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For a cruise in constant-speed prop aircraft, you can choose different combinations of the manifold pressure and prop rpm to generate the same amount of power. For example, in order to generate 117 BHP (65% Rated Power) at sea level in the chart above, you can select either the combination of 24.5 for MAP and 2100 for rpm or that of 22.9 for MAP and 2400 for rpm. Here, what's the difference (if any) in selecting different combinations of rmp and MAP to generate the same power? Any benefits using the combination of low rpm and high MAP or vice versa?

This other post answers my original question, yet I have a follow-up question:

  • How does the combination of low rpm and high MAP extend the flight range?

To my knowledge, whatever combinations are chosen, as long as the combinations generate the same power, the same amount of fuel is consumed. The above chart supports that: The same 10.3 Gal/Hr is flowed into the engine regardless of the combinations in the box.

For your information, I asked the same question to my instructor, and he told me there's really no difference when it comes to power and fuel efficiency. But I was not persuaded at first, because I thought it was the MAP that controlled power output (BHP), and therefore, the less the MAP, the less fuel required. And a more efficient use of props could supplement this loss of power, resulting in the same thrust power. But the above chart just contradicts my reasoning. According to the chart, it's not the MAP alone but the combination of MAP and rpm that determines the BHP. But if it's the case, I don't understand how the range of flight could be extended by decreasing MAP and increasing rpm.

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  • $\begingroup$ Does Skip Miller's answer to this question not cover it well enough for you? $\endgroup$ – UnrecognizedFallingObject Mar 11 '17 at 21:52
  • $\begingroup$ @UnrecognizedFallingObject Looks like Skip Miller's answer covers my question. I'll edit my question. Thanks $\endgroup$ – lemonincider Mar 11 '17 at 22:32
  • $\begingroup$ @RalphJ I edited my question. $\endgroup$ – lemonincider Mar 11 '17 at 22:56
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The advantages of oversquare (MAP > RPM/10) are:

  • less vibration, noise, heat, and wear
  • longer engine life as a result of this
  • much less noise emitted from the engine/propeller
  • more efficient cylinder charging and better combustion, gives lower fuel consumption if leaned properly
  • less power lost to friction inside the engine

Source

Since there are fewer loses to friction and high propeller RPM, the same fuel flow will yield slightly faster flight, and thereby range.


Another article from AVweb that discusses the topic: Why "Over Square" Is Good

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  • $\begingroup$ I usually trust what Mike Busch has to say. But you also need to consider your specific engine before choosing RPM. Continental has recommended running my TSIO 520 at no less than 2300 RPM in cruise. “Teledyne Continental Motors (TCM) has examined recent occurrences of crankshaft counterweight release and consequent engine stoppage with four high time IO and TSIO-520 engine models. Initial investigation and service history lead us to believe that these occurrences are associated with engine operation at sustained cruise engine speeds of less than 2300 RPM. ” $\endgroup$ – JScarry Mar 12 '17 at 17:57
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Some things to consider:

1) Engine Frictional Losses tend to be lower when turning slower for the same power output. (less sliding and rolling, less oil splash on the crank, less pumping loss from air in the crankcase)

2) Big engines turning slowly at wide open throttle tend to be more efficient. It is easier to draw in the air slower through a wide open throttle (WOT) than turning faster at partial throttle.

3) Propeller Efficiency changes with prop speed, pitch, and airspeed.

Not giving you a definitive answer, but, for a given airspeed, there will be an efficiency peak that is determined by the points above.

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  • $\begingroup$ Thanks. By your last comment, do you mean it's not necessarily the combination of the highest MAP and the lowest rmp (for the same power output) that brings the greatest efficiency? $\endgroup$ – lemonincider Mar 16 '17 at 6:19

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