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At what RPM is your engine happiest?

Mine seems to like 2400 best… 2575 (the max) is even smoother but the noise level goes up and the MPG goes down quite a bit.

The curious thing is that this differs between planes of the same model.

Administrator
Shoreham EGKA, United Kingdom

Never believed in this “happiest RPM” thing. If you engine doesn’t run smooth along the whole approved cruise RPM spectrum, get it checked over. Also, any speed differences will be minimal. This doesn’t mean that low RPMs aren’t usually more efficient.

Last Edited by boscomantico at 19 Jul 12:19
Mainz (EDFZ) & Egelsbach (EDFE), Germany

boscomantico wrote:

If you engine doesn’t run smooth along the whole approved cruise RPM spectrum, get it checked over.

Small imbalances of the propeller can cause resonances at certain RPMs which gives the impression that the engine doesn’t run smooth. Additionally there can be resonances with the aircraft structure at certain RPMs. (Just listen for that the next time you fly on a airliner or small jet!). No need to have the engine checked because of that, just avoid certain engine regimes.

Last Edited by what_next at 19 Jul 11:21
EDDS - Stuttgart

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.

5000 RPM feels about right as cruise setting :-)

EDLE

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

LSZH(work) LSZF (GA base), Switzerland

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.

EBKT

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).

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):

http://11hc.44rf.com/manuals/engine-prop/lycoming/aerosportspower-library/360_series_ops_manual/sec_3b-charts.pdf

[ local copy ]

EDDS - Stuttgart

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.

Last Edited by at 19 Jul 13:42
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