I'm trying to learn a little bit about power settings. When I flew a 172, I looked in the POH for a throttle setting that would get me what I wanted...climb, cruise, speed or better fuel burn...it was pretty simple and straight forward. With a constant speed prop, I'm a little more confused, and the lack of info for the M4-220C in the Pilots Manual doesn't help. I've included a performance chart that was with the plane when I first got it. According to the chart, at 65% power, there are several settings that achieve the same thing. My question is, for what reason would you select one over the other, unless it was to reduce noise? Is higher rpm, lower mp better than a lower rpm, higher mp? I also just finished an article that said "running over square" is not an issue like it used to be( P+P magazine). What would be good settings if you wanted to fly slow, down to 80-90 knots?

http://www.boatshopak.com/m4-performance-chart.jpg

Realistically all these power settings are kind of useless as if you look at the density altitudes involved, you probably won't be able to make the MP required to carry these power settings at altitude except on cold days... I'd use the lower manifold pressures, higher RPM's on the Franklin as it's a high RPM engine, and you'll make the lower MP's at altitude... I'd suggest normal cruz at 24" and 2600 to 2650 RPM or 25" squared, which is easy to remember and gives you somewhere around 73-75% power setting in cruz... I think that anyone who has run a Franklin for any period of time will tell you the same, it likes RPM's within reason. I believe in the KISS principle, and it's alot easier at my age to remember a couple of power settings that work most of the time then 9 or 10 settings that the majority of time almost never work? Running oversquare will not hurt most modern aircraft engines provided you have good, rated octane avgas. I wouldn't be so confident with auto gas though? That's my opinion based on my experience, I'm sure there are those out there who might disagree though... I think you'll also find that any power setting in the 70-75% range on the Franklin 220, when leaned properly will get you about 10GPH fuel burn and that won't vary whether it's High or Low MP vs RPM setting. Bottom line you've got to burn that much gas to make that HP...

aero101 wrote:Bottom line you've got to burn that much gas to make that HP...

Kinda boils down to that doesn't it.