O-320 Cruise Rpms

[TylerSC]

I have been doing a lot more cross country flying lately and want to get some feedback on my power management. Basically I fly at 2600-2700 (redline). I level off and then pull back the throttle for about a 100-rpm drop, maybe an inch from full open. Then I use the mixture for best power or best economy. I read about people setting RPM’s at 2400-2500, which is a lot lower than what I’m running, and would be only about 100mph for me.

Could it be an issue of prop pitch for why some people set RPM’s lower?

 

[Ted]

Some people prefer less noise or want to run at a lower power setting.

On an O-320 there's nothing wrong with running it the way you are.

 

[aftCG]

Agreed, the O-320 will withstand "rental power" setting indefinitely.

I have an O-320 on my Citabria. A tip from a pilot who used to fly it was that if I pulled it back to 2200 rpm and leaned it out I would get 90mph and about 4 gal/hr. On a long cross country where I was getting beat up I tried it out and sure enough, at that power setting I had the Aeronca Champ I always wanted.

Now typically I'm used to running an O-320 at 2400 or 2500 rpm, but at this point I'm not chasing logbook hours so I tend to enjoy the newfound slowness.

But when I get tired of the "champ" I shove it in and get C172 speeds.

 

[Tantalum]

I read about people setting RPM’s at 2400-2500

If I'm with a CFI or flying with a safety pilot to build time we'll go very easy on the engine and run around 2,400.. the goal is to build time anyway and it puts a little less wear on the engine. If I'm flying 250nm then cruising at 100 TAS vs 125 TAS has a pretty big difference on time.. Incidentally my CFI is also the owner the of plane, so he tends to baby it somewhat as well

If I'm flying with wife or friends and the objective is "hey flying is cool because it is faster than driving!" then I'll check the POH beforehand to get the 75% power RPM setting, which I believe in both the Archer and 172 conversion I fly is 2,650 RPM.. with fuel flow depending on altitude.. so I'll pretty much run it above 2,600 and below 2,700 (since these 1970s vintage RPM gauges make it hard to dial it in to exactly 2,650)

As far as noise goes... maybe that's plane and prop specific but they seem "quietest" or "smoothest" to me around 2,500 or 2,550. That seems to be the sweet spot. Above that I feel like I'm pushing it, and below that it just seems unhappy

*tangentially related. The SR20 I occasionally fly (which everyone knows has a prop that automatically changes pitch (but no blue knob)).. I find that at 65%-75% power setting the prop hangs out right around 2,700 RPM, or just under it.. which was surprising to me since it seems high

Also note that for the size of these engines and their displacements I think they could handle a lot more than we throw at them. Would be interested to set one up on a test stand and "tune it" like you would with a car and run it up into the 3K RPM range with some boost to see what it would handle and how much power you could coax out of it

 

[hindsight2020]

*tangentially related. The SR20 I occasionally fly (which everyone knows has a prop that automatically changes pitch (but no blue knob)).. I find that at 65%-75% power setting the prop hangs out right around 2,700 RPM, or just under it.. which was surprising to me since it seems high

That shouldn't surprise you. The Cirrus only flies two settings on the prop: 2500 and below (as dictated by pitch stops) and redline (2700). They are driven by a cam channel built into the throttle lever, below the quadrant façade. It's a mechanical channel. Black rod represents throttle cable, blue is the prop cable

*(illustration courtesy of NorthStar Aero)

At any rate, this means that as you set the power with MP, the physical throttle lever position will dictate if the RPM setting requires that last 200 RPM to make the % power you seek, or will 2500rpm be enough. You should be able to make 65% at a throttle position that caps the prop at 2500 when below 8k, certainly below 6k (NA engine). Above that, for sure you're gonna be running around at redline, which is an opportunity cost of note for those of us who don't find the blue knob an indignity.

I've heard the G1 are not particularly acoustic friendly, I know for a fact 2700 vs 2400 RPM in my Arrow is a significant acoustic difference. Turboprop owners (who often are able to run those 4 bladed food processors as low as 2000-2200) can attest to these non-linear improvements in cabin noise. To each their own.

As to O-320s, it depends on what kind of fixed prop pitch you actually have. Factory set, which is usually a compromise between cruise and climb prop, you're looking at 70% power at 2500 when WOT and lean down to rough or 2500, whichever happened first. That's about what it gave me on my Warrior II when I flew it. Consistently averaged 8ish gph at 6-10k which again, was like 70% power all over kingdom come. Carbs and fixed pitch props, two things I no longer accept in my personal aircraft ownership.

 

[Ted]

As far as noise goes... maybe that's plane and prop specific but they seem "quietest" or "smoothest" to me around 2,500 or 2,550. That seems to be the sweet spot. Above that I feel like I'm pushing it, and below that it just seems unhappy

That is very different depending on aircraft. The bigger 6-cylinders tend to be happiest around 2300 RPM. The 4-cylinders are typically 2400+ RPMs for their happy spot. The noise/smoothness will depend on the particular engine/prop combination, and

*tangentially related. The SR20 I occasionally fly (which everyone knows has a prop that automatically changes pitch (but no blue knob)).. I find that at 65%-75% power setting the prop hangs out right around 2,700 RPM, or just under it.. which was surprising to me since it seems high

Nothing wrong with that. Keep in mind Cirrus has their linkage (which handles both prop and throttle) set up for a particular profile. It's not ideal for all circumstances but works well for 90% of them. 2700 RPM on these engines is just fine. Not necessarily ideal efficiency, but nothing wrong with it.

Also note that for the size of these engines and their displacements I think they could handle a lot more than we throw at them. Would be interested to set one up on a test stand and "tune it" like you would with a car and run it up into the 3K RPM range with some boost to see what it would handle and how much power you could coax out of it

That depends on your definition of "handle" and how long you want them to last. RPM wise, keep in mind we mostly limit the RPMs where they are because of propeller efficiencies. Above 2700 RPM the prop efficiency tends to go down since you need to get the diameter significantly shorter. Geared engines have bigger props that turn slower, and then higher revving engines. Note the GTSIO-520s found in 421s, 404s, and Commander 685s which spin in the 3,200-3,400 RPM range at rated power, putting out significantly more power than a direct drive equivalent. The 414, for example, has 310 HP engines from TSIO-520s redlining at 38" and 2700 RPM. The GTSIO-520 in the 421 makes 375 HP at 38" and I want to say 3,200 RPM.

The higher power engines have other aspects put in to help their reliability. Piston oil squirters, different valve springs for higher RPM, in some cases a bigger crank, etc.

Just take an O-360 out of a 172 and then add a turbo to it, run it on a lot of boost and higher RPM, it won't last too well.

 

[RyanB]

I usually run the O-320 around 2500 in cruise.

 

[Tantalum]

Thanks @hindsight2020 and @Ted DuPuis I always learn stuff here! The simplicity of the Cirrus throttle is convenient. I've gone back and forth on this. Part of me likes the simplicity of it, but the other part of me that likes manual transmissions and control wants the ability to manage the prop

Cheers

 

[George Mohr]

I like to run mine at 65% power, which is more like 2400 at lower altitudes and 2500 up high. It does sound like the OP has a climb prop. 65% power usually gets me 105 to 110 KTAS.

 

[hindsight2020]

I like to run mine at 65% power, which is more like 2400 at lower altitudes and 2500 up high. It does sound like the OP has a climb prop. 65% power usually gets me 105 to 110 KTAS.

Depends on what Cherokee. Post-78 wheel pants are good for 8knots over older pants. His might actually be a stock prop if he's flying an older cherokee. My Warrior II (an 83) would do 110-113 at times at 65-70% power and that was with a tired 2200SMOH engine. Pipers leave a lot of money on the table on aerodynamic refinements between the introduction of the taper wing models and those with hershey bar. Pants, cowls, the wing finish quality itself (rivets et al). It's significant.

 

[George Mohr]

Yeah, I hear you, but he is saying more like 86 KTAS at 2400/2500. That's really low, don't you think? To get my Warrior that slow I need to turn 2200 or so. (60" prop)

It occurs to me that the OP might be quoting indicated speeds vs true. That would account for most of this gap, at least at higher altitudes.

 

[N3368K]

I typically cruise the O-320 in my RV-4 at 2450 if going someplace. If just playing locally, I'll pull it back to 2200-2300..and still be kicking along at 150mph IAS. If I am in a hurry to get somewhere quickly, I'll push it up to 2500-2550 and burn about 7.5gph.

 

[kgruber]

Anywhere in the green.

 

[SoCal RV Flyer]

Under 8000' DA and going on a half-hour breakfast run, I'll go 2400 rpm or so and fly 150-170 degrees ROP.

If I'm actually going somewhere above 8000' DA, I set it by fuel flow and let the revs fall where they may. 8.0 gph if I'm in a hurry (usually!) and 7.0 gph for economy cruise.

 

[dmspilot]

I have been doing a lot more cross country flying lately and want to get some feedback on my power management. Basically I fly at 2600-2700 (redline). I level off and then pull back the throttle for about a 100-rpm drop, maybe an inch from full open. Then I use the mixture for best power or best economy. I read about people setting RPM’s at 2400-2500, which is a lot lower than what I’m running, and would be only about 100mph for me.

Could it be an issue of prop pitch for why some people set RPM’s lower?

Can't answer the question unless you tell us what altitude and whether it's IAS or TAS you're talking about.

 

[kyleb]

I'm a 2400-2450 rpm kinda guy. That usually works out to 155 knots and 7.5 gph on the O-320.

 

[Dan Thomas]

If you're flying an old airplane, beware of the tachometer. The mechanical magnetic-drag type tachs, which are in about 98% of steam gauge airplanes, mostly start underreading as they get old. The little magnet in them gets weaker. In Canada we legally have to use a tach checker on them once a year and are allowed a 4% error. Beyond that, it's tach overhaul or replacement. 4% of 2700 is 108 RPM and you could be, if using 2700, be well over redline. The engine likely won't care that much but the forces on the prop go up, and props are already pretty highly loaded. A really worn/old tach could be 10% off. Very bad.

Replace those old tachs with digital units. They don't age like that.

I have a centrifugal tach in my Jodel. Very rare. Also very old and still accurate. It looks like this one:

 

[Dr. O]

mmm yup. The mechanical tachs in the Apache are off - one by 40 and one by a 100. I use an electronic optical tach in flight every now and then to see if they have changed (counts the blade flicker). The one that is off by 40 is set to the rpm I want (allowing for the error) and the one off by a 100 is matched to the first one by ear.
On cruise rpm, depends on what I am doing. For just burning gas for the heck of it it is any where from 1800 to 2200 rpm and lean till they shake.
For going somewhere it is 2400 - maybe 2500 into a headwind.
As far as mixture control - I do have an EGT - it is usually lean by my butt meter; lean till they shake like a wet dog then sweeten em up till smooth
Now, the way I fly these O-320 engines is not the way I fly a Commanche, Super Viking, Aztec, etc. A mans gotta know his limitations (the engine's, that is)
But for the low compression 4 cylinder Lyc's - which are refugees from a 1930 Farmall tractor - they like turning slow. Lean em out so they don't foul and they will run essentially forever at that power setting.

 

[N3368K]

I will carry my TruTach II with me occasionally to check my J-3 and RV-4 tachs. Both are is within 25rpm.

 

[edo2000]

Interesting thread. I've wondered about this myself. The RV9A I'm flying now has an 0-320 and I find myself running in the 2300-2400 range a lot. It has a fixed (cruise) pitch Sensenich and a placard that limits max rpm to 2600. Fuel consumption is considerably higher at the higher rpms. TAS is around 150kts at 2400 and the fuel consumption is just under 8 gph, not bad.