From: Brett Buck <buckbw@pacbell.net>
Subject: Re: What is silkspan?
Date: 12 Apr 1998
Newsgroups: rec.models.rockets

The Silent Observer wrote:
>
> Joe wrote:
> >
> > I looked in the FAQ and found references to its use, but no actual
> > description of the material.  Thanks.
>
> Silkspan is wet-strengthened tissue, intended for application in a
> prewetted state to model airplane frames.  While wet, it can be
> stretched very slightly to take curves well, and it shrinks some as it
> dries to ensure a taut covering.  Once dry, it's sealed and further
> strengthened with dope.
>
> It comes in three weights, light, medium, and heavy, and only in an
> single "white" undyed state.  It's sold, AFAIK, only by Sig, and even
> dealers that carry Sig free-flight supplies don't always keep it in
> stock, but they should be able to order it for you.

     The three commonly available grades are 00 (double-oh, light), GM (short
for gas model, medium), and SGM (super gas model, heavy). There are other
grades as light as 0000, but they are uncommon as the weight overlaps the
stronger and easier to fill Jap Tissue. GM is pretty strong, and SGM is very
stout indeed - one notch below real silk. It is available from Sig, K&B, and
several other sources. It isn't really a free-flight item - it's primarily
used for C/L stunt models anymore. There are that are more common in
free-flight - Jap tissue for light models, and Polyspan (like silkspan but
made with polyester fiber instead of wood fiber). I have seen color silkspan,
but it's rare. It's very easy to dye with Rit dye, so that's not a big
problem.

    It's used as a covering over open bay structures, and a grain filler and
strengthener for solid balsa. It's lighter to dope on silkspan and fill it
with dope than to try to fill the wood grain directly with sanding sealer and
fillercoat. It also add a tremendous amount of strength to the structure.
Heat-shrink plastic coverings like Monokote add very little rigidity to the
structure. Using silkspan allows much less underlying structure.

    Silkspan is wet and then doped onto the structure. When it drys out and
shrinks, it tightens up and smooths out. The grain then has to be filled with
more dope.

     For open-bay structures in model rocketry (like built-up boost-glider
wings if you're a glutton for punishment) Jap tissue is about as strong as you
need. For larger models, Silkspan or Polyspan is a good choice. For solid
surfaces, silkspan and Jap tissue is quickly being supplanted in other
disciplines by .2 oz/sq yd graphite matte. This is about the same weight as
Jap tissue, maybe a little lighter, but takes much less dope to fill. And it's
incredibly stronger, adding more strength than the much heavier silk. It also
allows very light underlying structure. It's also much more expensive.

      I've only built a few B/G wings and tails using graphite as a surfacing
agent, but I think it's a real improvement. When you can get a 18" C boost
glider with a 1/16" thick wing to go without flutter there must be something
good about it.

     Brett