Motor music in cold weather: what changes and what doesn't

Last Sunday it was cold enough at the field that I could see my breath, and the first thing that happened was that I couldn't get the XT30 into my toothpick. Gloves on, no chance. Gloves off, fingers numb within a minute. When I finally got it seated, the quad played its little tune exactly like it does on my desk at home, and I stood there wondering why I'd half expected otherwise.

That got me thinking about what cold actually does to motor music. Less than you'd think, but not nothing.

The pitch doesn't drift

Someone asked me once whether the melody goes flat in winter, the way a guitar does when you take it outside. It doesn't, and the reason is simple once you know how the sound is made. The ESC isn't vibrating a string or a reed. It drives the motor windings at a frequency set electronically by the firmware, and the bell just moves along with it. (This post goes through the details.) The microcontroller's clock doesn't care much whether it's mild or freezing, at least not in any range you'd want to fly in, so an A stays an A.

Can you hear a difference in tone? Honestly, I'm not sure. Cold air is denser, and maybe sound carries a touch differently, but on a still winter morning the tune seemed just as audible to me as in summer. Possibly more, because there's nobody else at the field and no crickets. I wouldn't read anything into that.

The battery is where cold bites

Cold lipos are the real winter problem, and anyone who's flown in December knows it. A cold pack has higher internal resistance, so it sags harder under load. Punch out on a frozen pack and watch the voltage dive.

The melody, though, happens at plug-in, before you've armed anything. The props don't spin and the current draw is small. So even a cold pack plays the tune fine. I haven't had a melody fail on a cold battery yet, and if you have, I'd like to hear about it.

What cold does change is the routine around it. I keep packs in an inside jacket pocket or an insulated lunch bag until right before they go in. Warm battery, quick tune, arm, go. And don't charge packs that are still cold from the car boot. Let them come up to room temperature first.

Long tunes get annoying

This one's purely about comfort. In July I don't mind a twelve-second Imperial March while I fiddle with my goggles. In November, with bare fingers and the wind going straight through my jacket, I want the quad to say hi and be ready. I've swapped two of my quads to shorter tunes for the winter and I don't miss the long ones. (Short songs beat long songs anyway, but winter makes the argument for you.)

The tune still does its job as a check. Plug in, hear all four ESCs start up, and you know power reached them. If one motor stays silent, look at that ESC and motor before you fly. A bad joint or a damaged motor wire is much nicer to find on the ground than at the top of a climb. The troubleshooting post has the usual suspects.

Gloves, plugs and condensation

A few smaller things I've picked up over the last couple of winters:

• Thin touchscreen gloves are a decent compromise for plugging in. Thick ski gloves and XT30s don't get along.
• A little pull tab on the battery lead makes plugging and unplugging easier with cold hands. Even a folded loop of electrical tape helps.
• When you bring a cold quad into a warm car or house, condensation can form on the electronics. I leave mine zipped in the bag for a while before opening it, and I don't plug a battery in until everything's dry.
• Don't reflash ESCs at the field in the cold. If you want a new winter tune, do it at the bench with a warm battery and enough time to fix a typo.

One last thing I learned the hard way on that Sunday: do the battery strap with your gloves still on, and only take them off for the plug. You get about a minute of useful fingers, and you want to spend it on the XT30.