The only comment I have to add is that it is generally wise with 'small'
repair areas to, after replacing what was there with same strength cloth and
ply orientation, overlapping plies half an inch is to add one ply of bid
over the whole repair area as a belt and braces, peal plying all the edges
or the whole repair if a small area. This covers the possibility of lack of
strength due to less than perfect layup conditions/lack of perfect
preparation etc, when done 'in the field'
Ivan
From: owner-europa-list-server@matronics.com
[mailto:owner-europa-list-server@matronics.com] On Behalf Of Duncan & Ami
Sent: 27 August 2012 08:46
Subject: RE: Europa-List: Re: Cracks on the cockpit module near to the seat
belts
Doesn't really matter which cloth, as the replacement cloth will be carrying
al of the load across the cracks.
Number of plies can be determined by burning a sample taken from the damaged
area.
D McF.
-----Original Message-----
From: owner-europa-list-server@matronics.com
[mailto:owner-europa-list-server@matronics.com] On Behalf Of Paul McAllister
Sent: 26 August 2012 15:15
Subject: Re: Europa-List: Re: Cracks on the cockpit module near to the seat
belts
Duncan,
Thanks for posting this, I have been meaning to post something similar so
you have saved me the trouble of going into the detail.
Jacques, the only thing I would like to add is that ideally you need to use
the same cloth that was used in the original construction. In this case it
might prove difficult because pre-preg bid was probably used in manufacture.
The reason you need to do this is that using a material that is
significantly different means that this material may carry more (or less) of
the required load and become a source of stress in of its self.
I would see if you can get a response from Nev or Ivan to to find out how
many layers of material were used and what sort of cloth was used. I know
that when I did a repair on my wing I was fortunate enough to be able to get
the exact cloth that was not pre-preg'd. Ask them is it the same cloth that
was used in the wing. If it was then I have a small amount of this and I
can send it to you.
Failing that, it wouldn't be horrible if you can't match the cloth, but do
try and find out how many layers were in the original construction.
Paul
On Sun, Aug 26, 2012 at 3:11 AM, Duncan & Ami <ami-mcfadyean@talktalk.net>
wrote:
<ami-mcfadyean@talktalk.net>
<<..Any advice is welcome. .>>
OK, I'll bite!
During a monowheel collapse, the weight of the aircraft (less tailwheel and
outrigger loads) is transferred from the monowheel in to the top of the
'tunnel'. Thereafter, the stress-path from the tunnel is mostly both
forwards and aftwards. In the aft direction the tunnel distributes loads in
to the seatback bulkhead then further in to the fuselage skin. This means
that the area that has cracked (being at along this stress-path is taking
abnormal loads and this is a possible cause of the cracking, as you have
surmised, compounded by the holes that have been cut at that point.
Assuming there is no evidence of any other overstress in that area, the
repair needs to put back the strength lost. This means replication of the
original strength of composite in that location, which can be determined by
counting the number of plies present. Allow a minimum 1/2 inch per ply
overlap on to the surrounding unaffected area (presuming a bond shear
strength of 500psi and cloth strength of 250lbs/in) with staggered edges. If
the holes must be recut, then double the reinforcement and cut the holes as
ellipses (at a length:width ratio of 1.414:1, if you want to be pedantic!).
Too much or overdesigned reinforcement may make the area too stiff and
introduce other issues, and makes the aircraft heavy too!
Your respective airworthiness and/or design authority will be the final
arbiter of any repair, which might be along the lines of the above.
Duncan McF.
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