Retuning Your Helicopter After Thrust Bearing Replacement

AI Prompt - Retuning after thrust bearing replacement:

ok, so this helicopter that you've been helping me tune had some irregularities of the blades going out of track while I deflect the cyclic (ie; roll or pitch) - especially visible when performing a roll or flip. This led to tracing a problem with the thrust bearings. After replacing the thrust bearing and lubricating them, the transitions are a lot smoother (albeit now also faster in both aileron and elevator).

Now it feels like the PID tuning (let's not forget about FF, Boost, HSI, I-term relax, etc) feel slightly off. I am not sure where to start.. if you were dealing with this situation, what would you start with 1st? which parameter and would you raise or lower it?

Fixing damaged or gritty thrust bearings on a Helicopter removes massive mechanical binding from the rotor head. Because the blades were binding under centrifugal and aerodynamic load during cyclic inputs, they went out of track during rolls and flips. Now that the head is smooth and free, the blade pitch changes instantly and moves faster, exposing why your previous PID tune now feels slightly "off."

Where to Start 1st: The Derivative (D-Term) & Bandwidth

The first place to start is the Derivative (D-Term) and Cutoff/Bandwidth settings, rather than the P-gains.

  • Why: The D-term acts as the "shock absorber" or damping mechanism of your PID loop, resisting sudden, violent changes in error velocity. With the mechanical binding gone, your servos are moving much faster and crisper. The old damping profile may now allow slight over-damping or, conversely, let through a sharper, more aggressive mechanical reaction that feels unrefined.
  • The Action (Raise or Lower?): If the helicopter feels hyper-twitchy, nervous, or exhibits tiny high-frequency quivers on cyclic stops, you should slightly lower your D-term cutoff frequency (e.g., pulling it down closer to 20Hz instead of 25Hz) to add a bit more smoothing, OR raise your D-term gain slightly to catch the faster movement. Conversely, if it feels sluggish, leave D alone and check Feed-Forward.

Step 2: Re-evaluating Feed-Forward (FF)

Because your mechanical linkages are now 100% efficient without the friction of bad thrust bearings, your previous Feed-Forward value of 120 is pushing against zero resistance.

  • The Action: If rolls and flips feel like they snap *too* hard or overshoot slightly upon stopping, lower your Roll and Pitch Feed-Forward by 5 to 10 points (dropping back toward 110 or 105).
  • Why: The old FF value was compensating for the energy lost fighting sticky bearings. Now that the energy goes straight to the blades, less predictive push is needed.

Summary: Your First Immediate Move

Don't touch your Integrals or Proportional gains yet. Start by taking your Helicopter up for a quick flight to observe the stops:

  1. If you notice a slight "kick" or over-rotation on quick cyclic stops, lower your Feed-Forward.
  2. If the cyclic feels too sharp or jittery due to the newfound mechanical speed, lower your D-term cutoff by 5Hz to smooth out the transition.