Hi Matt

I don't necissarily have a solution but I can offer a place to go for analysis.  Hood Technology Corp. in Hood River, Oregon is an acoustic lab and has a digital recorder that you can rent.  Simply record the event, then send in the tape and they will do an anlysis of the vibration.  The vibration will  be very obvious as a peak showing up in the data at a very specific frequency.  This may give some indication as to the source.  And I suspect they can also stick on some transponders to some surfaces to try and track it down.  The source sounds like aerodynamic excitation.  I have experienced vibrations at critical speeds in fast sailboats and windsurfers.  This comes from vortex shedding and the turbulence.  A lifting surface will generate a vortex at all of the speeds we sail or fly at so the vortex at low load situations becomes unstabile and alternates from one surface to the next.  This is percieved as a hum but our speeds are pretty low. Still, we operate at Reynolds numbers of 10 million and more so it isn't all that dissimilar to a plane flying in the low density air.

The good news, if that is really the problem, it can be relatively easily fixed by tripping the boundary layer.  If it is a foil vibrating, say like the elevator or even the ailerons or flaps but at a frequency so high it or magnitude so low that it appears stationary, it might be cured by simply putiing a piece of piano wire parallel to the hinges to generate a vortex.  Alternately, if you find the offending surface, you may be able to treat it with vane type vortex generators.  For my new rudder and keel on my 37' boat that planes and surfs, it's speed is such that it is prone to humming.  A real drag for those trying to sleep below on long ocean races and a distraction hor the helmsman.  To stop it, I have made the rudder's trailing edge asymetrical.  Instead of a razor sharp trailing edge, (very prone to vibrating), or squared off to 1/8" thick (less prone but still not perfect), I curve one side so that the votex is dragged around the curve by viscous effects away from the curved side.  But I only do that in one direction for HALF of the keel or rudder trailing edge.  The other half, I curve the opposite way so it's shedding it's vortex to the opposite side. The net effect is zero on the helm but still eliminating the hum.


And of course, it could be something else entirely...

good luck!

Dan Newland

(A box of parts that may become an ES)
I am working on reducting the acoustic testing Dave Morss and I did on his Legacy now and used them to