Prop not holding RPM Issue

I'm having an issue with a Mccauley prop holding the proper RPM. To recap the issue:

1. First takeoff of the day, the governor works fine. As speed increases the prop comes off the pitch stops (2660-2680 ish?) and goes to 2700. It holds it there pretty nicely.
2. Subsequent takeoffs (basically once the oil comes up to about 140-150 or more) once the prop comes off the pitch stops the governor will pull it down to 2600 or so. Sometimes it will surge up between 2600-2650ish.
3. On climbout and cruise and I’ll see approximately 30 rpm fluctuations, random but constant from wherever I set the rpm.
4. Once in cruise it will settle down after about 15 minutes and I’ll see a less than 10 rpm fluctuation. (At this point you can’t hear it anymore, but you can see it on a digital tach. For all I know they always did this and we just didn’t know it with old analog tachs.)

The governor was overhauled in 2018. In 2020 I bought the airplane and noticed the issue. I sent the governor in to the OH shop, they bench checked it, said it was fine and I reinstalled it. The issue remained, and I largely lived with it until now. Either it’s gotten worse lately or I’m just sick of dealing with it. So I sent it to another shop who felt it was the speeder spring. They checked it, couldn't find anything wrong, changed the speeder spring anyway, charged me a ton of money, and...no change once reinstalled on the plane.

The first shop felt it was the prop. It's got less than 1000 hours OH, but calendar wise it's been 15 years or so. However its tight, doesn't leak a drop, and is otherwise in good shape. I pulled the dome, cleaned out the crud, and put in a new oring...no change. The governor has about 300hrs on it since OH.

Any thoughts here? I really don't want to just shotgun this and send the prop in unless I can determine for sure that's where the problem lies.

What about the cable?

Given you have looked at the Governor and the Prop quite closely, what about the remaining link in the chain - the control cable?

I'm not discounting the Governor or the Prop as there are still a couple of things you can probably look at there, but the cable is often overlooked as a possible cause. It can depend upon the type of cable too as some are more susceptable to loss of movement than others. You state that the reduction of power is in the order of up to 100 RPM which might equate to a movement of the Governor Control Head of maybe 3/8" to 7/16" of movement. This could easily be something the cable could account for. A few quick checks while the engine is hot will easily answer this question for you. I'm assuming you don't need me to spell out what you need to do to check this but ask away if you need help.


Cheers,

Assuming it is a traditional prop that uses pressure to move the blades from fine to coarse (implied by your statements of coming off the stops on TO), I can't imagine any problems or failures inside the prop that would make it go coarser than commanded by the governor, with the exception of a problem with the springs becoming weak, but that would not match your symptoms, which are intermittent.

It may be worth following the Lyc SB for testing the gov -> prop oil path. Oil is sent to the prop by flooding the center of the nose bearing cavity and then getting to the crank core. Given the symptoms of working well with cold oil and not well with hot oil, kind of points in this direction. If there is a lot of leakage in the nose bearing that is forcing the governors pump to work at full capacity, it wouldn't be surprising that it is hunting and struggling and over shooting because of this. Cold oil temps allow higher pressures and therefore keeping the gov's pump just outside that fringe zone at the edge of it's capacity. I don't know much about the engineering of governors and it seems a bit odd that it would over pressurize with leaking bearings, but have seen all sorts of strange things when you take a complex device out to the edges of it's operating envelope. Also, IIRC, the #1 bearing is flooded in two area from the gallies and has a third, center area that transfers the governor oil to the crank core. If the bearing clearance are too loose, it is possible that there is more oil pressure bleed over and that would seem more likely when the oil is hot and thin. I have always assumed that problems here would cause the prop to go too fine vs too coarse, but not studying all of the potential failure scenarios I am not convinced it couldn't happen. That said, the hot vs cold scenario in your symptoms do seem to loosely point to oil being part of the issue.

Ok thanks guys. I assume you are talking SI1462A? I'm kinda almost afraid to do that one. If it fails, well.....

I'll check the cable also.

Ive been having a similar issue with my -6 for at least 7 years. I have rebuilt the governor, rebuilt the prop, replaced the governor cable, replaced the stainless line from the governor to the case, replaced the oil pressure spring in the engine, all oil lines, and switched from Ellison fuel injection to airflow replacing both fuel pumps in the process. Nothing has worked. I have a 900hr 0-320 with an Edo-air governor and a Hartzell prop. Im about to just buy a new governor and see if that solves it. It feels like im just throwing parts at it. The airplanes 23 years old so its not totally wasted money to upgrade but its getting old.

I had a problem with my prop holding RPM, and I was running Phillips XC (20w-50). I called Byam and talked to them about it, he said they had seen this a couple times and to switch to Aeroshell straight W100, that this had solved the problem for multiple people. I switched to it, and have kept it with that the last 2 years, haven't had any problems since. About a year after that however, I had the prop IRAN'ed for other reasons, so there's that.

The more I think about it that SB probably wouldn't apply. That appears to be for props that are running away (exceeding RPM) due to low oil pressure at the prop. Mine is not making RPM, which would mean that the prop is seeing excessive pressure (not insufficient pressure). In my mind, that would rule out any internal leaks in the prop as well.

I wouldn't be surprised if changing oil made a difference, but in my mind that's a bandaid. It should work with 20-50, and most everyone else's does.

The guy at the prop shop said they've seen issues where a particular governor continually tested fine but just didn't like a particular airplane. They'd replace it and problem goes away. However I just paid $500 to have mine IRAN'd and really don't want to keep shotgunning this.

Question. Are you really hearing it or just going by what you are seeing on your electronic tach? I have replaced 2 tach sensors on my airplane for a similar issue. The heat seems to break down the sensor over time and like you describe once the engine is warm the indication becomes unreliable. Not saying this is your problem but its another avenue to look at.
Ryan

Rallylancer122 said:

The more I think about it that SB probably wouldn't apply. That appears to be for props that are running away (exceeding RPM) due to low oil pressure at the prop. Mine is not making RPM, which would mean that the prop is seeing excessive pressure (not insufficient pressure). In my mind, that would rule out any internal leaks in the prop as well.

I wouldn't be surprised if changing oil made a difference, but in my mind that's a bandaid. It should work with 20-50, and most everyone else's does.

The guy at the prop shop said they've seen issues where a particular governor continually tested fine but just didn't like a particular airplane. They'd replace it and problem goes away. However I just paid $500 to have mine IRAN'd and really don't want to keep shotgunning this.

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As I see it, only three logical ways for prop to pull too coarse of a pitch. 1 - the springs in the prop are failing and allowing too little oil pressure to move the blades, but struggle to see that as intermittent via hot or cold. 2 - The governor is not working correctly, even though it tests fine on the bench. This is reasonable, given that it works fine for you when cold and assuming the shop tests it cold. 3 - something is allowing additional oil that did not come from the governor to get to the prop, causing it to go more coarse. While #2 seems most likely, after having 2 different shops tell you it is working as designed, you need to start exploring #3 or send it to a third shop or replace it. That nose bearing has two different oil circuits pushing oil into the somewhat shared space between the bearing and the crank. One from the oil pump to flood the bearing and the other from the governor that routes to the crank core. Suggest you do some research there and decide whether or not oil from the main galley can get to the prop and what might happen if it did. The fact that the SB applies to a different problem does not mean that your issue couldn't come from an issue in that bearing area. I am not for a minute suggesting that is the problem, but if the gov keeps coming back as "good" at some point you need to look at the more obscure issues.

I bet well over half the users here run 20w50, including myself, and above is the very first post I have seen claiming that 20w50 will cause a prop to hunt and misbehave. Sorry, but just not buying that. It's possible someone had an issue with something that was running on the very fringe of what would work and a different oil pushed it over the edge, but there are just too many people using that oil for it to be an issue.

One last thing. The governor has an oil return for bleeding off excess oil to reduce pressure. If that were partially clogged, the gov would struggle to go fine. That said, I would expect much more wild swing of RPMs than what you are experiencing and should be worse when cold.

The key to your issue seems to be oil temp. Cold oil works as designed and hot oil goes too coarse. Best to keep diagnosis efforts related to oil viscosity.

Excellent points. Didn't think about the return.

And yes, it's definitely doing it. You can hear it.

Well, it's not the cable.

Dumb question, but should the rpm be set on a hot or cold engine? Theoretically it shouldn't matter I guess but maybe I'm missing something.

Rallylancer122 said:

Well, it's not the cable.

Dumb question, but should the rpm be set on a hot or cold engine? Theoretically it shouldn't matter I guess but maybe I'm missing something.

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Well that's eliminated that potential cause. Well done.

When testing and setting up governors after overhaul we always did it with hot oil. After all, that's typically where the system is operating when you're flying - hot.

Rallylancer122 said:

Well, it's not the cable.

Dumb question, but should the rpm be set on a hot or cold engine? Theoretically it shouldn't matter I guess but maybe I'm missing something.

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Doesn't matter. Governor constantly observes RPM and dynamically adjusts the oil pressure to hold it. Most have their own oil pump to ensure they have enough pressure to get the job done. I take off with 80* oil and the Gov holds a dead solid RPM all the way up to 200* Now, if you have other issues between the gov and the prop, like extra oil being introduced or oil drained off or inability to bleed off excess pressure to the return, etc., then different temps create different viscosities which can create different issues at different times. But when all is right, oil temp or viscosity should have no bearing on a governor's ability to hold an RPM. That said, if your gov is operating on the fringe of it's operating range due to some problem, it is possible that viscosity changes could push it over the edge. Pretty sure they can normally handle a pretty wide range of conditions. I would like to think that the shop tested for that, but with the things I have seen I am not so sure.

lr172 said:

Doesn't matter. Governor constantly observes RPM and dynamically adjusts the oil pressure to hold it. Most have their own oil pump to ensure they have enough pressure to get the job done.

Now, if you have other issues between the gov and the prop, like extra oil being introduced or oil drained off or inability to bleed off excess pressure to the return, etc., then different temps create different viscosities which can create different issues at different times.

But when all is right, oil temp or viscosity should have no bearing on a governor's ability to hold an RPM. That said, if your gov is operating on the fringe of it's operating range due to some problem, it is possible that viscosity changes could push it over the edge.

Pretty sure they can normally handle a pretty wide range of conditions. I would like to think that the shop tested for that, but with the things I have seen I am not so sure.

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Yes, good points.

When testing them, I can attest to the fact they are sluggish with cold oil. They will work as you say, but they certainly don't react as quickly or as well as when the oil is hot. But they do still work.

The internal oil pumps are typically set to a working pressure of between about 210 PSI and 320 PSI depending on the goveror and prop combination. This is plenty to provide control over the props spring tension. This oil is then controlled via a pilot valve which is acted on by flyweights. If the flyweights are sticking or the feet on the flyweights are not perfectly radiused, the governor will tend to "stick" at an RPM and then hunt either side of that RPM as the flat spot on the flyweights influences the position of the pilot valve. While the rest of the governor might be in good order, flat spots on the flyweights can directly influence performance in the manner you describe. I would also like to think that the shop visit would have picked this up. The speeder spring can also affect performance however this is more likely to cause constant hunting as it doesn't exert enough pressure on the flightweights to dampen their control over the pilot valve.

If the clearence between the main governor driveshaft and the pilot valve is outside limits this could cause a loss of control too. Deep scoring of the pilot valve is easy to pick up on a visual inspection but its harder to be sure of the internal clearances between the internal bore of the driveshaft and the pilot valve. That said, unless the govenor was on its second life and had done a few thousand hours I wouldn't expect this to be an issue. But wear here would see a lack of control when the oil was hot.

The flow of oil through to the prop is a constant loss system so if you had a restriction of some kind on the outlet side of the governor this could definitely have an effect, however I would have thought that anything blocking this area would provide a constant restriction, not a variable restriction. I'm struggling to think what could do that, but lr172 is 100% correct. Definitely worth looking at.

Its a head-scratcher.

planenutz said:

YI would have thought that anything blocking this area would provide a constant restriction, not a variable restriction. I'm struggling to think what could do that, but lr172 is 100% correct. Definitely worth looking at.

Its a head-scratcher.

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While not having studied fluid dynamics beyond fuel flow in carbs, which have a surprising amount of it, I believe that the volume passing through a restriction is based upon the area of the restriction (also impacted by tapering shapes on entrance or exit), the fluid pressure, both before and after the restriction, and the viscosity of the fluid making the passage. Therefore, if dealing with a restriction problem, oil temp changes (and therefore viscosity changes) will affect the volume passing that restriction and therefore possibly the symptoms of the problem.

lr172 said:

While not having studied fluid dynamics beyond fuel flow in carbs, which have a surprising amount of it, I believe that the volume passing through a restriction is based upon the area of the restriction (also impacted by tapering shapes on entrance or exit), the fluid pressure, both before and after the restriction, and the viscosity of the fluid making the passage. Therefore, if dealing with a restriction problem, oil temp changes (and therefore viscosity changes) will affect the volume passing that restriction and therefore possibly the symptoms of the problem.

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Yes, very good point Larry. I was thinking more along the lines of trying to understand what could be physically sitting in the return ports that is randomly moving into and out of the oil flow, but you're right, viscosity changes might make a difference.

I'm looking forward to hearing what the final solution is.

planenutz said:

Yes, very good point Larry. I was thinking more along the lines of trying to understand what could be physically sitting in the return ports that is randomly moving into and out of the oil flow, but you're right, viscosity changes might make a difference.

I'm looking forward to hearing what the final solution is.

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Without a doubt a real head scratcher here. I really struggled to come up with a scenario that could cause the system to over pitch and the most logical was a restriction in whatever system/circuit the gov uses to bleed excess oil. However, thinner oil should bleed more through the same restriction than cold oil and therefore doesn't match the symptoms. But I just don't know a lot of details of the governor systems, at least beyond the speeder spring arrangement. Learned a lot about that when I converted my S series gov from 2575 to 2700.

Just on the 10 RPM fluctuations - as you say, the analogue guage is probably not going to identify this as they're dampened to prevent large swings of the needle, whereas the digital signal might well be less dampened by the signal conditioning. I don't believe either to be any more or less accurate than the other, just less dampening on the digital signal perhaps.

Any chance you could borrow another governor to try? It's really the next step towards resolving this.

planenutz said:

Any chance you could borrow another governor to try? It's really the next step towards resolving this.

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That would be ideal, and it did cross my mind. Unfortunately I don't know anyone with one laying around. I'm not real comfortable asking someone to take the governor off their flying plane.

Rallylancer122 said:

That would be ideal, and it did cross my mind. Unfortunately I don't know anyone with one laying around. I'm not real comfortable asking someone to take the governor off their flying plane.

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I have an old F series (F4 something IIRC) that came off my 540 core. You are welcome to use it. The engine was a prop strike core, so the gov was working when I got it. It has been sitting in my garage for 5 years and have no idea whether or not that will kill it.

flyboyeaa said:

Ive been having a similar issue with my -6 for at least 7 years. I have rebuilt the governor, rebuilt the prop, replaced the governor cable, replaced the stainless line from the governor to the case, replaced the oil pressure spring in the engine, all oil lines, and switched from Ellison fuel injection to airflow replacing both fuel pumps in the process. Nothing has worked. I have a 900hr 0-320 with an Edo-air governor and a Hartzell prop. Im about to just buy a new governor and see if that solves it. It feels like im just throwing parts at it. The airplanes 23 years old so its not totally wasted money to upgrade but its getting old.

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Just an update, I replaced my governor with another one same make and model. The surge stopped. I guess the old one had a problem that wasnt caught during overhaul. Time to put some hours on to see make sure its the real fix.

Check out the guts...

Well I'm pleased you've got it sorted out now.

If you get inquisitive and pull the old one apart, I'd be interested to see some photos of the flyweights and pilot valve.