4.
Repair
of
units constructed from GRP
If
you notice any damage to your glider,
you
should at once determine the extent of the
damage.
It
is
often found
on
inspection that the break extends much further than the
damage
visible
on
the surface.
Accomplish repairs with great care. With GRP gliders the outer skin
is
load bearing; a
failure of its structural integrity can lead to a crash.
Maintain the resin : hardener mixing proportion as correct
(±
0.
5%)
and use clean
containers. The weight of glass to the weight of resin mixture should
be
approximately
50
:
50.
Abrade and clean the repair joint surface immediately before applying the wet
laminate so that
no
contamination
can
occur to limit the adhesion.
As
with plywood, the direction of the fibres
in
the fabric layers (lengthwise or diagonal)
is
of great importance
in
the integrity of the structure. A guide to the number of
laminations required to re-establish the structure of the damaged area can
be
inferred
from the
simplified fabric construction diagram.
In
any case measure the thickness of
the damaged
laminate. Remove a piece of the damaged laminate and incinerate
it.
The
resin burns and the
glass fibre structure remains.
You
can identify the fabric weight,
number and direction of the
laminations from this sample.
Repair work
is
time consuming. Take care to cut the fabric patches accurately so that
when they are laminated into the structure they do not stand out from the required
profile. They will then not have to
be
sanded/ground down before restoring the surface
finish.
The resin hardening time
can
be
shortened,
by
increasing the ambient temperature
with a heater.
Caution: A high temperature will cause bubbles to develop
in
the resin . To minimise
the build
up
of local hot spots, construct a 'tent' from foil and direct the hot air jet into
the tent.
Ensure that when a
control surface
is
repaired, the weight does not increase, otherwise
there
is
the danger of flutter. Check the mass balance of controls after repairs.
5.
Damage
to
GRP - foam structures
It
may appear that only the surface (external) laminate
is
damaged, but
it
is
often the
case that the
whole structure, outside
and
inside laminate
and
the foam core
is
damaged.
a)
Simple surface lamination repair. (fig. 1 page 30)
Around a crack you often have separation of the surface laminate from the supporting
foam core.
You
can determine the extent of this foam separation by tapping. Remove
the laminate separated from the foam (grinding
wheel, sanding block or sharp knife).
With a sanding
block or flat plane cut a splice
in
the laminate around the damaged
area,
splice width a minimum of 20mm. Correct splice angle laminate thickness to
splice length 1 :
50
.
After the preparation of the repair, thoroughly clean the area - remove the abraded
dust with compressed air (also
clean from the foam pores) - wash the splice surface
with carbon tetrachloride or acetone if there
is
any grease or
oil
contamination.
Filler the damaged foam space, and the exposed surface pores of the foam with a
resin/microballoons mix. Now
lay
up
the correct number of glass laminations.
Important: - Insert the glass cloth
in
the correct alignment and with the largest patch
inserted first.
It
is
essential that the work
is
dust and grease free.
28
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