Is Super-STOL A Viable Alternative To Electric VTOL?
While many aviation startups are focusing on electric vertical takeoff and landing and urban air mobility, a small cadre believes extreme short takeoff and landing and regional services could be easier to certify and more economically viable in the near term.
It is a view that harks back to the 1970s and a vision of short-haul transportation using short-takeoff-and-landing (STOL) airliners as an answer to the growing problems of congestion and noise at major airports. Both NASA and the FAA saw quiet, clean STOL aircraft as a way to make better use of airports that were becoming hemmed in by urbanization. But with the technology available then, the economics did not work out.
Fast-forward to the 2020s, and proponents see in electric propulsion a way to enable the potential of STOL—and see in STOL a way to unlock the benefits of electric propulsion that is less technically challenging and more economically rewarding in the near term.
- Distributed electric propulsion reduces weight and cost of STOL
- STOL aircraft could fit between urban air taxis and regional jets
With electric vertical takeoff and landing (eVTOL), the opportunity is in bringing air transportation closer to the customer by enabling operations from convenient urban verti-ports. The challenges are in the energy required for vertical flight and the criticality of a propulsion system that provides both lift and flight control.
With electric STOL (eSTOL), the opportunities are in lowering the energy density and airworthiness certification hurdles. But the challenge is in providing field performance sufficiently short to enable operations from the same vertiports that eVTOL urban air taxis would use. Enter distributed electric propulsion (DEP) and powered-lift STOL, where propellers and aerodynamics interact to enable high lift at low airspeed. Together, they offer the potential for extreme STOL performance with lower weight and cost.
The Breguet 941 turboprop transport of the 1960s is a good example of both the capability and complexity of powered-lift STOL aircraft using conventional propulsion. Designed to take off in 185 m (607 ft.) carrying up to 60 people, the 941 used full-span slotted flaps to deflect the slipstream from four oversize propellers.
Power came from four Turbomeca Turmo turboprops, each with a free turbine driving a master shaft running through the wing. Connected by a shaft to each propeller, this master shaft ensured power from the engines was distributed equally to the props. If an engine failed, its free turbine was isolated, but the propeller kept turning.
The 941 first flew in 1961, and in 1964 demonstrated it could fly between heliports at Issy-les-Moulineaux in Paris and Allee Verte in downtown Brussels. The 941 and production 941S conducted two U.S. tours, in 1964-65 as the McDonnell 188 and again in 1968-69 as the McDonnell Douglas 188, including demonstrations for American and Eastern Airlines.
No airline order was forthcoming, cost of operation being one factor. But research continued, and NASA in 1978 flew the Quiet Short-haul Research Aircraft (QSRA). This was a de Havilland Canada DHC-5 Buffalo transport modified with a new wing and four Avco Lycoming YF102 turbofans providing upper-surface blowing. The QSRA was retired in 1993, but it contributed to the design of the Boeing C-17.
There are electric STOL aircraft flying today. California startup Pyka is using unmanned eSTOL aircraft for crop-dusting operations in New Zealand. Powered by three 20-kW electric motors on the wing and tail, Pyka’s production aircraft can take off and land in 150 ft. and carry a 625-lb. payload. The design does not use powered lift but has a sailplane-like wing with full-span flaps for high lift at low speed. With a cruise speed of 90 mph and battery swaps between flights, the aircraft can cover 85-135 acres per hour.
At the other end of the spectrum is the Metro Hop concept for a two-seat eSTOL able to take off and land in 200 ft. and fly a 990-lb. payload 100 mi. at 250 mph. The initial application envisaged is express delivery of medical supplies from central warehouses to local hospitals, says CEO Bruno Mombrinie. Being designed for the California startup by the e-Genius electric aircraft team at the University of Stuttgart, Germany, the Metro Hop is simple in concept but has some unique features.
To take off from a flight deck atop a warehouse, electric motors in the main wheels first accelerate the aircraft to 60 mph in 4-5 sec., then active landing-gear legs push the nose up to rotate for takeoff. To land with the precision required, the aircraft measures the distance to the flight deck and extends the motorized landing gear to meet the ground when over the landing mark. Wing lift is then spoiled to put weight on the wheels and brakes used to decelerate. Cargo and battery swap would be robotic.
UK startup Faradair is taking a different approach to STOL regional transport with its Bio Electric Hybrid Aircraft (BEHA) concept. Designed to take off and land in under 985 ft., the BEHA has a triple box wing for high lift—three staggered lifting surfaces joined at the tips by vertical stabilizers. “Our modeling analysis shows the aircraft is still lifting at 40 kt., at 16-deg. angle-of-attack and a lift-to-drag ratio similar to a [Boeing] 747,” says Neil Cloughley, Faradair managing director.
Propulsion is provided by a 1,600-shp turbine engine and 500-kW electric motor driving a contrarotating fan in a duct that increases thrust and reduces noise as well as providing vectored thrust for flight control. Takeoff is primarily done on batteries to reduce noise and emissions, transitioning to the turbine to cruise and recharge the batteries, which provides a reserve power capability in case of engine failure, he says.
Design cruise speed is 200 kt. At a higher speed, the drag penalty from the wings and duct would be too great. But Faradair aims to compete mainly with helicopters, flying faster, farther and more efficiently with less noise. “We see the BEHA doing 90% of the mission a large helicopter can do,” Cloughley says.
The planned first member of the family is the MH1, a 55-ft.-span aircraft capable of quick change between 18 passengers, three LD3 cargo containers or 5 metric tons of payload. Faradair is completing design optimization and hopes, funding permitting, to fly a full-scale prototype by early 2024 and certify the MH1 by 2025-26. The startup has struggled to secure private funding or government support in the UK, but Cloughley says interest has picked up recently, mainly from offshore sources.
John Langford, founder and former CEO of Aurora Flight Sciences, has launched Electra.Aero to develop a super-STOL (SSTOL) hybrid-electric aircraft for regional mobility using DEP and powered lift. He is working with a team from the Massachusetts Institute of Technology (MIT) that has been studying SSTOL as an alternative to eVTOL for urban air mobility (UAM).
Using propulsion for both lift and control in an eVTOL raises the criticality of power failure. This requires increased redundancy and complexity, which add cost and weight to the aircraft and time to the certification process, says Christopher Courtin, a graduate student at MIT. A fixed-wing SSTOL would be comparable to existing single-engine aircraft in a failure scenario, providing an established pathway to certification, he says.
Among the benefits of SSTOL over eVTOL, Courtin lists performance and the ability to use smaller motors, leaving more weight fraction for energy storage or passenger capacity. The lower thrust-to-weight and higher lift-to-drag ratios of an SSTOL versus an eVTOL aircraft are expected to increase speed, range and payload capability.
MIT has been studying a four-seat SSTOL as an alternative to eVTOL for infrastructure-constrained UAM. “If takeoff and landing distances can be made comparable to the size of a vertiport, there may be substantial benefit to using SSTOL aircraft for many of the proposed urban air mobility missions,” Courtin told the American Institute of Aeronautics and Astronautics SciTech conference in January.
DEP can enable extreme short-field performance by mounting propellers along the wing leading edge so that they blow the trailing-edge flaps along most of the span. Based on wind-tunnel tests, the MIT team believes takeoff and landing ground rolls of less than 100 ft. can be achieved using DEP-powered lift, short enough to be competitive with eVTOL.
“It needs a maximum lift coefficient [CLmax] of 7-12 to be feasible. That is well beyond what we can get without a blown wing,” says Courtin. “In the wind tunnel we saw a CLmax of about 9, but we were not confident we would see that in flight, so we decided to go to a subscale aircraft.”
With funding from Aurora, the MIT team in 2019 flew a 30%-scale unmanned model of their conceptual SSTOL. Weighing less than 40 lb. and with a span of 13 ft., this had eight propellers on the wing leading edge, single-slotted flaps and partially blown ailerons, as well as a conventional tail.
Blown-wing aircraft such as the Breguet 941 use wing flaps to deflect the propeller slipstream. This increases lift, both by turning the propeller jet and by suppressing the separation of airflow over the wing and flaps. Compared with turboprops, DEP allows the use of many small propellers. The small jets they produce increase blowing effectiveness. This allows use of a mechanically simple single-slotted flap, reducing the weight and cost of the high-lift system compared with previous blown-lift aircraft, he says.
The goals of MIT’s subscale testing were to determine if high lift could be achieved in flight, to assess the handling qualities and look at the effect of changing propeller diameter on the efficiency of turning the flow. Motors were evenly spaced along the wing leading edge. In takeoff mode, all eight were controlled together by a single throttle lever. In landing mode, the inner six motors—which provided most of the blowing—were controlled via a knob, while the outer pair were modulated by the throttle.
The team tested the aircraft with 9-in.-dia. two-blade and 7-in.-dia. five-blade propellers. Significantly slower flight speeds were achieved with the smaller props. “This was likely because the smaller jet is more effectively deflected by the flap, which enables more lift with lower power,” Courtin says.
At low speeds, the aircraft’s lateral handling qualities degraded, making it difficult to control. “It was hard to keep the aircraft in steady flight, which made it hard to measure CL,” he says. “But the five-blade prop gave the best high-lift performance, pretty repeatable up around a CLmax of 10.”
The subscale aircraft took off in about twice its own length, with a nearly level liftoff attitude. “This indicates that the takeoff distance of the aircraft was limited by the rotation rate that the unblown tail is able to generate at low airspeed,” Courtin says. Takeoff CLmax was not that high. “This suggests that the ground roll of this aircraft is limited by the control power of the horizontal tail,” he says.
The aircraft was difficult to handle because of low dynamic pressure over the control surfaces. At high flap deflections and CL, upwash from the flap ends induced the wing to stall at that location. As a result, a significant portion of each aileron was in separated flow, reducing roll control authority.
The tail was also a factor, its ability to trim the aircraft limiting the achievable lift coefficient. “Control strategies for this aircraft, both in how to generate sufficient control authority and how to translate pilot inputs into actuator movements, are an important area of research going forward,” says Courtin.
Langford’s Electra is working with the MIT team, led by professors Mark Drela and John Hansman, on the next steps. These will begin by flying a full-scale, two-seat concept demonstrator, within a year of COVID-19 restrictions lifting, to address control challenges and show the potential of hyper-STOL.
The team is looking at four-, nine-, 19- and 35-seaters and conducting market studies on which to launch first. Langford sees an opportunity between small urban air taxis and large regional jets for aircraft that can take off “in a couple of hundred feet” and fly 50-500-mi. stage lengths, a market now dominated by automobiles. The short-haul transportation promise of the Breguet 941 may yet be realized.