Editing Dewoitine D.520 (section)

==Development==
===Background===
On 13 July 1934, the French Air Force launched a new technical programme, under which the development of improved fighter aircraft to improve upon the [[Dewoitine D.500|Dewoitine D.510]], which was yet to enter service at that point but was already considered to be obsolete in the face of rapid advances being made in several European nations.<ref name="Danel 3">Danel 1971, p. 3.</ref> While French aircraft company [[Dewoitine]] initially responded with an improved design based on the D.510, designated as the [[Dewoitine D.500|D.513]], this quickly proved to be inferior to the in-development [[Morane-Saulnier M.S.406|Morane-Saulnier M.S.405]], a domestic competitor.<ref name="Danel 3"/>

In response to a specification for a new fighter promulgated by the French Air Ministry on 15 June 1936, [[Émile Dewoitine]], owner and founder of Dewoitine, formed a new private design office and ordered the firm's chief engineer, Robert Castello, to immediately study the development of a new fighter.<ref name="Danel 3"/> The envisioned aircraft would be as affordable as possible, be powered by the new {{cvt|900|hp|order=flip}} [[Hispano-Suiza 12Y]]21 liquid-cooled engine, and be capable of attaining {{cvt|500|km/h}}. However, the corresponding design was promptly rejected by the Air Ministry.<ref name="Danel 3"/> The design had been rejected by the French Air Ministry, which, after being impressed by the British [[Hawker Hurricane]] and [[Supermarine Spitfire]], had decided to respond by uprating the specifications to include a maximum speed requirement of {{cvt|500|km/h}}.<ref name="Dan30"/>

Accordingly, work on what would become the D.520 commenced in September 1936; according to aviation author Raymond Danel, the D.520 designation was a deliberate reference to the required speed of the aircraft.<ref name="Dan30"/> During January 1937, this revised design proposal was submitted to the Service Technique Aeronautique (STA); while the STAe found the design to be likely to conform with the specified requirements, no order for prototypes to be built was immediately forthcoming.<ref name="Danel 3 4">Danel 1971, pp. 3–4.</ref> By this point, official attention was oriented towards the MS.405, which had already been selected for the re-equipment effort.<ref name="Danel 4">Danel 1971, p. 4.</ref>

===Specification and rivals===
On 12 January 1937, the A.23 technical programme was launched by the Air Ministry.<ref name="Danel 4"/> The specifications called for a maximum speed of {{cvt|520|km/h}} at {{cvt|4000|m}}, the ability to climb to {{cvt|8000|m}} in less than fifteen minutes, with takeoff and landing runs not exceeding {{cvt|400|m}}.<ref name="Danel 3 4"/> The armament was to be two {{cvt|7.5|mm|3}} [[machine gun]]s and one {{cvt|20|mm|3}} [[Oerlikon FF|Hispano-Suiza HS.9 cannon]], or two HS.9 cannons.<ref name="Dan30">Danel and Cuny 1966, p. 30.</ref>

Other aircraft designed to the same specification included the [[Morane-Saulnier M.S.406#M.SE0|Morane-Saulnier M.S.450]], the [[SNCAO 200|Loire-Nieuport 60 (later C.A.O 200)]], and the [[Caudron C.714|Caudron-Renault C.770]], none of which either left the drawing board or entered service. Two other concurrent French designs, the [[Bloch MB.150#Variants|Bloch MB.152.01 and Bloch MB.155.C1]] series and the [[Arsenal VG-33]] entered service in small numbers with the French Air Force during the Battle of France, but too late to play a significant role.<ref>Danel and Cuny 1966, pp. 28, 295.</ref>

In addition to the lack of a prototype order, Dewoitine was absorbed into the larger Société nationale des constructions aéronautiques du Midi ([[SNCAM]]) state-owned manufacturing consortium. As a result of this organisational restricting, along with continued alterations within the French Air Force's established manufacturing programmes, work on the design of the D.520 was suspended throughout much of 1937, and it was not until January 1938 that a small number of draughtsmen started work on the first detailed drawings for the [[prototype]].<ref>Danel and Cuny 1966, pp. 30, 296.</ref>

However, Émile Dewoitine, now the deputy managing director of SNCAM, was keen to proceed with the project and decided to proceed to the detail design drawing phase with the aim of producing a pair of prototypes and a single structural test frame, confident that official interest would be found for the type.<ref name="Danel 4"/> On 3 April 1938, this private initiative was rewarded with the issuing of Air Ministry contract No. 513/8, which regularised the programme; by this point, the first prototype had almost been completed.<ref name="Danel 4"/>

===Modifications and first prototypes===
On 2 October 1938, the first prototype aircraft, D.520-01, powered by a [[Hispano-Suiza 12Y]]-21 engine that temporarily drove a fixed-pitch, two-bladed wooden propeller, performed its [[maiden flight]].<ref name="Danel 4"/> Completion of the prototype had been delayed somewhat by the need to incorporate modifications requested by the STAe following their examination of a wooden mock-up of the type. During early flight tests, the first prototype managed to reach only {{cvt|480|km/h}}, and suffered from dangerously high engine temperatures.<ref name="Car728">Caruana 1998, p. 728.</ref><ref name="Danel 4"/>

A large portion of responsibility for the encountered temperature issue was judged to have been a product of greater than expected [[Drag (physics)|drag]] resulting from the underwing [[Radiator (engine cooling)|radiators]], which exhausted across the upper wing surface and were relatively inefficient; this arrangement was replaced with a single radiator unit housed under the [[fuselage]] in a streamlined fairing.<ref>Danel and Cuny 1966, pp. 34, 296.</ref> Other aerodynamic improvements were made around this time, such as the minor enlargement of the [[fin]] and [[rudder]], for greater lateral stability.<ref name="Danel 4"/>

After sustaining minor damage in a landing accident on 27 November 1938, caused by the failure to deploy the [[landing gear|undercarriage]], further modifications were made to the prototype.<ref name="Danel 4"/> These included changing the engine to a newer -29 model and incorporating exhaust ejectors which provided added thrust, along with a three-blade [[Variable-pitch propeller (aeronautics)|variable-pitch propeller]]. These changes were enough to allow the aircraft to reach its design speed, achieving {{cvt|530|km/h}}. The maximum diving speed was {{cvt|825|km/h}}, as reached on 6 February 1939.<ref>Danel and Cuny 1966, pp. 34–36, 297.</ref><ref name="Danel 4"/>

===Additional prototypes===
During 1939, the first prototype was followed by two further examples: the D.520-02 and the 03, these were first flown on 9 January 1939 and 5 May 1939, respectively.<ref name="Danel 4 5">Danel 1971, pp. 4–5.</ref> The major differences between the first prototype and the second and third were the adoption of a new sliding [[Aircraft canopy|canopy]], the fitting of a re-designed and larger tail unit, and longer [[Oleo strut|Oleo]]-manufactured undercarriage legs; they also omitted the [[Handley Page]] [[Leading edge slats|slats]] fitted to the outer wings on 01.<ref>Danel and Cuny 1966, pp. 36–40, 297.</ref><ref name="Danel 4 5"/>

These prototypes were also armed with a single {{cvt|20|mm|3}} [[Hispano-Suiza HS.9]] cannon with the barrel was in a sleeve between the cylinder banks and fired through the propeller spinner.{{#tag:ref|This was a feature introduced by [[Hispano-Suiza]] during the [[World War I|First World War]] and later found on many German designs (as the ''Motorkanone''), as well as on Russian derivatives of the 12Y engine.|group=nb}} and a pair of [[MAC 1934]] {{cvt|7.5|mm|3}} [[machine gun]]s, each initially with 300 rounds per gun, housed in small pods under the wing.<ref name="Danel 4"/> The third prototype also introduced a small tailwheel instead of the original skid. The second prototype was later fitted with a [[Hispano-Suiza 12Y-31]] engine and achieved a maximum speed of {{cvt|550|km/h}} and reached {{cvt|8000|m}} in 12 minutes 53 seconds.<ref name="Car728"/><ref name="Danel 4"/>

In response to [[Belgium|Belgian]] interest in the D.520, who were at one stage preparing to negotiate for a [[Licensed production|licence to produce]] the type, several evaluation flights of the third prototype were flown by Belgian test pilot Captain Arendt.<ref name="Danel 5">Danel 1971, p. 5.</ref> During late September 1939, CEMA took charge of the third prototype to conduct armament trials. Overall, flight tests had proceeded successfully, and resulted in the issuing of a contract in March 1939 for 200 production machines to be powered by the newer -31 engine (later replaced by the -45). A contract for an additional 600 aircraft was issued in June, albeit reduced to 510 in July 1939.<ref>Caruana 1998, pp. 728, 730.</ref>

===Production===
In April 1939, the rate of production of fighter aircraft throughout France was far beneath official expectations.<ref name="Danel 5"/> With the outbreak of war, a new contract brought the total of D.520s on order to 1,280, with the production rate to be 200 aircraft per month from May 1940. During January 1940, the ''[[French Naval Aviation|Aéronautique Navale]]'' placed its own order for 120 aircraft.<ref name="Danel 5"/> Another French Air Force order in April 1940 brought the total to 2,250 and increased production quotas to 350 a month.<ref name="Car730">Caruana 1998, p. 730.</ref> In addition to domestic orders, [[Poland]] was interested in acquiring around 160 D.520s in order.<ref name="Danel 5"/>

On 2 November 1939, the first-production D.520 conducted its first flight.<ref name="Danel 5"/> On this and the other aircraft, the rear fuselage was extended by {{cvt|51|cm}}, the engine cowling panels were redesigned, the curved, one-piece windscreen was replaced by one containing an optically flat panel and armour plate was fitted behind the pilot's seat.<ref name="Car730"/> Most production examples were powered by the {{cvt|935|CV|kW hp}} [[Hispano-Suiza 12Y]]-45 with the new [[Joseph Szydlowski|Szydlowski]]-Planiol [[supercharger]], although later production versions used the {{cvt|960|CV|kW hp}} [[Hispano-Suiza 12Y]]-49.<ref name="Danel 5 6">Danel 1971, pp. 5–6.</ref> The production standard armament was a {{cvt|20|mm|3}} HS.404 cannon firing through the propeller hub and four belt-fed MAC 1934 M39 {{cvt|7.5|mm|3}} machine guns in the wings.<ref name="Danel and Cuny 1966, p. 249">Danel and Cuny 1966, p. 249.</ref>

[[File:D.520 Le Bourget Cckpit06.jpg|thumb|The D.520 on display at Le Bourget showing the pilot's seat and armour plate]]
As the first batch of D.520s rolled off the production line, they failed acceptance tests due to insufficient top speed and troublesome cooling.<ref name="Danel 6">Danel 1971, p. 6.</ref> Redesigned compressor intakes, a modified cooling circuit and propulsive exhaust pipes proved to be effective remedies of these shortcomings, but as early examples had to be retrofitted with these improvements, the type was not declared combat ready until April 1940, at which point the D.520's operational standards had been defined.<ref>Danel and Cuny 1966, p. 73.</ref> In order to speed up the redesign process, a total of four production aircraft were handed back to SNCAM to serve as special test machines.<ref name="Danel 6"/>

The manufacturing process was deliberately optimized, each aircraft consumed a reduced 7,000 man-hours each to produce, roughly half the time compared to the previous D.510 and MS.406, and far less than many other fighters of the time, such as the MC.200/202 (21,000 hours), but around 50% greater than a Bf 109E (4,500 hours). The French Air Ministry planned for over 300 aircraft a month to be built and managed to reach this goal, especially in June 1940, but it was too late to affect the tide of battle.<ref name="Gigli-Cervi"/><ref name="Danel 3"/> The armistice greatly curtailed production of D.520, which would have otherwise been a fighter aircraft produced in far greater numbers and with improved models.<ref name="Danel 3"/>

Additional plans had included another manufacturing line at [[Asnières-sur-Seine]], [[Paris]], for a lightened version of the aircraft, known as the ''D.521''. An initial batch of 18 pre-production D.521s had been produced at [[Bagnères-de-Bigorre]], [[Occitania (administrative region)|Occitanie]] and had been prepared for their maiden flight when the armistice came into effect.<ref name="Danel 3"/> During 1940, negotiations with American manufacturer [[Ford Motor Company|Ford]] had been underway with the aim of establishing the licensed production of an Americanised version of the type, designated as the ''D.522''. This model was anticipated to have been powered by an [[Allison V-1710]]-C15 engine, capable of generating {{cvt|1040|hp|order=flip}}, instead.<ref name="Danel 3"/>

In April 1941, during the aftermath of the armistice with Germany, a new programme was launched in which the production of 1,074 new aircraft were to be manufactured in the unoccupied zone of [[Vichy France]].<ref name="Danel 10"/> Of these, 550 were to be D.520s, which were confirmed as ordered under contract No. 157/41 on 8 August 1941. The intention was for the type to replace all other single-engine fighters that remained in service with the [[Vichy French Air Force]] and to eventually equip additional units that were to be reformed from September 1942 onwards.<ref name="Danel 10"/> An initial batch of 22 D.520s were delivered during August 1941. In total, a further 180 machines were constructed, bringing the production total to 905.<ref name="Jackson p. 17.">Jackson 2003, p. 17.</ref><ref name="Danel 10"/>