Vibration at lower RPMs

[bob865]

Reaching out to the community for an education.  

I flew on Sunday to do some practice and generally exercise my skills.  On my approach when I throttled back, the plane had a shake.  It was a vibration that to me felt like what I think an out of balance prop would feel like.  I didn't feel anything out of the ordinary when in flight (RPM 2350-2650) or at idle, only when maneuvering to land (RPM below 2k).  When I first noticed the shake, I was in the 'avoid continuous operation between' range to which I quickly got out of.  This was during maneuvering flight to land, so I don’t know how long I was in here, but I am sure it wasn’t long.

I talked to my partner on the plane and the A&P and they are going to pull the spinner to make sure nothing has nested, collected, or otherwise changed inside that could be throwing the prop out of balance.  Aside from that, nothing has changed in the plane (that I'm aware of, of course).  Any idea what could cause this kind of shake?  I'm concerned because I don't want any undue stress on the engine from vibrations and shakes.  Is there something other than the prop that could cause this kind of shake/vibration?

Becuase of this shake, I've been reading up on props.  One thing I discovered, that I didn't realize before, was the calendar time component on the prop overhaul time.  Asking those more experienced than me: I know the world is divided on engine overhauls, what is the consensus on prop overhauls?  Specifically the calendar time limit?  The 5 year deadline, I’m starting to approach if I haven’t already passed.  I have to grab the logs and find out the exact time frame to be sure. 

Thanks All!

[RobertGary1]

No reason to ever overhaul a GA propeller. However, opening it up and repair as needed every 10 years is a very good idea.

-Robert

[Andy95W]

If you have an engine monitor, I'd definitely check the data.  If you don't have one, then in addition to checking the prop and spinner, I'd also clean and gap the spark plugs and clean the fuel injectors.

[tmo]

Bob, people around the world go through sometimes great lengths to enjoy the on-condition approach to maintenance allowed by the FAA for part-91 operations... Don't let that advantage you have disappear by performing unneeded maintenance based on an arbitrary time limit, which more often than not based on a CYA approach... :-)

[philiplane]

the blades need to be checked for excess shake, by grabbing the tips and trying to shake them forward and aft, and side to side. This will detect loose bearings.

Attempt to rotate the blades from high to low pitch. This checks the pitch control forks in the hub. 

If you find less than 1/8" of shake, or less than 1/16" of rotation play, the prop is OK.

[kerry]

You can check the prop index.  If blade #1 is aligned with cylinder #1 at top dead center it can help with vibration.  It helped with my experimental with a Lycoming O-320.

[0TreeLemur]

Last spring I had my prop balanced, and the vibrations became worse at certain speeds.   The problem in my case was the engine mounts, the bottom two were flattened like pancakes.  Somebody before I bought it put in some washers to lift the engine back up.   After replacing the engine mounts at a MSC this summer, she purrs like a Singer sewing machine now.

[Guest]

By serial number your engine mount structure should be in compliance with the AD and SB’s from Mooney, but it never hurts to check it.  Engine Lord mount condition, bolt torque for engine to mount and mount to airframe are worth looking at as well.

Clarence

 

[carusoam]

Also check the engine mount welds...

 My M20C decided to break a few welds... it caused extra, noticeable vibrations...

and an engine droop that was there one day and gone the next... then back again...

Vibrations are common when something is touching, where it shouldn’t...

So flattened rubber mounts, and a broken engine mount... are worth chasing down...

 

Is there something being protected by the avoid rpm range like counterweights?  Then we would also be looking to see if they have moved or got stuck... (whatever is being protected by the avoid range)

Good luck on the hunt...

Let’s also invite our prop guy to the conversation... @Cody Stallings

PP thoughts only, not a mechanic...

Best regards,

-a-

[Cody Stallings]

Check to see if your blades are Snug(not tight). If they feel good or like other Props on your ramp you should be fine.
 

I would say it’s probably not going to be your propeller. A propeller that out of balance will shake at all RPMs with a change in Frequency, an the amplitude will fall slightly with RPM.

Calendar TBO should not apply to Part: 91 in my opinion.

5/6 Year TBO on a propeller with a 2000hr TBO. Most fly in the 100hr a Year range.

So 600hrs an it time to O/H?... Head Scratcher!!!

If you just wanna do something to your propeller. Re-Seal it. The Orings are what will go bad first, while the propeller is apart the Repairstation will be looking for any Safety of Flight issues. 
You will get more of your propeller back that way. 
My 2 cents 

 

 

 

 

[kerry]

http://www.knr-inc.com/shoptalk-articles/25-shoptalk/76-201501-dynamic-balancing

[bob865]

Thanks all for the feedback.  From the feedback and talking with my mechanic we stated looking at the engine too.  He believed it was a mis-firing cylinder.  We ground ran the engine trying to recreate the fault and could not.  All looked good so took her around the patch to see if in flight conditions could recreate it.  Still could not recreate it.  I'm now doubting myself on if I felt anything at all.  Oh well.  Closely monitoring to make sure it doesn't come back.

[Ragsf15e]

Bob,

sounds like you’re going down a good path with it, but I had a similar experience that I’ll share... I was in cruise, just after level off, 2500 rpm, maybe 22.0 map and I felt a slight vibration.  The exact same thought occurred to me... maybe I need the prop balanced?  Anyway, I ended up downloading and going over my engine data very carefully (jpg 930 engine monitor is great).  I found some anomalies with the egt in cruise -  about 4 minutes after setting it up for about 100 rop, the #3 egt slowly rose about 100 degrees and sort of cycled.  Anyway, between that slight vibration and the weird egt I showed it all to my mechanic.  He recommended we check the valves on #3 and sure enough, found a wobbly exhaust valve.  

So I guess what I’m saying is that it could definitely be an engine issue, and the digital engine monitors are definitely helpful if you can look at data during the problem.

Drew

[0TreeLemur]

3 hours ago, Ragsf15e said:

Bob,

sounds like you’re going down a good path with it, but I had a similar experience that I’ll share... I was in cruise, just after level off, 2500 rpm, maybe 22.0 map and I felt a slight vibration.  The exact same thought occurred to me... maybe I need the prop balanced?  Anyway, I ended up downloading and going over my engine data very carefully (jpg 930 engine monitor is great).  I found some anomalies with the egt in cruise -  about 4 minutes after setting it up for about 100 rop, the #3 egt slowly rose about 100 degrees and sort of cycled.  Anyway, between that slight vibration and the weird egt I showed it all to my mechanic.  He recommended we check the valves on #3 and sure enough, found a wobbly exhaust valve.  

So I guess what I’m saying is that it could definitely be an engine issue, and the digital engine monitors are definitely helpful if you can look at data during the problem.

Drew

"wobbly exhaust valve"?   That sounds expensive.

[Ragsf15e]

5 hours ago, 0TreeLemur said:

"wobbly exhaust valve"?   That sounds expensive.

Well I ended up with a rebuilt cylinder so yeah, it was expensive.  There is a way to clean out the valve guides with the cylinder still attached.  Search for “valve rope trick”.

[bob865]

On ‎11‎/‎6‎/‎2019 at 12:52 PM, Ragsf15e said:

He recommended we check the valves on #3 and sure enough, found a wobbly exhaust valve.  

Oh boy.  He did mention a valve when I described the problem.  For now it's wait and see so we can look for a cylinder to check.  I can't wait to get an engine montior installed so I have historical data to review.  Right now I have CHT and EGT for every cylinder, but just a readout.  No recording. *sigh* the stuggles of more wants that money and time. 

[steingar]

Every time I fly over open water I swear I hear new noises and feel new vibrations.