In the past few years, the excitement around electric airplanes has made it difficult to separate hype from facts. But rejoice! We finally have a real electric airplane to talk about. It’s not a prototype for airshows and venture capitalists. It’s for sale. It flies at real airports. And it’s built by a manufacturer who’s been in the business for decades. Writer and pilot Sarah Deener describes flying the Pipistrel Velis Electro in the January 2024 edition of AOPA Pilot magazine. If you can’t find a copy in your local library, check out your local municipal airport (the little one with flight schools, not the big one with commercial airlines). Look in the stack of magazines beside the comfy chair in the lobby. The article won’t take long to read and provides many of the numbers I discuss below.
Before we get into it, I need to back up and review three key points. The first is that batteries aren’t “gas tanks for electricity.” Gasoline tanks are simple, inert containers that last for decades. Batteries are expensive chemical reactors that decay each time they’re drained and recharged. They have limited lifetimes and need artificial cooling in high power applications.
The second point is that batteries are heavy. Seriously heavy. It takes a 50-pound lithium battery to match the energy in a single pound of gasoline (one McDonald’s medium drink cup). But since electric motors are 3 to 4 times better at converting energy into usable work, it takes 3 or 4 pounds of gasoline to do the same work as a 50-pound battery. That reduces lithium’s weight disadvantage from 50-to-1 down to about 14-to-1. In general, gasoline will take a vehicle 14 times farther than the same weight of lithium batteries. Electric cars compensate for that disadvantage by using batteries that weigh 800 to 1700 pounds, far exceeding the 96 pounds of gasoline in a typical 16-gallon tank.
The final point is that adding batteries to an airplane is nothing like adding them to a car. The weight of a car is supported by the road beneath it. You can pile on as much weight as the suspension will carry. But the weight of an airplane is limited to what the wings and power plant can lift. If you want to add batteries to an airplane, you must leave something else behind, either passengers or cargo. Or you can build a bigger airplane with more lift, but the bigger power plant will just drain the batteries faster.
With that understood, let’s look at the actual airplane.
The Electro is a light, two-seat, high-wing aircraft. Its liquid-cooled batteries weigh 300 pounds. The maximum weight for the pilot and passenger together is 378 pounds, and that’s all the Electro can carry. There’s no provision for cargo, not even an overnight bag. On the demo flight, the pilot did four takeoffs/landings and flew some common training maneuvers. The entire flight lasted 29 minutes, during which the battery went from Full down to 27%. (In other words, the flight used 73% of the battery capacity.) In round numbers, that’s 30 minutes of flying for 3/4 of the battery, which suggests a full battery could fly 40 minutes. The Electro’s advertised flight duration is “up to 50 minutes,” which I’m guessing they can achieve with only one person on board, a less-than-full-power takeoff, and gentle flying—the aviation version of hypermiling.
How much does an hour of flight cost? The Electro flew level using 30 kilowatts of power. (For comparison, that’s 16 household blowdryers running on high.) Doing that for an hour uses 30 kilowatt-hours. On average, that would cost $6 in the USA, which sounds fantastic. But the batteries also need to be replaced every 500 hours. The article didn’t give a price, but other sources in the trade press say the cost will be around $20,000. Divide that by 500 hours and you get another $40/hr for a total of $46/hr.
How does that compare to gasoline? The Electro’s nearly identical cousin–the Pipistrel Velis Club–has a combustion engine that burns 5 gallons an hour of unleaded automotive gasoline. That costs about $20/hr where I live. Regular oil changes and spark plug replacement raise the cost to $24/hr. The Club can fly 5.5 hours on 26 gallons (156 pounds) of gasoline. With 300 pounds of gasoline (the same weight as the Electro’s battery), it could fly 11 hours. Compared to the Electro’s 40 minutes, that’s a ratio of 17-to-1. If we believed the advertised duration of “up to 50 minutes,” the ratio might be 13-to-1. Either value is close to the 14-to-1 rule-of-thumb.
Why not add solar panels on the wings? Solar energy isn’t the panacea many people imagine. In the middle of a bright sunny day, solar cells on the Electro’s 10 square meter wings might produce 2 kilowatts of power. That’s less than 7% of the 30 kilowatts the airplane needs to stay aloft. At best, solar cells might extend a 30-minute flight by 2 minutes. But in aircraft, weight always consumes energy, and the added weight of the solar cells could actually reduce the range, especially on days without direct sunlight. [The contribution of solar cells is even worse for larger aircraft. Solar cells on the 500 square meter wings of a 747 would (at best) produce 100 kilowatts, less than 1/400th of the roughly 45,000 kilowatts the 747 needs.]
How far can the Electro go? The airplane is intended to fly in the immediate vicinity of an airport. Period. Pipistrel is very clear that if you want to fly cross country, you’ll need a gasoline-powered airplane. Even when flying close to an airport, the Electro is supposed to land once the battery is down to 30%, just like the flight in the article. People are trying to use the Electro for flight training since it can’t do much else, but I think it’s a bad fit. Having recently been a student, 30 minutes isn’t long enough for an effective lesson. There are too many logistical details in simply getting airborne (and landing again) without attempting to sandwich an effective learning experience in the middle. And since recharging takes one to two hours, the number of lessons per day is limited.
All in all, the Electro’s 40-minute range didn’t surprise me. I’ve been doing the battery math ever since I considered electrifying my first Subaru in the 1990s. And these physical limitations aren’t unique to the Electro. Any airplane with the same size and payload would need the same amount of power to stay airborne, and any decent 300-pound lithium battery would deliver that power for 40 minutes. That’s enough for a few demonstration flights in a day, like a jet pack, but it’s not practical aviation. A practical electric airplane will have to wait for the next big breakthrough in battery technology–either that or Mr. Fusion.
And that was supposed to be the end of this post. Then I thought again about landing with 30% of the battery. Out of 40 minutes in the battery, that leaves just 12 minutes of flight time. It gave me the wiggins, because landing is stressful enough without knowing you might not have enough fuel to go around and try again.
Lots of things can go wrong while landing. Even if you’re on a stable approach and on the glide path, an unexpected gust of wind can blow you to the side, forcing you to go around and try again. Complicated things can go wrong, too. If you’re second-in-line for landing and the airplane in front of you blows a tire on the runway, you don’t land beyond the wreckage. You stay in the air. The runway is closed until the obstacles have been cleared. You might even have to go to another airport, which is one of the reasons airplanes are required to carry at least 30 minutes of extra fuel (more for commercial and night flights). But an airplane on its last few minutes of battery couldn’t go anywhere else. The pilot would have to make a forced landing among the emergency vehicles and personnel on the ground.
Lithium batteries barely contain enough energy to provide the 30-minute safety margin, and people are shaving that margin to create the appearance of almost-practical flight times. I might as well say it out loud: “What could possibly go wrong?”