Prop governor- LOP

[The-sky-captain]

I didnt want to hijack the recent thread about power settings in which Pine Runner noted that running LOP causes the prop to fall out of governance. His comment got me thinking....I've noticed that randomly while LOP my RPM's will quickly drop 40 or so in a split second and pop back up to 2500. Barely noticeable to passengers but I can pick it up everytime. Some flights it will drop, other flights it will remain steady. FWIW it never does this on ROP flights. Reasons?

[DS1980]

Can you provide a link to the thread?

[The-sky-captain]

Here ya go...

http://mooneyspace.com/topic/10862-power-setting/#entry129645

[DS1980]

Got it. Post # 4.

 

"With a constant speed prop set at 2500 rpm you pull back the mixture until the rpm drops; ie you've gone well past peak and onto the lean side dropping power far enough to take the propeller out of governance."

 

I may be able to help, or be full of hot air. Lets go with hot air and it will be a good suprise if I help. I'm not sure of your mechanical background so sorry if this is basic for you. The prop governor is actuated by oil pressure and some basic ways (other things equal) to have an uncommanded drop in RPM are to have an oil pressure/supply problem, oil temperature problem, or not be producing enough power to keep the prop turning at the desired RPM. This is what Pine is saying-that you are going so lean the engine is loading up and slowing down. I don't think this is the problem. I'm assuming you're running at around 35-50 LOP. is this correct?

 

I think your problem is mechanical rather than a function of LOP. I understand it hasn't happened on ROP flights but I think that's a coincidence.

 

The bottom line here (well, not the bottom line because I still have more hot air) is that running LOP around best economy does not take the prop out of governance. I would like to know Pine's experience with this, as he has an injected engine and may be running far more lean than he thinks due to a number of things (most likely really good fuel distribution and an inaccurate EGT indicator). In that case, he's correct. But the fact that the revs come back without a decrease in airspeed (correct?) tells me it's not a power problem, but an oil supply problem.

 

But this is a doozie, and I would not like that random blip in RPM.

[pinerunner]

Actually I was just saying he had dropped the power so low that the prop was at lowest possible pitch. On the lean side power is a function of fuel flow and not MP. I wasn't suggesting any dire problems with his oil pressure.

[The-sky-captain]

It's puzzled me for a couple years and I've meant to post the question here for a while. The prop gov was overhauled 2 years ago which is also when I started running LOP so I don't have a baseline for before that. I've ran the issue past Maxwell and the governor has passed a bench test twice with no issues. The ROP steadiness is no coincidence as I fly approx 200 hours a year and every time I'm ROP it's steady Betty, whereas sometimes LOP is is steady and sometimes it will have the small drop in RPM, sometimes even on the same flight. Im running around 8.5-9 gph depending on the flight so I'm not too lean either as mentioned above to cause it to fall out of governance. Oil pressure and temps look good on the engine monitor as well. All that being said I'm going to have Maxwell take a good look at it, my annual is coming up soon

[pinerunner]

I think your getting power fluctuations in the LOP mode that are too quick for the governor to keep up with and maintain the rpms. Not bad enough to actually miss and run rough. I have no opinion on whether its something to worry about. Say, if you're Sky Captain did you ever run into Sky King? I used to love that show.

[DS1980]

Yea, I think Maxwell is a great bet. As Pine said, if it's not an oil issue it's a power fluctuation issue. Maybe an injector that's slowly going bad? This would cause roughness but it sounds like the RPM blip is quick enough that it smooths back out before it's noticeable. Ya got me captain. Might ask jetdriven or N201MKTurbo. They seem to be maintenance savvy folks.

[Cruiser]

what is happening with the FF when you notice the RPM fluctuations? Can you post an image of the engine monitor data?

[N601RX]

You can make the prop fall out governance at low power settings both lop an rop. Just raise the nose and let the airspeed decay. Once the prop is on the stop, the engine will start to bog down.

[The-sky-captain]

Cruiser, no difference on the fuel flow when the blip happens. I have a feeling it's a governor issue as I was having governor issues after my engine major at which time I had the governor OH'd as well. It was surging ROP initially until I had it looked at again. At that point it quit surging ROP and only surges LOP which in itself is a mystery. It will go months with no blips and all of a sudden start it up again. When it does surge it seems to go away after it "warms up" during a longer flight. Ill probably look at getting it OH'd again or at least replace the springs. One thing I forgot to mention is that on takeoff it sometimes does the same drop for the first minute or so after applying full throttle on the roll out and initial climb out.