What it takes to design an aircraft from scratch

Lilium Blog

  • There are many ways to design an eVTOL aircraft, here is how we are thinking about the key design principles and why our team at Lilium is striving for a different architecture. 

 

Revolutions are rare – particularly in the aerospace industry. Take the example of the Boeing 707. Introduced in 1957, it was the company’s first jetliner and was originally hailed as the ‘airliner of tomorrow’. 

In many ways it was exactly that, delivering range, speed and efficiency that had never been seen before, representing a revolution to the existing long-range aviation of the day. 

 

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An original Boeing 707 advert

Yet the ‘tomorrow’ they spoke of lasted a surprisingly long time: the revolutionary plane, first seen in 1957, was still in commercial operation as late as 2013 with Saha Air.

 

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A photo of the last operational Boeing 707, operated until 2013 by Saha Air. Credit: Sam Chui

Even newer jets like the Airbus A350 and the Boeing 787 look surprisingly similar to those earlier aircrafts, despite being considerably quieter and more efficient than their predecessors. That’s because the revolution in technology it represented, the introduction of the jet engine, has been replaced by the steady (and rather expensive) evolution of that same technology.

Only now, some 70 years after the shift from propellers to jet engines, are we reaching another genuine tipping point in aviation history, with the advent of technology that enables all-electric, vertical take-off and landing (eVTOL) passenger flights.

With the dawn of the enabling eVTOL aircraft technology, a new market for mobility will be created which can be classified as Regional Air Mobility (RAM). Connecting city centers and big metropolitan areas with a safe, fast and reliable mobility service will increase people’s radius of life and improve connectivity which will free up more time for what truly matters. RAM promises to be a substantial change to how we live. Where you live need not be dictated by how close it is to your workplace anymore. RAM allows for the first time, the ability to connect communities irrespective of size, with high speed (300km/h) direct flights of up to an hour without the need to build new roads, tracks and other expensive infrastructure.  

This all sounds good, but how are we going about designing such a new aircraft concept from scratch, and what are the tradeoffs in design decisions?

 

Bringing the vision to reality – designing an eVTOL jet

Lilium was founded with a vision to create a widely accessible and competitively priced Regional Air Mobility service. In doing so, we set ourselves a remarkable challenge. We needed to design an aircraft that performs well not just in one or two, but all five of the ‘degrees of freedom’ which guide the performance of an eVTOL aircraft; speed, range, payload, noise and safety.

First and foremost, the aircraft must be uncompromisingly safe. In addition, we needed an aircraft with a low enough noise footprint to fly into urban areas, a high enough range to connect entire regions and the speed to make it a true time saving customer proposition. Payload is also critical since the number of seats directly relates to the cost at which tickets can be offered to customers. The more seats (payload) and the faster one can fly, the better the cost efficiencies (pilot, maintenance, landing infrastructure, etc.) can be distributed among future customers.

The challenge with the five degrees of freedom is that they are strongly interlinked; the improvement in one parameter typically has a negative effect on another.

As such, there are four main archetypes of eVTOL aircraft configurations each with their own pros and cons: Multicopters, Lift + Cruise, Tilt Rotor and Ducted Vectored Thrust concepts.

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This chart shows various eVTOL architecture standards, with the yellow arrows indicating the direction of thrust generated

Multicopter architectures are relatively simple, they are very efficient during vertical take-off, landing and hovering due to the low disc-load. However, as they are missing wings, multicopters lack cruise efficiency which limits their application to use cases in Urban Air Mobility (UAM) markets. Furthermore, more battery is required to compensate the inefficiency during cruise flight, adding to overall aircraft weight.

Lift & Cruise concepts merge the multicopter for vertical take-off and landing operation with a standard aircraft for cruise flight. Doing so, the advantages of both architectures are combined. In order to maximize range for these concepts, the open propeller needed for VTOL is designed with less blades and short chords, in order to reduce drag during cruise fight. The smaller dimensioned open rotors for VTOL operation create a significant challenge in noise emission due to the resulting higher disc-load and blade tip speeds.

Tilt wing and tilt rotor concepts are capable of mitigating the aforementioned disadvantages, by applying slowly rotating open multi-blade props. The compromise of high range and low noise comes at a higher technological complexity as big propulsion systems need to be tilted. Due to the low tip speeds of the big open multi-blade props, low turning shaft torque motors are required, which may interfere with the structure. Thus, either heavy electric motors with high torque are necessary or an additional gearing system must be installed. Further challenges can occur in the design of the flight dynamics during the transition flight. However, overall it is a valid concept allowing for optimization across all 5 critical dimensions of an efficient eVTOL aircraft.


A fourth direction inspired by traditional aviation engines

Lilium is pursuing a different direction, based on ducted, vectored thrust. Although the concept of ducted turbo fans is not new, the implementation of the ducted fans in form of Distributed Electric Propulsion (DEP) in the rear of our four lifting wings is unique.

 

By breaking down larger, single-stage open propulsion systems into 36 smaller ducted fans, we have been able to design an aircraft that has a great compromise between the initial design trade-offs.   

Ducting the fans allows us to reach higher aerodynamic efficiencies, while the ducts themselves can be used to implement acoustic liners, eliminating blade passing frequencies. Ducted turbo fans emit the lowest noise proportional to their thrust and noise is additionally shielded by the ducts. 

 

Engines of change

Engines are the central performance enabler of an aircraft and are critical to the architectural eVTOL design. 

The ducted turbo fans work on the same principle as a traditional jet engine, yet is far simpler, relying on just a single ‘stage’ rotor/stator system driven by an electric motor. There is no combustion required. 

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Air is sucked through the front of the engine where it is compressed by a single fan stage and expelled from the rear of the engine at a higher speed than it entered

When compared to open rotors, the ducted design not only increases the engine efficiency but also delivers opportunities to reduce the noise footprint of the aircraft, via low blade-tip velocities at Mach numbers below Ma=0.5 and the inclusion of acoustic liners which dissipate the blade passing frequency.


Small is beautiful

Designing an eVTOL aircraft and employing ducted fans, one can choose between two different sub-architectures. 1) You can either choose a classical aircraft architecture with standard aerodynamic control surfaces and distribute, e.g. 6 big ducted fans - equally around the center of gravity, or 2) You can go with distributed electric propulsion (DEP) which distributes the same disc area nearly equally across the lifting wing system of the aircraft.

By relying on DEP, the Lilium Jet concept benefits from a 10-20% reduction in wetted nacelle surface compared to the first alternative architecture. This translates directly into increased range, while boundary layer ingestion also helps to decrease total pressure losses in the system and improve overall efficiency. The integration of DEP in the rear of the wings allows for a high lift system, which gets more efficient with the forward velocity squared. Traditional control surfaces like tails, rudders or ailerons become obsolete, which saves weight and complexity. The distributed smaller engines can almost immediately be adjusted to the required revolutions per minute (RPM) in order to control the aircraft safely through all flight phases of an eVTOL aircraft.

 

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A diagram showing the flow of air over the Lilium Jet tech demonstrator wings and through the ducted engines

The four wings contribute significantly to the overall efficiency of the aircraft, providing lift to support the weight of the aircraft during horizontal flight, and allowing the aircraft to maintain forward flight with minimal effort – using only around 10% of the power required for the hover flight phase. Due to the higher disc load of the Lilium based architecture, more power is required during the take-off phase. Yet, the aim of the Lilium Jet architecture is to efficiently enable high-speed regional travel with a minimum journey range of around 20km, thus, the flight profile of the Lilium Jet predominantly consists of full forward flight thereby placing even greater importance on cruise efficiency. This leads to less relative flight times in the energy consuming hover phase and more flight time in the aerodynamic efficient cruise phase. Its unique architecture also means that we can deliver the biggest customer value proposition in saving time.

 

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Hover vertical lift efficiency graph originally from NASA SP-2000–4517

Due to the higher disc load, the Lilium architecture requires approximately double the power than a comparable open rotor concept during VTOL phase, but this is intended. The figure above illustrates the typical phases of a mission flight profile. The energy consuming VTOL maneuvers with the respective high disc load are only considerable in the very first part of the flight profile (see red circle) and will last only seconds.

 

Aerodynamic efficiency is the art of any high performance driving or flying machine

The aerodynamic efficiency of the engine itself also plays a critical role in defining the range of the aircraft. Among others, the ducted design significantly enhances the efficiency of the engine by reducing the formation of tip vortices, a phenomenon that occurs in open rotors as a result of the pressure equalization at the tip of a blade and results in a decrease in efficiency known as 'tip loss'.

While the amount of real-life performance data available for our engine design is growing every day, a number of studies, along with our own calculations, simulations and testing all point to a technically feasible isentropic efficiency of between 0.85 and 0.90, based on an understanding that an electric ducted fan corresponds to a single stage axial compressor, albeit with a lower pressure ratio than standard axial turbomachinery.

Achieving these levels of efficiency is a key goal for the team and we’re pleased to see that progress of our flight testing and data analysis confirms our thinking about the technology and its application to Regional Air Mobility. 

 

Long term, general electrification of flight only depends on energy density in batteries

Like with any innovation, there are challenges to overcome in our journey to bringing the Lilium Jet and service to life. Some, like achieving full transition to horizontal flight, are more predictable and ones we will tackle ahead of achieving our goal of flying 300km in one hour on a single charge, while others are less so. As we face each challenge, we look forward to taking you with us on our journey towards regional air mobility.

This is a further step towards electrification of flight (some smaller fixed wing non-VTOL planes were only just recently approved by the relevant authorities). With improved batteries, our insights will help us go further and open new concepts for different missions; larger planes for high density routes, smaller ones for more individual trips. Long term, a lot will depend on how fast the energy density of batteries improve, as the aerodynamics won’t change in the same way. Doubling the energy density of batteries will either double the aircraft range or allow us to increase payload and add more passengers, without a complete redesign of the aircraft itself. 

For now, we are focused on the innovation around safe, efficient and quiet vertical take-off, as it opens up a new field of high-speed mobility currently only served by high-speed trains, which requires enormous infrastructure investment and decades to complete, rendering them unsuitable for most communities.

 

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Further reading:

More thorough analyses of the various eVTOL aircraft architectures are available here and here. 

You can read more about how the efficiency of turbofans is measured here and here. 

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