The Dewoitine D.520 is a French fighter aircraft that entered service in early 1940, shortly after the beginning of the Second World War.
The D.520 was designed in response to a 1936 requirement from the French Air Force for a fast, modern fighter with a good climbing speed and an armament centred on a 20 mm cannon. At the time the most powerful V-12 liquid-cooled engine available in France was the Hispano-Suiza 12Y, which was less powerful, but lighter than contemporary engines such as the Rolls-Royce Merlin and Daimler-Benz DB 601. Other fighters were designed to meet the specifications but none of them entered service, or entered service in small numbers, too late to play a significant role during the Battle of France.
Unlike the Morane-Saulnier M.S.406, which was at that time the most numerous fighter in the French Air Force, the Dewoitine D.520 came close to being a match for the latest German types, such as the Messerschmitt Bf 109. It was slower than the Bf 109E but superior in manoeuvrability.Angelucci and Matricardi 1978, p. 262. Because of production delays, only a small number were available for combat against the Luftwaffe. The D.520 proved to be relatively capable as a dogfighter against the Luftwaffe's inventory, but lacked sufficient numbers to make a difference.
Following the armistice of 1940, the D.520 continued to be used, being operated by both the Free French Air Force and the Vichy French Air Force. The type was also returned to production during 1942, although it was manufactured at a lower rate than it had been during 1940. Additional examples were operated by the Luftwaffe, Regia Aeronautica Italiana, and the Bulgarian Air Force. The D.520 saw combat service in North Africa, Bulgaria, and the Eastern Front, as well as use in France and Germany for training and defence purposes. During the type's later life, it was used as a trainer aircraft. On 3 September 1953, the last D.520s were finally withdrawn from service.
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.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.Danel 1971, p. 3. While French aircraft company Dewoitine initially responded with an improved design based on the D.510, designated as the D.513, this quickly proved to be inferior to the in-development Morane-Saulnier M.S.405, a domestic competitor.
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. The envisioned aircraft would be as affordable as possible, be powered by the new Hispano-Suiza 12Y21 liquid-cooled engine, and be capable of attaining . However, the corresponding design was promptly rejected by the Air Ministry. 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 .
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. 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.Danel 1971, pp. 3–4. By this point, official attention was oriented towards the MS.405, which had already been selected for the re-equipment effort.Danel 1971, p. 4.
Specification and rivals On 12 January 1937, the A.23 technical programme was launched by the Air Ministry. The specifications called for a maximum speed of at , the ability to climb to in less than fifteen minutes, with takeoff and landing runs not exceeding . The armament was to be two machine guns and one Hispano-Suiza HS.9 cannon, or two HS.9 cannons.Danel and Cuny 1966, p. 30.
Other aircraft designed to the same specification included the Morane-Saulnier M.S.450, the Loire-Nieuport 60 (later C.A.O 200), and the Caudron-Renault C.770, none of which either left the drawing board or entered service. Two other concurrent French designs, the 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.Danel and Cuny 1966, pp. 28, 295.
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.Danel and Cuny 1966, pp. 30, 296.
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. 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.
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. 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 , and suffered from dangerously high engine temperatures.Caruana 1998, p. 728.
A large portion of responsibility for the encountered temperature issue was judged to have been a product of greater than expected drag resulting from the underwing 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.Danel and Cuny 1966, pp. 34, 296. Other aerodynamic improvements were made around this time, such as the minor enlargement of the fin and rudder, for greater lateral stability.
After sustaining minor damage in a landing accident on 27 November 1938, caused by the failure to deploy the undercarriage, further modifications were made to the prototype. 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. These changes were enough to allow the aircraft to reach its design speed, achieving . The maximum diving speed was , as reached on 6 February 1939.Danel and Cuny 1966, pp. 34–36, 297.
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.Danel 1971, pp. 4–5. The major differences between the first prototype and the second and third were the adoption of a new sliding canopy, the fitting of a re-designed and larger tail unit, and longer Oleo-manufactured undercarriage legs; they also omitted the Handley Page slats fitted to the outer wings on 01.Danel and Cuny 1966, pp. 36–40, 297.
These prototypes were also armed with a single Hispano-Suiza HS.9 cannon with the barrel was in a sleeve between the cylinder banks and fired through the propeller spinner. and a pair of MAC 1934 machine guns, each initially with 300 rounds per gun, housed in small pods under the wing. 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 and reached in 12 minutes 53 seconds.
In response to Belgian interest in the D.520, who were at one stage preparing to negotiate for a licence to produce the type, several evaluation flights of the third prototype were flown by Belgian test pilot Captain Arendt.Danel 1971, p. 5. 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.Caruana 1998, pp. 728, 730.
Production In April 1939, the rate of production of fighter aircraft throughout France was far beneath official expectations. 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 Aéronautique Navale placed its own order for 120 aircraft. Another French Air Force order in April 1940 brought the total to 2,250 and increased production quotas to 350 a month.Caruana 1998, p. 730. In addition to domestic orders, Poland was interested in acquiring around 160 D.520s in order.
On 2 November 1939, the first-production D.520 conducted its first flight. On this and the other aircraft, the rear fuselage was extended by , 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. Most production examples were powered by the Hispano-Suiza 12Y-45 with the new Szydlowski-Planiol supercharger, although later production versions used the Hispano-Suiza 12Y-49.Danel 1971, pp. 5–6. The production standard armament was a HS.404 cannon firing through the propeller hub and four belt-fed MAC 1934 M39 machine guns in the wings.Danel and Cuny 1966, p. 249.
As the first batch of D.520s rolled off the production line, they failed acceptance tests due to insufficient top speed and troublesome cooling.Danel 1971, p. 6. 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.Danel and Cuny 1966, p. 73. 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.
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. 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.