[Kevin Horton]

I should get my RV-8 flying again in August, but I'll need to troubleshoot an issue with my prop governor/engine/prop. It works great initially, but as the oil warms up the max rpm slowly decreases. I'll have 2700 rpm for take-off, but as the flight progresses the max rpm will slowly decrease to around 2580 or so. The engine oil pressure is running in the low 80s. The prop is a counterweighted MT prop, which defaults to coarse pitch and needs oil from the prop governor to push it towards fine pitch. The prop governor is a PCU-5000, set up for a counterweighted prop..

The prop governor manufacturer wants me to do a check for internal oil leakage. They said:

>> The other
>> problem could be engine internal leakage, the oil transfer collar or
>> the crankshaft plug could be causing a loss in pressure as the
>> engine reaches operating temperature. The flyweight system is set up
>> to prevent the engine from exceeding the 2700 rpm, but the governor
>> can only maintain this rpm with good oil supply. The propeller
>> counterweights/internal spring are overcoming the governor pressure
>> and causing the rpm drop. The governor is fighting to overcome these
>> forces. Check with your engine shop on performing a differential
>> pressure check on your propeller oil transfer collar
>> to check for internal leakage and check crankshaft plug for damage.

Lycoming SI 1462A seems to apply:

It describes a complicated test setup which requires me to remove the prop governor and install a blanking plate. You then use a differential compression tester to measure the leakage in the prop control circuit.

I'm still trying to puzzle out why I need to remove the aft-mounted prop governor. If I understand correctly, the place they tell me to hook up the air is over the port that the governor uses to send oil from the governor back to the engine case, where it is then routed to come out a fitting next to the prop governor, and then into the oil line that goes to the front of the engine. I think I could run the same test by just hooking the differential pressure stuff to the fitting at the front of the engine where the prop oil line goes. I think that if I do the test this way, the only think it would miss would be an internal leak between the hole on the governor pad where the oil returns to the engine, and the fitting on the side of the governor mounting pad where the oil line to the front of the engine hooks up.

If I leave the prop governor in place, and it fails the test, that tells me I have a problem up at the front of the engine. Either a leaking crankshaft plug, or excessive clearance in the transfer collar. If it passes the test, that tells me I need to pull the prop governor - either it is bad, or there is a problem at the gasket, or maybe a leak between the prop governor base and the fitting where the prop oil line connects. I could check for that leak after I pull the governor off.

I had a terrible time getting that prop governor installed even when the engine was hanging from the engine hoist. There is very little clearance between the nuts and the governor body. I do not look forward to removing and installing it with the engine on the aircraft.

Comments?

 

[DanH]

<<...why I need to remove the aft-mounted prop governor.>>

Note SI-4462A includes a supply pressure check, not just the leakdown check. I'd be really curious about supply pressure. As I recall, you had an overspeed incident with the previous non-counterweight prop.

Supply pressure loss doesn't require anything exotic. Here's an example. Looks like oil supply into the governor adapter is via a passage at the top of the accessory case opening. A simple gasket failure would reduce oil supply pressure; the oil would dump into the drive gear cavity.

 

[Kevin Horton]

<<...why I need to remove the aft-mounted prop governor.>>

Note SI-4462A includes a supply pressure check, not just the leakdown check. I'd be really curious about supply pressure. As I recall, you had an overspeed incident with the previous non-counterweight prop.

Supply pressure loss doesn't require anything exotic. Here's an example. Looks like oil supply into the governor adapter is via a passage at the top of the accessory case opening. A simple gasket failure would reduce oil supply pressure; the oil would dump into the drive gear cavity.

Thanks for your thoughts Dan.

SI 1462A is pretty vague on the supply pressure check. It has you install a calibrated oil pressure gauge on the blanking plate. But it doesn't actually tell you to check the oil pressure, and it certainly doesn't give any limits. With no oil flowing between the engine and the governor, I'm not sure a pressure check tells you too much anyway. I'd be a lot more interested in knowing the supply pressure while the governor was actually controlling the prop, to see the effect of any restrictions in the input. But I see no way to check that, short of drilling into the oil passage way right at the governor base, and I don't plan to do that.

I know my earlier incident was caused by a supply pressure problem, as the engine oil pressure plummeted when I unported the oil pickup. The recorded data show it coming close to zero before it picked up again. The PCU guys say their governor should work properly as long as it has 40 psi oil pressure.

Certainly if the front end of the checks out OK in this test, then the next step has to be to remove the governor to check out the back end stuff. Hopefully it is just a gasket problem. I'll order a couple of those guys.

 

[Bob Martin]

Thanks for your thoughts Dan.

SI 1462A is pretty vague on the supply pressure check. It has you install a calibrated oil pressure gauge on the blanking plate. But it doesn't actually tell you to check the oil pressure, and it certainly doesn't give any limits. With no oil flowing between the engine and the governor, I'm not sure a pressure check tells you too much anyway. I'd be a lot more interested in knowing the supply pressure while the governor was actually controlling the prop, to see the effect of any restrictions in the input. But I see no way to check that, short of drilling into the oil passage way right at the governor base, and I don't plan to do that.

I know my earlier incident was caused by a supply pressure problem, as the engine oil pressure plummeted when I unported the oil pickup. The recorded data show it coming close to zero before it picked up again. The PCU guys say their governor should work properly as long as it has 40 psi oil pressure.

Certainly if the front end of the checks out OK in this test, then the next step has to be to remove the governor to check out the back end stuff. Hopefully it is just a gasket problem. I'll order a couple of those guys.

Kevin,
I used 1462A a few months ago to trouble shoot a similar problem while upgrading from FP to CS prop. The SB doesn't actually say much but does give you ways to check systems and point you to the problem by process of elimination. You are right that you can just connect the differential pressure
gauge to the front crankcase prop oil line fitting and preform the test. All of this testing is not easy but very doable even if it seems convoluted at times.
I got very good at removing and reinstalling the governor and drive adapter
and have the scars on my hands to prove it! The front pressure differential check is the first thing and easiest to preform. The problem is, the results are hard to believe sometimes as the pass/fail is in a range of pressures and then you second guess the instructions about the oil being cold or warm affecting the results, etc. Plus, the worst possible outcome is to think you have to tear the engine apart to correct a problem and you don't want to do that until you are really sure your trouble shooting efforts are very conclusive. The next thing to check is remove the prop and check the plug, look for looseness or anything else amiss with the plug that would let oil leak
by. While you have the prop off, check what you can see to make sure it all looks normal. The rear plate the SB has you make allows for several oil pressure and flow tests. I plugged off the oil line going to the prop and connected another line I made up about 3 foot long to the prop output fitting then ran the engine to see what oil pressure was going to the prop
and how it responded to cycling the governor. I bought a 0-400 fluid filled gauge from Northern Tools for this at about 15.00. This is dangerous as the prop is turning and you are standing next to the engine, so get lots of help for safety sake. Another fitting on the plate lets you check oil pressure going to the governor, this check helps confirm the passages are open from the oil pump to the governor, for the record the governor gets oil directly from the oil pump as it is just on the other side of the accessory case.
Not only can you check oil pressure, but we actually checked "flow" by running the temporary pressure line to a bucket and timed the amount of oil output for 15 seconds and got gallons for minute information, this test was my smoking gun result that explained my problem. So as you can see, there are many tests and results you can get from the SB. The plate isn't hard to make and there is a slim chance a local A/P would have one, but my local guys didn't have one so I made one! I will gladly send you mine to use if you need it or don't want to make one for yourself. I hope I never need one again! This is not a simple process, we spent days and days working on this and countless calls to shops and manufacturers for information. We swapped props and governors from known working planes to rule out defective parts, etc. It was not fun and we wound up overhauling the engine, so it turned out to be the worst possible outcome but a blessing in disguise to get it fixed. I hope you can find your problem right away, but be prepared to keep digging to find the problem as I know you will.
Good Luck.

 

[DanH]

<<it turned out to be the worst possible outcome but a blessing in disguise to get it fixed. >>

Well done Bob.

So what exactly did the problem turn out to be?

 

[Bob Martin]

The problem was.......

<<it turned out to be the worst possible outcome but a blessing in disguise to get it fixed. >>

Well done Bob.

So what exactly did the problem turn out to be?

Purchased RV-6 with OH'd engine.
There were several problems.
10 years on Overhaul and 600 hours flying RV-6 behind the engine with a FP prop.
Cases had been overhauled and somehow the front thrust bearing bore was 15/1000 over sized, but standard bearings were installed.
The normal 15 to 20/1000's running clearance plus the 15/1000's oversized
bearing bore, let one side of the thrust bearing shift during assembly.
The one side that shifted lifted up onto the dowel pin in the case which really closed the bearing to crankshaft gap to "0" and let oil pass from the engine oil galley under the bearing and into the prop oil galley!!! So the governor was pumping it's heart out but the oil pressure would bleed off under the bearing and back into the crankcase!!!
Luckily no damage to the crankshaft!! and crankcases were repairable also.
The smoking gun that I referred to was the normal oil pressure to the governor but low volume........that told us for sure the oil was going somewhere, but we couldn't get the governor output to stabilize. So we pretty well knew the oil was going somewhere and we had to look inside to actually find the bearing issue. No telling how long the engine would have gone with the bearing like that.....maybe TBO.....but maybe not??
We went ahead and did a total OH on the engine to get better logbook value for our efforts! It was surely a learning experience. What started out as a mildly hard project of swapping from FP to C/S turned into a big deal and 5 month of down time! Now that we are up and flying again......it is just a memory....... Hopefully I can help other's like Kevin with the information learned.

 

[Kevin Horton]

Thanks for all the info Bob. It is very helpful.