The promise of all-electric commuter aircraft is smooth, carefree travel to and from busy urban centers. But as the engineers who are designing such aircraft can attest, it takes a lot of work to make such flights appear effortless.
In November, students at the U.S. Naval Test Pilot School (USNTPS) and others interested in the future of aviation technology had an opportunity to learn about some of those challenges from Gerno Loots, the lead flight test engineer at Lilium, a company in Munich that is building a five-seat electric regional air mobility aircraft called the Lilium Jet.
“I’m excited to be working on an aircraft that represents the future of how we will commute,” Loots told his audience, who logged in from sites around Naval Air Station Patuxent River and remotely via CVR Teams.
USNTPS hosted the presentation as part of its quarterly Innovation Speaker Series, which features military and civil aviation pioneers discussing cutting-edge technologies and exploring how innovation and risk are balanced in unique ways.
The Lilium Jet is powered by 36 ducted electric motors mounted on four wings, which pivot to provide vertical thrust for takeoff and landing, and horizontal thrust for forward flight. The aircraft does not have traditional control surfaces such as ailerons or rudders, relying instead on the engines to provide nearly instantaneous thrust in the desired direction. It’s quiet, too; Lilium’s promotional materials say it will be as loud as a passing truck on takeoff and inaudible from the ground when cruising above 1,300 feet. The operational version of the Lilium Jet will have a range of 186 miles and cruise at 186 miles per hour.
Loots said that the Lilium Jet uses a fully fly-by-wire flight control system and is equipped with a stability and control augmentation system to dampen out undesirable motion throughout the operating envelope. The aircraft’s flight control laws are designed to allow for a smooth and intuitive control in both vertical and horizontal flight.
The Lilium team began flying a two-seat technology demonstrator in 2017, two years after the company was founded. Loots and his colleagues used the lessons learned from test flying that aircraft to build the five-seat technology demonstrator undergoing test now. Bad weather forced the engineering team to innovate an indoor testing regime for the initial ground testing of the aircraft — an option made feasible by the Lilium Jet’s zero-emission electric motors.
“We were able to actually test 24/7,” Loots said. “Our only constraint was manpower.”
Unmanned test flights followed, with each series of tests gradually expanding the aircraft’s speed envelope. The tests allowed the Lilium engineers to assess hover flight characteristics, including ground effect, forward speed, bank angles, and climb and descent performance. Loots said that two factors made the envelope expansion tests difficult: the flight control mechanical characteristics, and the latencies of the download links.
One of the biggest challenges that Loots and his team faced was energy management and its effect on predicting remaining flight time. “Managing voltages and temperatures are critical for flight time estimation,” Loots explained. They needed to find a way to keep the motors operating within safe temperature and voltage boundaries — especially during hovering flight, when the energy drain on the batteries is very high. The Lilium engineers developed a sophisticated engineering simulator which allows them to develop flight profiles and estimate power consumption for each test flight.
Test pilots have a very important role to play in the development of innovative aircraft systems in the private sector, said Loots, a former Royal Australian Air Force research and development engineer and a 2014 graduate of the Empire Test Pilots’ School. Efficiency is a primary consideration when developing a flight test strategy, but it should never compromise a methodical approach.
It’s also important to understand risks and to clearly communicate them to the engineering team and to management, Loots added. “Ultimately, if you want to push the boundaries of technology, you have to take risks somewhere,” he said. “We’re very selective about where we take risks, and careful to only take risks in areas where we can afford it and fully understand it.”
The multi-million dollar company currently employs over 600 people from 50 countries. Unlike many other electric commuter aircraft manufacturers, Lilium’s strategy emphasizes regional transportation rather than local; they argue that for flights shorter than 12 miles within densely populated urban areas, any time saved in point-to-point air travel will be lost by the commute to and from the aircraft itself.
Lilium anticipates its jet becoming operational by 2025 and is currently working with regulators and government officials in the United States and Europe on defining regulatory requirements and scoping out locations for modular “vertiports.”
Loots encouraged his audience to seek out the strengths and knowledge from all members within their own teams. “Some colleagues may have a completely different way of thinking about problems and identifying solutions which may benefit your test program.”
As technologies and capabilities continue to advance, old assumptions may no longer be valid, and flight test engineers need to be open to new ideas and new ways of doing things, Loots said — though safety should always remain a bedrock of any flight test program, no matter how innovative.
“Always challenge yourself to be open to new ideas and new ways of doing things,” Loots said.