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Why is 75% power in Aircraft engines not 75% of Max RPM

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Lendo

Well-Known Member
Joined
Feb 6, 2013
Messages
935
Location
Brisbane
As we all know HP is Cubic Inches x RPM/ 5252 = HP.

For Instance the Cessna 172 Lycoming O -360 rated at 180 HP, is at Max Power at RPM of 2700 and Cruise at 75% power is at 2,500 - however 75% of 2,700 is 2025 RPM not 2500 RPM. We see that 2,500 rpm is only 92.599%.

Looking further we can find that 3333.3333 RPM is 100% if 2,500 rpm is 75%.

Now I can assume that running the Engine at max RPM may not be good for it - hence to reduce the RPM for longevity, and/or max power may drop-off over 2,700 rpm. There is the issue of Prop Tip speed and prop Diameter to consider as well.

I'm also wondering if keeping the Max Power at 2,700 and Cruise at 2,500 RPM keeps the Propeller specs closer together - for better overall efficiency of fixed props.

I would be interested in any other suggestions.
George (down under)
 
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