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Given atmospheric condition, a typical common engine (say, Lycoming 4-cylinder), and a fixed-pitch normal prop:

What are normally the best altitudes for:

  1. ground-speed and
  2. fuel efficiency (range)?

Is it always highest (reasonable) altitude (say >=10,000')? the air resistance is low but so is the engine power, or is it usually some intermediate altitude?

Does it depend on the aircraft? Is a Cirrus SR-20 different from a Vans RV-9 from a Cessna 182 from a Piper Cherokee?

How does the efficiency of a piston aircraft change with altitude? covers a lot of ground about the primary forces (air resistance and (normally aspirated) piston/prop efficiency) and secondary forces. But it does not answer how these forces typically combine to create "best" altitudes (for mpg and gs). The question ultimately was about how these forces could create typical optimums at different alternatives, and whether they could be different for an SR-20 or a 182.

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First off this highly varies by aircraft. Piston aircraft that fall under the GA header these days have a pretty wide range of altitude capabilities. The Piper Archer I fly wont really get over 10,000 but you can get your instrument rating and grab a Mooney Acclaim which will take you well into the flight levels and get you up to 25,000 Ft.

best altitudes for ground-speed....

The simplest answer here is the altitude at which your tail wind component is the best. Wind speed and direction does shift with altitude and in many cases your [(increased performance/cruise time) + tail wind - (reduced climb speed/climb time)] is best where ever the tail wind component is greatest and your climb does not take impractically long.

Very Generally speaking performance increases with altitude to an extent. However climb rate usually decreases so there is a break even but it for sure varies by airframe and configuration.

fuel efficiency...

Again this varies by aircraft as well as how you chose to operate the plane. Things like running lean of peak can effect efficiency at the cost of speed typically. Some planes have little to no change in endurance at varying altitudes (although your change in airspeed may allow you to cover more distance). At 75% power my archer has almost no change in endurance as I climb.

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Variable pitch propellors also offer a great deal of power setting options that can allow you to tune your efficiency even further.

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  • $\begingroup$ thank you, dave. (per my question, the acclaim is not normally aspirated, however, and variable-pitch props are not fixed pitch.) but very important, wind speeds at altitudes will be of major importance here. going east, higher is probably better. I guessed that different airplanes may have different "best" altitudes, but for the same one, I had hoped to learn "higher within its max altitude is always better" or "usually intermediate is best." your answer suggests usually the former. thank you for the advice. $\endgroup$
    – ivo Welch
    Dec 12, 2017 at 2:06
  • $\begingroup$ @ivoWelch the east west prevailing winds is a factor generally speaking although in my personal experience day to day weather at lower altitudes (GA altitudes) and over shorter GA capable distances varies heavily. $\endgroup$
    – Dave
    Dec 12, 2017 at 2:27

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