I understand that the "Rudder Common Gain" is a multiplier, but I am not clear what it actually multiplies. Does "Rudder Common Gain" multiply ONLY the "Rudder Gain" exclusively, or does this "Rudder Common Gain" also multiply other "background" rudder parameters as well?

Hello,
the rudder gain is only the prortional part of the rudder controll-loop - the rudder common gain is only an factor of the porportional rudder gain - In some configurations the value with the multipier x1 is to less to get an proper behavior so you must the multiplier to 1,5 for example - afterwards you have to correct the rest of the control-loop (with I-Gain and D-Gain) - In the spirit you have here to set the rudder delay (among other things, servo-dependent) and the rest of the loop is the Advanced/Piro Consistency -

Very well described how to set rudder and cyclic see here:
http://manual.spirit-system.com/index.php?title=Tuning_Guide

Best Regards
Matthias

So to be 100% clear, does it seem fair to state that the "Rudder Common Gain" directly multiplies only and exclusively the "Rudder Gain" value without directly multiplying any other value whatsoever?

Yes, this is what is also written in the manual and Spirit Settings software tooltips. Only simple multiplier of the Gyro Gain.

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Spirit System developer