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RPM Surging Issue at Cruise

219PB

Well Known Member
I had an issue with my RPM fluctuating on my flight this past weekend. The aircraft was at 12000 feet, 20 MAP(WOT), at 2300 rpm. The rpm was slowing surging back and from from 2280 to 2340. The oil pressure was remaining steady at 89 psi. I reduced the propeller rpm to 2200 and then slowly raised it back to 2300. This seemed to help and the variation in the rpm went down to 2290 to 2310.

Another piece of information is that since new, the engine had to be at at least 1900 rpm to be able to cycle the prop as part of the preflight checklist. At 1800 rpm, moving the governor control would not change the pitch of the prop. Don't know if these are related but wanted to raise this issue. Spoke to Hartzell awhile back and they said that this is normal. This is my first constant speed prop so I did not have any personal background. Everyone else that I spoke with stated their prop would cycle a much lower rpms.

My engine is a Lyc 360 with the Hartzell governor (PROP GOV HZ 320/360) and prop (PROP C2YR-1BFP/F7497) bought from Vans.

I would appreciate any advice. Don't know if I have an issue with my governor or propeller.
 
Governor gasket

If you have a prop governor gasket with part nr: MS9144-01 it has a
built in filter screen. If this is clogged the flow is reduced.
Good luck
 
RE: cycling at runup. I have exact same setup. I do runup checks at 1700. Mag check and prop cycle. Couple of years ago I checked to see how low of an RPM I could cycle prop. Fairly certain it cycled at 1500. Could have even been lower, maybe 1400. But 1500 for sure.
 
If you have a prop governor gasket with part nr: MS9144-01 it has a
built in filter screen. If this is clogged the flow is reduced.
Good luck

I remember this screen when the governor was installed. If the screen was dirty, shouldn't the problem occur continuously? When I reduced to 2200 and then moved it back up to 2300, the fluctuations were significantly reduced.

Later in this same flight, the rpm became very stable and no discernable fluctuations were noted.
 
Gov Arm Spring Tension

A thought came to mind yesterday. During the build, I had to reclock the arm on the governor to accommodate the control cable and the mounting bracket. Cannot remember which way I moved the arm but it is possible that I relaxed the tension on the return spring.

Is it possible that the spring tension needs to be increased?
 
A thought came to mind yesterday. During the build, I had to reclock the arm on the governor to accommodate the control cable and the mounting bracket. Cannot remember which way I moved the arm but it is possible that I relaxed the tension on the return spring.

Is it possible that the spring tension needs to be increased?

No. Governor cover is designed to spin. As long as you didn't pull the cover assembly off when you did this, that is not your problem. If you pulled it out too far, that could explain the low control RPM range being high (though Hartzell says 1800 for the S series, so you are pretty close), if the washer fell out. Did you leave the cover pushed in when you did the rotation?

If if used to work well and now surges intermittently, the issue will likely require work at a prop shop. Oil blockage at the inlet should create a variety of odd behavior across many areas of performance, not just occasional hunting at one RPM.

You can't compare low RPM control ranges with other airplanes that have different governors, as they are all a bit different.
Larry
 
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7 yrs ago I did my first flight and my prop was surging +/- 100. I sent my MT Gov in for a SB and it helped. for the past 6 yrs it would hold +/- 20. it was a little better if it was calm air. this year I swapped it for a PCU 5000 Gov and it is much better as it usually holds +/- 5 rpm. I have both an optical sensor and and a hall effect sensor. the optical sensor is very accurate. anyone need an expired MT Prop Gov?
 
7 yrs ago I did my first flight and my prop was surging +/- 100. I sent my MT Gov in for a SB and it helped. for the past 6 yrs it would hold +/- 20. it was a little better if it was calm air. this year I swapped it for a PCU 5000 Gov and it is much better as it usually holds +/- 5 rpm. I have both an optical sensor and and a hall effect sensor. the optical sensor is very accurate. anyone need an expired MT Prop Gov?

I suspect each gov model is a bit different. I have the Hartzell S-1 series on my 10 and it had holds RPM rock solid for the last 130 hours. I have never seen it drift or oscillate once set in cruise configuration. My RPM comes from EI (hall effect and mangets on flywheel) and is very reliable
 
I suspect each gov model is a bit different. I have the Hartzell S-1 series on my 10 and it had holds RPM rock solid for the last 130 hours. I have never seen it drift or oscillate once set in cruise configuration. My RPM comes from EI (hall effect and mangets on flywheel) and is very reliable

all constant speed propellers will have some fluctuation. some of these fluctuations are filtered out by hardware/software when displayed for the pilot. optical sensors generally do not filter and will provide more accuracy. I don't see the fluctuations for my hall effect sensor but I do with the optical sensor.
 
all constant speed propellers will have some fluctuation. some of these fluctuations are filtered out by hardware/software when displayed for the pilot. optical sensors generally do not filter and will provide more accuracy. I don't see the fluctuations for my hall effect sensor but I do with the optical sensor.

You may be right, but i see very rapid changes in rpm when down at idle making me think there is very little smoothing in my system.
 
No. Governor cover is designed to spin. As long as you didn't pull the cover assembly off when you did this, that is not your problem. If you pulled it out too far, that could explain the low control RPM range being high (though Hartzell says 1800 for the S series, so you are pretty close), if the washer fell out. Did you leave the cover pushed in when you did the rotation?

If if used to work well and now surges intermittently, the issue will likely require work at a prop shop. Oil blockage at the inlet should create a variety of odd behavior across many areas of performance, not just occasional hunting at one RPM.

You can't compare low RPM control ranges with other airplanes that have different governors, as they are all a bit different.
Larry

My memory recollection is that I removed the screws and reclocked it without splitting the flanges at all. Why would lowering the rpms to 2200 and then advancing on 2300 have a different effect than just decreasing to 2300 from 2700?
 
My guess would be hysteresis caused by a little bit of stiction in the governor. If you're holding a valve closed with a spring and try to open it by lowering the amount of load on the spring, you may have to lower the load more than anticipated in order to get the valve moving and overcoming the stiction. If you try closing the valve by adding force, you may have to increase the force over whats expected in order to get the valve moving closed. These two situations result in the valve being in the same position (RPM), but with 2 different levels of preload on the spring. This could lead to 2 different levels of control based on design.
 
My guess would be hysteresis caused by a little bit of stiction in the governor. If you're holding a valve closed with a spring and try to open it by lowering the amount of load on the spring, you may have to lower the load more than anticipated in order to get the valve moving and overcoming the stiction. If you try closing the valve by adding force, you may have to increase the force over whats expected in order to get the valve moving closed. These two situations result in the valve being in the same position (RPM), but with 2 different levels of preload on the spring. This could lead to 2 different levels of control based on design.

The spring jujust applies pressure to the cantilevered arms of the flyweights. No valves near the spring. The spring just resists the pressure from centrifugal force on the flyweights. The area is flooded with oil so striction isn't likely,
 
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My memory recollection is that I removed the screws and reclocked it without splitting the flanges at all. Why would lowering the rpms to 2200 and then advancing on 2300 have a different effect than just decreasing to 2300 from 2700?

Because something inside is not working correctly. That gov, when working correctly, will do either without hunting.
 
You are at full throttle and high.
Have you leaned sufficiently, surging or hunting is often a sign of rich mixture. This could only apply if the governor was incapable of finely controlling revs because of full open throttle.
 
You are at full throttle and high.
Have you leaned sufficiently, surging or hunting is often a sign of rich mixture. This could only apply if the governor was incapable of finely controlling revs because of full open throttle.

The engine was aggressively leaned. Was burning 6.2 gph and the cylinder head temps were ~320F.

This does get me thinking though. I am running an EFII system and I have set 2300 as my cruise RPM and adjusted my fuel curves to be lean at this RPM. The fuel flow was varying from 6.2-6.5 while these RPM fluctuations were occurring. This data was retrieved from the Dynon log files.

At exactly 2300, the fuel flow is 6.2. Any fluctuations from that point, the fuel flow increases slightly. This would cause an increase in power acerbating the RPM swings.

I am going to take the 2300 lean point out of the fuel curve and manually lean the aircraft and see if the problem goes away.

view


The link above shows the RPM fluctuations along with the oil pressure. Hartzell wanted to make sure that the oil pressure was not fluctuating during these times.
 
As mentioned, be sure to cycle the prop during your run-up per your prop manual. This removes trapped air which will manifest itself as RPM "wondering." I had the exact same issue. Next flight I cycled the prop well, never saw it again. May not be the solution, but do the simple things first before you go in for major surgery!

$$
 
cycle prop

As mentioned, be sure to cycle the prop during your run-up per your prop manual. This removes trapped air which will manifest itself as RPM "wondering." I had the exact same issue. Next flight I cycled the prop well, never saw it again. ...
The MT-Propeller ATA manual agrees with you - in the troubleshooting section one of the causes listed of "Surging RPM" is "Trapped air in propeller piston." and the Remedy is "Move propeller control at least twice every time before flying at about 1800 rpm with a drop of about 500 rpm."
 
Just to close out this thread. I removed the lean mapping from the 2300 portion of the curve and flew the aircraft to 9500 over the weekend. Manually leaned the engine and the rpm remained within 10 for the flight. This is only one data point and I will keep operating the engine in the manner to make sure that this fix has eliminated the surging.

Thanks to the post by Yen to get me thinking about the mixture richening.
 
If you run LOP and don't have your cylinder mixtures very finely tuned to each other (search GAMI spread) then as you get lean you'll have one cylinder stop firing first - likely on just one plug - which changes the amount of power you are producing. The prop tries to repitch to keep the RPM constant with the new power available, and as that cylinder catches and loses and catches ignition from both plugs in the lean mixture then your produced power can go up and down, and the RPM up and down as the prop tries to compensate.

If it's a question of engine versus prop governor, go rich on the mixture to give a stable power level for further testing.
 
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