What is the force to change the pitch of a variable pitch propeller?
In a well-designed propeller the hinge axis is very close to the line connecting the centers of pressure, so the aerodynamic pitch moment is rather small. The main force is friction; note that the centrifugal pull of the blades at the hub will put a considerable load on the blade bearings, so friction goes up with rotation speed. Regular propellers have plain bearings; if you know the mass of the blade, the rotation speed and the coefficient of friction, you can calculate the breakaway torque of one blade.

Since propeller blades have some (but not much) camber, the center of pressure moves forward with increasing angle of attack. This will drive the blade away from a medium angle of attack; in other words, the aerodynamic moment is destabilizing the propeller pitch.