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There are two kinds of needle/ball/tc instruments in a real plane...in one, the needle is only affected by the real turn rate ("no roll") - in the other, BOTH roll and turn rate affect the needle. In a real plane the gyro angle is the determining factor on which one you have. A no-roll TC is really just a rate-of-turn indicator...the 'theory' is that if you have an artificial horizon failure (or a vaccuum failure that nukes your azi art horizon but not your TC) then having roll and turn rate in the TC is useful. I say 'theory' because my aero instructor was a big critic of this. Anyway, enough background - perhaps I am just stating what you arleady knew. :-)
Anyway, both are in degrees per second...according to Austin's notes, a 3 deg/sec turn will cause a 20 degree deflection on the no-roll variant and a 34 degree deflection on the roll variant - why...I have no idea. He just seemed to pick this. Basically these datarefs take the plane's raw roll rates ("P, Q and R") and map them based on yaw and some scaling constants.