Experimenter is a magazine created by EAA for people who build airplanes. We will report on amateur-built aircraft as well as ultralights and other light aircraft.
Issue link: http://experimenter.epubxp.com/i/339662
22 Vol.3 No.7 / July 2014 FLYING AN E-GULL motor he fl ew at Oshkosh in 2013 didn't generate enough power for his needs. Mark sat down and laid out the motor he would want. It would be around 10 inches in diameter to fit into a variety of cowlings, turn low enough to not need a gear box, have about 20-kilowatt output, and be driven by at least 100 volts. Like many other aircraft electric motors, Mark's would be a brushless outrunner configuration, where the 36 inner coils remain stationary while the casing rotates. This design cools well and provides an easy mount for the ground-adjustable propeller. He searched for a motor like the one he envisioned and found none. Mark describes the coincidence that happened next. "About a week later, I got an e-mail from Thomas Sankel, who is a German physicist and a freelance motor designer," he said. Sankel sent Mark a 3-D model of the motor he had been designing for aviation use. It was very similar to Mark's concept: a brushless outrunner and 15-kilowatt output. (Mark had wanted 20.) Mark replied to the e-mail and told Sankel exactly what he had in mind. Sankel had access to MotorSolve, a program for detailed motor design. Motor- Solve models and analyzes several configuration variables, including the key one: the design of the windings. There are many choices in the winding specification: fill factor, con- vective area, winding area, angular spread, winding factor, and gauge parameters. The program computes motor torque, efficiency, and loss. Using this MotorSolve program, Sankel worked until he honed in on the desired performance. He e-mailed Mark an updated 3-D model of the motor. Sankel's price for doing the detailed design on Mark's concept was attractive—flight les- sons. In addition, Sankel required that he himself wind the coils. Sankel came to northern California for three months to help with the motor; Mark produced the motor, hiring out some of the components. The motor worked well on the bench. Sankel stayed on in northern California to evaluate the motor's flight characteristics and make some modifica- tions. Mark flew the airplane again to the Arlington Fly-In. This electric airplane had performance similar to the Hirth-powered Soaring Gull: 65 mph versus 63 mph top speed; 22 mph versus 25 mph stall speed; 115-foot take- off distance; 70-foot landing distance; and 245- versus 254-pound empty weight. In the past few years, Mark has experimented with 10 electric motors and modifications. Since he had achieved acceptable performance, he turned his attention to affordability. Mark had imagined that a diverse market for his Rad- Cam engine would keep the cost down. This time he ap- proached JoeBen Bevirt. Founder of Joby Energy Inc., Bevirt has a passion for renewable energy. Five years ago, Bevirt started developing airborne wind turbines that required a new type of motor and controller for airborne applications. Mark recognized an opportunity to collaborate. He wanted inexpensive motors, controllers, and batteries; Bevirt wanted to get into electric airplane propulsion. Mark made Bevirt an offer he couldn't refuse: He would give Joby the design for the motor. Joby could sell the mo- tors to anyone as long as Mark could buy them also. Bevirt agreed. Joby Energy Inc. tested the motor and refined it. It weighs 26 pounds and has a 20-kilowatt output, producing around 27 hp. Mark flew the plane full throttle to 9,000 feet without any cooling problems. In June 2013, Zero Motorcycles approached Mark. Zero produces electric motorcycles, with their own 16-pound motor, 170-pound battery, and Sevcon controller. The system was reliable, integrated, and rugged. The only non-Zero component was a 2.58-to-1 Rotax gearbox. It was this con- figuration that Mark chose to fly to EAA AirVenture Oshkosh 2013, although he intends to fly the Joby this year and offer both motors for sale. At AirVenture 2013, Mark flew often in the "beehive"— the morning and evening parade of airplanes in the Ultra- light area. The crowd was mightily impressed with how quiet the airplane was and also with the e-Gull's rate of climb, which was most certainly above the 500-fpm perfor- mance in the company literature. Two questions are commonplace for those who bring electric airplanes to AirVenture. The first is the flight dura- tion. The e-Gull can fly an hour with a 10-minute reserve. Squeezing out more reserve time could be difficult; the bat- teries are heavy and expensive. Which leads us to the other common electric propulsion question: How much does it cost to fly electric? Recharg- ing the batteries costs $1. Mark says the batteries will cost $10,000 to replace. Since batteries are good for 3,000 hours, the battery replacement cost is a little more than $3 per hour. But Mark points out that the used batteries can be repurposed for golf carts or as solar-panel backup batteries. What is the future for Earthstar Aircraft? Mark currently sells five types of ultralights as kits or ready-to-fly aircraft. But his interest is clearly in advancing the electric e-Gull. Just as he envisioned the Rad-Cam engine to have appli- cations in motorcycles, personal watercraft, RVs, electric generators, and snowmobiles, he knows that adapting a mass-market motor will make electric flight more affordable. He is forging a path to low-cost electric airplanes. He envi- sions solar panels on hangars to further reduce the cost and increase a flier's freedom. We may not be as free as Mark was in his childhood dream, but he's helping more of us get into the air. For more information about Mark's projects, visit www.ThunderGull.com . Lynne Wainfan is a private pilot. An aerospace engineer and former manager at Boeing Space, she now teaches at California State University, Long Beach. E A A E X P _ J u l y 1 4 . i n d d 2 2 EAAEXP_July14.indd 22 7 / 1 / 1 4 9 : 5 5 A M 7/1/14 9:55 AM