From: svoboda@rtsg.mot.com (David Svoboda)
Subject: Re: Foam Cores
Organization: Motorola Inc., Cellular Infrastructure Group
Date: Sat, 22 Aug 1992 00:56:54 GMT

In article <1992Aug19.192113.29240@sequent.com> patch@sequent.com (Pat Chewning) writes:
|
|One thing you SHOULD NOT DO: DO NOT USE POLYESTER RESIN ON FOAM!  It will
|melt the foam.

Uh, yeah.  If you are very conscientious about filling all gaps thouroughly, 
though, you can get away with polyester-fiberglass  over BALSA over foam.  That 
is my preferred method of covering foam wings.

|Attachment methods:
|	Special very thin double-stick tape (.002" thick)
|	3M 77 spray contact adhesive (works with all of wood coverings)

You can use any kind of contact adhesive that doesn't attack foam.  Sig
sells a water-based contact glue called Core Bond specifically for the
purpose.

|If I assume the following:
|	The model in question has a thick airfoil.
|	You are new to foam-core construction.
|	You do not have a vacuum-bag setup.
|	The model has a center-of-wing mounted aileron servo.
|   Then I would suggest the following:
|	Sheeting with 1/16 balsa applied using a thin (very thin)
|	application of epoxy.  Do 1/2-span at a time.  

A good method of doing this is to use an old credit card to squeegee the
epoxy onto the foam.  Remember, epoxy does not get lighter as it cures, so
every ounce you leave on the foam is an ounce added to the weight of the
airplane.

Wing Mfg ships foam cores in the cutting blank, so there is a very good
method of insuring alignment during sheeting.  Before you begin sheeting,
use a marking pen to VERY carefully draw a line down the center of the
wingtips and roots, leading edge to trailing edge.  Now, find a perfectly
flat table to do the work on (the flatter the table, the straighter your
wing will be).  Place your foam cores (in the blanks) on the flat table,
and make sure the centerlines that you drew are parallel to the table
surface.  If not, you can sand one side of the blank, or use some strips
of masking tape on the bottom of the stack to shim up a corner.  Get this
VERY precise.

Cut your favorite sheeting material to size (I like 1/16" balsa, edge 
glued together), leaving about 1/4" around the whole outside edge that 
you'll cut off later.  Now, spread the epoxy VERY thinly on both sides 
of each wing surface.  Place the bottom foam cutout blank on the table.  
Put several sheets of waxed paper in the blank.  Put one of the peices 
of balsa sheeting on the waxed paper.  Now, take the foam core, oriented 
properly, and place it gently on the stack.  Take another balsa sheet and 
place it on the gooey foam core.  Now, another several sheets of waxed 
paper, then the center blank, and keep on stacking things in order until 
you have a full sandwiched stack.  Now, stack many pounds of books (hardback 
preferred, classics not necessary :-) on the foam core stack.  Re-check
the alignment of the lines that you drew on the cores.  You have a while
yet that you can adjust things if you find anything amiss.

Just let that stack sit overnight, and in the morning,

|Glue on 1/4 X 1/4
|	leading edge (spruce or balsa).  Plane and sand to shape.
|	Cut, shape, hinge ailerons.
|	Join sections together at dihedral angle (use 1/8" ply joiners at wing
|	bolt-down areas, avoid servo mounting location.)  

You can also use those lines you drew on the cores here, with a little
ingenuity.

|Cut out servo
|	mounting area on underside of wing (topside for low-wing).
|	Fiberglass the center section using epoxy and 2 layers of 3oz glass
|	cloth.  Sand. 

And fill any gaps with DAP, or your favorite filler.  Filling and sanding
is the secret to a good looking airplane.  Sand the crap out of it, using
as fine a grit as you can stand.

|Monocote.

Or, just fiberglass the whole shebang, and paint it with your favorite
paint.


	Dave Svoboda, Palatine, IL
Newsgroups: rec.models.rc