REFLECTOR: FW: Chasing Down the Source of a Vibration
John Dibble
aminetech at bluefrog.com
Thu Jan 26 17:57:15 CST 2006
I think any tape should never be re-applied. The last time I replaced
the ss leading edge tape on my IVO prop, I tried using propguard over
the top of the ss tape. The propguard stuck to the ss real well, but
after 1/2 hour flying one of the ss tapes pulled loose from the back
side of the prop. I removed the propguard and reapplied the ss tape.
It seemed to stick ok. My next flight was 2 hours and when I landed,
the ss tape had completely detached from the back side, was all shredded
and the prop had many dings and scratches from the tape slapping it.
Never the slightest hint of vibration however.
John
Christopher Martin wrote:
>
>
> 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 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
> Martinchristophercmartin at earthlink.net
>
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