By William Pearce

In the early 1930s, Avions Farman (Farman) built the F.1000-series of aircraft to break altitude records. On 5 August 1935, the F.1001 reportedly achieved stable flight at around 10,400 m (34,120 ft). However, one of the small windows in the aircraft’s pressure vessel soon failed. The sudden decompression incapacitated the pilot, Marcel Cognot, and the aircraft crashed.

In late 1936, France began a program of nationalizing its arms industry, which many aviation manufacturers fell under. In early, 1937 Farman was combined with Aéroplanes Hanriot to create the state-run Société Nationale des Constructions Aéronautiques du Center (SNCAC or Aérocentre, the National Company of Aeronautical Constructions of the Center).

SNCAC initiated development of some advanced aircraft and designed other aircraft to serve as technological testbeds. One of these aircraft was the NC 130 (NC standing for Nationale Center), a twin-engine monoplane built around a cabin pressure vessel. The NC 130 was designed by Marcel Roca, the former head of the Farman design office, and it had an anticipated service ceiling of 34,777 ft (10,600 m). The NC 130 made its first flight in 1939 but was destroyed in the early part of World War II. Roca and his team also designed the NC 160, a monoplane dive bomber with contra-rotating propellers. The NC 160 did not progress beyond the design stage.

After the German invasion of France on 10 May 1940, SNCAC personnel and offices were relocated south from Boulogne-Billancourt, near Paris, and untimely to Cannes on the Mediterranean Sea. SNCAC, along with SNCASE (Société nationale des constructions aéronautiques du Sud-Est, National Company of Aeronautical Constructions of the South East), SNCAO (Société nationale des constructions aéronautiques de l’ouest, National Company of Aeronautical Constructions of the West), and CAMS (Chantiers Aéro-Maritimes de la Seine, Aero-Maritimes construction sites of the Seine) were combined with and operated under SNCASO (Société nationale des constructions aéronautiques du sud-ouest, National Company of Aeronautical Constructions of the South-West). At the time, southern (Vichy) France operated as an independent and unoccupied ally of Germany, but the state’s “independence” from Germany was certainly not absolute. The German overseers allowed the continued development of commercial and civil aircraft in Southern France.

Roca and the SNCAC team were put in charge of the special aircraft division, which would use the SO 3000-series to designate their designs, SO standing for Sud-Ouest (South West). The SO 3020 was an experimental high-altitude research aircraft designed to observe stratospheric and meteorological conditions, and the basic layout of the aircraft was based on a scaled-up version of the pre-war NC 160 dive bomber design.

The fuselage of the large, taildragger aircraft consisted of three sections. The forward fuselage housed two 1,400 hp (1,030 kW) Hispano-Suiza 12Z engines placed side-by-side and mounted on a tubular frame. Each engine powered half of a six-blade, coaxial, contra-rotating propeller via a SNCAC-designed combining gearbox designated NC T1.

The central fuselage was built around a welded pressure vessel that was 17 ft 1 in (5.20 m) long and 5 ft 7 in (1.70 m) in diameter. A bulge atop the pressure vessel was the cockpit that protruded above the fuselage. A bulge in the lower part of the pressure vessel contained two viewing stations for observations and photography of the stratosphere. The lower bulge was contained within the aircraft’s fuselage, but the fuselage was glazed around the bulge. The cabin pressure vessel accommodated five people: the pilot, a radio operator/navigator, a mechanic, and two scientists/observers. Pressurization of the cabin was achieved by two SNCAC-designed NC 41 positive displacement compressors that were driven directly from the engines. The cabin was accessed via a door in the rear fuselage that led to a hatch at the back of the pressure vessel.

While the forward and center fuselage sections were all-metal monocoque designs, the rear fuselage had a spruce frame that was covered in plywood. The vertical and horizontal stabilizers were also made of wood, but the rudder and elevators had metal frames that were covered with fabric.

The SO 3020’s three-spar wing was of mixed construction, and the main spar attached to a bulkhead that was mounted to the pressure vessel in the central fuselage. The structure of the wing was made mostly of metal, but spruce was used for the front and rear spars of the outer wing sections. The wing was covered with metal. The ailerons had metal frames and were covered in fabric. When retracted, the landing gear was fully enclosed with the main gear in the wing and the tailwheel in the fuselage. The main gear had a wide track of 18 ft 9 in (5.71 m). Tanks within the wings held the aircraft’s 1,836 US gal (6,950 L / 1,529 Imp gal) of fuel.

The SO 3020 had a wingspan of 73 ft 3 in (22.32 m), a length of 65 ft 3 in (17.90 m), and a height of 19 ft 2 in (5.83 m). It was anticipated that the aircraft would cruise at 311 mph (500 km/h) at 33,793 ft (10,300 m) and have a ceiling of 45,932 ft (14,000 m). The SO 3020 had an empty weight of 13,382 lb (6,070 kg) and a gross weight of 26,015 lb (11,800 kg). This would allow the aircraft to carry 11,023 lb (5,000 kg) of fuel, 1,014 lb (460 kg) of freight, and five crew members. Range was 4,169 miles (6,710 km) with an endurance of seven hours.

Work on the SO 3020 was allowed to move forward in mid-1941, but progress was slow due to the war situation. A full-size wooden mockup was built toward the end of 1942. When Germany invaded Vichy France in early November 1942, progress on the SO 3020 slowed even further. In March 1943, the letter designation ‘B’ was assigned to SNCASO aircraft, and the SO 3020 was given the name “Belphégor,” for the demon who seduces people by suggesting to them ingenious inventions that will make them rich.

Construction of the SO 3020 continued throughout the war. The aircraft, its design team, and other SNCASO operations were moved west to Le Flayosquet in early 1944. This move was a result of a British air raid on Cannes in November 1943 and was finally completed in May 1944. However, after the Allied landings and subsequent liberation of France, everything was moved back to Cannes between November 1944 and January 1945. With the liberation of France, the nationalized aircraft manufacturers were restored, and SNCAC broke off from SNCASO. The SO 3020 and everything else associated with SNCAC was moved back to Boulogne-Billancourt.

By early 1946, the SO 3020 was complete with the exception of its engines. The war had delayed work on the Hispano-Suiza 12Z, and it would be some time before the engines would be available. As a result, the decision was made to switch to a single 2,950 hp (2,170 kW) Daimler-Benz DB 610 engine. The DB 610 consisted of two coupled DB 605 engines and was similar to what was planned with the two 12Z engines and the NC T1 gearbox. DB 605 engines were available to France and SNCAC in the immediate post-war era. With this new configuration, the aircraft was redesignated NC 3021. At the time, a number of experiments were planned for the aircraft to study cosmic rays and their interaction with the atmosphere.

In March 1946, the NC 3021 was transferred from Boulogne-Billancourt to the Toussus-le-Noble airfield for final assembly. With the DB 610 engine, the contra-rotating propellers were discarded, and a single, four-blade propeller was used. This VDM propeller was 14 ft 9 in (4.5 m) in diameter and was most likely the same as that used on the German Heinkel He 177 bomber. The DB 610 engine was mounted on a tubular frame at the front of the aircraft, and an annular radiator was installed around the propeller’s extension shaft. Ducts on each side of the cowling delivered air to the transversely-mounted superchargers at the rear of the engine. Air for the cabin and its pressurization was brought in from a duct under the spinner. This sealed duct passed around the lower exhaust stacks which helped heat the air.

The NC 3021 had a wingspan of 73 ft 3 in (22.32 m), a length of 65 ft 3 in (17.90 m), and a height of 19 ft 2 in (5.83 m). The aircraft’s estimated performance was a maximum speed of 348 mph (560 km/h) at 19,685 ft (6,000 m) and a cruising speed of 280 mph (450 km/h) at 39,370 ft (12,000 m). The aircraft had a landing speed of 87 mph (140 km/h), an initial rate of climb of 1,968 fpm (10 m/s), and a ceiling of 41,995 ft (12,800 m). Compared to the SO 3020, the NC 3021’s empty weight had increased 3,880 lb (1,760 kg) to 17,262 lb (7,830 kg), and its gross weight had decreased 3,073 lb (1,394 kg) to 22,941 lb (10,406 kg).

The NC 3021 was completed at the end of May and registered as F-WBBL. Taxi tests were initiated at the beginning of June, and the aircraft made its first flight on 6 June 1946 with Joanny Burtin as the pilot. The aircraft suffered from directional instability, and a dorsal fairing was soon added in front of the tail to increase its lateral surface area. Testing was brought to a halt later that summer when the right main gear collapsed. The landing gear manufacturer was slow to provide a new main gear leg, and SNCAC resumed flight tests as best as it could with a temporarily repaired main gear fixed in the down position.

The landing gear was eventually repaired, but the DB 610 engine proved to be difficult to service and maintain. To make matters worse, SNCAC was having financial issues and did not have the funds to spend on an experimental project that offered little in return. When SNCAC delivered the NC 3021 to the Centre d’essais en vol (CEV, Flight Test Center) at Brétigny-sur-Orge on 12 October 1948, the aircraft had only made 45 flights for a total of 40 hours of flight time.

The CEV worked to maintain and test the NC 3021. By April 1949, the CEV had put in 1,500 hours of work on the NC 3021 but had only flown the aircraft for 2 hours and 45 minutes. The CEV did not want to continue to operate the aircraft, and SNCAC declared bankruptcy in July 1949. There were no other parties interested in funding the expensive and difficult to maintain experimental aircraft, and the NC 3021 was most likely scrapped in late 1950.

Sources:
– “NC-3021 Belphégor: le monstre de la haute altitude” by Philippe Ricco, Avions #207 (September/October 2015)
– “NC-3021 Belphégor: le monstre de la haute altitude” by Philippe Ricco, Avions #208 (November/December 2015)
– Les Avions Farman by Jean Liron (1984)
– Jane’s All the World’s Aircraft 1949-50 by Leonard Bridgman (1949)
– https://aviation-safety.net/wikibase/194708