Andy and others,
I have just recently done my leading edges and ended up with agood result
with one exception. The skin that I wrapped around the underside of the
spar did not stick well in a couple of places. These places were not as
well supported by the plastic angle as the others, obscurred by my spar
jig. I know that the uppermost surface, and the spar sides, were well
sanded prior to bonding and layup work respectively. I do however think I
may have omitted a good sanding of the underside of my spars, other than a
cursory light sand some time ago. The areas not bonded to the spar equate
to about 5% of each surface, if that. If I insert a chisel or knife and
lever at the sides of the bond between the cloth and the spar, and lever, I
can migrate outward the "unbonded area".
Now my problem is that I don't recall my original sanding was a thourough
one, and yet 95% of the cloth has bonded well it apprears to the spar. If I
try and lever underneath any other area it is nearly impossible as I used
peel ply and have a good resin transition onto the spar. Of course, if I
lever outward from an already unbonded area, I can exacerbate my problem.
I see my option as being:
1) inject resin underneath the unbonded area not well supported during
cure, and continue with the trailing edge block attachment
2) inject resin and place BID straps 1" wide, overlapping ontop of the
bottom and top surfaces that are wrapped behind the spar locking the skins
together at an arbitrary 300-450mm apart. These straps would not wrap onto
wing skins, but would sit in recesses sanded into the trailing edge blocks.
3) cut away the skin that has been wrapped behind the spar and apply the
bid brackets as per the new manual. It would seem a pity to negate the
whole wrapping process, which as a generalisation "went quite well". At the
same time I would end up with bumps on my skins at the spar that would
require more filling.
4) Apply the same amount of BID that the new manual requires as a bracket,
however rather than apply it ontop of the skins and around onto the rear
face of the spar, have it overlap the already wrapped skin that now sits
behind the spar (with questionable bonding), and then transition it down
onto a thoroughly sanded spar. This results in the same amount of keyed
bond between spar and skin, albeit this area now bridges over wing skin
that may not have ideal bonding. Any force of the skins wanting to depart
the spar will want to tear the wrapped cloth upward/downward, therefore if
the bridged skin cloth is a stable structure in itself (which it is), the
force is simply transferred closer to the middle of the rear face of the
spar where it would be bonded. I should end up with equivalent strength and
equivalent weight to the new manual method without the bumps on the skins.
The only drawback is my trailing edge blocks will sit about 1-1 1/2 mm off
the spar, so I will need to trim them nominally.
These steps are in order of weight gain to my wings and I am hopeful that
suggestion 1) will be adaquate. I can only say that I feel, "even well
keyed layups may well separate at overlapped joints if you lever them apart
at their union", so I am probably being overconservative. Because I don't
know of the required strength of this bond I feel however that I should ask
and end up with the best result.
I await your advice at your earliest convenience.
Reg
Tony Renshaw
P.S. with such a wealth of knowledge and opinion out there, I have sent
this also to the forum to see what others think of my dilemna. (Any
personal response however I will obviously keep confidential!) Thanks Andy.
Reg
Tony Renshaw
Builder No.236
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