Prop / Governor RPM Variations

[Charles Schefer]

ICS Friends:

Ever since I first bought my PA-30B, I've occasionally seen slight variations in the left engine RPM. Usually both are rock solid and I have no problem keeping the props in sync. But on occasion, I would get brief periods of the props going in and out of sync (audibly) and the left RPM needle would bounce up and down +/-50 RPM... then become steady, the bounce again. No rhyme or reason just occasionally.

Both of my engines are pretty tight in terms of little oil seepage but recently I've seen more from the left side which we tracked down to the prop governor.

Because of the slight leak and the occasional wandering RPM, I looked at my 8130-3 file and sent the governor to the shop who overhauled it for the previous owner about 700 flight hours ago (when the engines were overhauled so were all the accessories). I was pretty impressed with Mike Grow of Professional Aircraft Accessories, Inc. talking to him on the phone. His colleagues tell me he has 40+ years experience in these governors and it showed from the conversation.

The Mike and his shop bench tested it for me and much to my surprise said the governor was perfect. The only leak was a very slight one in the middle section so he replaced that gasket, ran it again for hours on the bench at full temp and said it was working perfectly no issues. He said an overhaul was not necessary and would be a waste of money (always impresses me when a shop turns down extra work because it's truly not necessary even tho they could easily get it if they wanted to...). Anyways... back the governor came and we reinstalled it with new gaskets between the governor and accessory adapter and for good measure - new gasket between the accessory adapter and the engine casing.

Flew to Chattanooga over the holidays (5.4hrs round trip) with no problems but there was still the very occasional left engine wandering RPM and looking in the engine nacelle the governor is once again a little wet with oil (tho less than before). Really it's mostly the Bendix RSA unit and cowl flap below that get wet but the oil seems to be coming from what I've been told is the pressure relief valve on the governor above as seen in the attached pic.

Of bigger concern, on Thursday this past week (2 days ago) I flew the plane to Asheville, NC and back. About 1/2 way to AVL the left engine RPM seemed to start bumping around more than normal and If found myself doing a lot of work to try and re-sync the props. Now it was also a very windy day and flying over the mountains (East Coast version not the Rockies) it was fairly smooth but there were a fair number of up and down drafts so maybe this contributed...?

On the flight back from AVL the left engine prop RPM was even worse. I tried to set them at 2400 for cruise and the right engine stayed fairly solid but the left engine seemed to bounce between 2350 and 2500. It would center around 2400 but then constantly surge back and forth creating a very annoying yawing affect on the aircraft as I flew home. The last 40-50 min of the trip it got a lot better but far from perfect.

After landing, inside the cowl - once again oil coming from that relief valve (I'd throughly cleaned it with isopropyl alcohol and dried it before so as to be clear in terms of new oil seepage). We checked the prop governor cable to see if there might be play in it such that it would allow the governor to be moved around by engine torque even tho the control in the cockpit was fixed in place - that did not seem to be the issue but we are going to take a second look.

Mike Grow the prop gov guru was out of the shop on Friday but I hope to chat with him next week for advice. Before he did tell me that such RPM variations can also be caused by the prop itself having issues. We are considering swapping the left and right governors to see if the problem moves to the right side but that's a bit of a headache. i'd like to know why I'm losing oil from the pressure relief valve when Mike told me there were no leaks and all was good on his test bench.

I've also been told that the oil line from the governor to the prop (runs under the cylinders on the right side of the engine) can be an issue - particularly at the flange where it meets the governor. They are known to crack and leak according to Mike but we checked mine carefully and that does not appear to be the source of the leak.

Anyway I am not sure if my oil seepage has anything to do with the RPM issue or not - perhaps connected issues, or perhaps not. Any thoughts or feedback or experience on this would be appreciated.

UPDATE: I've posted some iPhone videos of the RPM issue in action from my flight two days ago to Drop Box. There are three videos that can be found here -> . Not that it matters but the first video is from the trip down to AVL and the other two were on the way home when I felt the problem was worse Looking at the videos I am not sure any one looks better than the other but just FYI.

ANOTHER UPDATE: I just read on this page -> under the section near the bottom entitled "Other Factors" that:

"The Hartzell used on the Aztec and some twin Comanches relies on just the air-charge in the prop, the ATM (aerodynamic twisting moment) and the internal spring to counteract or balance the governor pressure. There are no counterweights. If the dome air charge is low, the RPM will be airspeed sensitive; that is, the RPM will increase with airspeed and decrease with airspeed. This is true of most Hartzells but especially of this particular model because of the lack of counterweights."

This may be relevant as I do find the left dome leaks down over time whereas the right dome holds it's pressure well. I just set both dome pressures before my Thanksgiving Chattanooga trip. I typically check them every month or so per the book. I did not check them before my AVL flight last week...

- Charles

[N3322G]

Have had low dome pressure produce these results in past.

Be sure correct table is used to charge. Hartzell manual requires massive attention to detail to choose table 6-2 over 6-3 and they are both temperature dependent. Know your style is will fit right in with this requirement. Note everyone makes mistakes and learned in Oct Twin's props were significantly overcharged.

Still may not be the problem.

Another thought, oil goes into the prop chamber but does not circulate. Over time sludge builds. Have the props been off lately to clean the sludge accumulation? This is called for on Piper inspection checklist but in 20 years of my ownership, the last IA was the first to point this out and none have done it prior.

[Charles Schefer]

Pat,

Thanks for all this info.

Props have not been off in a long while. They were last overhauled or resealed less than 1K hrs ago but a long time (calendar) ago. I had them off to change alternator belts but nothing to do with sludge...

Dome pressures - as I mentioned the left side constantly leaks down and so maybe an overhaul or reseal is needed. Just read about the revised pressure table. Didn't know about it. We've just been going from a card/placard inside the prop dome and setting based on temp. Typically we are in the 40-50 psi range based on the temp per the placard...

I wonder if some blockage (eg sludge etc) in the prop or perhaps incorrect dome pressure is forcing more work on the governor, causing higher pressure and causing the relief valve to weep oil...

Will check out table 6.2 vs 6.3. Are those in the Piper or Hartzel manual?

Thanks Much,

- Charles

[N3322G]

That's the 6-2 is the Hartzell manual but it is also in service manual. IIRC the top of that is 53 psi so looks like you are in range.

See the middle of this long page for sludge photos http://www.comancheflyer.com/forum2/vie ... dge#p42463 double click to view photos and enlarge and back button to return to post.

Hope it helps.

[Charles Schefer]

Thanks Pat.

The other day, before Thanksgiving and about 10-11 flight hours ago, I'd set the domes to 48.5 psi each. That was the interpolated correct pressure for the temp that day. We just checked the domes again. It's colder today and the right dome is now at 47 psi but the left has dropped to 44 psi. The left definitely leaks down more slowly but I don't think the delta is significant enough to be the root cause of my RPM issue on the left.

I'm beginning to think maybe your sludge in the prop hub comment is the issue. I'm going to make some more calls today and see what I can find out.

One other tidbit... In the three years I've had the plane I've noticed that when cycling props on the ground the left always responds more slowly. Not too slow IMHO just slower than the right.... Maybe this is also related.

- Charles

[N3322G]

after some work, right gov had to be removed and adj as it was slower for Prop cycling after O/H at Hartzell. Took multiple gos at this. A bother as oil filter had to be removed each time.

[Charles Schefer]

[Will Berninger]

[Charles Schefer]

[Will Berninger]

That is all good news Charles! Did not mean to scare you or suggest your engine was/is toast. Just passing along some information and a suggestion. You are very correct about the analysis being all about trends however if you did have a gizmo chewing itself up, trend or not it should show as out of the normal for the series engine which is also a bench mark they watch.

I have seen prop governors act strange although working normally on the bench many times. They just can't match the real world conditions on the bench. Although a pain in the back side, your idea of swapping GVN's left to right would at least narrow it down.

There is also a collar in the engines that can leak causing issues but that fix is not an easy one either. This helps direct the oil from the GVN through the crankshaft and into the propeller dome. There is a procedure for checking it for excessive leakage. Something like a differential compression test type thing, so many PSI in and you must retain so many PSI as I recall. Like 30 PSI in and you need to retain 8 minimum.

I personally have never ran across a propeller needing overhaul to correct this type of issue but I am sure there is always a first time. One thing I do watch out for though (which may or may not be popular here) is that you can only overhaul these propellers so many times before you wear them out of limits by processing them. Running a propeller through a tear down, inspection and reseal if your just up against 6 years can save the propeller for an overhaul later when it really needs it. They can inspect it for binding, corrosion, cracks just not necessarily run it all the way through the process so to speak.
Just food for thought, you have to do what your comfortable with.

Also, maybe you mentioned it, but if you change your power settings does that have an impact on this situation? How about in a decent with unchanged power settings? Curious if there is one sort of sweet spot that something is binding at...

Another thing which may have zero impact, have you tried switching off one mag at a time in cruise to be sure you do not have some sort of cross firing from a mag that potentially has an issue your not seeing on the ground? Just run on say the left mag a bit, then back to both, then the right mag to see if it is acting worse, better, same...

Last suggestion, have you tried cross feed, different tanks and using the boost pump? Change it all up in case your sucking air somewhere and getting actual power surges rather than a propeller control issue. All easy enough to do and basically free. Just be sure you know how to do all that before your accidentally shutting down engines with selectors in the wrong spot

Again, food for thought.
Good luck.
Will B.

[Charles Schefer]

Will, these are great thoughts and suggestions - Thank You!

- Charles

[Will Berninger]

[Charles Schefer]

Will I tried some of your suggestions....

I flew 2.8 hours on Saturday and another 2.6 hours last night and uncovered something interesting in the process...

First off on the first flight (2.8hrs Sat) I had NO problems whatsoever. Both props behaved well, and the left-hand did not vary abnormally. I had intended to check dome pressures before flight but didn't have a chance. At any rate the plane and both engines / props performed flawlessly.

Sunday morning, before my second flight I checked the dome pressures. Ambient temperature in the hangar was 65F so the proper dome pressure should be 48.5 psi per the placard inside the nose cap. Using a calibrated digital pressure gauge, I found the right hand dome pressure right about where it should be, it was 48.7 psi. The left hand dome had done it's normal routine of dropping down to about 35 psi. It's a small pressure cavity in those domes and even just checking the pressure causes a 1 psi loss in pressure for each check with the gauge. I serviced both with nitrogen setting them to 48.5 (+/- 0.2) psi.

Before setting the left dome I did try your suggestion of rapidly releasing the pressure to see if any oil came out. I did not want to pressurize the dome to a level higher than listed on the placard. I only went to 50 psi max but I then pressed the Schrader valve open to rapidly drop the pressure to 20 psi or less. I did this 2 or 3 times and there was never any sign of any oil coming out the valve. I then set the final pressure and put the dome caps back on.

I then did my second flight last night (Sunday night - AFTER bringing the left dome back up to correct pressure). In the climb, after I pulled back to 25-squared the left prop started doing it's RPM variation thing again. Generally what I would see is that the RPM would hold steady at 2500 but then periodically "surge" up to say 2600 then drop back to 2500. In the process of dropping back it will overshoot and drop to say 2450 before bouncing back to 2500. I tried switching off just the left or right mag on the left engine but it did not make much difference (my mags were overhauled not that many hours ago). I tried increasing rpm and decreasing and experimenting. Not much difference tho I think the left behaved better at higher rpm settings than at 2500.

Upon reaching cruise, and pulling back to 2400 rpm I was able to get the props in sync and did not have much issue. I did see some occasional variation but not as much in cruise as in climb.

To get an idea of what I am talking about here's a link to 3 videos in my dropbox. These videos are ones I should 3+ months ago when I first had the problem. These videos were in cruise at 2400 rpm targeted whereas last night I had the problem largely in the climb. The point is it lets you see the general behavior...

https://www.dropbox.com/sh/y4ld847odmgx ... B9w-a?dl=0

The link above is publicly available, nothing special to do, just click the link and you can see the 3 videos there.

In descent the rpm also behaved fine, it was really just in the climb at a targeted 2500 rpm I was having the issue. I checked the mags as suggested but I can't say I experimented much with power setting changes in the climb (I forgot that part of your suggestion in the moment).

The one thing that really strikes me here... perhaps a coincidence but maybe not.... on Saturday the left side behaved perfectly... but the dome pressure was low at about 35 psi. On Sunday AFTER I set the dome to 48.5 psi per the correct spec, I then had the problem in the climb. It's another data point that may give some ideas. I am tempted to go drop the left dome back to 35 psi (it will soon get there on it's own anyway) and fly again and see if it behaves normally.

If in fact the problem occurs when dome pressure is correct but does not occur when dome pressure is low what might that indicate?

Thanks,

- Charles

[Will Berninger]

Charles,
I will have to think about that, not sure what it would mean off of the top of my head.

Yes, those domes are tiny, great you noticed your loss per check!

That gauge is moving all the time, I can see why your annoyed! If you did not say you were feeling the plane yaw I would have guessed (by the video) that the dampening fluid in the gauge is bad. Also noted the RPM does seem to go up first. Hard to say for sure but a it looks that way (as you indicated).

I need to think more on it Charles!
Will

[Charles Schefer]