The Path to Motor and Rotor Certification for eVTOL

EASA Special Condition Basics

With EASA’s Special Condition for Small-category VTOL Aircraft published last July, following the proposed version from October 2018, we now have a more transparent path for certification for eVTOL. Although the EASA text was not met with unanimous elation, especially to the west of the Atlantic, it is much better to have a clear way forward, properly tailored to the realities brought about by the new technologies and proposed theatres of operation that are being spawned in the eVTOL ecosystem.

While the U.S. airframers appear to favour certifications under the CFR 14 Part 23, CFR 14 Part 27/29, or some sort of hybrid, the pathway provided by the EASA Special Condition appears to be better suited to provide the safety level that the general public, represented by the Authorities, will expect of an aircraft flying commercial missions over densely populated urban areas. This is achieved by satisfying comparable safety objectives currently met by CS-25 and to some extent by CS-27 and CS-29. CFR 14 Part 23 / CS-23 simply does not pass muster.

The Preamble to EASA’s Special Condition explains this succinctly. Even if you are not an expert in certification, this section of the document is a must-read for a better appreciation of EASA’s thought process behind this Special Condition for anyone with a stake in eVTOL.

Some speakers at this year’s Uber Elevate appeared to promote the idea that the industry could start off under the safety objectives at CFR 14 Part 23 / CS-23 levels, and then graduate to a higher standard as eVTOL operational hours ramp up. I disagree with this approach. Although eVTOL and urban operations combination will in a way be a greenfield, one can still extrapolate operational safety data from fairly similar commercial operations by mainstream rotorcraft. Additionally, when you are looking at 10?? Function Development Assurance Levels (FDALs), you should not assume that all will be fine until you approach the 10? hours and at that juncture ‘upgrade’ to a higher safety objective, because with this methodology one is looking at acceptable rates.

Motor and Propeller Certification Specificities

One of the items that did not change between the proposal and current version of EASA’s SC VTOL is that each aircraft engine, propeller and auxiliary power unit (APU) must be type certified, or meet accepted specifications. The approach from EASA to take as much material from CS-23 as possible would reasonably indicate that the same will happen for engine and propeller certification standards where we can expect the Authority to base some requirements on CS-E and CS-P to put some flesh on the requirements of Subpart E of the Special Condition. The yet to be published Acceptable Means of Compliance (AMC) will provide more clarity.

Taking a look at what is currently prescribed in CS-25, CS-E and CS-P, we can postulate what text could find its way into the VTOL AMC. Following this, one can take a more positively critical look at some designs that are being proposed.

Currently, the count of proposed designs has surpassed the 200 mark. Darwinian evolutionary processes will take their course but the designs that will be discussed in this article are those that I consider to be the top-drawer selection either by design, the industrial and financial clout behind the project or a combination thereof.

The observations are naturally an outsider’s look and are based on publicly available sources and should be considered as a back-of-a-napkin sketch; a starting point for a discussion, rather than a definitive ruling on the designs’ ultimate operational or financial viability.

 The Multicopters

In Uber’s Complexity × Criticality / Performance chart, multicopters live in the lower left corner, indicating a low complexity and criticality but also a lower performance compared to the tilt-rotor and tilt-wing concepts and aircraft with separate lift and thrust motors. This lower efficiency has led Uber to forgo this design route. However, while they favour a higher complexity and efficiency combination, as have other established airframers such as Bell, this increased efficiency will come at a cost and potentially increases the risk of delays in certification which greater complexity will almost certainly entail.

Volocity

Volocity and CityAirbus have however embraced the potentially simpler multicopter design. They have clearly positioned their aircraft for the urban mission unlike other designs which also seem to target intercity missions in addition to the urban, 15 to 25-minute flights.

Volocity has garnered a lot of media attention with their public flights. It is an aesthetically pleasing design and has an unmistakable futuristic allure. At first glance, this aircraft’s forte would appear to be its ample motor redundancy. It would however be fair to ask if the number of motors had been mandated by the lifting power of affordable motors at the inception of the first prototype, rather than a design requirement with certification and commercialisation forethought.

In commercial aviation we have seen a move away from a high number of power units to a world where twins practically rule the skies. The lower maintenance of the least number of units, feasible with today’s gas turbine engine reliability, has become a primary factor. As electric motor reliability is already high at the outset, it is anticipated that this progression to a lower number of more reliable, more powerful and more efficient motors could happen even quicker in eVTOL design.

Returning to redundancy, although Volocity’s design appears to provide ample, there is the question about segregation.

In his presentation during the Uber Elevate 2019, Jacek Kawecki just very briefly cited the criticality of propeller or blade separation. It is surprising that SC VTOL is very specific on protection from ice being shed from a propeller, but ostensibly less prescriptive about a blade or other propeller debris release, unless this is being planned for the AMCs. While exposure to icing conditions may be restricted by certification or operational limitations not to operate the aircraft in icing conditions, the possibility of a blade release or other propeller structural failure will be something that all eVTOLs will be subject to. On the subject of blade de-icing, the electrical power requirement would almost certainly make it only feasible for hybrid power systems, but that is another topic altogether.

In CS-25 AMC 25.905(d) 2.1, which deals with Impact Damage Zone, states that all practical precautions should be taken in the aeroplane design to minimise, on the basis of good engineering judgement, the risk of catastrophic effects due to the release of part of, or a complete propeller blade. These precautions should be taken within an impact zone defined by the region between the surfaces generated by lines passing through the centre of the propeller hub making angles of at least five degrees forward and aft of the plane of rotation of each propeller. Within this zone the first consideration is the vulnerability of critical components and systems (e.g. location, duplication, separation, protection).

One could argue that firstly, this text is not part of VTOL regulation just yet and these are lift rotors rather than specifically propellers, but one would not be surprised to find similar text finding its way to the SC VTOL AMCs to supplement VTOL.2400 (c)(1).

With this in mind one can consider the Volocity design, which includes a number of closely spaced rotors, could have some exposure as any debris release has a potential of affecting its neighbour thus triggering a cascading failure. The initiating failure could be an internal structural failure of a rotor blade or hub, or precipitated by a foreign object such as a bird strike.

EASA’s SC VTOL does in fact set out the requirement for bird strike resistance although there is no prescribed bird mass mentioned. From my previous article, eVTOL, The Case for a Bigger Bird, I put forward that the 1.8 kg (4 lb) bird mass could be a reasonable figure as regards to the general structure, but this figure is also prescribed in CS-P 360, which may be seen as a signal for a value that could be stipulated in the SC VTOL AMCs.

On the plus side, the larger number of rotors means that any released debris would possibly be smaller and lighter thus the impact energy transfer to anything struck would be less.

Further analysis of passenger count / payload capability, endurance, battery charging time vs. battery swapping are possible, but this article will be limited to possible risks at the motor and motor configurations at the certification level.

One derived advantage for the inherently slower multicopters is that if the Regulators impose a bird mass requirement, the impact energy that the structure will need to demonstrate for continued safe flight will be Iower than for the faster aircraft. The Rotorcraft Bird Strike Working Group in the U.S. also observed that no bird strikes were recorded with aircraft speeds less than 55 kt. One theory is that these speeds are closer to speeds that birds experience in nature such as other birds in flight and so are able to detect and avoid the aircraft.

CityAirbus

The CityAirbus is another striking design and one that appears to have some ingenious approaches to certification challenges. This is not at all surprising as Airbus engineers have been certifying both rotorcraft and fixed-wing aircraft for years. Even if SC VTOL was not available at the time of the project’s origins, the design is forward-looking in this aspect.

At first glance one could question why the two superimposed rotors are not both protected by a duct with only the lower rotor having this protection. There could possibly even be some potential performance gains by a more prominent duct due to a more marked Coand? Effect.

However, and this is pure supposition, it is possible that the current configuration was selected with propeller debris release in mind. Had both superimposed rotors been enclosed in a duct, if the upper rotor failed, the debris could presumably stay confined in that enclosure and drop by gravity onto the lower rotor thus disabling a complete ‘corner’ of lift which would not allow safe continuation of flight. With the configuration that Airbus have chosen, if debris is liberated from the upper rotor it should be ejected clear of the lower rotor and would affect the other upper rotors only if it is liberated in approximately one quadrant of its described rotation. If a lower rotor fails, the debris would be kept confined in the duct and presumably ejected from the lower opening without impacting the upper rotor. One question is if by putting the rear sets of rotors at a higher position than the front pair could even increase this segregation. Although there would be an impact on the aerodynamics and weight distribution of the current prototype, the increase headroom could also allow space for the ICE or turbine component of a hybrid unit, that Airbus seem still to be resisting, even if Guillame Faury has acknowledged that battery technology still needs work to become a viable unique power source for urban aircraft.

EHang 184

This programme from China appears to have a very strong financial backing and has logged an impressive number of flights, many of them in public. The configuration selected in this project does however raise some questions on passenger accessibility and how the airframer can ensure that there would be no inadvertent operation of the rotors while the passengers are embarking or disembarking. There of course countless ways of tackling this issue, but the final design must meet the required FDAL. Not unlike the Volocity, there are the questions about the number of closely spaced, unguarded rotors and additionally, in this case, sections of the passenger cabin must also be strengthened as it is in the area that could be impacted by rotor debris.

Workhorse SureFly

The SureFly is the last multicopter in this overview. It is strikingly similar to the CityAirbus in concept, if quite a bit smaller. In its current design iteration, none of its rotors are protected by a duct. The main differentiating element of this design is that Workhorse have decided to go for a hybrid system rather than pure electric, surprisingly the only the second such example from the projects described in this article. This pragmatic approach results in a claimed endurance of more than 2 hours, eminently more practical than the 15 to 30-minute flight time capability of the battery powered eVTOLs with the current cell technology.

Vectored Thrust

Stepping back to Uber’s Complexity × Criticality / Performance chart and the presentations from Uber Elevate 2019, the performance advantage of the vectored thrust aircraft over the multicopters is well explained.

However, the increase in complexity could bring with it an increased risk at the certification stage. One has only to look back to the vectored thrust programmes in the recent past including the military applications to see that this family of VTOL aircraft has seen an arduous journey. The most prominent civilian project, the AW609 has yet to reach the certification finish line, and this with a prototype having first flown in 2003. Hopefully, this will be achieved in time to see this engineering marvel at Dubai’s EXPO 2020.

We cannot overlook the advantage that distributed thrust made possible by electrification which helps decrease both complexity and criticality, yet some of the issues with this approach still linger.

UberAir eCRM-004

UberAir’s eCRM-004 is an impressive design by all counts. It is a blend between vectored thrust and lift + thrust model which looks good in the promotional videos and more importantly promises range and performance that the multicopters would find hard to match.

Like with the Volocity, although the number of rotors and motors provide a level of redundancy, the proximity of the rear lift rotors and the tightly stacked lift rotors could pose some additional concerns at the certification level.

When one looks at the same aircraft from an engineering perspective, you can appreciate that each moving part is one or more actuators, and that each actuation mechanism needs one, possibly more, position monitoring sensors. None of this is rocket science, but although we have been using this technology for years in the aerospace industry, the fail-proof actuator and sensor are still a utopia.

While actuator and sensor redundancy is one way of mitigating this, even in the CS-25 world we have to take into account scenarios where multiple layers of redundancies are lost and the operator has to deal with an abnormal configuration such as jammed flaps, landing gear etc. As with other any mechanical device, each moving part will require more inspections, maintenance, lubrication etc. The matter is amplified once any operations in icing conditions certification is requested as ice accumulation that could jam any moving surface has to be taken into account as well as possibly prescribe a de-icing capability that would be a huge draw on available power.

One scenario that vectored thrust aircraft may have to demonstrate is if the aircraft has a failure which has it is blocked in airplane mode. This would already be a challenge in a conventional VTOL, if there is such a thing, as the geometry of the rotors would probably result in some released debris at touchdown. There would also be a requirement to find a suitable landing area with sufficient dimensions to allow the aircraft to come to a stop after landing at the minimum safe speed in airplane mode. At first this might seem a fairly straightforward scenario as the operator would just need to divert to a conventional airport. However, when you factor in the limited endurance of battery-powered eVTOL, finding a suitable landing area in the remaining range, when you are at the end of a mission in the middle of a busy city, could well present a bigger challenge. This might not impact certification as such but could become a concern when operational regulations for required endurance reserves are set.

Having said all that, the eCRM-004 design may actually have this scenario catered for, provided its lift rotors can support continued safe flight without the two tilting motors in lift mode. The wheeled landing gear is also a bonus.

Bell Nexus

When in multicopter mode, the Bell Nexus looks rock solid. The six rotors provide the required redundancy and while the larger rotors could release heavier debris after a failure, they are enclosed in what appears to be a very rugged duct, which would also shield the rotor from oncoming foreign objects.

The picture changes when the aircraft is in airplane mode. The hexacopter layout introduces some concerns when in this mode. In aircraft design, you rarely see in-line power units as any released debris from the front engines could impact the aft engines. While CS-25 AMC 25.905 seems to concentrate on protecting the areas from debris that may be released radially ±5°, it would not be unreasonable for Regulators to look at other debris paths if it is considered they may impact critical components, to satisfy VTOL.2400 (c)(1) where any released material will effectively be a foreign object. With the Bell Nexus there is some protection provided by the small wing, and other design features may mitigate this risk but one can expect some scrutiny of this chosen layout during certification.

Although possibly more specific to turbine engines, CS-E 800 Bird Strike and Ingestion, also prescribes different bird masses according to the inlet area. We will see if the Regulators weigh up a similar way forward for the SC VTOL AMCs as otherwise, for example, the safety benefits of a multicopter design that has fully enclosed motors and rotors compared to the large area of frontally exposed rotors such as in the Bell Nexus, will not be recognized.

As for other vectored thrust VTOL designs, having the aircraft stuck in airplane mode is an additional issue and due the relatively small wing, it can be deduced that the higher minimum safe speed in this configuration would require a longer landing distance.The analysis may also have to be done also for intermediate positions of the vectored units.

Lilium Jet

If looks were the main deciding factor for eVTOL design, the Lilium Jet would be hard to beat. Lilium appear to be positioning their product for a longer-range mission and one of the factors that will need to be considered is the higher cruise speed that would increase the energy of any foreign object impact. The power unit design and layout does however appear to have had quite a bit of certification, operational and maintenance minimalism foresight.

Although the number of power units will increase maintenance activity, the location and comparatively small size of each separate unit should qualify them as Line Replaceable Units (LRUs), increasing the ease and speed of maintenance while minimising the logistical support required.

Due to their small size (mass?), even if most probably rotating at higher speeds, the cowling provided around each unit should capture any released debris, protecting the adjacent unit and the rest of the aircraft structure. Having the front bank of power units at a lower level than the rear bank should help avoid any debris exiting the front bank from impinging the aft ones.

There is also redundancy with the power unit banks that appear to have a degree of independent movement and this, aided by a higher aspect ratio should help in a scenario with one or more banks stuck in airplane mode. Any operational regulation to have a suitable landing distance with range at the end of a mission could still impose some mission length limitations.

A3 Vahana

The last vectored thrust aircraft in this list is the A3 Vahana. Some could think this to be Airbus’s ploy to have an aircraft readily available to display in air shows etc. and stay in the urban aircraft buzz and limelight. In the meantime, the engineers at Manching and Donauw?rth continue to work undisturbed by disruptive media outings, quietly honing the CityAirbus project which, significantly, is the one that carries the Company’s name.

This would not do justice to the broad development work and quantity and quality of data that the A3 project must be producing in battery technology, autonomy, and horizontal to vertical flight transitions, among other fields. On the latter point, however, A3 have always been a bit cagey. The publicly released videos of these flight transitions have either been ultra-wide shots or extreme close-ups of the motors and rotors from which you cannot get a feel of what the passenger experience could be during these phases.

The main certification pathway obstacles that are manifest are similar to those described earlier, mainly the lack of physical segregation of the rotors and the aircraft being blocked in airplane configuration. From the operational viability aspect, being a single-seater, the aircraft in its present form would probably be more conducive for recreational use which could lessen the certification burden. Any commercial missions would probably need to wait for autonomous flight certification which is further down the development path. A3 have since announced that flight testing has been completed.

Lift + Thrust

Making one last reference to the Uber’s Complexity × Criticality / Performance chart, the Lift + Thrust aircraft are the ones which are the closest to the ideal lower right corner. Uber Air’s eCRM-004 would live squarely in this area were it not for the two tilting pods.

A pure lift + thrust aircraft could have some advantages from the outset, provided that the lift and thrust components are effectively segregated as regards power supply as well as physical separation.

Aurora PAV

The Aurora PAV would seem to tick a lot of boxes but the problem with the segregation of the lift rotors is also present. Not unlike the EHang 184, the fuselage would also have to be strengthened to cater for the eventuality of released propeller debris. Any strengthening could be achieved by innovative materials but could also entail an increase in weight, the bane of all aircraft designers.

Aurora’s backing from its Chicago-based parent is not a negligible asset by any measure. The PAV for the moment has all the vestiges of a pure prototype so it would be interesting to see how this aircraft evolves with the second prototype next year.

Pipistrel 801

Although there are many other exquisitely ingenious eVTOL projects, I felt compelled to narrow the selection to keep the length of this article in check, but this does not mean other designs cannot find a niche, or even become a dominant force the eVTOL ecosystem.

Nonetheless, I could not conclude without mentioning the Pipistrel 801 from Dr Tine Toma?i?’s stable, unveiled at Uber Elevate 2019. The project has many bright spots, and while some futurists may not like that, if only superficially, it looks so much like a mainstream aircraft, the conventionality of some of its characteristics could pave the way for a smoother certification avenue.

The lift and thrust components are well segregated and the eight lift rotors are ensconced in the body, shielded from most foreign object threats. The interesting operational aspect comes from the use of doors for the lift rotors that give further protection and mainly increase aerodynamic efficiency in airplane mode. Certification will undoubtedly look at the reliability of the actuation of these doors although the number of available lift rotors could make it less vulnerable to the scenario where the aircraft is forced to land in airplane configuration than in the vectored thrust model case.

Like the Lilium Jet, this design appears better tuned for the longer missions. If the lift rotor doors are closed, it is likely that there will be a minimum altitude at which the aircraft can cruise in airplane mode that would allow enough time for the lift rotor doors to open and sufficient lift rotors to come online in case of a failure of the single thrust motor or propeller. Contingent on this minimum altitude, shorter trips could be less practical and also not so comfortable unless the aircraft stays in multicopter mode, which would make the thrust component virtually redundant or as a minimum, not exploited as efficiently.

Following the initial publication of this article Pipistrel have announced a change of focus to cargo drones showing the fluidity of this sector.

First Off the Blocks, the Multicopters

Although there are a number of challenges linked to certification, we must keep these in perspective. If we look at say the A380; this is a nearly 600T aircraft, cruising at 85% the speed of sound at up to 43,000 ft while carrying more than 500 customers, some of whom will be enjoying a shower or having a drink in the lounge, flying from Dubai to Los Angeles in just over half a day. If we can achieve this while complying with the voluminous CS-25 and other applicable tomes, eVTOL aircraft are certainly within our grasp as each engineering problem has an engineering solution.

Without ascribing myself any superforecasting capabilities, it is probable that the first eVTOL to achieve certification for commercial operations will be a multicopter. The simplicity of the design will trump the lower performance and efficiency but which will still be sufficient to start off operations with short, urban trips. Airframers putting forward the more complex designs, striving for the added range and performance could face the risk of development and certification delays though we have to see how the regulations supplemented with the soon to be published AMCs will ultimately shape this path. The more complex designs may lose the first-to-market race, but large-scale operations like Uber Air may in fact require this performance premium to make their business model viable.

While some eVTOL aircraft could do demonstration flights for Dubai’s EXPO 2020, it is unlikely they would have reached certification and operational approval for passenger-carrying flights in the remaining months as we are now less than a year away.

The first multicopter to enter commercial service could well be the CityAirbus, potentially modified with a hybrid power unit as battery capacity and charging profile / time limitations would not have converged with the mission demands as fast as originally forecast. The strengths that Airbus brings with it, including the engineering and specifically, certification and industrial experience, added to the financial backing of the group, will be strong drivers.

This, together with the planned timeline of having urban air taxis ready and in operation by the 2024 Paris Olympics following the agreement signed by Airbus, Groupe ADP and RATP, will inject the additional impetus into the project that will have all the cards in place to set off the eVTOL urban air mobility transformation.

à bient?t!

References and Suggested Reading

• EASA - Special Condition for small-category VTOL aircraft
• EASA - CS-25 / Amendment 23
• EASA - CS-E / Amendment 5
• EASA - CS-P / Amendment 1
• Video: Uber Elevate Summit 2019 - Day 1 Presentations
• Video: Uber Elevate Summit 2019 - Day 2 Presentations

By the same author

• eVTOL, The Case for a Bigger Bird?
• The B737 MAX, a Cautionary Tale for the eVTOL industry?
• eVTOL, Revolution or Evolution?

Note: The views expressed in this article are those of the author and do not necessarily reflect the policy or position of any other agency, organization, employer or company, past or present. The information contained in this article does not constitute investment advice.