Aviation – Starting all over Again

For the aviation industry, sustainable flight is the task of the century. Of course, alternative propulsion systems with batteries, hydrogen, and fuel cells also play an important role. But engineers can’t simply pack tons of batteries into an airplane. What we need now are radical and revolutionary aircraft and propulsion concepts.

World Air Transport Association (IATA): Global aviation should be climate-neutral by 2050. Photo: Shutterstock/Shine Nucha.

Global aviation is at the beginning of a transformation to climate neutrality that makes the automotive industry’s efforts look like small potatoes. The World Air Transport Association (IATA) wants to see this ambitious goal achieved by 2050. In fact, the social pressure for change is enormous. Even the guardians of the German language have added the term “Flugscham”, or “flight shame”, to their official vocabulary. The term is defined as the guilty conscience of polluting the climate when traveling by air, primarily through high CO2 emissions. So should the motto for emission-free aviation be: Decrease carbon dioxide?

If only it were that simple. After all, the effects of nitrogen oxides, water vapor, soot, sulfate aerosol particles, and contrails are also among the costs of aviation. An international study published in 2020 with the participation of the German Aerospace Center (DLR) in Cologne concludes that only one-third of the climate impact of aviation consists of CO2 emissions, while two-thirds is attributable to non-CO2 effects. For zero-emission aircraft that emit no pollutants in flight or on the ground to become a reality, the industry must pull out all the stops.

The demands on drive technology are higher than in the automotive industry

The Maker from Californian startup Archer is an electric vertical take-off vehicle with twelve propellers for lift and propulsion. The two-seater serves the urban mobility segment; it has a flying speed of 240 kilometers per hour and a range of 100 kilometers. Photo: Archer

Alternative propulsion systems based on hydrogen and electricity will play an important role in aviation in the future, as will hybrid systems and synthetic fuels. DLR – the German Aerospace Center – has comprehensively described where the journey is headed in its white paper on zero-emission aviation published at the end of 2020. According to this paper, it would be foolhardy to try to transfer the technological strategies that have proven themselves in ground transportation to aviation. Unlike in the automotive industry, the sector has no blueprints on which new aircraft concepts might be built. In addition, the requirements for the power-to-weight ratio of a drive system and the energy density of the energy sources are many times greater than in automotive engineering. In any case, aeronautical engineers would be loathe to pack tons of batteries, tanks, and engines for an alternative propulsion system on board. Modern aircraft are perfectly balanced systems – if you make changes for new propulsion concepts, you have to accept tangible effects on design, safety, flight performance, and aerodynamics.

The aviation industry faces a Herculean task

But what does this mean for the development of emission-free aircraft? The technological maturity of alternative propulsion solutions for aviation is currently very low. The aviation industry is therefore faced with nothing less than the task of reinventing flight. However, due to the high safety requirements and associated hurdles for approval, new flight concepts need a lot of time – so it’s five minutes to midnight for setting a course for the future. We need radical and revolutionary aircraft and propulsion concepts. There is still a great deal of research and development work to be done before aircraft manufacturers can roll out the first alternative models on the runway.

Battery-electric concepts, for example, enable good power density and high efficiency – but the decisive disadvantage is the batteries’ low energy density. In this respect, hydrogen clearly holds the better cards. According to the hydrogen fuel cell drive specifications, the fuel cell’s power-to-weight ratio is high. And the storage of hydrogen also places high demands on the integration of voluminous tanks in the aircraft structure. In its assessment of the near future of aviation, DLR sees several development paths: Battery-electric regional aircraft will be used for travel within metropolitan areas. Short- and medium-haul routes will be the domain of aircraft based on fuel cells, while for the time being long-haul routes will be dominated by new gas turbine concepts in conjunction with sustainable fuels. In the long term, however, hydrogen and fuel cell propulsion systems are also conceivable.

Startups take off with bold and spectacular flight projects

The upheaval in aviation is taking place on a scale that can hardly be overestimated. At the same time, it is unleashing creative forces. In a market report published in September 2021, the international consultancy Roland Berger analyzes that more than 280 aviation projects with alternative propulsion systems were carried out around the globe at the beginning of this year. Around 85 percent of them happened in Europe and North America. Even today, the industry has a gold-rush atmosphere. In urban air mobility in particular, bold projects that would have delighted aviation pioneers like Otto Lilienthal or the Wright brothers are taking off. Like the Californian startup Archer, which unveiled a prototype of an electric vertical take-off aircraft with twelve propellers for lift and propulsion in mid-2021. On board the “Maker” are six batteries for a range of 100 kilometers. The Israeli aviation company Eviation Aircraft believes it has already found the magic elixir for regional aviation – they expect their battery-electric propulsion system to take to the air in the first half of this year. In the same segment, the startup Wright Electric from California is planning to develop an all-electric powertrain based on hydrogen fuel cells or aluminum fuel cells, which will be designed for a range of around 740 kilometers. The industry giant Airbus, in turn, is relying on hydrogen for long-distance travel, with direct combustion as a possibility, as well as the use in a fuel cell technology. The company will make the decision on the most suitable system by 2025.

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First intra-European flights with HY4 hydrogen aircraft in ten years

The HY4 project can boast the first successful flight with a hydrogen aircraft. Among other things, it is backed by the combined expertise of DLR and the University of Ulm, which launched the four-seater with fuel cell propulsion in 2016. The approximately 7.50 meter long aircraft with a double fuselage has a wingspan of around 21 meters and can carry a pilot and three passengers. Thrust is provided by an 80 kilowatt electric motor that drives the aircraft’s propeller. Lithium-polymer batteries provide additional power during takeoff and landing. The latest generation of the HY4 can fly around 750 kilometers and reach a top speed of 200 kilometers per hour. A permit for Europe-wide test flights is also in place. According to DLR, intra-European flights of up to 2,000 kilometers should be possible with the hydrogen aircraft in ten years.

DLR builds emission-free Do 228 technology carrier as research aircraft

DLR is taking the next step in emission-free flying with a new research aircraft. In mid-November 2021, aviation company General Atomics AeroTec Systems delivered a Dornier “Do 228” to DLR, which they will develop into a technology platform at the Oberpfaffenhofen site over the next few years. Another partner is engine manufacturer MTU, which is responsible for developing the propulsion system designed for liquid hydrogen. The fuselage will reportedly house the fuel cells as well as the hydrogen tank. As a first step, one of Dornier’s two conventional powertrains will be replaced by a 600-kW electric powertrain. The partners will develop this powertrain by 2025 and then test it under real operating conditions.

With the new “Do 228 D-CEFD” test platform, DLR intends to put a wide variety of components and propulsion systems under the microscope in real flight operations. Photo: DLR

Harbour Air plans electric commercial aircraft based on the DHC-2 Beaver

The Canadian seaplane airline Harbour Air probably added the world’s first purely electric commercial aircraft to its fleet of around 40 units. This is a converted six-seater DHC-2 seaplane from Canadian aircraft manufacturer De Havilland of Canada with a battery-electric propulsion system. The first flight near Vancouver in late 2019 supposedly went smoothly. The 560-kilowatt electric motor comes from US company Magnix. With the batteries installed, they expect the Beaver to achieve a range of more than 160 kilometers. Extensive tests with the prototype are underway, providing data on flight behavior and operation. In the meantime, Swiss battery manufacturer H55 has joined Magnix as a partner of Harbour Air. The plan is to obtain certification for the electric propulsion and battery system this year. Subsequently, Harbour Air will convert its entire fleet to electric propulsion.

The Cassio 1 demonstrator from Voltaero flies with a hybrid module

The French aviation company Voltaero is committed to developing a highly efficient aircraft for four to nine passengers for traveling and commuting. The current demonstrator for the Cassio 1 project is based on a Cessna 337 Skymaster and has successfully completed over 40 test flights, according to the manufacturer.  At the heart of the propulsion concept is a drive module developed by Voltaero, which combines a 300-kilowatt internal combustion engine with three 60-kilowatt electric motors in a single unit. The combustion engine has two tasks in this hybrid concept. In cruise flight, it serves as a range extender that recharges the buffer batteries. If required, it also drives a pusher propeller, which is additionally used in special flight situations. Reportedly, test flights to date – most recently a mid-December 2021 crossing of the English Channel from Calais to Bedfordshire in the UK – show that the engines enable safe and efficient flight operations in all-electric mode. In addition, takeoffs and landings in this mode are almost completely silent. The hybrid propulsion system allows up to 3.5 flight hours and under good conditions saves up to 50 percent fuel compared to a conventional aircraft of the same type.

Rolls Royce and Tecnam develop hybrid module for electric aircraft in Norway

Norway is putting its money where its mouth is when it comes to regional air traffic: By 2040, all flights lasting less than one and a half hours and taking off in the country will only be operated by electric aircraft. The regional airline Widerøe is working with engine developer Rolls Royce and Tecnam, an Italian company specializing in light aircraft, to develop the necessary hardware. The “P-Volt” test aircraft used in the joint research program is based on eleven-seater Tecnam P2012 Traveller with two engines. The H3PS hybrid-electric drive serves as the propulsion system, which Rolls Royce has already been developing together with Austrian engine manufacturer Rotax since 2018 and which has been used in the test aircraft since 2020. According to Rolls Royce, the system is a parallel hybrid – the combustion engine and electric motor can each independently drive the propeller. However, a division of labor is also possible, with the electric component providing additional power. The e-aircraft is scheduled to enter service at Widerøe in 2026.

Eviation Aircraft makes aviation history with battery-electric “Alice”

Israeli aerospace company Eviation Aircraft could soon make aviation history with “Alice”. Unveiled at the Paris Air Show in 2019, Alice is the first aircraft with electronic flight controls designed since the very beginning to be powered by electricity, according to the manufacturer. Following successful engine tests in January at Arlington Municipal Airport near Seattle, Alice is expected to take off for its first flight in the first half of 2022. A spectacular unique selling point is the purely battery-electric propulsion system. On board is a lithium-ion battery pack weighing around 3,720 kilograms with a capacity of 820 kW/h. The batteries fully charge within 30 minutes. Two 640-kilowatt electric motors with five-bladed traction propellers at the rear provide the propulsion. Technical data for Alice indicate a range of up to 815 kilometers, a maximum speed of about 460 km/h and a maximum payload of 1,200 kilograms. Eviation Aircraft is planning three versions of Alice: a feeder aircraft for nine passengers, an exclusive business jet, and a cargo version. Deutsche Post DHL Group’s Express Division has already ordered twelve cargo units for delivery in 2024. 

Airbus is planning a giant jet as hydrogen research aircraft by 2025

Airbus is currently setting the technological course for flying long distances with large jets without emissions. In mid-February, the Group announced that they will equip an Airbus A 380 as a test carrier for hydrogen propulsion. This takes Airbus’ hydrogen initiative, launched in early 2020 under the name “ZEROe”, to the next level. The first step is a cooperation with engine manufacturer CFM, which belongs to Safran from France and General Electric (GE) from the USA. Specifically, they will equip an Airbus A380 with a hydrogen tank and an auxiliary power unit. This is designed as a hydrogen-powered direct combustion engine, mounted on the rear upper fuselage section of the aircraft. Phase one is basic research. For example, the auxiliary engine’s emissions will be measured separately and compared with those of regular engines. The demonstration aircraft should be ready for takeoff by the middle of the decade. Airbus plans to launch the first hydrogen-powered aircraft in passenger service as early as 2035.

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