Junkers G 31

Junkers
Industry	Aerospace, Aircraft manufacturing
Founded	1895
Founder	Hugo Junkers
Headquarters	Dessau, Germany
Products	Aircraft including the Junkers G 31 airliner

The Junkers G 31 represented a sophisticated three-engine airliner crafted by the German company Junkers. It was notable for being the inaugural aircraft in the Deutsche Luft Hansa fleet to include a flight attendant, thereby elevating passenger service standards.

Conceived in the mid-1920s as a successor to the smaller Junkers G 24, the G 31 featured an all-metal airframe and aerodynamic styling that was progressive for its era. Although retractable landing gear was initially considered, this feature was ultimately discarded because the weight increase outweighed the aerodynamic gains. Various powerplants were tested since the originally planned Junkers L5 engines proved underpowered.

Development

Following the commercial achievements of the tri-motor Junkers G 24, Professor Hugo Junkers embarked on designing a new, larger airliner focused on enhanced comfort and simplified maintenance and operation. The G 31 pioneered onboard catering services delivered by flight attendants, earning it nicknames such as 'flying dining car' and 'flying moving van' due to its roomy fuselage.

Early design concepts included retractable landing gear; however, wind tunnel experiments overseen by Ernst Zindel led to its rejection in favor of fixed gear to avoid added weight, expense, and mechanical complexity. The tail skid incorporated a roller to protect grass landing strips.

While originally intended to use three Junkers L5 inline engines, testing revealed insufficient power output. More potent options like the Napier Lion engine were evaluated but dismissed because of high cost and currency limitations. Consequently, most G 31 units were fitted with foreign air-cooled radial engines around 500 horsepower, frequently produced under license in Germany.

Design

The Junkers G 31 was a three-engine all-metal low-wing monoplane designed for passenger transport, sharing fundamental layout elements with its predecessor, the Junkers G 24. Its primary construction material was duralumin, complemented by cast electrum and silumin alloys for components subjected to lower stress such as bearing housings. The wing surfaces were clad in corrugated duralumin sheets supported by tubular frames typical of Junkers engineering. Three external doors provided access: one designated for passengers and two for baggage handling.

The fuselage was segmented into distinct sections: the front housed the nose-mounted third engine; the central section contained the cockpit, passenger cabin, baggage area, and auxiliary systems. Similarly, the wing comprised a central section supporting the fuselage and two outer panels carrying wing-mounted engines along with six integrated fuel tanks. This modular design facilitated transportation and repairs despite increasing overall weight. Sections were fastened with removable screws, while trailing edge flaps reduced landing speeds by approximately 10%.

Piloting duties were performed by two crew members using hand wheels to control elevators and ailerons, pedals for rudder operation, and an instrument panel centrally located featuring stabilizer and rudder position indicators, manual pumps, and radiotelegraphy controls. Optional equipment included individual compasses and inclinometers for each pilot. The right-hand pilot had access to a radio switchboard positioned behind his seat. An auxiliary equipment compartment housed radiotelegraphy devices, an auxiliary power unit supplying compressed air for engine starts, and an electric generator. Forward of the cockpit was a cargo compartment intended for lightweight freight and airmail.

Passenger accommodation consisted of three compartments plus an access corridor, seating eleven passengers in paired leather seats alongside four folding jumpseats for additional passengers and one jumpseat reserved for the steward. A fully enclosed lavatory was located at the rear. Each pair of seats corresponded with sliding triplex glass windows; ventilation systems and electric ceiling lighting enhanced comfort. Heating was provided through hot air pipes beneath seats. For nocturnal flights, up to ten beds could replace seating arrangements; configured as an air ambulance, up to fifteen beds could be installed. Cabin walls were mounted on removable metal frames allowing straightforward cleaning.

Both cockpit and cabin areas were reinforced with robust tubular structures designed to absorb impacts during hard landings; below these lay a baggage compartment serving as additional cushioning. Wing-mounted engines were positioned to minimize risk from propeller debris to passengers or critical systems. The central fuselage incorporated reinforced bulkheads and transverse frames sheathed in corrugated duralumin. Cargo distribution within the main hold was managed carefully to maintain aircraft balance. Flight control linkages ran beneath the cabin floor through accessible channels. Access to the rear fuselage was provided via a door situated behind removable rear seats.

The horizontal tail assembly was notably large to ensure stability even with aft center-of-gravity positions; its stabilizer was adjustable during flight while elevators featured aerodynamic balancing. The vertical tail consisted of twin fins equipped with rudders. The aircraft could sustain straight flight on two engines; rudder deflections required during prolonged single-engine operation were compensated by an adjustable mechanism. All movable control surfaces were mounted on adjustable ball bearings. The landing gear lacked a continuous axle; each wheel had independent axles, steering rods, and shock absorbers resulting in a simple yet robust construction.

Equipped with three engines delivering between 1,100 and 1,200 horsepower depending on variant, the G 31 was capable of maintaining level flight on just two engines. Firewalls installed behind each engine reduced fire hazards. Fuel stored across six wing tanks was pumped via centrifugal pumps either into a gravity-fed tank above the cockpit or directly into carburetors. Engine cooling employed radiators designed by Junkers themselves. Compressed air typically initiated engine starts.

Operational history

Originally, the G 31 was designed to operate for Junkers' own airline, Junkers Luftverkehr, but the 1926 consolidation into Deutsche Luft Hansa changed its intended use. Deutsche Luft Hansa purchased merely eight G 31 aircraft, which they primarily assigned from May 1928 to extended routes heading to Scandinavia. This model remained active until 1935, when it was succeeded by the more widely adopted Junkers Ju 52.

An additional four G 31s were marketed for cargo transport in New Guinea. These freighters, managed by Guinea Airways and the Bulolo Gold Dredging Company, were equipped with Pratt & Whitney Hornet powerplants and featured open cockpits alongside sizable fuselage hatches to facilitate loading of large freight. Remarkably, they were employed to airlift eight dredges, each weighing 3,000 tonnes, from Lae to Bulolo. On 21 January 1942, a Japanese air attack on Bulolo destroyed three of these aircraft; the sole survivor was soon incorporated into the Royal Australian Air Force (RAAF). This particular plane (construction number 3010, registration VH-UOW) experienced significant damage in an accident at Laverton, Victoria on 31 October 1942 but was eventually restored for cargo duties in New Guinea after World War II.

Despite its innovative engineering, the G 31 did not achieve commercial success; only thirteen units were sold in comparison to fifty-four civilian G 24s and over thirty military variants. It provided enhanced passenger comfort along with greater speed and safety than contemporary biplane airliners due to its all-metal design. Nevertheless, elevated acquisition and operational expenses restricted its market penetration. A further development of the G 24 might have yielded stronger sales than this newer G 31 model.

Accidents and incidents

25 September 1928: The Deutsche Luft Hansa G 31de (c/n J3004; registration D-1427; named Deutschland) crash-landed and was destroyed by fire following an engine malfunction.
11 December 1928: The Deutsche Luft Hansa G 31fi, bearing registration D-1473 and named Rhineland, crashed near Letzlingen due to adverse weather conditions, resulting in three fatalities out of four occupants.
May 1936: A collision occurred between an RLM G 31fo (c/n 3008, D-ABIL) and a Junkers Ju 52/3m registered D-APUT.

Variants

G 31.1 – initial prototype powered by three Junkers L5 engines
G 31.2 – similar to G 31.1 but with the central engine replaced by a BMW VI
G 31ba – production model based on the G 31.2
G 31de – fitted with three Gnome et Rhone-manufactured Bristol Jupiter VI engines, featuring an enclosed cockpit and an additional tailfin
G 31fi – equipped with three Bristol Jupiter engines built by Siemens, incorporating larger wing and fuselage dimensions
G 31fo – powered by three BMW-built Pratt & Whitney Hornet engines
G 31ho – similar to G 31fo, but with the center engine replaced by a Pratt & Whitney Hornet
G 31go – freighter variant designed for New Guinea operations, featuring an open cockpit and a large cargo hatch measuring 3.60 m × 1.50 m

Operators

Deutsche Luft Hansa

External links