Revolution Curve also known as revo mix.
Used in helis with no gyros or with a gyro in normal-rate mode only.
It adds opposite rudder to changes in head speed and or pitch from main blades. Not to be used with gyros in heading hold setting.
It "acts" like a gyro in heading hold, although not for 3D flying.
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SUB TRIM is used to make up for centering a servo horn to a specific degree such as 90, since not all servos centering correspond to the spline to servo horn angle needed.
Given the adjustable linkages and or the ability to turn a horn to find that perfect angle, sub trim is not critical. Also, newer FBL units have a means to do such in their programming.
If the manual to your gyro states to not use sub trim, then dont.
Not to be confused with TRIM, which is seen as an active stick input.
~~Enjoying the hobby one flight at a time~~