Is the purpose of variable pitch propellers to maintain an optimal angle of attack? 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?
What you know about variable pitch propellers is generally correct. If you make a slight change, your flight instructor would not have had any reason to object: They keep propeller blades near their optimum angle of attack across a wide speed range.

As the distance from the hub increases, the blade section gains circumferential speed while being in the same forward speed as all other sections. Therefore, the twist of the propeller blade should change linearly from the hub to the tip. If you twist the full blade at its root, you add a constant change to each cross section, so at high speed the root of the blade sees too high an angle of attack and the tip has too little angle of attack. Normally, the section at approximately 75% of propeller span is kept at its ideal angle for the best overall compromise. That is good enough for practical use.

Ideally, the propeller would spin faster with increasing flight speed, but that would require a gearbox and switching gears with the limited speed ranges of combustion engines, would deliver little thrust at the beginning of the take-off run and at high speed you would run into compressibility problems at the tips much sooner - practical high speed propellers spin more slowly than typical low-speed propellers.

If you look at the efficiency chart of a three-bladed variable pitch propeller below, you will see that efficiency peaks for one speed (given as an advance ratio in the plot below) and one pitch angle. This is when the full blade is flown at the optimum angle of attack for each section. This optimum would shift to higher speeds for less twist and lower speeds for more twist. Operating the propeller away from its design point incurs small losses since not all of the blade is exactly at its optimum angle of attack, but if you compare the overall efficiency with that of a single pitch setting, the advantage of variable pitch becomes obvious.



Efficiency chart of a variable pitch propeller. Source: McCormick B.W. Aerodynamics, Aeronautics &amp;Flight Mechanics. John Wiley &amp; Sons, Inc., 1979.