Well, the CSDs I’m familiar with do have hydraulic motors (some IDGs do it all electronically but I won’t even take a stab at describing what kind of black box magic they use). CSUs/PCUs are something completely different and don’t. Excellent description of the latter though.
Except for one minor nitpick: Clouds typically aren’t denser than the surrounding air. If they were, they’d sink but they don’t. In fact, air inside clouds tend to be rising... at least until they become CBs, and by then their density probably isn’t the property you’re most concerned about.
But as you said yourself, it is really secondary and does not invalidate your description of the inner workings of a PCU.
With the clouds out of the equation, increased airspeed and/or increased shaft power are the two main reasons for increasing the blade pitch to keep the prop RPM constant.
I’d like to add that propeller control oil pressure can work towards either fine or coarse pitch in a single-acting prop. In multi-engine installations, you want the prop to revert to coarse/feathered if propeller control oil is lost. You have one more engine to fly on, and if the engine dies it won’t get stuck in fine pitch with the associated windmilling and drag. You’ll want to prevent this from happening on shutdown though, as starting a recip engine (or a turboprop engine without a free turbine) with a feathered prop puts too much strain on the engine.
In single engine installations, you want the prop to revert to fine pitch if oil pressure is lost. Your maximum airspeed will drop, but you will be able to continue flying and you will have full power available at low speeds, should you need it for say a go-around.
You can have centrifugal weights on the prop assy, pulling the blades to a predetermined pitch should oil pressure be lost.
Finally, a few questions which might or might not be stupid. How would a single acting CSU be used on a double-acting prop? What would drive the prop back? Would you apply a constant reduced oil pressure to the other side? And if you are prepared to go through that hassle, what would prevent you from using a double-acting CSU on a single acting prop, leaving the “spring side” oil pressure free to drain to the sump (preferably through a restrictor).
I’m fairly sure that the gas coming out of the combustion chamber is what drives the turbines - which in turn transfer power the gear boxes. Interesting typo.
The exception would be that in some engines, power to spin up the engine during engine start might come through a gear box. E g, you can use a generator or hydraulic pump as an electrical or hydraulical engine to get the engine to turn.
I thought I was doing good trying to avoid those airport hotels... and look at me now.