New occurrence - Slight but noticeable vibration - Cause?

[Rick A]

A very cold day. Landed in update Pennsylvania and dropped off an A&P friend near his hunting cabin. On the trip up, he commented on how smoothly the Mooney's engine (IO-360-A3B6) ran. The two blade original prop has always been balanced, and it's been a super smooth runner. The propeller and hub are original, with nearly 6000 hours since new (along with the airframe) and almost 1000 hours since the engine and propeller were overhauled.

On the return flight climbed out normally, then leveled off. After leveling and progressing into normal cruise I noticed a very slight vibration that was definitely new. Nearly identical (but less) to the vibration that occurs when a little thin strip of rime ice develops on the leading edges of the propeller. Every other indicator was as to be expected; cylinder temperatures, EGT's/peak points, fuel flow, TAS etc. were all completely normal.

Changing the prop pitch there was a touch more vibration in the 2350 to 2400 rpm range where I normally operate in cruise. Increasing the rpm's higher resulted in a little less vibration, but still there.

After landing, I checked the prop for any sloppiness. The blade seals were tight with no oil leakage. However, when I positioned the propeller vertically and pushed in /pulled back hard on both blades there was a small "clunk" heard. I can only imagine some wear is occurring in the propeller hub. If this is the case, how dangerous is this? Would hate to have something major break in flight.

Thoughts, opinions and recommendations as to "what's next" would be greatly appreciated.

 

[FORANE]

Clunk may be just the crankshaft which I've been told is normal.
Regardless I would send that prop for reseal if in question. Also, you don't want to push pull on the blades less you might induce some runout.

 

[Bell206]

Thoughts, opinions and recommendations as to "what's next" would be greatly appreciated.

Take your A&P friend on another flight and see what he feels. Sometimes the power of suggestion alters our perception especially on the next flight after he told you how smooth it was.........

 

[bonefishpete]

My 201 had slop in the crank backlash for and aft normal. Blade hub shouldn't have any.

 

[AV8R_87]

commented on how smoothly the Mooney's engine (IO-360-A3B6) ran

He jinxed it right there.

Check the engine mounts as well.

 

[Rick A]

Take your A&P friend on another flight and see what he feels. Sometimes the power of suggestion alters our perception especially on the next flight after he told you how smooth it was.........

I wish it was that simple. Noted during the discovery of, then awareness, was the wingtips and instrument panel were also now vibrating (which never happened since the last engine/prop overhauls and dynamic prop balance). Increasing rpm's lessened it, then when reduced back to typical cruise increased the vibration a little. Unfortunately, it's really there.

He jinxed it right there.

Check the engine mounts as well.

Yeah, jinxed it. Next time I'll change the subject, or just shut up...

Following a couple of calls with my IA, the plane is getting dropped off tomorrow at his facility. What bothers him too (my guy) is the suddenness of the onset.

He called the local prop shop and they said my prop should be (but not always) good for 6 or 7 rounds of overhaul before the dimensions are worn out. There's been 3 complete prop overhauls, plus occasional inspections and prop seals during its 5900+ hour life. At a minimum it looks an inspection is in order, and depending upon what's found, a complete overhaul may be in order. Rust is the biggest issue, and they claim being kept in an unheated hangar is often not much help. They've seen twins with identical prop histories, with rust on one side, and no rust on the other side.

We'll also take a good look at the engine mount and the rubber donuts. They were addressed nearly 9 years ago (new rubber and frame sent out) at the last engine/prop overhaul. The engine is sagging slightly, which I figured would / could be shimmed at the next annual. He wasn't too concerned about the slight "clunk" heard when pulling/pushing on the prop tips.

 

[bnt83]

The "clunk" sounds like typical crankshaft-to-crankcase end clearance to me.

 

[Bell206]

What bothers him too (my guy) is the suddenness of the onset.

Well considering now its not "a very slight vibration," did he mention to pull the cowling and give a good tactile check to all the items around the engine before flying to his facility?

 

[Rick A]

Well considering now its not "a very slight vibration," did he mention to pull the cowling and give a good tactile check to all the items around the engine before flying to his facility?

I've considered it slight because the same type of vibrations have been evident in other aircraft, and considered normal. Just not in mine until now. Fortunately, the plane's maintenance is done on the home field, so it's only a taxi (or tow) away.

 

[Gary Ward]

When my old engine started to foul a lower spark plug it created a slight vibration. Clean the plug and it's smooth again for another 50 hours.

 

[Rick A]

When my old engine started to foul a lower spark plug it created a slight vibration. Clean the plug and it smooth again for another 50 hours.

Understood, I've had that happen too. In this instance all 8 spark plugs are fairly new fine wires, and all EGT readings were level and normal. I'd expect the fouled plug would cause an unusual, and noticeable EGT increase in temperature.

 

[Rick A]

Follow up report: Prop balance checked, hasn't changed since last done nine years ago. Under a 1.0 (whatever that means, but supposedly excellent balance).

All fine wire plugs were checked and cleaned (no issues), and the compressions checked (all high) and even.

The vibration seems to have been caused by thick gook on the surface of a couple of the GAMI injectors. All soaked for 48 hours, blown out, and re-installed. They hadn't been removed in years. Now super clean.

Test flown today and the vibration gone. Back to normal! Happy Holidays to all.

 

[Bell206]

Under a 1.0 (whatever that means, but supposedly excellent balance).

It refers to the inches per second or IPS which is the unit of measurement for vibrations. However, I believe you probably have under a .1 IPS as a value of 1.0 will have your teeth rattling.