Beech Duchess - RPMs mismatched at reduced MP settings

Fearless Tower

Fearless Tower

Touchdown! Greaser!
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Fearless Tower
I've got about 70 hrs in the Duchess and observed something recently that I have not seen/experienced before:

Last trip I made, when reducing power (throttle only not touching the props) while being vectored for the approach to around 14-16", the right engine is maintaining its cruise RPM setting of 2500, but the left engine is dropping about 300 RPM. When increasing power, the left engine RPM increases to match the right.

Almost all of my BE-76 time is in this particular airplane and I have not seen this before. Any ideas what the issue is? Is there an adjustment problem with the left governor, or is this a sign of an impending governor failure?
 
Skylane81E said:
Any yawing tendencies?
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Nothing significant, but I wouldn't expect to see much with both engines at around 15". The only real noticeable yaw is a slight yawing motion when the left engine resumes the cruise RPM when you apply power.
 
Ok I was just wondering if you could notice if one engine was making more or less power than the other.

At idle do they have the same RPM and MAP?
 
Skylane81E said:
At idle do they have the same RPM and MAP?
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Idling on the ground - yes. The only real difference between the two is that you can lean the left engine a bit more than the right....IOW you can bring the left mixture lever back noticeably further before getting an RPM increase on the ground, or rough running in flight as compared to the right. However, on this particular plane, the engines have always been like that, whereas the RPM difference is a new thing.
 
This is somewhat doubtful, but do you have access to the logs, or the person in charge of maintenance? I've seen situations like this where, for example, the governor on one engine was O/H'd or repaired and the other governor was not. The governor is dependent upon an internal spring to resist the action of the flyweights. You could have one governor with a new spring, and another with a 50 year old spring (your airplane doesn't have to be that old to have a OLD spring). The old spring just doesn't have the resistance that the other spring has, so you get a difference in RPM because the old spring can't control the flyweight with the authority that the new spring can. You can test this by "bumping" the prop control forward a little to see if the situation improves.

Another thing that could be happening here is engine time. Is one engine either just out of O/H, or near (or beyond) O/H? If that is the case, it could be the transfer bearing leakage rate. A very low time engine, or a very high time engine may have a high transfer bearing leakage rate, and at low RPM, the prop governor can't keep up with the leakage, so the prop goes to a higher pitch (lower RPM) simply because the when the throttle is lowered the governor can't pump enough oil to keep the prop in low pitch. Another indication of this is when the "slow" prop feathers on shutdown as opposed to catching on the startlocks as normal, especially after a long flight, or any other time when the oil is hot. Please note that this is not visible leakage - oil will not be spewing out of your engine. It's an internal leak that you can't see.
 
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Been thinking about it, seems one engine is making more power than the other for a certain mp. Can't see how a governor could cause this,
 
I'd like to see what is happening with MP when this is occurring. That would tell the story. It could be a rigging issue, or it could be the governor.
 
I'm still stuck on the gov having no effect here, it's not going to make one spin any faster and also won't make one suddenly slow down when the throttle is reduced.

Now perhaps the props low pitch settings aren't the same?
 
The governor is nothing more than a regulated pump. When you reduce throttle, the pump spins slower, but the prop settings are telling the governor to maintain a constant speed. With a feathering prop, oil pressure drives the prop to low pitch, but if the system is leaky, or there is a weak speeder spring, the pump can't keep up.

I always use this illustration. Remember in the old WW2 movies? A bomber is flying along and gets the engine shot up. They'd always show a shot of oil leaking from the engine, then the engine would stop, and the prop would feather. You lose oil, you lose pitch control, and the prop defaults to feather (high pitch, low RPM). Thats not really how the old Ham Std. props work, but don't let that get in the way! If throttle is reduced, the pump may not be able to keep up, either because of leakage or spring weakness.

Not saying you might not be right, could be a power issue, but in my experience it's usually an old/new engine or a governor maintenance issue.
 
Interesting.....it is a flying club airplane, so I don't have easy access to the maintenance records, but I do know this:

The engines are definitely not same time - one was overhauled a couple years ago due to a prop strike. One prop/governor (not sure if the same one) was overhauled about a year ago due to having a tendancy to go into feather on the ground.

Also , when doing the feather check during the runup, the left prop cylces much more slowly than the right.

As far as the MP, I haven't noticed a difference between engines. When I pull the power on both back to around 15", that is when the left RPM falls off and lags behind the right. I didn't notice anything in the MP when adding power, but I'll look again next time I fly it.
 
KRyan said:
The governor is nothing more than a regulated pump. When you reduce throttle, the pump spins slower, but the prop settings are telling the governor to maintain a constant speed. With a feathering prop, oil pressure drives the prop to low pitch, but if the system is leaky, or there is a weak speeder spring, the pump can't keep up.

I always use this illustration. Remember in the old WW2 movies? A bomber is flying along and gets the engine shot up. They'd always show a shot of oil leaking from the engine, then the engine would stop, and the prop would feather. You lose oil, you lose pitch control, and the prop defaults to feather (high pitch, low RPM). Thats not really how the old Ham Std. props work, but don't let that get in the way! If throttle is reduced, the pump may not be able to keep up, either because of leakage or spring weakness.

Not saying you might not be right, could be a power issue, but in my experience it's usually an old/new engine or a governor maintenance issue.
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That actually makes alot of sense....at the low RPM, the engine driven pump may not be putting out enough pressure to maintain the set RPM.
 
Good points on the gov, but the slow speed here is an effect not a cause, both were spinning 2500 then at about 15" one engine couldn't hold that rpm.
 
I may have a "fix" for this problem, but it's really only a band aid. Eventually, the underlying problem will need to be addressed, but I'll have to check it out in the morning. If you'd like, PM me in the morning and I'll see what I can come up with.

By the way, as you may have seen, I'm a new poster on this forum, but have been a "lurker" for a long time. You (Fearless Tower) have always seemed one of the more astute, reasonable, members of the group.

For some strange reason (I haven't able to quite get my finger on it yet) I seem to like Henning's views as well.
 
Yeah, I might know a thing or two about the props and governor on a Duchess. :D
 
I bet, we used to send ours up every 100 hrs for eddy current inspection before the plane and I parted ways
 
What were your oil pressures at the time? Anytime a CP prop acts funny, always look at your oil pressure.
 
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