Total Electrical
(continued) The ratio of weight to energy content -- the so-called energy density or specific energy -- of electric propulsion systems is the sticking point. The best electric motors, with their controllers, weigh between one and four pounds per horsepower. (Incidentally, the output of electric motors, like the input, is often reported in kilowatts rather than horsepower. One kilowatt is 1.34 horsepower.) But that's just the motor; there is still the question of energy storage -- what we currently call the fuel.
Gasoline and batteries do the same thing: they hold energy in a retrievable form. Electric motors have the great advantage over internal combustion engines that they apply electromotive force -- magnetism -- directly to a propeller shaft, whereas the energy of gasoline is extracted in a roundabout way by combustion, heating of a gas, and eventual conversion of the molecular motion of heating into linear motion of pistons and, ultimately, rotation of a crankshaft. The result of all the detours taken by energy on its way from the fuel tank to the propeller, to say nothing of the fact that very high temperatures are involved, is the loss, in the form of waste heat, of three-quarters of it. Electric motors, on the other hand, actually turn 90 percent of the energy that drives them into useful work.
Battery technology is the object of intense research for many commercial reasons having to do not with aviation but with cell phones, PDAs and laptops. The amount of energy that practically available batteries can store, and the rate at which it can be retrieved from them, continue to improve. The best lithium-ion batteries have 10 times the energy density of lead-acid batteries, but gasoline has about 60 times the energy capacity, per pound, of current lithium batteries. Supposing that the electric motor uses stored energy four times as efficiently as a small gasoline engine, that ratio drops to 15. In homely terms, these numbers mean that if you usually take off with 60 gallons, or 360 pounds, of fuel aboard, an electric airplane with the same weight in batteries could do the work of just four gallons of gasoline -- the first 20 minutes of a flight. The low power and short duration of present electric powerplants limit them to use in highly efficient airplanes, which means, among other things, rather slow airplanes with abnormally large wingspans. Boeing's fuel-cell prototype, for example, is a modified Diamond Dimona motorglider with a span of nearly 54 feet.
With airplanes, as with automobiles, there is disagreement about the form that an electric powerplant would take. Battery storage is not the only option. The solar route is practical, given the present limitations of solar cells, only for extremely large and light airplanes flying above cloud cover and economic concerns. Various hybrid arrangements are possible, as with automobiles. Some sort of internal combustion engine, running steadily at its best efficiency, can drive a generator which in turn drives electric motors. This seems circuitous, but it is a reasonable approach when multiple propellers, but not multiple engines, are needed. A one-third scale version of AeroVironment's Global Observer, an observation plane intended to remain airborne for a week at a time, flew in 2005 powered by fuel cells and eight electric motors. Fuel cells are batteries that consume a chemical fuel in order to renew their charge; we have been hearing about them for a long time, but their breakthrough into everyday practicality has not yet taken place. The electric motors of the full-scale Global Observer will get their power from a generator driven by a hydrogen-burning internal-combustion engine -- a propulsion system reminiscent of those of many ships.
These are some of the technical aspects of the subject. But the participants in the Electric Aircraft Symposium ventured into other areas, including the incompatibility of present light sport aircraft rules with optimal use of electric power (EAA has petitioned FAA for some rule changes to encourage the use of electric powerplants) and the potential of small autonomous airplanes for urban commuting -- autonomous both for collision avoidance and for relieving their users, who are imagined to be as numerous as flies, of the need to be skilled instrument pilots. Apparently quite a bit of study has been expended on this chimera, which, IMHO, ranks, on the attainability scale, right up there with the colonization of outer space and peace in the Middle East. The connection to electric propulsion, however, though tenuous, is apparent: It would be appropriate for short range and intermittent use between charging stations. I suspect that virtual reality would make aerial transport of people on this scale unnecessary, however, even if it were possible.
As peripheral to aviation as electric power seems today, it's not improbable that in 10 or 20 years it could find widespread use in recreational aircraft, just as it has for model airplanes. Its near-silence, mechanical simplicity, reliability, potentially low operating costs, small carbon footprint and (relatively) nonpolluting nature are all attractive. No doubt practical experience will uncover some disadvantages as well. High-performance electric power systems have their own failure modes, some involving overheating, fire and electrocution, and we have little experience of their crashworthiness and toxicity.
Not all technologies enjoy limitless progress. It may very well be that batteries will never appoach the power density of chemical fuels; but it may also be that for certain applications the electric motor will become the powerplant of choice. After all, if electric motors had been the original powerplants for aircraft and someone had later come along and suggested using internal combustion engines instead, some people might have called them unsuitable for flight vehicles because of their sheer complexity and the great variety of their potential modes of failure. New ideas take some getting used to.
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