Originally Posted by
Lumps
Can someone explain this to me:
In the Piper Chieftain POH, for 65% power (230BHP) it lists 'best economy' fuel flow as 30.8 GPH (both engines). Seeing as the POH specifically prescribes LOP setting, one has to assume that 'best economy' in this case is LOP.
Problem is, these are 7.3:1 CR engines, so using 230 bhp ÷ 13.5 you get 17 GPH, or 34 GPH both engines LOP. How does Piper do it!?
* My guess is the 65% power in the title of the chart refers to the Best Power column.. seeing as TAS is lower for best economy. But it's a guess
I'm about four years late, but I can give it a crack.
In my PA31-350 manual (early), it describes in the footnote for the cruise performance chart that at best economy power, power is reduced by 4.5%.
so 60.5% of 350 is 211.75 hp, using 13.5 to find hp, so that's<!--td {border: 1px solid #ccc;}br {mso-data-placement:same-cell;}--> 15.68 gallons or 31.37 total gph.
However, I have a feeling the best economy the manual refers to is NOT lean of peak, given this plane is from 1973 and the reference to LOP is an update to the flight manual. I bet if you sit on 1650 egt (on the original factory gauge, which really is TIT, based on where the jpi probe sits) you could probably get to 30.8 gph... as your JPI is screaming at you with egt's over 1680 and hot CHTs. I've never tried it, but somebody let me know how you go
