Aircraft electrification – a real revolution?

Air transport accounts for about 3% of global CO2 emissions. As they are released at high cruising altitudes, the effects of CO2, fine dust, nitrogen oxides, and other pollutants on the climate are five times stronger than on the ground.

?

Aircraft manufacturers have long been working on making flying more environmentally friendly. Technologically optimised jet engines, special coating for improved aerodynamics; investment in innovative fuels such as SAF (Sustainable Air Fuels) or low carbon aviation fuel have significantly reduced energy consumption and pollutant emission. Yet are these enough to offset the effects of air travel that, according to IATA, is recovering quickly on a global scale after the drop during the pandemic? Strict emission targets such as the “Fly Net Zero” commitment of airlines to achieve net zero carbon by 2050 or EU’s “Flightpath 50” program (75% CO2 reduction compared to 2000 by 2050) will most likely only be achieved with the massive research and investment in hybrid or fully electric propulsion systems.

?

The innovations which will make this next generation of sustainable aircraft possible are going to involve changes to all elements of the aircraft: propulsion systems, structures, and flight strategies. HBK enables these innovations by empowering engineers to confidently test and validate their designs to ensure airworthiness.

Battery power – the limiting factor?

Electric propulsion systems had long been considered unsuitable for air transportation. Yet innovative start-ups managed to demonstrate feasibility on a small scale, thus putting pressure on big aircraft manufacturers and their propulsion system suppliers to think twice and invest in research and development. Siemens, for example, launched an electric motor of 50 kg with 260 kW output that can operate smaller aircraft of up to two tons directly.

?

The advantages of electric motors over gas jet turbines are obvious:?

• Zero-pollutant emission locally (by the aircraft);
• Considerable noise reduction;
• Reduced production, maintenance, and repair cost because of fewer moving parts and less complex designs;
• Reliable power output independent of air density, speed, or temperature;
• Distributing the propulsors along the wing increases the lift and drive efficiency; wings and tail units can be smaller thereby reducing weight and drag.

?

The trade-off is of course energy supply and storage. Energy stored in commercially available lithium-ion batteries is not only 60 times heavier than the equivalent in fuels. Energy density (250-270 Wh/kg) is significantly lower, too (250-270 Wh/kg vs 12,000 Wh/kg). Add to that a difficult temperature management. Although battery efficiency is improving at about 5-8% per year and now suitable for short to medium-haul flights, experts say developing new battery technology to be used for all-electric long-haul flights will take decades and huge budgets.

Fully electric or hybrid? Range as the decisive factor

Many a fully electric or hybrid project was launched during the last decade – most of them still under development. Compared to their target dates, entering service is delayed for many, some got cancelled, but some have been certified and are in production.

?

Fully electric propulsion is used, for example, in VTOLs (vertical take-off and landing), which will open up a completely new market segment as air or transport taxis with a low seating capacity of up to 5, but an MTOW (maximum take-off weight) of 2,000 kg and a max range of 300 km.?

?

Another example of a fully electric-powered experimental aircraft is?NASA’s X-57 “Maxwell. Powered by lithium-ion batteries, 12 high-lift electric motors get the plane airborne; 2 large wingtip motors propel it in cruise mode. Ground tests will be complete in May 2022, and the project is moving closer to the first test flights.

?

Initiatives for large commercial aircraft are focusing on hybrid-electric propulsion systems that enable longer ranges while saving fuel and reducing emissions. However, their development will take more time, and they are unlikely to enter service before 2030. And while they show promise, they won't necessarily catch on, like the E-FanX project initiated by Airbus, Rolls-Royce, and Siemens in 2017. The idea was to replace one of 4 gas turbines with a 2 MW electric motor. The remaining jet engines powered a generator to produce electricity. The project was discontinued in 2020.

?

In summary, gas jet turbines will remain the standard long-range propulsion system for the foreseeable future. The use of SAF will help to reduce the climate impact to a certain extent. Longer-range aircraft will use hybrid systems. Their fuel consumption is expected to drop by about 25%. However, fully electric short- and medium-range aircraft with low seating capacity are already operational or will be very soon. And although product life cycles of about 30 years will most likely prevent electric or hybrid propulsion systems from being used comprehensively before 2050, experts predict about 45% of all aircraft will at least be hybrid electric by 2035. The technology seems likely to fundamentally change air transportation.

Testing Electric Aircraft Propulsion Systems

As with conventional aircraft propulsion systems, testing and validation are critical throughout the design and prototyping phases. However, there are major differences between testing conventional and distributed electric propulsion systems. Batteries of?NASA’s X-57 plane, for example, had to be validated to ensure they safely power a whole flight profile and temperature increases do not result in thermal runaway. Engineers had to test and validate every component of the electric propulsion as well as the whole electric system to predict any system integration challenges. While there is a lack of standardized tests for all-electric or hybrid propulsion, others like GVT (Ground Vibration Test) are mandatory to achieve airworthiness – regardless of the propulsion system.

Efficiency Testing

An electric propulsion system consists of a power source (battery), an inverter, an electrical machine (motor), and a propeller. Evaluating the individual components and understanding how they work as a system is crucial for optimizing efficiency. The more efficient the overall system, the longer the flight range. To evaluate its efficiency, tests are run during motor start-up, steady-state, and failure events.?Running these tests in addition to traditional efficiency maps will allow engineers to optimize the operation throughout the flight. HBK offers tools for quickly and accurately measuring the efficiency with the eDrive power analyzer. The eDrive is unique because it can make traditional electrical efficiency measurements, but also look at torque, speed, thrust, and other aircraft-specific measurements for efficiency.

Electromechanical Testing

Electric machines create thrust to propel the aircraft, but also create vibration and heat. These mechanical properties need to be handled to ensure the safety of the aircraft. The vibration and thermal properties of the powertrain are both created by the machine but influenced by the voltages and currents from the inverter. An inverter output can create more or less torque ripple which results in different vibrations. Less efficient inverter controls will result in increased heating. The relation between the inverter and the mechanical operation has resulted in many engineers characterizing the electrical and mechanical signals in one place. The HBK eDrive power analyzer allows engineers to measure all of these signals in one place so they can optimize efficiency while allowing for a safe mechanical operation of the aircraft as well.

Voltage

Higher voltage lowers the current and reduces the size of the cabling needed to distribute the power across an electric aircraft. Electric aircraft propulsion currently operates at up to 900V but is looking to the future where it could operate as high as 5kV and early investigations have started into 10kV. These high voltages pose measurement challenges, and also safety concerns for the aircraft. To develop a high voltage powertrain that is safe, there will need to be significant durability and failure testing. This testing will be destructive and need to be safe for both the engineers and test equipment. The HBK eDrive power analyzer has high acquisition rates that can be optically separated from the user at a long distance. This allows destructive testing to be safely executed and have the data integrity to ensure safe flying conditions.

Structural Integrity

Electric Vertical Takeoff and Landing (eVTOL) aircraft have radical, or non-conventional designs where they may have multiple motors on each wing. The motors may rotate vertically for takeoff and then rotate horizontally for forwarding motion.?

How does distributed propulsion impact aerodynamics? What are the implications on classical flutter and whirl flutter??

Because of these new non-conventional designs, the structural characteristics of the aircraft are totally different from the conventional designs (no motors or one to two motors on the wings). The added mass and distribution of these motors can cause the classical flutter modes to be lower in frequency and since these motors are propeller-based for efficiency, these aircraft are more susceptible to whirl flutter. Most of these motors have an RPM range with a frequency content that is in the same frequency range as the structural modes of the aircraft. If a flutter mode is excited too long the aircraft could incur a catastrophic failure. Therefore, making flutter predictions based on comprehensive measurements is pivotal to adapting the airplane design at an early stage of development.

HBK Testing Solutions

As seen above, electromechanical testing is important to understand complex systems such as hybrid or fully electric propulsion. At HBK, we can provide a test system that fits your needs: time-synchronized data acquisition, continuous recording, visualization, and real-time data transfer to storage, automation, or Hardware-In-The-Loop system. This streamlined approach helps development engineers perform system-level analyses and share data across development departments – for quicker testing and validation at a lower cost.

Our Solutions comprise:

??Ground Vibration Testing and Analysis?– Correlate the flutter model to reality

??Static and Fatigue Testing?– Ensure structural integrity throughout the vehicle’s operation

??Electric Powertrain Testing?– Test performance, stability, and efficiency from the battery to the propeller

??Noise Testing and Analysis?– Optimize aircraft noise in the air,?on the ground?and in the cabin

??Engineering Services?– Experienced professionals available to augment your team's capabilities