I flew a short cross-country up to College Station and back the other day, and was a little bored, so I started playing around with the mixture control and watching speeds and mpg. You see, I usually fly with the mixture as lean as I can get it (always LOP, even with the carb) without undue roughness. The bottom line is that I really like having lots of RANGE – I guess I tend to maximize range because I hate losing all the time it takes to descend and climb back after a fuel stop. At an altitude like 8,000’, where WOT gives you roughly 75% power on a standard day, you can (as has been discussed many times), use the mixture knob like a speed control. I hadn’t really realized how much though – as I played with it the other night on my way home, I was getting about 160 knots (True) leaned as far back is it would run. That gave me about 7.8 gph, a good 1.0 gph below the Lycoming fuel flow/percent power lien on the “Best Power” graph. Richened up to “Best Power” (a touch under 9.0 gph, and essentially at peak) and I was doing 173 (True). The GRT EFIS gives me real-time miles per gallon as well – running as lean as I could go, I was up to about 20.0 mpg, and as I richened it up, that dropped to 18.9 mpg. I had a little headwind, which affects mpg directly of course.
So if I really want to go as far as possible on a tank of gas, I want to lean as aggressively as I can (duh!) – it does affect the “bottom line” when it comes to range. Giving up 10 knots sounds like a lot – but if you can get rid of a descent/fuel/climb cycle, you will save a lot of time in the long run – especially if that fuel stop is south of Phoenix on a summer afternoon, and your climb back to altitude has to be modulated to keep the oil temp in check!
Paul
