RPM and MPG

Robert, when flying lean of peak it is a bit more efficient, so 8GPH according to Mike Busch can be multiplied by 14.9 to obtain horsepower. This is about 47% with the 260hp rated engine that I drive in the Comanche.

But yes you have to set a lot of things right. For example, the Comanche – like a Cirrus or practically any plane – does have two speeds where it levels off. I can perfectly fly at around 120 knots indicated on level flight, and when I push it, after some seconds it levels out again at 130 knots. You cut the glide polar twice. But this is yet another topic. Downside is, that flying at the slower speed of these is much easier, because on either pushing or pulling you get a positive climb rate, so that maintaining altitude hasn’t to be monitored so closely. I used to really fly a lot in gliders in earlier days, for which I’m quite on the nose on this.

Udo

Peter, thanks for the nice words. I found it easier to measure increase in speed (in still air) and keeping FF constant, as I see it digitally displayed on the glass panel it is plain simple to read.

I’m quite disappointed to see that so few pilots concern about efficiency. Of course you could say that some Gallons more or less don’t pay the bill or make any difference, and it is true speaking money-wise. But then you burn double the Gas per mile on higher engine stress levels, implying higher temperatures and pressures on the cylinders, which all seems quite a waste of resources to me.

Surely, if you rent a plane all this calculation is different. You normally go for higher speeds, and I did the same, because you’re billed by what the Hobbes meter counts (time airborne).

I don’t have much experience yet with my Comanche, I have to admit. But I can read and understand the publications of GAMI and Savvy, and I find it convincing. So I’ll continue flying efficient.

Udo

I won’t go through the “Flying on the step” topic, but you just have to talk to glider pilots (edit: maybe you should choose one who seeks deeper understanding of this, like one flying in competitions :-) ) to learn that for given lift to drag ratios you can end at different speeds. Planes having a laminar or quasi-laminar wing tend to show this more distinctly, as does the Comanche or a Cirrus.

Udo

Peter, of course you made me read the “Flying on the step” topic now. I won’t add anything there because it’s dead for five years now. What I could add to this is, that not only it affects laminar foil wings more, but another thing is that you won’t see that effect if you try to fly full power maximum speeds. There, drag takes over and you get a single speed. Or spoken a bit simplified: Thrust and drag equal out. What I also missed is, that you don’t have to climb above target altitude. You simply start with more thrust and reduce thrust in level flight to cruise setting, thus making it “IFR compatible”. Of course you can only do that if you have excess thrust available.

Udo

RobertL18C wrote:

30 lph at 150 KTAS on a turbocharged Lycoming 540 is seriously impressive. 8 usgph is around 40 % power on a normally aspirated variant, the turbocharger heating means it is below 40% on a turbocharged engine.

I burn about 48-49l hour at FL100 in my normally aspirated Comanche. However I true between 167 and 172Kts on an indicated of around 143. That is with about 20-21inches and In between 24-25RPM. I rarely reduce RPM below that. I like the speed of the Comanche’s and with that setting with the tip tanks full still get about 8.5hours of useable…

LFHNflightstudent wrote:

I burn about 48-49l hour at FL100 in my normally aspirated Comanche. However I true between 167 and 172Kts on an indicated of around 143. That is with about 20-21inches and In between 24-25RPM. I rarely reduce RPM below that. I like the speed of the Comanche’s and with that setting with the tip tanks full still get about 8.5hours of useable…

It’s a decision on either stay below the power up to where flying lean of peak is possible, which by the numbers I gave can be extended up to 130 indicated in FL100; Or going above that speed, where you have to fly ROP. I, too, like the possibility of speed as well and with the family on board on long trips will most probably fly faster as well. However, there are in fact quite some flights where it doesn’t make such a big difference whether flying 130 indicated or 150 indicated and this is where I go for Lean of Peak operation.

In fact, even with high power cruise recommended engine revolutions can be chopped down to 2000 RPM (or maybe with more caution might be 2200 RPM) according to the manual, but so far I know of no one doing this – despite “old-man tales” of Max Conrad, who insisted that full throttle and 1900 RPM was the best way of riding a Comanche.

The tip tanks are nice, I don’t have them but if I could get my fingers on a set of these maybe I would even think about installing them.

Yup really interesting aircraft that Turbo Comanche! Which kind of turbo control system does it have? Manual, fixed or automatic wastegate?

In our Conti TSIO-520P we have a sweet vibration spot at 2500RPM (that is also where we do dynamic prop balance) so that is preferrable to 2400RPM. Lower than that I can only do low MP or else it is not as vibration-free. Also LOP above 19000ft dens altitude (usually FL180 or above) I cannot get my desired 33" MP for LOP ops. In the end, other than for low-altitude or low-power ops (descent and hold) I end up using 2500rpm all the time (2700 for T/O). So considerations other than efficiency drive me there.

We now have a freshly OH prop with hub balancing weights so maybe the vibration signature will change…we are yet to fly it, so we shall see how it goes!

It’s a manual wastegate, providing me with the real joy of a second throttle in the cockpit. However, downside is using it wrong can cost an engine. I can go up to 29” MP (summing up to 30” including system loss of 1”) which is of course normally aspirated sea level pressure.

I have a new prop installed, and my Comanche is a real low-timer, don’t know if that’s the difference. but I really don’t see vibration in my plane from 2700 all the way down to 1800.

Efficiency for me is MPG which is a sum of many factors, but multiplied by some personal factors like Carson thought about it. Because if not, obviously MPG is best at V_bg, but then it starts to get a bit lengthy up there.

In my personal experience I’ve found one sweet spot, using low RPM and LOP, where MPG is quite nice and this is what I wanted to share, because it is somewhat “hidden”. I only got there by reading, understanding and trying.

Antonio, why do you need 33” for operation on the Lean side?