Wait, Electric Airplanes Have Radiators?

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I’ve covered this budding electric aviation thing just enough to understand this: The idea that an electric motor has so few moving parts that all you do is keep feeding it power and it will run smoothly forever is just wrong. The minimal parts count is right, but the simplicity isn’t.

But it’s nice to have it demonstrated. I was in a hangar the other day in which a small brushless DC motor—probably under 50 HP—was being tested. It ran smoothly and silently up 2000 RPM, the only noise being the whoosh of the prop blades cutting air. Then, just as suddenly as the motor had spun up to operating speed, it stopped with a thunk as loud and certain as a rod being thrown in a piston engine. Some little bit in the operating software wasn’t happy and pulled the plug on the test. Twice. Code writing skills will be much in demand in the emerging age of electric aviation.

So I made a note while at Aero to prepare a report on how a brushless DC motor actually works. The go-to guy for that is Frank Anton, Siemens’ lead guy for the company’s aircraft electric motors. In this brief video, which ran on AVweb a couple weeks ago, he explains how it all works.

That 350-HP motor Siemens had on display was quite a marvel, in my view. At only 110 pounds, it’s the most powerful electric motor of its kind yet developed. But users of this motor will face their own developmental challenges and it’s a mistake to think of them as trivial. As Dr. Anton explains, a brushless DC motor has to know the position of the rotor at all times in order to reverse magnet polarity that would normally be done with brushes in an AC motor. If it gets out of phase, the motor simply won’t run and that’s what I saw in the motor test I observed.

So the motor requires inverters that are robust and reliable if they’re going to even match piston-engine reliability, never mind exceed it. They also need cooling systems and pumps to run them, so if you were thinking electric aviation will be free of leaks, sorry. When you examine an electric propulsion motor closely, it’s astonishing how much power comes out of so little mass. And that means heat that has to be carted off. So yes, electric airplanes will have radiators. They probably won’t need antifreeze, however, because oil is the cooling fluid of choice.

I trolled our YouTube channel to see if anyone would take the bait on claimed power-to-weight ratio of electric motors. And, it being the internet, someone, of course, did. That big Siemen’s behemoth weighs but 50 kg for a power-to-weight ratio of 7 hp/kg or 3.2 hp/lb.

The closest thing to compare it to is the Lycoming IO-540 series which, at 350 hp, weighs about 450 pounds for a power-to-weight of 0.78 hp/lb. Of course, the electric motor enjoys a big advantage and just as obvious, it’s less so when the batteries are considered. Here, it gets complicated, because to be fair, you have to include the weight of the gasoline for the Lycoming, not to mention how long you wish to fly.

Pick a number: 30 minutes. In that case, at current battery energy densities, you’d need a ton of batteries to sustain that power. Literally, a ton. Do the same for 200 pounds of avgas. You can see how far we have to go before a mechanic confronts a leaky radiator in a new electric airplane.

I think just as every technology has, electric propulsion in airplanes will go through teething pains. We’ll see our share of them quit in flight before everything is sorted out. It’s inevitable. And, so what? That’s the price of progress and it’s never been easy. But now you know a little about what’s involved.

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