when the Mustang X left the ground. Designated the XP-51B, the «home grown» Merlin Mustang first flew on November 30 with eye-watering results. The conversion became standard with the Packard automobile plant-producing Merlins as the V1650, rated at nearly 1,400 horsepower.

Most of the early North American tests were flown by Robert C. Chilton, who recorded a 441mph top speed—about 50mph more than the Allison models. But as with most aircraft, an increase in power translated into more climb than level speed, and the B model out climbed the P-51A nearly two to one. Chilton remained the leading Mustang engineering test pilot from early 1941 onward.

NAA’s Inglewood plant began building P-51Bs in early 1943 with a new Dallas factory delivering the first C models that August. (The two products were identical: there was no need for different designations, but they did it anyway.)

The RAF called the B model the Mustang III, as the interim Mk II resembled the A-36 dive-bomber without dive brakes. Though the Merlin version enjoyed advantages of greater range and speed over the Allison models, the British continued using the new fighter in low-level missions into 1944.

In November 1943, the AAF’s first Merlin Mustang unit arrived in Britain: the 354th Fighter Group, which had trained on P-39s. Though assigned to the Ninth Air Force, the «Pioneer Mustangs» were immediately assigned long-range missions, escorting Eighth Air Force «heavies» into Reich airspace beginning in December. The «Mighty 8th» began receiving Mustangs early in the new year, with bubble-canopy D models (Mk IVs) appearing just before D-Day. Meanwhile, test and evaluation continued in the U.S. and Britain. Tommy Hitchcock, then a lieutenant colonel, was killed in a P-51D in April 1944 due to wing failure. He was 44 years old, having lived an exceptional life including flying in both world wars. With Ronnie Harker, «Willie» Freeman, and thousands of others, he helped turn a contender into a champion.

Range: The game changer

Apart from its speed and performance, the Mustang was all about range. In late 1943, when P-47s and 38s were unable to take deep-penetration bombers to the target and back, Mustangs proved an immediate game changer in German skies. No less an authority than Reichmarshal Hermann Goering conceded the point. During the 8th Air Force’s first «Big B» missions in March 1944, the Luftwaffe commander said, «When I saw American fighters over Berlin, I knew the war was lost.»

More astute combatants such as fighter chief Adolf Galland had reached that conclusion much earlier: when Hitler attacked Russia in June 1941 and declared war on America in December.

More fuel? Drop tanks to the rescue The P-51A/B/C provided 180 gallons of fuel in wing tanks, usually enough for more than four hours cruising. Two 75-gallon drop tanks increased total capacity to 330, a substantial increase, especially with the Merlin’s greater fuel efficiency at altitude.

However, in aviation there is no such thing as too much gas («unless you’re on fire»), so North American found room for more internal fuel. An 85-gallon tank was shoehorned into the fuselage behind the cockpit from late B models onward (you can see the fuel gauge peeking over their shoulder)—a total of nearly 270 gallons internal fuel. Thus, depending on type of drop tanks, a combat-loaded Mustang took off with more than 400 gallons of high-octane fuel—enough to fly a round-trip mission of 1,500 miles.

Despite the obvious advantage of drop tanks, they were not an easy sell. Before the war, the Army thought them unnecessary, as fighters were envisioned for relatively short-range missions. Doctrine developed from the 1920s onward centered upon the self-defending heavy bomber, presumably rendering long-range fighters unnecessary.

Experience proved otherwise. Early Thunderbolts had no provision for external tanks, an oversight that quickly was recognized. Even then, the 1943 efforts proved inadequate, and development continued throughout the war.

Drop tanks came in two varieties: ferry and combat. The former were of larger capacity but their size and bulk impeded operational use. The sleeker combat «drops» mostly were plastic-laminated paper, operated under pressure, for relatively brief periods. They were not intended to store gasoline for much more than the’ duration of a single mission, and generally were jettisoned before landing.

Probably the largest drop tanks routinely fitted to Mustangs were 110-gallon capacity. The additional 660 or so pounds was well worth the weight and drag penalty when translated to tactical radius.

Managing a P-51’s fuel system required the fighter pilot to become his own flight engineer. The fuselage tank was especially troublesome in turbulence, as 400-plus pounds of sloshing liquid did bad things to the fighter’s center of gravity.

Fuselage tanks: Welcome fuel in the wrong place

Mustangs seldom penetrated a storm front. When possible, they remained in the clear to avoid major turbulence, as the 85-gallon fuselage tank became a critical factor. Said Maj. Harry Crim, an

Iwo Jima-based squadron commander, «In rough weather the ’51 with the fuselage tank full didn’t fly like anything resembling an airplane.» Before entering weather, standard procedure was to run the fuselage tank down to no more than 40 gallons to put the center of gravity on the near side of controllability. Even then, it was no fun flying a P-51 in turbulence. When the drop tanks were partially empty, the gas sloshed from front to back, creating a roller-coaster sensation. It was almost impossible to fly straight and level visually; far less so on instruments.»

Most Mustang pilots stress how quickly they tried to burn up the fuselage fuel on the way to the combat zone because the last thing they wanted was fuel in that tank when they engaged the enemy.

For return flights, Harry Crim explained, «We dropped our tanks, shot up all our ammo, and tested the relief tube.» Then it was a matter of managing fuel for the 750-mile flight home. Cruising at lower power settings using only 40 gallons per hour, rather than the usual 60-65, could burn up a set of plugs but the hardy Merlins didn’t seem to mind.

Armament: Lethal and otherwise

Fighter ace and best-selling author Robert L. Scott wrote, «Guns are the soul of a fighter plane.» So it was with the Mustang.

The standard U.S. fighter weapon of World War II was John M. Browning’s timeless M2 .50 caliber, a 69-pound recoil-operated weapon nominally cycling at 800 rounds per minute. A two-second burst from the standard six-gun battery of an American fighter put more than 150 heavy bullets, nearly 15 pounds of steel, down range, often a mixture of ball, tracer, and armor-piercing incendiary.

Browning became the standard

Because the Mustang began as a British aircraft, it originally reflected the RAF (and European) armament philosophy with a mixture of heavy and light weapons. But unlike the Spitfire (typically with two 20mm cannon and four .30 cal. guns) the North American fighter debuted with four ,50s and four .30 calibers mounted in the wings and nose. P-39s and early P-40s also packed .30s and .50s, but thereafter, the Army Air Forces narrowed the option. The P-51/Mustang IA upgraded to all-cannon armament with four 20 millimeters (mostly operated by the Brits), but from the P-51 A/Mustang II onward, the type relied exclusively on the .50 caliber.

Fitting the big Brownings and sufficient ammo into the narrow wing took some doing, as the laminar-flow design was crucial to the Mustang’s performance. The four-gun battery in the P-51A/B/C typically was loaded with 350 rounds per gun inboard and 280 outboard, totaling 1,260 rounds. The A-36 added two .50s firing through the propeller arc for a total of six.

In 1943, the AAF began standardizing fighter armament. The P-40 Kit-tyhawk/Warhawk series settled on six .50s, and the Navy already had done so for the F4F-4 Wildcat, F6F Hellcat and F4U Corsair. Consequently, the P-5 ID packed three Brownings in each wing, loading 400 rounds per gun on the two inboard .50s and 270 for the outboards, a total of 2,120 rounds, or a 68% increase. It boosted the B/C models’ nominal 43 seconds of firing time to nearly 50 for the bubble Mustang. As always, however, there was a weight penalty as two additional guns and ammo added 344 pounds.

Lt. Col. Richard E. Turner commanded a squadron in the 354th «Pioneer Mustang» Group and wrote a well-regarded memoir titled Big Friend, Little Friend. Describing P-51 armament he said, «An operational bug which…gave us temporary trouble with the four .50 caliber machine guns. They were mounted on an angle canted’inboard around 30 degrees…due to the thin profile of the wing. This angle-off mounting caused a binding effect to be exerted on the ammo belts during turns of any appreciable G-force, which in turn tended to cause at least three of the guns to jam. Since the pilot had no clearing mechanism…this problem gave us great concern at first. The only recourse a pilot had, since one firing gun usually rendered the gunsight useless, was to ride the rudders and spray the bullets from the operable gun like a hose, hoping for a lucky hit.

«The deficiency was corrected by a modification figured out and installed by our ingenious, hard-working ground crewmen. They liberated some ammo-booster motors from a nearby B-26 group and rigged them on our fighters’ ammo belt which, for all practical purposes, eliminated all such stoppages in the future. In the later P-5ID there were six guns, but all were mounted upright, effectively solving the problem.”

More guns or more ammo?

Analysis of European Theater aerial victory claims indicates that the six-gun armament was unnecessary. A study of 675 P-51B/C claims yields 408 credited kills, or 60.4% of the total assessed as destroyed, probables, and damaged. A similar evaluation of 352 D model claims shows 245 destroyed, or 69.6%. So: the 50% increase in guns provided less than a ten-point improvement in lethality. Mediterranean Theater figures fall within 5% the ETO data.

In comparison, ETO kills-to-claims figures show 66.3% for eight-gun P-47s and 59.6% for P-38s with four .50 calibers and a 20mm cannon. Unsurprisingly, Thunderbolts and Lightnings were more lethal than P-5 IDs against Japanese aircraft.)

Whatever the benefits of the extra guns, the additional ammo did make a difference. Of the 20 Mustang aces in a day, 19 flew D models. The first was 22-year-old Lt. Carl Luksic of the 352nd Fighter Group who had two previous kills when he tied into a gaggle near Brunswick on May 8, 1944. In a prolonged combat he downed three Fw 190s and two Bf 109s flying his P-5 IB. He was the only non-D pilot to achieve that feat.

The gunsight view

Mustang pilots saw the war through a variety of gunsights, from simple ring-and-bead «iron sights» to sophisticated gyro-stabilized computing devices.

Because the 4th Fighter Group was originally composed of former Eagle Squadron pilots, many of the RAF veterans preferred British sights even after joining the U.S. Army. Consequently, photos of Debden-based Mustangs sometimes reveal RAF optical (usually Mk II) and Ferranti computing sights in American aircraft.

Probably the most common gunsight in the A through C models (including the A-36) was the N3, a reflector sight with an aiming reticle projected on an inclined plate of glass. To the pilot, the yellow-white reticle appeared superimposed on infinity. It had an adjustment knob to change the angle of the glass so the aiming dot and surrounding ring (typically subtending 30 mils) could be raised or lowered for shallow-angle bombing. Evidence from manuals indicates that many P-5 IDs were delivered with N9s, permitting the pilot to replace the light bulb in flight. Some retro-fitted D models and many late-pro-duction Mustangs received the British-developed K-14 lead-computing sight, with gyro-stabilized target tracking.

Air to ground

Other than its guns, the Mustang packed other punches beneath its wings. Dive-bombing became a frequent mission, especially for P-5 Is of the Ninth Air Force, which supported Allied armies in Northern France after D-Day. The standard loadout was two 500-pound general-purpose bombs, typically used against bridges and defensive positions. However, it was easy to get too steep and therefore too fast, occasionally with calamitous results. Consequently, the 354th reluctantly swapped its Mustangs for Thunderbolts in November 1944, as the ’47’s robust structure withstood high-G pullouts better than the stang. However, the group returned to its beloved ’51s early in the new year.

P-5 IDs also could load high-velocity aerial rockets with 5-inch warheads. It was a potent system, giving a fighter the equivalent of a destroyer’s broadside. HVARs usually were employed against hard targets such as bunkers and armored vehicles.

Shutterbugs: Their ammo was film

Seldom acknowledged as a weapon system, the cameras in tactical reconnaissance aircraft could be even more useful than guns or bombs. The RAF first employed the Mustang as a low-level «tac recce» platform, and the Americans took note. A variant of the original P-51 variant was the F-6A, reflecting the WW II designation system when P meant Pursuit and F meant «Foto.» Two Fairchild K-24 cameras in the fuselage were typical, providing oblique photographs.

P-51C, D, and K models all were adapted to the recon mission, permitting a K-17 and K-22 in the aft fuselage for horizontal, vertical, or oblique coverage, or a single K-24. Some units also mounted smaller cameras behind the pilot’s headrest. The cameras provided various focal lengths from six inches upward, typically with 250 exposures per roll.

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