Uneven RPM output across motors, how to fix?

Question

After enabling the RPM measurement on a Quad-X craft that flips on takeoff, I see that the front motors (#4 and especially #2) consistently deliver higher RPM for the same throttle input. Here are screenshots from Betaflight Configurator:

My first thought was that this could be related to the voltage drop in the wiring (the rear motors, #1 and #3, have longer wires), however, that doesn't explain a particularly high RPM value from the motor #2.

Is there a way to "equalize" the RPM output of all my motors in software? Why is such discrepancy even possible in the first place, when Electronic Speed Control must by definition achieve constant RPM, increasing the voltage on a motor with higher friction or poor wiring?

PS. My ESCs are 20A LittleBee modules flashed with Bluejay v0.16.

Answer 1

A flipping quad is not the result of such rather minor differences in RPM, it is more likely that one of your motors is spinning in the wrong direction, a prop is on wrong or the orientation of your flight-controller is wrong.

With Bluejay, the ESC has no internal feedback loop for input throttle and RPM. It merely response with the measured eRPM in case you are running bi-directional DSHOT. Everything else is controlled by the flight controller, it will compensate accordingly and adjust for the "weakest" motor.

As an anecdote: I once had an AIO where one of the ESC's turned bad - a shot FET most probably. It lead to one of the motors performing with a RPM difference of 70% in comparison to the other motors. You could feel that the punch was gone, but apart from that, the quad would still fly fine, everything adjusted down to the "wekest" motor.

Answer 2

If it's flipping on take-off, it's much more likely to be a reversed motor. Check which way each motor is actually blowing, then try to work out if the problem is the prop, the motor direction or the gyro orientation.

A minor imbalance in motor speeds won't cause a flip, as the flight controller should notice the roll and adjust the speeds to level the quad. It only flips when its correction makes things worse because something is reversed.

I've run quads with different size motors, different size props, and have even flown back from a crash with one blade missing (and seen a friend do it with 2 blades missing). The flight controller is amazing at handling small imbalances.

Minor differences at low speed are probably just due to slight differences in drag in the bearings. You're running at very low power, so it wouldn't take much to change the RPM. At higher speed it will probably even out.

Answer 3

I don't know your setup but if you are talking about a quadcopter then what you are experiencing is the controller actuating to keep it stable in a hovering position. It will never be the same rpm, not even at take off, might be similar but never the same cause it depends on many things being computed simultaneously like orientation, throttle input, angular velocity, calibration, etc.

From my point of view looks fine, if you are connecting directly without flight controller and just connecting the output to the ESC then something is off.