From: ntaib@silver.ucs.indiana.edu (Iskandar Taib)
Subject: Foam wing building hints n' tips
Organization: Indiana University
Date: Wed, 19 Aug 1992 20:20:47 GMT

Buzz McDermott asked me to write a little missive about building 
foam wings, since there has been a fair amount of interest in the
techniques used for the building of the Astro Blaster and Phoenix
R/C Rocket Gliders.

What follows is a series of dos and don'ts about building foam 
wings, as well as some general overviews and tips on foam wing 
construction. 

I'd like to point out right away that most of my experience is 
with Control-line Combat aircraft, the wings of which are usually 
left unsheeted and simply covered in a shrink wrapped plastic. 
Techniques for sheeting foam wings are easily found in model air-
plane magazines, however, and are readily described.

The first thing to know is that certain paints and glues dissolve
foam. Both the stuff made out of white beads (referred to as "bead-
board") and the blue (Dow Styrofoam (tm) ) or pink (DuPont Foamular)
extruded foam will behave in the same way. Once sheeted a foam wing 
can sometimes be finished in a paint that ordinarily dissolves foam 
if one is careful about not putting too much on at a time. Here is 
a list of what will dissolve styrofoam and what won't:

	Will dissolve foam:

		Nitrate and butyrate dope
		Ambroid
		"Model Airplane Cement" (you know what I mean)
		Polyester resin (sold as "fiberglass resin" at K-Mart)
		Thick and thin cyanoacrylates (excepting UFO)
		Paints from spraycans
		Dope and paint thinners
		Gasoline
		Dope thinner, acetone
		Solvent-based contact cements

	Won't dissolve foam:

		Polyurethane paints and varnishes (inc. Rustoleum)
		White or aliphatic glues (Elmer's, Titebond)
		Epoxies
		Ethanol or methanol (sometimes used to thin epoxies)
		UFO superglues
		Water-based contact cements (eg. Southern Sorghum)

Follow the instructions provided and you won't go wrong. Most struc-
tural building is done with white glue and epoxy is used for sheeting
the wing and/or putting down fiberglass, graphite or kevlar cloth.


Sheeting the wings:

The Phoenix kit requires that you sheet the wing with balsa using epoxy
as the glue. Aerotech also recommends that you vacuum-bag the wing for 
the lightest wings possible. Vacuum bagging is a fairly new technique 
that I will describe later. 

The process involves preparing the wing skins, mixing the epoxy (need-
less to say, the 24 hour laminating variety, spreading it on the skins 
with a squeegee, scraping most of it off, applying the skins to the 
core, then assembling everything together in the core beds (the pieces 
left over after the core is cut), and putting lots of weight on top 
of the whole thing. Oh yeah.. the wing has to be kept straight so 
you'd have to do this on a very flat surface. The more pressure you 
can put on this, the better glue joint you'll have, and the less glue 
you'll have to use, which makes for a lighter wing. 

This is where the vacuum bagging comes in. The core bed/sheeting/core
assembly is put into a large bag which is sealed on all sides. Then the 
air is pumped out of the bag. This is supposedly the equivalent of pi-
ling hundreds of pounds of weights on the core. In fact they tell you 
to limit the vacuum to so many inches Hg otherwise the cores will crush.

Vacuum bagging is also useful if you are going to lay up fiberglass 
on top of the balsa wing skins. Fiberglass cloth is now available in 
very light weights and people often use it in lieu of a covering film
or fabric. 

The way it used to be done was that the cloth was laid down and a thin-
ned (with alcohol) epoxy brushed into it. Then excess epoxy was removed
using rolls of toilet paper (discarding layers as they became saturated). 

With vacuum bagging one lays down a sheet of drafting mylar on top of 
the wet glass cloth, then puts the assembly in core beds. The assembly 
is then vacuum-bagged. After curing the mylar sheets are removed and 
you end up with a glass-like finish that is extremely light since all 
excess epoxy has been squeezed out. This also obviates the need for 
lots of the filling and sanding usually necessary before painting.

There was a kit review for the Phoenix in the July 92 issue of Model 
Builder magazine. They mentioned an unusual aspect about the construc-
tion - the Phoenix uses fiberglass tape under the sheeting at the 
trailing edge. This allows one to sand the trailing edge down to a 
razor sharpness.

The Astro Blaster kit (September 92 Model Builder) uses contact 
cement for sheeting the wings. The cement is of the water based 
variety. Itis applied to both skin and core and is allowed to dry. 
After this has occured, the skins and core can then be brought 
together. This is a little trickier, since you don't get a second 
chance.. Once the core touches the skin you can't separate them 
without breaking something. The skins are just 1/32" thick so one 
has to be gentle with them. 

Other skin materials are sometimes used. Dense cardboards (eg. Sig's
Kromcote, Tiger Skins (tm), etc) are often used for cheap, "disposable"
wings. Some ducted fan fliers have been using pre-cured fiberglass 
sheet. Hardwood veneers (usually obechi) are also often used, as is
1/64" aircraft plywood.

There is also a double sided tape that can be used for attaching skins.


Miscellaneous topics:

Repairing foam is fairly easy. One simply hacks out the damaged piece, 
glues in a block of foam and carves and sands to shape. Carving is best 
done with a brand new utility knife (the kind that has break-off points)
and sanding can be done with a sanding block. Sheeting is replaced in 
the same manner - cut out the damaged piece and glue on a repplacement. 
A little glass cloth or carbon fiber matte over the break helps too. 

Foam is interesting stuff to play with. You can cut wing cores using a
hot wire and 1/16" ply or formica templates. Parts for rockets can be 
made by simple carving and sanding. 

Even more interesting is making lightweight wings and other parts using
foam, silkspan and thinned white glue. Someone called Ron St. Jean built
lots of competition free flight models in this manner. The silkspan is 
applied wet over the foam, and thinned white glue is brushed on. When 
the silkspan dries it shrinks, and the result is an incredibly strong and
stiff structures. One could conceivably use this method for nose cones
or complex scale models. In England, foam and brown wrapping paper is 
used for complex ducted fan models (someone actually flies a seven foot 
long scale Concorde constructed like this). 

If one uses heavier paper (eg. grocery sacks) perhaps one can dissolve 
the foam once the white glue is set (use acetone or dope thinner for 
this). There was an article in Model Aviation where someone was laying 
up Gee Bee cowls (maybe a foot in diameter) on a mould. The result was 
a stiff, very smooth part. For rockets imagine something shaped like a 
V2 made like this. Once the foam was dissolved you'd end up with a light-
weight craft paper tube of the proper shape, boat tail and all.

I'll be posting a list of suppliers once I have a chance to peruse the
magazines. Most model airplane magazines have ads for vacuum bagging 
supplies.



-- 
-------------------------------------------------------------------------------
Iskandar Taib                          | The only thing worse than Peach ala
Internet: NTAIB@SILVER.UCS.INDIANA.EDU |    Frog is Frog ala Peach
Bitnet:   NTAIB@IUBACS                 !