Prop Governor Issue

[Doug G]

I just had the prop governor overhauled in my M20J. It’s going back to the shop to be bench tested but I have seen situations where everything tests fine but the problem remains. So thoughts from people who know more about constant speed props would be appreciated. Bottom line, when MP is reduced, usually below 22, the RPM drops along with the MP. Since I’m in an approach environment I push the prop full forward and don’t get more than 2500 rpm. From then on the prop seems to be behaving like a fixed pitch prop, I use MP to maintain approach speeds.

Engine oil pressure and temperature are where I expect them to be and the oil is fresh. There are no leaks or issues around the prop and I get 2700 rpm on departure.  This doesn’t happen 100% of the time but I can’t think of anything other than the governor that could be causing it. It also wasn’t an issue until the overhaul. My thought, tell them to keep looking if the bench test says all is good. Any other opinions?

I appreciate everyone’s input!

 

[Fly Boomer]

55 minutes ago, Doug G said:

From then on the prop seems to be behaving like a fixed pitch prop, I use MP to maintain approach speeds.

Sounds normal to me.  If the throttle is pulled way back on approach, you can't make the prop turn faster than the engine by pushing the prop control in.  As you point out, it behaves like a fixed pitch prop.

[Doug G]

5 minutes ago, Fly Boomer said:

Sounds normal to me.  If the throttle is pulled way back on approach, you can't make the prop turn faster than the engine by pushing the prop control in.  As you point out, it behaves like a fixed pitch prop.

Agree on final but I’m just pulling back to 22 MP on approach, that seems a bit high for this to happen. I used to go to 15-18 MP before I dropped the gear,

[IvanP]

I ususally see the prop behaving like a constant pitch below 20in MAP on my Bravo. I ususally descend at about 20in MAP/2400 RPM and on approach I am ussually at 17-18in MAP and see the RPMs drop. If all else seems to be OK and the RPMs are stable in climb and cruise, I would not be too concerned about it. 

Edited July 19 by IvanP

[cbarry]

Is it possible that the oil flow through the governor is restricted or not maintaining the proper pressure as the MP starts decreasing (now at a higher MP than before)? I’m not a mechanic, but it sure sounds like the governor oil control valve is not opening well to increase oil flow or the check valve is not “holding”  well as  MP drops. You know your airplane, so if it doesn’t seem normal, it probably isn’t.  

[Fritz1]

You know your airplane, if you feel something has changed it probably has, bench testing the governor is the logical next step. Only other thing I can think of is debris in the oil system, did your oil pressure change at all?

[Doug G]

Is it possible that the oil flow through the governor is restricted or not maintaining the proper pressure as the MP starts decreasing (now at a higher MP than before)? I’m not a mechanic, but it sure sounds like the governor oil control valve is not opening well to increase oil flow or the check valve is not “holding”  well as  MP drops. You know your airplane, so if it doesn’t seem normal, it probably isn’t.  

Hopefully the bench test will tell us what’s wrong. It’s unsettling because it’s intermittent, the usual “it worked fine here.”


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[Doug G]

To close out this topic, it was a configuration problem in the governor. It took two tries to fix it, after the first it wouldn’t cycle at all. Second time was a charm after they consulted with someone with more expertise. Now, after removing and installing it three times, I just want to get even with whoever decided the back of the engine was a good place to put the governor.

Thanks for the input!


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[Paul Thomas]

What was the configuration problem?

[Doug G]

The output shaft length which puts pressure on the speeder spring was set to the wrong RPM.


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[47U]

8 hours ago, Doug G said:

The output shaft length which puts pressure on the speeder spring was set to the wrong RPM.

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Huh?

My money was on the muffler bearing, and I lost again.

Glad you’re back to ops normal.  And concur that the back of the engine is a challenging location for the prop governor, even on my lowly C.

[Andy95W]

Safety wiring the high speed stop has been the bane of my existence on multiple airplanes.

[47U]

52 minutes ago, Andy95W said:

Safety wiring the high speed stop has been the bane of my existence on multiple airplanes.

Concur.  It might be more fun than safety wiring the prop bolts.

I bought a shaft collar from the local ACE Hardware, drilled a hole in it, and replaced the set screw with a knob.  I thread one strand of the safety wire through the hole in the collar and give it a couple twists with the free end of the wire.  That way I can keep tension on the wire while twisting it.  The length of the wire to the anchor point is fine-tuned by sliding collar on the shaft.  Just for the prop governor high RPM stop screw.    

 

[EricJ]

2 hours ago, 47U said:

Concur.  It might be more fun than safety wiring the prop bolts.

The last few times I've done props I've used safety cable instead of safety wire.   The tools and the material are expensive, but it's much easier and a time saver.