It seems like an oxymoron: Lightweight Mustang. Just when the Mustang was beginning to establish itself as a heavyweight fighter, its very own designer tried to take its title away. Good as the latest version of the Mustang was in mid-1942, the Allison-powered P-51/Mustang Mk.IA was deemed too heavy and ineffective for aerial combat against the best Luftwaffe fighters. So its architect, Ed Schmued, invented a theory: ‘If we lighten its load, it’ll excel [read accelerate].’

A Second World War fighter pilot wanted, as all fighter pilots want today, a fighter that is agile (responsive), manoeuvrable and one that does not lose energy (speed) in turns. In other words, a fighter that cannot be beaten under any circumstance and altitude. The upcoming B variant with the Packard-built V-1650-3 Merlin engine promised to address this situation.

Chief of aircraft design Schmued never tired of making improvements to ‘his’ P-51 family of airplanes throughout the entire Mustang development programme. In this instance, Schmued wondered why the Supermarine Spitfire Mk.V – roughly the same size as the P-51/Mustang Mk.IA – was lighter and more manoeuvrable than the latest version of the Mustang. He, of course, realised that a lighter version of the Mustang would improve both its manoeuvrability and overall performance. He discussed this with USAAF officers and NAA management and was given a go-ahead to investigate further. NAA simply referred to this programme as the ‘light weight version’ of the Mustang.

Weight Differences between the Spitfire Mk.V and Mustang Mk.IA/P-51

Empty weight: 5,090 lb (Spitfire Mk.V), 6,500 lb (Mustang Mk.IA/P-51) 

Gross weight: 6,770 lb (Spitfire Mk.V), 8,933 lb (Mustang Mk.IA/P-51)

To better put the Mustang development programme in perspective, the first of two experimental XP-51B Mustangs was not to make its first flight until 30 November 1942 and it would not be until June 1943 that the USAAF first took possession of its production P-51B when the first twenty airplanes were delivered. Moreover, its P-51A predecessor did not make its first flight until 3 February 1943, some two months after the XP-51B.

In late 1942, Schmued produced several preliminary designs to use three different propulsive systems for what he thought would create a new breed of Mustang that would have much improved performance. One propulsive system in particular, the new Rolls-Royce RM.14.SM Merlin boasting of 2,200 hp at 120 inches of manifold pressure, dominated his interest. These designs were well received by the USAAF and on 2 January 1943, under NAA Charge Number NA-105, the Lightweight Mustang programme officially began with a letter contract in hand for the manufacture of five experimental airplanes designated XP-51F and XP-51G.

The US War Department approved this contract (AC 37857) on 20 July 1943. This very same contract was amended and approved on 30 June 1944 to include two XP-51J airplanes (44-76027 and 44-76028). But this is getting ahead of the story.

The aircraft were issued USAAF serial numbers 43-43332, 43-43333 and 43-43334 (NA-105, XP-51F); 43-43335 and 43-43336 (NA-105A, XP-51G); and 43-43337 and 43-43338 (NA-105B, XP-51J). For some reason, however, the serial numbers 43-43337 and 43-43338 were cancelled and later replaced with 44-76027 and 44-76028 (XP-51J). Most likely, for a very short time, the two cancelled serial numbers were assigned to what would have been two XP-51H airplanes, but this has never been substantiated and no XP-51H airplanes were built.

The first of three experimental XP-51F Lightweight Mustang airplanes after it was rolled off the assembly line in Inglewood. The three XP-51Fs were each powered by the 1,280 hp Packard-built V-1650-3 Merlin engine that gave them a top speed of 491 mph at 21,500 feet. (San Diego Air & Space Museum via John Melson)

Two experimental XP-51G airplanes were produced with the rather unique 1,700 hp Rolls-Royce RM.14.SM Merlin engine installed. Moreover, at first, both examples featured the use of five-bladed Rotol propellers which proved inadequate for the Lightweight Mustang programme and were replaced by Aeroproducts four-bladed propellers. (San Diego Air & Space Museum via John Melson)

Excellent view of the first of two XP-51G airplanes (43-43335) flaunting its short-lived five-bladed Rotol propeller installation. The blades were made of wood and did not handle the awesome power produced by the Rolls-Royce RM.14.SM Merlin engine efficiently, especially when its war emergency power or WEP was used that produced nearly 2,000 hp. (San Diego Air & Space Museum via John Melson)

With the blessing of NAA management, Schmued was sent to Great Britain on what became a rather lengthy two-month trip to visit USAAF commanders, Supermarine and Rolls-Royce. He departed Inglewood on 9 February 1943. During his trip, he first visited the Rolls-Royce aircraft engine factory in Derby, Great Britain, to meet with its people. At the behest of Dutch Kindelberger, he was authorised to arrange for the procurement of two RM.14.SM engines for shipment to Inglewood which he did. Schmued next met with several USAAF commanders who in part asked him why British fighters were lighter than American fighters. Schmued did not have a ready answer. So he then had NAA’s Field Service Department in Great Britain visit British aircraft manufacturers, especially Supermarine, which was building the Spitfire series of fighters.

The two experimental XP-51J airplanes were powered by a single 1,500 hp Allison V-1710-119 (F32R) engine with two-stage superchargers. None of these experimental Lightweight Mustang aircraft, the XP-51F, XP-51G or XP-51J, led to production contracts. The production Lightweight Mustang, the P-51H, was never an experimental airplane but rather a production airplane from the start. There were no XP-51H airplanes produced although two USAAF serial numbers 43-43337 and 43-43338 had been assigned to the Lightweight Mustang programme but were cancelled. (San Diego Air & Space Museum via John Melson)

Inexplicably, there were no weights of fighter aircraft parts available to the field service personnel. So they set about weighing individual parts of certain fighter aircraft, especially those of the Spitfire. They discovered that the British Air Ministry did not require specific manufacturing standards to meet airframe stress requirements as did the USAAF Materiel Command. Therefore, British aircraft parts were lighter meaning that airframe weights were also lighter.

Schmued returned to Inglewood in early April 1943, followed shortly thereafter by two Rolls-Royce RM.14.SM engines and several Rotol five-bladed constant speed electrically actuated propellers for use on the two XP-51G airplanes. Rolls-Royce had insisted upon the use of these propellers to acquire data for its own benefit which NAA agreed to. As planned, the XP-51F was powered by the Packard-built V-1650-3 Merlin engine; the XP-51G by the Rolls-Royce RM.14.SM engine; and the XP-51J by the Allison-built V-1710-119 engine with a two-stage blower to enable high-altitude operation and water injection for additional power. All of them, like the upcoming P-51D variant, had the bubble-top canopies installed from the outset.

The premier XP-51F (43-43332) made a successful first flight on 14 February 1944 from Mines Field with NAA test pilot Bob Chilton at the controls. The third example (44-43334) made its first flight on 20 May 1944 while the second example (43-43333) first flew on 22 May 1944. Chilton made all of these first flights. The Rotol propeller did nothing to improve performance during the XP-51F flight test programme and was replaced by a four-bladed Aeroproducts Unimatic electric constant speed propeller. After it was flight tested several times, the third XP-51F was disassembled and crated up to be shipped by sea to Great Britain and left Inglewood on 30 June 1944. When it arrived in Great Britain, it was assigned RAF serial number FR409 and designated Mustang Mk.V.

The first of two XP-51G airplanes (43-43335) made its first flight on 9 August 1944 with NAA test pilot Joseph Linus ‘Joe’ Barton under glass. Joe Barton was first to fly the second XP-51G (43-43336) as well on 14 November 1944.

The second example of the XP-51G was shipped to Great Britain in February 1945 and was assigned RAF serial number FR410, and like the XP-51F sent over there earlier, it too was designated Mustang Mk.V. Both were thoroughly flight tested by the Aeroplane and Armament Experimental Establishment (A&AEE) at RAF Station Boscombe Down.

The first XP-51J (44-76027) made its first flight on 23 April 1945 with NAA test pilot Joe Barton in command. The second XP-51J (44-76028) soon followed, but made only two test flights by NAA. The first XP-51J made only seven test flights by NAA.

Both were delivered to the USAAF on 15 February 1946 and were returned to Allison to test the V-1710-143 engine for the upcoming P-82E Twin Mustang programme.

XP-51F, XP-51G and XP-51J Specifications

Length: 32 ft 11 in. (XP-51F, XP-51G and XP-51J) 

Height: 13 ft 8 in. (XP-51F, XP-51G and XP-51J) 

Wing span: 37 ft (XP-51F, XP-51G and XP-51J) 

Wing area: 235.6 sq ft (XP-51F and XP-51G); 233 sq ft (XP-51J) 

Propulsive system: one 1,380-hp V-1650-3 (XP-51F), one 2,200-hp RM.14.SM (XP-51G), one 1,500-hp V-1710-119/F32R (XP-51J) 

Propeller: Hamilton Standard 11-ft-diameter four-bladed constant speed propeller (XP-51F and XP-51G); Rotol constant speed five-bladed propeller (XP-51G); Aeroproducts 11-ft-diameter four-bladed Hydromatic propeller (XP-51J) 

Empty weight: 5,634 lb (XP-51F), 5,750 lb (XP-51G), 6,030 lb (XP-51J) 

Gross weight: 7,340 lb (XP-51F), 7,265 lb (XP-51G), 7,400 lb (XP-51J) 

Maximum speed: 491 mph at 21,500 ft (XP-51F), 498 mph at 25,000 ft (XP-51G), 495 mph at 25,000 ft (XP-51J) – all speeds recorded in level attitude flight

This exploded view of P-51H clearly shows its cockpit, engine installation along with its major- and sub-assemblies. All P-51H aircraft were powered by 1,380 hp V-1650-9 Merlin engines spinning four-bladed constant speed 11-ft-1-in.-diameter Aeroproducts propellers. NAA produced 555 P-51H airplanes and the final example rolled off the Inglewood assembly line on 9 November 1945. (NAA via Peter M. Bowers)

Chronological order of First Flights

02/14/44 – XP-51F number one 

05/20/44 – XP-51F number three 

05/22/44 – XP-51F number two 

08/09/44 – XP-51G number one 

11/14/44 – XP-51G number two 

02/03/45 – P-51H number one 

04/23/45 – XP-51J number one 

xx/xx/45 – XP-51J number two

Enter the P-51H Mustang

The last ‘new’ mass produced version of the P-51 Mustang, the lightweight P-51H (44-64160), made its first flight out of Mines Field on 3 February 1945 with Bob Chilton at its controls. Its propeller failed during a test flight three days later on 6 February and was damaged beyond repair when it crash-landed. (It is interesting to note that the P-51H flew before the first XP-51J that did not fly until 23 April 1945.) According to official USAAF Materiel Command documentation, under Materiel Experimental number MX-356, the Lightweight Mustang programme is simply listed as an Offensive Fighter covering the experimental XP-51F, XP-51G and XP-51J airplanes. While there is no mention of the lightweight P-51H airplane, it is more than likely covered by the MX-356 umbrella. It is presumed that NAA set aside the suffix H for the production model of the lightweight Mustang if the experimental F, G and J variants proved to be worthwhile. It seems rather odd, however, that no XP-51H airplane(s) were ordered considering that the first P-51H was an outright production airplane.

An early production P-51H banks away from the company cameraman in a chase plane before it returns to Mines Field after a contractor’s functional check flight. Initially some 2,500 P-51H airplanes had been ordered, but 1,945 were cancelled due to war’s end. A large number were to be produced as P-51L airplanes under NAA Charge Number NA-129 in Dallas, Texas. (Stan Piet)

This phantom view of a P-51H shows the inner workings of a complex yet functional fighter from the 1944 era of military aircraft design. It was drawn by NAA staff artist Reynald Brown. To reduce weight, the P-51H featured smaller and lighter main landing gear assemblies in addition to lighter interior sub-assemblies. Moreover, the structural strength of the P-51H was ten per cent greater than its P-51D/K predecessors. (NAA via Peter M. Bowers)

The fifth production P-51H-1-NA (44-64164) was turned over to the NACA (now NASA) for a series of flight test evaluations from its Langley, Virginia facility. NACA-Langley received numerous Mustangs during the Second World War for various evaluations including wind tunnel tests. (NASA)

The P-51H was the final result of the NAA lightweight Mustang programme. It was powered by a 1,380 hp (2,220 hp war emergency power rating) Packard-built V-1650-9 Merlin engine with water/alcohol injection, the offspring of the Rolls-Royce RM.14.SM Merlin 100 engine. The -9 engine was another USAAF Materiel Command project designated MX-438. The P-51H was the fastest production Mustang having been clocked at 487 mph at 25,000 ft during flight testing. The lightweight H model – based upon the lightweight experimental F, G and J models – was of a completely new design. Almost none of the parts from the P-51B/C and P-51D/K production lines were compatible on the P-51H.

The P-51H featured a number of differences and improvements such as the use of the then new lighter, thinner and stronger 75ST aluminium skins; redesigned canopy so that the pilot could sit higher for better visibility; smaller and lighter main landing gear/wheel and tyre assemblies; disc brakes; taller vertical stabiliser; longer and sleeker fuselage; smaller chin air inlet scoop for the carburettor; smaller fuselage fuel tank; relocated oil tank (from inside the belly scoop to a mounting on the forward side of the firewall); four instead of six machine guns; redesigned ammunition loading doors; removable ammunition boxes; redesigned engine cradle; and the leading edge forward-swept kink at the wing apex was now removed. The wing span of the H remained the same as the D/K models. Also, to further reduce weight, the 11-ft-1-in.-diameter four-bladed propeller was now a constant speed Aeroproducts Division of General Motors Aeroprop dubbed the ‘H prop’ with rounded rather than square tips.

The P-51H Lightweight Mustang featured two different vertical stabilisers: a short stabiliser on the first twenty airplanes and a tall one on the remaining 535 airplanes. The first twenty airplanes were all retrofitted with the taller tail by mid-1946. Another major change between the H and the earlier D/K airplanes was the incorporation of a fifty-five-US gallon fuel tank in the fuselage rather than the eighty-five-US gallon tank employed by the former. This change, in addition to the taller tail, contributed to the demise of the directional stability problems of the D/K Mustangs.

In mid-1945, the USAAF Materiel Command at Wright Field authorised a feasibility study designated MX-533 whereby four .60 calibre machine guns would replace the standard four .50 calibre machine guns, but this action never materialised.

As shown in this official USAF document dated 22 March 1949, the P-51H is 33-ft 3-in. long, slightly longer than P-51D/K airplanes; 13-ft 3-in. high, slightly higher than P-51D/K airplanes with a wing span of 37-ft 0-in. (236 sq ft wing area). It also details the exact location of the belly-mounted engine coolant radiator. (Photograph courtesy of USAF)

Without a great deal of fanfare at Inglewood on 9 November 1945, the 555th and last P-51H Mustang (44-64714) rolled off the assembly line. The last of 15,586 Mustangs conceived had been born. None of the P-51H Mustangs saw combat in the Second World War although some were assigned to squadrons just prior to VJ Day. After the war, a large number of these lightweight Mustangs were assigned to front line USAF units, Air Reserve units and Air National Guard units.

When the Korean War erupted, it was the heavyweight P-51D and P-51K Mustangs that went back to war, not the lightweight P-51Hs. As it happened, according to the pilots that flew them, the heavier six-gun P-51D/K Mustangs were preferred over the lighter four-gun version. Of the 555 P-51H Lightweight Mustangs manufactured, just five remain in existence. Two are in flyable condition, one is on display, one is being fully restored for display, and the other is undergoing a complete restoration to flyable condition.

P-51H Specification

Length: 33.3 ft 

Height: 13.7 ft 

Wing span: 37 ft 

Wing area: 236 sq ft 

Propulsive system: one 2,220-hp Packard-built V-1650-9 Merlin engine with a two-stage, two-speed supercharger and water/alcohol injection 

Propeller: four-bladed constant speed 11-ft-1-in.-diameter Aeroproducts propeller 

Empty weight: 6,551 lb 

Gross weight: 13,000 lb 

Maximum speed: 474 mph at 22,700 ft with water injection 

Rate of climb: 1,100 ft per minute – 24 minutes to 25,000 ft 

Combat range: 437 miles at 237 mph cruise at 10,000 ft (with two 165-US gallon drop tanks) 

Armament: four .50 cal Browning M2 machine guns – two in either wing; 390 rounds per gun (inboard) – 500 RPG (outboard)

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