eSTOL vs eVTOL: The jury’s still out

This week, tensions rode high at the American Institute of Aeronautics and Astronautics’ (AIAA) Aviation Forum when debate emerged over the comparative performance viability of electric short take-off and landing (eSTOL) and eVTOL aircraft for urban air mobility (UAM) missions.

UAM has been popularized as a concept meant to be served by eVTOL aircraft, which like helicopters will be able to land in compact spaces in high-density urban environments. Recently, however, several emerging eSTOL companies have been trying to make the case for why their electric airplanes can fulfill the same general mission with less complex aircraft for a lower cost.

Heated discussion over this arose during an Aug. 3 panel after Mark Moore, previously a leader at Uber Elevate and now CEO at Whisper Aero, presented on why he believes the claims of eSTOL companies are unattainable (access the slides for his presentation here). This, he said, creates a dangerous predicament for the industry as UAM vertiport planning is now at a critical stage of development.

“If we don’t have infrastructure for eVTOL, it’s going to be a big problem and the market may flounder,” Moore emphasized. Developers of eSTOL aircraft including Electra and Airflow are claiming that their aircraft can land with as little as 100 feet of ground roll on a 300-foot balanced field length — figures that would seem to make them compatible with many of the vertiports being planned for eVTOL aircraft. “Whether or not these companies should be claiming to use future vertiports is creating confusion,” Moore said.

Airflow and Electra plan to use a technology known as “blown lift” to reach performance numbers that allow for such short take-off and landing distances. Normally, the associated low approach and take-off speeds create maneuverability problems for fixed-wing aircraft as they approach a stall — or the point when air isn’t flowing over the wing fast enough to sustain lift. Blown lift aims to solve this problem by blowing air from the propellers over the wing, which “tricks the wing into thinking it’s much bigger than it really is,” as Electra’s website puts it. Electra is claiming its aircraft will have an approach/departure speed of 30 mph and a minimum powered stall speed of about 23 mph.

Moore, during his time at NASA and before breaking off to help start Uber Elevate, conceived and led the LEAPTech study, which aimed to demonstrate how a fixed-wing aircraft may perform when combined with distributed electric propulsion technology, as is the goal of many eSTOL companies. That goal can be quantified as eSTOL developers are working to increase their aircraft’s coefficient of lift (CL), a measure of how much lifting force is being generated by an airfoil.

A normal general aviation aircraft — such as a Cessna 172 or a Piper Cherokee — has a CL under normal conditions of about 2.0. Moore’s LEAPTech project achieved a maximum CL of about 5.5. Moore claimed that today’s eSTOL developers will require a nominal CL north of 10 to reach their performance targets, which he said is unattainable without sacrifices in acoustics and maneuverability. He suggested that eSTOL companies will require at least double the field lengths they’re claiming, which will call for “fundamentally different infrastructure” than that required by eVTOL aircraft.

Today’s eSTOL manufacturers are building on an approach to aircraft design that has been around for some time, as STOL planes have often been utilized by general aviation bush pilots in remote areas with little to no airport infrastructure, such as Alaska. Even today, at airshows and aviation meetups, STOL competitions are often held as a spectacle, where pilots try and see how short they can get their aircraft to land, often getting ground rolls well below 100 feet. However, those numbers are achieved by experienced aviators operating under ideal conditions in terms of weight and balance, wind, and g-loading, generally in more forgiving locations than dense city centers. Now, the name of the game is large-scale urban commercialization of that same technology.

Airflow CEO Marc Ausman was quick to defend his company’s approach, suggesting that not only can eSTOL aircraft meet their claims, but that the performance assumptions surrounding eVTOL aircraft may be inaccurate. “I think a lot of people think that you’re going to take off with eVTOL vertically and climb up to altitude straight up in a city, but they are just not going to do that. It’s too much energy and too much of an impact on the range,” he said, referencing the high power demands associated with hovering flight. “So, in most scenarios there’s going to be a glideslope of some sort.”

Ausman’s comments reinforce the idea that eSTOL and eVTOL operations will be able to share the same approach and departure corridors, which has become a contentious topic as companies fight for real-estate leases and work to quickly build enabling infrastructure for their vehicles. Shared corridors would make the case for having eSTOL aircraft that can land both at a vertiport environment, as well as something more remote with a short field — such as a road or a vacant lot — with their presumably lower cost and less complex designs.

Electra CEO John Langford added his view on this angle as well: “I don’t know where this idea that you can’t maneuver a STOL airplane comes from — they are very highly maneuverable,” he said. The topic of maneuverability came back when the presenters sparred over how weather restrictions may impact each type of aircraft differently. “Trying to land a VTOL in unstable winds, that airplane is going to be rocking around quite a bit, presenting significant challenges,” Ausman said. The presenters never came to a consensus on whether VTOL or STOL technology will perform better in high-wind conditions, such as those present in urban environments where tall buildings can funnel wind and create significant mechanical turbulence.

Martin Peryea, CEO of Jaunt Air Mobility — an eVTOL company designing a “Reduced rotor Operating Speed Aircraft,” capable of autorotating to the ground like a helicopter in the event of a total power loss — sided with Moore in the low-speed maneuverability requirements discussion. He argued that any eVTOL that gets through certification will have “demonstrated the low-speed handling requirements that are levied on rotorcraft today. And you see rotorcraft operating in cities all the time, climbing in and out vertically,” he said.

This debate brought into public view a now-heated conversation going on in the advanced air mobility space, as companies work to prove out their technologies and differentiate themselves from various competitors. The advertised topic of the panel was to determine whether eVTOL and eSTOL aircraft will compete with or complement each other as the UAM market develops, and it appears that no conclusion was made during this session.

Moderator Michael Patterson — an aerospace technologist at NASA’s Langley Research Center — chimed in as the panel’s time limit approached: “Good points all around — let the prototypes and the aircraft developed prove that out,” he said.