A Battery-Powered Cessna 172 Skyhawk

Bye Energy has settled on the venerable Cessna 172 as its electric platform.

(February 2011) — It has been seldom in the history of aviation that a single technology has revolutionized the way we fly by addressing multiple problems facing the industry and solving them all. The most important such event was the widescale adoption of the turbine engine in the 1940s. Turbines, as you know, remedied (and continue to remedy) commercial, military and business aviation problems of reliability, cost, range, speed and power, while also bringing with them a number of infrastructure advantages associated with using a single fuel type.

Though it’s still in its infancy, electric propulsion seems to promise to solve a similar range of problems in light GA. But in the case of the electric motor, the potential advances might be more sweeping and more compelling, though almost certainly on a smaller scale economically.

The most visible electric airplane project these days is Bye Energy’s Green Flight Project. The man behind the program is George Bye, an engineer whose background as an Air Force transport pilot and instructor doesn’t seem to lend itself to his new role as a high-tech entrepreneur and self-described futurist, the head of a company that is trying to do no less than bring electric power to the mainstream of light general aviation.

Bye has even bigger ideas for Green Flight, but the first step for his company is nevertheless an ambitious one: to convert the most popular airplane in history, the Cessna Skyhawk, to electric power. On Bye’s team is a guy who certainly knows Skyhawks, former Cessna CEO Charlie Johnson. Bye and Johnson gave a product update on the Green Flight Project at a press conference during the AOPA Summit in Long Beach, California, in November. There was much promising news, including endorsements and partnerships with several industry heavy hitters, such as Jeppesen and Cessna. Now nearly done with the detailed engineering phase of the project, Bye Energy could be flying the first electric Skyhawk soon.

Skyhawk as Platform
While some might argue that next-generation aircraft should use next-gen airframes, Bye argues that the Skyhawk is an “ideal” platform and was chosen only after long and “careful deliberation.” The airplane is the most popular model in aviation history, with more than 40,000 produced and a good percentage of those airplanes still flying. And the 172 is the model of a perfect trainer, many believe, a relatively lightweight four-seater that flies beautifully and lasts a long time.

It’s hard to overstate the importance of the fact that the 172 is a certified airplane, one that has no questions surrounding it, other than those having to do with it being too safe a choice. With the airframe question out of the way, Bye can simply concentrate on what is a tough enough task, creating a safe and satisfying electric power solution.

Moreover, the company’s choice of the Skyhawk infers instant credibility upon the project, something that’s hard to imagine happening for a start-up purpose built electric airplane project.

A Trainer, to Begin With
The choice by Bye to focus, at least at the start, on the training market seems as well thought out as its choice of airframe. Because the current state of the electric art will limit the first “Byehawk” to about two hours’ absolute endurance, the most suitable role for the airplane is that of trainer. There are good reasons for this.

Most training flights last around an hour, and very few go as long as two hours, so the projected endurance of the electric Skyhawk is workable. Just as importantly, most training flights are out-and-back affairs, so the electric trainers will be returning to their home airports to recharge, instead of having to search for a friendly source of current elsewhere.

We often think of battery weight and capacity as being the holy grail of electric vehicle progress, and that’s true, but there’s another key factor: recharging time. This just happens to be an area where the state of the art is rapidly improving. Bye says that, within the very near future, recharge times, which currently run at around a 1:1 ratio — that is, it takes an hour to put an hour’s charge into the batteries — could soon be down to a ratio of 0.25:1, or 15 minutes for a one-hour charge. This improvement would change the game.

Imagine a typical training flight. The student and an instructor start up the airplane, taxi out, take off and proceed to, let’s say, practice ground reference maneuvers before returning to the airport to do a few touch-and-goes and before calling it an hour. Afterward, the two repair to a briefing room to briefly go over the day’s flight and cover the next lesson. During that 20- to 30-minute time period, the airplane is being charged. By the time the instructor is ready to take to the sky with his next student, the airplane is ready to go.

With two hours’ range, even the long cross-country requirement for the Private would be easy to accomplish, with no need to “refuel” at any of the three required legs along the way (that is, as long as there’s minimal time spent getting lost and then found again). At least for the Private, it seems as though the training role would work well for an electric-powered 172. In fact, until battery life improves as storage technology advances, it might be one of the only realistic roles.

Skyhawk vs. Skyhawk
While it might look that way at first glance, upon closer inspection it’s clear that Bye’s electric Skyhawk isn’t a 172 with an electric motor bolted on the nose and some batteries thrown in.

The biggest difference in the illustration of the electric Skyhawk supplied by Bye Energy is that strange prop. The six-blade prop is very light in weight and is designed to be the perfect complement to the electric motor. (Note: Though usage varies widely in general, an electric power plant is not referred to as an engine, a term that is usually reserved for some kind of combustion device. In fact, “power plant” is certainly off target, as well.) More on the motor in a bit.

There might be a few other outwardly noticeable differences between your father’s Skyhawk and George Bye’s. Bye’s wings could very likely at some point be fitted with solar panels, to help recharge the batteries, even in flight. The panels would be located on top of the wings, except, presumably, for aerobatic models. This is another reason the Skyhawk is an excellent candidate; it is a high-wing airplane, so the solar panels are out of harm’s way, unlike with, say, a Cherokee. They will also be light and will add little additional drag, Bye says.

There are also going to be, at some point, small generators at the tips of the wings. These mini turbines will capture the energy of the wingtip vortices and turn that energy back into electrical power, to be used by the airplane to enhance endurance.

Another source of power is the prop, which would generate electricity when in descent, for instance, or even while the airplane is sitting on the ramp on a particularly windy day. It’s like free gas. Bye isn’t saying, at least at this juncture, just how much free “gas” these systems will provide.

Weight vs. Weight
Figuring out the weight differences between the two airplanes is an interesting exercise, because how they derive their power is so fundamentally different.

The electric motor in the Bye Energy Skyhawk will weigh just 45 pounds, the same as just 7½ gallons of avgas. This is compared with the approximate 350-pound weight of a four-cylinder Lycoming power plant. In addition, with the Lyc you need to add the weight of fuel, which is 42 gallons times 6 pounds per gallon, for a fuel weight of 252 pounds, which, added to the engine’s weight, gives you a grand power-system total of around 600 pounds. There are small additional savings in the removal of fuel drains, tanks and lines as well.

If it weren’t for the fact that the electric motor needs batteries to run, the swap would be a major win in terms of weight. But it does need batteries, and those batteries, while getting lighter and more energy dense all the time, are still heavy.

The current battery of choice is the lithium polymer (sometimes referred to as a lithium-ion polymer) battery, often called LiPo. LiPos have the advantage of being much lighter than the lithium-ion batteries they have largely replaced because instead their electrolyte is not held in an organic solvent but in a solid polymer, allowing the cells to be housed in thinner, more flexible and much lighter housings, while also allowing the batteries to be shaped in a variety of ways.

The downsides for LiPo batteries are significant. Their recharge cycle characteristic, that is, the number of times they can be recharged before degrading, is improving but is still marginally workable for airplanes. Recharge figures are approaching a thousand cycles before the batteries can no longer hold 80 percent of their charge.

Bye’s design will not feature removable batteries, as some other emerging electric-powered airplanes have. Instead, Bye says that the design will be optimized for recharging. In fact, if recharging times do get down to 15 minutes to half an hour, the slight time difference between swapping batteries and charging them will make the issue moot. It might even be faster just to charge them.

Many pilots are concerned about the fire issues associated with lithium polymer batteries, and it’s true that they are highly flammable and explosive, very much like gasoline. The batteries are safe, however, as long as they are not overcharged, something that the onboard charging and power conditioning system would ensure. LiPo batteries react with nitrogen, a plentiful atmospheric gas, and with water. So in a crash, a compromised battery housing could lead to fire. The same is true, again, for gasoline, when it is exposed to a source of ignition.

In addition to the motor and the batteries, the power system on Bye’s Skyhawk will need a power conditioner and new controls and engine (motor) monitoring displays. Bye won’t say with whom he’s working to develop the displays, but it’s not hard to imagine that the Garmin G1000 in new Skyhawks could be modified to work nicely for power monitoring.

Upsides One of the big benefits of the system will be its low noise. It’s likely, in fact, that the airplanes will require a horn so that pilots can alert people on the ground of an electric Skyhawk’s approach. The neighbor-relations benefits to flight schools, especially busy ones in noise-sensitive areas, would be great. Pilots too would reap the rewards of flying in a very low-noise cockpit.

Other benefits have to do with the characteristics of electric motors compared with gas-piston ones. Starting is automatic. Mixture control is a no-brainer, because there is no mixture. There’s single-lever power control — a quick glance will tell you what the power setting is and how much more flying time you have.

Electric motors also don’t run on oxygen, so they will be efficient at high altitudes and density altitude won’t be as big an issue — wings, however, will still rely on the density of air to do their thing. While it’s not a major perk for training, an electric motor’s indifference to air means that it would be a great high-altitude power plant, with all of the upsides of a turbocharged gas-piston engine without the increased fuel consumption or overheating concerns.

With growing concern over the cost of training, the allure of electrics is very strong. Just think of it. Go flying for an hour, get back, plug in, pay a few bucks for that electricity versus around $40 for avgas (eight gallons for an hour’s flight at $5 per gallon) and head back up again, literally for pennies on the dollar.

There are additional savings to be had. The costs of gas-piston engine maintenance and overhaul are very high, but electric motors need much less maintenance and their TBOs are nearly unlimited. For the short term, cost of operation of an electric Skyhawk will be much, much less than for a conventional one.

But airplane owners aren’t in it for the short term. And long term, there are big questions about electrics. First, battery cost and battery life (two closely related subjects) are hard to predict, and Bye Energy isn’t going out on a limb here. How long will the batteries last? And how much will it cost to replace them? Both are unknowns at this point, but suffice it to say, the cost will be high and the time between replacement (TBR) will not be long enough to make owners as happy as they would like to be.

At this juncture, it’s likely that electric Skyhawk owners will be spending a great deal more on battery replacement than they would have spent on engine overhaul, probably in the neighborhood of three times as much. But then again, the savings in fuel and routine maintenance will balance that out to a great degree.

Until the details are known, however, it’s impossible to say just how much more or less it will cost to operate an electric Skyhawk compared with a conventional one. But one thing is sure. As battery technology improves and prices come down, which will very likely happen over the next few years, the economics of electrics will begin to outweigh the airplanes’ limitations, which for now require a return to home base within a few hours.

One potential option is hybrid power, which Bye Energy refers to as APU use. In this case a small generator engine (powered by jet-A, says Bye) would provide additional power to charge the batteries. This solution, however, would require the installation, care and feeding of two separate power systems, and it would require carrying an additional fuel source and additional weight, all of which undermine some of the gains that electrics promise going in.

Still, the promise of electric power is great. It seems to be dependent on the development of better batteries. Today’s batteries are, in fact, much improved over those available commercially just a few years ago. And they do seem to be good enough now to make everyday electric flight, if not an economic godsend, at least a realistic alternative in some segments of the industry to the gas-piston paradigm.


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