• 05 May 2021 02:00 AM
• 1

Lilium’s Technology Approach Explained

Sidebar to Lilium Goes Big, Vertiflite, May/June 2021

Although the previous Lilium Jet full-scale demonstrators were not flown extensively — and neither transitioned to fully wingborne flight — Lilium has conducted extensive analysis and ground testing that supports their technology.

With the announcement of their Qell SPAC merger, the company has begun to review details on their development efforts. In a 4,700-word blog post on April 8, Lilium’s Chief Technology Officer, Alastair McIntosh, provided additional information on the architecture and technology of the Lilium Jet. McIntosh joined Lilium in December, after previously serving as the Chief Engineer and Managing Director at Rolls-Royce Germany, responsible for delivering a number of turbofan jet engine programs, including the BR725 for the Gulfstream G650 and the Trent XWB for the Airbus A350. Several passages are highlighted here.

McIntosh explained that the aircraft wingspan was limited to less than 46 (14 m) to enable the use of existing helipads, noting that there are approximately 14,000 possible locations in the US alone. The main wings generate about 60% of the lift, the canards about 20%, and the remaining ~20% is generated across the fuselage. The canards and wings are positioned as far apart as practicable, to enable the aircraft to be stable in pitch. The aircraft has a fly-by-wire control system, with directional stability provided by active electronic differential thrust control.

Lilium calls the aircraft “simple by design,” with no ailerons and no need for vertical stabilizers. The landing gear is fixed and the aircraft has no hydraulics.

The propulsion system consists of 36 individually controllable flaps, which also serve as lifting and control surfaces and each flap contains a ducted electric fan. Embedding the 36 ducted fans into the wings eliminates the need for dedicated nacelles, reducing weight and minimizing aerodynamic drag loss. Each flap is rotated by an integrated servo unit, which can change the angle of the whole flap unit for controllability during hover and cruise flight. The flaps only receive two signals — fan speed and flap angle — by which the aircraft can be controlled throughout the flight envelope via thrust vectoring.

By embedding the ducted fans in the rear of the wing, the aircraft benefits from boundary layer ingestion, which reduces the total pressure loss. Although the assessment of this effect is heavily coupled to general drag housekeeping, due to the low disc exposure during cruise flight, boundary layer ingestion benefits offset blocking effects from the distributed fans, McIntosh wrote.

Lilium argues that despite requiring twice the power consumption in hover as an equivalent open rotor concept, the increased power demand is compensated by optimizing cruise flight performance — the Lilium Jet in cruise needs only one-tenth its required hover power — which will typically comprise 90-95% of the flight time.

McIntosh also discussed the battery approach for Lilium. Batteries for an eVTOL aircraft need sufficient energy density to enable longer operations and sufficient specific power to support the high energy demand in hover. The total range of the aircraft is challenged by two factors of the battery cell, he noted: total energy in the cells and the minimum accessible state-of-charge (SOC), which represents the total accessible energy in a battery cell. One of the challenges in designing the battery system for the Lilium Jet was providing high power by the battery cells at low SOC.

Battery technology from earlier than 2014, which was used in the 5-seat Phoenix, allowed for a minimum SOC of only 30-40. Today’s state-of-the-art cell technology, using more advanced anode material, such as silicon, increases the discharge capacity of the battery cell and, thus, the power provision at low SOC is significantly improved, enabling a minimum SOC of 10-15% for the 7-Seater serial Lilium Jet.

In addition, the performance of battery cell development has rapidly improved over the past decades, with today’s cells available with energy densities of greater than 300 Wh/kg and power density more than 3k W/kg. This enables the mission range of greater than 155 miles (250 km) plus reserves.

In its investors briefing, Lilium said it had evaluated more than 50 battery technology companies. In addition to high energy and power density, the company also evaluated approaches based on a long cycle life, low cost and fast charging capability. Although it didn’t name the supplier, Lilium stated it had selected silicon-anode lithium-ion pouch battery cells with 330–350 Wh/kg that could charge to 80% in 15 minutes and to 100% in 30 minutes. It had measured the battery chemistry beyond 800 cycles.

In the blog post, McIntosh also went into depth on the acoustic signature of the Lilium Jet. But he also more succinctly explained the advantages in a video released on March 30. There, he explained that the significant number of blades in the fan is to optimize mainly for noise and also for aerodynamic efficiency and performance. Aerodynamic optimization of the fan and matching them with the stators behind the fans is good for efficiency and noise.

The duct also contains the noise, so it can only be projected forward or rearward, stopping it from radiating in all directions, as it does with open propellers. Within the duct itself, Lilium has applied acoustic liners, which absorb more of the noise energy and dampen those down or tune them out.

A fan has to work quite hard in comparison to an open propeller for a given diameter, he said. However, it has the advantage that it’s a very efficient system because there is reduced leakage around the blade tips, producing an efficiency benefit of about 40%.

The ducts also help match the flow through the “electric engine” throughout its mission. A significant amount of air flow is required for takeoff, but much less is needed in cruise. To match the fan performance at cruise, Lilium changes the geometrical area at the back of the fans with a variable nozzle; as the angle of the ducted fan changes, there’s a simple mechanical link, which then closes down that area, optimizing the air flow both in hover and in cruise, McIntosh explained.

Lilium Fact Sheet

Courtesy of Lilium GmbH, March 30, 2021

Lilium 7-Seater Jet

• Carries six passengers and one pilot
• Uses proprietary Ducted Electric Vectored Thrust technology
• Powered by 36 low-noise electric ducted turbo fan engines
• Cruise speed of 175 mph (280 km/h)
• Range of 155+ miles (250+ km) including reserves
• Sound footprint of ~60 dBA at 100 meters in hover flight, inaudible during cruise from the ground
• Ability to scale without significant increase in sound or footprint
• Certification basis CRI-A01 received from EASA and in concurrent type certification with EASA & FAA
• Targeting commercial launch in 2024 and operating in multiple regions in 2025

Business Model

• Total addressable market of between $1.5T and $3T by 2040
• Lilium-branded passenger mobility network and a turnkey enterprise solution, leasing aircraft to corporate and government customers
• The 7-Seater Lilium Jet is projected to deliver the best unit economics, with market-leading capacity, low noise and high performance
• Projected 1,000 jets in operation by 2027 with 30,000 tickets sold per jet per year
• Projected $5M in revenue per jet, per year
• Targeting profitability by end of year 2025

Network

Florida:

• Received ≈$200M in commitments to develop Florida network from infrastructure partners including Ferrovial and Tavistock Development Company
• Planned network of up to 14 sites — 1,200 miles (1,930 km) of connectivity
• Projected annual revenue of $600M from ≈125 jets

Germany:

• Agreement with Cologne Bonn Airport and Dusseldorf Airport [plus Munich and Nuremberg, announced April 19, 2021]
• Projected annual revenue of $900M from ≈190 jets