The Dewoitine D.520 Story
The Dewoitine D.520 was 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.[1] 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, 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, 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 On 3 September 1953, the last D.520s were finally withdrawn from service.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.[2] 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.[2]
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.[2] The envisioned aircraft would be as affordable as possible, be powered by the new 900 hp Hispano-Suiza 12Y21 liquid-cooled engine, and be capable of attaining 500 km/h (310 mph). However, the corresponding design was promptly rejected by the Air Ministry.[2] 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 500 km/h.[3]
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.[3] 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.[4] By this point, official attention was orientated towards the MS.405, which had already been selected for the re-equipment effort.[5]
Specification and rivals
On 12 January 1937, the A.23 technical programme was launched by the Air Ministry.[5] The specifications called for a maximum speed of 520 km/h (320 mph) at 4,000 m (13,000 ft), the ability to climb to 8,000 m (26,000 ft) in less than fifteen minutes, with takeoff and landing runs not exceeding 400 m (1,300 ft).[4] The armament was to be two 7.5 mm (0.295 in) machine guns and one 20 mm Hispano-Suiza HS.9 cannon, or two HS.9 cannon.[3]
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.[6]
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.[7]
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.[5] 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.[5]
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.[5] 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 480 km/h (300 mph), and suffered from dangerously high engine temperatures.[8][5]
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.[9] Other aerodynamic improvements were made around this time, such as the minor enlargement of the fin and rudder, for greater lateral stability.[5]
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.[5] 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 530 km/h (330 mph). The maximum diving speed was 825 km/h (513 mph), as reached on 6 February 1939.[10][5]
Additional prototypes
During 1939, the first prototype was followed be two further examples: the D.520-02 and the 03, these were first flown on 9 January 1939 and 5 May 1939, respectively.[11] 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.[12][11]
These prototypes were also provisioned with armaments, being armed with a single 20 mm Hispano-Suiza HS.9 cannon in an engine mount in which the barrel was mounted in a sleeve between the cylinder banks and fired through the propeller spinner.[nb 1] and a pair of MAC 1934 7.5 mm (0.295 in) machine guns, each initially with 300 rounds per gun, housed in small pods under the wing.[5] 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 550 km/h (340 mph) and reached 8,000 m (26,000 ft) in 12 minutes 53 seconds.[8][5]
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.[13] 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.[14]
Production
In April 1939, the rate of production of fighter aircraft throughout France was far beneath official expectations.[13] 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.[13] Another French Air Force order in April 1940 brought the total to 2,250 and increased production quotas to 350 a month.[15] In addition to domestic orders, Poland was interested in acquiring around 160 D.520s in order.[13]
On 2 November 1939, the first-production D.520 conducted its first flight.[13] On this and the other aircraft, the rear fuselage was extended by 51 cm (20 in), 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.[15] Most production examples were powered by the 935 CV (922 hp) Hispano-Suiza 12Y-45 with the new Szydlowski-Planiol supercharger, although later production versions used the 960 CV (950 hp) Hispano-Suiza 12Y-49.[16] The production standard armament was a 20 mm HS.404 cannon firing through the propeller hub and four belt-fed MAC 1934 M39 7.5 mm (0.295 in) machine guns in the wings.[17]
As the first batch of D.520s rolled off the production line, they failed acceptance tests due to insufficient top speed and troublesome cooling.[18] 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.[19] 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.[18]
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.[20][2] 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.[2]
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, Occitanie and had been prepared for their maiden flight when the armistice came into effect.[2] During 1940, negotiations with American manufacturer 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 1,040 hp, instead.[2]
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.[21] 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.[21] 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.[22][21]
Design
Overview
The Dewoitine D.520 was a French fighter aircraft, intended to be a capable contemporary of types such as the Messerschmitt Bf 109 and Supermarine Spitfire. An all-metal structure was used, except for fabric-covered ailerons and tail surfaces. The wing, even if single-spar, was a solid and rigid unit with a secondary spar and many reinforced parts. The inwardly retracting undercarriage had a broad 2.83 m (9.3 ft) track, and was fitted with wide, low-pressure tyres.[23]
The D.520 was designed to be maintained easily with many inspection panels, a rare feature for its time. Recharging the D.520 ammunition was swift and easy; the machine gun magazines required five minutes each and three minutes for the 20 mm cannon. To fill the machine gun ammunition boxes took 15 minutes, while five minutes were needed to empty the 20 mm box (the cartridges were not expelled). The D.520's cockpit was set well back in the fuselage, aft of the trailing edge of the wings. This gave the pilot good downward visibility, but the long nose in front of him was a drawback when taxiing on the ground.[24]
A self-sealing fuel tank with a capacity of 396 litres (87 imperial gallons) was mounted between the engine and cockpit, along with two wing tanks which, combined, carried another 240 L (53 imp gal), for a total of 636 L (140 imp gal);[25] this was considerably more than the contemporary Bf 109E, Spitfire I and early Italian fighters, each with about 400 L (88 imp gal) fuel capacity. The ferry range was from 1,300 km (810 mi) to 1,500 km (930 mi) at 450 km/h (280 mph) which, from June 1940, allowed D.520s to escape to North Africa when France fell.[20]
Engine
The Hispano-Suiza 12Y-45 engine was an underpowered, older design, with 850 CV (840 hp) at takeoff at 2,400 rpm, or 935 CV (922 hp) emergency power at 2,520 rpm and at a height of 1,900 m (6,200 ft).[26] The Hispano engine had some advantages over some later engines; for example, its weight was only 515 kg (1,135 lb), compared to the 620 kg (1,370 lb) of the Rolls-Royce Merlin III. Development of the engine had not been straightforward, and had delayed overall production of the aircraft.[16] According to an aviation author, it was alleged by American aviator Charles Lindbergh that secret negotiations were conducted between Dewoitine and Daimler-Benz during 1939 to obtain the DB.601 engine for the D.520, but that these did not come to fruition due to the war.[13]
Fuel was fed via six Solex S.V. 56 carburettors mounted on an inlet manifold which directed compressed air from the supercharger to the engine cylinders; the 12Y-45 and -49s fitted to production D.520s used either 92 or 100 octane fuel.[27] The -45 drove an electrically operated Ratier Type 1606M three-bladed, 3-metre-diameter (9.8 ft), variable-pitch propeller, while D.520s from No. 351 were fitted with the 12Y-49 960 CV (950 hp) engine driving a Chauvière type 3918 pneumatically operated propeller, also 3 m in diameter.[28]
By 1940 a version of the 520D was flying with a Rolls Royce Merlin X engine. Ernest Hives stated that agreement had been reached with the French Government for the manufacture of the Merlin in France. ("The Merlin in perspective" - A Harvey-Bailey -Rolls Royce Heritage Trust, p130)
Fire suppression system
The D.520 had a fire-suppression system with a fire extinguisher activated from the cockpit. The engine was started by a simple but effective system, operating with compressed air. A Viet 250 air compressor charged several air bottles (one with a 12-litre capacity, as well as another eight litre tank, three smaller one litre units were matched to the weapons). The 12-litre air bottle was used for the brakes and later, for the Chauvière propeller's constant speed adjustment. The small air bottles provided up to 12 seconds at 9,000 m (30,000 ft) or 20 seconds at low level, before the Viet air compressor recharged them. The pilot had a complete set of cockpit instruments, and a ten-litre oxygen bottle located in the fuselage just behind his seat, with either a Munerelle or Gourdou oxygen regulator system mounted on the right instrument panel.[29] Equipment included a Radio Industrie Type Rl 537 radio-receiver set, an OPL RX 39 reflector sight (less effective than the Revi system), a height-adjustable seat, and a sliding canopy with large, clear panels.[30] Except over the long nose, the pilot's view was good, since he was seated quite high over the forward fuselage; however, no rear-facing mirrors were fitted.
Armament
Production-standard armament consisted of a 20 mm HS.404, which had an ammunition capacity of 60 rounds, firing through the propeller hub, and four belt-fed MAC 1934 M39 7.5 mm (0.295 in) machine guns in the wings, with 675 rounds per gun.[17] The MAC 1934 machine guns had a high rate of fire of 1,200 rounds per minute (rpm), while the effective HS.404 fired at 600 rpm and was accurate up to 500 m (1,600 ft); the ammunition capacity meant that the machine guns could be fired for a total of 30 seconds, while the cannon had ten seconds' worth of ammunition.
In combat, the MS.406 had only a pair of 7.5 mm machine guns and was, therefore, at a disadvantage when the HS.404 had used up its ammunition, while a D.520 could continue to fight effectively because it had four fast-firing machine guns (over 80 rounds/sec), with 20-plus seconds of ammunition still available. The D.520 had provisions for two BE33 "illuminating bombs", useful for nocturnal interception missions, but these were seldom used because French fighters rarely flew nighttime missions.[31]
Flight performance
Although employing a modern design philosophy for its time, the D.520 was considered more difficult to fly than the older MS.406.[20][32] Captain Eric Brown, commanding officer of the Royal Aircraft Establishment's Captured Enemy Aircraft Flight, tested the D.520 at RAE Farnborough, saying that "It was a nasty little brute. Looked beautiful but didn't fly beautifully. Once you get it on the ground, I was told not to leave the controls until it was in the hangar and the engine stopped. You could be taxiing toward the hangar and sit back when suddenly it would go in a right angle."[33]
The handling changed according to the amount of fuel carried; using the fuselage tank alone, fuel consumption had no appreciable effect on handling because the tank was at the centre of gravity, but with full wing tanks, directional control was compromised, especially in a dive. The flight controls were well harmonized and the aircraft was easy to control at high speed. The maximum dive speed tested was 830 km/h (520 mph) with no buffeting and excellent stability both in the dive (depending on fuel load) and as a gun platform.
Variants
D.520
The main production version, sometimes designated the D.520 S (for série – production) or the D.520 C1 (for chasseur – fighter, single-seat).
Direct derivatives
In 1940, SNCAM had several projects to fit the D.520 airframe with more powerful engines. These developments were halted by the June armistice.[63]
D.521
engine replaced by a Rolls-Royce Merlin III, one example was built, but the project was cancelled.
D.522
engine replaced by an Allison V-1710 C-1, project abandoned after the armistice (22 June 1940).
D.523
engine replaced by slightly different sub-variants of the 820 kW (1,100 hp) Hispano-Suiza 12Y-51, with Szydlowski-Planiol supercharger. D.523 Prototype was completing pre-production trials in June 1940.
D.524
Version powered by Hispano-Suiza 12Y-89ter engine. One prototype built, but it never flew.
D.525
development version of the D.523
D.530
planned version with a 1,044 kW (1,400 hp) Rolls-Royce Merlin or a 1,342 kW (1,800 hp) Hispano-Suiza 12Y.
Related prewar projects
HD.780
Floatplane derivative of the D.520, one prototype built, but it never flew, the development was cancelled with the armistice.
D.790
Carrier-borne derivative project, none built.
D.550
Unarmed aircraft built for speed record attempt, with an airframe loosely based on the D.520, but using weight-saving construction techniques. One example was built.
D.551 and D.552
Military developments of the D.550. 12 examples were built, but none flew. The development was resumed in 1941, but was quickly terminated by the Germans.
Post-armistice developments
Several projects were initiated after the June 1940 armistice. They were all terminated with the German occupation of Southern France in November 1942.
D.520 amélioré
Single production D.520 experimentally fitted with minor improvements to improve top speed with an unchanged engine.
D.520 Z
D.520 airframe with 12Z engine and minor improvements. One example was built. The development resumed after the war (as SE.520Z), but was eventually cancelled in 1949.
M.520 T
Different airframe loosely based on the D.520. None built.
Postwar derivative
D.520 DC (double commande – dual control)
Two-seat trainer conversion, at least 13 built.
Related development
D.521 - D.522 - D.523 - D.524 - D.525 - D.530
Markings
Apart from the first prototype and postwar examples, D.520s sported the usual French camouflage of dark blue-grey, khaki, and dark brown with light blue-grey undersurfaces. The camouflage pattern was not standardized. National markings were the standard light-blue-white-red roundels on the wingtips, as well as on the rear fuselage, and the rudder flag.
Specific markings were applied during the Vichy era, consisting of white outlined fuselage roundels with a white fuselage stripe, and from mid-1941 on, the infamous "slave's pajamas" with red and yellow stripes on the engine cowling and tail surfaces.
Operators
Kingdom of Bulgaria - Bulgarian Air Force
France - French Air Force (Armée de l'Air) and French Navy (Aéronautique navale) and later the Vichy French Air Force (Armée de l'air de Vichy)
Free France - Free French Air Force
Germany - Luftwaffe
Italy - Regia Aeronautica
Intended operators
Romania - Royal Romanian Air Force
Surviving aircraft
n°603 – D.520 on display at the Conservatoire de l’air et de l’espace d'Aquitaine in Bordeaux–Mérignac.[64]
n°650 – D.520 under restoration at the Musée National de la Marine in Rochefort.[65]
n°862 – D.520 on display at the Musée de l’air et de l’espace. It is painted as n°277, which was used by GC III/6 in June 1940.[66]