Advanced Air Mobility: Using a short takeoff and landing aircraft design

By Manfred Hader and Stephan Baur

Whereas the usual focus of air mobility is on creating global connections and long-haul routes, Electra Aero takes a new approach: The company's goal is to establish air mobility on a regional level to minimize travel times for people as well as for cargo. The key starting point is the use of an electric short takeoff and landing (eSTOL) aircraft design. In our interview with Diana Siegel, Director of Strategy at Electra Aero, she explains how this vision can be realized in an environmentally friendly way, what technological advantages this aircraft offer and how this approach differs from the common electric vertical takeoff and landing (eVTOL) aircraft design.

Can you briefly describe to our readers what Electra Aero's vision is when it comes to Advanced Air Mobility?

Diana Siegel: Aviation has done an incredible job in connecting us all globally and over longer-distance domestic routes. However, it provides limited benefit on shorter, regional routes since getting to and through an airport presents a significant time overhead. On routes between 50-400 km, we are typically stuck using ground transport. On such routes, air mobility can save people and cargo hours, while reducing fuel burn and emissions vs. inefficient ground routes. Electra's vision is to enable this form of air mobility with its unique aircraft design that allows aircraft to get much closer to where people and cargo actually need to go, requiring no more than the space of a parking lot (300x100 ft) to take off and land.

Electra Aero has chosen an eSTOL aircraft design – what are the benefits and drawbacks of such a design compared to an eVTOL (vertical takeoff and landing) aircraft design?

Diana Siegel: An eSTOL design, in contrast to an eVTOL design, uses a technique called "blown lift" to become wing-borne at very low speeds after a small amount of ground roll on the order of 100-150 ft. This approach dramatically reduces the power required for takeoff and landing and allows an eSTOL to carry more people or cargo and fly longer distances compared to an eVTOL of equivalent size. A recent study by MIT found that an eSTOL aircraft is able to carry more than twice the payload of an eVTOL for the same aircraft weight. This means that it can be operated at less than half the per-seat operating cost with less than half the per-seat emissions of an eVTOL design, which we think is crucial to sustainably scale this new form of air mobility. Given the substantially lower sensitivity to weight and its fairly conventional design, an eSTOL aircraft is also much less risky to build and certify than an eVTOL.

Can you share some details of the technological advantages (e.g. distributed electric propulsion, blown lift) of your aircraft design?

Diana Siegel: Blown lift is a technique that has been extensively studied in the 1970s and 80s by NASA and a few aircraft OEMs. It has been shown that the technique enables increasing the lift a wing creates by many multiples. This lift enhancement allows takeoff and landing at speeds as low as 25 knots, resulting in an order-of-magnitude reduction in ground roll vs. conventional winged aircraft. However, conventional propulsion technologies have only been able to take advantage of this phenomenon so much. For example, NASA's QSRA that used four turbojet engines could only blow half of the wing. The French Breguet 941 had to work with larger than optimal rotors given a limit on how many engines could be practically placed on the wing.

Distributed electric propulsion (DEP) does away with these limitations and we now have the opportunity to size and place the propulsors in such a way as to achieve optimal aerodynamic performance. DEP also eliminates the age-old problem of how to control an eSTOL in slow-speed flight, where control surfaces such as ailerons and rudders lose effectiveness. With DEP, we can use the electric motors to create quick changes in differential thrust to maintain precision control also during slow-speed approaches and landings.

What are the intended use cases and missions based on your aircraft design?

Diana Siegel: Given the payload-range performance achievable with an eSTOL, we are planning to serve a variety of use cases from short- to medium-range passenger and cargo mobility, as well as special government missions and medical transport.

We have recently signed partnership agreements and aircraft commitments with three launch customers that are each planning to open up new markets with Electra's aircraft. Bristow, one of the world's largest helicopter operators, is looking to expand its services into regional and urban air mobility as well as middle-mile logistics. Flapper, Latin-America's first on-demand private aviation platform provider, is looking to offer sustainable, lower-cost passenger air mobility services between multi-pad heliports and small airfields, as well as opening up new destinations in the region. Skyportz, an Australian AAM infrastructure provider, and its operating partners are planning to directly connect city centers or dispersed logistics facilities, and deliver cargo to remote regions.

Will the eSTOL compete with eVTOL aircraft on shorter missions in the future or are those separate markets?

Diana Siegel: We expect eSTOL and eVTOL to largely complement each other and to compete on select routes. Electra's aircraft is designed to carry a larger number of passengers or higher cargo load on highly frequented routes connecting larger V/STOLports. eVTOLs typically carry fewer than four passengers and are designed to support single-pad to single-pad operations. While we expect there to be a market for short-range, single-pad to single-pad operations, we also expect this market to emerge at a later stage, given the need for dense infrastructure and lower operating costs than initially feasible. Instead, we envision the market to start with fewer trunk routes flying between larger ports. Larger heliports, such as the Manhattan heliport, have sufficient space to support VTOL as well as STOL takeoffs and landings or could easily be extended to support STOL operations.

What is your vision on the ecosystem required for Electra Aero and how does it differ from, e.g., an eVTOL ecosystem?

Diana Siegel: Electra's aircraft is designed to not require changes to existing aviation operations (helicopter and fixed-wing), but rather to fit within existing operational rules. The aircraft is designed to be flown by a single pilot under visual and instrument rules. Due to its hybrid powertrain, it does not rely on charging infrastructure, which simplifies the setup of a STOLport, particularly in non-urban areas.

While new guidelines are required to enable the setup of compliant ports, Electra does not require changes to the airspace structure, or navigation and communication infrastructure, which quickly becomes cost prohibitive to roll out at scale.

Electra views the setup of suitable V/STOLports at attractive locations for passengers as most critical to success for both VTOL and STOL air mobility. Electra is actively working with its launch partners to ensure compliant infrastructure is available in its partners' launch markets.

How important do you consider the building of partnerships for the ecosystem to be?

Diana Siegel: We view partnerships as essential to effectively serve a wide range of end markets across a wide range of regions globally. As Electra, we are focused on developing, certifying, producing and supporting eSTOL aircraft. We are partnering with operators and end users to deploy eSTOL aircraft in their respective markets, leveraging our partners' understanding of local transportation needs as well as expertise to safely operate air vehicles in their region. As an example, our launch partner, Bristow has decades of experience safely operating helicopters in the most challenging environments. We are working with Bristow to extend their current transportation offering to existing customers, as well as expand into new markets, such as middle-mile logistics. In our collaboration with Flapper, we are leveraging Flapper's knowledge of traffic flows in Latin America to offer flights on the most attractive passenger routes.

Which partnerships along the value chain do you see as more important than others and why?

Diana Siegel: For Electra, early partnerships with operators are key to ensure that we are building the right aircraft for their respective markets and use cases. As part of these partnerships, we are also working with local infrastructure providers to ready the ground infrastructure for future eSTOL operations. Furthermore, we are building partnerships with key suppliers whose technology is essential to the program's success. This includes the propulsion units, flight automation and structures.

When can we expect a "production-like" prototype and "proof-of-concept" test flights?

Diana Siegel: We have flown a 1/3-scale demonstrator and are currently building a two-seat manned tech demonstrator that will be flying in 2022. We have already built the hybrid-electric propulsion system and are testing this on an Iron Bird setup, which will be integrated into the airframe early next year.

What are your planned next steps up to entry into service?

Diana Siegel: Our next major milestone is the flight test of our tech demonstrator and demonstration of the ultra-short takeoff and landing capability. Subsequently, we plan to demonstrate our aircraft in core markets on representative routes to showcase its ability to serve the envisioned future use cases. The tech demonstrator will be manned and having a range of greater than 100 miles, we can demonstrate point-to-point routes in existing airspace without needing special permits for autonomous flight, providing great flexibility for early testing.

What will be the biggest challenges to overcome? What keeps you awake at night?

Diana Siegel: We have purposefully minimized technical risk on the aircraft and have proven the technical concept of blown lift via earlier sub-scale and wind tunnel tests, which makes us confident that we can achieve our performance targets. The tech demonstrator will allow us to validate the ability of pilots to repeatedly land the aircraft safely within a small footprint. There is some remaining risk related to certification. Our aircraft, as well as eVTOL and several novel CTOL aircraft programs, are the first to certify a hybrid-electric propulsion system and fly-by-wire system for a Part 23 aircraft. This “first” is an inherent risk for the collective advanced air mobility industry, even though Electra's architecture minimizes that risk. What keeps us up most at night is public acceptance and support for new infrastructure and novel air mobility routes, be that for cargo or people. As an industry, we need to continue to collaborate to win the public's trust and support, in order to realize the potential of this promising form of mobility.

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