When you first sit in the cockpit of an electric-powered airplane, you see nothing out of the ordinary. However, touch the Start button and it strikes you immediately: an eerie silence. There is no roar, no engine vibration, just the hum of electricity and the soft whoosh of the propeller. You can converse easily with the person in the next seat, without headphones. The silence is a boon to both those in the cockpit and those on the ground below.
You rev the motor not with a throttle but a rheostat, and its high torque, available over a magnificently wide band of motor speeds, is conveyed to the propeller directly, with no power-sapping transmission. At 20 kilograms (45 pounds), the motor can be held in two hands, and it measures only 10 centimeters deep and 30 cm in diameter. An equivalent internal-combustion engine weighs about seven times as much and occupies some 120 by 90 by 90 cm. In part because of the motor's wonderful efficiency-it turns 95 percent of its electrical energy directly into work-an hour's flight in this electric plane consumes just US $3 worth of electricity, versus $40 worth of gasoline in a single-engine airplane. With one moving part in the electric motor, e-planes also cost less to maintain and, in the two-seater category, less to buy in the first place.
It's the cost advantage, even more than the silent operation, that is most striking to a professional pilot. Flying is an expensive business. And, as technologists have shown time and again, if you bring down the cost of a product dramatically, you effectively create an entirely new product. Look no further than the $300 supercomputer in your pocket.
At my company, Bye Aerospace, in Englewood, Colo., we have designed and built a two-seat aircraft called the Sun Flyer that runs on electricity alone. We expect to fly the plane, with the specs described above, later this year. We designed the aircraft for the niche application of pilot training, where the inability to carry a heavy payload or fly for more than 3 hours straight is not a problem and where cost is a major factor. But we believe that pilot training will be just the beginning of electric aviation. As batteries advance and as engineers begin designing hybrid propulsion systems pairing motors with engines, larger aircraft will make the transition to electricity. Such planes will eventually take over most short-hop, hub-nd-spoke commuter flights, creating an affordable and quiet air service that will eventually reach right into urban areas, thereby giving rise to an entirely new category of convenient, low-cost aviation.
I will never forget my first experience with electric propulsion, during the early days of Tesla Motors, in the mid-2000s. I was a guest, visiting Tesla's research warehouse in the San Francisco Bay Area, and there I rode along with a test driver in the prototype of the company's first Roadster. Looking over the electric components then available-the motor was large and heavy, and the gearbox, inverter, and batteries were all relatively crude-I found it hard to imagine why anyone would take an electric car over a gasoline-powered one. But then the driver's foot hit the accelerator, the car lunged forward like a rocket, and I was a believer.
Electric flight has advanced on the backs of such efforts, themselves the beneficiaries of the cellphone industry's work on battery technology and power-management software. I founded Bye Aerospace in 2007 to build electric planes and capitalize on three advances in particular. The first one is improved lithium-ion batteries. The second is efficient and lightweight electric motors and controllers. And the third is aerodynamic design-specifically a long, low-drag fuselage with efficient long-wing aerodynamics, constructed with a very lightweight and strong carbon composite.
Our first project was the Silent Falcon, a 14-kg (30-lb.) solar-electric fixed-wing drone. We optimized the power system for long-duration flight by including only enough lithium-ion batteries to supply peak power for climbing. We designed and built a pneumatic rail launcher so that the plane does not have to take off under its own power. When it reaches the desired altitude, it can cruise for 5 to 7 hours, supplementing a trickle of battery power with electricity from solar panels spanning the 4.2-meter (14-foot) wings. The solar panels turn sunlight into electricity with 11 percent efficiency, effectively doubling the flight time that the batteries alone could provide. Nowadays, the best solar cells are rated at 26 percent efficiency, and they will allow the plane to stay up for 10 to 12 hours.
The Silent Falcon can carry various payloads, including conventional and infrared cameras and sensors useful for surveilling border areas, inspecting power lines, gathering information on forest fires, and many other uses. It flies with a completely autonomous plan. You give it a general order-where you want it to go, how high, and over what location-and then hit the Send button. The Silent Falcon started production in 2015, becoming the world's first commercial solar-electric unmanned aerial vehicle, or UAV.
Our next project was to develop, with the help of subcontractors around the world, an electric propulsion system for use in an existing full-size airplane: the Cessna 172 four-seater, the most popular airplane in the world. After flying the converted Cessna for a few dozen short hops, we followed up with a purpose-built, single-seat electric airplane. We've taken each of these test planes on 20-odd test flights.
Our first problem was finding a suitably light, efficient motor. Years ago, in the early days of electric flight, we encountered aviators who considered dropping (or actually did drop) a conventional electric motor into an airplane. But it weighed too much because of the heavy motor casings, the elaborate liquid-cooling systems, and the complex gearboxes. Our approach has been to work with such companies as Enstroj, Geiger, Siemens, and UQM, which have designed electric motors specifically for aerospace applications.
These aviation-optimized motors differ in several respects from the conventional sort. They can weigh less because they don't need as much starting power at low revolutions per minute. An airplane has far less inertia to overcome while slowly accelerating along a runway than a car does as it kicks off from a stoplight. Aviation motors can dispense with the heavy motor casing because they don't need to be as rugged as auto motors, which are frequently jostled by ruts and potholes and stressed by vibration and high torque.
In a Tesla, the power might peak at around 7,000 rpm, and that is fine for driving a car. But when you're turning a propeller, you need the power curve to peak much sooner, at one-third the revs - about 2,000 rpm. It would be a shame to achieve the shape of that power curve at that lower speed by adding the deadweight of a complex gearbox; therefore, our supplier furnishes us with motors that have the appropriate windings and a motor controller programmed to deliver such a power curve. At 2,000 rpm, the motor can thus directly drive the propeller. As a result, we've been able to progress from power plants that developed just 1 to 2 kilowatts per kilogram to models generating more than 5 kW/kg.
Even more important was the lithium-ion battery technology, the steady improvement of which over the past 15 years was key to making our project possible. Bye Aerospace has worked with Panasonic and Dow Kokam; currently we use a battery pack composed of LG Chem's 18650 lithium-ion batteries, so called because they're 18 millimeters in diameter and 65 mm long, or a little larger than a standard AA battery. LG Chem's cell has a record-breaking energy density of 260 watt-hours per kilogram, about 2.5 times as great as the batteries we had when we began working on electric aviation. Each cell also has a robust discharge capability, up to about 10 amperes. Our 330-kg battery pack easily allows normal flight, putting out a steady 18 to 25 kW and up to 80 kW during takeoff. The total energy storage capacity of the battery pack is 83 kWh.
That peak power rating is generally most needed toward the end of a flight, when the state of charge drops and voltage gets low. Just as important, the battery can charge quite rapidly; all we need is the kind of supercharging outlets now available for electric cars.
To use lithium-ion batteries in an airplane, you must take safety precautions beyond those required for a car. For example, we use a packaging system to contain heat and at the same time allow the venting of any vapors that may be created. An electronic safety system monitors each cell during operations, avoiding both under- and overcharges. Our battery-management system monitors all these elements and feeds the corresponding data to the overall information-management system in the cockpit.
Should something go wrong with the batteries in midflight, an alarm light flashes in the cockpit and the pilot can disconnect the batteries, either electronically or mechanically. If this happens, the pilot can then glide back to the airfield, which the plane will always be near, given that it is serving as a trainer.
A key precaution, pioneered in the original Tesla Roadster, is to separate the individual cells with an air gap, so that if one cell overheats, the problem can't easily propagate to its neighbors. Air cooling is sufficient for the batteries, but we use liquid cooling for the motor and controller, which throw off a lot of heat in certain situations (such as a full-power takeoff and climb-out from a Phoenix airport).
The airframe design takes advantage of advanced composites, which allowed us to produce a wing and fuselage design that is both lightweight and strong. We used advanced aerodynamics design tools to shape the fuselage airfoils and wing for a very low drag without compromising easy handling.
Much of the aerodynamic payoff of our electric propulsion system is centered in the cowling area in the airplane's nose. The motor sits in this space, between the propeller and the cockpit, and it is so small that we could squeeze the cowling down to an elegant taper, smoothing airflow along the entire fuselage. This allows us to reduce air resistance by 15 percent, as compared with what a conventional plane such as a single-engine Cessna would offer. Also, because the electric motor throws off a lot less heat than a gasoline engine, you need less air cooling and thus can manage with smaller air inlets. The result is less parasitic cooling drag and a nicer appearance (if we do say so ourselves).
The sleek airplane nose also increases propeller efficiency. On a conventional airplane, much of the inner span of the propeller is blocked because of the large motor behind it. In a properly designed electric airplane, the entire propeller blade is in open air, producing considerably more thrust. A bonus: The airplane can regenerate energy during braking, just as electric cars do. When the pilot slows down or descends, the propeller becomes a windmill, running the motor as a generator to recharge the batteries. In the sort of airport traffic pattern typical for general aviation and student-pilot training, this energy savings comes to about 13 percent. In other words, if a plane lands having apparently used 8.7 kWh during the flight, it has actually used 10 kWh?-the propeller-recoup system put back roughly 1.3 kWh while flying in the traffic pattern.
The commercial rationale for training aircraft like the Sun Flyer is the projected crisis in the supply of qualified airline pilots. Last year, Boeing made a staggering projection: The world will need an additional 617,000 commercial pilots by the year 2035. To put that in perspective, the total number of commercial pilots in the world today has been estimated at 130,000.
The growing scarcity of pilots has various causes. Fewer trained pilots are coming out of the world's large militaries. At the same time, it's increasingly expensive to obtain a commercial airline pilot's license from civilian pilot schools, as more hours of flight time are now required, some 1,500 flight hours in total. On top of that, the age of the typical training aircraft-in the United States, it's probably a Cessna or a Piper-now averages 50 years, according to the General Aviation Manufacturers Association.
The Sun Flyer, manufactured by our Aero Electric Aircraft Corp. (AEAC) division, is currently one of a kind, but it won't be much longer. NASA has announced a project to develop an experimental electric airplane, the X-57 Electric Research Plane, which would be the first new experimental aircraft the agency has designed in five years. (Because NASA is a government agency, its plane would not be a commercial competitor of the Sun Flyer.) Airbus has flown a small experimental electric aircraft several times over the last few years, but it now focuses on hybrid-electric commercial transport (which I'll discuss in a moment). Pipistrel, a Slovenian maker of gliders and light-sport aircraft (LSA), has flown experimental electric prototypes for several years. However, the future of such craft is unclear because the U.S. Federal Aviation Administration and the European equivalent, EASA, do not now allow any LSAs, electric or otherwise, to be used as commercial trainers.
For now, we are sticking to our niche in training. AEAC is working with Redbird Flight Simulations, in Austin, Texas, to offer a comprehensive training system. The Spartan College of Aeronautics and Technology, in Tulsa, Okla., has placed a deposit toward the purchase of 25 of our Sun Flyers, and it has also signed a training-related agreement that will help us to develop a complete training system. Other flight schools and individual pilots have made deposits and options to buy, bringing the total to more than 100 Sun Flyer deposits and options; another 100 deposits are in various stages of negotiation.
The Sun Flyer aircraft will be FAA certified in the United States according to standard-category, day-night visual flight rules with a target gross weight of less than 864 kg (1,900 lb.). And the airplane will not compromise on performance: We are aiming for a climb rate of 430 meters (1,450 feet) per minute; for comparison, a Cessna 172 climbs at about 210 meters (700 feet) per minute.
Why aren't we pursuing a larger commercial electric airplane? The main reason is the energy-to-speed ratio. The bigger and faster an electric airplane gets, the greater the number of batteries it needs and the greater the share of its weight those batteries constitute. The underlying problem is the same for any moving object: The drag on a vehicle goes up as the square of speed. If you double speed, you increase drag by a factor of four. In a relatively slow airplane, like a flight trainer, electric aviation is a serious contender, but it will take years before batteries have enough energy density to power airplanes that are substantially faster and heavier than our models.
While we wait for pure-electric technology to mature, we can use hybrid-electric solutions, which operate in planes on the same principle as they do in cars. Because you need about four times as much energy during takeoff as when cruising, you can get that extra burst of energy by running the electric motor at peak power; this is possible because motors have such a wide band of efficiency. Then, we could use a small internal-combustion engine running at optimal rpm to recharge the battery and sustain cruising speed. As a side benefit, relying on the electric motor for takeoff spares the neighbors a lot of noise.
We are in the midst of the monumental task of making the two-seat Sun Flyer 2 and the four-seat Sun Flyer 4 a viable, commercial reality. Some still say it can't be done. I counter that nothing of any fundamental and lasting value can be accomplished without trying things that have never been done before. Thanks to visionaries and pioneers, electric airplanes are not just an intriguing possibility. They are a reality.
Reprinted with permission of the author. This article appears in the September 2017 print issue of IEEE Spectrum as "Fly the Electric Skies."
First Published: 2017-08-24
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