Editing North American P-51 Mustang (section)

==Design and development==
{{Main|North American P-51 Mustang variants}}
[[File:NA-73X-NX19998-.jpg|thumb|North American NA-73X, with a short carburetor air-intake scoop and the frameless, rounded windshield: On the production Mustang Mk Is, the frameless windshield was replaced with a three-piece unit that incorporated a bullet-resistant windshield.]]
[[File:Musas 2.jpg|thumb|P-51D on the Inglewood assembly line]]

In 1938, the British government established a purchasing commission in the United States, headed by Sir [[Henry Self]].<ref>{{cite journal |url=https://www.aerosociety.com/media/10277/on-the-planning-of-british-aircraft-production-for-second-world-war-and-reference-to-james-connolly_2018-09.pdf |title=On the planning of British aircraft production for the Second World War and reference to James Connolly |journal=Journal of Aeronautical History |last=Brinkworth |first=Brian |pages=233–299 |access-date=14 September 2021}}</ref><ref>Pearcy 1996, p. 15.</ref> Self was given overall responsibility for RAF production, research, and development, and also served with Sir [[Wilfrid Freeman]], the Air Member for Development and Production. Self also sat on the British Air Council Subcommittee on Supply (or "Supply Committee"), and one of his tasks was to organize the manufacturing and supply of American fighter aircraft for the RAF. At the time, the choice was very limited, as no US aircraft then in production or flying met European standards, with only the [[Curtiss P-40 Warhawk|Curtiss P-40 Tomahawk]] coming close. The [[Curtiss-Wright]] plant was running at capacity, so P-40s were in short supply.<ref>Pearcy 1996, p. 30.</ref>

North American Aviation (NAA) was already supplying its [[North American T-6 Texan|T-6 Texan]] (known in British service as the "Harvard") trainer to the RAF, but was otherwise underused. NAA President [[James H. Kindelberger|"Dutch" Kindelberger]] approached Self to sell a new [[medium bomber]], the [[North American B-25 Mitchell]]. Instead, Self asked if NAA could manufacture P-40s [[Licensed production|under license]]<!--Engvar?--> from Curtiss. Kindelberger said NAA could have a better aircraft with the same [[Allison V-1710]] engine in the air sooner than establishing a production line for the P-40.

[[John Leland Atwood|John Attwood]] of NAA spent much time from January to April 1940 at the British Purchasing Commission's offices in New York discussing the British specifications of the proposed aircraft with British engineers. The discussions consisted of free-hand conceptual drawings of an aircraft with the British officials. Self was concerned that NAA had not ever designed a fighter, insisting they obtain the drawings and study the wind-tunnel test results for the P-40, before presenting them with detailed design drawings based on the agreed concept. NAA purchased the drawings and data from Curtiss for £56,000, confirming the purchase with the British Purchasing Commission. The commission approved the resulting detailed design drawings, signing the commencement of the Mustang project on 4 May 1940, and firmly ordering 320 on 29 May 1940. Prior to this, NAA only had a letter of intent for an order of 320 aircraft. Curtiss engineers accused NAA of plagiarism.<ref name="Martyn Chorlton 2012">{{Citation |title=Allison-Engined P-51 Mustang |first=Martyn |last=Chorlton |publisher=Osprey |series=Air Vanguard 1 |date=2012 |isbn=9781780961514 <!-- |asin=1780961510 --> |pages=11, 12}}</ref>

The British Purchasing Commission stipulated armament of four [[.303 British|.303 in (7.7 mm)]] machine guns (as used on the Tomahawk), a unit cost of no more than $40,000, and delivery of the first production aircraft by January 1941.<ref name= "Delve p. 11">Delve 1999, p. 11.</ref> In March 1940, 320 aircraft were ordered by Freeman, who had become the executive head of the [[Ministry of Aircraft Production]] (MAP) and the contract was promulgated on 24 April.<ref name= "Delve p. 12">Delve 1999, p. 12.</ref>

The ''NA-73X'', which was designed by a team led by lead engineer [[Edgar Schmued]], followed the best conventional practice of the era, designed for ease of mass manufacturing.<ref name="Martyn Chorlton 2012"/> The design included several new features.{{#tag:ref|Because the new fighter was designed to a British, rather than an American or USAAC specification, it was allocated a private-venture civil designation instead of the more usual XP- (eXperimental Pursuit) group.|group=nb}} One was a wing designed using [[laminar flow]] airfoils, which were developed co-operatively by NAA and the [[National Advisory Committee for Aeronautics]] (NACA). These airfoils generated low drag at high speeds.<ref>Jackson 1992, p. 3.</ref>{{efn|The Mustang, with its relatively thin laminar flow wing, had a tactical Mach number (the maximum speed at which combat was feasible) of 0.78. This compared favourably with the Messerschmitt Bf 109 and the Focke-Wulf Fw 190, which both had a tactical Mach number of 0.75. The other escort fighters available to the Allies were the Lockheed P-38 Lightning, with a tactical Mach number of only 0.68 and the Republic P-47 Thunderbolt, at 0.71. At the high altitudes at which fighter escorts operated in Europe made this an important performance difference.<ref name="ArmouredCarriers">{{cite AV media |people=Eric 'Winkle' Brown |title=Where US fighters failed over Europe |url=https://www.youtube.com/watch?v=VQzSQUQ33OQ |date=13 May 2023 |via=YouTube}}</ref>}} During the development of the NA-73X, a wind-tunnel test of two wings, one using NACA five-digit airfoils and the other using the new NAA/NACA 45–100 airfoils, was performed in the [[University of Washington]] Kirsten Wind Tunnel. The results of this test showed the superiority of the wing designed with the [[NACA airfoil|NAA/NACA 45–100 airfoils]].<ref>Wagner 2000, pp. 16, 18.</ref>{{#tag:ref|For more specific information on the P-51's airfoil, known as the NAA/NACA 45–100 series, see<ref>Selig, Michael. [http://www.ae.uiuc.edu/m-selig/ads/afplots/p51droot.gif "P-51D wingroot section."] {{webarchive|url=https://web.archive.org/web/20080408233751/http://www.ae.uiuc.edu/m-selig/ads/afplots/p51droot.gif |date=8 April 2008 }} ''uiuc.edu.'' Retrieved: 22 March 2008</ref>|group=nb}}

[[File:XP-51, serial number 41-039 (cropped).jpg|thumb|left|XP-51 ''{{USAF serial|41|039}}'' is one of two Mustang Mk I aircraft handed over to the USAAC for testing]]
The other feature was a new cooling arrangement positioned aft (single ducted water and oil radiators assembly) that reduced the fuselage drag and effects on the wing. Later,<ref>"We also found out, later on, that the heat from the engine actually produced thrust... That horsepower gained by the radiator was only discovered by wind-tunnel investigation..." Mustang Designer, Edgar Schmued and the P-51, p. 61. The comparison was between wind-tunnel tests of full-scale models, with normal heating (thermic engine) and without heat production (electric engine).</ref> after much development, they discovered that the cooling assembly could take advantage of the [[Meredith effect]], in which heated air exited the radiator with a slight amount of [[jet thrust]]. Because NAA lacked a suitable wind tunnel to test this feature, it used the [[GALCIT]] {{convert|10|ft|m|adj=on|abbr=on|order=flip}} wind tunnel at the [[California Institute of Technology]]. This led to some controversy over whether the Mustang's cooling system [[aerodynamics]] were developed by NAA's engineer Schmued or by Curtiss, as NAA had purchased the complete set of P-40 wind tunnel data and flight test reports.<ref>Yenne 1989, p. 49.</ref><ref>The British Air Ministry sent over the Canadian aerodynamicist [[Beverley Shenstone]] in early 1941, who also assisted with the cooling airflow (Ray Wagner ''Mustang Designer'')</ref> The NA-73X was also one of the first aircraft to have a fuselage [[lofting|lofted]] mathematically using [[conic section]]s; this resulted in smooth, low-drag surfaces.<ref name ="Jac4">Jackson 1992, p. 4.</ref> To aid production, the airframe was divided into five main sections—forward, center, rear fuselage, and two wing halves—[[Modular design|all of which]] were fitted with wiring and piping before being joined.<ref name ="Jac4" />

The prototype NA-73X was rolled out in September 1940, just 102 days after the order had been placed; it first flew on 26 October 1940, 149 days into the contract, an uncommonly short development period even during the war.<ref>Kinzey 1996, pp. 5, 11.</ref> With test pilot [[Vance Breese]] at the controls,<ref>[[Illustrated Encyclopedia of Aircraft]], p. 6.</ref> the prototype handled well and accommodated an impressive fuel load. The aircraft's three-section, [[semi-monocoque]] fuselage was constructed entirely of [[2024 aluminium alloy|24S aluminum alloy]] (a type of [[Duralumin]]) to save weight.<ref>{{cite journal |last1=Ouissi |first1=Toufa |last2=Collaveri |first2=Gilles |last3=Sciau |first3=Philippe |last4=Olivier |first4=Jean-Marc |last5=Brunet |first5=Magali |title=Comparison of Aluminum Alloys from aircraft of four nations involved in the WWII conﬂict using multiscale analyses and archival study |journal=Heritage |volume=2 |pages=2788, 2791, 2792 |date=November 2019 |doi=10.3390/heritage2040172 |doi-access=free |url=https://mdpi-res.com/d_attachment/heritage/heritage-02-00172/article_deploy/heritage-02-00172-v2.pdf?version=1576405224}} See also [https://mdpi-res.com/d_attachment/heritage/heritage-02-00172/article_deploy/heritage-02-00172-s001.pdf?version=1576405227 Supplementary material.]</ref> It was armed with four .30 caliber (7.62&nbsp;mm) [[M1919 Browning machine gun#Aircraft|AN/M2 Browning machine gun]]s in the wings and two .50 caliber (12.7&nbsp;mm) [[M2 Browning#.50 Browning AN/M2|AN/M2 Browning machine gun]]s mounted under the engine and firing through the propeller arc using a [[Synchronization gear|gun-synchronizing gear]].{{#tag:ref|This was one of the last US applications of gun synchronization – later American single piston-engined fighters, including later models of the Mustang, all had their gun armament concentrated in the wings.|group=nb}}

While the USAAC could block any sales it considered detrimental to the interests of the US, the NA-73 was considered to be a special case because it had been designed at the behest of the British and all dealings were directly between the BPC and NAA, and did not involve the US Army or Wright Field in any way. In September 1940, a further 300 NA-73s were ordered by the MAP.<ref name= "Delve p. 11" /> To ensure uninterrupted delivery, Colonel [[Oliver P. Echols]] arranged with the Anglo-French Purchasing Commission to deliver the aircraft and NAA gave two examples (41-038 and 41-039) to the USAAC for evaluation.<ref name=gml2007>[http://www.gml2007.com/history3.asp "P-51 History: Mustang I."] {{webarchive|url=https://web.archive.org/web/20080815221501/http://www.gml2007.com/history3.asp |date=15 August 2008 }} ''The Gathering of Mustangs & Legends''. Retrieved: 26 March 2009.</ref>{{#tag:ref|One of the NA-73s given to the army, s/n 41-038 is still in existence and last flew in 1982.<ref>[http://museum.eaa.org/collection/aircraft/4North%20American%20XP-51%20Mustang.asp "North American XP-51 Prototype No. 4 – NX51NA ."] {{webarchive|url=https://web.archive.org/web/20131029203038/http://museum.eaa.org/collection/aircraft/4North%20American%20XP-51%20Mustang.asp |date=29 October 2013 }} ''EAA AirVenture Museum (Experimental Aircraft Association)''. Retrieved: 23 July 2013.</ref>|group=nb}}

It is important to note that the Mustang I (NA-73 and NA-83) and the Ia (NA-91), produced for the British, were not equivalent to the P-51A which was a later model (NA-99). Two British Mustang Is were held back by the USAAF and given the provisional model number XP-51. The USAAF held back 57 Mustang Ia aircraft armed with 4 x 20mm Hispano cannon, from the third British order, converting most of them to tactical reconnaissance aircraft and designating them P-51-2/F6A. North American retained the second aircraft of this batch to help develop the P-51A.<ref>Willis Matthew – ''Mustang The untold story'', 2020, Pages=261–264, Key Books ISBN 978-1-913295-88-2</ref>

The Allison engine in the Mustang I had a single-stage supercharger that caused power to drop off rapidly above {{convert|15000|ft|m}}. This made it unsuitable for use at the altitudes where combat was taking place in Europe. Allison's attempts at developing a high-altitude engine were underfunded, but produced the V-1710-45, which featured a variable-speed auxiliary supercharger and developed {{convert|1150|hp|kW}} at {{convert|22400|ft|m}}. In November 1941, NAA studied the possibility of using it, but fitting its excessive length in the Mustang would require extensive airframe modifications and cause long production delays.<ref name="Marshall 2020" /><ref name="ModDesig" /> In May 1942, following positive reports from the RAF on the Mustang I's performance below 15,000&nbsp;ft, Ronald Harker, a test pilot for [[Rolls-Royce Limited|Rolls-Royce]], suggested fitting a [[Rolls-Royce Merlin#Basic component overview .28Merlin 61.29|Merlin 61]], as fitted to the [[Supermarine Spitfire (late Merlin-powered variants)#Mk IX .28type 361.29|Spitfire Mk IX]].<ref name="Marshall 2020">{{cite book |last1=Marshall |first1=James |last2=Ford |first2=Lowell |title=P-51B Mustang: North American's Bastard Stepchild that Saved the Eighth Air Force |date=21 July 2020 |publisher=Osprey Publishing |location=Oxford, UK |isbn=978-1-4728-3967-1 |chapter=The Birth of the Merlin Mustang}}</ref> The Merlin 61 had a two-speed, two-stage, intercooled supercharger, designed by [[Stanley Hooker]] of Rolls-Royce.<ref>{{cite book |last1=Hooker |first1=Stanley |title=Not much of an engineer |date=1984 |publisher=Airlife Books |location=UK |isbn=978-1-85310-285-1 |pages=56}}</ref> Both the Merlin 61 and V-1710-39 were capable of about {{convert|1570|hp|kW}} [[war emergency power]] at relatively low altitudes, but the Merlin developed {{convert|1390|hp|kW}} at {{convert|23500|ft|m}} versus the Allison's {{convert|1150|hp|kW}} at {{convert|11800|ft|m}},<ref name="Hazen memo" /><ref name="RDE 66-41">{{cite web |title=Merlin 61, 65 & V-1650-3 Power Curves. R.D.E. 66-41 |url=http://www.wwiiaircraftperformance.org/mustang/V-1650-3_Merlin61_65_curves.jpg |access-date=10 September 2020}}</ref><ref name="ModDesig">{{cite book |title=Model Designations of U.S.A.F. Aircraft Engines |date=1949 |publisher=U.S.A.F. Air Material Command |page=29 |url=http://www.enginehistory.org/References/ModDesig/jpg/I29.jpg |access-date=10 September 2020}}</ref> delivering an increase in top speed from {{convert|390|mph|kn km/h|abbr=on}} at ~{{convert|15000|ft|m}} to an estimated {{convert|440|mph|kn km/h|abbr=on}} at {{convert|28100|ft|m}}. In the end the Merlin 61 was never fitted to the Mustang X, (or any other Mustang). The 65 series (a medium altitude engine) was fitted to all Mustang X prototypes.<ref>Rolls Royce and the Mustang, Birch David, Rolls Royce Heritage Society, page=40</ref>

Initially, the Mustang's steadfast champion, USAAC/F Assistant Air Attaché Major Thomas Hitchcock, was concerned that the USAAF had little or no interest in the potential of the P-51A and its development with the Merlin engine. He wrote: "Its development in this theatre has suffered for various reasons. Sired by the English out of an American mother, the Mustang has no parent in the Army Air Corps to appreciate and push its good points. It does not fully satisfy good people on both sides of the Atlantic who seem more interested in pointing with pride to the development of a 100% national product..."<ref>Birch, David. Rolls-Royce and the Mustang Page=38. Derby, UK: Rolls-Royce Heritage Trust, 1987. ISBN 0-9511710-0-3.</ref>

Nevertheless, during the British service development program of the Mustang I at [[Hucknall Aerodrome#Rolls-Royce Flight Test Establishment|Rolls-Royce's airfield at Hucknall]], a close relationship was developed between NAA, the RAF Air Fighting Development Unit and Rolls Royce Rolls-Royce Flight Test Establishment at Hucknall.

Following extensive communication between Hitchcock (based in England), Rolls Royce engineers and Phillip Legarra at NAA regarding the promising outlook of a Merlin Mustang, along with the subsequent work in progress by Rolls Royce on the Mustang X, NAA representatives including Mustang designer Schmued visited the UK to examine and discuss the project in detail.<ref>Birch, David. Rolls-Royce and the Mustang. Derby, UK: Rolls-Royce Heritage Trust, 1987. ISBN 0-9511710-0-3</ref>

The promising calculations and modification progress by Rolls Royce led in July 1942 to a contract being let for two NAA Merlin prototypes, briefly designated XP-78, but soon to become the XP-51B.<ref>{{cite book |last1=Gruenhagen |first1=Robert |title=Mustang: The Story of the P-51 Fighter (rev. ed.) |date=1976 |publisher=Arco Publishing |location=New York |page=68}}</ref> Based on the Packard V-1650-3 duplicating the Merlin 61's performance, NAA estimated for the XP-78 a top speed of {{convert|445|mph|kn km/h|abbr=on}} at {{convert|28000|ft|m}}, and a service ceiling of {{convert|42000|ft|m}}.<ref name="Marshall 2020" />

Initial flights of what was known to Rolls-Royce as the [[Rolls-Royce Mustang X|Mustang X]] were completed at Hucknall in October 1942.<ref name="Marshall 2020" />

The first flight of the US version, designated XP-51B took place in November 1942, but the USAAF had become so interested in the Merlin Mustang project that an initial contract for 400 aircraft was placed three months beforehand in August.<ref>{{cite book |last1=Gruenhagen |first1=Robert W. |title=Mustang: The Story of the P-51 Fighter (rev. ed.) |date=1976 |publisher=Arco Publishing |location=New York |page=74}}</ref> The conversion led to production of the P-51B beginning at NAA's Inglewood, California, plant in June 1943,<ref>{{cite book |last1=Newby Grant |first1=William |title=P-51 Mustang |year=1980 |publisher=Paul Hamlyn Pty Ltd |location=UK |isbn=0-7296-0183-8 |pages=24}}</ref> and P-51s started to become available to the 8th and [[Ninth Air Force|9th air force]]s in the winter of 1943–1944. Conversion to the two-stage supercharged and intercooled Merlin 60 series, over {{convert|350|lbs|abbr=on}} heavier than the single-stage Allison, driving a four-bladed Hamilton Standard propeller, required moving the wing slightly forward to correct the aircraft's [[Center of gravity of an aircraft|center of gravity]]. After the USAAF, in July 1943, directed fighter aircraft manufacturers to maximize internal fuel capacity, NAA calculated the P-51B's center of gravity to be forward enough to include an additional {{convert|85|gal|abbr=on}} fuel tank in the fuselage behind the pilot, greatly increasing the aircraft's range over that of the earlier P-51A. NAA incorporated the tank in the production of the P-51B-10, and supplied kits to retrofit it to all existing P-51Bs.<ref name="Marshall 2020" />