FW: TRIM_PITCH vs PWM_MAIN_TRIM2

#1

Hello all,

What would be the correct way to apply trims to the config of a fixed wing? TRIM_PITCH, PWM_MAIN_TRIM2 or even RC2_TRIM?

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#2

Hi @MBarros , it depends of what you’re looking for,

  • RCx_TRIM applies trim to the signal received from the RC transmitter. These parameters are automatically set during RC calibration.

  • PWM_MAIN_TRIMx applies trim to a PWM channel, after mixing. Use that to finely align the control surfaces to default angles before flying.

  • FW_PSP_OFF applies an offset to the pitch setpoint. Use that to set the angle of attack at which your aircraft need to fly at cruise speed.

  • TRIM_PITCH applies trim before mixing. For example, if you have two servos for the elevator, it will apply trim to both of them. Use that when your control surfaces are aligned but the aircraft pitches up or down during manual (not stabilized) flight or if the control signal has a constant offset during stabilized flight.

The correct order is then:

  1. Trim the PWM channels (PWM_MAIN/AUX_TRIMx) your aircraft on the bench to properly set the control surfaces to their theoretical position

  2. Fly in stabilized mode at cruise speed and set the pitch setpoint offset (FW_PSP_OFF) to desired angle of attack.

  3. Look at the logs and set the pitch trim (TRIM_PITCH)

Step 4 can be performed before step 3 if you don’t want to have to look at the log or if you feel comfortable flying in manual mode. You can then trim your remote (with the trim switches) and report the values to TRIM_PITCH (and remove the trims from your transmitter) or ask someone to update TRIM_PITCH directly during flight via telemetry and QGC.

Tell me if anything is still unclear,
Good luck!

#3

Thank you so much for the response. It is really well explained and everything makes sense.
I don’t know if it is possible, but I really think your answer or something similar to it should definitely be in the official docs.

Thanks again!

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