Engines typically achieve highest fuel efficiency somewhere near their peak torque rpm, which is a pretty flat rpm range for aircraft engines, but below the rpm that may require rich mixture for reliable operation. Regardless of when the engine was designed, this is a key design issue and was the product of forethought.
My O-320 and CS prop are smoother when running slower, so fully leaned out at say 2200 rpm makes for a nice experience, with maybe 23 inches MP and economical fuel flow.
Alexis wrote:
Depends on the engine version. Max. continous power for IO-550N is 2700 rpm, for IO-550G it’s only 2500. The “N” version is very happy at 2700, also that’s only 8 percent more revolutions per minute.Whatever “happiness” means in this case, I doubt it’s the same as “best efficiency”.
IO 550G & N are exactly the same engine. The prop governor limits the G to 2500 but with a STC you can bump it back to 2700.
Depends on the engine version. Max. continous power for IO-550N is 2700 rpm, for IO-550G it’s only 2500. The “N” version is very happy at 2700, also that’s only 8 percent more revolutions per minute.
Mooney_Driver wrote:
That is where the engines are most efficient and give most power.
No way are these 70 year old engines designed in a way that they are most efficient at max RPM. Max power at max. RPM: Yes. Best efficiency at max. RPM: No.
Just look at this graph of one of those engines I picked at random (Lycoming O-360):
[ local copy ]
The lower limit for cruise on the big bore Contis is 2300 rpm, by TCM
My IO-550N is happy at 2400, 2500, 2600 and 2700 … smooth across the whole range (with the 4-balde MT prop anyway).
This has probably more to do with the propeller efficiency.
On the big Continentals you have to avoid 2100RPM because of counterweight resonance. Above or below are OK but not 2100, there is a big spike.
Higher RPM means more efficient power pulse since we have fixed ignition timing, it also means more internal friction, mostly less efficient propeller (depends on type).
Find the smoothest and most efficient setting for your combination.
I fly two A36s, one is smooth at 2500-2200, the other is only smooth at 2500-2400. Different prop.
From a technical point of view, most of our non-turbo engines are happiest at full open throttle and with max continous RPM. That is where the engines are most efficient and give most power. Bob Kromer is a strong advisor of this and he is mostly right, so is John Deakin.
Taking an example for max cruise at 10’000 ft:
At 2600 RPM, the C Model will do 155 kts TAS @ 10.6GPH. At the more moderate 2300 RPM, it will do 153 kt and 10.8 GPH so almost identical but 2 kts less and burn 0.2 GPH MORE. At these regimes, 45’ range will be at around 650 NM.
Best range power is at 2300 RPM and 135 kt TAS @ 7.8 GPH. There range is 100 NM more.
Most piston planes are usually flown at reduced RPM, as the noise level decreases. We have operated ours mostly at 2300 RPM
5000 RPM feels about right as cruise setting :-)
I tend to be more concerned with getting a sensible flow rate in the twin at any rate because pressing the speed doesnt get a whole lot of time saving but a significant increase in fuel burn. 24 works just great as well. Getting the prop sync just right has the biggest impact on a pleasant ride. I am not sure my ear is good enough to always get it spot on mind you.
