REFLECTOR: FW: Chasing Down the Source of a Vibration

[Scott Derrick]

I've heard so many bad experiences from loose tape, its starting to loose
its luster..

thanks Chris,

Scott

>
> From: Christopher Martin
> 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 piece 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_______________________________________________
> To change your email address, visit
> http://www.tvbf.org/mailman/listinfo/reflector
>
> Visit the gallery!  www.tvbf.org/gallery
> user:pw = tvbf:jamaicangoose
> Check new archives: www.tvbf.org/pipermail
> Check old archives: http://www.tvbf.org/archives/velocity/maillist.html