Prop / Governor RPM Variations

[Will Berninger]

Charles,
It just seems that it basically has to be a control issue of some sort (as you indicated early on). Whether it is the prop binding, GVN sticking, transfer collar leaking whatever, just seems the most likely.

In my first comment about the camshaft on the PA31-350 going out, we noticed such similar propeller variations. I am glad your not reporting poorer than normal performance or high metal in your samples!

You can swap props LT to RT on this plane if you want but it is a bit of work on the Lycomings. Might be a tick easier than swapping the GVN's but I think I would try the GVN's first.

In any case, I hope you find someone that can help you. If I think of something, I will surely pass it along.
Will

P.S. hard to tell from the tiny Avatar picture but do you have the new scimitar props on this plane? If so, curious if you swapped from the old 2 blade to the new 2 blade and have some thoughts to share (maybe you have a thread somewhere on this already to direct me to?).

[Charles Schefer]

Thanks Will. You're making me a tad nervous regarding your Chieftain comparison but other than from the #3 cyl I changed and broke in last time there has been no sign of metals in the oil or filter. Also the problem seems to go away with lower dome pressure - weird eh? As noted in previous posts, I just noticed this but when I piece together what I've seen on past flights this seems to fit. I may experiment by lowering the dome back to 35psi and then see what happens on a test flight.

The governor has been bench checked twice. I'm suspicious of the prop. These props are the originals (not scimitar) and they've only been overhauled once! In fact one of the overhauls looks more like a reseal not an overhaul. I agree with your earlier comments regarding not wearing the props out by excessive overhauling but in my case I think they may just be due.

In terms of help, my Director of Maintenance who is an A&P / IA is in the office next to mine here in the hangar. He and I do all the work together. We've had the props off before putting new alternator belts on. Doesn't seem that hard to swap them. We could swap governors as well and that may be the next step. One advantage of having 2 of everything.

- Charles

[Will Berninger]

Not trying to scare you, sorry about that.
Will

[Charles Schefer]

No worries Will. My DOM and I talked this over today. I checked the left dome and its holding at 48-49psi where I set it Sunday morning before the flight where I had the issue. Sometime this week I think I'm going to:

1) With correct dome pressures fly and check behavior. If the problem reoccurs then;
2) I will lower the dome pressure to 35psi and fly again. If the problem goes away then;
3) I will raise it back to 48.5 (or correct press for temp) fly one more time to see if the issue comes back again.

If I'm right I'll have the issue at higher dome pressures and not at the lower pressure. I'll talk to prop shops and see what they say.

- Charles

[Will Berninger]

Sounds like your making excuses to fly

Curious how this turns out as if it works better with low dome pressure, that would be opposite of what normally happens. Hope your onto something there.
Good luck!
Will

[Charles Schefer]

[Charles Schefer]

Ok... so yesterday was nice day and I went out and did a little test, just in the airport pattern, making dome adjustments in-between. The ambient temp was 59F making the correct dome pressure 49psi and I set the left dome there. It' hadn't lost any pressure since the previous weekend but I went ahead and made sure it was right at spec.

On the prior weekend when I had the "surging" RPM at 2500 it happened in the climb right off the deck so I figured I could reproduce this right in the pattern. I took off and I did have some variation but nowhere near as much as the other evening on Sunday after I'd set the dome. Nevertheless there was enough variation and "surging" to be noticable but again nowhere near what it was before.

I landed, taxied back to the hangar and lowered the dome pressure to 35psi. I took off again and this time the left RPM was ROCK SOLID... did not waiver at all. Strange eh?

I landed and went back to the hangar. I decided to split the difference and set the dome at around 40-42 psi. The other day I did engine shut downs in flight and had no problem feathering the prop when the dome was down at 35psi so I know it will feather there.

So even tho at normal dome pressure I did not have as much variation as I did the other evening the bottom line is I seem to have confirmed my theory that a slightly reduced dome pressure results in a steadier holding of the RPM by the governor. Seems odd. I am going to call some prop shops and the folks that overhauled the governor and get their thoughts.

Oh one other thing... I did try reducing power to see what that did. At much lower power setting (say 15-17" MP) the RPM became very steady and varied more with higher power.

- Charles

[Will Berninger]

[Charles Schefer]

[Charles Schefer]

Another 5hr XC flight this past weekend. RPM steady as a rock.

- Charles

[Will Berninger]

[Charles Schefer]