Hi, my name is Gabe and I’m an unabashed electric airplane nerd. I have built and flown two of my own electric aircraft, and I have assisted in another dozen or so electric aircraft efforts, both recreational and professional.
I was a founding team member and ran R&D for Zero Motorcycles for about five years. I worked at JOBY when they were just a small team of a dozen engineers or so, I did a five-year stint as an industrial drone designer, and I now work for ZeroAvia as head of drivetrain, where we are developing hydrogen fuel cell electric powerplants for commercial aircraft. Oh, I also designed the electric powerplant you might have seen in the Aerolite 103 EV flying around at Sun ’n Fun and AirVenture this year.
While that might sound like a lot of high-tech nerdiness, rest assured, I am just like you, and I just want to build and fly airplanes!
After Paul Dye and Marc Cook visited and wrote the article on my Electric Xenos, and Dean Sigler had done a few articles on my E-Gull, it dawned on me that perhaps there is enough interest for me to start a series of articles on electric aircraft, components and technology, and how they are relevant to kitbuilding today.
One of the more amusing/frustrating aspects of electric aviation (and all EVs in general) is just how misunderstood the basic fundamentals are. There’s a good chance you know how an internal combustion engine (ICE) works, right? Pistons, cylinders, carbs, valves, ignitions, CHT, EGT, 100LL, etc. But if I tell you my electric plane has a 55 kW motor and a 15 kWh battery, does that mean anything to you? If it does, you may find some of this material boring. if not, read on and I will do my best to elevate your knowledge on the subject of electric flight.
I plan to write a series of articles demystifying and explaining many aspects of electric aviation. I’ll cover electric powerplant basics, motors, batteries, controllers, safety, cost, charging, etc. But I’d like to start by helping you expand your knowledge of ICE powerplants and electric powerplants. Once you wrap your mind around a few simple concepts, you’ll find they are not so different after all. The propeller doesn’t know or care what’s making it spin around.
So let’s get started on “power” and “energy” (and apologies in advance to users of the metric system).
Translating Watts to Horsepower
What’s the number one metric everyone wants to know about an ICE powerplant? How about power, specifically horsepower? When I tell you my ICE has 75 hp, you probably intuitively know what that means, right? You know that’s plenty of power for a Piper Cub or similar small aircraft but probably not enough for much more. An ultralight or self-launching sailplane can fly on as little as 20 hp or so. A Cessna 140 has an 85-hp engine, a 150 has 100 hp, a 172 has 160 hp, and so on.
When I tell you my electric Xenos has a 55 kW motor, this translates to approximately 75 hp. Watts translate to horsepower directly; they are both units of power. Don’t be confused or led astray by the fact they have different rpm and torque characteristics. Power is power. To convert from watts to horsepower, you just divide the number of watts by 745 to get horsepower. Using the more normal metric of kW, you can simply multiply by 1.34 to get horsepower.
Because 55,000 watts is a cumbersome number, we condense to kilowatts (thousands of watts) and then abbreviate kilowatts to kW. Thus 55 kW; simple, right? You’d be amazed how many genuinely intelligent folks get this wrong. One of the most common mistakes I see is folks using kWh in error and mixing up kW and kWh. A kilowatt hour is a unit of energy, not a unit of power. Saying you have a 55 kWh motor would be like saying you have a 75 gallon engine, which is of course just silly.
This provides us a nice segue to energy. In electric energy storage systems, it is customary to use watt hours as the measurement for capacity. This is simply the amount of power your energy storage could provide in one hour and would be just like measuring your ICE fuel in horsepower hours. This is actually quite convenient. If your plane cruises at 50 hp and you have 100 “horsepower hours” remaining, you’d have two hours duration. Get it? To keep the units reasonable we use “kilowatt hours,” and this is generally analogous to saying “gallons of fuel.” Once again, there is a very direct conversion of energy from gallons (or liters) of gas to kWh of energy. One gallon of gas is equivalent to 33.7 kWh of energy.
So now, when I tell you my electric Xenos has a 55 kW motor and a 15 kWh battery, you can do some simple math and determine that this means my airplane has approximately 75 hp and a bit less than half a gallon of gas equivalent energy storage. So you might say, “It’s got plenty of power, but not much gas,” and you’d be right.
Powerplant and Airframe Efficiency
Electric powerplants have no problem delivering lots of power. It’s energy storage that is their weakness. But electric powerplants have an incredible performance edge that allows them to claw back some of that deficiency, namely efficiency. Efficiency is the ability to turn stored energy into motive power. Power is what makes the propeller spin, and an electric drive can do this about three times more efficiently than an ICE powerplant. A good electric powerplant is about 90% efficient, while traditional aircraft ICE engines are about 30% efficient. This means my half gallon of energy storage in my electric Xenos, is actually equivalent to about 1.5 gallons of stored energy in a traditional ICE powerplant. “Well that’s still not very much,” an astute reader might say. And you’d be right again.
And here we must use our last trick to make electric airplanes work: efficient airframes. How much power does your Cessna need to fly 100 mph (or knots, or whatever), I’m pretty sure the Cessna 150 I learned to fly in burned about 4.5 gph at just about 100 mph. So that’s 4 gallons at 33.7 kWh equivalent energy to equal approximately 150 kWh of energy total for one hour of flight at 100 mph. We then apply the abysmal 30% efficiency of the ICE to give us approximately 45 kW (about 60 hp) of power required. We just showed that a Cessna 150 needs about 45 kW to fly 100 mph by calculating the energy used through fuel consumption rate. Are you still with me? Good. If not, apologies for not being a better teacher. Regardless, let’s carry on.
Back to the topic at hand. What if I told you my electric Xenos needs only about 21 kW (approximately 28 hp) to fly 100 mph? It’s true and I can prove it. Below is a video and data log (Figure 1) of a flight in California from Watsonville (KWVI) to Hollister (KCVH). After leveling at about 2500 feet, I just let it run and clocked approximately 110 mph airspeed at about 24 kW and approximately 100 mph at about 21 kW.
That’s a bit more than double the efficiency of energy turned into range (compared to the Cessna 150) based purely on aerodynamics and has nothing to do with the electric powerplant. Aerodynamic efficiency is like magic and makes all airplanes better regardless of power or energy source.
OK, so now we’re getting somewhere, right? Because of our efficient airframe, we can turn that 1.5-gallon equivalent of energy into about 3.5 gallons of equivalent performance. “Well that’s still not much,” you’re probably saying, so now say it with me: “You’d be right.”
Consuming 21 kW from our 15 kWh battery, we only get 15/21 of an hour: about 43 minutes of flight. “Not much after factoring in 30 minutes of VFR reserves,” you might say, but this is where we push the efficiency even higher. We fly slower, we fly at best glide (Vbg) speed. Did you know Vbg is also your most efficient speed for level flight? It’s true! But because of the non-linear power-efficiency curve of an ICE powerplant, this does not mean your Cessna will be most efficient at this speed. But because electric powerplants do have fairly linear efficiencies across most of their rpm range, it does hold true for electric airplanes.
So, what if I told you (sorry, last time, I swear) that the electric Xenos can fly at 60 mph with only 9 kW of power? It’s true and I can prove it. Figure 2 is a speed to power graph I created using actual data-logged measurements.
As you can see, when flying at 100 mph, our duration is about 43 minutes and our range is about 72 miles. By reducing our airspeed to 60 mph, our range goes up to 100 miles and our duration to 100 minutes. Now that works out to 70 minutes over VFR reserves—not bad, eh?
I never claimed this was truly practical, and I never said we were going very far or very fast, but those are purely subjective sentiments, and everyone will have their own opinion. All I can say with authority is, I took an existing off-the-shelf electric powerplant and combined it with an existing off-the-shelf efficient airframe and, in my opinion, it’s lots of fun!
Your assignment, if you choose to accept it, is to do some quick calcs on other aircraft. How much power and energy do they have? You may notice if you Google an aircraft, it frequently lists the powerplant power in kW already. Welcome to the wonderful world of the metric system!
Here’s an example:
A Cessna 172 has 160 hp and 56 gallons of fuel capacity.
This works out to:
160 hp / 1.34 = 119.4 kW
56 gallons x 33.7 = 1887 kWh (or 1.887 MWh)
Now, 1.8 MWh is a lot of energy! A big Tesla battery is about 100 kWh these days. But also don’t forget that the Cessna has a very inefficient engine. If you stuffed a 100 kWh Tesla battery into a Cessna 172 (assuming equivalent weight and space of the fuel it replaced) and converted its energy into motive power at 90% efficiency instead of 30%, you’d get about 300 kWh equivalent energy, or about 1/6 a full fuel load. That’s good for about an hour. Only that battery actually weighs more than double the fuel it replaced. That means you can only load half the battery, which means you only get 1/12 of the equivalent fuel load. Plus you now have a big chunk of battery you have to package somewhere in your airframe. Sigh, electric airplanes are hard…