Peter wrote:
A lean mixture burns slower so you need to have a later spark, and on our engines which have fixed timing you achieve that with a lower RPM.
I think there’s a typo there – earlier spark is required at a given RPM to accommodate the slower flame front travel that comes with lean mixture. Lower RPM with fixed timing does indeed achieve the same end of providing more time to burn the fuel completely.
Timothy wrote:
I wonder whether we can put numbers on that? I mean, develop a rule of thumb as to where the crossover lies
I suspect the crossover would lie at the point where the difference (in percentage) between fuel flow and groundspeed is zero.
IE if you burn 10% more fuel per hour but only get 5% more groundspeed, throttle back.
Set GPS to show fuel on board at destination. Tweak throttle to maximise. While that does not take into account how the wind might change during the rest of the flight, this gives you maximum “miles per gallon” at that moment, no calculation required.
Cobalt wrote:
Tweak throttle to maximise.
I think it’s a bit more subtle than that!
Apart from anything else, when LoP it’s more a question of tweaking the mixture control to maximise, as fuel is the limiting parameter, not air. Also, as Peter has pointed out, it’s not just a question of fuel burn, but also CHT, and so you might be tweaking RPM to minimise. Next, you might have very different winds at different altitudes (look at France today for a fine example) so you might want to tweak the altimeter. But while you tweak the altimeter, you’ll find that the TAS is affected, which also changes the SFC.
Oh that life were that simple!
Set GPS to show fuel on board at destination. Tweak throttle to maximise. While that does not take into account how the wind might change during the rest of the flight, this gives you maximum “miles per gallon” at that moment, no calculation required.
I sort of do this on longer flights. Display the landing fuel on board (LFOB) and tweak the mixture to maximise. It has to be done really slowly; minutes per change. The throttle is already wide open (I have no turbo) and the RPM is left at 2400 unless I am really going a long way and that is not common (Shoreham to Granada was one such flight which I did at 2200rpm).
I use the following iterative process:
Assumptions (base case for turbocharged twin, RAM VII TSIO520): 30" MP, 2300 rpm, 200 KTAS, LOP FF 14.5 gph per side (around 25F LOP, but I am looking at the TIT), FL180
First I select the cruising level for best GS.
Once in the cruise, lean to 14.5 gph, adjust as required to give 200 KTAS (at FL180).
If headwind, increase MP and consider change in rpm. If tailwind, reduce MP (or leave it and enjoy the ride). Check FF indication (Shadin) for mpg.
That’s about it. 13.5 gph is a good compromise for range in still air or tailwind. Reduction below 13.5 gph gives a significant drop in TAS. Increasing FF above 15.5 gph is limited by TIT. The mpg indication on the Shadin (gps input) helps – I can change any parameter, pause, and see what happens. Range drops considerably with ROP so I never go there in the cruise.
Once I came back from Iceland at 3,000 ft into ferocious headwinds, but that was another ballgame 
Leaning to book tables usually doesn’t harm engines. In effect set power and lean to the GPH in the table.
Timothy wrote:
I think it’s a bit more subtle than that!
.. but it doesn’t get more practical…
My comment was responding to “So, the faster the aircraft, the less true it is that reducing power increases range – I wonder whether we can put numbers on that? I mean, develop a rule of thumb as to where the crossover lies.” So let me change my statement a bit – what I meant was ’Tweak throttle power to maximise".
And while of course the fuel remaining at destination will not be accurate because the wind will change, it will definitely give you most miles-per-gallon for the air mass you are currently in. At least if the destination is more than vaguely where your track is pointing to.
I wouldn’t use the calculated fuel at destination to optimise mixture – for that, the best would be to tweak mixture for maximum IAS, which settles a bit more quickly.
RobertL18C wrote:
Leaning to book tables usually doesn’t harm engines. In effect set power and lean to the GPH in the table.
The key word here is usually. And as long as power is 65% or so, there really is little scope to do damage…. but:
This strongly depends on the engine and accuracy of the fuel flow indicator. Conti TSIO-550 in a Columbia or Cirrus with proper fuel flow meter leaning to the lean side – any time. Just big mixture pull to target fuel flow.
Lyco TIO-540 in a Saratoga with fuel pressure indicator, only a single CHT gauge and leaning to peak – no way. Too much risk to end up a bit on the rich side and running too hot; for this dinosaur, one should pull the mixture quickly to the target, but follow up by establishing the peak and then using that.
Until today, I have been flying 32”, 2400, 25°F LoP and the results have been good, but today I put the 65% settings in from the book.
I was surprised how far LoP it was – about 80°F – and there is also a 55% that is even leaner, but I was a bit pressed for time.
The RPM had no effect on CHT. There was a big interaction between RPM and MP, though. 200 RPM took 7” off.
