I recently had a discussion with an aviation expert, who works as a Flight Instructor, about constant speed propellers - variable pitch propellers - on light piston aircraft.
I have always known that these kind of propellers are capable of changing their pitch angle in flight in order to keep propeller blades at their optimum angle of attack across a wide speed range, while maintaining constant RPM. Therefore, they are very efficient over a wider airspeed range compared to fixed pitch propellers. A change in TAS in flight, for example, would lead to a change in pitch, in order to restore the optimum angle of attack of the propeller blades.
To my surprise, the instructor told me that what I knew about variable pitch propellers was not true: he explained to me that a change in TAS during the flight would lead to a change in the angle of attack of the blades, but that doesn’t mean that the resulting pitch change would lead to the optimum angle of attack.
His answer really surprised me, so I am now wondering what is the purpose of variable pitch propellers then. I understand that there may be a point where the PCU and CSU won’t be able to adjust the pitch to the most optimum angle of attack, but I thought that during normal operating speeds the purpose of this kind of propellers was to provide the blades with the most efficient angle of attack. Am I wrong?