Why does one motor behave differently?

The symptom

One of your four motors is not like the others. Pick whichever version you have:

• It is noticeably hotter to the touch after landing.
• It sounds different — a different pitch, a rougher note, a whine the other three do not have.
• In a hover, it sits at a visibly higher or lower output than its neighbours, in the Configurator's motor readout or in a Blackbox log.

Everyone's first instinct is that the motor is bad. Sometimes it is. Usually it is not.

Check this in 60 seconds

1. Balance the aircraft on your finger, or on the edge of a table, under the frame's centre. Does it sit level, or does it tip? Do this before anything else. It takes ten seconds and it is the most common answer.
2. Spin all four motors by hand, slowly, with props off. One that is stiff, notchy, or gritty compared to the other three has a mechanical problem you can feel.
3. Look at the per-motor outputs in a hover. Blackbox motor[0..3], or the Motors tab. Write down the four numbers.

That third step is the point of this whole article, and almost nobody does it.

The key insight: the FC is correcting for something

In a stable hover, with nothing wrong, all four motors should be doing roughly the same work. That is what a symmetrical aircraft with a centred mass looks like: four equal outputs, wobbling by a few counts around the same average.

So if the flight controller is commanding one motor consistently harder than its neighbours — not for a moment, but for the whole flight — that is not the motor misbehaving. That is the FC applying a permanent correction. The integral term has wound up to hold an offset, and it is holding it because something on the airframe is asking for it.

The question is therefore never "what is wrong with that motor?" The question is "what is the FC fighting?"

Motor output differences in a Blackbox log are one of the most informative diagnostics available to you, and they are almost entirely ignored.

The causes, in order of likelihood

1. The centre of gravity is off

A battery mounted 15 mm to one side. A payload bolted on where there was room, rather than where the mass balances. A camera, a gimbal, a dropped-in module.

The FC must run the motors on the heavy side harder, continuously, forever. There is no tune that fixes this and no filter that hides it — it is physics, and the FC is doing exactly the right thing about it.

This is extremely common on heavy platforms, precisely because heavy platforms carry payloads, and payloads are rarely mounted at the exact centroid. Balance the aircraft on your finger. If it tips, you have found it, and you should move the mass rather than trim it away — the same offset also shows up as a drift you cannot calibrate away.

2. A twisted frame, or an arm that is not coplanar

A motor tilted a couple of degrees out of the plane does not just push slightly sideways. It puts a component of its thrust into yaw, and the FC has to cancel that yaw for the entire flight.

Here is the diagnostic that earns its keep: look at whether the harder-working pair is a diagonal pair.

The two CW motors and the two CCW motors sit on opposite diagonals. A yaw correction is made by running one diagonal harder and the other softer — that is the only way a quad can produce yaw torque. So if your motor outputs come out as two high and two low, and the high pair is diagonal, you are looking at a permanent yaw bias. Something is twisted: a bent arm, a motor mount that is not flat, a frame plate that took a knock, a motor sitting on a burr or a bit of grit under one mounting foot.

If the high pair is adjacent — front two, or left two — that is a centre-of-gravity offset, not a twist. Go back to cause 1.

That distinction, diagonal versus adjacent, is the single most useful thing in this article. It costs you one glance at a log and it tells you which half of the fault tree you are in.

3. Prop damage or imbalance on that arm

A chipped tip, a bent blade, a prop that has been repaired, or — very common and very silly — a different prop model on one corner because that was the one you had spare. Different pitch means different thrust at the same RPM, and the FC compensates.

Fit four fresh, identical props. It takes two minutes and it eliminates the cause outright.

4. A mechanically stiff motor

A dying bearing, a bent shaft, grit inside the bell, a bell rubbing on the stator. The motor still produces thrust, but it burns more current doing it, and it runs hot. This is the one that genuinely is the motor's fault, and you can usually feel it by hand.

Note the difference in symptom: a stiff motor is hot but its commanded output may be perfectly normal, because the FC commands thrust, not current. Heat without an output offset points here.

5. A weak motor

A partially shorted winding, or a magnet that came loose in the bell after a crash. The motor produces less thrust for the same command, so the FC pushes it harder to keep the aircraft level — an output offset on that one corner alone.

6. An ESC out of step with the others

Four ESCs are only interchangeable if they are actually the same. Different firmware versions, different timing, different motor-direction or startup-power settings across the four will give you four slightly different responses to the same command. Read all four back and compare them field by field. If one was replaced after a crash, suspect it.

7. Motor screws that are too long

Screws that bottom out against the windings will chew into the enamel and cause a hot, weak, or eventually dead motor. Classic after any motor swap that used the screws that came in the bag rather than the ones that came off. Check the screw length before you buy a new motor.

The definitive test: swap

Once you have a suspect corner, stop reasoning and start swapping. Move the suspect motor to a different arm.

• If the problem follows the motor — it is the motor. Bearings, windings, magnets, shaft.
• If the problem stays on the arm — it is not the motor. It is the frame, the ESC, the wiring, or a bent mount.

This single test cuts the fault tree in half and it costs you eight screws. Do it before you order parts.

How to know you actually fixed it

Not "it feels better." The bar is a number.

• The four motor outputs in a steady hover are back to within a few percent of each other, in a log — not in your imagination.
• All four motors are the same temperature after a two-minute hover.
• The fix survives a different flight: forward flight, a yaw, a hard climb. A centre-of-gravity offset that hides in a hover will not hide in forward flight.

If the outputs are still uneven, you fixed a real problem but it was not the one causing this. Keep going.

A note on simulating this

The reason this signature is hard to learn is that in the field you only ever see the result, never the controlled experiment. You get one log, one aircraft, one unknown fault.

In a simulator that models mass and centre of gravity properly, you can run the experiment the other way round: put the payload deliberately off-centre, fly a hover, and look at what the four motor outputs actually do. Then twist an arm and look at the diagonal signature. You are not learning to fly — you are learning to read, and knowing the answer in advance is precisely what makes the lesson stick.