Once you, the pilot, have selected a specific rpm, the system takes over — Page 164, Lesson 220

Right, so we've just covered the idea that the propeller is operating between its internal fine and coarse pitch stops. That's the physical range of movement the blades have. Now, the key point I want you to take from this next part is the automatic control loop. Once you, the pilot, have selected a specific rpm, the system takes over. That's the job of the control unit. It goes by two names depending on the installation: the CSU, which stands for Constant Speed Unit, or the PCU, the Propeller Control Unit. They're the same concept, just different manufacturers' names. Here's what that unit does. It automatically varies the propeller pitch angle. Remember, pitch angle is the angle between the blade chord line and the plane of rotation. By changing that pitch angle, it changes the blade's angle of attack relative to the prevailing relative airflow. That's the airflow the blade actually sees as it moves through the air. The whole purpose of this automatic variation is to maintain the selected rpm. So if airspeed changes, or manifold pressure changes, the unit senses the rpm trying to drift and adjusts the pitch to hold it steady. If the aircraft speeds up, the propeller wants to windmill faster, so the unit coarsens the pitch to keep the rpm where you set it. If you slow down, it fines the pitch off to keep the rpm from dropping. That's the constant speed action. Now, one more thing this passage introduces. Variable pitch propellers can also incorporate a feature called "Feathering." I'll just name it for you now, and we'll get into the full advantages of it later in the chapter. But for now, just know it's a feature available on variable pitch propellers, and it's significant enough that we'll dedicate time to it shortly.