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Governor/RPM Issue

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Joe Enzminger

Joined: 06 Feb 2006
Posts: 26

Posted: Mon Jun 09, 2014 5:45 pm Post subject: Governor/RPM Issue

Looking for advice.



Encountered a new issue flying my (regularly flown) CJ this weekend.  In cruise, climbed up to 7500 feet, throttled back to 620, and then tried to set the RPM to 1900.  The lowest it would go is about 1950 (prop control full aft).



At that setting, RPM would change with airspeed.  Pushing the prop forward, about about 60% travel it would "catch" and go back to constant speed mode.



I wasn't comfortable with that so I turned around and headed home.  Oil pressure, fuel flow, etc remained normal throughout.



No obvious oil leaks around the prop hub.  I haven't pulled the screens yet, but I thought I'd see if anyone has any insights into what to check first (hub, governor, etc).  



Also, it got me curious about how the system actually works - any insights appreciated.



Thanks,



Joe Enzminger

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wlannon(at)shaw.ca
Guest

Posted: Mon Jun 09, 2014 7:37 pm Post subject: Governor/RPM Issue

What was the OAT at 7500 ?

It is a fairly common problem with counterweight props in cold weather if 

you have been cruising at a steady pace in smooth air.   The oil in the 

cylinder gets thick enough that it's return flow through the governor 

passage is restricted.   RPM would tend to change with airspeed since, with 

frozen oil, it is essentially a fixed pitch prop.  Good practice to cycle 

the RPM occasionally.

It even happens to double acting Hydromatic types and some have an electric 

heater unit attached around the dome.

Should have returned to normal on descent.



Walt

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Joe Enzminger

Joined: 06 Feb 2006
Posts: 26

Posted: Tue Jun 10, 2014 7:06 am Post subject: Re: Governor/RPM Issue

Thanks Walt,



I'm in Texas, so probably not a cold oil problem (oil temps were normal) and I was flying with the canopy cracked.



Joe

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wlannon(at)shaw.ca
Guest

Posted: Tue Jun 10, 2014 8:29 am Post subject: Governor/RPM Issue

Hi Joe;



Did it function OK after descent?



It is a matter of OAT at altitude and whether the prop was responding to 

rough air or maneuvering.   If smooth and steady prop may be sitting in a 

fixed pitch position for some time.  The oil trapped in the pitch change 

cylinder will soon be at ambient OAT (even with the Chinese blanket in 

place).   Matters not what the operating oil temp is.   Of course could be 

exacerbated by excessive blade turning torque but if it returned to normal 

during descent you likely do not have a problem.



Walt



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