Bothell-based startup Zunum Aero hasn't nailed down exactly which lithium ion battery technology it will use, complicating the design of the airframe. The planned schedule for commercial service has slipped out a year to 2023.

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Bothell-based startup Zunum Aero, which is developing hybrid electric airplanes, depends crucially upon the evolution of Tesla-style lithium ion battery technology. Yet the rapid changes to that technology are complicating the effort.

Zunum engineers, awaiting the best available battery system that might be available when the plane is ready for passenger service five or more years from now, cannot yet settle upon which batteries will power the first airplane. And they anticipate having to redo the battery certification process multiple times even after that, as the technology improves.

Matt Knapp, Zunum’s chief technical officer for the propulsion system, said that without a firm battery selection, mechanical engineers are today working on the airframe design without a precise idea of the shape, size or weight of the battery pack that will be fitted inside the wings.

“It’s driving my mechanical engineers nuts,” said Knapp in an interview. “They are having to figure out how to accommodate a changing energy source and size over the lifetime of the aircraft.”

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He said that because battery technology, being developed largely in the auto manufacturing world, is evolving quickly, “We don’t have a crystal ball to lock down a (battery) production partner for 2022.” That’s when Zunum wants to start final Federal Aviation Administration (FAA) certification of the airplane.

And Zunum’s engineers expect the state-of-the-art in battery technology to continue to shift, which will mean any improvements developed over the coming years will need to be recertified by the FAA.

“This is the blessing and the bane of the business,” said Knapp. “As the batteries keep changing they keep getting better. It means we’ll be in a perpetual cycle of requalifying the batteries.”

Knapp said the projected schedule for commercial passenger service has slipped out a year to mid-2023.

A helicopter engine backs up the batteries

Zunum, which won early funding from Boeing and airline JetBlue, as well as an $800,000 research grant from the state of Washington, has ambitious plans to develop a family of hybrid electric aircraft. And it vows to do final assembly of the aircraft in Washington.

The initial, smallest model, designated ZA10, is designed to carry up to a dozen passengers. In practice, it will be restricted to nine passengers in the U.S. because FAA rules would require a second pilot for 10 or more passengers.

Aircraft of this size typically are used as air taxis or as commuter planes for business people on high-traffic, short-hop routes such as Silicon Valley to Los Angeles.

Zunum has about 70 employees, with another 10 or so part-time consultants.

About 35 full-time employees are at the Bothell headquarters. An additional 20 work at Zunum’s electrical power facility in Elgin, Ill., near Chicago. About 15 employees work in Indianapolis, developing the plane’s ducted fan propulsion system in a partnership with Purdue University.

Knapp said Zunum will seek a second round of “more substantial” venture capital funding early next year, which could also affect its timetable.

Safety certification of the battery technology by the FAA is critical in light of the problem that caused the grounding of Boeing’s 787 Dreamliner fleet in 2013.

On the Boeing jet, when individual lithium ion cells within the battery pack overheated, damage spread to adjacent cells and threatened a potential “thermal runaway,” which could have engulfed the entire battery pack in fire.

Boeing engineers fixed the problem with a redesign that enclosed the battery in a heavy steel box, with a system of exhaust tubes that vent any gases outside the aircraft if the cells overheat.

Knapp said the fundamental design requirement for Zunum’s battery system is that two or more cells can overheat without spreading to the entire pack and creating a fire.

“How to do this is pretty well known,” he said. “It’s a matter of doing it in a lightweight and consistent manner.“

Knapp said the hybrid plane’s other significant power technology choice has been nailed down: Zunum has chosen a variant of a helicopter engine built by Safran of France to provide the jet-fuel portion of the ZA10’s power system.

In a long-term contract, Safran will develop a variant of its Ardiden turboshaft engine to integrate into the aircraft’s hybrid power system, delivering 500kW of electric power to supplement the battery packs in the wings.

Should the battery system go down, that’s sufficient power for the plane to cruise and land, Zunum said.

Zunum is considering various aerostructures suppliers to design and assemble airframe components, among them Spirit AeroSystems of Wichita, Kan.; Triumph Group, based in Berwyn, Pa.; Daher of France; Aernnova of Spain; and Bombardier Aerostructures in Belfast, Northern Ireland.

Knapp said Zunum will make decisions on basic airframe configuration, including the materials to be used and the window size, early next year and then take bids from supplier partners.

However, Knapp said, the current plan is that “the extensive systems integration and final assembly will be done in Washington state.”

Zunum also announced that it has acquired a Rockwell Turbo Commander turboprop aircraft as a flying testbed for the propulsion system, starting next year. Knapp said engineers will progressively modify the Rockwell plane to test each aspect of the system.

Knapp acknowledged that “the timeline has moved back a bit” on Zunum’s project.

Flight tests for certification of the first production aircraft will start in late 2021, he said.