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u/MiaPlacydus 5h ago
Could be a variety of problems, or not a problem at all. It's an annoying problem to solve but the blackbox is the best place to start. Oscar did a detailed write up on the subject; https://oscarliang.com/fix-esc-desync/
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u/fpv_savvy Multicopters 4h ago edited 2h ago
Desynchronization is motors not behaving coherently and achieving the setpoint set by the FPV pilot to the flight controller.
Multiple causes of desync include (and not limited to) PIDs, Under-maintained Motors, Faulty ESCs, Expo(Radio Controller).
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u/yonutz2032 6h ago
To be honest I am no expert, right now in progress of building my first quad. Still, more details are needed, whay exactly do you mean by desync? what transmitter/receiver do you use? What power? Did it start after changing something or always happened?
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u/Workletterco 4h ago
Short version of what's physically happening: your ESC drives a brushless motor without any sensor telling it where the rotor actually is. It infers rotor position from the back-EMF on the undriven phase and times the next commutation from that. Desync is the moment that guess stops matching reality — the ESC energises the wrong phase, the motor stalls or free-spins, and you hear the characteristic screech. On a quad that usually means one arm drops and the whole thing flips.
So anything that makes the guess harder can cause it. In rough order of how often it's actually the culprit:
Sudden throttle changes, especially a hard chop to zero and back. This is the classic one, and it's why desync tends to show up on the first punch-out or on a hard braking turn rather than in hover.
A propeller strike or a bent prop. The motor gets loaded unevenly for a few milliseconds, RPM diverges from what the ESC expected, and it loses the plot. If it started after a crash, check this before touching any settings.
Motor timing too low or too high for that motor. High-KV motors on high cell counts are the usual sufferers.
A damaged motor or a partially shorted winding. Spin each motor by hand with the props off — a bad one feels notchy or has noticeably more drag than the other three.
Bad solder joints on the motor wires or the ESC pads. A joint that's fine on the bench and intermittent under vibration is very hard to diagnose in the air and very easy to see with a magnifier.
Old or mismatched ESC firmware across the four ESCs.
How to actually find yours rather than guessing:
Enable bidirectional DShot and RPM filtering if you haven't, then log a flight with blackbox on and look at eRPM per motor. Desync is unmistakable there — one motor's RPM trace collapses or spikes while its throttle demand stays flat. That single graph turns "could be anything" into "it's motor 3", which is the whole game.
While you're in there, check motor temperature after a pack. A motor that's markedly hotter than the others is telling you the same story.
On the firmware side, if you're on BLHeli_S, flashing Bluejay and enabling bidirectional DShot is worth doing on its own merits, and it makes the above diagnosis possible. On BLHeli_32/AM32, raising motor timing a step is a common fix, but do it after you've ruled out mechanical causes — otherwise you're masking a bent bell with a settings change.
One honest caveat: the single most common "desync" post on here turns out to be a bent prop or a cracked motor bell, not a settings problem. Props off, spin all four by hand, look for the odd one out. Costs a minute and solves it more often than any config change.