Clutchless servos based on stepper motors all have a bit of drag. Our servos are fairly low in this regard despite fairly strong motors.
The reason for this is that they contain an electronic version of a "clutch" which disconnects the motor electrically so any voltage generated by the motor (which you do by turning the shaft) can't result in generation of power (i.e. no current can flow).
It's largely academic though. Once installed your leverage on these shafts is enormous with a corresponding reduction in required force needed. On our aircraft the controls are extremely light, not just on the ground but also in flight - I can just feel the motor on the ground if I move the stick rapidly - in flight there is simply nothing I can notice.
This is about the only (very minor) downside of a stepper motor based servo plus perhaps the weight (which can be a bit more compared to a DC motor based system). Everything else is on the upside:
Much reduced mechanics - almost nothing to go wrong. Results also in less "slop" or play due to gear tolerances. Much better positioning ability as the motor moves in discrete, easily controlled steps. Makes it easy to precisely control speed and position changes. If you can control the motor current you can control the "breakout" point - the point where your force can overdrive the servo - much like a clutch does but with no wear and exact control.
I like DC motor based servos as well - make no mistake. Well designed servos of this nature can be extremely good. But they have to be designed and made very well to be good.
Rainier
CEO MGL Avionics
