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Sonex Aircraft and AeroConversions Show Electric Propulsion System for Sport Aircraft

The basic elements of the E-Flight electric propulsion system. Click to enlarge.

At EAA AirVenture Oshkosh 2007, Sonex Aircraft and AeroConversions Products introduced their E-Flight Initiative, including a proof-of-concept prototype electric powerplant installed in a Waiex airframe. (Earlier post.)

The most significant portion of E-Flight is the development of the electric motor powerplant, controller, battery pack and charging system. The companies’ goal is to determine the feasibility of commercializing a line of electric Sonex and AeroConversions products.

The concept for the electric sport aircraft project pre-dates the 1997 founding of Sonex Aircraft. In 1994, John Monnett, the founder of Sonex, and Pete Buck devised the concept to design, build, and fly a small electric-powered and manned aircraft that would be capable of a short duration flight in order to set or establish speed records for this new class of aircraft.

The system installed in an airframe. Click to enlarge.

Pete Buck prepared a detailed feasibility study for the project dubbed “Flash Flight”. Buck, who works full-time as an engineer at Lockheed Advanced Development Company Skunk Works and is Sonex Aircraft’s Chief Engineer, also spent two semesters of his engineering degree analyzing and building the battery/power system for a Hybrid Electric Vehicle (HEV) sponsored by Ford Motor Company.

Buck’s study concluded that Flash Flight was feasible using many off-the-shelf components at relatively little risk. The aircraft would fulfill its record-attempt mission; however, it would only have an endurance of approximately 10 minutes. Other tasks associated with the founding of Sonex Aircraft, LLC took priority, and Flash Flight never came to fruition.

Since 1994 and Flash Flight’s feasibility study, the popularity of radio-controlled (RC) electric powered toy vehicles, gas-electric hybrid cars, and the boom in wireless electronic devices such as cell phones and PDA’s have pushed the state-of-the-art in battery, electric motor and controller technology.

Brushless DC cobalt motors are now lighter and more efficient. Advances in microprocessors have allowed motor controllers to be smaller, lighter and more efficient. Lithium Ion and Lithium Polymer battery technology has pushed the endurance, efficiency and power output of electronic devices, while shrinking in physical size and weight. The Sonex R&D team concluded that the time for this endeavor had arrived.

E-Flight’s proof-of-concept prototype will use the flight-proven Waiex airframe, flown single-pilot only, so that the emphasis can be placed solely on powerplant research and development. Initial top speeds will reach approximately 130 mph, and endurance is expected to range between 25-45 minutes or longer, depending upon power usage on each individual flight.

The motor. The design team, in collaboration with Bob Boucher of Astro Flight, Inc., has designed and built a completely new AeroConversions motor. The brushless DC motor is a 3 phase, 270 volt, 200 amp motor that will be more than 90% efficient. It uses elegantly designed CNC machined anodized aluminum and nickel-plated steel parts in combination with “off the shelf” bearings, races, snap rings, and magnets.

The prototype AeroConversions motor is slightly larger than a 35-ounce coffee can and weighs approximately 50 pounds (22.7 kg).  The motor is a modular, scalable unit. The motor core’s design has modular sections that can be reduced to a lower-output, smaller motor (shortened in length), or added upon to make a larger motor with a higher power output.

The controller. The research and development team, in cooperation with a key electronics expert, designed a proprietary AeroConversions electronic motor controller. The controller can commutate the motor in two different ways: using Hall effect sensors to determine the magnet core’s position in relation to the coils, or using the motor’s back-EMF to sense rotor position, eliminating the need for Hall sensors. The AeroConversions controller will initially employ a Hall effect sensor-equipped motor, but back-EMF controlling will also be explored to potentially further simplify the AeroConversions motor design. The AeroConversions controller will also provide in-cockpit monitoring of battery power levels to the pilot.

The battery system. The E-Flight project is using lithium polymer cells. To address safety and thermal management issues, the E-Flight design team has engineered and constructed 10 battery “safe boxes” intended to contain 8 Li-Poly battery packs per box and consolidate their charge/discharge and balancing wiring into two sets of multi-pin connectors.

The boxes will accommodate natural cell expansion and contraction while safely securing each cell pack and facilitating cell cooling with “cooling foam” padding. Cooling will further be aided by heat sink surfaces on each box that will have cooling inlet air directed over them. Additionally, the boxes are designed to contain and safely direct fire or explosion within the box through a “blow hole” in the box that will be connected to a small exhaust manifold.

For the proof-of-concept aircraft, the battery boxes will be removed from the aircraft and charged individually. The charging units need to be configured to safely keep all cells balanced during charging. Lessons learned from the proof-of-concept systems will lead to the design of more advanced charging and balancing systems allowing safer battery handling by consumers, including a single-plug charging system that may remain in the aircraft at all times, featuring easy exchange of battery boxes to enable consecutive back-to-back flights in a short period of time by pilots who wish to invest in spare batteries.

By developing a viable electric motor and controller system for this proof-of-concept aircraft, we will open a door to future flight that we have only been able to dream of. Self-launching electric powered gliders already exist. The potential of electric power goes beyond that single use and relates directly to sport flying, aerobatics and high altitude flight in purpose-built airframes. It is essential that our proof-of-concept vehicle is a conventional aircraft that the majority of aviation enthusiasts can relate to.

—John Monnett, Founder and President, Sonex Aircraft, LLC

(A hat-tip to Dick!)



AeroVironment knows very well about Altairnano NANOSAFE capabilities for this kind of application and I believe Lockheed contracted w ElectroEnergy (who partnered w Altairnano for the nanosafe technology).


Sounds like a natural for zinc-air. The maintenance hassle of sucking out the zinc oxide afterwards is relatively minor compared to the normal maintenance on an aircraft. Now is there a way to structurally load a zinc-air cell?


For flight, energy density is everything. Zinc air is too heavy, but lithium-air is definitely viable for zero-emissions flight (similar energy density to fossil fuel).

Bob Tasa

Can solar or wind generators be added to return some
charge to the system without large amount of drag?
Sounds fantastic!
A great buisness idea would be to rent the packs and
not own them.
Make the battery packs easily dropped and replaced.

Then there could be a new buisness in pack rentals to
go cross country.
Touch down in airport X and refuel with a new charged rental pack.
Seems possible..

joe padula

Solar yes, wind no except for the prop as a regen source on landing rather than spoilers.

P Schager

Zinc-air should be a good choice. ReVolt is developing rechargeable ones. They are promising much more energy density than lithium-ions. Thus they should be a good upgrade for this plane.

One downside to be aware of with metal-air batteries is that the weight will increase as they discharge. This is the opposite of what pilots are used to, with the fuel weight declining to zero as you exhaust your energy supply. That will impact the average weight for the flight. Nonetheless the high efficiency and many advantages of electric drive ensure that such batteries will find application in aircraft.

Another possible battery type to look forward to is lithium-sulfur. Then of course there is hydrogen-air....


The nice thing about using multiple battery cells is that they could be potentially placed in different parts of the aircraft to put weight wherever you want it. They don't all have to be in the same place. (I assume that you are trying to have the center of gravity over the wings)


They talk like they are re-inventing the wheel.

Everything they say has been done by the Radio Controlled model airplane industry. Just on a smaller scale.

In fact, RC has done it even better with external rotors.

More power to them, but unless somebody invents a power storage system capable of twenty times the best we have currently, it will never fly.

Roger Pham

Electric propulsion with retractable propeller is great for a glider, since it affords very streamline application without much need for cooling, quietness and no vibration.
Use power only to get to thermal-hunting altitude, then soar with the eagles or albatross with the motor shut down and propeller retracted, and only use power sparingly to find another thermal (rising air current) or to limb home. A large glider wing may sports flexible and lightweight solar electric panels to recharge the battery while you are gliding, giving you a hope of cross-country capability, if weather condition permits. Aeroenvironment Inc. has done great research in this respect. Solar powered drones have been able to cruise at very high altitudes for days at a time, using only solar energy.

However, an inefficient Piper Cub design like this with flight duration of only 10 minutes is not the best demonstration of the technology, except for quick research purpose for the propulsion unit.

Roger Pham

Oh, sorry, it's not a Piper cub, it's rather a Sonex aircraft, hopefully a Xenos with longer and more efficient wings. But, still, a true glider with retractable propeller would be best.

Reality Czech

A sailplane in flight could easily windmill the propeller to regeneratively charge a battery.  If there is enough lift in a thermal or other rising air current, this could be done very quickly.

Roger Pham

Reality Czech,
If you want to glide efficiently, then you would want to fold the propeller, or feather it (keep the propeller still by advancing the pitch until the blades are parallel to the flight path). A spinning propeller produces a lot of drag against the flight path that will make the airplane sink a lot faster. Sure, you will be able to recharge the battery but you will lose altitude, and considering the mechanical loss in friction of air against the propeller and internal friction of the motor, and electrical resistance in the motor and the battery, there will be a significant net loss of energy as much as 50%.

It would be far more efficient to gain altitude as much as possible in a strong thermal, then use that altitude as a source of stored energy in order to glide to another thermal. Storing energy in the form of altitude gained is ~100% efficient. Besure to carry Oxygen bottle in case the thermal carries you above 12,000 feet.


well the motor is not to impressive (only ~71hp), and 25-45 min flight time at 130mph does not mean much for getting to places, but its a interesting project, perhaps a ancestor for something greater.

Its good to hear of metal-air fuel cells, zinc is the easiest (and cheapest) to work with but aluminum, magnesiums and lithium have better energy densities (but are technically harder and more dangerous to operate)


Roger - "Electric propulsion with retractable propeller is great for a glider, since it affords very streamline application without much need for cooling, quietness and no vibration."

Indeed and you can buy one right now:

"The electric self-launch version of the Silent Club sailplane is the world's first series production electric-powered aircraft. It is the most environmentally friendly solution available, satisfying the mutually exclusive objectives of minimum noise, 42 dB max, and an exceptional climb rate. The DC motor and the folding two-blade large-diameter propeller provide maximum efficiency. In less than 5 minutes of quiet climbing, the electric Silent Club can reach an altitude of 600 - 700 meters (2000 - 2300 ft) without the need for headphones to use the radio. Engine retraction is rapid, allowing soaring flight to begin almost immediately. "

Roger Arnold

Reality Czech's suggestion of recharging the batteries in an updraft is not off the wall. It actually makes sense.

What you'd have is a 3-mode system: (1) powered flight for climbing or cruising in non-rising air; (2) folded for minimum drag when coasting between thermals or maintaining altitude in a weak thermal; and (3) windmilling to recharge batteries in a strong thermal or ridge wave.

All that circling would make for slow cross-country travelling, but it would beat straight thermal hopping.

Roger Pham

Thanks, Ender, for the web reference.

I see your point, Roger Arnold, if you don't have enough Oxygen on board to climb any higher, and the thermal is real strong, then you theorically can recharge your battery by windmilling the prop...But, you must have pitch control system in the propeller in order to maximize windmilling efficiency. Your electric control system must also be engineered for regenative braking, so to speak.

So, if you like adventure and can carry enough Oxygen on board, climb as high as the air will take you! That's the thrill of flight! The higher you can get to, the faster you can fly, and the calmer the air will get.

sae gozashti

It seems like the rational direction is a HYBRID engine with efficient/fast regen capability(possibly an ultra capacitor storage) in addition to high efficiency solar co generation. The e-plane is a great research vehicle for future development, but Airbus and Boeing should be working on at least some regen(imagine the energy capture of massive thrust turbines slowing a fully loaded FedEx jet landing) + replacing APU with a hybrid(HC/electric+solar). Thanks for the link Ender, I was looking for a way of doing away with the tow. And Roger Pham maybe the batteries/solar could provide the oxygen?

Gordon Foat

There are so many things you could do with the correct team of people and backing. I do like the idea of producing more oxygen, and I like the idea of using solar /wind power to do that to go even higher! Nice Green MotorSport.

Iain Adam

Some very interesting comments. I have flown sailplanes for many years, yes oxygen will be required above 1000 feet. AS for speed, well all gliders have a do not exceed speed (DNE) and most heve speed limiting brakes/spoliers. I was looking at Pipistrel's
all electric powered 2 seater self launching motor glider - The Taurus with a 50 HP Rotax 503 engine with large pylon mounted retractable prop with twin landing gear. Would love to buy one - need $$$$$$$$$$$$$$

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