|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!)