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I haven't flown an aircraft with a CSU so I am unfamiliar with them and know only what I have read.
1. During runups of an engine fitted with CSU, if the pitch lever is positioned just before the point of RPM decrease, is this point the pitch full fine position (lever fully forward), or is it somewhere between the lever's full fine and full coarse position (lever fully back)?
2. If the lever were placed at this position, and the power was increased, would the RPM remain the same?
3. Finally, what does it mean if a CSU is "cycled"?
Hi. With a CSU, the propellor has a mechanism driven by oil in / oil out, and a system of counterweights. Basically for various engine RPM the CSU will keep the prop spinning at constant revs, thus makimng it more efficient - "bigger bites of the airflow".
During runups, the pitch is always 'full fine' to begin with" ie normal 1:1 relationship to the engine RPM.
The reason we cycle the prop is to get the warm oil flowing into the CSU and to check we get a 'drop in RPM as we make a severe adjustment into course pitch' and 'an appropraite small drop in oil pressure' during this test. Thus checking for correct operation of both the CSU and Oil Pressure. ie: we are severely lowering the propellor RPM relative to engine RPM deliberately. This is much a much greater than you would do in flight, and it is mainly to lubricate the CSU; and it is always at a specific engine RPM around 1800RPM or thereabouts to avoid over boosting the CSU as the pitch is placed into 'Full Fine' again. Generally three cycles of the prop are sufficient. The Pilot Operating Handbook gives specifics for each aircraft.
1.) The CSU will attempt to keep the RPM of the engine set to whatever RPM you set with the prop lever in the cockpit. During runups, you first set the prop levers to fully fine which locks them in that position. You then increase the MP until you are getting 1800 RPM (or whatever value the POH recommends).
If you now start pulling back on the prop lever, you unlock the fully fine pitch position and the CSU will now start to try and maintain whatever RPM you set it to. So, "just before the point of RPM decrease" would be the fully fine position since you setup 1800RPM using the MP with the props fully fine and any reduction in the prop setting will tell the CSU to start reducing the RPM.
2.) You do run ups with the prop fully fine. If you increase the MP you increase the engine torque. The props are locked in the fully fine position with the lever full forward so the increased engine torque will increase the RPM.
