I flew a planned 160 miles out to remote coast, a hour of touring out in that area, and 280 miles back to the nearest fuel stop. I planned fuel carefully, and allowed for forecast winds. But they came up more and earlier than forecast. 2.5 hours out on my flight inbound for fuel, I could see that my reserves would be well used making up for the increased headwind. My back up plan would be to land on a lake back of my runway destination, and ask my flying partner to fly some fuel back out to me, which he had agreed to do if needed. But, getting to the runway and fuel was certainly the preferred outcome.
So I optimized the power setting, and played around with Lean of Peak. I have never been a proponent of LOP. I always lean 50 ROP, and the engine seems to like it. I think that 50 ROP is simply cheap insurance against engine maintenance. But, I’m not hard over one way or the other, to each their own. However, this flight, I would want every drop at the end, so I may as well start saving it from the beginning. I had the power setting such that the plane was flying at the best endurance speed, and if I wanted to climb, I enrichened the mixture a little. The resulting increase in power would offer a 100-200 FPM climb, then back to LOP when I had climbed as required. I did not touch the throttle or prop.
I carefully emptied three of my four tanks, slipping slightly for the left main, to get as much as I could – 15 minutes after “empty” was indicated. With GPS distance and time, vs the rate of consumption of fuel, I kept setting “halfway points”, which if not achieved, would dictate a water landing, and Sat phone call to my friend, who was awaiting if needed.
Within 5 minutes of the airport, and with one more “last ditch” lake to pass yet, I knew I would make it. The remaining tank still had a “bouncing” fuel indication as I turned final, and I could have slipped for a bit of that fuel if I needed to, I was purposefully high. A perfect touchdown, and roll out, ahhh…. As I turn off the runway, with the tail down, the engine stumbled. I had enough to park, in three point attitude, but nothing more, the tank pickup was exposed with the tail down.
Upon fueling, I had had 15 liters useable yet to go, but still much too close for comfort, and out of the question not good had a water landing not been a option. But LOP operation got me through, I was just glad I started it three hours early, that made the difference!
The tests I have done have failed to indicate that LOP gives a better MPG, relative to simply operating at peak EGT.
There is a possible second order effect which you do get with LOP: the mechanical losses in an engine are lower at lower rpm (e.g. 2200 versus 2400) but the ignition timing is then sub-optimal; LOP gives a slower burning mixture which improves the “crank angle advantage” at the lower rpm. I think this may be worth of the order of 2-4% on MPG.
50F ROP is the point at which you get the highest CHT so perhaps not a good point for long term operation.
The best power point is about 120F ROP and that is obviously useful for, ahem, best power…
So I cruise only at peak EGT, or best-power if I need to climb during cruise, or fly really high.
If you believe the chart in the Lycoming SI, the specific fuel consumption is optimal at 25 degF LOP, but changes very little between peak and 50 degF LOP. By the time you get to 50 degF ROP the specific fuel consumption is substantially worse, and at 125 degF ROP (best power) it starts to look quite extravagant. At the other end, more than 25 degF LOP, power starts dropping off very rapidly, and roughness is almost inevitable in the absence of perfect mixture distribution. Moreover, you burn more fuel for the same power.
For that reason, like Peter, I use peak EGT, as it is insignificantly different in SFC from 25 degF LOP. But the difference between peak and 50 degF ROP is significant.
All the charts and my experience with Lyco TIO-540 and Coni TIO-550 show pretty much the same – going from peak to LOP does not improve specific fuel consumption preceptively. But the CHT goes down a bit, maybe 10-15 degrees. At FL180+, where cooling at high power settings can be an issue in the cruise, leaning a bit more (even beyond 25 LOP, if the engine is well balanced) and recovering the power with the throttle gives lower CHTs at the same speed, although at 50 LOP the fuel consumption is a bit higher. Of course you could just throttle back by a couple of inches…
Just one note – it can be a bit strange when a well-balanced engine just drops power without any sign of rough-running when you play the “let’s see how lean I can go” game…
I usually run 40 degrees LOP.
Reasons:
- the charts say it is marginally more efficient than peak
- it puts me further away from the red box (which, admittedly is not much of a factor when running at only ~60% power, which is ~13GPH in the SR22) but still, it feels “better” for the engine
- some people say that running at maximum EGTs is not good for the valves and exhausts – true or not, I just stay 40 degrees cooler than max*
By the way, I can run 100 degrees or even 130 degrees LOP without any roughness – it’s merely a question of how well balanced the engine is. When running 100 degrees LOP (which, as we all know, is always measured with reference to the richest cylinder), the leanest one is 120 or maybe 130 degrees LOP – not enough to cause roughness. However, as noted, it just doesn’t have any advantage to run that lean (as opposed to 40 LOP), so I never do it.
* the exception of course is when flying very high up (say FL140 and above), where one usually wants to maximize power output from a non-turbocharged engine.
Looking around the other day re an unrelated topic, I was suprised to find this debate is still being done in the USA 
I think everyone has a couple of those flight where you had to use every drop of fuel to make it work. They really stand out in your memory over all the others. You just feel you know the aeroplane well when you land.
The amphibian advantage is clear in your story!
Isn’t slowing down the thing to do in the circumstances described in the original post?
My understanding is that for most aeroplanes the ‘most miles per gallon’ speed is quite a bit slower than the advertised or commonly accepted cruise speed. The ‘most time aloft’ speed is even slower, isn’t it?
Flying the TIO-550 (Corvalis), I had two power setting options for climb: everything full forward, giving ~1300 fpm at 120 KIAS burning 36 gph as I recall, or I could reduce "MP a little and leaning to below max TIT on the lean side. That would correspond to 100-120 degrees LOP EGT, halving FF to 17-18 gph and climbing about 800 fpm. The engine ran very cool and smooth LOP and since the FF advantage was so significant, I always operated LOP for climb and cruise. For descent the challenge was to keep the CHT high and so peak EGT was better.
I agree there is no first-order SFC improvement when leaning beyond peak. But as a tool for keeping outside the red box at high power, LOP is perfect when engine balancing allows it.
As for stretching the fuel, I agree very much with @Graham that the best speed is much lower than any setting normally listed in the POH. While best-glide speed would theoretically give the best range, it does not work in practical terms to fly quite that slow, because the engine-prop is optimized for higher speeds, and also because the speed stability is marginal at or just above best glide-speed. In just the slightest turbulence or inaccurate flying, you will drop below your selected speed and will need additional power to regain speed, or the speed (or altitude) will drop further.
Except if flying for range in a headwind, I would (and have) set Vg + 10%, a low rpm (like 1800, to reduce internal friction), and lean to peak EGT.
some people say that running at maximum EGTs is not good for the valves and exhausts
People do say all sorts of things, but assigning any significance to +/- 20F for a component designed to operate up to about 1,450F looks to me like the sort of OWT which we should hesitate to promulgate on EuroGA.
