REFLECTOR: FW: Chasing Down the Source of a Vibration

Terry Miles terrence_miles at hotmail.com
Thu Jan 26 20:47:16 CST 2006


Chris,
Thank you very much.
Terry

-----Original Message-----
From: reflector-bounces at tvbf.org [mailto:reflector-bounces at tvbf.org] On
Behalf Of Christopher Martin
Sent: Thursday, January 26, 2006 11:37 AM
To: reflector
Subject: REFLECTOR: FW: Chasing Down the Source of a Vibration


 

From: Christopher Martin <mailto:christophercmartin at earthlink.net>  
To: reflector-admin at tvbf.org
Sent: 1/25/2006 11:53:57 PM 
Subject: Chasing Down the Source of a Vibration




I want to share a little interesting problem I had because we all learn
from one anothers mistakes and experiences. It's the nature of the
Reflector and it is a very good thing.
 
I have been flying my Velocity XL RG since November of 2000. I now have
620 plus hours on it and many happy experiences. I used to be a more
frequent contributor to the Reflector and am overdue in making a
contribution. So, it's time to share a story.
 
Last May, I was departing Fresno, California on a return flight to my
home base in El Monte , California. I was climbing out in a left cross
wind departure at 3000' AGL. Suddenly, a vibration emanated from
somewhere in the plane. It sounded like a bearing in some engine
component. It lasted for what seemed like a full 20 seconds. I was
debating on wether to return to Fresno or land in Visalia when it, just
as suddenly, disappeared. I climbed on up to 9500' and leveled off in
cruise at 185 KNTS. Suddenly, the vibration appeared again. This time I
tried changing engine RPM and throttle settings. At times the vibration
would shut off and then suddenly reappear. This was a very abrupt
vibration. Meaning it was either on or off. And, when it came on it
lasted for as much as a minute and then might go off for as long as 20
minutes. I concluded that it must be engine related and that I would
debug it back at home. The first thing to check would be the engine oil
and filter. 
 
Back home I started the investigation. Engine oil and filter were fine.
So, how about vacuum pump. It was time to rebuild it anyway. After all,
the pump was approaching 600 hours. So, I rebuilt the vacuum pump. With
the rebuilt pump installed, I took off and all was fine. I flew out to
Camarillo and it was perfect. On the return flight all was well until I
started my decent into El Monte and suddenly, the vibration reappeared.
Lasted for a few minutes and then just as suddenly disappeared. It must
be the alternator I concluded. Had to be. I had nothing else with
bearings that could make that noise. I have dual electronic ignitions
so, it's not magnetos. It has to be the alternator. So, I send the
alternator in for new bearings and brushes. After all, like the vacuum
pump, 600 hours is time enough for a rebuild.
 
The alternator comes back. I bolt it on. Take off and immediately hear
the vibration. It lasts for 20 seconds and then disappears. After all of
this I am dumbfounded. I can't reproduce the problem on the ground. It
only appears in flight. It is intermittent and I'm baffled. So, I
continue to fly the plane with this occasional vibration sound. There is
no other manifestation of the problem other than the sound. I conclude
that there is only one other possibility. With 600 hours on the plane
and 6 years, the engine is starting to sag on the mounts. Is it possible
that in flight the engine is causing some vibration to be transmitted to
the cowling? Is something hitting the cowling because the engine mounts
are sagging? Ok, I'll change the rubber bearings. This is expensive and
difficult and I really don't want to do it, but I have time at Christmas
when 'I'm doing the annual.
 
So, last Christmas, I changed the engine mounts. First of all, it does
look better to once again have the engine sitting squarely in the engine
cowling. The 6 years and 600 hours of use had allowed the mounts to
creep so that the engine had sagged a full 1/2 inch. Now, with the new
mounts, it was back where it was supposed to be; squarely in the engine
cowling. The prop perfectly aligned with the prop opening and cowling. A
piece of sculptural beauty. But, did it solve the problem?
 
I took off from El Monte and was just turning down wind when the
vibration appeared. It lasted 20 - 30 seconds and, again, disappeared. I
landed.... I was pissed..... What else could it be...? I am such a
moron..... I can't find this dumb sound..... I decided that I had become
so focused in the chasing down of the vibration that I needed to stop
and walk away. Take 100 steps back and look at the plane entirely. Is
there anything that appears to be out of place..... There wasn't.
 
I walked around the plane and checked it again, in total. Everything was
exactly as it was supposed to be. As I completed my walk around, I
passed by the co-pilot side rudder and, almost out of reflex,
straightened the tape I had placed on the rudder gap. You know how we
have talked about the merit of gap sealing the ailerons and the rudders.
Well, I had placed some white electrical tape over my aileron and rudder
gaps. It works great. All of my control surfaces have it. And, I do
think the control surfaces are more responsive and must have less drag.
But, every time I go by this co-pilot side rudder, I need to realign
this piece of tape. It is as if it has come loose at some point and then
reattached itself. Is it possible that this tape is loose in flight and
creating a vibration? I check it out..... It seems that 2 to 3 inches of
the tape is loose and is not adhering well. The rest of the tape is
holding strong. It is very possible that this 2 to 3 inch pi! ece of
tape is trailing in flight. So, I pull off the tape. After all, it is
not flight critical. In fact it is more cosmetic than anything. 
 
I take off........ not a sound.... Perfectly quiet..... I can't believe
it! I flew all over the area and and into the desert and back without a
sound! I have now flown 6 hours and not one time has the vibration
reappeared. The problem is solved.....
 
As I reflect back on my preflights. I had always checked my rudders and
I had always straightened that same piece of tape on the co-pilot side
rudder. I had been looking at the problem before every flight and
couldn't see it because I was so certain it was engine related.
 
I'm not drawing any other conclusions from this other than : These
planes are very fast and anything that gets into the air stream that is
not tied down is going to flap and vibrate. Appreciate the aerodynamics
and drag associated with everything that is hanging out there in the
breeze. At 185 KNTS a 2 inch piece of tape can generate a great deal of
vibration energy.
 
And, that's my little story for "Share Time" today.
 
Build it straight and fly it safe....
 
Christopher Martin
christophercmartin at earthlink.net
 
 



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