Pilot on board
By Paul Brinkmann|January 2024
In its initial flight test campaign, Electra established a distinctive course for itself in the emerging field of advanced air mobility through its lift-enhancing design and avoidance of remote piloting. Paul Brinkmann tells the story.
Correction: An earlier version of this story listed an incorrect number of seats for Electra’s planned production aircraft. The plan is to seat nine passengers, not eight, plus two crew stations for customers who want two pilots aboard.
When Electra’s demonstrator aircraft, the hybrid-electric EL-2 Goldfinch, spun up its eight rotors and took off on its inaugural flight, there was a notable difference compared to first flights by most other companies working on aircraft electrification: Lead test pilot Cody Allee climbed aboard the two-seat aircraft.
Others in the field have tended to fly their aircraft remotely from the ground, often numerous times, before putting a pilot aboard. Having a person on the Nov. 11 flight from Manassas Regional Airport in Virginia ruled out a development approach in which lots of technical risk would have been accepted.
“It’s a business decision — are you willing to crash [the plane] to learn whatever you’re trying to find out on that first flight?” J.P. Stewart, Electra’s vice president and general manager, tells me. “You can’t take that trade when you have a person on the airplane.”
Electra invested more time in modeling, simulation and ground testing to be confident about the odds of success and safety for that flight and the five additional ones in the initial test campaign, which concluded last month.
The flights gradually increased in duration and complexity. The first flight was powered only by
electricity running to electric motors from four lithium-ion batteries, and Electra did not reveal the duration. On the second flight, conducted Nov. 19, the hybrid-electric powertrain was engaged, demonstrating the range-enhancing capability for the planned production versions. This flight lasted 23 minutes, with Allee taking the aircraft to an altitude of 3,200 feet and covering 30 miles [48 kilometers] over the Washington, D.C., suburbs, according to Electra. The longest of the six flights lasted 50 minutes.
Allee emphasizes the safety of the design: “With eight motors and essentially five power sources — a generator and four batteries — there is incredible redundancy, meaning a failure of a single component has less urgency compared to a single or twin-engine airplane,” he says by email.
As far as Electra knows, the Nov. 19 flight marked the first by a hybrid-electric, short takeoff and landing aircraft. Specifically, the company calls the planned production version an “ultra-short takeoff and landing” aircraft, and this year intends to demonstrate the ability to take off in as little as 45 meters. Such a short takeoff would be made possible by the eight rotors positioned across the wing. These rotors accelerate the air before it reaches the leading edge, augmenting the lift produced by the wing’s shape and the plane’s forward motion, in a “blown lift” technique.
Allee says blown lift, sometimes called blown wing, provided aerodynamic and handling benefits during the test flights.
When those eight motors are producing thrust and blowing over the wing, “it becomes a unique machine” and “more responsive,” he says, adding that the company has “just begun to explore” the effects of the blown wing.
The Goldfinch was able to fly as slow as 35 knots, or 64 kilometers per hour, and “handled just the same as it did when flying faster, with no buffet or shaking that typically accompanies those speeds,” Allee says.
Such stability at slow speed meant the aircraft could tackle a steeper angle of attack at the upper range of the altitude that the test flights reached, than conventional airplanes, he says. Slow speeds will be explored more in the upcoming test flights.
“We will begin to deeply explore the benefits of blown lift. We’ll work to design approach and departure paths and techniques to maximize those benefits,” he says.
The test flights on battery power only showed that the Goldfinch could take off and land without the louder turbogenerator, which could make such aviation a “better neighbor” to people living around airports, he says.
Allee might have kept the aircraft up longer on the second flight, but the near-freezing temperatures
that day resulted in the lithium-ion batteries getting too cold for optimum operation, an effect that was expected based on the ambient temperature, says Stewart, the vice president. The demonstrator doesn’t have the full thermal management system that future versions will have, particularly the ability to regulate the coolant.
“We’ve shown that our computer modeling and the whole design match pretty well. We thought that was true, but the rubber meets the road in a test like this,” Stewart says.
Regarding power, electricity on the EL-2 sometimes flows only from the turbogenerator in its nose, sometimes only from the batteries, or sometimes from both, he says. When and how the right balance between these sources is achieved is something the company intends to study in future flight tests. The turbogenerator reflects Electra’s business strategy, which is to expand from the small EL-2 Goldfinch to an nine-passenger production version that could be flown up to 800 kilometers without having to be recharged. By contrast, the purely electric aircraft in development have so far topped out at about 240 kilometers.
The longer range means Electra’s aircraft could replace today’s long car trips, which Stewart says amount to “one very promising market segment.”
NASA’s canceled X-57 all-electric plane would have tested a version of the blown-lift technique in f light, but the agency decided to end the project earlier this year without flying, partly due to problems with its electric motors.
Stewart says the aircraft share only very basic similarities. “We blow the wing all the time, using all the rotors,” he says, whereas NASA had intended to turn off and stow some of the rotors after takeoff.
There also will be significant changes from the EL-2 to the as yet unnamed production version. The demonstrator has cables and pulleys connected directly to the flight control surfaces, whereas the production model will send electronic commands from the cockpit through software to the control surfaces, meaning it will be fly-by-wire.
Flying with mechanically controlled surfaces has shown that “you don’t need fancy fly-by-wire systems in order to make the airplane safe and flyable,” Stewart adds.
Future test flights will “drive into every detail of stability control and of aircraft performance,” while the first two flights were designed to demonstrate “basic functionality of all of the systems, mostly that the hybrid system moves power through it as it should, and that temperatures stay within the ranges that they should” despite changes in altitude and range and outside temperatures, Stewart says.
“We’ve shown that our computer modeling and the whole design match pretty well. We thought that was true, but the rubber meets the road in a test like this.”J.P. Stewart, Electra vice president and general manager