Message-ID: <374D03AF.5A23@erols.com>
Date: Thu, 27 May 1999 04:34:55 -0400
From: "Jeffrey B. Chipetine" <abcrental@erols.com>
Reply-To: abcrental@erols.com
To: lancair.list@olsusa.com
Subject: Aileron Vibration
Mime-Version: 1.0

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Guy Buchanan correctly points out the ability for sharp cut offs on the
trailing edges to help carry airflow further back on a foil before
turbulating.  

If I may contribute my 2c worth,  I would refer you to "Abbot and
VonDernoff's Theory of Wing Sections".  This very old text is still a
basic building block for any aerodynamics student(this means us).  The
sharpness of the trailing edge should be a function of the type of foil
AND a percentage of the chord length.  Further sharpening beyond this
theoretical ideal is tough because any deviations become super critical
in relation to their propensity for any error to increase the
turbulating of air further forward on the foil. While written for
engineers,  it is understandable, if a little boring,  even for non
engineers.(like Me) 

According to them, it is better to SQUARE your trailing edges than to
carry them further back to a sharp point that may include minor but
fatal errors.

When modifying boats for racing,  a square trailing edge inevitably
shows flow to stay attached further back than the sharp edge.  While the
mediums are different,  I think builders should consider this
information. I have done quite a few of these mods,  and my boats
inevitably have done very well with the squared edges.  

I have to forward my concern that this  Vibration as described could
easily propogate throughout the minor structure on a sympathetic basis,
and cause a catastrophic failure.  Despite everyone's best efforts, 
voids of even a minor nature are inevitable in a wet layup technique,
and can become amplification points, depending on their size and
relation to the frequency about which the vibration is based.  Also, 
vibration such as described can quickly flex a structure millions of
times in a short period.  All multiple ply layups are life limited, and
have a definate fatigue and cycle lifespan.  Vibration can easily cause
these structures to reach their fatige lifespan during testing,  leaving
little or nothing left for actual flying.  Finally, (whew, I thought
this guy would never shut up),  Repetitive flexing of the structure with
vibration such as this can cause an incredible amount of very localized
heat build up,  with the accompanying loss of integrity. Composite
structures using wet layup techniques have failed through repeated
flexing on a microscopic scale.  Such flexing appears to break down the
layup starting at a void, powdering the cloth and glass mix and creating
a very effective internal pumice that accelerates the wear around it.

The problem of vibration in a project like this is that it is difficult
if not impossible to substitute an aileron from another project to see
if it is the aileron itself,  or the attachment methods that are the
cause. 

While I hate to bring it up,  you need to check the wing area ahead of
the aileron for any deviations.  Turbulated air reaching the leading
edge of the aileron sets up all kinds of nasty surprises.

It is possible that wind tunnel testing may be the only way to closely
observe and correct this problem, as you essentialy have a foil , "the
aileron" functioning in the lee of the main wing foil. The gap between
the two must be correct, and consistant for best air flow. If you can
duplicate the problem with a tufted wing while safely on the ground, 
you will be in a better position to deal with it.If many others have had
this problem,  the factory may wish to help/observe. 

The suggested solution of the addition of weight may stop the vibration
through damping action.  I believe this may only shift the critical
frequency of the vibration.  If this shift is to a point that is outside
the failure range of the structure,  OK.  If however this shift just
goes to a point that is on the edge of the envelope,  you may wish to
reconsider  You may need that aileron to be there to the very last
instant, even if it is just before the spar overloads on a hi-g pullout
or thunderstorm burst....

My personal  LAYMAN'S OPINION would be to correct the deviations in the
surface,  double check the leading edge thickness entry and  width for
correctness,  review the internal structure/bracing,  trailing edge
sharpness and square, and of course, the integrity of the
hinge/attachment systems.

I wish you well.  Be safe..

Regards to all....Jeffrey B. Chipetine


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