I'd like to start a discussion about power settings.
When I first bought my C180, I'd read a few pireps that said using 21"/2300 was the way to go so I tried it. Other people I've talked to said "just use 22 squared", so I tried that.
The old rule of thumb (aka old wives tale) was to avoid over-square operations, but nowadays it seems like plenty of people run over-square & it works just fine.
Here's a clip from a recent BCP thread about installing an IO-550 in a C185:
"We all spoke to MT at Airventure this year. Their advice was to dial back to 2200 RPM in cruise, whatever MP we desired, and enjoy a significant efficiency improvement."
I'm assuming this is recommending over-square settings, such as 24" / 2200.
yesterday I tried 23"/2100, didn't notice any big change in speed from my usual 22/22 or 21/23, and too short of a flight to see much difference in fuel burn (no fuel flow gauge).

I checked out the 2500' performance table in my 1957 C180 owner's manual:
21"/2300 = 62% bhp, 145 mph tas, 11.5 gph
22"/2200 = 63%, 146 mph, 11.4 gph
so extrapolating, it seems that 23"/2100 should be 64%, 147 mph, 11.3 gph.

Not too many over-square settings shown, except
23"/2200 = 67%, 149 mph, 12.1 gph
about the same as
22"/2300 = 67%, 149, 12.2

Does anyone routinely run over-square settings, and if so what's your reasoning for doing so?

I cruise at 2250-2300 rpm and 21 inches MP at 5000 ft. I'm typically cruising at 130 knots at 11.5 gph at this setting. My engine seems to be quite happy with this. If the engine is happy then I am happy with this setting as well.

Depends on the engine and prop. Nothing wrong with running an O-470 over square. Years ago, I was interested in running a Husky over square (O-360 Lycoming), but, as is the case with your engine, Aviat doesn't provide much in the way of over square power information, nor does Lycoming. So, I called Lycoming Tech Services (which, by the way is well worth doing if you need information on your Lycoming engine), and asked the tech rep whether oversquare power settings were approved on these engines. He told me he'd have to do some research, so I gave him my email address. Next day there was a message from him....."Okay to run up to 9 inches over square". Which is physically impossible with one of these engines. Essentially, the message was "go for it".

What I've found is that some engine/prop combinations don't seem to like over square power settings. Won't hurt anything necessarily, but vibration can be annoying. Other, virtually identical engine/props seemed to work just fine at the same power settings.

My philosophy on running oversquare is simply what gives me the best fuel economy (best and easiest determined by using a fuel flow computer) and power setting. To me, oversquare MAY offer a LITTLE better fuel economy, but it takes a little testing to find that out. Also, you may wind up running power so low that the plane is kind of dragging along.....which makes little sense.

So, do some experimentation and if the plane seems to like the power setting, and it works, go for it. I seriously doubt you're going to hurt anything. You also may not gain a whole lot either, however.

MTV

My take is from reading a lot about propellers (former design engineer, non-aeronautic type) and having the pleasure of knowing a retired aerodynamicist and structures guy. Which is just another way to say "I heard."

Basically, any given propeller-airframe combo is going to have a sweet spot as it relatetes to RPM, pitch and how the thrust created interacts with the fuselage. Again....I was told that short of a lot of detailed testing, more RPM yields smoother, less drag-inducing thrust on the fuselage. Obviously, huge number of variables.

That said, on my 55 model with the J engine, 2450 seemed to be the ticket. 2400 seemed to be right when I went to the U engine. On my '75 180 with the 88" C203, 2400 again. On my 185....I have not found it yet. I just adjust the throttle for speed/desired fuel flow.

Another separate driver for me is that I'm also a believer in more engine RPM for engine longevity reasons.

When my airplane was stock(O-470R/88" C66) it liked 2200 RPM, with 24" or whatever I could get depending on altitude.

Now with a Pponk engine and an 86" 401, it prefers 2450-2500. Down low I use 24" if I want go fast, or as low as 18" for loafing around. Up high max throttle, then pulled back just until I see the MP gauge move a hair. This helps to lean more efficiently.

I believe this SB from Continental was missed by some; it recommends cruise RPM of at least 2300 for many big bore Continentals. I believe many Pponk engines( including mine) started life as the included TSIO-XX engines.


CONTINENTAL MOTORS® AIRCRAFT ENGINE CRITICAL SERVICE BULLETIN
COMPLIANCE NECESSARY TO MAINTAIN SAFETY
SUBJECT: Minimum Cruise RPM
PURPOSE: To establish minimum engine RPM cruise.
COMPLIANCE: Upon issuance of this bulletin
CATEGORY 2 CSB09-11A
Supersedes CSB09-11
TECHNICAL PORTIONS FAA APPROVED
MODELS AFFECTED:
New, Rebuilt, and Overhauled: O-470-G; IO-470-N; IO-520-BB, CB, MB, P; IO-550-A, B, C, D, E, F, G, L, M, N, P, R; IOF-550-B, C, D, E, F, L, N, P, R; TSIO-520-AE, BB, BE, CE, DB, EB, JB, KB, LB, NB, UB, VB, WB; LTSIO-520AE; TSIO-550-A, B, C, E, K, N; TSIOF-550-D, J, K, P; TSIOL- 550-A, B, C
I. Background Information
Continental Motors, Inc.(CMI) examined occurrences of crankshaft counterweight release and subsequent engine stoppage in two, high time IO-520 and two, high time TSIO-520 engine models. Investigation and reported service history led us to believe that these occurrences are associated with engine operation at a sustained cruise engine RPM of less than 2300 RPM.
Engine model specifications, for many of the affected engines indicate power settings of less than 2300 RPM, are within the recommended cruise operating range. The reported population of aircraft (equipped with the affected engine models that operate using an RPM less than 2300 RPM for extended cruise operation) is limited. CMI will continue to evaluate any counterweight releases reported to CMI in an attempt to establish a root cause, including any possible connection with power settings.
II. Action Required
Effectively immediately, Continental Motors strongly recommends the following action be observed for all affected engine models:

Engine cruise RPM settings should be no lower than 2300 RPM
NOTE: This service document applies only to cruise operation and does not supersede the aircraft manufacturer’s recommendations for other operational modes such as emergency or holding procedures.
Engine models identified in the “Models Affected” section of this service bulletin with a history of consistent cruise operation below 2300 RPM should contact CMI Customer Service at 1-888- 826-5465 or 1-251-436-8299 for further information and instructions.

I've done a bunch of fiddling with my O-360, and I seem to get the best combination of speed and fuel economy at 21" and 2400 rpm, using my fuel flow meter and analyzer to aid the adjustments. Lower rpm causes a vibration that smacks of lugging, and speed drops off very noticeably. Higher rpm just increases fuel flow with very little increase in speed. At 21" and 2400 rpm, at an altitude of 7000' to 9000', it burns about 9.8 gph. Of course, with my airplane, "speed" is an oxymoron, and sometimes 21" is simply not available at the altitudes I routinely fly.

I'm not about to say that 21" and 2400 rpm would be best for other similar engines in different airframes, or even in another airplane substantially like mine.

Cary

hotrod180 wrote:Does anyone routinely run over-square settings, and if so what's your reasoning for doing so?

Yes I do - the reasons - because low RPM means less friction and a cooler engine, also less vibration, and I still want enough power output to cruise fast. So given those considerations, think cruising over square is a good idea. That logic applies to most any of our older air-cooled engines.

I use 25" and 2350 RPM which is about 75% in my Lycoming, but then I back off the fuel mix to 65% LOP.

Another thought on prop RPM, which may or may not mean anything...... Designers of Turbine engines can essentially run the propeller at whatever rpm they want, since the turbine itself is running very high rpm, a reduction is required. Turbo prop engines typically run pretty low rpm, compared to piston engines--like 1800 to ~ 2000 rpm. That suggests to me that slower rpm MAY be more efficient in developing thrust. Of course, turbine engines also run somewhat different propellers, but....

Or not.

MTV

Agreed. Even on the GTSIO engines, the designers choice is to keep prop RPM low. Of course, this may not benefit you if you have a propeller designed to run at 2500

In my case, MT says they designed the sweet spot at 2200. I was not aware of the CMI recommendation to stay above 2300. It won't make much difference to the prop, so I'm going to wind it up another 100 RPM for a while and see how that works. Next cross-country is here to Whistler on floats.

I'm also going to search for a power chart for the IO-550.

Another side-thought, which you probably already know - there is no scientific reason why over-square should actually matter - by that I mean:

The number of times the engine turns over per minute (RPM), and the air pressure in the manifold measured in inches of Mercury (inHg - such a random measurement!) multiplied by a factor of 100 - why should those two things bear any relationship to each other?! The whole idea is hilarious.

I have been told it was originally related to radial engines, and was just an easy rule of thumb for pilots to remember. There's really no firm basis for those to numbers relating to one-another by an exact factor of 100.

Underwood Aerial Patrol found the same performance as Cary on a stock O-360 powered Cardinal. Pipeline pilots liked the five hours of fuel. I didn't like the fatter cowl that made visual reconnaissance out the front windscreen more difficult than 172s. My bladder had only two hours endurance anyway.

O-360 A1A
MT prop
31" ABW

23" mp
2150 rpm
8 - 8.5 gph
110 - 115 mph

.

Cary's and akgreg's numbers are interesting. My C172 is dragged through the air by a hartzel CS prop balanced for 2350 rpm, which is hung on a Lycoming O-360 A1A. I burn 100LL, and fly the engine at 20-21" MP at cruise altitudes from 4500-8500', leaned with my JPI to about 50 d ROP on the leanest cylinder. That gives me about 7 gph of fuel flow and consistently around 100 KIA. So far, after about 900 hours on the engine the cylinders look good (no scorch points), all of the valves are clean, and bore scoping at annual shows a happy engine. I fly in the 60-65% power range because the Lycoming tech notes suggest lower RPM and lower power settings give the best longevity. I really don't need to go fast, and "fast" in a C172 is a meaningless term.

PapernScissors wrote:Cary's and akgreg's numbers are interesting. My C172 is dragged through the air by a hartzel CS prop balanced for 2350 rpm, which is hung on a Lycoming O-360 A1A. I burn 100LL, and fly the engine at 20-21" MP at cruise altitudes from 4500-8500', leaned with my JPI to about 50 d ROP on the leanest cylinder. That gives me about 7 gph of fuel flow and consistently around 100 KIA. So far, after about 900 hours on the engine the cylinders look good (no scorch points), all of the valves are clean, and bore scoping at annual shows a happy engine. I fly in the 60-65% power range because the Lycoming tech notes suggest lower RPM and lower power settings give the best longevity. I really don't need to go fast, and "fast" in a C172 is a meaningless term.

I'll be getting a new digital tach soon--the OEM analog tach has finally bitten the dust--so I'll be able to more accurately determine rpm. I'll see what 2350 rpm does. Otherwise, I lean the same as you (50 ROP on the leanest, since any leaner tends to make it run rough). Our speeds are the same (115 mph IAS/100 KIAS). But as I've been running it with the formerly wobbly tach indicating somewhere around 2400 rpm, 9.8 gph on the EI FP5L fuel computer has been pretty consistent until the higher altitudes, where it comes down some, but never as low as 7 gph--that would be nice!

Cary

Just did a scouting flight through the mountains. 6-6500 feet, 22" and 2300 RPM. Leaned out to 12.5 GPH and 120KTS. I'm pretty happy. I know I can get lower fuel burn, but for that speed at gross weight, and some varying power settings because I was scouting, I didn't feel comfortable going LOP. I like the WOT and 2300 RPM in my 180. I have a 3 blade top prop Hartzell on my stock 470R.