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Momentary Prop Over-Speed on Takeoff

twcroy

Active Member
All,

I was wondering if you ever had much problem with the prop governor? It surges sometimes, just for a second or two. The last several flights it has been over-speeding momentarily before the governor grabs it and brings it back into range (<2700RPM) during takeoff. My A&P has adjusted the high RPM stop down twice, and it appears to have limited the over-speed range down to 2740-2800 before the governor brings it back to 2640 during take off. It was spitting a little grease at me too on the windshield, but now that it tops out at 2800, it doesn't do that.

I was wondering if you had any thoughts on it. My A&P plans to adjust it down further a third time to see if it will not momentarily surge above 2700RPM.

Thanks,
Tim

p.s. I had it bench-tested two annuals ago, and the tester said it checked out fine. Maybe it's worn since then?
 
What prop, what gov, and how warm do you get the oil prior to runup and prop cycle??
 
If it is a typical Hartzell, your adjusting the wrong thing.

Adjusting the low pitch stop is the key here....The Hartzell manual details this procedure. That is if it is a Hartzell...
 
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If it is a typical Hartzell, your adjusting the wrong thing.

Adjusting the low pitch stop is the key here....The Hartzell manual details this procedure. That is if it is a Hartzell...

I believe this is correct. I have a MT prop and governor and haven't actually started the engine yet.......... However, my reading and research indicates the following:

- full power static run should be limited by the prop fine pitch stop to something less than 2700rpm. In other words, the governor has not yet started to control the RPM. If the RPM is over 2700 then the physical fine pitch stop needs to be adjusted.

- as the aircraft accelerates and the RPM increases due to the windmill effect, the governor starts to bring the prop off the fine stop and coarsen the blades. If the RPM settles above 2700, then the prop control should be brought back to 2675 and left for the rest of the flight. After landing, the governor high RPM screw should be adjusted against its stop.

So, at the start of the TO run, the governor should be doing nothing and the blades should be at full fine against the prop stop. It appears that this is set too fine so the prop initially overspeeds until the governor can coarsen the blades and bring it back under control. Sounds like the governor is working exactly how it should and it's the prop fine pitch stop that needs adjusting.
 
As above, it is the static RPM wrongly (or just not) set on the prop.

My suggestions:
  1. Set Governor (what your A&P is adjusting) to ~2680-2690 RPM
  2. Set Static RPM to 2600-2650 RPM
  3. Unless you are brave, I would not advise setting these in a "static" scenario i.e. aircraft tied down. You have the info now to incrementally tweak the settings - the static RPM needs to come down by ~150RPM and the governor +50RPM.
  4. The manual(s) will give guides as to how many turns adjust by what RPM. I'd use about 1/2 - 3/4 of what they advise, then fly again i.e. anticipate taking 2-3 adjustments to get it just right ;)
 
I would first recommend you re examine your procedures. The only time my prop ever surged in 13.5 years was when I failed to cycle the propeller properly during run up (removing air from the system). The other question I would ask is how fast are you advancing the throttle at takeoff? Have you changed your procedures? I sometimes hear people slamming the throttle forward so fast that there's no way the propeller control for the first couple of seconds. (Probably the same kind of people who start their Bonanzas and Cirruses up at 2200 rpm, but that's another discussion.)

Just rule out a simple procedural change before you start taking your plane apart.
 
I'd also recommend you check the actual rpm with an optical tach. If you fly up to X35, we have an optical tach as part of our Dynamic Balancer (which we just got trained on and are having a blast balancing props with!).
 
Thanks for the help on this. I'll discuss it with my A&P and move ahead. The original owner thinks I should have it overhauled, but I'm not sure this is necessary at this point.

Tim
 
Tom

On the 3 hartzell prop's I have setup they all needed about 1 1/2 turns on the fine pitch stop. This was with a 180 hp IO-360

Cheers

Peter
 
I would first recommend you re examine your procedures. The only time my prop ever surged in 13.5 years was when I failed to cycle the propeller properly during run up (removing air from the system). The other question I would ask is how fast are you advancing the throttle at takeoff? Have you changed your procedures? I sometimes hear people slamming the throttle forward so fast that there's no way the propeller control for the first couple of seconds. (Probably the same kind of people who start their Bonanzas and Cirruses up at 2200 rpm, but that's another discussion.)

Just rule out a simple procedural change before you start taking your plane apart.

This is right on the mark. Exactly where in the takeoff roll is the overspeed happening? Any C/S prop can only react so quickly.

Another point - static RPM tests need to have the plane securely anchored - see other threads on this subject. A light RV will not sit still on its own at full power.

One could make the argument that static RPM is almost not relevant, since as the plane picks up speed, the governor starts controlling. More important is if you are getting full rpm on rotation and climb, and that is a function of the governor limits, not the prop's hard adjustments.

I too constantly hear idle to full throttle takeoff power changes in less than a second, and it almost always is accompanied by a momentary overspeed. And yes, it does seem to be the above mentioned airplane brands...
 
Hi Alex...
Another point - static RPM tests need to have the plane securely anchored - see other threads on this subject. A light RV will not sit still on its own at full power.
I too constantly hear idle to full throttle takeoff power changes in less than a second, and it almost always is accompanied by a momentary overspeed.
You have answered your own question :D

Whilst opening the throttle rapidly on takeoff might be seen as not an ideal practice, I cannot see any real harm coming from it? So use the "rapid opening" technique, rather than a static tie down, to determine the need to adjust the static RPM.

One should not, IMHO, be "opening the throttle slowly" to avoid the overspeed. The overspeed is there because the prop is not set up iaw the manual.

In my RV test flying, this static RPM setup is invariably required on a C/S prop, and I do not use the tie down technique, but the takeoffs as above.

Just my view ;)
 
Hi Alex... You have answered your own question :D

Whilst opening the throttle rapidly on takeoff might be seen as not an ideal practice, I cannot see any real harm coming from it? So use the "rapid opening" technique, rather than a static tie down, to determine the need to adjust the static RPM.

One should not, IMHO, be "opening the throttle slowly" to avoid the overspeed. The overspeed is there because the prop is not set up iaw the manual.

In my RV test flying, this static RPM setup is invariably required on a C/S prop, and I do not use the tie down technique, but the takeoffs as above.

Just my view ;)
Sorry to pile on and trying not to be harsh but - dusting off and putting my old rotating machinery engineer hat on - rapid (slamming) throttle advance does the moving parts no good and can only cause unnecessary harm. There is no reason to slam the throttle open, short of an emergency condition.
 
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Yeah, there is at least one good reason....

The rush of suck you back in the seat acceleration!

:)
 
Hi Alex... You have answered your own question :D

Whilst opening the throttle rapidly on takeoff might be seen as not an ideal practice, I cannot see any real harm coming from it? So use the "rapid opening" technique, rather than a static tie down, to determine the need to adjust the static RPM.

One should not, IMHO, be "opening the throttle slowly" to avoid the overspeed. The overspeed is there because the prop is not set up iaw the manual.

In my RV test flying, this static RPM setup is invariably required on a C/S prop, and I do not use the tie down technique, but the takeoffs as above.

Just my view ;)

Andy, good points. If the throttle is indeed slammed open, and there is not much wind down the runway, the hard stop on the prop will govern.

But, with strong winds down the runway and the additional velocity due to any initial ground velocity of the aircraft, the prop will likely overspeed.
 
Thanks again everyone. The overspeed occurs in about the first third or so of the take-off roll with me advancing the throttle slowly and smoothly. The governor is older and as the bench tech said it looks beat up but passed the tests two annuals ago. I'm not sure where to go at this point or why we've been adjusting the high RPM stop on it per the tester's instruction. I'm starting to wonder where he gets his information from. Tim
 
I have found in the past year that it is not uncommon for mechanics and pilots to misunderstand how all of this prop/governor business works in certain phases of the flight. Ask 2 people and you will get conflicting advice most of the time.

The manuals of the common props and governors all tell the same story....

As long as everything is operating more or less as it should, overspeed early in the takeoff roll is a function of the low pitch stop in the prop.

Talking about non-counterweighted pressure to increase pitch props here.
 
Brian,

I checked with my A&P who said he's adjusting the low pitch high RPM stop. Is this the same one you recommend? We will likely tweak it a bit again tomorrow.

Thanks,
Tm
 
Thanks, I think I'm tracking :) . He puts the wrench in the center of the prop (in front and in the middle of the four safetied nuts) and adjusts from there. It sounds like we're talking about the same adjustment. Glad to know that.
 
Yep, loosen the locknut, turn in/cw to increase low pitch and retorque locknut.

My Hartzell out of the box required 1.5 turns in/CW.

Works like a dream!
 
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