Conference vmszoo::rc

1139.0. "BUILDING WITH CARDBOARD" by CTD024::TAVARES (John -- Stay low, keep moving) Tue Oct 31 1989 13:02

I'VE BEEN WORKING ON THIS FOR SEVERAL MONTHS NOW, AND ITS GETTING
LONG, SO HERE'S THE FIRST INSTALLMENT.  THE NEXT NOTE IS A
BIBLIOGRAPHY, AND I'LL FINISH THE NOTE UP LATER.  MEANWHILE,
HERE'S SOMETHING TO CHEW ON...

INTRODUCTION

I've been experimenting with corrugated cardboard as a building
material over the past year, and I thought I'd enter some notes
on using this material for those wishing to try it also. 

My first project was a .10 sized version of the famous free
flight model Dakota. This was a popular cabin biplane in the
'50s, and in my humble opinion, one of the most beautiful models
ever designed.  This model was never completed; I made my
mistakes on it, and it turned out too heavy.  Basically what went
wrong was that I tried to make the model out of only cardboard,
and I used conventional building techniques.  It came out
hopelessly heavy, and when I hit the wings I came to a dead stop. 

Rule 1: Forget cardboard for wings.  Chuck Felton does it, but I
think that the technique he uses to fold cardboard around the
leading edge produces a thick, unsophisticated wing.  Use foam
wings. I will, however be experimenting with cardboard as the
bottom covering material on a foam wing.

After licking my wounds and studying further with the material, I
incorporated it into the ill-fated Quick Stick I.  I used
cardboard for the "cabin" area of this model, unfortunately, when
falling into the ground after a mid-air, it fails like anything
else. Actually, the cardboard part held up pretty well
considering the crash.

Next came Son of Quick Stick.  This model was entirely cardboard,
pine, and ply, with the exception of a small piece of balsa under
the tail.  That's right folks, from the firewall (engine is beam
mounted to the pine longerons) to the back of the wing (that's
all the "cabin" it has), the stabilizer and fin, including the
control surfaces: all cardboard.  It held up with reasonable
strenth during the hard landings of a duffer pilot learning to
fly a taildragger, finally becoming beyond repair after a very
hard season. 

My next model was the Cardboard Special, my first design, and my
first full body cardboard model.

WHERE TO USE CARDBOARD

Where should you use cardboard as a building material?  On any
flat surface part, such as any of the typical box (including a
complex, more than 4-sided box) type sport fly planes.  I am
currently designing a Tri-Pacer (the Cardboard Overcast) at 1/5
scale in cardboard.  I anticipate building and flying this plane
with minimal impact on the building schedule of my Cessna 172
project, a conventional model.  Cardboard is that fast and easy. 

And where is cardboard unsuitable?  Though I wouldn't swear to
this, I think it would be more trouble than its worth to build a
complex scale model, such as a Corsair, from cardboard.

Less certain is a pattern ship. This is one of those things I'd
like to see tried; cardboard will have the strength needed, and
it will finish, I think, lighter because it doesn't require the
surface preparation of balsa.

Electric? Forget it, at least until the cardboard industry
cooperates -- more on that later.  Same with gliders, though a
slope ship would be ok.

Small parts, say something smaller than 2 inches square, are best
made out of balsa or plywood.  This is also true for parts that
are frequently handled, such as hatch covers.

I have articles and material from popular magazines, notably RCM,
showing that this material has been experimented with since the
'60s.  At the present time Chuck Felton is the only person
publishing models of cardboard.  Refer to the bibliography of
articles on this material as a part of this note.

THE MATERIAL

Corrugated fiberboard, as it is known in the industry, is the
common material that we see in cartons.  It has been around for
over a hundred years (how's that trivia fans), but has only been
in widespread use since World War 2.

It can come as "single faced", a flat sheet of paper glued to
one side of a sheet of corrugated (wavy) paper, or "double
faced", having flat paper glued to both sides of the corrugated
paper.

Another term used in describing cardboard is "single wall"; this
means that there is only one layer of corrugations in the
material.  You will see a face, a sheet of corrugations, and
another face.

I should also mention that there are "double-wall" and
"triple-wall" forms of this material; if you think of the wall as
the corrugated part, you can visualize these.  Double wall has a
face, then a sheet of corrugations, another face, another
corrugated sheet, and a final face.  Triple wall repeats this one
more time.  DEC uses this material in heavy shipping containers,
but it is worthless for model building (so far!). 

The flat sheet of paper glued to the corrugated paper varies in
thickness according to the bursting resistance of the material.
The corrugated paper is for practical purposes made up of .009
inch thick paper that weighs about 26 pounds per 1000 square
feet.

The most common corrugated fiberboard is of the 200 pound/per
square inch bursting pressure variety.  The facings on this
material are .011 inch thick, nominal.

To compare fiberboard with balsa, which is rated in pounds per
cubic foot: very light balsa weighs between 4 and 7 lbs./cu. ft.,
medium balsa weighs from 7 to 10 lbs./cu. ft, and heavy balsa
weighs upwards of 10 lbs./cu. ft.  In comparison, 200 pound
cardboard weighs between 10 and 11 lbs./cu. ft. But don't go away
yet, there's a good side to this material that I will show you in
this note. 

Also commonly seen are bursting pressures from 175 pounds down to
125 pounds, and lighter fiberboard, down to 65 psi is made.
Unfortunately, these lighter weights are mainly seen in small
cartons -- and are generally unavailable without special order
(read large quanities) from suppliers.  If you look carefully,
you can find the 175 pound stuff in boxes, though it doesn't
offer any significant weight savings over 200 pound.

By now you're probably wondering how you can determine the
bursting pressure of a given piece of cardboard.  Real simple;
its printed on the box!  Most boxes have a circular label,
usually on the bottom, that gives the bursting pressure, and the
loads for which the box is rated.  Simply read the label.

But some boxes, like many DEC boxes, don't have this label.  In
this case you're on your own, and you can assume 200 pound
cardboard if not stated.  Of interest is the box that DEC
software comes in.  This is a nice thin cardboard of about 1/16
inch.  Also check out the box that the wonderful box of goodies
for servicing the LN03 printer comes in.  Great stuff!

Before we leave this discussion on fiberboard as a material, I
should touch on an additional point; flutes.  You need to know
about flutes if you talk cardboard to any packaging engineer or
cardboard salesman.  Flutes are simply the name for the waves in
the corrugated central section of the material.  They come in
sizes of:

Flute          Flutes per foot     Nominal Thickness
A              36                  3/16
B              51                  1/8
C              42                  5/32
E              96                  1/16

These are important when determining the crush resistance of the
cardboard for packing purposes, and are used by us when 

specifying what cardboard you're using or wish to buy. Common 200
pound fiberboard is known as "200 PSI B-Flute" fiberboard.  If
you'd like to take a quick look at this, its what is used for the
ordinary Xerox box.

DEC uses a lot of 275 pound BC flute, but I don't use
the stuff, and have never counted the flutes in that material.

The very last item we should discuss is the material of the
facings.  Facings, as I described earlier, are the proper name
for the flat sheets that are glued to either side of the
corrugated paper center section.  We also know that facings are
what determine the bursting pressure of the material.  In the
lighter bursting pressures, such as 125 pound, this material is
thin enough that the corrugations will show through after we
prepare the fiberboard for use. 

There are two materials used for facings: brown Kraft paper, and
white Kraft paper.  Fiberboard using the white paper puts it on
the intended "outside" surface.

The white facing is primo for us because it takes finishing with
minimal preparation.  Unfortunately, most boxes that use this
white facing are also heavily printed - such as our Xerox box. 

When you first discover fiberboard, you go willy-nilly to every
box, brown or white, and see a model.  After a while, you become
jaded and only want the white faced material.  But you can't
find enough unprinted surface in boxes, so what do you do?  You
buy it from your local friendly paper distributor, remember
reference to the fiberboard salesman above?  I can locally buy
corrugated fiberboard panels for 6.5 cents a square foot (7.5
cents if I want it cut into a box).  That's $2.76 for a 5x8.5
(standard) panel! 

If that doesn't get your interest, think of the last time you
built a big high wing model with a deep cabin area and a long
fuse.  Think of how you bought balsa or plywood, and how much
that cost, and remember how you glued several sheets together to
get the depth, and all the heavy doublers you put behind the
joints to hold it together...All that is gone with fiberboard,
and you get Cheap too.

You can buy the white faced fiberboard at art stores, but they
charge a premium for it; our local packing materials distributor
sells both white and brown for the same price.

So from now on, when we say "cardboard", understand that this
means double faced, single wall, 200 pound, B-flute corrugated
fiberboard, unless explicitly stated.  This is the common form of
the material, and the one we're most interested in. 

This material is nominally 1/8 of an inch thick, plus or minus
1/32 inch.  The deviation is caused mainly by the load placed
into the box before you got it.  It is unfortunately significant
enough that we must deal with it when we build.

PREPARING CARDBOARD FOR BUILDING

Cardboard is conventionally prepared for model building by
spraying or painting on a mixture of 75% paint thinner and 25%
varnish.  The varnish is specified to be urethane or oil base. 
Just slob it on both sides so it soaks into the material.  This
is the classic method, and it is indeed the best way to do it.

I've used a brush and an airless sprayer for this; its a messy
job any way you do it.  I think the brush is easiest, but only
because the airless sprayer is so fussy to work with.  While I'm
writing this I just thought of using a roller or painting pad --
good idea for next time!

However, there is more to this story.  I've experimented with up
to 50% varnish and have found that as the amount of varnish
increases the material becomes stiffer, so much so that at 50%
varnish I believe you could make a firewall of it!

But as you increase the amount of varnish, you decrease the
holding power of glue, particularly at critical joints made
across the corrugations (that is, the end view shows the
zig-zag of the corrugations), where you have minimal exposed area
for the glue to grip.  Joints are the most critical part of
building with cardboard, and this across-the-corrugation joint
represents the major limitation.  The cardboard must be able to
soak up a little of the adhesive to make a strong bond, and too
much varnish prevents this soaking.

We will have to back up these joints with balsa or hardwood
strips when we go to build our model.  And this will also become
a factor when we go to repair our model -- the soaked-in glue
makes the second joint much weaker.

On the other end of the scale, as you decrease the varnish below
25% it does almost nothing to the material.  What you want is
something a little more rigid than the basic material, so it will
cut easily, yet stiff enough to not tear.  I believe the optimum
ratio is a little less than 30% varnish.

One last point.  I figured that since varnish is good,  "liquid
plastic", such as Flecto Varathane must be better: wrong.  The
Varathane does not soak into the cardboard, and you wind up with
a hard facing surface, but a soft corrugated layer.  This is very
wicked when you try to make a straight cut without tearing the
material.  

Ray Vandewalker, "PINATA", RCM April 1969 -- Construction of a
cardboard Formula II Racer.  Contains interesting comment that
cardboard is strongest parallel to the flutes, not across them.

J W Headly, "Try Cardboard", RCM November 1972 -- A slope racer
resembling the ME262 Komet.  Uses single-wall cardboard which he
measures at 1 oz/sq ft versus 1.5 oz/sq ft for double wall.

Don Dewey, "Cardboard", RCM March 1976 -- This is in my opinion,
the definitive article on cardboard as a construction material.

Chuck Felton, "1929 Heath Parasol", RCM April 1976 -- Designed by
Bob Miller, this project follows the March article by Don Dewey
to illustrate an application of this material.

Chuck Felton, "Ki-61 Hein", Model Aviation September 1986

Chuck Felton, "Super Cruiser", Flying Models May 1989

Chuck Felton, "Cardboard P-51B", Model Aviation September 1989