Qalupalik wrote:
To abstract from this remark, based on research I did a decade ago using proprietary performance data for a FAR25 aeroplane there are still savings to be made by optimising the climb trajectory which is by far the more interesting case.
Certainly in turbines there is a lot of optimisation possible here. Best rate of climb, cruise climb, constant pitch.
It really relates to to power curve all the way up and the fuel consumption at different altitudes. I tend to climb at a fixed pitch now then when the FF has dropped sufficiently around FL380, climb at the highest airspeed I can. This can result in being at the cruise altitude, for the same fuel burn but 60nm further downrange. But if time is the consideration, I try to climb at just under Vmo/Mmo for as long as I can.
Peter wrote:
Does not SL = 0ft achieve max range on SEPs?It achieves the highest IAS and the highest TAS, but not the best range, because you are flying with a partially closed throttle (high pumping losses) and aren’t getting any TAS gain. If you go to full throttle, a TB20 does 165kt IAS and TAS down there, but burning 24 USG/hr because at full throttle you have to be very rich on the mixture!
Yes, I looked at TB20 POH fuel/range numbers you get 8.6Gal/100nm at FL100, 8.7 at FL50 and 8.9 at FL020 so the pumping loss is not that much, but I bet it would explain the 0.3Gal/100nm 3% gap between FL020 and FL100, the rest on TAS/IAS will just cancel out for “max distance”, but to get to FL100 you will have to climb first 
Shutting down one engine? For jets at normal (high altitude) cruise, shutting one down will reduce altitude to a less economic altitude. The associated range reduction is part of jet fuel planning, notably ETOPS.
But piston aircraft? Obviously asymmetric flight has some trim drag penalty, but does that always outweigh the savings in internal friction in the shut down engine?
The BN Trislander reportedly flies very economically one the one center rear engine at light weights, with the two wing engines shut down.
Peter wrote:
For a non turbo piston, IO540 type of engine, certified, Vs=60kt, it is around 8-10k feet, and it doesn’t vary much. You just need to be flying with WOT (wide open throttle) for the minimum pumping losses, and peak / lean of peak on the combustion.
Maybe I’m missing something but isn’t the optimum altitude (in terms of the mentioned NM/Gal) for a non-turbo piston as high as it can operationally go? The IO-550N seems to have the best range stats near its ceiling at 16-17k feet.
Not taking account for the fuel required to climb to that level (ROP) which requires a lengthy cruise to offset the expense.
TimR wrote:
Maybe I’m missing something but isn’t the optimum altitude (in terms of the mentioned NM/Gal) for a non-turbo piston as high as it can operationally go? The IO-550N seems to have the best range stats near its ceiling at 16-17k feet.
No, neglecting pumping losses, there is no optimum altitude for piston engine propeller aircraft.
In theory, the max range of a piston aircraft is independent of altitude. But the speed for best range increases with altitude, so to get a good range without sacrificing to much speed, a higher altitude is desirable.
huv wrote:
In theory, the max range of a piston aircraft is independent of altitude. But the speed for best range increases with altitude, so to get a good range without sacrificing to much speed, a higher altitude is desirable.
Couldn’t agree more, as long as what you gain as speed (or time) in cruise (1) offsets the speed (or time) you loose in climb/decent (2) but for max range in theory (1) should dominate and climb/decent are always included in the perf charts, see C150 poh
the max range of a piston aircraft is independent of altitude
That is certainly not the case, as put, because above a certain altitude, well short of the ceiling, you have to go ROP. I can fly peak/LOP up to FL160 (ISA) and to get to FL200 I have to go some 150F ROP which burns 20-30% more fuel.
When I wrote above “and it doesn’t vary much” I meant that the altitude is not critical to MPG, so anywhere from say 8k to say 14k the MPG is close to constant. Below and above that you definitely lose.
It thus follows that if you fly ROP all the time (as indeed many do) then the statement “the max range of a piston aircraft is independent of altitude” will be valid over a wider altitude range than if you fly at peak/LOP.
A quick look at the PDF posted by a_kraut above and searching it for “lean” shows just this passage
Indeed!
That is from 1993, when almost nobody knew about engine management.
I made a real life experiment last time when flying to Quimper with really strong headwinds. Best level was FL50 otherwise we would have been flying backwards. (Okay I‘m joking, but higher was worse, we gave it a try first.) Then I tried ROP and LOP in several RPM and MNP configurations. After each change I checked the remaining fuel and the fuel efficiency on this setting on the GTN750. With the strong headwind best was 24/2400 ROP. LOP there was no chance to reach the destination without refuelling stop.
On the other hand, when going from Malta to Regio last year we had to be very careful with our fuel, because Malta has run out of Avgas, we had to go 23/24 LOP to get best efficiency and make it to the destination without running dry and keeping reserve for a possible go around. In this case no wind influenced our powersettings.
So maybe another factor might be, how the aircraft design deals with wind? I don‘t know.
Peter wrote:
That is from 1993, when almost nobody knew about engine management.
Well, engine management is best done by a computer, and there is no need to know anything about it
