Elysian tweaks E9X design, boosting weight and wingspan of battery-powered aircraft

Dutch start-up Elysian Aircraft has tweaked the configuration of its planned E9X regional aircraft as part of the conceptual design review phase, extending the wingspan beyond 50m (164ft), trimming the number of electric motors from eight to six, and upping the maximum take-off weight to more than 80t.

Dr Reynard de Vries, Elysian chief engineer, says the changes were implemented after the company “revisited” its initial design choices and reanalysed the underlying calculations and assumptions.

“Overall the outcome of these studies was positive in the sense there were no showstoppers,” he says. However, certain tweaks were nonetheless required based on a better understanding of the overall concept and the maturity of key technologies.

Those in-house studies built on earlier analysis of the more challenging aspects of the E9X – a list of 10 areas Elysian referred to as “hot potatoes”.

Conceived as an all-electric design – albeit featuring a 4-5MW turbogenerator in the rear fuselage to provide a 45min reserve – the E9X should be capable of flying 88-100 passengers of routes of around 400nm (750km), a slight reduction on the previous 430nm goal.

Energy comes from 36t of batteries stored in the aircraft’s wings which power six 1.6MW electric motors rather than the eight 1.3MW units of the previous iteration – a 0.8MW total peak power difference. The propellers are now also six- rather than five-bladed units.

Arguably the biggest change is the wing, which is slightly reshaped and grows from an already substantial span of 42m to 52m.

Comparatively, the E9X is now closer in size to the 64m wingspan of the Airbus A330neo widebody than the 35.8m of the A320neo single-aisle or the 27m of the ATR 72-600.

De Vries says the longer wingspan serves two goals: improving the lift-to-drag ratio and providing more internal volume for battery packs.

“One thing we learned from the studies was the volume needed for the cells at pack level,” he says. “That was the one number which we looked over too quickly in the first design iteration.”

The revised wing shape has a straight inboard section and a slightly tapered outboard section, plus a larger overall area.

Although the E9X retains the folding tips of the previous design iteration, allowing it to access ICAO Category C gates with a 36m wingspan limit, the length of the folding portion increased to around 8m – far in excess even of the 3.5m-long tips on the similarly equipped Boeing 777X widebody.

De Vries describes the long folding portion as “one of those bite-the-bullet kind of challenges”, requiring a compromise between the size of the aircraft and its intended range.

And while the size itself is a challenge de Vries notes that with an aileron located on the folding portion, discussions will need to take place with the regulator on how it will be certificated.

Further, the large folding part will require a heavy locking mechanism to prevent accidental deployment in flight, adding yet more weight to the aircraft.

“We account for the fact that we are going to have a heavier wing because of this, but in all the trade-offs, although it’s a challenge, it is well worth the benefits it brings.

“If the alternative is reducing wingspan then you have to compromise on a lot of other fronts,” says de Vries.

Potential customers have offered “mixed opinions” on the longer wingspan. While some are accepting of the need for folding tips to access Category C gates, others think a first generation of the E9X without the feature would be feasible, with the aircraft instead operating from remote stands.

Additionally, Elysian has now taken a “more conservative” view of battery performance, basing its calculations on an energy-density target of 400Wh/kg, down from 450Wh/kg previously.

While that lower battery performance estimate in turn reduces range to 400nm, it is still sufficient to address most regional routes and will also grow as batteries improve in future.

Elysian says the switch from eight to six electric motors reduces “system complexity while preserving distributed thrust and redundancy”.

De Vries says its analysis showed that from an “integration perspective – the aerodynamic and volumetric packaging of everything in the nacelles” it was “slightly better” to reduce the motor count, while staying within the 1MW-class range to broaden supplier choice.

Although there is a total peak power difference of 0.8MW between the two iterations, de Vries says the lower figure is acceptable based on “improved modelling” of the E9X’s take-off performance and the beneficial effects of the larger wing.

“The funny thing is if it wasn’t for the span constraints at the gate and the folding tips, even bigger wings would have been even better, he adds.

In combination, the changes also push up the E9X’s MTOW to 82.5 tonnes, a 6.5t increase on its previous target, putting it broadly in line with modern narrowbodies, but almost four times as much as the 23t of the ATR 72-600.

Accordingly, airport charges will be higher than those for a typical regional aircraft, but operator feedback is that providing overall operating costs are lower “then it’s a beneficial trade-off”.

Elysian co-chief executive Daniel Rosen Jacobson says the latest iteration of the E9X “really, at the core of it, has all the same design principles” as the original concept and that “the original design ideas stand”.

With the concept design now effectively frozen, Elysian is moving into a technology demonstration phase over the next year, feeding back those findings into its preliminary design, which should be signed off in 2028.

It has identified maturation of five core technologies as critical to the overall project. These are: a reusable battery pack; wing-battery integration; the high-voltage electrical system; distributed propulsion and flight controls; and the reserve energy system.

In addition to the civil E9X programme, Elysian sees potential for these “dual-use technologies” to be offered to the defence sector, citing “strong interest” in them.

Flights of a 1:13-scale demonstrator of the E9X have recently commenced, kicking off with a “very successful and beautiful first flight”, says de Vries. It plans to “eventually” field a larger demonstrator, although its size and timing have yet to be determined, he says.

Other projects Elysian is involved in – such as with Dutch fuel cell powertrain developer Conscious Aerospace – will also help to validate certain technologies in flight, he says.

A ‘copper bird’ electrical system test rig is now in operation and a wing-battery integration mock-up is “pretty much up and running”, de Vries adds.

However, a further consequence of the changes is a slight shift to the E9X’s planned schedule: “We are stretching the programme a tiny bit,” says Jacobson.

First flight is now set for the early 2030s, with entry into service in the middle of the decade, moving from previous targets of 2030 and 2033, respectively.

Elysian’s capacity to move faster is limited by the size of its team and inevitable funding constraints, adds Jacobson.

“From our side, our development is going as fast as it can but we have a relatively small team and it is going to be relatively close to the timeline.”

“We could have grown faster but I am happy with where we are,” he says, pointing to the company’s transition from a “small, relatively academically focussed team to really a strong engineering team, a very active workshop and a lot of physical technology development.”

Elysian has so far largely relied on finance from the private sector, plus some from the Dutch government, and is in the process of a Series A raise, which it expects to close in the second quarter.