Best Power vs Best Economy

[Charles Schefer]

I debated whether this really belongs on the chat forum as an "operational" vs maintenance issue but I ultimately decided to put it here since how we treat our engines ties into longevity and maintenance. Sorry if that's a stretch..

Do you ever feel you know and understand something and then later start to question it? That's the case for me with this question. To frame the question and for reference here is a picture of the PA-30 performance chart from the ICS (Killough) POH though I believe it is the same as Piper published.

As we can see from the chart, for each given % power setting we have for a given pressure altitude and standard temperature the expected manifold pressure (MP) and engine RPM settings required to achieve that % power as one would expect... However when it comes to expected fuel flow we have two options... the best economy and the best power settings which vary by as much as 2.6gph (in the 55% chart for instance).

So the question is... how can there really be both best economy and best power settings for the same % power? What does this really mean? Surely if I am flying along at say 65% power using "best power" settings and I lean the engine further to "best economy" then surely I am taking power away. How can "best power" and "best economy" BOTH be 65% power and deliver the same actual speed / performance? According to the other charts, 65% power delivers a specific speed at a specific altitude, and temperature. So if that's always the case (in theory) and I can have 65% power at two different fuel flows then why not always take the more economical fuel flow? That is of course assuming best economy AT peak EGT is not harmful to the engine - hey there's the maintenance angle If if it were harmful why would Piper publish it that way...

Thoughts appreciated...

- Charles

[Ed Asmus]

Hi Charles,

From what I understand about power charts, the power settings and percentages are likely based on "best power" mixture settings. So, if you select some given RPM, MP, at some OAT and altitude that the chart says is 65% power, then the mixture setting must be at "best power" to obtain that actual percentage of power in this case 65%. If you choose to lean to best economy without changing anything else then, as you indicated in your post, the actual percentage of power will be something less than 65%.

My guess is mixture management has become so much more important to pilots these days then it was in the 1950's and 60's that what we pilots would really like to see is one power chart based on all mixture settings being set at the "best power" setting and then a whole other power chart with mixture settings being based on the "best economy" setting. This way we might actually know what percentage of power we are actually at while using the "best economy" mixture setting.

As I'm sure you have heard or observed, there are multiple ways to obtain a given percentage of power. I don't want to start the whole ROP/LOP debate but you could set X% power at some RPM, MP setting leaned to ROP "best power" or the same X% power with some higher RPM and/or MP leaned to "best economy" likely LOP. The airspeed indicator would read the same in both examples but the fuel flows would likely be different, and so would the CHT's. I think the entire ROP/LOP debate is based on selecting higher MP/RPM leaned to LOP and cooler CHT's being better for the engine then some lower MP/RPM leaned to ROP "best power" and higher CHT's.

This is what I think I understand. Hope it helps a little!

Ed

[Charles Schefer]

Thanks Ed, that is helpful. I honestly hadn't thought of the idea they just set the chart based on best power but then offered alternate best economy settings. Thinking about it further I bet you're right and that makes sense.... Especially when you consider the likely thinking of Piper and their pilots of the 1960s.

Regarding ROP/LOP - that's a whole different (but related) issue that I have a lot of thoughts on and experience with but I'll have to share that later as I'm on a tight schedule this morning. I operate Cirrus aircraft LOP and the results are excellent but it must be done properly / correctly. I do NOT operate my PA-30 LOP - that's not possible (at least with my set-up). More on that another time and I imagine there are already threads on ROP/LOP.

- Charles