STOL(en) the show? – Electra.aero’s short take off vision

Deep Dive

by Yves Le Marquand

16 May 2024

The majority of hype and investment appears to centre around aircraft that take off vertically on battery power alone. Electra.aero has backed a different horse in both races. Founded in Autumn 2020, the firm has already flown piloted test flights with its hybrid-electric short take off and landing (STOL) demonstrator, EL-2 Goldfinch.

The initial tests, which began with two separate missions conducted on November 11th and 19th, 2023, saw the aircraft first fly on pure battery power and then hybrid-electric for the latter flight. It marked the first time a blown lift aircraft using distributed electric propulsion and a hybrid-electric propulsion system took to the skies. Since then, Electra has completed just under 20 flights in its ongoing test campaign.

In short, the aircraft uses eight electric motors to significantly increase wing lift and enable ultra-short take offs and landings (within the size of a football field), and hybrid power provides long ranges without the need for ground-based charging stations. All whilst making big reductions on noise and emissions relative to conventional aircraft and helicopters.

The startup has made swift progress towards its entry into commercial service target of 2028. After first joining the US Air Force’s Agility Prime programme in 2021, last year Electra was awarded a strategic funding partnership valued at $85m. The cash raised so far, made up between private investments, government funding and matching Small Business Innovation Research (SBIR) funding, will enable Electra to develop a full-scale pre-production prototype electric STOL aircraft.

Defence potential

eSTOLs, as Electra puts it, are characterised by helicopter-like operational flexibility with performance and operating costs better than comparable fixed-wing aircraft.The US military has been interested in STOLs for more than half a century. Back in 1972, the US Air Force launched the Advanced Medium STOL Transport competition, asking entrants to design a C-130-class aircraft with STOL capability. Entrants included Boeing’s peculiar looking YC-14 twinjet (worth a look if you haven’t seen it before). There was also NASA’s Quiet Short-Haul Research Aircraft (QRSA), a modified 1974 De Havilland C8.

Fast-forward 52 years and the US Department of Defense is just as interested. Late last month, the US Navy awarded Electra a contract under the Naval Air Systems Command (NAVAIR) to design an eSTOL for ship-based logistics. A week earlier, the US Army awarded the Virginia-based startup a $1.9m SBIR contract to fund powered wind tunnel testing of its aircraft.

“We are seeing strong interest for eSTOL technology serving national security applications because in addition to its ultra-short take off and landing capability, it has relevant payload and range, which is not always the case for many of the other advanced air mobility platforms out there,” Diana Siegel, chief financial officer at Electra.aero tells us. “We are not talking weaponised aircraft, but mainly providing logistics support, for example in the Indo-Pacific region, independent of traditional runway infrastructure – whether that be by taking off from land or ship, delivering personnel or cargo.”

The current fleet of helicopters, tilt-wing and tilt-rotor aircraft, such as the Bell Boeing V22 Osprey or Sikorsky UH-60 Black Hawk, are large assets with a big payload and high value, but they are also small in number. “The DoD often speaks about the need for high-in-number low-in-value aircraft, supporting the DoD’s high value aircraft and helicopters, something which our eSTOL technology could provide. Imagine a C130 coming into an airport and you can have a number of eSTOLs to fan out to serve surrounding dispersed locations,” says Siegel.

Civilian market vision

Despite a significant focus on the DoD side, Electra’s mainstay is the commercial market (more on this in a moment), where it plans to operate under FAA Part 23 regulations for small airplanes. That said, the contracts and relationships they foster with various segments within the DoD are “highly valuable” to Electra’s intentions for the civilian market.

“Firstly, they provide a seat at the table early on with a potentially large customer later down the line. Secondly, those contracts support paying for elements of the eSTOL technology of interest to the DoD. The government does not take an equity stake in the company, so it is a non-dilutive equity source of funding which our venture/equity investors very much like,” says Siegel.

Commercially, the Electra team has a vision not dissimilar to many of the other regional air mobility hopefuls out there. The first port of call will be airport-to-airport connectivity replacing and supplementing turboprop services and also helicopters in specific locations. Electra then wants to expand the route network to include locations which are not well served either because of a lack of aviation infrastructure or because noise and emissions of today’s aircraft are not accepted by the local population.

Among other regions, Australia has been identified as an initial launch market. “It is attractive Down Under simply because of the widely dispersed population centres.” says Seigel. “I used to study at the University of Queensland and getting to the Gold Coast, the Sunshine Coast or the Queensland hinterland could be pretty painful to do by car. If you think about Sydney to Canberra, that is a three-hour drive and not much faster via commercial flight. But if you fly direct from Sydney Harbour and land near Lake Burley Griffin in Canberra, you can cut that journey time to little more than one hour.”

Blown-lift promise

There is a considerable amount of hot air in amongst the serious innovations promising to change airborne mobility, where does blown-lift fit in that landscape? Electra by its own admission is not reinventing the wheel, it is taking what often proves to be the wisest approach by augmenting an existing, proven technology with a cutting-edge advancement.

Pre-second world war, some 30 years or so prior to the aforementioned Boeing YC-14, flap blowing tests were being performed by the Royal Aircraft Establishment (now dissolved into the UK’s Ministry of Defence). Similar tests were also performed in Germany during the war with Messerschmidt and Dornier aircraft which used an ejector- or engine-driven single flow of air sucked over part of the trailing edge span of the wing and blown over the remainder. But the downside to these higher wing loadings is at low speed they do not provide enough lift to keep the aircraft airborne. Even with the addition of large flaps, many early examples landed at higher than suitable speeds and were prone to accidents.

The challenge for STOL aircraft of the past was that they had to make this effect work with traditional engines. Firstly, that makes it very difficult to optimally blow the wing with only a few engines. Either the rotor radius ends up too large, or you end up blowing the wing only partially. Second, traditional engines aren’t well suited to provide fast-acting differential thrust. “It was very difficult to accurately land STOL aircraft of the past since the aerodynamic surfaces alone did not provide fast enough response at this slow speed. Distributed electric propulsion coupled with fly by wire flight control completely changes that. The electric motors can respond quickly to flight control commands and keep the aircraft on its trajectory during precision approaches and landings,” says Siegel.

Blown-lift technology holds great promise for decarbonising aviation as a whole, according to Siegel. “A small passenger aircraft is a fantastic place to start, but we all well know that the greatest portion of emissions comes from larger passenger aircraft with 100 seats or more. There’s no near or mid-term path for all-electric propulsion to serve that mission profile, but hybrid-electric and blown lift are very interesting technologies for those flights. If you ask: ‘How do you smartly integrate aerodynamics and propulsion to get better performance and make the aircraft use less energy to fly in the first place?’ We think blown lift and hybrid are very promising elements to decarbonise aviation at a grander scale.”

Funding

Backing different horses has not had a detrimental effect on Electra funding wise. As well as DoD contracts, the startup has attracted equity investment from Lockheed Martin Ventures and Statkraft Ventures among others. Electra is not a vertically integrated company and does not plan to be its own customer. Plus it has funding needs four to five times lower than an electric vertical take off and landing (eVTOL) developer.

“Also, as a later entrant there is advantage to being a fast follower because you can leverage investments in the underlying systems that others have already made,” says Siegel. “The supply base has really stepped up and put significant development dollars into systems like light-weight, high performance flight controls, batteries and motors which we can now buy at a lower cost than we could have five-to-10 years ago.”