Skippy, it is extremely simple.
You have 3 control levers, Throttle (black), Mixture (red) and the Prop control (blue).
The Throttle and the Mixture levers are connected to the carburettor or the injector and the Prop control is connected to the CSU (Constant Speed Unit).
In order for you to understand the way that you control the prop I have to give you a quick explanation on the operation of the CSU.
The CSU is made of three components (there are more but they are not relevant for this discussion)
1. Weights
2. Spring
3. Valve Selector
There are two weights by the shape of ‘L’ facing each other (one looks like an ‘L’ and the other will be a mirror image creating two vertical lines and one horizontal line).
In the centre there is a valve selector that seats on the horizontal lines of the ‘L’ shaped weights. This selector is fitted in 90º to the weights (in our case vertically). The spring is fitted in the centre and pushes against the horizontal line of the ‘L’ shape i.e., pushing the weights and the selector valve down around a shaft that is fitted in the corner between the vertical and the horizontal lines of the ‘L’.
When the engine runs the weights of the CSU turn around the selector valve, by doing so, a centrifugal force is created and the weights want to be thrown out, as a result, the vertical part of the ‘L will move outwards, by doing so it lifts the valve selector. When the engine slows down, the centrifugal force will be reduced and the weights move inwards (by the force of the spring), letting the valve selector down.
When you operate the Prop Control you do one thing, you change the spring’s force against the horizontal portion of the weights.
When the Prop Control is fully forward, you mechanically lock the CSU weights and the valve selector at a position that does not allow the engine oil to enter or exit the prop causing it to stay in fine pitch at all times. Any other position of the control will cause to a different spring pressure against the weights. Both forces (spring’s and centrifugal) wants to be in an equilibrium.
On take off, the control is fully forward, hence, the CSU is locked and the prop is in a fine pitch. You throttle back to 25” and than pull the prop control to 2500rpm. What you actually do, you reduce the pressure of the spring on the weighs, allowing them to open, that in turn causes the valve selector to go up which allows some oil to escape from the prop, causing the prop’s pitch to become coarser (and vice versa).
On the ground, the engine power and RPM are controlled by the throttle only, however, in the air, power is controlled by the throttle (hg”
and the RPM by the Prop Control. Yet, you must be careful not to overboost the engine above its limits (usually max 3” over square). You will find that every engine has a power setting table that will provide you with the correct throttle/RPM combination for 55%, 65% and 75% power settings, it also will tell you what is the fuel flow for each settings. As a rule, if you don’t know what is the fuel flow for 75% power use full rich or you will damage the engine.
Prop Control forwards = fine pitch
Prop Control backwards = coarse pitch
If you are still confused, find a diagram of a CSU and ask your mechanic to explain you how it works.