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How do cruise speeds change with altitude (standard piston prop)

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Aerodynamics, General Aviation

Ignoring all other factors (winds, time to climb, practical considerations like O2, etc...), how can you relate the maximum cruise speed to the cruise altitude for a normally aspirated (non-turbo), piston/propeller aeroplane? A C172M for example. I know that as the air becomes thinner, the engine cannot produce as much power, and the aerodynamic drag decreases. One of those makes you go slower, and the other faster, and I'm sure the relationship between the two produces a curve that may have a sweet spot that one should aim to cruise at to maximise their speed. Can you help me understand this relationship, and how to determine that sweet spot using the POH?

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2 Answers

  1. Mark Kolber on Jun 25, 2017

    Doesn’t your manual have a nice chart or graph that tells you this? It’s usually entitled “Cruise Performance.”

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  2. k-west on Jun 25, 2017

    Here is a link to the POH, it’s not immediately apparent to me how to find the sweet spot in the performance chart. I’m also interested in some more insight into the relationship, rules of thumb, general concepts, etc…


    Thanks everyone!

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