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Sonex electric power: It looks simple, but sparks challenges…
By Peter Lert
What could be simpler than an electric-powered airplane? A motor with just one moving part, a battery, a control box, some wires to connect them—just hook ’em up and fly, right? Radio-controlled model folks have been doing it for years.

Unfortunately, that’s not how it works; as size and power increase, difficulties and complexities scale up exponentially faster.

John Monnett, founder and president of Sonex Aircraft in Oshkosh, and the company’s chief engineer Pete Buck discussed the possibility of building an electric-powered airplane to break FAI class records as long ago as 1994.

Work started in earnest in 2006, with the focus changing from record breaking to the development of a product that could be marketed to sportplane builders and pilots.

Along the way, the Sonex team learned a great deal, gained significant experience, and produced—according to John and Pete during a presentation in the Aviation Learning Center on Tuesday—“a pile of blown-up IGBTs.” For those not conversant in electronics lingo, those are the $1,000 power transistors at the heart of electronic motor control systems.

Rolling with the punches
They originally planned to use off-the-shelf components from a motor manufacturer.

That plan morphed through a phase when a manufacturer developed a motor and controller with Sonex input—and on to another phase in which Sonex developed a power system with help from manufacturers.

Finally, John and Pete concluded that the only way to get the product they really wanted would be to develop it almost entirely in-house.

The result, following years of trial, occasional error, and refinement, is an elegant motor in the 70-hp class—or, as we have to learn to think in this electric age, 52 kilowatts (kW). It’s compact, about the size of a big coffee can; think Costco, here. It weighs about 65 pounds and is 90 percent efficient; by comparison, 30 percent efficiency represents the best of internal combustion engines.

A diversely sited collaboration
Most of the physical work of machining and assembling components has been done in Oshkosh, but in this electronic age, other elements are spread out.

Pete Buck, the chief engineer, works a day job in California (at the Lockheed Skunk Works) and commutes electronically.

Andrew Pearce, the designer of the electronic controller package, lives and works in North Carolina.

Not only can he upload new software to the motor controller via the Internet; he can actually run the test motor (in a Sonex airframe strapped down in the Oshkosh hangar) remotely, all while observing via Skype video and recording the tests in real time.

Even if no one is present. But having no one around can hold up progress.

After uploading new software late at night, Andrew found out the hangar lights hadn’t been left on to let him see the motor. So, that test run had to be aborted.

Firing up, ready-to-go…
At this point Sonex’s complete integrated power package is ready to begin flight testing.

The difficulty of developing such a system underscores the fact that high-voltage battery packs must be treated with great respect and caution. A battery that could run the Sonex motor at full power for an hour contains as much raw energy as about 5 gallons of gasoline (or, for that matter, several sticks of dynamite).

This writer had the chance to observe an inadvertent demonstration of this at another manufacturer’s display on Tuesday; a battery pack was incorrectly connected to a controller, which immediately blew out internal components with the flashes and bangs of a string of Chinese firecrackers—and then it burst into flame.

It was a graphic reminder that “there’s a lot of lighting stuffed into that little box—and it really would like to come out and play.”

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