In preparation for Fleet Week, the U.S. Navy’s Blue Angels F/A-18s roar over San Francisco. Office workers unaccustomed to military aviation dive for cover. Professors halt their lectures. Everyone wonders how those little planes can make so much noise … much more than much bigger commercial aircraft.
The answer, of course, is in the muffling. Commercial jets, saddled with noise abatement regulations, have their engines silenced. To an extent. A modern jet engine still registers up to 160 decibels up close—louder than the loudest heavy metal band.
But for super quiet planes, where you can have a normal conversation standing next to the engine, we have to go electric.
An electric plane has other benefits, too. There’s little vibration. The motor is so small, you can pick it up and carry it, according to one company that is trying to build and fly a Cessna-size electric airplane. And so efficient is electric power (95 percent) that an hour of flight costs only $3—compared to $40 with a combustion engine.
Bye-Bye Fossil Fuels?
Do you think it will cost more to get an electric plane? Nope. Bye Aerospace’s Sun Flyer is projected to cost less than comparable Cessna. With fewer moving parts, it should also be less expensive to maintain.
Electric? Go Ask Alice
Looking more like a sleek, corporate flyer but minus the jets is Eviation’s (“E” for electric) Alice Commuter. The Israeli startup showed a prototype of the aircraft at the Paris Air Show. Larger than the Sun Racer, Alice has a 44-foot wingspan and seats nine. Three propellers (two wingtip and one in the tail) provide the thrust, each with locally placed electric motors and a 980 KWh lithium-ion battery. That’s about 10 times the juice of a Tesla. It’s enough to give Alice a range of 600 miles at speeds of over 250 mph.
The batteries, weighing over 10,000 pounds, are about two-thirds of the maximum takeoff weight of the electric plane. Looking at Alice’s sleek shape, it’s hard to say where the batteries would be hidden. Details are not forthcoming.
Energy Density
So, even after years of Priuses and Teslas roaming our roads, why don’t we have more electric planes in the skies?
A Tesla Model S has an 85 kWh battery and weighs 1,200 lbs (540 kg) —roughly one-quarter of the total weight of the car—attached to its underside. A ground vehicle’s mass is supported by the ground, extracting a penalty only on acceleration. By contrast, an electric airplane’s battery must be not only accelerated but also lifted thousands of feet off the ground, requiring a huge change in potential energy.
To lift large mass off the ground and move it at speeds over 10 times the speed of cars, aviation has relied on petroleum-based fuel. Overlooking its other faults, this liquid fuel is volatile and dangerous and leaves a trail of pollutants. It must be exploded in heavy engines that have to be carried aloft, and which generate a sound that has to be muffled and vibrations that must be absorbed. All for one single virtue: energy density. Gasoline packs a whopping 44 megajoules, or about 12,000 watt-hours per kilogram. When energy has to be stored in batteries, you are going to get a lot less energy.
Bye has been developing special lithium-ion batteries that produce 260 watt-hours per kilogram—2.5 times the density of batteries that were available when they first started the project. That’s still about a 50th of the power density of gasoline. The 330kg battery pack has an 80 kW capacity.
While still not a match for the power density of aviation fuel, it is enough for slower speeds. All electric airplane manufacturers—current or those waiting in the wings—anxiously await improvements in battery technology.
Weight: How Low Can You Go?
The wait for the energy density of batteries to reach even half that of petroleum-based fuel is still decades away. In the meantime, engineers have no choice but to work on lowering mass. There is less room to work in lightweighting, as the aviation industry is already quite weight conscious and margins of safety are already lower than they are for land- and water-based transportation vehicles. Electric planes risk being and appearing flimsy compared to their all-metal predecessors, which, due to energy-dense fuel, have power to spare.
Nevertheless, an electric plane can take advantage of several areas of weight savings. Since an electric plane can be created from a clean slate, the latest in lightweight materials, like composites and engineering thermoplastics, can be extensively employed.
Another advantage to an electric aircraft is the relative compactness of electric motors, which can contribute to better aerodynamics. Drag is less than 15 percent, according to Bye.
Because electric drive systems are inherently cooler than engines in which fuel is burned, there can be less need for cooling equipment, which could further reduce weight.
Regeneration can be built into an electric plane. As an electric plane decelerates, the propellers will act as windmills and run the motor to recharge the batteries.
Hybrids
Zunum, a Kirkland, Oregon-based startup, is trying to make a hybrid plane, one that uses internal combustion engines and electric motors, like the Chevrolet Volt. Zunum’s hybrid flyer plans to take 10 to 50 passengers a distance of 700 miles. About one-quarter of the distance will be due to batteries, the rest from the generator, a proportion Zunum will keep increasing as batteries are improved.
A number of research projects are underway for electric planes in the private sector with startups as well as in academia. Boeing and Airbus have both been working on electric planes for a while, though most of the working models currently in development are hybrids.
Airbus’ two-seat all-electric plane, the E-Fan, went 136 mph in a flight across the English Channel in 2015.
There’s a reason why many pictures of electric planes are artist’s depictions or renditions: the planes have not been able to leave the ground. NASA experiments with electric planes are embodied in the X-57, nicknamed Maxwell, which is perfect for something that is chubby, technically gifted and destined to be made fun of. With too many propellers and not enough landing gear (see Figure 5), its wings are still being dragged around on a truck on NASA proving grounds to test the far-out design. The design aims to solve the second biggest dilemma (the first being energy density) of electric flight: slow speed needs big wings, but that makes more drag. NASA decided to make the Maxwell’s wings super thin and use the extra propellers to generate faster flow across the wings, essentially tricking the wings into thinking they are going faster than the plane. That will come in handy during takeoffs and landings—done at the slower speeds of the electric plane.
Sources
IEEE Spectrum, September 2017
Wired, June 28, 2017
Popular Mechanics, July 20, 2017