Post-curing is essential to achieve optimal bond strength. This applies to
laminated
parts and also part to part glued joints. The regime for the post-curing
depends on the system used. Some systems achieve full strength when exposed
to RT (ambient 20 deg C) other require higher temperatures. Generally higher
temperatures result in full post-cure properties being achieved in a shorter
time.
Ideally, Europa (composite airframes in general) should be post-cured in their
entirety - achieved in an oven with a controlled environment. A Paintshop
facility
can provide these conditions.
It should be noted that the Achilles heel of composite aircraft (at least those
approved via the LAA) has been the requirement to use a 'Composites
Super-Factor'
(see LAA TL 1.17). This takes into account the significant variability in
composite strength, stiffness, etc at non-RT conditions. It also includes a
variability factor that accounts for variations in the processes of laminating,
jointing, etc. This CSF can be as high as 1.50. A reduction in CSF may be
negotiated, this is more likely if documented processes and post-cure is
followed.
Recent discussions with LAA Engineering indicate that CSF of 1.20 may
be possible in some circumstances. This can have a significant influence on RFs
(margins of safety) calculations and even MAUW determination.
My advice to Europa builders is to post-cure their entire airframe. Survey the
airframe to ensure no items are present that will degrade at the chosen
post-cure
temperature.
A typical post-cure process will look something like the following (BUT adhere
to the supplier instructions and limitations):
1. RT to 80C, ramped up at 1-2C per minute.
2. Hold (dwell) for 120 mins (typically 80C).
3. Ramp down at 1-2C/min.
The structure should be free (no clamping or holding things down), I would
assemble
the airframe but this is rarely possible.
--------
John Wighton
Europa XS trigear G-IPOD
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