Designing a better electric plane
First in a series. One thing about mature markets is they spawn opportunity through pure complexity. What does the press do but sit around discussing the size and depth and pimples on the bum of mature markets? But we spend so much time discussing the implications of what has already happened that we don’t give much space to what’s coming in the form of new ideas. So for the next week or so I’ll be doing a series of columns about new ideas, especially new technologies, that ought to interest us all.
Leading by example I’ll start with a project my kids and I have been working on this summer -- an electric airplane.
Electric planes are becoming more and more popular. In April there was here in Santa Rosa, Calif an international symposium on electric flight led by, of all people, my ophthalmologist. It’s a small world, eh?
The Power Problem
As an emerging technology, electric flight is full of new ideas as well as a lot of old ones that need to be forgotten. We’ve attacked one of these old ideas in our design, throwing away the concept that an electric plane has to have enough battery capacity to get where it is going.
Remember Steve Jobs saw as his design strength always asking why things were done the way they were. The boys and I took this to heart when designing our electric commuter: why do battery packs have to be the size and weight they are?
Batteries are the major design feature of any electric airplane. They comprise most of the weight and cost and making the wrong battery decisions can easily doom any electric airplane project. Our design philosophy was “simplicate and add lightness” and toward that end we wanted to build our plane around the lightest (and cheapest) battery pack possible. Even better would be a battery pack that was seemingly inadequate to the task -- so small as to be impossible. That’s the ticket!
Our electric plane (we haven’t named it yet) has the simple mission of flying from Santa Rosa to Palo Alto, a distance of 71 nautical miles (81.7 statute miles or 131.5 kilometers) carrying one person (me) and 10 lbs of stuff. The budget for this project is $10,000 or less. Our biggest cost by far is batteries so the fewer of those we need the more likely we are to stay within our budget.
But battery size goes beyond just cost, because the more batteries we carry the more airplane we have to build to carry those batteries. So even a slight reduction in battery size can lead to significant changes in the size of the overall aircraft.
The most obvious way to make batteries smaller is by using better batteries -- those with higher storage density. Alas, our budget won’t allow that, because bleeding edge batteries are very expensive. We’re using very good nickel-zinc batteries from a San Diego company called Powergenix. While not quite having the power density of Lithium-ion or even Lithium-polymer batteries, the Powergenix cells are far less expensive and have the advantage of higher voltage and higher discharge rates. They are great batteries for this application. But since they don’t have greater storage density, the only thing we can do to make our battery pack lighter is to cheat.
Remember we’re carrying about 180 lbs of fighting fury (me) for 81.7 miles, which we’ll round up to 90 miles. That’s our target range -- 90 miles. It’s easy to figure the weight of the airframe required to carry such a load that distance. We need energy to accelerate, takeoff, climb to cruising altitude, cover some distance to the destination, then reduce power, land and taxi to the recharging station. All this plus a reserve for higher than expected winds, air traffic control delays, or even rerouting to an alternate destination. Since this is only a 45-minute flight we’ll add another 15 minutes for the reserve.
Sparing you the calculations, this mission profile will require a nickel-zinc battery pack weighing approximately 270 lbs. The structure to carry that many batteries as well as me will weigh about another 270 lbs for a maximum gross weight of 720 lbs.
While this doesn’t seem like much weight, it is too much for the Cringely boys who demanded a more aggressive mission profile. Nickel-zinc batteries have high voltage, low impedance and can flow gobs of current (hundreds of amps). Unlike internal combustion engines, batteries don’t lose power with increasing altitude. And since drag diminishes with altitude about two percent per thousand feet and we have plenty of excess power for climbing, our ideal mission profile would be to climb as fast as we can as high as we can then head toward our destination. That’s why jet airliners fly high because the fuel efficiency is greatest at altitude.
Go Short to Go Longer
The real design innovation came when we decided to use this new mission profile then add one more twist: we decided to run completely out of power at 60 miles -- about 12 miles short of our destination.
By designing for 60 miles rather than our original 90 miles plus 15 minutes equals 120 miles, we’ll need a battery pack only half as large or 135 lbs. That would not only save half our battery budget (our most expensive component) it would reduce our structural weight by at least 150 lbs! That’s 150 lbs less to buy, build, and carry. At 570 lbs gross our tiny electric plane will be even tinier.
How do we do this without landing in San Francisco Bay?
There’s always a trick. In this case the trick starts with running out of power at 10,000 feet (maximum efficiency altitude) not near the ground. Our plane can glide 12 feet for every one foot of altitude it loses. So from 10,000 feet we can glide up to 120,000 feet or 22.7 miles. That gives us a no-wind range of 82.7 miles, just enough to reach Palo Alto.
But what if there’s a headwind? What if there’s an air traffic delay? What if?
Electric cars and hybrids gain efficiency by using regenerative braking where the electric motor is used as a generator to slow down the car, making electricity in the process that is stored back in the battery pack. But electric airplanes don’t use regenerative braking because, well, there’s no brake pedal and because regenerative motor controllers are more expensive.
Our plane uses a regenerative motor controller, though. Our mission profile is such that in the relatively steep descent to Palo Alto from 10,000 feet we’ll put enough power back into the battery pack for almost 15 minutes of low-power flight after reaching our destination -- enough for a headwind and a couple go-arounds if necessary.
With this design dodge that allows us to buy only half the batteries we might combined with revolutionary fold-a-plane construction technology, our little commuter is likely to come in under budget after all.
This is an example of technologies and a technical approach that won’t change the world, maybe, but it’s cool and someone smarter than us might really turn it into something. Until then we have five little airplanes to build. Imagine a small flock of birds...
Reprinted with permission