From: patch@sequent.com (Pat Chewning)
Newsgroups: rec.models.rc
Subject: Re: Foam Cores
Date: 19 Aug 92 19:21:13 GMT
Article-I.D.: sequent.1992Aug19.192113.29240
Organization: Sequent Computer Systems Inc.

In article <1992Aug18.214800.1782@iex.uucp> djones@iex.iex.com writes:
>cores will form the wing for my Jensen Stick. Having never worked with foam
>before I am curious what folks use to cover them. I have considered the
>following options:
>
>1) Completely sheeting them with 1/16" balsa and the using Monokote.
>2) Completely sheeting them with 1/16" balsa and then glassing and
>   painting.
>3) Using leading and trailing edge 1/16" balsa sheeting with cap 
>   strips and then covering with Monokote.
>4) Glassing over the raw cores and then painting.
>
>If I should try glassing (something I have never done before...at least not
>like this) what weight of glass? And do I use epoxy thinned or polyester resin?
>
>What are the techniques that have been tried. Please post or send
>email and I will post a summary.

How you chose to cover the foam cores depends on a lot of factors, some of
which are:
	A) strength -- how big a span, how heavy a load, thickness of
	airfoil, etc.  
	B) will the foam cores be covered in 1/2-span sections and then
	joined together?  Or will the wing be one-piece?
	C) how about aileron/flap linkages and/or embedded servos?
	D) will there be a spar system, ballast tubes, etc?
	E) how about finish of the leading edge? (glue on after sheeting, or
	fiberglass over the radius of the leading edge?)
	F) was the foam core cut out with a specific sheeting thickness
	allowance?  If you cover with thinner or thicker material it will
	affect the resultant airfoil shape.

One thing you SHOULD NOT DO: DO NOT USE POLYESTER RESIN ON FOAM!  It will
melt the foam.

Here are some techniques I have used with foam-core wings:

Materials approx in order of strength:
	  foam with vacuum-bagged fiberglass/carbon-fiber cloth (6oz)
	  foam with 3oz glass and 1/16 balsa
	  foam with 1/64 plywood
	  foam with 1/16 balsa
	  foam with 3oz glass
	  foam with Obechi
	  foam with 1/32 balsa

Attachment methods:
	Special very thin double-stick tape (.002" thick)
	3M 77 spray contact adhesive (works with all of wood coverings)
	epoxy to glue on the wood covering (apply VERY thin)
	epoxy to impregnate the cloth coverings (vacuum bag technique)

Variations:
	4 servos embedded in wing (2 aileron, 2 flap)
	2 servos embedded in wing (2 spoileron)
	1 servo embedding in wing ( aileron)
	no servos in wing (polyhedral)
	no servos in wing (dihedral)
	vacuum-bag 1/2-span sections
	vacuum-bag full-span sections with dihedral
	aluminum spar (tubular, for balast loading also)
	spruce-carbon fiber spars
	balsa spar
	no spar

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.  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.)  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. Monocote.