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Asked by: 1663 views Private Pilot


Some questions RE the CSU, preferably in a Cessna 172RG

1. How/can a pilot select a setting, ie: MAP, RPM, etc., that will provide the angle of attack which produces the best thrust/torque propeller ratio in a similar way to how an IAS is designated for the best lift/drag ratio, eg: the POH's performance charts?

2. Once the aircraft levels out in cruise, & a particular RPM is set, under what conditions (eg: climb, descent, other?) would the TAS increase or decrease significantly (thereby changing blade angle), for the purposes of proving to the textbook student that the prop's most efficient angle of attack is preserved?

3. From it's very definition, Constant Speed Unit, can you please confirm that it's the prop RPM & not engine RPM that's been maintained?

Sorry if the questions we're too confusing.



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1 Answers

  1. Bob Watson on Feb 05, 2014

    I’m not sure I completely understand the nature of questions 1 & 2, but it seems to me that with max thrust, you’d have max speed in level flight or max rate-of-climb in non-level flight (all other things being equal). Unless I’m missing something, you’d look for those numbers in the POH.

    Re: 3, if the propeller is bolted to the engine’s crankshaft, how could the engine RPM be different from the Propeller RPM? I suppose if there’s a reduction gearbox, but then the two speeds are still linearly proportional. I don’t know of any viscous gearboxes on a plane where the two RPMs would be different. Maybe in a turboprop engine, you might have a case where the engine RPM could be non-linearly proportional to the prop RPM. In any case, the prop governor changes the propeller’s blade angle to vary the load on the engine in order to maintain a constant RPM. So, while it directly varies the propeller RPM, the fact that the propeller is mechanically coupled (bolted) to the engine (in most piston engine installations) means it also varies the engine RPM.

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