The Dornier Do 31, a VTOL transport, was designed in response to German Air Force interest in STOVL aircraft. NATO’s NBMR-4 specification further fueled development.
Three aircraft, including two flight-ready ones, were built. On February 10, 1967, the Do 31 had its maiden flight, and in July 1967, it performed its first hovering flight.
Dornier showcased the prototypes at events like the 1969 Paris Air Show and set several world records. However, due to high costs and technical challenges, the German Air Force ceased VTOL trials, including those involving the Do 31, VJ101, and VFW VAK 191B.
Lacking sales prospects and state support, these VTOL projects eventually turned into research endeavors before being abandoned. To date, the Do 31 remains the only jet-powered VTOL transport aircraft ever flown.
Feared Vulnerable Airfields
During the late 1950s and 1960s, the German Air Force feared vulnerable airfields in a conflict with the Eastern Bloc. To address this, they explored dispersed operations, including using Autobahns.
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Trials were conducted, modifying F-104 Starfighters for rocket-assisted takeoff (ZELL). These aircraft would land on short strips using carrier-type arresting gear, a concept also intended for the Do 31.
In 1959, Dornier began informal work on VTOL concepts, which formalized into the Do 31 design in 1961. They collaborated with Bristol Siddeley and studied VTOL-capable utility transport aircraft. A team led by Gustav Wieland at Friedrichshafen facility engaged in this.
Dornier recognized the flight control system’s critical role in vertical flight, especially in managing control failures. To aid development, they built a specialized flight control test rig for exploring various attitude control laws and flying qualities.
For detailed modeling, they created the Dornier DO-960 hybrid computer to solve the necessary differential equations. Despite accommodating vertical flight control, the Do 31’s control philosophy resembled that of a conventional aircraft rather than a helicopter.
Two Prototype Aircraft
In February 1962, the Do 31 program was officially launched with a development contract from the West German government. By early 1964, Dornier initiated the construction of two prototype aircraft, primarily at the Oberpfaffenhofen plant.
Three test prototypes were built: E1, E2, and E3, denoted as “E” for Experimental. E1, focused on horizontal flight, was powered solely by Pegasus engines. E2 was a static test airframe and never took flight. E3, equipped with both Pegasus and RB162 lift engines, aimed to assess the design’s vertical flight capability.
The Do 31’s design heavily relied on its engine setup. Dornier chose the British Bristol Pegasus vectored-thrust turbofan engine, known for powering the Harrier jump jet. Each of the two inboard nacelles housed a pair of Pegasus engines. During vertical flight, four Rolls-Royce RB162 lift engines in each outer nacelle provided extra lift.
To maximize cargo space, the engines were placed in pods, allowing for a spacious hold accessed via a rear-facing loading ramp.
Initially, more than four Rolls-Royce RB162s were used, but as more powerful Pegasus versions became available, the number of supplemental lift engines was reduced to four. The Pegasus engine required special modifications for the Do 31 due to its nacelle placement instead of being within the fuselage like the Harrier.
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Besides lift and control considerations, other factors influenced the propulsion system. Noise, particularly near the airframe’s natural frequency, worried Dow. Managing hot exhaust gas re-ingestion was complex, given 16 gas “fountains” during vertical hover, 12 of which were hot.
Intensive flight testing determined that positioning the nozzles at an 85-degree angle, instead of 90 degrees, prevented issues during takeoff, with no problems observed during landings.
Various air intake types were tested to prevent ingestion issues and uneven lift engine starts. Bleed air from the Pegasus engines was used to address ingestion problems, and ground erosion effects were closely studied.
Successful Transitions Between Vertical and Horizontal Flight Phases
On February 10, 1967, the first prototype (E1) had its inaugural flight, powered solely by the two Pegasus engines. In July 1967, the third prototype (E3), equipped with all ten engines, conducted its first hovering flight.
In December 1967, successful transitions between vertical and horizontal flight phases were achieved. On February 28, 1968, the first flight featuring multiple transitions took place. Although some initial challenges were encountered, confidence in the aircraft grew rapidly, as noted by aviation author Andrew Dow.
As the flight envelope expanded, test pilot Drury W. Wood conducted several daring maneuvers in the Do 31. He intentionally flew it backward on one occasion to demonstrate its capability and executed a barrel roll on another.
To generate publicity for their new aircraft, Dornier flew one of the prototypes to the 1969 Paris Air Show, showcasing it to the general public. The ferry flight to the event set multiple Fédération Aéronautique Internationale (FAI) world records for the aircraft type.
Public recognition held great significance for Dornier, given their long-term aspirations for the Do 31, including potential civilian applications as a commercial VTOL transport. They even explored collaborations with Douglas Aircraft and Ling-Temco-Vought (LTV) for the Do 31 program, at one point declining an approach from Douglas.
Do 31 Made its Final Public Flight
The Do 31 stands as the first and, to date, the sole vertical takeoff jet transport ever constructed. In April 1970, it was officially announced that the project had been terminated, though the Do 31 made its final public flight on May 4, 1970, during the Internationale Luft- und Raumfahrtausstellung (ILA) in Hannover.
One purported factor leading to the Do 31’s cancellation was the relatively large drag and weight caused by the lift engine pods.
These factors diminished the useful payload and range compared to conventional transport aircraft. According to Dow, the German government had grown frustrated by the lack of commitment from other NATO nations and was unwilling to shoulder the substantial funding required for full-scale development on its own.
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In a later phase of development, Dornier considered eliminating the Do 31’s outer nacelles and engines. Instead, they planned to use larger RB153 turbofan engines, each capable of producing around 5,000 lbf (22 kN) of thrust, once these engines became available.
Dornier also explored a subsequent evolution of the Do 31, known as the Do 131, which was designed to be powered by either twelve or fourteen liftjets. However, no prototypes of this variant were ever built.
Do 31, Where Can You See Them?
Germany has preserved both flying prototypes, but the whereabouts and status of the non-flying testbed (E2) remain unknown.
Dornier Do 31 E1, D-9530, is preserved and displayed at the Dornier Museum Friedrichshafen.
Dornier Do 31 E3, D-9531, was initially put into storage at Oberpfaffenhofen, then for several years on display in the open in the courtyard of the Deutsches Museum in Munich and after restoration is now on display at the Deutsches Museum Flugwerft Schleissheim at Oberschleissheim near Munich