The Dornier Do.31, which was developed in the FRG by engineers at Dornier in the 1960s, is a truly unique aircraft. It is the only vertical take-off and landing transport aircraft in the world. It was developed by order of the German military department as a tactical jet transport aircraft. The project, unfortunately, did not go beyond the experimental aircraft stage; in total, three prototypes of the Dornier Do.31 were produced. One of the prototypes built today is an important exhibit at the Munich Aviation Museum.
In 1960, the German company "Dornier" in strict secrecy ordered by the Ministry of Defense of the Federal Republic of Germany began designing a new tactical military transport aircraft for vertical take-off and landing. The aircraft was to receive the designation Do.31, its feature was a combined power plant of lift-sustainer and lift engines. The design of the new aircraft was carried out not only by the engineers of the Dornier company, but also by representatives of other German aviation companies: Weser, Focke-Wulf and Hamburger Flyugzeugbau, which in 1963 were merged into a single aviation company, which received the designation WFV. At the same time, the project itself of the Do.31 military transport aircraft was part of the FRG program to create vertically taking off transport aircraft. In this program, the tactical and technical requirements of NATO for the military transport VTOL aircraft were taken into account and revised.
In 1963, with the support of the German and British Defense Ministries, an agreement was signed for a period of two years on the participation in the project of the British company Hawker Siddley, which had extensive experience in designing the Harrier vertical takeoff and landing aircraft. It is noteworthy that after the expiration of the contract, it was not renewed, so in 1965 Hawker Siddley returned to developing its own projects. At the same time, the Germans tried to attract US companies to work on the project and production of the Do.31 aircraft. In this area, the Germans have achieved some success, they managed to sign an agreement on joint research with NASA.
In order to determine the optimal design of the transporter being developed, the Dornier company compared three types of vertically taking off aircraft: a helicopter, an aircraft with rotary propellers, and an aircraft with lifting and cruising turbojet engines. As an initial task, the designers used the following parameters: transportation of three tons of cargo over a distance of up to 500 kilometers and subsequent return to the base. Studies have shown that a vertically taking off tactical military transport aircraft equipped with lift-cruising turbojet engines has a number of important advantages over the other two types of aircraft under consideration. Therefore, Dornier focused on work on the chosen project and took up calculations aimed at choosing the optimal layout of the power plant.
The design of the first prototype Do.31 was preceded by rather serious tests of the models, which were carried out not only in Germany in Göttingen and Stuttgart, but also in the USA, where NASA specialists were engaged in them. The first models of the military transport aircraft did not have gondolas with lifting turbojet engines, since it was planned that the aircraft's power plant would consist of only two lifting and cruising turbojet engines from Bristol with a thrust of 16,000 kgf at afterburner. In 1963, in the USA, at the NASA research center in Langley, tests of aircraft models and individual elements of its structure in wind tunnels took place. Later, the flying model was tested in free flight.
As a result of studies carried out in two countries, the final version of the future Do.31 aircraft was formed, it was supposed to receive a combined power plant from lift-sustainer and lift engines. To study the controllability and stability of an aircraft with a combined power plant in hover mode, Dornier built an experimental flying stand with a cruciform truss structure. The overall dimensions of the stand repeated the dimensions of the future Do.31, but the total weight was significantly less - only 2800 kg. By the end of 1965, this stand had passed a long test path, in total it performed 247 flights. These flights made it possible to build a full-fledged military transport aircraft with vertical take-off and landing.
At the next stage, an experimental aircraft, designated Do.31E, was created specifically for testing the design, testing the piloting technique and checking the reliability of the systems of the new device. The Ministry of Defense of the Federal Republic of Germany ordered three similar machines for construction, with two experimental aircraft intended for flight tests, and the third for static tests.
The tactical military transport aircraft Dornier Do 31 was made according to the normal aerodynamic design. It was a high-wing aircraft equipped with propulsion and lift engines. The initial concept involved the installation of two Bristol Pegasus turbofan engines in each of the two internal nacelles and four Rolls-Royce RB162 lift engines, which were located in the two external nacelles at the wing ends. Subsequently, it was planned to install more powerful and advanced RB153 engines on the aircraft. The fuselage of the semi-monocoque aircraft was all-metal and had a circular cross-section with a diameter of 3.2 meters. In the forward fuselage there was a cockpit designed for two pilots. Behind it was a cargo compartment, which had a volume of 50 m3 and overall dimensions of 9, 2x2, 75x2, 2 meters. The cargo compartment could freely accommodate 36 paratroopers with equipment on reclining seats or 24 wounded on a stretcher. In the rear of the aircraft there was a cargo hatch, there was a loading ramp.
The landing gear of the aircraft was retractable tricycle, on each rack there were twin wheels. The main supports were retracted back into the lift-sustainer engine nacelles. The nose support of the landing gear was made steerable and self-orienting, it also retracted.
The first experimental aircraft was completed in November 1965 and received the designation Do.31E1. For the first time, the plane took off on February 10, 1967, performing the usual takeoff and landing, since at that time the lifting turbojet engines were not installed on the plane. The second experimental vehicle, Do.31E2, was used for various ground tests, and the third experimental transport aircraft, Do.31E3, received a full set of engines. The third plane made its first vertical takeoff flight on July 14, 1967. The same aircraft made a full transition from vertical takeoff to horizontal flight followed by vertical landing, this happened on December 16 and 21, 1967.
It is the third copy of the experimental aircraft Dornier Do 31 that is currently in the Munich Aviation Museum. In 1968, this aircraft was first presented to the general public, this happened as part of the international aviation exhibition, which was held in Hanover. At the exhibition, the new transport aircraft attracted the attention of representatives of British and American companies, who were interested in the possibilities of not only military, but also its civilian use. The American space agency also showed interest in the aircraft, NASA provided financial assistance to conduct flight tests and study the optimal approach trajectories for vertical takeoff and landing aircraft.
The following year, the experimental Do.31E3 aircraft was shown at the Paris Air Show, where the aircraft was also a success, attracting the attention of spectators and specialists. On May 27, 1969, the plane flew from Munich to Paris. Within the framework of this flight, three world records were set for aircraft with vertical take-off and landing: flight speed - 512, 962 km / h, altitude - 9100 meters and range - 681 km. By the middle of the same year, 200 flights were already performed on the Do.31E VTOL aircraft. During these flights, test pilots carried out 110 vertical take-offs, followed by a transition to horizontal flight.
In April 1970, the experimental aircraft Do.31E3 made its last flight, funding for this program was discontinued, and it itself was curtailed. This happened despite the successful, and most importantly, trouble-free flight tests of the new aircraft. At that time, the total cost of Germany's expenditures on the program to create a new military transport aircraft exceeded 200 million marks (since 1962). One of the technical reasons for the curtailment of the promising program could be called the relatively low maximum speed of the aircraft, its carrying capacity and flight range, especially in comparison with traditional transport aircraft. At the Do.31, the flight speed decreased, among other things, due to the high aerodynamic drag of the nacelles of its lifting engines. Another reason for the curtailment of work was the ripening at that time of disillusionment in the military, political and design circles with the very concept of vertical takeoff and landing aircraft.
Despite this, the Dornier company, based on the experimental aircraft Do.31E, developed projects for improved military transport VTOL aircraft, which had a higher carrying capacity - Do.31-25. They planned to increase the number of lifting engines in the nacelles, first to 10, and then to 12. In addition, the Dornier engineers designed the Do.131В vertical take-off and landing aircraft, which had 14 lifting turbojet engines at once.
A separate project of the civil aircraft Do.231 was also developed, which was supposed to receive two Rolls Royce lifting and cruising turbofan engines with a thrust of 10,850 kgf each and 12 more lifting turbofan engines of the same company with a thrust of 5935 kgf, of which eight engines were located in four. nacelles and four by two in the nose and aft fuselage of the aircraft. The estimated weight of this model of aircraft with vertical takeoff and landing reached 59 tons with a payload of up to 10 tons. It was planned that the Do.231 would be able to carry up to 100 passengers at a maximum speed of 900 km / h over a distance of 1000 kilometers.
However, these projects were never implemented. At the same time, the experimental Dornier Do 31 was (and remains at the present time) the only jet military transport aircraft built for vertical take-off and landing in the world.
Flight performance Dornier Do.31:
Overall dimensions: length - 20, 88 m, height - 8, 53 m, wingspan - 18, 06 m, wing area - 57 m2.
Empty weight - 22 453 kg.
Normal takeoff weight - 27,442 kg.
Power plant: 8 Rolls Royce RB162-4D lifting turbojet engines, takeoff thrust - 8x1996 kgf; 2 Rolls Royce Pegasus BE.53 / 2 lift and cruise turbofan engines, thrust 2x7031 kgf.
The maximum speed is 730 km / h.
Cruising speed - 650 km / h.
Practical range - 1800 km.
Service ceiling - 10 515 m.
Capacity - up to 36 soldiers with equipment or 24 wounded on a stretcher.
Crew - 2 people.
