I have an O-360 A1A with a CS prop (McCauley).
The RPM gauge has a caution range (marked yellow) in the 1950 to 2350 range.
There is also the following placard:
"AVOID CONTINUOUS OPERATION
BELOW 15" H.G. BETWEEN
1950 AND 2350 R.P.M."

My first question is that placard referring to the possible prop or the engine issue - i.e. what happens (or what can one damage) when one operates in this range and how long is continuous.

The second question is that the Lycoming manual recommends 21.6 H.G. and 2200 R.P.M at 2,000' ASL for 65% power cruise at 8.8 gph., putting me firmly in the caution, yellow zone for the prop R.P.M.. Am I correct that the placard for MP and RPM is for when both conditions are present, not either one separately.
Or am I missing it all together?

AKclimber wrote:I have an O-360 A1A with a CS prop (McCauley).
The RPM gauge has a caution range (marked yellow) in the 1950 to 2350 range.
There is also the following placard:
"AVOID CONTINUOUS OPERATION
BELOW 15" H.G. BETWEEN
1950 AND 2350 R.P.M."My first question is that placard referring to the possible prop or the engine issue - i.e. what happens (or what can one damage) when one operates in this range and how long is continuous.

The second question is that the Lycoming manual recommends 21.6 H.G. and 2200 R.P.M at 2,000' ASL for 65% power cruise at 8.8 gph., putting me firmly in the caution, yellow zone for the prop R.P.M.. Am I correct that the placard for MP and RPM is for when both conditions are present, not either one separately.
Or am I missing it all together?

Maybe that meens Below 15" and the rpm limit at the same time. Why would you not want to run 2200 RPM at 20" all the time. Never ever flown an O360 so maybe I should not post.

Tim

I believe you have 2 issues going on. one is the prop harmonics, and the counter weights in the engine the 0-360 has a very hard or what ever word you want to use Pulse when it fires.
Might be for the protection of the Hub on the prop?
The tack is marked for the engine and the placard I believe is for the prop.
Just makes it easy, do not run it between those RPMs unless you want something to come apart, mainly rods and crank!!
My understanding and that is very minimal to say the least, or is that most??
GT

That is a strange rpm restriction for an O 360 A1A. What flavor of McCauley prop do you have? I've never heard of a McCauley CS prop on an O-360 (which may also be why that rpm restriction seems strange ).

Most of the O-360s have a restricted operating range, due to third and fourth order harmonics related to the COMBINATION of the prop AND the engine. This isn't just a prop thing,nor is it just an engine thing, it's about the rotating assembly.

The O-360 A1A does not have mass balancers on the crank, which is why so many of these engines have rpm restrictions.

I've asked Lycoming for the definition of "continuous" in this context, and they basically told me that anything over five minutes would be considered "continuous".

Remember that with a constant speed prop,there are always two, and sometimes more, ways to achieve a particular percentage of engine power. If you want 65% power, that may be done with, say 2350 and 20 inches (I'm making up numbers here, so don't quote me) OR, it might be done with 1950 and 23inches.

If the placard is in fact accurate, and as you noted, I'd say I wouldn't worry about that restriction UNLESS I was running at near idle power. You can check the wording on the placard by looking at the STC for the engine installation. That'll list placards that must be installed.

MTV

AKclimber wrote:I have an O-360 A1A with a CS prop (McCauley).
The RPM gauge has a caution range (marked yellow) in the 1950 to 2350 range.
There is also the following placard:
"AVOID CONTINUOUS OPERATION
BELOW 15" H.G. BETWEEN
1950 AND 2350 R.P.M."

My first question is that placard referring to the possible prop or the engine issue - i.e. what happens (or what can one damage) when one operates in this range and how long is continuous.

The second question is that the Lycoming manual recommends 21.6 H.G. and 2200 R.P.M at 2,000' ASL for 65% power cruise at 8.8 gph., putting me firmly in the caution, yellow zone for the prop R.P.M.. Am I correct that the placard for MP and RPM is for when both conditions are present, not either one separately.
Or am I missing it all together?

If I read this right it said below 15HG, wouldn’t 21HG be above? Putting you out of the caution range. Could there be a carb icing issue involved?

I have the O-360 A1A engine (AVCON conversion), but the prop is a field approved McCauley model # C3D36C415-C/G82NGA-8 in place of the Hartzell model HC-C2YK-1B/7666A-2.
I have yet to find in the paperwork where that restriction came from.
The previous owner only ran it at 24 squared and planned 10 gph at 125 mph.
I am trying to find out my best economy cruise by using the different MP and RPM combinations, as well as a nice, slow combination for just tooling around site seeing. I did note that by having lower MP to RPM ratio I go slower than when the ratio is more equal...

172heavy wrote:If I read this right it said below 15HG, wouldn’t 21HG be above? Putting you out of the caution range. Could there be a carb icing issue involved?

That's what I think, as well, but just want to confirm it's a 15" HG *and* 1950-2350 restriction not a 15" HG *or* 1950-2350.

I have an Avcon too. That is where the restriction comes from. The STC papework specifies that you place the Red Scale on the Tach between 1950 & 2350 RPM. I know the same engine/prop in other planes don't have the restriction so I always thought it was from mounting the engine GG further out on the motor mount. Basically the Prop is in the same place and the two back cylinders are gone.

I wonder about it too because when your going through that range up or down it feels smooth. I just live with it because I don't want to find out "why" as I'm plummeting to the earth after the engine flys off.

When I was taking my BFR from a 20,000+ retired Airline pilot (he also owns small planes and a twin), I asked him what settings I should use to get the best economy. He told me that first you decide what fuel burn you want. Then go to the performance table and set the RPM and Maniflod pressure to get that fuel burn. Then you fly at the highest altitude you can maintain while using those settings. He said, that will give you optimum performance. Sounds good to me

PS: AK climber, I have and Avcon power chart if you need a copy.

Jaerl wrote: I have an Avcon too. That is where the restriction comes from. The STC papework specifies that you place the Red Scale on the Tach between 1950 & 2350 RPM..........

The Avcon conversion uses the Hartzell prop, so with a Mac the mp/rpm restriction may not be applicable. You need to look through all the paperwork from the field approval for the Mac prop to see if that restriction is addressed. That restriction may no longer be applicable, OR there might be a whole new restriction.

Well, I'm getting more confused now....
The AVCON STC calls for an RPM restriction (red arc) on the Hartzell propeller for 2000 to 2250 and does not mention the MP restriction.
The field approval 337 form for Mac propeller does not mention any RPM restrictions, and neither does the prop manual.
I cannot find anywhere in the logs or paperwork where this MP and RPM restriction came from.

My placard adds the words "while descending" or "during descent" or some such--don't recall exactly. I asked my IA why, he said he didn't know, just that the STC requires it, so I follow it. I normally run the rpm at 2400 anyway, so it's not a big deal. Below 15" typically means descending, so perhaps it's to keep the pilot from cranking back the prop during a long descent. Except when I'm descending in the pattern, I typically descend from cruise by just nosing over and leave the power setting the same as at cruise, 21" (or less depending on altitude) and 2400 rpm. To answer the original question, I think it's pretty obvious that it's a "both" proposition.

Cary

I just pulled out the STC for my Avcon conversion. It's for 175, 175A, B, C & 172D. There is no explanation anywhere why not to use that RPM range. It states "Avoid continuous operation between 2000 and 2250 RPM" In two more places it refers to the Placard required "In Close Proximity to the Tachometer" and "Install all necessary Placards". Actually doesn't say I need it on my tach but my mechanic made me do it anyway. Maybe I missed it.

When I had the Prop balanced I found my Tach was reading 50 RPM slower than the engine was turning. We balanced the prop at 2350 indicated so I was running 2400. I run at 2350 - 2400 when I'm cruising because that is where my engine runs the smoothest. If I'm in a long decent, I drop the RPM's below 2000. I get more vibration if I run 2250. It is really smooth running slow but I don't like to run low RPM's and high(er) Manifold Pressure.

AKclimber wrote:I have the O-360 A1A engine (AVCON conversion), but the prop is a field approved McCauley model # C3D36C415-C/G82NGA-8 in place of the Hartzell model HC-C2YK-1B/7666A-2.
I have yet to find in the paperwork where that restriction came from.
The previous owner only ran it at 24 squared and planned 10 gph at 125 mph.
I am trying to find out my best economy cruise by using the different MP and RPM combinations, as well as a nice, slow combination for just tooling around site seeing. I did note that by having lower MP to RPM ratio I go slower than when the ratio is more equal...

I would get in touch with McCauley, ASAP. They should be familiar with that setup. They probably are the ones that did the vibration survey on that engine/prop combination.

GIve them a call on Monday.

MTV

Jaerl wrote:I just pulled out the STC for my Avcon conversion. It's for 175, 175A, B, C & 172D. There is no explanation anywhere why not to use that RPM range. It states "Avoid continuous operation between 2000 and 2250 RPM" In two more places it refers to the Placard required "In Close Proximity to the Tachometer" and "Install all necessary Placards". Actually doesn't say I need it on my tach but my mechanic made me do it anyway. Maybe I missed it.

When I had the Prop balanced I found my Tach was reading 50 RPM slower than the engine was turning. We balanced the prop at 2350 indicated so I was running 2400. I run at 2350 - 2400 when I'm cruising because that is where my engine runs the smoothest. If I'm in a long decent, I drop the RPM's below 2000. I get more vibration if I run 2250. It is really smooth running slow but I don't like to run low RPM's and high(er) Manifold Pressure.

The reason for that restriction is that during the vibration survey of the propeller/engine combination, they found destructive 3rd or 4th order harmonic vibrations in that range. Since they specify "no continuous operation" it's apparent that it took a while at that rpm range to create an issue.

A word to the wise: If there are rpm restrictions on your engine/prop combination, abide by them. Third or fourth order vibrations are VERY bad, and you probably won't FEEL them.

MTV

does the MT prop get rid of the restriction?

Just a thought but the Hartzel has potential to crack the hub. It could have something to do with the restriction. The vibrations that MTV is talking about could aggravate the situation. My Hartzel works OK for now but if I could, I would switch to a MT prop in an instant. I think the two blade gives you 60 lbs more thrust.

mtv wrote:...... Third or fourth order vibrations are VERY bad, and you probably won't FEEL them.

You've mentioned "third or fourth order vibrations" several times. And that you can't feel them. I guess I don't know what you're talking about--how about a brief education?

Cary wrote:My placard adds the words "while descending" or "during descent" or some such--don't recall exactly. I asked my IA why, he said he didn't know, just that the STC requires it, so I follow it. I normally run the rpm at 2400 anyway, so it's not a big deal. Below 15" typically means descending, so perhaps it's to keep the pilot from cranking back the prop during a long descent. Except when I'm descending in the pattern, I typically descend from cruise by just nosing over and leave the power setting the same as at cruise, 21" (or less depending on altitude) and 2400 rpm. To answer the original question, I think it's pretty obvious that it's a "both" proposition.

Cary

I have slept on it and once again this is only a guess, but here goes, I think that any thing below 15HG and the RPM settings will not develop sufficient heat in the heat exchanger to melt or prevent carb and manifold ice, prolonged periods @ these settings could leave you with carb ice and no way to remove it. Imagine after a long decent your engine looses power and there is no carb heat to melt it with, now wouldn’t that be a bitch.
Once again this is a guess, but I doubt that the placard is for harmonics, generally harmonics are the tuned length of part assembles and fed from other sources, beat frequencies that feed and help amplify these vibrations, reducing power or low power normally has the opposite effects on harmonics or none at all, perhaps exception to that is for gear driven props where low power settings in decent will cause the gears to chatter and break parts.
Call Avcon and ask them.

hotrod150 wrote:

mtv wrote:...... Third or fourth order vibrations are VERY bad, and you probably won't FEEL them.

You've mentioned "third or fourth order vibrations" several times. And that you can't feel them. I guess I don't know what you're talking about--how about a brief education?

A third order vibration is one that pulses three times for each revolution , a forth order vibration pulses pulses 4. Each cylinder of a 4 cylinder 4 cycle engine will produce fourt order vibrations, and are usually expected and tamable, I suspect a vibration of a third order on a 4 cyl 4 striker to be a PITA to deal with. Maybe not so bad on a 6 cyl. Though....
Vibration is a weird beast, and props are pretty much voodoo... Ask any good prop guy about vibes, it is almost always the vibes you can't feel that do the most damage, first order, tenth order... No matter...

There is no doubt in my mind this restriction is due to harmonics. I suspect the FA for the Mac was granted during a friendlier period (FAA wise) and as such, no vibration tests were done on the combo. It is likely the restriction was just maintained as a CYA. And yes the MT would rid you of it for good. Most banner tow Huskies go this way because that restriction is exactly where they want to tow....

Take care, Rob