In helicopters what factors decide the horizontal tail stabilizer angle? By angle I mean that in some helicopter they are -6 deg relative to tail boom while in some -3 deg. These angle were remain fixed although provision is there to change the angle.
The stab angle influences the pitch attitude in cruise, allowing the body of the machine to be flatter (more nose up) than it would be without it, by pushing down on the tail with speed. It also dampens pitch oscillations of the body under the rotor disc. A designer will come up with a stab area and incidence that is estimated to provide the necessary down force for a desirable cruise attitude for a particular machine, then adjust as required during flight testing.
The Blackhawk takes it much farther; the stab is a moveable control surface that allows attitude control over a very wide speed range.