REFLECTOR: Front Vibration revisited

[Andy Millin]

Rene',
 
Just thinking.
 
No change with engine and prop rpm ... must be airframe
No change with controls and no vibration felt in the controls ... not the
controls
Nothing loose on the airframe ... not the cowl or gear doors
 
Sounds like an aerodynamic issue.  Harder to find.
 
Now the challenge seems to be to find a clever way to locate the affected
area.
 
Alex Balic might have an idea on how to measure vibration.  Every time I try
to figure out what his background is he comes up with a new tale of
experience in a different discipline.
 
It would be helpful if we could place a sensor or sensors on the airframe
and measure vibration.
 
You have looked at the obvious things.  We still need to find out just what
is vibrating.  Then we might find out what is making it vibrate.
 
FWIW,
 
Andy
 
Just an afterthought.  Could you take up a passenger and while the vibration
is going have them feel around the plane.  Check the center spar, gear legs,
firewall, windows, etc.  Might give a direction...

-----Original Message-----
From: reflector-bounces at tvbf.org [mailto:reflector-bounces at tvbf.org] On
Behalf Of Rene Dugas
Sent: Monday, November 08, 2004 7:38 PM
To: reflector at tvbf.org
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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