The Search for a Fuel-Efficient Aircraft

And the winner is...

Taurus G4

Taurus G4

** Despite the huge advantage of four seats,
Pipistrel's purpose-built Taurus G4 barely
edged Stuttgart U's two-seat e-Genius.**

(January 2012) The news cycle is like a carousel whose riders are ever changing. For a brief moment in September, the Green Flight Challenge swept past: NASA handed a prize of $1.35 million to an airplane that had achieved an efficiency of 400 passenger-miles per gallon. Huh? said the world — and then along came Jen and Angie.

The contest that had produced this surprising but ephemeral result was set in motion by the CAFE (Comparative Aircraft Flight Efficiency) Foundation, which for the past three decades has encouraged innovation and refinement in general aviation with a series of programs and competitions. It started with the CAFE races, efficiency contests that took place in northern California from 1981 to 1990. Based on an evolving formula combining speed, fuel consumption and payload, they had quite an impact: Several airplanes, including Burt Rutan's Catbird, were specially designed as CAFE racers. But toward the end it seemed as though the universe of hyperefficient CAFE winners had shrunk to the VariEzes of two monomaniacal modifiers, Gary Hertzler and Klaus Savier.

After the CAFE races came the Triaviathon. The all-time champion of that contest, which measured only minimum and maximum speed and rate of climb, was the Lycoming IO-360-powered RV-4 of Dave Anders, which managed to stay aloft at less than 39 knots, hit a top speed of 218 knots and climb at more than 3,300 fpm.

It is interesting that the most successful contestants in the CAFE races and the Triaviathons were not purpose-built airplanes but highly optimized instances of stock designs. If this means anything, it seems to be that persistent experimentation and fanatical attention to detail, rather than basic design, are the keys to
exceptional performance.

In recent years, the CAFE Foundation has been agitating for a paradigm shift in airplane design. To suggest a clean break with all that we take for granted, it calls the still imaginary airplanes of the future PAVs — personal aerial vehicles. They should be capable of operating out of tiny airports or no airport at all; should be quiet, nonpolluting and affordable to operate; should have simple, standardized controls; and should incorporate robotic and all-weather capabilities that reduce the skill level required of pilots to about that needed to operate a rented Chevrolet.

A tall order, certainly, but the CAFE Foundation is prepared to proceed incrementally. The first goal — the one, I suppose, that seems most nearly within reach today — is to achieve very high, and pollution-free, efficiency.

Aviation has a long tradition of spurring innovation with cash prizes. The Spirit of St. Louis, the Gossamer Albatross, the Solar Challenger and SpaceShipOne were all engendered by prizes. The purse at the old CAFE races was a paltry $2,000, but things have changed. The purse at last year’s Green Flight Challenge, co-sponsored by NASA, Google and CAFE, was $1.65 million, of which $1.35 million would go to the first-place finisher and smaller amounts to a few others.

The rules of the contest were simple. To qualify at all, an airplane had to fly a 200-statute-mile course in less than one hour using less than one gallon of fuel per occupant, and land with a 30-minute reserve. The “fuel consumption” of electrically propelled airplanes was defined in terms of the energy equivalence of gasoline and electricity: One gallon of fuel was equal to 33.69 kilowatt-hours, or 45 horsepower-hours. (It’s interesting that the costs of a gallon of auto gas and its equivalent in electricity are about the same.) There were some details; for example, entrants had to be able to fit into the 44-foot-wide CAFE hangar for weighing, but folding and removable wings were permitted, and so this was not a severe constraint. The winner would be determined by a simple formula combining speed and mileage.

Thirteen entrants registered. Though the contest, originally scheduled for July, was postponed because several teams were not ready, one after another dropped out until in the end only four remained. One of these, a gasoline-electric hybrid sponsored by Embry-Riddle Aeronautical University, participated but did not compete because it could not obtain insurance for anything remotely resembling an "air race" — this was in the aftermath of the terrible accident at Reno. That left three: a Phoenix motorglider; the e-Genius, an adaptation of a German design intended eventually to incorporate a fuel cell; and the Taurus G4, a one-off four-seater from the firm of Pipistrel in Slovenia.

Of these, the Czech-built Phoenix, with a 15-meter (about 49-foot) wingspan, was the only commercial product. With fixed landing gear and a featherable propeller, it has a 32:1 glide ratio. It was supposed to have an 80 hp electric motor, but that wasn’t ready, and so it appeared with a 100 hp Rotax 912 instead.

The e-Genius is a battery-powered version of Hydrogenius, a University of Stuttgart program for developing a hydrogen-powered airplane. A motorglider with a 17-meter (about 56-foot) wing partly made in Pipistrel tooling, it has an 80 hp electric motor mounted at the top of the vertical fin. It reported a glide ratio of 34:1, which seemed so conservative as to border on gamesmanship.

The Pipistrel entry was something quite different, consisting of portions of two two-seat Taurus sailplanes spliced to a centersection packed with half a ton of batteries and sprouting a 195 hp electric motor in a central nacelle. Freakish-looking, with a wingspan of 75 feet and a gross weight of 3,300 pounds, it is said to be the largest electric airplane yet to fly.

Two hundred passenger-miles per gallon is not an extremely high bar. A six-seat A36 Bonanza, cruising on a lean 14 gph, gets 70. Plenty of homebuilts, including my own (so I know this to be true), get between 80 and 100. And they are optimized for payload and speed, not efficiency. Even a Skyhawk, which is optimized neither for speed nor for efficiency but rather for safety and ease of operation, gets 60.

The most aerodynamically efficient airplanes are sailplanes. Their design basically consists of minimizing parasite drag with a very clean shape and small surface area, and minimizing induced drag with a very low span loading. To generalize, therefore, the formula for aerodynamic efficiency is a clean, compact airframe with a wing that combines a large span with a small area. Aerodynamic efficiency is most readily expressed as the lift-drag ratio, or L/D. Most sailplanes have values of around 30 to 40; a few claim 60.

Propulsive efficiency is a different matter. It has two components: the engine and the propeller. (In the present state of the art, only propellers, not jets, can achieve high efficiency at low speed.) Propeller efficiency, at best, is around 85 to 90 percent. The efficiencies of internal-combustion engines range from 25 to 35 percent. What this means is that, of the energy in the fuel you use, only about one-fifth propels your airplane; the rest merely warms the atmosphere in your wake. Some of the heat lost is in the exhaust; some extracts a double penalty, because it adds cooling drag as well.

Electric motors, on the other hand, are 95 percent efficient and require very little cooling.

It would seem at first glance that electricity has an insuperable advantage over liquid fuel, because the electric motor gets three times as much work out of a given amount of stored energy. The problem is weight. The weight of gasoline required to carry two people 200 miles at 100 mpg is negligible; the weight of batteries to store the energy to do so is not. The same principle applies to additional passengers. For a liquid-fueled airplane, more passengers are almost pure gain under the GFC formula. For an electric airplane, however, more payload means more batteries, which mean still more weight, and so on.

The contest results were interesting. The two-seat Phoenix with its Rotax engine achieved 94.3 passenger-miles per gallon; e-Genius, also with two seats, made 376; the G4, with four seats, edged e-Genius by a very slender margin, scoring 404 p-mpg. (Reduced to airplane miles per gallon, these values are 47, 178 and 101 respectively.) That the simple and conventional-appearing e-Genius lagged only a few percent behind the huge and specialized G4 was very impressive.

What did it all prove? Nothing we didn’t already know. Electric power yields superior efficiency, but the weight (not to mention the cost) of batteries severely limits payload. Efficiency and practicality are competing, not to say internecine, criteria. Most airplanes can’t be completed on schedule even for a million bucks.

The 200 p-mpg criterion may have been motivated by a desire for newsworthiness; it’s too high to be meaningful in global-warming or oil-depletion terms, because the amount of fuel saved by getting 100 mpg rather than 50 is much smaller than what would be gained by getting 30 rather than 15. And although the electricity used for the GFC was said to have come from nearby geothermal plants, for the foreseeable future most electricity will be generated by coal- or oil-burning plants whose thermal efficiency is — you’ll never guess — 33 percent.

Send reader mail to: edit@flyingmag.com or P.O. Box 8500, Winter Park, FL 32789.