Intermittent vibration

Hi all,

I've recently been getting some vibration on my RV-7 (O-360 + HC-C2YK-1BF). It kind of follows the RPM, so I thought that it could be a prop out of balance. However the confusing thing is that it seems to come and go with engine parameters unchanged. I wasn't quite able to pinpoint what causes it to start and stop, but it seems that it diminishes during and after entering some chop, but it may just be perceptual effect. It also does seem to be less on the ground even during runup to high rpm.

I found that my exhaust pipe (Vetterman crossover) was loose. I replaced some clamps and now I can't move the pipes so that they touch either cowling or the airframe, although there is still some play.

Not sure what the next step should be. My idea is to find someone with a vibration analyzer and try to record it in flight or do a prop balance just in case, but the intermittent nature of it seems to hint that it won't help?

Any suggestions?

Thanks!

Check to see if it changes with how you position your feet on the cabin floor. That makes a real difference in my -6.

Thanks, I'll try it. I can also feel it in the control stick though.

Check prop spinner mounting bulkheads, both front & rear for cracks
Check prop blades for looseness & track
Check front of cowl for spinner rubbing
Check cowling mounting hinges for broken loops, especially bottom sides
Check baffling for rubbing on cowling
check baffling for cracks, side panels
check baffling for cracks around lower cylinder skirts
check (carbureted or vertical FI) air filter mounting plate for cracks
check alternate air door for looseness
check oil cooler if mounted on baffling for cracks
check crank breather tubing for rubbing
check engine mounts for wear, engine sag
check air snorkel for rubbing interference with cowling, or rubbing on lower side cowling
check for anything rubbing on engine mount
check loose hot air baffle
check gear leg fairings for looseness
check gear leg rear edge hinges for looseness
check wheel pants for looseness or wear
check w/shield fairing for separation from plexi
check canopy skirts for looseness or dynamic vibration
check all wing access covers for looseness
check gear mounting covers (rv8) for looseness
check flap to fuselage fairing for looseness
check flap hinge & aileron bearings for wear

I've found all these conditions on various RV's over the years & the owners common complaint was vibration, weird feel, or noise during flight.

& yes the prop balance could cause a varying harmonic. The tail pipe hangar would be the first thing I would look at, I've rotated the pipe around to get the bend in a better angle in some cases & have trimed the cowl in some others to gain more pipe clearance on others. In a couple, I've revamped the tail pipe hangar arrangement to better float with the engine & not hang from the engine mount.

Hope this helps you find your vibe!

Thanks, this is very helpful! Hopefully one of these will resolve the problem, will try this weekend.

kaa said:

Hi all,
....
I found that my exhaust pipe (Vetterman crossover) was loose. I replaced some clamps and now I can't move the pipes so that they touch either cowling or the airframe, although there is still some play.

......
Any suggestions?

Thanks!

Click to expand...

That might be a hint.

If the aft end of your pipes are connected to the airframe and the ball swivel joints in the Vetterman system are sticky/frozen you will most likely get vibrations through the airframe and tend to loosen the exhaust system.

A few drops of Mouse Milk applied every time your lower cowl is off keeps the ball joints working.

I had a similar transient vibration on my Beech Musketeer with an O-360 that turned out to be a loose exhaust valve guide in one cylinder. Required replacement of the cylinder. The engine had about 3000 hours tt and 1100 smoh. The transient vibration was most pronounced in cruise at 2450 rpm.

Thanks everyone! I checked most of the things suggested in the thread so far, and didn't find anything obviously criminal. I did tighten the exhaust pipes some more and the vibration seemed to get better (I don't have a way to objectively measure it). Maybe the pipes were resonating and transmitting the vibration to the engine mount? They are mounted using a "hose plus steel clamps" method which seems to be standard.

How much play should there be in the tail pipes? My plane had them moving quite a bit ever since I bought it, and now there is maybe 3/4 inch motion in one of them (it doesn't touch the cowling though). The ball joints are free - I lubricate them fairly often.

Thanks again.

I get this same kind of thing in my airplane, occasionally I feel it in the stick. Next time I take it up , nothing. Sometimes it’s on part of a flight and not another part. Thing is, I get the same thing in my boat going up river I’ll get it and I’ll turn around down river and boom it’s gone, or in a following sea.

I’m not convinced it has anything to do with the airplane at this point as it’s come and gone since I’ve owned it and had it apart many times and nothing seems to make any difference. I think I’m just more in tune to vibration because half the pilots who fly with me don’t notice it until I point it out. If it definitely comes and goes.

Also would like to add that my club airplane seems to do it as well, so I really just think it’s how the blades act on the air in certain conditions.