open combaT has been growing at a fantastic rate in the last
several years. One of the reasons it has been so successful is that the
models can be built very quickly, decreasing the amount of time you
have to invest in the model you fly.
The first goal is to find a competitive design; e.g., not a wallowing
dog in the air. The rest of the goals (in order of most people’s
preference) are that the design be quick to build, cheap, and durable.
Toward those ends, several fliers have been looking at building
their models with inexpensive plastic materials. Corrugated plastic,
polyethylene foam, PVC (polyvinyl chloride) pipe, and HDPE
(high-density polyethylene) have been borrowed from industries,
and applied to kits and used by scratch-builders.
Although most modelers are familiar with balsa, plywood, and
polystyrene foam, many builders are unfamiliar with some of the
new materials in the way they cut and the way they fasten—in short,
how to build with them.
As more modelers try to build with these alternate materials, several
have found them extremely frustrating to work with; I have been one
of those people. So this column, and future columns as time and space
permits, will be dedicated to building tips for alternate materials.
selecting which material to discuss first was easy; my attempt to
make a Coroplast™ wing was a total disaster!
With my model overweight and coming apart at the seams, I
admitted to myself and to others that I needed help. The people at
SPAD (Simple Plastic Airplane Designs) came to my assistance.
Coroplast™, a corrugated polyethylene used extensively in the
sign industry, looks much like corrugated cardboard in its structure,
but is very durable.
For more than a decade, U.S. Core and the Airplane Factory have
recognized the potential for making wings from corrugated plastic.
However, most of those designs were quite heavy when compared to their
balsa or foam counterparts; therefore, they required larger, heavier engines.
More recently, several Combat fliers have spearheaded the
development of Open Combat designs that use lighter versions of
these materials. That allows them to produce competitive models
that easily meet the weight and power requirements of Class B Open
and even Class A Open Combat.
I called SPAD and asked for a crash course in building with
Coroplast™. I was asked to “come on up” and see how they do it.
The first thing I noticed when I stepped into Collin McGinnis’s
workshop was rafters full of earlier Coroplast™ designs. He explained
that the development of SPAD designs was based on a great deal of
trial-and-error, and most of the learning had come from the errors.
SPAD’s earlier designs had wings made from the heavier 4mm
Coroplast™, but more recent designs used the 2mm for the wings
and saved weight by only using the 4mm for the ailerons and the
tail, where greater stiffness was needed.
Another method of weight savings was to use lighter Canadianproduced
PVC gutter pipe for Combat designs.
Collin took the time to build a complete Derelict Class B Combat
Greg Rose, 1312 NW 196th St., Edmond OK 73003
RADIO CONTROL COMBAT
From raw materials to flight-ready in three hours! Collin
McGinnis holds the SPAD Derelict design he constructed.
Small dots of cyanoacrylate (CyA) adhesive are all that is needed
to bond the Derelict’s ailerons.
When flame-treating Coroplast™, be sure to keep the torch in
constant motion and clear the workbench of any flammables!
118 M ODEL AVIATION
May 2001 119
model, and he let me take notes and
photograph each step.
Coroplast™ can generally be purchased
locally at sign companies, and I found
several in my area. Although they only
carried the heavier 4mm material in stock,
each one offered to order the 2mm.
With a four-ounce difference between a
Derelict wing built with 4mm material and
one built with 2mm, it’s important to choose
the proper thickness.
After the wing is cut, it is folded and
glued with Zap-A-Gap.
This is the step that requires the most skill;
Zap-A-Gap, and cyanoacrylates (CyAs) in
general, will not stick to untreated Coroplast™.
Each inch of bonded area has to be flame-treated
for the material to be glued successfully.
it is critical to understand that if you are
planning to glue Coroplast™ with any CyA,
you are going to have to learn to flametreat—
and nothing other than practice,
practice, and more practice will do.
You flame-treat Coroplast™ by running
a propane torch back and forth across the
area to be bonded. As with working with a
MonoKote™ heat gun, you cannot let the
torch set on an area for too long or it will
quickly melt through the plastic.
(I learned that Coroplast™ will burn, but
only if you light it intentionally; even then,
it does not burn rapidly.)
Plan your actions before you light the
torch; make sure the materials behind and at
the sides of your flame-treating area are free
of anything that could be hazardous. For
instance, I removed the bottle of isopropyl
alcohol from my worktable.
As you move the torch back and forth, watch
for a slight dulling of the surface that indicates
that the area has been flame-treated. However,
this dulling occurs just before the plastic melts,
so practice repeatedly on scrap material before
you move onto treating the wing.
While you are practicing, be aware that
4mm and 2mm Coroplast™ behave
differently under the flame treatment, and
that flame-treating across the corrugations
versus flame-treating in the same direction
as the corrugations gives different results.
Treating 4mm across the corrugation is
easiest to work with, and treating 2mm with
the corrugation (this is the way you have to
be able to do it to make the Derelict wing) is
the most difficult to get just right.
The only way to judge if you are flametreating
right is to make test bonds. There
are some tricks to making a good bond with
CyA on corrugated plastic.
When CyA is used on balsa, a large
portion of the adhesive is wicked into the
wood; although it strengthens the general
area, it removes adhesive from the bond
line. There is no wicking action with
corrugated plastic, so builders who are
familiar with gluing balsa tend to put way
too much adhesive on the bond.
To put it in terms any modeler should be
able to understand, all you need to apply is a
small drop—roughly as large around as the
red Sullivan flexible control rods—every
half-inch or so.
More is not better, and a bead of CyA on
Coroplast™ tends to stay wet and useless in
the center of the bond. Two more differences
are that the impervious Coroplast™ limits the
effectiveness of kickers, and the lack of
moisture in the material slows its cure time.
Brush-on, solvented contact cements,
such as DAP Weldwood, can work on
Coroplast™ without flame treatment, but
they have some tricks of their own.
I tried several spray-on contacts and a
water-based material, but I could not get
them to perform. Weldwood did give a good
bond, but definitely requires good
ventilation to use properly.
In addition, the contact cement, which must
be applied to both sides, has to dry completely
before you mate the parts, since the Coroplast™
will not let any remaining solvent out. I prefer
the results I got with flame-treated material.
Although the bond to properly flametreated
Coroplast™ is very good, Collin
recommends that control horns also use
mechanical fasteners (1/2-inch-long #6 selftapping
screws screwed into a scrap piece of
drain-spout PVC) as a backup.
although developing the skills to bond
Coroplast™ does require some time, the
results can be well worth it.
I was impressed by the speed of
assembly for the Derelict that Collin
120 M ODEL AVIATION
built for this column. In slightly less
than three hours, the process went
from measuring out the dimensions on
a sheet of Coroplast™ to being ready
to toss the model into the air for its
first flight!
Admittedly, Collin had some nice
fixtures he had built to help him, and he
was very familiar with the design, but an
average scratch-builder who has
thoroughly practiced flame-treating
Coroplast™ could probably build this
airplane in a weekend.
Equipped with an O.S. Max .20, three
full-size servos, and a large Futaba battery
pack, the 42-inch-span model weighed
two pounds, five ounces.
All of the SPAD designs, including
the fantastic CoroSpit Spitfire designed
for the Scale 2610 RC Combat event,
are available as free downloads from
the company’s Web site:
www.feist.com/~mcmcginnis/SPAD.htm/.
With the success SPAD and other
Combat model designers are having with
Coroplast™, we should see many more
plastic airplanes in Open and Scale
Combat this summer.
That’s all for now. Thanks to Dean
Tuinstra and Collin McGinnis for showing
me how to flame-treat Coroplast™.
Remember to fly safe, fly Combat, and
be sure to check your six! MA
