How to get rid of propwash? This is one of the last big questions in PID tuning. Raising D gain helps get rid of propwash, but high D gain can overheat and even smoke your motors. Up to now, there’s not been a great way to overcome this. In Betaflight 4.0, a new feature called D_min promises to give great propwash handling without the usual bad effects of high D gain.
But what is D_min? How do you enable d_min? How do you make sure d_min is working? How do you tune D_min? Let’s find out.
A PID controller’s job is to make the quadcopter do what the pilot is commanding via the transmitter sticks. The PID controller consists of a P, I, and D term, each of which has a different role in making the quad fly well. The D (short for derivative) term helps fix bounceback after sharp movements such as snap rolls, flips, and other such maneuvers. In addition, the D term helps prevent propwash oscillation. The problem with the D term is, when it is too active, you can get hot motors, or even smoked motors, and the quad can try to fly away at high throttle uncontrollably.
What is D_min?
D_min is a dynamic D gain introduced in Betaflight 4.0. When you are flying in a straight line and not making abrupt maneuvers, you don’t need much D gain. In those situations, D_min lowers D gain so that motors can stay cool. When you are making sharp turns and snappy flips and rolls, Betaflight boosts D to prevent bounce backs and propwash oscillations. With D_min, you can use a much higher D gain without risking damage to motors, since D will be low most of the time, except when it’s actually needed.
You can find the D_min feature under PID Tuning > PID Settings > D_min
The D_min value is the D gain Betaflight dials down to when you are flying in smooth straight lines. There are separate values for pitch, roll, and yaw. When you do sharp turns and snappy flips and rolls, Betaflight increases the D gain up to the value in the Derivative column (where the D gain is usually set).
Two other settings are related to this value. D gain controls how sensitive the flight controller is to ramping up D term, making it more or less likely to ramp up D term. The default value is probably the best value for most quadcopters.
D Advance controls how quickly the D term ramp up is when Betaflight feels D term must be increased. Again, the default value is probably the best value to use. Focus on setting D term and D_min appropriately.
How to tune D_min
If you have bounce back on flips and rolls, and/or propwash oscillation, raise D gain until those are eliminated. Should you raise pitch or roll? You can use blackbox logging if you want to precisely figure this out, or use trial and error until you fix the issue. (D gain on yaw should almost always be zero.)
As always when tuning D gain, make sure you test for hot motors frequently. If motors get uncomfortable to the touch, stop raising D gain! With the default D_min values, typical mini quads should be able to raise D gain as high as needed without getting hot motors. If motors do get hot, lower D_min to allow them to cool down. However if you want to cool your motors down, lower D_min. If this brings back bounceback and propwash oscillation, raise D_min Gain.
If your quadcopter is flying great with no hot motors, you can disable D_min by setting the D_min value on the three axis to 0. D_min can sometimes cause mid-throttle oscillations, so if you can disable D_min without getting hot motors, you should. If you reach a perfect tune and your motors are cool, consider raising D_min. If motors stay cool as D_min approaches D gain, you are probably safe to disable D_min entirely.
To tune D_min precisely, enable the debug OSD element and place it on your OSD where you see fit.
Next, go to the CLI and type:
This displays four numbers on screen. The 3rd and 4th number from the left are what we care about. The 3rd number is the current D gain for the roll axis; the 4th is the D gain for the pitch axis.
When flying in a straight line, you should see the D values be close to the D_min value. If they are resting at the D_min value without raising at all, raise D_min Gain.
When you enter a sharp turn, you should see the D values increase to about half to 2/3 the max value. If propwash is not well controlled, consider increasing either D_min Gain or D gain.
When you perform snap rolls and flips, the D values should always hit the max value. If they don’t, then D_min Gain is too low, or D_min is too low.