Electrified Propulsion for Aerospace: Where to Start?
The advent of electrified propulsion in the aerospace sector, captured in microcosm by the fast-emerging eVTOL market, both threatens to upset the establishment of major aerospace players and offers significant new opportunities for start-up companies. In all cases, it is forcing a marriage of system simulation and architecture definition techniques from markets already meeting these challenges, such as automotive. The demands of these aerospace applications are causing engineers on both sides to find the best blend of tools and approaches to meet their goals.
In the eVTOL marketplace there is no common ground yet regarding the number of propulsion modules, their disposition or the underlying electrification technology to convert stored energy into thrust. As a result, there are several approaches, each with its own set of perceived benefits in addressing key attributes like mass, range, safety, and efficiency. Taking this to a deeper level invites a myriad of questions and challenges. Among the most hotly debated are those related to the overall architecture options available, including rim driven fan (RDF) or hub driven fan (HDF), whether a gearbox has a rightful place in the overall optimized system and how certification hurdles can be overcome by intelligently integrating systems.
To address the architecture question, Drive System Design (DSD) has applied its architecture definition tool, ePOP, with the intent of providing a way forward, or at least quantifying the tradeoffs affecting the eVTOL industry. The high-level architectural questions that eVTOL engineers are asking themselves are the proper starting point in taking a full-system approach to electrified propulsion in aerospace. Additionally, it enables designers to leverage the learnings and technology from other market segments already deploying systems of this nature.
Taking a given range, vehicle, payload and flight profile for an eVTOL application, we can start to answer some key questions.
How Many Propulsion ‘Modules’ Are Optimal?
Aside from avionics, controls, and inherent redundancy, asking how many propulsion modules should be deployed is not a simple question to answer. While more modules, distributed across a wing or around the aircraft structure, offer an inherent thrust generation (Thrust = Mass Flow Rate × (V2 – V1)) efficiency improvement by increasing air mass flow rather than air acceleration, this comes at the cost of additional mass. This is a fine balance to strike, but DSD has seen industry players willing to absorb the additional propulsion system mass to unlock the efficiency improvements.
Figure 1 shows a near halving of the energy consumed over a flight cycle at the cost of doubling the powertrain mass, when moving from 4 to 8 propulsion modules. However, there are permutations of a 6-module architecture that offer over 75% of the same energy consumption reduction at a cost of 50% increase in system mass. While there are many other variables that contribute to a final answer to this question, this analysis shows directionally that there is a ‘sweet-spot’ to be found that is well worth the cost of looking for it during initial concept design.
Rotor Radius – Is Bigger Really Better?
In short – yes. The inherent improvement in efficiency offered by higher air mass flow rate compared to higher air velocity delta, for a given thrust produced, is still a compelling driver in the architecture of eVTOL systems. Figure 2 depicts this with the pareto line for energy consumption vs. powertrain mass firmly defined at the left edge by the maximum rotor diameter included in our study. However, in simple terms, higher rotor radius can be directly at odds with trying to package a greater number of propulsion modules in the given aircraft volume, so a balance is required.
Where larger rotor radius can’t be accommodated and engineers have to downsize, the thrust produced needs to be maximized via a given propulsion module by operating with speeds that allow fan tip speeds to reach near their sonic limits. For electrified systems, we can leverage the high electric motor speeds enabled by recent generations of commercial vehicle motor technology with speeds of more than 20,000 rpm. This will drive the use of SiC and GaN inverter technologies to bring switching losses down, unlocking the potential of higher speed and smaller rotor radius systems. In parallel, thermal management benefits become valuable as the aircraft will need to carry smaller, lighter heat rejection systems.
A question worthy of consideration at this point is how a RDF arrangement might offer some additional benefits compared to an HDF system. DSD sees this partly as a question of accepted risk and price of market entry. An RDF system gets the motor away from the rotor downwash and can offer aerodynamic efficiency gains, alongside the ability to more readily stack fan stages and introduce contra-rotating designs; however, it necessitates a custom and clean sheet design. This can be a frightening prospect, driving companies toward using a conventional motor topology that can be leveraged from other sectors as part of an HDF system.
That’s not to say that an HDF with a commercial off-the-shelf (COTS) motor is an inherently compromised arrangement – the COTs motors’ potential can be maximized and its characteristics matched to the eVTOL needs by using a transmission, bringing us to our next question.
Do I Really Need to Carry Around a Transmission...and What Would I Gain?
Answering this question depends on a sound understanding of the overall system interactions – to understand the specific cases where carrying the mass ‘penalty’ of a transmission could be worthwhile. Figure 3 highlights some specific points that represent a direct drive arrangement. It’s clear that in matching the requirements of a given motor type to the needs of the overall aircraft propulsion system, constraining the design to a direct drive can limit the energy consumption benefits that might be available.
If a transmission can be combined with a more power dense motor, the overall combination is seen to offer potentially lower energy consumption over the flight profile, with plenty of lower mass system combinations available to offset the additional transmission mass.
If we’ve determined that a transmission might be worth the perceived mass penalty, complexity and cost, then we’ll want to know how powerful the overall transmission ratio is as a way of balancing cost and system efficiency. Figure 3 tells us that there’s a spread of different transmission ratios across the pareto front, so while introducing a transmission gives us more flexibility, it is really giving flexibility in how to achieve the overall energy consumption target while influencing the system cost. Greater ratio directionally drives greater system cost as the supporting gear, shaft and bearing structures are designed to withstand greater loads.
I’ve Concluded What I Think is the Optimal System – How Does Redundancy and Certification Influence It?
With the urban air mobility eco-system in its infancy, confusion tends to reign, as the milestones and requirements around the eventual critical need for aircraft certification are often forgotten. What is clear is that the robust engineering approach to activities like Design Failure Mode & Effect Analysis (DFMEA) carries over, with the focus upon severity even greater once the functional hazard analysis (FHA) is complete.
The mitigation strategy of choice is often redundancy, which is no different to other aerospace applications and comes at the price of increased mass where hardware redundancy is the chosen path. In some cases, such as additional propulsion modules, we can provide redundancy while optimizing the system, but often we must double up on primary systems. An example of this is where an RDF is used and a secondary stator and power electronics can be easily packaged, while still paying the price of increased mass.
Intelligent system integration can sidestep some traditional certification obstacles. For instance, the myriad of requirements around every fluid on an aircraft can be cut down if fluid commonality can be achieved. One example is the current drive to investigate the cooling of power electronics using transmission (and potentially motor) oil, where a transmission is present. This is a great example of when the close integration of subsystems that is commonplace in consumer ground vehicle applications, and the toolchains needed to deliver that integration successfully, can be readily leveraged and adapted for the benefit of the eVTOL community.
I Can Now Answer Some Critical Architecture Questions, But How Do I Combine All This Information Together?
A system-level architecture trade-off tool is a key component in the toolkit of an electrified propulsion engineer.
Even though there is no one-size-fitsall solution, the case study here has shown relevant points around use of a transmission, the number of propulsion modules, and how primary and secondary variables interact to produce an array of viable options in the mass and energy consumption space – summarized in Figure 4. For our specific study, we could select from the pareto front the propulsion architecture and system highlighted in Figure 5, with 6 propulsion modules using a common 57kW motor design. As is always the case, no clear system type or technology will win the day and remain in place for the foreseeable future, so we need to be comfortable with continued change as eVTOL technology matures and evolves and we are able to project how these changes might influence an overall system.
This article was written by Ben Chiswick, Director, Engineering Business Development, Drive System Design (Farmington Hills, MI). For more information, visit here .