REFLECTOR: Front Vibration revisited

[Al Gietzen]

Rene;

 

Since you have pretty much checked everything mechanical/structural it seems
that it must be an aerodynamic phenomenon.  Is there anything different
about your air inlets – NACA scoops or engine air scoop, than on other
Velocitys?  What would be your guess of the frequency of the vibe, and is it
constant with the amplitude varying with speed?

 

Not sure where I’m going with this, but some kind of vortex shedding comes
to mind; you know the old Von Karmen thing.

 

Al

 

-----Original Message-----
From: reflector-bounces at tvbf.org [mailto:reflector-bounces at tvbf.org] On
Behalf Of Rene Dugas
Sent: Wednesday, November 10, 2004 11:14 AM
To: amillin at sbcglobal.net; 'Velocity Aircraft Owners and Builders list'
Subject: RE: REFLECTOR: Front Vibration revisited

 

Thanks Andy, Scott and Herb,

I will be taking the plane down for annual next month for a more thorough
shake down(pun intended).  Propeller tips are within 1/16” and prop balanced
6 weeks ago.  What kind of sensor are you proposing Andy?  I have hade three
pilots feel around in the plane but not with the rear bulkhead removed.
They could not localize it.  The rear compartment cover has a window and the
retracted gear  brackets are not moving.  Front gear is solid after
retraction.  I have removed the cowl and plenum examined the nose cone and
interior areas.  I have removed the wing bolt covers and find nothing loose.
I do not have any KNOWN tools missing that have been left anywhere in the
plane.  My next attempt will be to forward weight the elevators a little
since the elevator is the only control surface that changes significantly as
speed increases.  I agree that control surfaces seem to not be involved but
my gut tells me it is the canard somehow vibrating.  It may even be some
harmonic.  Tightening the elevator hinges before helped a little.   I do
have a front oil cooler that is snug.  Only my engine air scoop is hanging
out in the air.  It is tight.   My cowl and engine plenum touch but they
have since new over the #1 cylinder.   I do have foam caulk under my canard
and along the outer body to lower the cold air entering but  removing it did
not decrease the vibration and changing it to another type had no effect.
Nose door and “dog house” are tight.

 

-----Original Message-----
From: reflector-bounces at tvbf.org [mailto:reflector-bounces at tvbf.org] On
Behalf Of Andy Millin
Sent: Tuesday, November 09, 2004 11:05 AM
To: 'Velocity Aircraft Owners and Builders list'
Subject: RE: REFLECTOR: Front Vibration revisited

 

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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