Tracking down vibrations

[phxcobraz]

Just had my M20C dynamically prop balanced this past weekend, we removed a stack of weights to get it running smoother. It absolutely is smoother on the ground but once in the air at full MP/RPM, the vibration I had seems to have changed to a more fine buzz in the controls, so the airframe vibration is mostly gone.

Honestly I wasn't super happy with the person doing the balance for a few reasons, but he was the only person in the area that even lists still offering this service, every shop I called said they don't do it or don't know anyone.

I have a set of new Lord engine mounts I might swap out as well with my A&P, but looking at any other suggestions anyone might have to reduce control vibrations?

I had gotten used to the old vibrations since I bought it a year ago but it was bad enough my Gopro video was always shaky, so decided to sort out the root cause.

Thanks for the help!

[0TreeLemur]

I experienced something similar a few years ago.  During balancing, they removed a bunch of weight (four or five washers) from one side of the spinner base plate and installed three or 4 on the opposite side.  The first flight after prop balancing there was still vibration, but different.  My first reaction was that it was worse.   Flew back and met with the shop.   Talked it over.   Decided to fly it for a while.   Got used to it.

My gut feeling is that I'm not very good at discriminating vibration.  

[A64Pilot]

vibrations are tough, I used to chase them on helicopters, you have 1 to 1, 2 to 1, then also harmonics. Did you see what he IPS reading on the prop was after balance? If you did and they were under .2 then it’s balanced well enough.

How old are the mounts? Most are very old and that can be a problem

Many balance at cruise RPM , but honestly balance shouldn’t change with speed, your car tires for instance, if they are balanced, they are for all speeds.

Edited May 26, 2022 by A64Pilot

[phxcobraz]

3 minutes ago, A64Pilot said:

vibrations are tough, I used to chase them on helicopters, you have 1 to 1, 2 to 1, then also harmonics. Did you see what he IPS reading on the prop was after balance? If you did and they were under .2 then it’s balanced well enough.

How old are the mounts? Most are very old and that can be a problem

Many balance at cruise RPM , but honestly balance shouldn’t change with speed, your car tires for instance, if they are balanced, they are for all speeds.

I didn't get the IPS, another reason I was a bit unsatisfied with the guy, but he said it went down a lot. We did about 4 runs to get it dialed in.

Mounts were replaced in 2008 when the engine/prop were overhauled. At annual in March, the IA said the mounts seemed good and no excessive sag or cracking was found.

[47U]

Is your prop a 2-blade or 3-blade?  Is it indexed properly on the crankshaft?  Do you have a lens cover over your landing light?  Have your tried a different RPM/MP settings?  Is the compass fluid turning to foam?  Random screws loosening all by themselves?  Baffles cracking?  Improperly installed weights on the spinner backplate can induce cracks, so watch for that.

Lemur makes a good point, it might take awhile to get use to it.  But, your prop balance guy should have printed up the report for you.  And made a logbook entry.   

[phxcobraz]

36 minutes ago, 47U said:

Is your prop a 2-blade or 3-blade?  Is it indexed properly on the crankshaft?  Do you have a lens cover over your landing light?  Have your tried a different RPM/MP settings?  Is the compass fluid turning to foam?  Random screws loosening all by themselves?  Baffles cracking?  Improperly installed weights on the spinner backplate can induce cracks, so watch for that.

Lemur makes a good point, it might take awhile to get use to it.  But, your prop balance guy should have printed up the report for you.  And made a logbook entry.   

2-blade McCauley. No lens cover, it a Whelen LED landing light with what looks to be the stock trim/cover. Vibes are worse at high MP/RPM but when reduced power for landing they pretty much go away. Compass is fine, no issues there or foamy bubbles(had that on a 182 I used to fly). No screws working loose anywhere. Took the spinner off again to make sure it wasn't mounted wrong and lined it all up, didn't change anything.

Maybe I am just being picky, it is an older aircraft with control rods lol. I am flying to ID and WA from AZ next week, figured I would try to smooth out anything I could.

The guy made a log book entry, unfortunately he didn't list anything and I think he just forgot to give me the info or document it, without naming anyone or throwing someone under the bus, I will just say I won't use/recommend that guy anymore. Even the MSC locally didn't have any suggestions on anyone else to do prop balancing, must be a dying trade.

[carusoam]

Find the report…

There should be before and after numbers for IPS….

There should be an RPM that it is balanced at…

If you are getting vibrations that make bubbles appear in your compass, or shaky video….

This is not normal…. Or the video guys would have fixed it….

 

Expect something to not be right….

A squashed rubber engine mount…

A broken weld on the tubular engine mount…

 

My M20C broke welds on its engine mount leaving it connected by three bolts instead of four….

The higher vibration than normal was not outstanding…. Only slightly noticeable….

 

Keep both eyes open…

Dynamic balancing comes after everything else is demonstrated to be correct…

 

Take a pic of your spinner compared to the cowling…. Is there any engine sag?

 

PP thoughts only, not a mechanic…

Best regards,

-a-

[phxcobraz]

14 minutes ago, carusoam said:

Find the report…

There should be before and after numbers for IPS….

There should be an RPM that it is balanced at…

If you are getting vibrations that make bubbles appear in your compass, or shaky video….

This is not normal…. Or the video guys would have fixed it….

 

Expect something to not be right….

A squashed rubber engine mount…

A broken weld on the tubular engine mount…

 

My M20C broke welds on its engine mount leaving it connected by three bolts instead of four….

The higher vibration than normal was not outstanding…. Only slightly noticeable….

 

Keep both eyes open…

Dynamic balancing comes after everything else is demonstrated to be correct…

 

Take a pic of your spinner compared to the cowling…. Is there any engine sag?

 

PP thoughts only, not a mechanic…

Best regards,

-a-

I am leaning towards engine mounts at this point, like I said in the initial post, it seems like the airframe vibrations have reduced significantly and it is mostly buzzing in controls now. That is a good point on the welds, I will take a closer look at those, perhaps the 14yr old engine mounts could be enough to add additional vibration, even though they aren't sagging or cracking too much.

[carusoam]

There is a measurement for engine sag…

and methods for swapping out engine donuts…. And their spacers….

14yr old engine donuts… are probably lower cost to replace, then spend any time fighting with them…

When you get them out and compare to new…. It may be obvious why they aren’t working any longer….

Rubber flows slowly over time under temp and press…

PP thoughts only,

-a-

[larryb]

When I owned my J I had the engine mounts replaced. They were old and the engine was sagging a bit. There was a very noticeable reduction in vibration after the swap.

Same thing with landing gear pucks. I had them replaced and taxiing was much smoother as well.

These rubber parts deteriorate slowly so we don't notice it. But you sure notice the improvement when fixed. These parts were all changed at my direction. They were not deteriorated enough for the shop to notice and say we had to do it at annual.

Larry