Mysterious Vibration Source: Thoughts?

[Jeff_S]

I've debated about posting this since I haven't had time to do my own thorough investigation and facts are sparse, but decided to open this up to see if others have thoughts.  Here's the situation:

Flying home from SnF on Saturday evening, my J was performing flawlessly running LOP at 8000' and purring like a kitten. As we near Atlanta we get step downs to 6000', then 4000', then finally 3000' for the last turn westbound to PDK. All of a sudden at 3000' a loud pulsing sound, accompanied by a vibration, immediately fills the cabin. JPI isn't showing anything dramatic but I put in full rich mixture and fiddle with the prop to see if I can get any change. The plane still seems to be making plenty of power but it's dark and we're only at 3000' by this time so I don't want to mess with throttle to experiment.

We're only 8 minutes from home so the decision is easy just to take it in. We did notice that we'd turned into a massive headwind, as we were making 155KTAS but only 125 GS. I got preoccupied with just landing so I didn't notice at what point the sound and vibration dissipated but it seems like eventually they did. We landed fine, and I did a run-up before parking...no problem on either mag, propellor cycled correctly, etc.

I was tired and it was dark so I didn't investigate too much more around the plane, but one thought I had was that it could have been something on the airframe vibrating. I did notice that the right cowl flap seems a bit loose in it's fully-closed position. Is it possible this could be fluttering? Other than that, I won't be able to get back for more investigation until later this week.

Not much to go on, I realize, but any ideas?

[carusoam]

Thoughts for your consideration...

Was the vibration constant (like the cowl flap always fluttering)?  Or, was it  always changing with each up and down of turbulence?

With that much wind, there may have been gusts, up/down drafts, related changes of attitude and resulting change in altitude...

Is it possible that the govenor was challenged in keeping control over the engine rpm?  As the load changes with each attitude bump, are you hearing changes in engine sound?

While checking the cowl flaps, also consider checking the landing gear doors.  At SnF you can drag through alot of sandy ruts and grass clumps to get to parking and camping sites.

Flying up the east coast on Saturday morning, I got bumped around so much it was extremely difficult to twist knobs on the GPS for an ammended clearance.

-a-

 

[N201MKTurbo]

I had my alternator fan come apart once and it was kind of like what you describe. It was on my F model and the piece that left the alternator hit the oil cooler and made a small hole. I landed about 8 min. later to see what was going on and had only 2 Qts of oil left. There was a massive oil streek alonge the left side of the plane just below the window, I couldn't see anything from the cockpit.

[Cruiser]

when did you open the cowl flaps? That will change the sound and vibration.

[laytonl]

A bearing failure in the alternator can cause a similar reaction.

Lee

[Jeff_S]

Thanks all for your responses. I haven't been back to the plane yet but I spoke to the prior owner yesterday and he said the exact same thing had happened to him in 2006, and in fact it was that right cowl flap as the culprit. So while I can't confirm that yet, it seems a likely cause...after all, the same thing can break on an aircraft more than once, can't it?  (hah!)

I shall do a thorough check today or tomorrow and hopefully be able to report back with final results soon.