Germany as pioneer: The dream of vertical takeoff

Vertical takeoff air taxis are high up on the aviation agenda for the 2020s. Soon you could be catching one from outside your front door, or at least from downtown locations previously unsuitable for air traffic. But let’s first go back over half a century, when Germany was poised to usher in the golden age of vertical takeoff aircraft for commercial passenger flight. Striking colorful illustrations document how the planners of the time envisioned vertical takeoff aircraft that would depart from locations like the roof of Munich’s central train station.

Engineers have always been fascinated by the notion of building machines that combine the advantages of helicopters with those of fixed-wing aircraft. The appeal lies in aircraft that require neither a runway nor momentum to get airborne, but rather take off under their own power before switching to efficient cruising flight. In 1922, a French-made quadcopter became the world’s first aircraft to take off vertically and fly reliably. But it would be another 40 years before the first vertical takeoff jets appeared.

Vertical takeoff from the highway

Development was driven by the Cold War and by the fear that tensions could erupt into a full-blown conflict. Germany would then have been on the front lines, with many airfields located close to the inner German border, which at that time separated NATO allies from the members of the Warsaw Pact. To be able to quickly relocate fighter jets to more remote areas and launch them if needs be from highways, the West German Air Force turned to vertical takeoff aircraft.

As early as 1959, the West German government green-lit three experimental industrial programs: a supersonic interceptor, a subsonic reconnaissance fighter and a transporter jet. These efforts produced the VJ101 C (a vertical takeoff fighter jet, maiden flight 1963) and the VAK 191B (a vertical takeoff reconnaissance and strike aircraft, maiden flight 1971). But the crowning glory of these programs was the only transport aircraft with vertical/short takeoff and landing (V/STOL) capabilities ever built: the Dornier Do 31. It was designed to fly at around 700 km/h and carry a payload of up to six metric tons, which is roughly equal to a three-ton truck complete with troops, or up to 36 fully equipped soldiers.

Lift and cruise engines: complex mechanical construction

The engineers decided against pivoting engines. Instead, they installed wingtip nacelles, each with four Rolls-Royce RB162 turbojets as lift engines—weighing just 127 kilograms a piece but producing two metric tons of thrust. Under each wing, there was a cruise engine equipped with pivoting thrust nozzles, which also aided vertical takeoff. These two engines were Bristol Siddeley (today Rolls-Royce) Pegasus 5-2 turbofans, which were also used in the Hawker Siddeley Harrier—still the most successful V/STOL aircraft ever produced. Not even the most powerful helicopter available at the time had the lift capability of a Do 31 with its maximum takeoff weight of 24.5 metric tons.

On February 10, 1967, the Do 31 took off on its maiden flight from Oberpfaffenhofen near Munich with an initial takeoff weight of 19.3 metric tons. This milestone is made all the more impressive considering that the highly complex challenges concerning the aerodynamics, engine arrangement, flight control and the control-pilot interface were solved using 1960s technology. The control system, which had to manage a precision interaction of lift and cruise engines close to the ground, was a complex mechanical construction. It featured an early version of fly-by-wire, the electronic flight control system, which at the time was being adopted for the Concorde and would later become the backbone of the Airbus cockpit.

Setting still unbroken world records on the way to Paris

The Do 31 came of age in what in any case was a most eventful year in the history of global aerospace. In 1969, both the Concorde and the Boeing 747 had their maiden flights, while the moon landing captivated the world. It seemed anything was possible. Shortly before the moon landing, the Do 31 had set multiple world records for jet-powered vertical takeoff aircraft in its weight class on its ferry flight from Oberpfaffenhofen to the Paris Air Show. One of these was completing the journey in just one hour 19 minutes—a record that has yet to be beaten. Like all vertical takeoff aircraft, however, the Do 31 did have some major drawbacks: it consumed vast amounts of kerosene, which was as objectionable as the noise level both inside and out.

“People in and around Oberpfaffenhofen got scared when the ground shook and they heard the plane’s unbelievable roar, which was nothing like normal aircraft noise,” said German test pilot Dieter Thomas, who died in 2013, in an interview. His colleague Drury Wood, who died in 2019, noted: “This ten-engine aircraft got so incredibly loud that it caused damage.” He was referring to damage to both health and property.

Nevertheless, at the time there were big plans to use vertical takeoff aircraft to transport passengers. Indeed, Dornier had already presented a modified design of the Do 31, dubbed the Do 231. Designed as a shoulder-wing aircraft, the Do 231 was earmarked for Lufthansa as a 100-seater with a maximum speed of 900 km/h and a range of just under 3,000 kilometers. Equipped with a total of 12 lift engines, it could have managed a takeoff weight of 59 metric tons and was set to enter service in 1977/1978.

The oil crisis dashed all dreams of commercial vertical takeoff aircraft

Unfortunately, the international situation at the time was anything but conducive for vertical takeoff aircraft. The West German government started cutting research budgets as early as 1967, including for the Do 31. Vertical takeoff aircraft also lost strategic importance for the military, and the 1973 oil crisis dashed all dreams of commercial applications. Decades later, Airbus Group historians stated that, “From today’s perspective, it’s really hard to understand the hype that surrounded vertical takeoff aircraft in the ’60s. The technology of the time would be in no position to comply with today’s efficiency and noise standards.

Today’s engines perform considerably better while also being much quieter and more efficient.” They recognized, however, that considerations are different for special missions, as demonstrated for instance by modern tiltrotor aircraft like the Bell-Boeing V22 Osprey. Germany’s engineers of over 50 years ago were definitely ahead of their time, and their early concepts could well inspire present-day practitioners. The buildings in Oberpfaffenhofen in which the Do 31 was designed have since been modernized. As it happens, they are now home to the company Lilium, which is working on an electric vertical takeoff and landing (eVTOL) aircraft.