REFLECTOR: Front Vibration revisited

Thu Nov 11 14:04:41 CST 2004

Thanks,

Vibration is similar to the one caused if one pushes down on the canard
tip and releases it.  Don’t know the frequency.  This vibration is new.
It did not do this for the first 200 hours.  My scoops are NACA left and
right front and two top.    I talked with the Factory about vortex and
they state the vortex does transfer from below to above near that
airspeed but they are unaware of a plane in which it can be detected.  I
plan to attach yarn strips to try to determine this weekend.  Fairly
constant in amplitude speeding up slightly with speed up to 200 knots.

Can rotate prop 120 degrees but since prop is balanced I will wait on
this ‘till other suggestions exhausted.  It does not change based on
prop or engine speed or power.

Thanks and keep them coming.

Rene’

-----Original Message-----
From: reflector-bounces at tvbf.org [mailto:reflector-bounces at tvbf.org] On
Behalf Of Al Gietzen
Sent: Thursday, November 11, 2004 1:10 AM
To: 'Velocity Aircraft Owners and Builders list'
Subject: RE: REFLECTOR: Front Vibration revisited

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

-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://www.tvbf.org/mailman/private/reflector/attachments/20041111/3f92720b/attachment.html