REFLECTOR: Front Vibration revisited

[Ronnie Brown]

Rene,

Since it sounds like you have exhausted everything else, try this:

There is a program on the internet at http://www.tunelab-world.com/rpmsound.html which is an audio spectrum analyzer.

It actually works - I have checked RPM's on aircraft engines, even from several video tapes that I had from cross country trips.  You can use this to try to determine what frequencies are occurring in your plane.  It has high and low rpm settings (to trap unwanted harmonics), harmonic settings, etc to help trap these frequencies.

You can run it on your laptop while you are flying.  Perhaps a better way would be to record the sounds using a high quality audio recorder or a analog cam corder with stereo hi fi capability.  You could attach two good quality remote mikes - taped to the areas that may be generating the vibrations.  Then analyze these at home.  You should be able to isolate engine rpms, prop pulses, and in your case, perhaps the randomly generated air pressure pulses - or what ever - then using the frequencies found, figure out what the source of your vibration might be.

Happy Hunting!!!!
Ronnie Brown


  ----- Original Message ----- 
  From: Rene Dugas 
  To: reflector at tvbf.org 
  Sent: Monday, November 08, 2004 7:38 PM
  Subject: REFLECTOR: Front Vibration revisited


  Folks,

  Five weeks ago I began chasing a vibration in my XL that only occurs over 160 knots.  Some helpful suggestions have been received and pursued to no avail.  I spoke with the factory today and they don't know what it could be but suggested I keep looking.  This symptom is a shimmy that starts at over 160 kts and gets slightly worse all the way to 200 knots.  It is not uncomfortable but VERY annoying.  It does not change with pull ups or ½ g unweighting or turns.  Prop speed or engine RPMs eo not affect the shimmy.  No vibration can be felt in the stick ( I have 230 hrs of experience in this plane and this is new.)  No visible shake is noted in the canard winglets or wings.  Slight pressure on the rudders does not change it and no vibration is transmitted to my feet.  No increase vibration is felt in the torque tubes to the elevators or in the canard with my feet or hands.  Back seat passengers can feel the vibration.  

  I have 

  Removed the wings and confirmed incidence and tight bolts.  

  Removed the canard and confirmed bulkhead is ok and bolts are tight.

  Elevator matched and balanced and hinges snug.

  Elevators adjusted up 1/8th inch and then down 3/8th inch no changes.

  Front gear doors tightened -Very snug.

  Main gear doors adjusted x five no change

  Main Gear doors removed no change

  Doors confirmed snug and vibration the same as entire plane

  Cowl removed and realigned x five

  Left wing incidence increased 0.2 degrees no change

  Discussed canard vortex hitting wing - Why a change and factory states a non problem.

  No glass cracks found.  Fuselage and wings and canard all very solid.

  No fuel leaks.

  Spinner tight rotates very centered.

  Prop balanced.

  Windows not loose.

  Heated Pitot tube snug no slack.

  Door handles not moving.

              Same problem solo or with slightly over gross wt.

  Only external scoop is ram air to engine just behind the firewall  very stable.  Unable to move on the ----ground at all.

  Not expecting cures but ideas?  



  Rene' Dugas



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