Conflicting quotes from theA Dash-1:
#1 PITCH AND ROLL TRIM CONTROL SYSTEMS
The pitch and roll trim control systems are similar in function. Both systems act on the artificial feel devices to vary the zero load position of the stick and equivalent surface positions.
Trim is provided by two independent, electrical circuits: the normal pitch/roll trim control circuit and the emergency override
pitch/roll trim circuit. These circuits lead to a trim motor which acts on the artificial feel devices to reposition the actuators and move the entire elevator surface.
Trim is provided by two independent electrical circuits: the normal pitch/roll trim control circuit and the emergency override pitch/roll trim circuit. These circuits lead to a trim motor in each wing. The trim motor acts on the artificial feel device, which in turn repositions the actuator to move the entire aileron surface.
The ROLL CONTROL SYSTEM SCHEMATIC is another source of study. Linkage path is mechanical stick-fuselage-wing-tab shifter-artificial feel device and then interfaces with the hydraulic servo-actuator and then finally the control surface.
Quote #1 suggests that if you hold the stick in a given position and actuate the trim switch, the only changes will be to the feel force. The positions of the linkage, stick, hydraulics, and surfaces will not change. Quote #2 suggests that (maybe?) trimming actually repositions the actuators even if the linkage/stick aren't allowed to move.
My money is on the #1 interpretation. I believe #2 is saying "Changes in artificial feel neutral position will result in actuator motion given that the stick is allowed to move with the changing forces.
" Otherwise I cannot reconcile the two.
Testing can be tricky in DCS with a spring-loaded joystick. Joystick input is pilot arm force and not direct manipulation. The A-10 stick can move when forces change even if the joystick is held stationary.
Aside: The genius of the artificial feel device being located outboard of the tab shifter and before the aileron is that it is only in effect when the tab shifters are normal. When the tab shifter shifts, the artificial feel device is not in the linkage path anymore. Stick forces would then be solely due to air loads on the aileron tabs. Such is not the case for the elevator and one feels both real and artificial loads combined in MRFCS. For the rudder there doesn't appear to be an artificial feel at all.
Your explanation certainly describes the behavior seen in A-10C for a non-forcefeed back stick (i.e., adjust trim and the stick moves along with the control surface), although I think this occurs even before the hydraulic system is powered up. I don't know whether the P-51 has a hydraulic control system, but I see the same observation: stick and control surface moves with application of trim.
Below a certain hydraulic pressure pitch and yaw control automatically revert to MRFCS mode, roll control however does not (different mechanism). This explains why that yaw and pitch control is possible even with a completely "off" jet while roll control is not possible.
Try trimming the P-51D on the ground with the engine off and see what moves. While in flight trim will alter the air loads which will make it difficult to hold the stick stationary.