For Jeff: Percent Power Calculation

Post Reply
revenson
Posts: 31
Joined: Thu Mar 21, 2013 12:00 pm

For Jeff: Percent Power Calculation

Post by revenson »

Question for Jeff: How do you calculate % power when lean of peak?

What I think I know: The numbers we enter to our EFIS for 55% and 75% power, as well as the altitude numbers, are based on engine manufacturer power curves that assume leaning to "best power". I'm sure you use these numbers to calculate percent power when operating rich of peak. I've 'heard' it said from other sources, that percent power drops off linearly with fuel flow once on the lean side.

How does my EFIS calculate percent power when we operate peak or lean of peak?

Thanks, Roger Evenson.
GRT_Jeff
Posts: 802
Joined: Tue Dec 11, 2012 12:11 am

Re: For Jeff: Percent Power Calculation

Post by GRT_Jeff »

The EFIS doesn't keep track of if you are rich or lean of peak and it doesn't adjust the percent power calculation for it.
revenson
Posts: 31
Joined: Thu Mar 21, 2013 12:00 pm

Re: For Jeff: Percent Power Calculation

Post by revenson »

Thanks for confirming that. I had noticed whether leaned to best power or anywhere lean of peak, the display of percent power does not change. Conclusion: If not leaned to best power mixture, I should ignore the display of percent power and SFC since those numbers can't be reasonably accurate.
revenson
Posts: 31
Joined: Thu Mar 21, 2013 12:00 pm

Re: For Jeff: Percent Power Calculation

Post by revenson »

For me, this has been mostly an exercise in understanding power in normally aspirated engines. My only practical application applies to flights at and above 8000 density altitude. Since I had been using lean of peak operation, even at quite high altitudes, I wondered how much power and speed I was giving up when operating lean of peak EGT. Peak EGT is where best economy starts, but power and speed drop off rapidly when leaner than that.

I recently found an article by Lycoming (Experts are Everywhere to Help You...The "New" Old Leaning Technique, SSP700), that gives a power number for operation lean of peak..."If leaning is initiated at 75% power and continued through 50 degrees F lean side of peak, the actual power output at that point will be approximately 69%."

This article also shows a generalized chart of engine trends vs. fuel flow. In the range between Peak EGT and 50 LOP, the power dropoff looks to be close to linear. From this chart I've estimated 2 additional power levels when leaning is started from 75% power:
Lean to Peak EGT: 75% x .96 = 72% Power
Lean to about 25 LOP: 75% x .94 = 71% Power
Lean to 50 LOP: 75% x .92 = 69% Power

This gives me a better idea of power levels and for calculation of SFC when leaning to peak or somewhat leaner.
Post Reply