Variable pitch propellers
B737Dude, the primary reason for having a variable pitch propeller is to enable the engine/propeller combination to operate at the most efficient engine RPM/Propeller Blade Angle of Attack. At low airspeed the propeller is most efficient at lower pitch settings and high RPM. As airspeed increases the propeller needs to move toward a coarser pitch to maintain the most efficient blade angle of attack, hence the Constant Speed Unit (CSU) provides a means of selecting and maintaining a constant RPM. This is achieved by setting the CSU, via the pitch control in the cockpit, to maintain the desired RPM. The propeller pitch is able to be varied by the action of the Pitch Change Mechanism (PCM). Engine power is set by adjusting the throttle to obtain the desired Manifold Air Pressure whilst at the same time being able to maintain the selected RPM. You will have noted in an aircraft fitted with a variable pitch propeller that on descent with a lower than cruise power set, RPM will remain as selected as long as airspeed is sufficient to maintain that RPM when the propeller reduces pitch (blade angle) to the fine pitch stop within the propeller. You select Fine Pitch with the Pitch Control during the approach so that full RPM, and thus full power, is available in the event of it being required e.g. in the case of a missed approach and go around.
Last edited by Old Fella; 3rd Apr 2012 at 11:50.