Is My Prop Governor on the way Out?

Discussion in 'Maintenance Bay' started by MBDiagMan, Jan 12, 2021.

  1. MBDiagMan

    MBDiagMan En-Route

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    I keep my Mooney with IO360 hangared and have a small heater that blows into the engine cowl. I shoot it with an IR gun for a temperature reading and I usually have 60 or 70 F when I get it out and start it. Saturday morning early and this morning early I rolled it out and started it in about 35 F ambient temperature. I kept the Rpm very little above idle and slowly taxied to the run up area and waited for min oil temp before doing a run up. Both days when I made my first takeoff with everything to the firewall, I only got about 2,560 RPM on take off. Both days I flew about an hour before coming in, then did another takeoff. Both days on the second takeoff I got a normal 2700 RPM.

    In Winters past, regardless of oil temp I always had max RPM on take off. Is this pointing toward a prop governor that is asking to be serviced?

    Thanks in advance for sharing your experiences.
     
  2. Salty

    Salty Final Approach

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    Are you using a direct engine driven tach, or a tach off the mag? If it's engine driven, I'd bet your gauge is more likely the problem rather than the governor.
     
  3. Salty

    Salty Final Approach

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    I didn't catch this in first reading, that doesn't fit my previous post.
     
  4. MBDiagMan

    MBDiagMan En-Route

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    Thanks for the response anyway Salty!

    Answer to your question is that it has a JPI 930.
     
  5. Clip4

    Clip4 Final Approach

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    Could be the gov, could be a prop control cable binding in the cold.
     
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  6. charlie

    charlie Pre-takeoff checklist

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    Governor pressure keeps the prop from overspeeding. I vote failing control cable.
     
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  7. Stewartb

    Stewartb Final Approach

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    What oil are you using?

    Improve your preheat to warm the oil to a higher temperature. Cycle the prop a couple of extra times. See if that makes any difference.
     
  8. Bill

    Bill Touchdown! Greaser! PoA Supporter

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    My thought as well. Control cables, especially older ones, get super stiff in cold weather. Let me ask you a question, after pulling the knob to do your prop checks and you push it in, do you spin the vernier to ensure it's all the way in?
     
  9. Stewartb

    Stewartb Final Approach

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    Make sure the governor arm hits the stop. Simple. The cable would be the last thing I'd suspect.
     
  10. MBDiagMan

    MBDiagMan En-Route

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    Answer to question, Phillips Crosscountry multi grade. The engine has been preheating to about 70F. I turned up the thermostat a little after I flew this morning.

    I rolled it out into 40F ambient. Taxied slowly to run up area. Ran up after the oil temp hit 100F. When exercising the prop I would say it would lose RPM about a second or less on every cycle. I did four and they all felt the same. I made sure everything was forward and it made 2700 while approaching rotation speed.

    I will be watching this closely, but I expect that it was indeed a stiff cable.
     
  11. Dan Thomas

    Dan Thomas Final Approach

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    The CS prop on a single defaults to the lowest pitch/high RPM position. It takes oil pressure to move the blades to a higher pitch/lower RPM. Cold oil therefore won't limit the RPM in a single-engined airplane. There's another problem somewhere, possibly a slipped cable housing. Stuff that should be checked at annual is the security of the control cable at its housing clamp and rod end at the governor. If that housing clamp slips and lets the housing move back, the governor can't return to its max-RPM position.

    Same goes for throttle, mixture and carb heat controls.

    Screwing a vernier control in tightly can put unacceptable forces on that cable and can slip the housing.
     
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  12. Bill

    Bill Touchdown! Greaser! PoA Supporter

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    I didn't say put a pipe wrench on it, just be sure it's all the way in.
     
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  13. Dan Thomas

    Dan Thomas Final Approach

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    Thre should be some "bounce" in the control. That is, the knob should stop an eighth of an inch or so from the panel fitting. This is to ensure that the stops are being contacted at the engine end. No bounce means a misrigged or slipped control. I have seen pilots screw those verniers in tight, which forces things and which is why I mentioned it.
     
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  14. Magman

    Magman Pre-takeoff checklist

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    I can understand issues that would prevent reaching 2700 RPM on first TO.

    How do they heal for subsequent though?

    Low Pitch Stop moves predictably?

    Engine develops Less Power first TO?

    Hmmm.
     
  15. George Mohr

    George Mohr Line Up and Wait

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    I think it can't be power output related. If the engine wasn't developing enough power to keep the prop off the fine pitch stops at the start of the takeoff roll, then the RPM would quickly build to 2700 as airspeed increased on the takeoff roll. If the RPM was constant, the governor is controlling it.
     
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  16. Magman

    Magman Pre-takeoff checklist

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    Assuming Prop Control is Full Increase.

    Shouldn’t Prop be on Low Pitch Stop until it has reached 2700 RPM

    and then pitch increases to absorb power to keep it @ 2700?
     
    Last edited: Jan 14, 2021
  17. George Mohr

    George Mohr Line Up and Wait

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    Yes, and if the low pitch stop results in some RPM less than redline, then the unloading of the prop over the takeoff roll should result in an increase in RPM. If you don't see that steady change, you don't have low power as the main cause of low RPM.