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2 Answers

Manifold pressure vs RPM settings

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Aircraft Systems, Commercial Pilot, Flight Instructor, Instrument Rating, Student Pilot

I know that for any given RPM, there is a manifold pressure that should not be exceeded. I tried to search the C172 RG POH but I didn't find any table that shows me the max MP for a given RPM. My question, how can I determine this limitation?

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2 Answers

  1. Kris Kortokrax on Nov 20, 2014

    If it were a limitation, it would be found in the Limitations section of the Airplane Flight Manual (or POH). It is not a limitation, although I would not pull the prop all the way back and shove the throttle to full. You will see in some manuals a warning to not let the RPM drop more than 500 RPM when exercising the prop during a runup.

    Are you talking about the Old Wive’s Tale that says that manifold pressure should never exceed RPM/100? (i.e. 25″ and 2400 would be an invalid power setting)

    Consider that on every takeoff, your manifold pressure exceeds RPM/100.

    In the Performance section, you should find a chart that lists MP/RPM combinations that will yield a certain percentage power at given altitude/temperature combinations.

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  2. Mark Kolber on Nov 20, 2014

    There may be some ultimate limits that an engine guru can teach us all but I’m not aware of any POH “limitations” on the subject. “Oversquare” settings exist in all the power tables I’ve seen, some pretty large ones in the case of turbocharged engines. The normal takeoff in a Turbo Arrow, for example, calls for 2575 RPM and 41″ MP, more than a 15″ difference. . I have seen a few aircraft placarded against certain combinations, but other than that…

    Unless you are interested in the purely academic exercise, remaining Within the tolerances of the POH should keep you comfortably within the “safe” area. Beyond that, even if not an engine health issue it’s certainly an engine efficiency issue.

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