Tune INAV PIFF controller for fixedwing

Description of PIFF controller:

The FF-gain should do most of the work steering the airplane, leaving only P and I controller to fight turbulence and drift.

1: Figure out the maximum rates your airplane can do for each axis (pitch, roll, and yaw)

• Fly in MANUAL mode (called PASSTHROUGH mode up to version 1.8.1) with the manual_roll_rate, manual_pitch_rate and manual_yaw_rate settings set to 100%. Have some way of recording the flight: blackbox, DVR or both. Do hard rolls, hard loops and one 360° yaw turn. Use full stick deflection on all these maneuvers.

  • To calculate an axis' (approximate) rate from a DVR recording you'll need to count the number of frames it took for your aircraft to do a complete maneuver (roll/flip/yaw turn), determine the average FPS of the recording, and then use this formula: 360 / (number_of_frames / FPS). You can take multiples samples and average them for a better accuracy.
    You can also use a Python script to help automating the process.

• Note down the maximum rates. Typical values are 360°/s on roll, 100°/s on pitch and 60°/s yaw.
  Enter these values as your rates in configurator.

2: Zero out P and I gain on Roll, Pitch and Yaw controller and set tpa_rate to 0. Increase FF-gain (D column in the PID tuning tab) until you get 90% of full servo throw when having sticks at full throw in ACRO mode (no flight mode enabled) compared to manual mode.

• This is so the FF-gain does most of the work turning the airplane, but leaving some for the P and I gain to work with.
• For this step it is convenient to have the two modes MANUAL (called PASSTHROUGH mode up to version 1.8.1) and ACRO available on a switch to be able to switch easily between the two to compare the throws.
• The 90% deflection value can also be calculated by dividing 13950 by the maximum rate for the axis, e.g. 360deg/s maximum roll 13950/360=38.75 FF. For 80% deflection, divide 12400 by rate.

Now set a little P and I gain as a starting point, for example: 10 P-gain and 15 I-gain to Roll, Pitch and Yaw axis.

3: Go out and fly in acro mode.

• If airplane drifts to one side or up and down add I-gain to the axis it drifts in.
• If you want more stabilization against wind try and add more P-gain.

4: Want to calm your airplane down? Now is the time to reduce rates to fit your needs.

• Note: It's normal to get reduced servo throw when reducing rates at this point, if you got full servo throw at this stages you would overshoot the target deg/s you wanted.

5: Tune Angle / Horizon mode

• Enter Angle mode. If your aircraft doesn't fly straight and level your FC is probably not mounted flat relatively to the aircraft's natural attitude when flying (most planes and wings actually fly with a few degrees of nose-up attitude to maintain their altitude). You'll need to trim your board's alignment (align_board_roll, align_board_pitch, align_board_yaw) accordingly. After each adjustment fly again and check if the behavior has improved.
• If you are unhappy with the value of maximum bank/pitch angles, you can adjust them via the max_angle_inclination_rll and max_angle_inclination_pit. Be aware that if you want the same amount of maximum angle for poshold/althold you will also need to increase their values (nav_fw_bank_angle, nav_fw_climb_angle, nav_fw_dive_angle).
• If you are unhappy with the strength of the Angle mode, for example if it levels out too quickly/hard, adjust P-gain of the level controller via fw_p_level.

Other tuning tips:

• Setup your TPA correctly. PID Attenuation and scaling

• If your plane over corrects when RTH is engaged (symptom is a wave-like flight path), try increasing nav_fw_pos_xy_p and/or increasing nav_fw_pos_xy_i. Good values to start: set nav_fw_pos_xy_p = 50, set nav_fw_pos_xy_i = 5. You can also try lowering nav_fw_pos_xy_d.
  When P & I are too high the symptom is fast wandering left-right by a small amount (less than 5 deg). In that case you should try to decrease nav_fw_pos_xy_p and/or nav_fw_pos_xy_i or increase nav_fw_pos_xy_d. The behaviour of the plane is very different with or w/o wind, so it is necessary to test and tweak parameters in both scenarios.