Electric Aircraft to Play Increasing Role in the Future
Advances likely to come from homebuilders, experimenters
April 27, 2009 — The CAFE Foundation, well-known to aircraft homebuilders and experimenters for its CAFE 400 air races and aircraft efficiency comparisons, held its third annual Electric Aircraft Symposium on April 24 at the Hiller Aviation Museum in San Carlos, California.
Some 150 attendees heard from 15 presenters from the fields of aircraft design, alternative energy, battery and power system development, climate, and autonomous vehicles—among others—during the daylong seminar. Participants and presenters came from as far as Germany as well as from all over North America.
Reporter Peter Lert, who attended the symposium, provided the following overview of what was presented.
Electric power will become ever more important in the future
With increasing demand for fossil fuels for both aviation and non-aviation uses, finite and limited supplies, and ever-increasing scientifically validated concern about climate change, the “niche market” for 100LL fuel will continue to contract, while prices will continue to increase. In addition, current internal combustion engines (ICEs) for light aircraft are “gross polluters:” while there’s been an almost 200-fold decrease in the pollution levels of automobile engines over the last few decades (offset to some extent, of course, by the constantly increasing size and number of cars on the road), aircraft reciprocating engines incorporate no pollution control technology at all. Moreover, the pollutants they produce end up high in the atmosphere, where they can have much more significant effect than those near the surface.
Power systems should be considered as a whole
For equal power, an electric motor is both significantly more efficient, and often considerably lighter, than the equivalent ICE. The very best ICEs are no more than 30% efficient (comparing energy input with power output at the prop shaft), and most light aircraft engines aren’t even that good; typical electric motors are 90% efficient or better, with many approaching 95%. However, when one factors in the weight of stored energy—whether in the form of hydrocarbon fuels or in the form of batteries or fuel cells and their associated equipment—the equation changes dramatically. The “energy density” of the best batteries currently available — i.e., how much energy they store per unit of weight—is about 200 times less than that of hydrocarbon fuels like avgas or Jet-A.
“Not yet, but soon”
Future batteries under development will gradually improve this ratio. It’s expected that by 2015, energy density will improve by about 60%, developments in nanotechnology could improve that up to fourfold within not too many years after that. At this point, the equation begins to look a little less lopsided: the energy density of gasoline would only be about 35 times greater than that of batteries and since an electric motor is some three times more efficient than an ICE, the ratio of overall power system efficiency begins to approach 10 to 1.
“It won’t be easy”
For a battery-powered electric airplane to begin to compete against current ICE technology, energy storage will have to increase at least 20 times, with total powerplant and energy storage costs remaining comparable. Fuel cells may ultimately provide an attractive option, but they’re likely to require onboard storage of either gaseous hydrogen under very high pressure (necessitating very sophisticated tanks, as well as a not-yet-developed distribution and supply system), or liquid hydrogen, which must be kept at a temperature of -423 deg. F and which constantly boils off when it’s not being used—and has even thornier supply, storage, and distribution issues.
“There ain’t no such thing as a free lunch.”
Electric motors—even with the sophisticated electronic controllers they require—often cost significantly less than ICEs of similar power. Electricity, of course, is often cheaper per unit of energy than avgas. However, at present (and quite probably well into the future), batteries are expensive: one experimental aircraft currently flying uses less than a dollar’s worth of electricity per hour (its normal flying range)—but its battery, which if conservatively operated and carefully maintained might last for 1,000 discharge/recharge cycles, costs $8,500…so figure $8.50/hour, about the cost of a couple of gallons of gas, for battery replacement reserve, and that’s for a system of less than 20 hp.
For the present: small, slow, short-range ... and skinny
Given the current state of technology, those aircraft that will fly successfully and practically on electric power — and, despite the bleak picture I’ve painted in the last few paragraphs, there will be some in the next few years — will tend to be small (no more than two-place), relatively slow (cruising speeds around 100 knots or less), and relatively short range — ultimately, perhaps, around 300 miles in still air before recharging will be necessary, but for the moment probably less. They’ll also validate the old aerodynamicist’s rule of thumb that “for efficiency, wingspan is king.” Or, to be more exact, aspect ratio: the most successful designs, including a couple already flying and more on the drawing board for commercial production, tend to look quite sailplane-like, with long, narrow wings to minimize induced drag and thus allow cruise flight with the least required power. In fact, a couple of them are self-launching sailplanes: Greg Cole demonstrated a very slick integrated electric power package for the SparrowHawk sailplane he builds at Windward Performance Ltd. In Oregon, and Lange Flugzeugbau in Germany offers the Antares self-launcher, a single-place ship with a 66-foot wingspan and a 60:1 glide ratio…at a price, unfortunately, of over $250,000. And, equally unfortunately, many of the airframe optimizations that would contribute to an efficient electric airplane would be equally applicable to one powered by a small ICE.
Advances will come - and they’ll likely come from us
Cast your minds back to 1903. Internal combustion engines were just beginning to become widely available for automotive use; the technology was becoming available and accessible; and a couple of bicycle mechanics were available to put it to good use. The major drivers for the development of better and cheaper batteries, better and cheaper electric motors, and better and cheaper ways to operate and control them will come from the automotive industry, driven by rising fuel prices, increasing concern about environmental issues, and increasing consumer demand for hybrid, “plug in hybrid,” and all-electric cars. The “bicycle mechanics” will be waiting; I’d hazard a guess, even now, that if some unfortunate wraps a hybrid or electric car around a tree near an experimenter’s hangar and goes away for help, there might not be much left of it by the time the tow truck arrives! All joking aside, electric powered aircraft are already here, and more will follow over time; and it’s very likely that their developers will come from the ranks of aircraft homebuilders and experimenters.
Efficiency, electric or otherwise: Aviation’s Green Prize
The CAFE Foundation also utilized the symposium’s forum to announce the latest details of their planned AGP (Aviation Green Prize) competition, successor to its original CAFE 250 and 400 aircraft efficiency races. At $1.63 million, the prize is very worthwhile, but it won’t be an easy one to win. Criteria will include the following:
- 200 seat/miles per gallon, i.e. an aircraft with a minimum of two seats that gets 100 mpg or better.
- 100 mph cruise speed at 4,000 feet.
- 200 (statute) mile range at 4,000 feet.
- 52-mph (45 knot) stall speed.
- Takeoff over a 50-foot obstacle of 2000 feet or less at sea level.
- “Realistic” payload and passenger accommodations with specified minimum seating dimensions
- Capability of being FAA certified (experimental or LSA)
- “Acceptable” handling qualities not requiring unusual pilot skills
- A full-aircraft emergency parachute system
It may take a few years for this prize to be claimed – first competition is slated for mid July 2011 - and it may not be claimed by an electric airplane…but it shows that homebuilders and experimenters continue at the cutting edge of civil aircraft research, development, and improvement.