More Than a Science Project

News from Wisconsin: Sonexs E-Flight Initiative project takes wing, and development of the Onex continues.


With E-Flight team leader John Monnett at the controls, Sonex Aircraft’s proof-of-concept electric-powered Waiex made its maiden flight on December 3, 2010, lifting off of Runway 27 at the Wittman Regional Airport in Oshkosh, Wisconsin. On this short hop, it climbed out of ground effect before returning to the 5647-foot-long pavement with room to spare. Reflecting on the flight, Monnett said, "I experienced just a glimpse of what the Wright brothers must have felt like flying an unproven system for the first time."

From left, Andrew Pearce, John Monnett and Pete Buck inspect the battery unit.

That unproven system was the electric powerplant, not the Waiex airframe, so the true kinship must be with Charles Taylor. Guided by the Wright brothers’ rough sketches, Taylor designed and built the first successful piston aircraft engine, a 12-horsepower, water-cooled straight-four that powered the Wright Flyer into history. There’s similar kinship with Sir Frank Whittle and Dr. Hans von Ohain, who upset the status quo by independently creating the turbojet engine. Now the Sonex E-motor is the disruptive technology upsetting aviation’s status quo.

History shows that people initially dismiss game-changing innovations as "science projects." That seems especially true with powerplants, which rarely embody the sensuous allure of the airframes they propel. The Sonex E-motor is no different. Between the prop and the big, black battery box is a bare aluminum cooling shroud, a featureless can that contains the E-motor, painted a cheery AeroConversions red. But to Monnett and his engineering alter ego, Pete Buck, it is the elegant realization of a project first attempted in 1992. Recognizing that "there’s only so much fuel in the tank," Monnett said, referring to the Earth’s petroleum reserves, they launched Flash Flight. It ended with a feasibility study because, like the Wrights, they could not find a suitable off-the-shelf motor and power supply.

In 2006 Sonex had the time and capital to fund the E-Flight Initiative that would allow the design, development and building of its own electric powerplant. As project manager, CEO and General Manager Jeremy Monnett kept the team on track. John Monnett provides the common-sense reality check of innovative designs. Buck and Andrew Pearce, a Sonex customer and electrical engineer, are the "geek squad." A printed circuit board design wizard, Pearce’s wife, Rosemary, turns the Sonex’s proprietary electrical designs into finished motor control, high-voltage commutation, battery management and instrumentation units.

Scalable Power

All of the involved electronics are production-ready printed circuit boards, said team spokesman Mark Schaible. With the flight-test program underway, Sonex is putting the finishing touches on the fourth version of its motor and its new consumer-ready Electric Sport Aircraft (ESA) controller, the 12th iteration of the E-Flight controller design.

Flight testing will carefully expand the flight envelope, gradually increasing duration with extended patterns that include touch-and-goes and multiple full-stop landings. As motor and controller systems prove their reliability, the E-Flyer will venture away from the airport. Static ground tests prove only so much because the motor is under a different load in flight, Monnett said. In an electric system, that can affect voltage and amperage levels as well as the motor temperature. Success is the reward for methodical testing.

The E-Flight motor, battery and controls at the time of the December 3 first flight. The red-anodized motor is amazingly compact, while batteries are sandwiched between the mount and the firewall.

Part of the challenge is programming the E-Flight controller to replicate the power response of a piston engine. Aware of rpm and power delivery, Jeremy said, the software actively controls the prop so that it delivers thrust on the same curve as an AeroVee, Jabiru, Lycoming or Continental. "Regardless of where these settings are, the pilot is still in complete control of the application of power and can feed power gradually via the throttle," he said. "We expect the pilot will naturally and easily adjust to it."

On the first flight, John Monnett found the E-Flyer’s performance "startling." In rough terms, the 54-kilowatt motor equates to 72.5 horsepower. This is an inadequate comparison, he said, because the E-motor produces torque instantly and constantly. That’s why it turns the prop used on the 120-hp Jabiru 3300-and turns it at the same static rpm.

How much power the scalable E-motor delivers depends on the stator’s length. It’s created by laminating thin, laser-cut stainless-steel slices that stand together like a hollow loaf of sliced bread wound with wire. Inside it spins the rotor, the E-motor "crankshaft" that is surrounded by eight rows of magnets whose polarity alternate north and south. Commutation, the rapid change of polarity from north to south in the stator’s windings, is what makes the motor turn. Opposites attract and like polarity repels, Monnett explained. A three-phase design, the E-motor has a null polarity between north and south. "Think of it as a three-man relay pulling a wagon (the rotor’s magnets) instead of passing a baton, with each one handing off motive force to the other. This endless three-man relay is constantly pulling, creating constant torque. It’s not like an internal combustion engine that goes bam, bam, bam."

In a simple electric motor, brushes run over a commutator, a mechanical switch that reverses polarity, which "creates the sparks you see in older electric drills." A brushless motor, the E-motor’s control box changes the polarity electronically.

System Safety

It takes a substantial amount of electrons to propel the E-Flight Initiative. Filled with 720 lithium-polymer cells, the 280-pound battery pack delivers 330 volts at 220 amps. It’ll last an hour, give or take, unless "you’re going full-tilt acro," Monnett said. That will expend a full charge in 16 or 17 minutes, but "a normal aerobatic routine runs around 6 minutes, and you don’t need an inverted [oil or fuel] system."

Batteries make or break electric flight, and the lack of a suitable power source stymied Monnett’s Flash Flight project in the 1990s. Today, researchers are working on some interesting battery technologies that hold hope of a 10-fold increase in energy density, Schaible said. But E-Flight’s goal is the motor, and it will run on any system that delivers the proper power, even the banana-peel-and-eggshell-consuming Mr. Fusion that Doc Brown brought home in Back to the Future.

Protecting the electronics from voltage spikes has been the project’s biggest challenge. "We lost a lot of expensive semiconductors early in the project working through these issues," said Andrew Pearce. "Electrical noise has also been a learning adventure," as it can affect systems, from the throttle to the digital sensing of the motor position. Because they’re committed to an air-cooled engine, heat has been another primary focus, Pearce added. All have been difficult problems to solve, and flight testing will verify the solutions proved on the bench and through ground runs.

When installing a lethal amount of electricity in a metal airframe, safety is paramount, Monnett said. His mantra is "isolation." With the new ESA [Electric Sport Aircraft] controller, all high voltage will be forward of the firewall. Behind it the max is 12 volts, with all cockpit motor control units working on 5 volts. "DC-to-DC converters totally isolate the system, so we cannot get high voltage into our low-voltage systems-or the airframe," he said.

The E-Flyer panel is an exercise in simplicity. Power management is front and center, with pitot-static instruments to the left.

Sonex and the E-Flight team have been deeply involved in helping create the ASTM Standards for Electric Propulsion of LSA, said Pete Buck. Approved in November 2010, they are awaiting final approval and publishing. "The next step…is to modify the LSA standard to incorporate the changes necessary for electric propulsion such as minimum fuel requirements, fire and electrical shock suppression, and the basic electric propulsion system."

Even with these important changes to the standards in place, "The FAA still needs to complete a rule change for electric propulsion in lieu of the reciprocating engine," Buck said. "While Sonex and the E-Flight team have not committed to producing a LSA-compliant electric propulsion system, we are committed to producing an electric propulsion system for Experimental/Amateur-Built aircraft that comply with the LSA rules."

Monnett illustrated the safety systems by talking through the preflight and engine-start procedure. There are no wires to check. "Reaching into the cowling and wiggling wires is not a good idea," he said with a chuckle. In the cockpit, a single instrument controls the system and reports such parameters as battery state of charge, cell balancing, motor rpm and component temperatures. Before the system, which is powered by a separate 12-volt battery, arms the main battery pack, it runs through a power-on self test.

With John Monnett at the controls, the E-Flyer climbs out of ground effect.

When the diagnostic routine verifies the airworthiness of the E-motor system, it begins a precycle charge. "If you just turn on the battery pack, the huge surge or in-rush of power can blow up everything, so we bring up the power slowly to keep the system in balance," Monnett said. This process activates several relays. "We call them contactors…they are the only semi-mechanical components in the system, and if anything out of the ordinary happens-a wire breaks or we have an impact-the contactor says, ‘OK, we’re done!’ And it immediately shuts down the battery pack, isolating it from everything."

Jeremy Monnett said Sonex plans to patent its proprietary system, once it’s finalized after the flight-test program. They’ll complete the consumer packaging at the same time, said Schaible. "Our goal is to deliver an E-Flight system that will be a drop-in aftermarket replacement or OEM option for many existing Experimental and LSA airframe designs, just like our AeroVee engine kit is today."

Then Sonex can start working on an airframe optimized for electric propulsion. The E-Flight Waiex is stock, "because we didn’t want to run two experiments at the same time," Monnett said. The ESA aircraft concept is described on Sonex’s R&D web site, the Hornets’ Nest ( "That’s probably our first purpose-built electric-powered airframe," said Jeremy Monnett. "From there, the sky’s the limit. You can bet we’ll be focusing not only on long-winged, efficient airframes such as the Xenos, but also on thoroughbred single-place aerobatic machines similar to our Onex design to really take advantage of the tremendous torque and power offered by an electric motor."

For more information, call 920/231-8297 or visit

Scott Spangler A pilot since 1976, Spangler was the founding editor of Flight Training magazine. In 1999 he launched and edited NAFI Mentor for the National Association of Flight Instructors, and for seven years was editor in chief of EAA publications. As a freelancer hes written for Air & Space Smithsonian, Overhaul & Maintenance, Aviation for Women, Twin & Turbine and a number of non-aviation titles.

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Scott Spangler
Scott Spangler A pilot since 1976, Spangler was the founding editor of Flight Training magazine. In 1999 he launched and edited NAFI Mentor for the National Association of Flight Instructors, and for seven years was editor in chief of EAA publications. As a freelancer he’s written for Air & Space Smithsonian, Overhaul & Maintenance, Aviation for Women, Twin & Turbine and a number of non-aviation titles.


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