2200 or 2400 cruise

[astaib]

Hello Guys,

Can you tell me the difference of a cruise at 2200 or 2400 RPM?

I never find a clear reason when to cruise at 2200 and when to cruise at 2400 RPM.

For example at 2000 ft @ 75% or 7000 @ 65%.

Thank you!

[Roxi5m9]

I think a lot of people could chime in with different opinions here but I believe it is personal preference. I actually always run 2500 RPM in climb above 1000 feet agl always run 2300 in cruise. Those numbers to me seemed like a good middle ground on speed and performance, but more importantly, I feel the level of vibration and noise of my motor and prop are the least at 2500 RPM with wide open throttle on climb, and at 2300 in cruise, usually with 23” MP, unless I’m too high to hold 23”, then wide open throttle. The lower you set your RPM for a given manifold pressure setting, the less power you will actually be developing, and less TAS and fuel flow, etc.

[BRS]

RPM and heat are related. More combustion events for the same HP equals more heat.
Higher RPMs spend more time rubbing parts together so there is more mechanical drag. Most noticeable in the oil temps.
Lower RPM at the same HP% can be done more efficiently especially LOP
In loiter mode at 2000 rpm and 18" (90 kts?) I can fly for less than 6 gph.

[CFIDave]

For 2007 and newer Lycoming DA40s -- all of which have Powerflow exhausts -- the Powerflow exhaust is "tuned" to an RPM of 2450, i.e., it maximizes scavenging of exhaust gasses at this RPM for increased power and efficiency.

Obviously you can reduce fuel consumption by flying at a lower RPM where the engine will produce less horsepower at WOT (wide-open throttle; cruising at WOT will reduce engine induction losses). The lower HP and resulting airspeed reduction at reduced RPM reduces parasitic drag, which is proportional to the square of velocity.

But if you really want to get somewhere on a X-country flight, 2400 RPM is a good compromise between frictional losses vs. the horsepower needed to overcome drag.

The only exception I make to flying at 2400 RPM in cruise is when flying at higher altitudes (e.g., above 8000 feet), where I'll cruise at 2500-2600 RPM to counter the horsepower loss due to thinner air and resulting lower manifold pressures. If you think of the engine as a big air pump -- where horsepower is proportional to the amount of fuel/air mixture drawn into the cylinders -- higher RPM at high altitudes helps counter the inevitable power loss up high.

[Rich]

In spite of the DA40 POH wanting to have us use (in some cases restrict to) 2400 RPM, the IO-360-M1A manual describes max power cruise at 2450 RPM. I've taken to using this latter number, along with WOT (I'm almost never below 6,000 ft. DA these days - usually more like 7-8 K) for "going places". This yields 141-145 Kt, depending on altitude. The lower number at about 11K-12K ft DA.

There are times when I'm not going anywhere distant or just sightseeing. Then I'll often run 22x2200. Of course I'm slower then but don't particularly care. In some cases I'll go LOP.

One other scenario is when traveling and trying to speed up by slowing down - i.e., to eliminate a fuel stop. Then it's LOP, 2200 RPM, and WOT. This is always at higher altitudes, so MP is whatever it turns out to be.

[astaib]

That’s interesting for me.
Because I’m always trying to follow the grid that is in the POH for MP and RPM (I adjust the fuel flow either LOP OR ROP according to Mike Bush advises).
But I never use the WOT position when cruising.
I religiously follow 26,5/2200 when I’m at 2000ft for tourism and each value for each significant change of altitude.
Maybe I’m wrong ?

[Rich]

That’s interesting for me.
Because I’m always trying to follow the grid that is in the POH for MP and RPM (I adjust the fuel flow either LOP OR ROP according to Mike Bush advises).
But I never use the WOT position when cruising.
I religiously follow 26,5/2200 when I’m at 2000ft for tourism and each value for each significant change of altitude.
Maybe I’m wrong ?

You have to understand I live at some altitude. Field elevation is 3250 ft. So when I say 6,000 ft. it's really just ~2500 ft AGL and WOT has me at 24-ish inches. Takeoff full power MP is usually around 25.5 in. MP.

[astaib]

Ok. Got it.

[CFIDave]

That’s interesting for me.
Because I’m always trying to follow the grid that is in the POH for MP and RPM (I adjust the fuel flow either LOP OR ROP according to Mike Bush advises).
But I never use the WOT position when cruising.
I religiously follow 26,5/2200 when I’m at 2000ft for tourism and each value for each significant change of altitude.
Maybe I’m wrong ?

There's a school of thought that considers that the "red box" combination of mixtures at lower altitudes should be avoided to reduce the cylinder pressures. You can read about this in John Deakin's classic column:
https://www.avweb.com/features/avweb-cl ... -3-cruise/
But RPM has very little to do with it.

You can operate at WOT to reduce induction system losses due to a partially-open throttle plate. And once you get above 7000 feet or so (where the manifold pressure is limited by altitude), you can run any mixture and not hurt the engine.

[Chris B]

But RPM has very little to do with it.

Mostly true.

But a common misunderstanding is that reducing RPM is always "easier" on the engine.

When Rich-of-Peak, lowering RPM greatly increases stress on the connecting rods and crankshaft. Lower RPM pushes peak cylinder pressure much closer to Top Dead Center. At low manifold pressures this is not an issue, but for typical (lower altitude) WOT operation always use high RPM when Rich-of-Peak.

Lean-of-Peak operation significantly retards & reduces peak cylinder pressure, so high manifold pressure at low RPM is not an issue. Some engines have minimum RPM limitations due to counter-weight harmonics, but not the Lycoming IO-360.

The Advanced Pilot Seminar (co-founded by John Deakin) online engine management training course is the best $400 I have spent in aviation: https://www.advancedpilot.com/

Chris