Calculations seem good, but the situation on the Mk 1 spar is more
accurately described as thick cloth overlaps that have to faired out,
plus the fact that the foam cores are just a bit narrower than the
spar. On mine, the depression generally was about 1/16", but tapered
quickly moving fore/aft.
The amount of filler used is most dependent upon the visual result
desired, followed by techniques used to minimize sanding. The above
mess at the spar will take much more filler if one desires the
preserve the curvature of the airfoil at the spar area to the extent
that the gloss coat shows no distortion (requires a dial indicator
tool to achieve). Or you can just get it smooth to the swipe of the
hand. A 24" sanding spline will take more filler than a 12", but the
better result might be desired. Go really wild and use a 48" metal
straightedge to check you work, preselect a spot for the fly-in
trophy, but order up more filler.
On my wings and flying surfaces, the glass fibers are just about
visible through the SuperFil on about 40% of the surface area, so the
average thickness is likely less than than 1 mm (ignoring the spar
problem). This implies also that you have to be careful about trying
to sand off too much, because sand through glass fiber and the
structure has unknown integrity.
Something has to be used, and it appears nothing is lighter than
SuperFil, except as pointed out here, other formulations that are
about as dry as practically possible. But too dry and you run the
risk that there's insufficient epoxy for a good bond a few years down
the road. Or maybe not, who knows. You might also value ease of use,
as especially on the Mark 1, the job is no stroll in the park. Just
like Bondo on an auto repair, its the many finishing passes that take
the time and energy.
XS builders have a break in that should 1 gal. be enough, subtract
what winds up in the ShopVac, and relative weight comparison of any
lightweight filler comes down to a few ounces.
Just opinions mindya, but no longer a virgin in this area.
Regards,
Fred F., A063
Richard Mole wrote:
>
> Would someone verify my calculations that Superfill is like plastic wood, in
> the sense that it weighs about 28 lbs/ft3, the same as Spruce?
>
> Its advertised as 3.68 lbs/US gallon. So if my density conversion is
> correct, then anyone putting 1/8'th of an inch above the spar is adding the
> equivalent of a 1/8'th inch wooden spar cap - as far as weight is concerned
> - without any structural benefit whatsoever. Another cut on this is to say
> that it weighs about 0.8 lbs/yd sq when 1mm thick.
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