Its almost a truism that aircraft performance numbers are a smoke-and-mirrors act where best-case scenarios are pushed forward as simple facts. Looking at the numbers we have to remind ourselves that real-world compromises-no, you cant fill the tanks and the seats-are kept off the table. Were beginning to smell the distinct aroma of aviation optimism, spiked with a twist of marketing, in the new arena of electric aircraft. With an opportunity for those potential buyers to put deposits down on kits or power systems within the next 12 months, its time to take a close look at where this new industry stands.
Its almost a truism that aircraft performance numbers are a smoke-and-mirrors act where best-case scenarios are pushed forward as simple facts. Looking at the numbers we have to remind ourselves that real-world compromises-no, you cant fill the tanks and the seats-are kept off the table.
Were beginning to smell the distinct aroma of aviation optimism, spiked with a twist of marketing, in the new arena of electric aircraft. With an opportunity for those potential buyers to put deposits down on kits or power systems within the next 12
months, its time to take a close look at where this new industry stands.
The Good News
Electric flight in enclosed light airplanes is practical with todays technology. Yuneec International and Electric Aircraft have both flown proof-of-concept aircraft on battery power for dozens of hours. Sonex has a couple of years invested in systems development. None of these companies have hit the two-person, two-hour, 85 MPH mark, but they are close enough that we can believe its within the grasp of incremental improvement.
The bad news is that we have no solid way to benchmark these designs. This is a new arena where we can only make educated guesses.
Risks and Compromises
Anyone who has seen YouTube video of lithium battery fires should wonder about fire risk with electric airplanes. The short answer here is that the odds are on our side. Batteries will overheat and potentially ignite when being overcharged or over-discharged. Overcharging is only likely to happen in the hangar, while youre not in the airplane. Were going to let this dog lie, perhaps pointing to how many laptop batteries are getting charged right now and how few of them are on fire.
A much bigger concern is fire during discharge-e.g., in flight. There are two risks here: too high a power draw from the whole battery pack and a simple short circuit. Companies we spoke with are all are developing smart systems that precisely manage the charge and discharge of individual cells. The occupants safety may rest largely on that controller. This could be a non-issue, but we have no data on reliability, so we dont know. In evaluating a design, however, protection and isolation of that controller from heat and water should be high on the list.
Shorts at the batteries themselves are probably a non-issue. These tend to be sealed systems with few opportunities for exposed wire. Chafing exposing a 300-volt cable somewhere in the airframe 2000 hours into its life may be more of an issue. Designs that make annual inspection of these runs easy would be a plus. Randall Fishman, President of Electric Aircraft, actually had a cell fail in his prototype because a loose cable bouncing while the aircraft was being trailered cut into a cell (it was a big cable). The result was only a small overheat and minimal damage.
Perhaps this is why Electric Aircrafts Electraflyer X will house its batteries in stainless steel. Were not sure that would make a difference if a cell went nuclear. The box might not melt, but it would be so hot that other aircraft components probably would. “Isolation” is going to be a crucial term when it comes to safety in electric aircraft.
Heat is probably more of an issue with battery life. LiPo batteries heat up when charged or discharged. A temperature of 90-100 degrees is ideal, but hotter than that and their lifespan is reduced. Of the designs weve seen so far, we like the Electraflyer Xs design with controllable airflow through the battery boxes. Were skeptical of designs that pack several cells together tightly. Hotter cells in the middle might not last.
Interestingly, cold is an issue too. None of these aircraft will likely have enough power to take off when its zero degrees outside. The batteries will just be too weak even if fully charged. Cruising at cold temps is fine as discharge will self-heat the cells and the power demand is less.
Cooling for the motor may be an issue, but we think this wont amount to much. The Electraflyer will have water cooling for the motor, but the Yuneec e430 and the Sonex system will both be air cooled. The potential issue is rain. According to Clive Coots, Managing Director of Yuneec, the e430 “is not something youd want to fly in a monsoon, but we will seal it sufficiently for all normal flight, including rain.”
We asked all the prospective manufacturers if theyd thought about any heat source for the passengers-theres no burning fuel to provide even lousy aircraft heat. No one had thought about that yet.
Electric airplanes need to maximize use of limited power. Putting some numbers on this, the one-seat Electraflyer C, which was a proof of concept aircraft, flew on an 18 HP (13.5kW) motor. A low drag profile meant it could do 70 MPH on that power. Thats not bad.
But doubling the speed requires eight times the power. This means were skeptical of anyone saying we can go fast and go electric. The specs for the Electraflyer X of going 80 MPH for two hours and Yuneecs goal of 90 MPH and flight times of 1.5-2 hours for the e430 sound reasonable to us. Were more skeptical of Sonexs goal of a 130 MPH aircraft with a more conventional aspect ratio wing. The motor exists, but pulling enough power from the batteries to turn that motor for more than minutes would be a serious discharge rate. High discharge rates mean shorter battery lifespans (see sidebar). Jeremy Monnett implied to us that we would be proven wrong on this count. Wed like to see that.
Pilots of electric airplanes might have to change their style in more ways than top speed. David Morss, the test pilot for Yuneec, got 10 minutes flight time until the batteries were dead on his first flights. By the end of the program he could do over an hour. The difference was in power consumption.
Using the batteries at a lower rate nets more cumulative power-more time in your “tanks.” Halving the climb rate in the Yuneec could increase the cruising time by a factor of six. A continuous descent to the airport might seem efficient, but it isnt when flying electric. The better plan is to turn off the motor and stop the prop. Glide down to pattern altitude and then turn the motor back on to get to the runway.
Heres another oddity: If you pitch up in a fixed-pitch prop, electric airplane the RPM wont change, but the power consumption to keep that RPM will go up. Pitch down and the consumption goes down-until the controller starts drawing current to actually slow down the prop.
Monnett of Sonex told us, “Part of the problem is how you tell this story to the pilot.” We think hes spot on. New instrumentation needs to somehow give us the tools to make good decisions on using power, as well as monitoring heat and
system status. This is something well be looking for in new designs.
Down to the Dollars
The biggest glossy-eyed handwaving we see is on the cost of operation of these aircraft. Sure it costs about $1 to recharge your electric aircraft, but the battery packs can only take 600-1000 charge/discharge cycles before they need to be replaced. All three companies are taking the official stance of offering these packs for between $10,000 and $15,000.
Those numbers arent typos and theyre not real, either. The retail cost of such packs could top $25,000 if you tried to buy them as an individual. Weve certainly seen before aviation companies trying to fill the holes in a leaky business plan by cranking up the price of parts. The only plus we see here is that as the years pass before you need replacement, battery technology should improve. Each replacement might be like buying a bigger gas tank.
Assuming an optimistic 1000 hours at $15,000 in batteries, thats $15/hour So we think a ball park of $20/hour in consumable costs is probably reasonable for these birds. Still not bad for recreational flight.
No designs are available for purchase just yet, and the first ones in the U.S. will be kit-only because the FAA doesnt yet have a plan for electric airplanes. There are standards in Europe, and Yuneec, a British company, does plan to offer ready-to-fly aircraft. Aircraft plus batteries for the e430 and Electraflyer X will be approximately $90,000 and $80,000, respectively. Both companies hope to deliver kits sometime next year. Sonex hasnt flown their system yet and wont commit to a timeline, but plans to offer a complete kit or just the propulsion system to buyers.
Well keep an eye on all these designs and update you as they prove (or dont) their flashy promise.