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11-30-2010, 09:29 PM
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Join Date: May 2009
Location: Bowling Green, KY
Posts: 293
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Bouncing Needles
I bought a flying RV-9A with the full complement of Van's steam gages. The ASI, VSI and altimeter are, of course, connected to the pitotstatic system. These three gages have a small amount of needle bounce, about +/- one needle width, even in steady flight. Is this normal? If not, is there a way to put a snubber in the system to smooth out the fluctuations? Where should it go?
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11-30-2010, 10:25 PM
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Join Date: Oct 2006
Location: Garden City, Tx
Posts: 5,122
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Sounds like laminar turbulence next to the skin rolling over the static ports. Do you have dual static ports Tee'd together?
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Greg Niehues - SEL, IFR, Repairman Cert.
Garden City, TX VAF 2020 dues paid 
N16GN flying 700 hrs and counting; IO360, SDS, WWRV200, Dynon HDX, 430W
Built an off-plan RV9A with too much fuel and too much HP. Should drop dead any minute now. 
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11-30-2010, 11:19 PM
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Join Date: Jul 2008
Location: Camas, WA
Posts: 481
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Not to hijack the thread, but mine does that too. And yes, mine are two ports tee'ed together... Been looking for the leak forever... :-/
DM
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12-01-2010, 08:29 AM
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Join Date: May 2009
Location: Bowling Green, KY
Posts: 293
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Yes, the static ports on the fuse are tied together.
As an aside, after I bought the plane, I found both the static ports were plugged with the partial mandrels still in the rivets. There was enough leakage that the instuments worked, but reacted very slowly. I drilled out the mandrels to open up the ports, and got instant relief, but now I have more fluctuation than I would like. Makes you wonder how this got past the FAA inspection...
I can't remember what drill size I used to drill out the mandrels. It was no bigger than the hole already in the rivet. Is it critical? Would this be part of the problem?
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12-01-2010, 05:58 PM
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Join Date: May 2009
Location: Bowling Green, KY
Posts: 293
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A bump for the evening shift.
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12-01-2010, 07:03 PM
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Join Date: Oct 2008
Location: Arlington, WA
Posts: 799
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Try disrupting the airstream
Use some duct tape and create an airdam either in front of, or behind the static ports. Do the same on both sides. Test fly to see if it helped. Make a couple of different airdams to see if you can steady out the flow of air near the static port.
Just a suggestion - can't guarantee that it will work...
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Scott "Grumpy" Stewart
RV-7 N957RV (First Flight on Dec 18, 2009 )
RV-14 N144P (Empennage complete, wings almost complete, fuselage almost complete)
#866 on the Van's RV-7 hobbs
#6563 on Van's generic hobbs
Arlington, WA
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