Final answer:
Collective/airspeed to yaw control effectiveness peaks at moderate speeds and declines at very high speeds, due to an increased relative air flow over control surfaces at moderate speeds and factors like compressibility effects or control surface saturation at high speeds.
Step-by-step explanation:
The question you've asked pertains to the relationship between collective/airspeed and yaw control effectiveness in flight. This concept is related to a field of physics known as aerodynamics, which is the study of the motion of air and its interaction with moving objects, such as aircraft. The effectiveness of yaw control is strongly related to the airspeed of the aircraft. At lower airspeeds, the collective (or thrust vector in helicopter terminology) and the airspeed have a greater influence on the yawing motion of the aircraft because of the relative motion between the air and the control surfaces. As the airspeed increases, the control surfaces have more air flowing over them, and thus they are more effective at controlling the yaw. However, at very high airspeeds, the effectiveness can start to decrease due to factors such as the onset of compressibility effects or control surface saturation. Therefore, effectiveness may peak at moderate speeds and then begin to decline as the airspeed continues to increase until a certain critical airspeed is reached.