Quo vadis, multicopter?

From the time of the much-missed presentations at the Uber Elevate summits, many leading figures of advanced air mobility (AAM) — including Mark Moore, to whose drive and energy AAM owes a lot of its current élan — were and still are very clear about their technical criticisms of the multicopter design.

This interpretation of the distributed electric propulsion design philosophy occupies the lower left corner of Uber’s complexity and criticality vs. performance chart. Although great for the first variable pair, it is the lowest performing configuration of the various eVTOL designs. With battery energy density still not growing at the hoped-for rate, this deficit is possibly even more penalizing.

Many of the eVTOL designs that now appear in the lead are in the vectored thrust category, including the Joby Aviation S4 and Archer Maker, which all incorporate some number of tilting rotors/propellers. The striking Beta Technologies ALIA-250 has taken the simpler lift-plus-cruise design approach, doing away with the tilt mechanisms and using independent propellers for lift and cruise.

To the west of the Atlantic, these aircraft have secured impressive tranches of financing. When you combine this with the seemingly more flexible certification regime that is available under U.S. Federal Aviation Administration rules, one can understand how both the whale and shark investors think they are onto the scent of potential big gains from betting on a first-to-market design.

In its 2020 earnings call on April 16, EHang confirmed it is pursuing development of a new winged, lift-plus-cruise, “passenger-grade” eVTOL with a target range of 300 kilometers (185 miles), even as the company remains committed to certifying its EH216 multicopter. This perseverance with the EH216 may owe more to the desire to have an aircraft available for flight demonstrations and pre-existing commitments, rather than a steadfast, unyielding support of the multicopter concept.

Backed by an aerospace giant, Airbus’s CityAirbus is one of the largest and highest-profile multicopter designs, but in an interview last December, Eric Ferreira da Silva, head of engineering of Urban Air Mobility (UAM) at Airbus Helicopters, was emphatic that CityAirbus is a technology demonstrator, rather than a prototype on its way to commercialization.

The last stand of the multicopter as an urban air taxi thus seems to be Volocopter’s VoloCity. Yet even here there are some questions as a patent application from last December shows that the German company is, at a minimum, considering the lift-plus-cruise configuration. Admittedly, not all patent applications lead to an actual project, but Volocopter’s filing is a plausible signal of a potential shift from the company’s emblematic 18-rotor design.

The end of the road?

So where does this leave us? There are many stakeholders in the AAM sphere who still believe that the multicopter has a place for specific missions. Serving an area such as the Dallas-Fort Worth metroplex or the Los Angeles Basin might stretch the range envelopes of today’s multicopters. However, more compact urban agglomerations and cities such as Paris would have a number of key locations and transportation nodes at short distances from each other.

In their 2019 paper, “Electric VTOL Configurations Comparison,” Alessandro Bacchini and Enrico Cestino compared the EHang 184, the Kitty Hawk (now Wisk) Cora, and the Lilium Jet. While these particular models have evolved or been replaced by more capable versions, the comparison and methodology still stands.

Bacchini and Cestino compared three types of trips: a 7-km urban mission, a 30-km extra-urban mission and the 100-km long-range mission. One of their conclusions was that the short-range missions are best performed by multirotors, because they require less energy for flights where the hover portions (take-off, approach, and landing) represent a greater proportion of the trip time. The lift-plus-cruise and vectored thrust models tend to have a higher disk loading and thus a higher power requirement for this phase of flight, and their time savings for a 7-km trip were minimal.

The counterargument would be that this very short urban trip distance does not provide enough flexibility, especially when considering that aircraft will probably have to use predefined corridors rather than optimal straight-line flight paths between departure and destination pairs. However, even if battery capacity has not evolved as much as forecast, the achieved improvements make it possible to stretch this distance for the sweet spot of multicopter operations.

The study was limited to battery-only applications and any use of range-extending hybrid applications would change the mix entirely. That would merit a whole discussion by itself. However, most of the best-funded eVTOL developers are committed to battery-only solutions because their low noise and sustainability are seen as key enablers for large-scale urban air mobility.

Certification and the future

Being in the lower left corner of Uber’s complexity and criticality vs. performance chart, while not being great for performance, does bring the distinct advantage of lower complexity and in general terms this should equate to a less arduous pathway to certification.

Any aircraft that relies on winged lift will have to demonstrate all the handling qualities expected of a fixed-wing aircraft for stall characteristics, stability, etc. in addition to all the handling qualities required for hovering flight. When it comes to analyzing risks from high-energy fragments that might result from a rotor/propeller failure, this must be done for all positions of a tilting thrust lift unit, such that no catastrophic effect should result from a single failure.

Applicants may even be required to demonstrate that safe flight can be continued to a landing if the aircraft cannot transition to the vertical flight mode, in which case the aircraft would need to have sufficient energy reserves to reach a site suitable for landing in airplane mode. All of these subtleties add layers of complexity not afflicting the multirotor design.

In a January 2020 article in Aviation Today, Jacek Kawecki, at the time Uber Elevate vehicle components lead, stated, “Outside the Uber network, there may be short-range markets where a multicopter is the right choice.” From a business insider who clearly favors configurations with an element of wing-borne lift, I think this sums up the subject perfectly.

The UAM/AAM system will have place for a number of aircraft designs, tuned to specific missions. Some will evolve into exceptional multi-mission platforms and will be well placed to win the lion’s share of the market and be the backbone of a highly integrated transportation system. As in today’s aviation scene, others will find specific niches that may be just out of reach of what will become the mainstream designs.

In December 2019, I predicted that the first AAM commercial operations would be in a multicopter. I still stand by that evaluation, even if I now look at Joby, Archer, and Beta with some trepidation! However, projects such as the VoloCity and EH216 are still well positioned to bring the multicopter first across the line, albeit as a limited-use aircraft with a short range and single passenger capability. The next two exciting years will provide us the answer.