P51 Mustang! What a fighter. Just the name brings to mind George Preddy, Don Gentile, Bud Anderson, and John Landers…to name only a very few. But as it seems to be with many legends, the facts as they were are not the story that they became…

It all started with the Brits (as so much has). In 1940 they were having a rough go of things (which would get much worse shortly when France fell and they lost a LOT of equipment left behind at Dunkirk). They needed fighter aircraft desperately and in more numbers than they could produce. They approached the Curtiss corporation to see if they could get their hands on some P-40s, but Curtiss didn’t have the production capability to meet their contractual demands and the British demands. The Brits contacted North American Aviation (NAA) with the idea that NAA could build the P-40 under license from Curtiss. NAA had a counter-proposal. NAA wasn’t at all interested in producing a Curtiss product and suggested to the Brits that they could design a better fighter using the same engine that the P-40 was using (the Allison V12 liquid cooled V-1710). Legend has it that they promised the first prototype in 120 days and, though it’s a nice legend, it’s just a legend and that was never promised. Facts, as they often are, are more interesting…

I’m sure you’ve all heard of Eisenhower, Patton, and so on. Have you ever heard of 1st Lt. Benjamin Kelsey? Kelsey was an aeronautical engineer trained at M.I.T. and worked at Wright Patterson as the America’s chief fighter projects officer. To quote from P-51 Mustang in Detail and Scale, Volume 50 by Bert Kinsey (published by Squadron/Signal Publications):

On 4 May, 1940, when North American obtained release to sell the NA-73 [NAA’s internal designation for the new design] to the British [the British variant had different armament and exhaust tips and was called NAA-83], Kelsey had included the stipulation that two aircraft from the first production batch would be turned over to Wright Field for testing. This meant that the British would buy the U.S. Army two aircraft which it did not then have the funds to purchase for itself.

Kelsey was also aware of studies done on a new wing design, “laminar flow,” which decreased drag substantially by moving the thickest portion of the wing from the front where conventional design had placed it more towards the rear. Curtiss had also been ordered to turn over design studies that included a new design for a belly scoop for the radiator, also moving it back from the nose, where it produced drag, more rearwards towards the middle of the aircraft. (How much of Curtiss’ design studies were used seems to be up for debate and varies by who is asked.)

102 days after the contract was signed, NA-73 was rolled out, complete except for engine. When the 1120 hp V12 showed up a couple of weeks later, engine and taxi tests started. The first of what would be called the P-51 Mustang (named by the Brits, BTW) took to the air on October 26, 1940.

Another part of the Mustang legend revolves around that Allison engine. Legend has it that the Brits really liked the Mustang’s low and mid altitude performance but the engine ran out of beans at about 15,000 feet. The Allison engine had a single-stage supercharger and didn’t produce its rated 1120 hp above 15,000 feet, so somebody decided to put the Merlin V12, with a two-stage supercharger that the Spitfire used, in a P51…and the legend was born.

Not quite. Well, not at all. The Brits wanted the P-40 which was known to be a low and mid altitude fighter. Engineers, the most clever of clever monkeys, also knew that any air-frame that used an engine with a single-stage supercharger would be a low to mid altitude aircraft. The fact that two air-frames of the original production batch of the P-51, ordered even before the first flight of an XP-51 (which is what the Army called NA-73), had been set aside to be fitted with the Merlin engine (being built under license by Packard) showed that engineers were well aware of the Allison’s performance capabilities and were going to “fix” that by using an engine with a two-stage supercharger.

Something else not widely known was that at 10-12,000 feet, Mustangs with the Allison engine were faster than Mustangs with the Packard/Merlin engine.

Once the Packard/Merlin engine was installed, the Mustang was given the nomenclature P-51B. The radiator scoop was redesigned, making it simpler to build and operate as well as improving cooling (and the design of the scoop actually used the escaping hot air, heated by cooling the radiator, as additional thrust, which overcame any drag the newer scoop created), the carburetor intake was moved from above the engine to below it, and the three blade Curtiss Electric prop was replaced by a Hamilton Standard four blade cuffed prop.

As Kinsey states, both aircraft were very good at the altitudes they were designed to operate at.

One thing that pilots didn’t like about the early Mustangs was the canopy. The canopy opened in two places; the top was hinged at the right and opened to the right and the left side was hinged at the bottom and opened to the left, and when it was closed did not offer very much room. As if that wasn’t annoying enough, it wasn’t easy to see out of and bloody impossible to see rearward very well. B variants were fitted in the field with what’s been termed the “Malcolm” canopy. The Malcolm canopy, very similar to the canopy of the Spitfire, was a blown canopy of a single sheet of plexiglass (“perspex” as it was called in the ’40s) that did away with both sides and the top as well as all the framing for them, opening by sliding to the rear and making getting in and out of the cockpit much easier (a welcome feature to a pilot who had to get out in a hurry). Some of the B variants were fitted with it (there weren’t really a lot of the blown canopy to go around), but the real improvement in view came with the introduction of the bubble canopy. It had been tested (along with other improvements) on a late-production P-51B and introduced into production with the advent of the P-51D.

Shortly after the P-51B was deployed, it was given something else that turned the aircraft into a real game-changer. An 85 gallon fuel tank was installed in the fuselage behind the pilot.

An interesting thing about Mustangs and fuel…

THE definitive Mustang was the P-51D. With 184 gallons of fuel inside the wing’s fuel tanks, and 85 gallons of the fuselage tank, it could also carry up to 110 gallons in each drop tank. With a full fuel load of 489 gallons, I’ve read that the P-51D was 50% over maximum designed weight. (This may be part of The Legend, though…do your own research.) Carrying that much fuel did bad things to the carefully calculated center of gravity. When initiating roll maneuvers with a full load of fuel, it was nigh onto impossible to keep the Mustang from flipping over onto its back. I’ve read that recovery of stable flight was also nigh onto impossible. In practice, the fuselage tank was limited to 60 gallons, and even with less weight behind the pilot, roll maneuvers had to be initiated with a fair amount of forward stick to maintain control of where the aircraft went (and how it went there).

The first P-51, the Mustang Mk I, was built for the British and was armed with four .50 caliber (two of which were mounted in the “cheeks” of the engine cowling) and four .303 caliber machine guns. The first 51 for US use was armed with four 20mm Hispano Suiza cannons. The 51A and B used four Browning M2 .50 caliber machine guns, two in each wing, and the P-51D had six, three in each wing.

Three different types of belly scoops, two different designs of wings, three different styles of canopies… Oh. Did you know that there was a dive bombing version of the P-51? It was called the A-36…and it was armed with six .50 caliber machine guns (two in each wing, two in the nose in the same manner that British Mustang was) and had dive brakes above and below the wings.

Pretty legendary, huh?*

A36

Mustang MkI

P-51

P-51A (With a P-40 in background; modern photo)

P-51B

P-51B (Malcolm canopy; modern photo)

P-51D

*Another bit of the Mustang legend that’s not exactly as told…

The Legend has it that with the addition of the fuselage fuel tank to the P-51B, the US now had a fighter that could escort bombers all the way to and from their target. Well, the US already had a fighter that could do that with the P-38 Lightning (a project also initiated by 1st Lt. Ben Kelsey). So why wasn’t the P-38 used for bomber escort? General Ira Eaker didn’t believe the bombers needed fighter escort and that their defensive armament would be enough to protect them. And in spite of the high rate of bomber losses, Eaker didn’t change his mind (he sent the P-38s in England to North Africa). I wonder how much that decision affected his being relieved of command…

In order of production, this is how many Allison-engined Mustangs were built:

US designation XP-51; British designation Mustang I; 4 .50 cal. machine guns, 4 .303 machine guns; 620 built (2 taken by US for evaluation)

US designation P-51; British designation Mustang IA; 4 20mm cannons; 150 built (93 to British, 55 to USAAF, 2 for Merlin engine tests as XP-51B)

US designation A-36; no British designation; 6 .50 caliber machine guns; 500 built (one was shipped to England for evaluation, none were ordered)

US designation P-51A; British designation Mustang II; 4 .50 caliber machine guns; 310 (50 to England)