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An Introduction to Electric Airplanes

By David Ullman, EAA 0446096, ullman@robustdecisions.com

 

This article is being written before EAA AirVenture Oshkosh 2010. There is promise of many developments during AirVenture, and this article along with its companion “The Electric-Powered Aircraft: Technical Challenges” (May 2010 Experimenter) will be followed up after reviewing the Oshkosh developments.

In the “Technical Challenges” article, I made some predictions about electric planes. In the next five years, before 2015, I predict:

  • An electrically powered airplane will stay aloft for two hours carrying two people.
  • An electrically powered airplane will be cost competitive for special-light-sport aircraft and trainers.
  • Electric airplanes will be powered by batteries with energy densities at least 50 percent better than those available today.
  • An electric airplane will land at an airport near you! 

The state of the art is advancing so rapidly and that there are sure to be advancements yet this year that may make these predictions too conservative. Readmore

The Definition of an Electric AirplaneThere is no firm definition of an electric airplane. So here is mine: An electric airplane is a human-carrying, heavier-than-air vehicle with the motive force supplied by an electric motor. The motor might get the electrical energy from batteries, solar collectors, fuel cells, or a combination of these. (Additionally, one old-timer mechanic suggested a long extension cord.) An electric airplane might be a motor glider, which gains lift from thermals and updrafts and only uses propulsion periodically, or it might use the motor for continuous power. Further, it might also use electric power for lift as does an electric helicopter – not that I know of anyone who has tried this, but I’m sure someone is out there considering it. A little on the gray side of this definition are airships. To me these aren’t airplanes. Yet an electric airship flew in 1884, a feat discussed in the history section below. Finally, electric airplanes may also be capable of energy regeneration, regaining energy for storage by descending, decelerating, or circling in thermals.

History
Even though the definition above states “heavier than air,” I’ll start the history with an airship, the La France, just because it was developed a surprisingly long time ago. It was designed by ArthurConstantin Krebs and Charles Renard, with the bulk of the technology by Krebs, a fascinating man with many inventions to his credit. The La France was 51 meters (167 feet) long, and during 1884 and 1885, it made seven flights. Five of these were round trips – the first for any kind of aircraft. Its longest flight was 8 km (5 miles) which it covered in 23 minutes. The electric motor was 9 hp, and the batteries weighed 440 kilograms (880 pounds), giving 97 pounds/hp. The figure showing a side view is a tracing from the 1911 book The Conquest of the Air (by Alphonse Berget [in English]) which is available to read online and has more details on the flights.

Drawing

Not much happened in electric airplane technology until the late 1970s when Larry Mauro built a solar-powered hang glider called the Solar Riser. The solar energy stored in its battery could power it for 3 to 5 minutes, enough to launch it to gliding altitude. In 1981, the Solar Challenger, built by Paul MacCready’s AeroVironment Company, successfully completed a 163-mile (262 km) man-carrying and record-breaking flight from France to England. The Solar Challenger had an empty weight of 200 pounds (90 kilograms), and the 16,128 cells powered two 3-hp motors using solely photovoltaic cells that covered its wing and stabilizer. It had no batteries on board and would slow down if passing under a cloud. MacCready also developed unmanned electric aircraft such as the Pathfinder, Pathfinder-Plus, and Helios for NASA during this period. These aircraft (there were five of them) were powered by six to fourteen 2-hp motors mounted in pods on a single flying wing. They were designed for high altitude loitering with the solar cells collecting energy during the day to run the motors and recharge the lithium batteries for uninterrupted flight during the night. This series culminated with the Helios which had a wingspan of 247 feet (75.3 meters) and flew to 95,000 feet (29,000 meters). The Pathfinder-Plus is now on display at the Smithsonian’s Steven F. Udvar-Hazy Center at Dulles Airport outside Washington, D.C.

Pathfinder

In 1990, Eric Raymond flew the Sunseeker across the United States much as the Wright-VinFiz flew across in 1911 – in short hops. The Sunseeker was a purpose-built solar motor glider that could take off and climb under power and then soar while recharging. It could fly up to 150 miles on solar energy and batteries, and Raymond also logged flights of 250 miles with additional thermal hopping. It was powered by an 8-hp brushless direct-current motor. In 2002, Sunseeker II was launched with many improvements. It’s been on a tour of Europe, and in 2009it became the first solar-powered aircraft to cross the Alps, 99 years after the Peruvian pilot Georges Chavez did so in a Bleriot. Sunseeker II has a gross weight of 230 kg (506 lbs) and a span of 56 feet (17 meters).

At the 2008 AirVenture, two hits were based on John Monnett airplanes; Randal Fishman’s ElectraFlyer, an electric Moni fully capable of flight, and the E-Flight Initiative’s Waiex static display.

Fishman’s ElectraFlyer-C was designed as a prototype and based on a converted Moni motor glider. It’s powered by an 18-hp (13.5-kilowatt) electric motor, a pulse width modulation controller, and a 5.6-kilowatt-hour lithium-ion battery pack. It flies for 1 to 1.5 hours at a cruise speed of 70 mph (61 knots) with a maximum speed of 90 mph (78 knots). Its empty weight (with batteries) is 380 pounds (172 kilograms), and maximum takeoff weight is 625 pounds (283 kilograms). The second article in this series will explore the meaning of, and inter-relationship of these values. Fishman has founded the Electric Aircraft Corporation to develop and sell power systems for electric aircraft.

Skyspark

SkySpark
In June 2009, the SkySpark made its first public appearance. The SkySpark is a Turin Polytechnic University project, aided by many technical sponsors. The SkySpark team’s goal is to design and build a high-performance, single-seat, electrically powered light aircraft able to fly at 300 kilometers/hour (186 mph) for over 2 hours. In the June flights, the aircraft was powered solely by batteries, but it’s designed as a fuel cell powered airplane with batteries only for auxiliary power. During the June test flights, it reached a top speed of 250 kilometers/hour, a record (so they claim, and to the best of my knowledge, true) for a 100 percent electrically powered aircraft. It’s powered by a 75-kilowatt electric motor using brushless technology and lithium polymer batteries.

The team is now focusing on the fuel cell system, designed to produce 60 kilowatts of power from 22 liters (5.8 gallons) of stored liquid hydrogen. As shown in the drawing, hydrogen is stored in a tank (yellow) behind the pilot. The fuel cells are in the copilot position, and batteries are under the cowling, alongside the motor (green cylinder).

Skyspark

In June 2009,Yuneec E430, a 54-hp two-seat Chinese electric airplane made its first EAA AirVenture appearance. What makes it unique (pun intended) is that it’s the first electric aircraft designed to be commercially produced. The E430 is a special light-sport aircraft (S-LSA) that seats two and is made with lightweight composites. The 230-volt battery system charges in three hours, and they claim it will be able to fly for about 2.5 hours on a single charge. Yuneec says it will sell for $89,000.

Yuneec
Photo by Brett Brock
Tom Poberezny welcomes Clive Coote and Yu Tian of Yuneec to EAA AirVenture 2009. Yuneec International's two-place electric LSA is slated to enter production in late 2010 at a price of $89,000.
 
Finally, in July 2010 the Solar Impulse achieved a 26-hour flight solely on power from the sun. This solar-powered airplane is meant to fly around with world, using only the sun for fuel. Its duration during the record-setting test flight was only limited by the pilot, not the plane and its power.

Aviation Green Prize
The Aviation Green Prize (officially called the 2011 CAFÉ Aviation Green Prize Challenge) is a $1.65-million prize cosponsored by the CAFÉ Foundation and NASA. The goal is to develop an aircraft before July 2011 that can cover 200 statute miles at greater than 100 mph with better than 100 miles per equivalent gallon of gasoline. Due to NASA’s involvement, only U.S. citizens need apply.

The goal of the 2011 Challenge is to encourage the development of new technologies for small aircraft that are potentially applicable to unmanned aerial vehicles (UAVs), air-taxi operations, homeland security surveillance, and personal transportation. New technologies that may be competitive include, but aren’t exclusively limited to, electric airplanes.

The score for each aircraft is a function of miles per hour (MPG), number of passengers carried, and MPGe - the equivalent miles per gallon relative to 87 octane regular unleaded auto gasoline. The equation to calculate the score is:

Equation

“Passenger MPGe” = number of passengers (pilots, passengers, or seats with equivalent ballast) multiplied by the calculated vehicle miles per gallon equivalent for the fuel and/or electricity referenced to the average energy content of 1 gallon of gasoline. For example, if there are two passengers and the airplane gets 100 MPGe, then Passenger MPGe = 200. Further, if the plane averages 100 mph over the course, then the total score is 50.

To see what this means for an electric airplane or some other technology, consider Table 1 which contains data from the CAFÉ website. Say you flew an electric airplane that had 66 kilowatt-hours of energy stored in batteries. In terms of 87 octane gasoline, this is about 2 gallons (66/33.703), and the MPGe is 100 as in the previous example. To get a feel for 66 kilowatt-hours of energy, your car battery can store about 1 kilowatt-hour, but as I’ll discuss in the second article, there are battery packs that can provide this amount of energy for a more reasonable size and weight.

 

Fuel type

British thermal units per gallon

Density pounds/gallon

Energy ratio to 87 octane gasoline

87, 89, 91 unleaded
auto gasoline

115,000

6.09

1.0

Bio-diesel B20

127,250

pending

1.1065

Avgas 100 LL

120,000

6.02

1.0435

Petro-diesel

129,500

7.09

1.1261

Hydrogen, liquid

34,644
(51,532/lb.)

0.567

0.3013

Electricity: mile/kilowatt-hours

N/A

N/A

33.703 kilowatt-hours per gallon

Energy Equivalents for Aviation Green Prize (partial list)

For full details on the rules, refer to the CAFÉ website. In summary they are:

  • Passengers: upright seats with adequate volume for a 6-foot-tall, 200-pound adult
  • Efficiency: ≥ 200 passenger miles per gallon energy equivalency
  • Speed: ≥ 100 mph average over racecourse
  • Range: 200 miles with FAR mandated reserves (30 minutes)
  • Minimum Speed: ≤ 52 mph in level flight without stall
  • Takeoff Distance: ≤ 2000 feet from brake release to clear a 50-foot obstacle on takeoff
  • Community Noise: ≤ 78 A-weighted decibels at full power takeoff, measured 250 feet perpendicular to point of brake release
  • Wingspan: must fit inside 44-foot-wide hangar for weighing
  • Vehicle Weights: ≤ 6,500 pounds on main gear and ≤ 2000 pounds on nosewheel or tailwheel
  • Payload Carried: 200 pounds/seat. Dual pilots if two or more seats. 200 pounds/seat sandbag ballast in all seats not occupied by pilot/copilot
  • For electric-powered aircraft, a power meter, provided at the competition, will accurately determine energy used during the race.

Early bird registration fee was $4,000 before November 30, 2009, and $6,000 after that time. A maximum of 12 competitors will be accepted, so hurry up and get your registration in.

Summary
Things are happening fast. I’m sure there will be more interesting hardware yet this year as this is a fledgling and rapidly growing industry. The technological challenges that will be faced are discussed in a subsequent article.
 
For now, I will stick with my predictions made at the beginning of the article, and I expect you’ll be seeing and flying electric airplanes soon.

Sources of More Information
Solar Impulse: http://www.SolarImpulse.com
Aviation Green Priz:e http://cafe-green-prize.blogspot.com/
Electric Airplane Symposium: http://cafefoundation.org/
SkySpark: http://www.SkySpark.eu/web/eng/index.php
Electric Aircraft Corporation: http://www.ElectraFlyer.com
E-Flight Initiative: http://www.SonexAircraft.com/research/E-Flight

David Ullman is a retired mechanical engineering professor and author of books on mechanical design and decision making. He’s building a Velocity SE-FG and expects to be flying in 2011.

 
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