Editor's note
This article took a
roundabout path on its way here. It was originally written in English and
first appeared in MHR #45, Winter 1991. Later on it was translated into
German and appeared in Bumerang Welt issue III/92. The German version was the
one I first encountered, but reading it was difficult without a grasp of the
language. I resorted to one of the online translators, but could only guess
at the meaning of several parts.
In search of a better copy, I found out about the MHR version, but had
gleaned enough from reading the German version to see that it had been
abridged. I contacted Jonas to see if he had a copy of the original, but none
was to be found. Several dead ends were followed and it began to seem that there
was no hope.
Finally, with the help of one of his colleagues, Jonas was able to recover a
copy of it from the hard drive of his old 386 machine, which had been struck
by lightning! Jonas included these comments on the article:
"When reading the article,
please keep in mind it was written 10 years ago using the knowledge and
experience I had acquired at the time. Most of it is correct, but does not
necessarily reflect the way I would do things today. Nor does it accurately
describe the details of my MTA production methods developed through making
another 350-odd MTAs after the article was written."
So, it has been a long, strange trip, but here's the
unabridged version in English. Now I know that when the
online translation of the German version said, "Additionally MTAs lean
to it, in the case of the feed or with longer storage too forgiven," the
original said, "MTAs also tend to detune during transport or prolonged
storage". Oh, well, I guess that should have been obvious!
While the article is over a decade old, it's a classic. Those getting
started making their own composite MTAs can save a lot of trail-and-error.
Plus, we can always hope for a sequel one day. Right, Jonas?
:-)
-- Carl
Space age boomerangs - fiber composite MTAs
Jonas Romblad, 1991
What is wrong with plywood?
Well, nothing basically, but I got tired of MTAs that do
not stay in tune. The problem with wooden boomerangs is they aren't stable,
and what do we expect, wood is a "living" material. Birch, for
instance, shrinks 14% as it goes from green to "dry"; wood. This
problem became very evident to me during my trip to the US -89. Sweden had,
by the time I left about 20� C and rather dry air. I got on the more moist
and certainly cooler air-conditioned airplane and ended up in a (to a Swede)
sauna-like New York. The temperature was at least 5�-10� hotter than I was
used to and the air was so humid I was humming "Singin' in the
rain" and my wooden MTAs flew in the most crazy fashion. No, seriously.
I really do have problems with the weather affecting my wooden MTAs, not only
on extreme occasions like the one mentioned. MTAs also tend to detune during
transport or prolonged storage. Am I the only one who has taken out my
favorite MTA after the winter, just to find out it doesn't fly at all like
expected?
So, I started to think about making MTAs from some sort of
"dead" material. Since a lot of people I have talked to mean an MTA
must be tuned each throwing session to fit the weather conditions I figured I
had to make a set of good booms, five or so, each one with its special
"window". This way I would be able to go to a tournament, look at
the winds and pick a boom with just the required tune. Sounds great, doesn't
it?
Well, first I had to find a substitute material for the birch plywood. As
wood is strong and light relative to most other materials, I soon figured I
had to use a combination of several materials to match the Baltic birch. If
one looks close enough at a piece of wood, one sees that it consists of a lot
of fibers with some sort of matrix material to keep them together. This is a
very good idea, using a combination of light and strong materials to get the
benefits of both.
Strength
The boomerang has to withstand both twisting and bending
forces. This must be taken in account during the choice of building methods.
The torsion strength is too easily forgotten
First, let's make a list of
materials that are strong, and that we can use without too much trouble (this
rules out metals, most ceramics and a lot of other goodies):
- Fiberglass
- Carbon fiber
- Kevlar (aramid)
All these materials are heavy compared to wood, but if
the boom is constructed with a shell of some of these materials and a light
core, it is possible to get a boom much stiffer and not heavier than a wooden
one.
Fiber materials are strong in one direction only - pull. To get
sufficient torsion strength some of the fibers have to be oriented 45 degrees
to the arm, while fibers must run along the arm to make it resistant to
bending.
Low weight
The only reason we have a core is to keep the shells from
separating or being pressed together. In other words it doesn't have to be
that strong. Styrofoam or balsawood would probably do the job, but they proved
hard to use when molding MTAs the way I do.
Another very light material is
called microballoons. Microballoons are tiny, gas filled bubbles of glass
(white) or phenolic plastic (brown) that looks like a fine powder with a
density that makes you think the jar is empty (open carefully!).
Matrix material
Now we have a piece of Kevlar cloth, a length of carbon
fiber tow and some white powder on the workbench. How to make these materials
stay together? We need a matrix material.
I have tried two different ones;
epoxy and polyester resin. The epoxy is the one to use unless many booms are
to be made during a short time. The polyester is a little more brittle, the
smell is strong and dangerous and it is harder to mix to the right ratios.
The epoxy must be a slow (12-24 hour) setting type suitable for laminating
high strength fiber materials. Watch out for temperature-sensitive,
rubber-like types often used for surfboards. Surfboards must not get cracks
if they hit a rock, but I have yet to make an MTA too brittle. Impact
resistance is simply not an issue for MTAs.
To mold, or not to mold
I have made composite boomerangs both freehanded and in
molds and I can assure that making a good mold is really worth the labor, but
actually it all depends on what materials you want to use for the boom.
No-mold buildup
Simply make a core of a material of your own choice
(balsa, foam etc.) and put on the strong stuff. Just putting epoxy and fibers
directly on a core usually gets heavy as the core absorbs the resin and it's
hard work to get a smooth finish. Vacuum bagging has been used to produce
high quality MTAs.
Making a mold
This is not all that hard. All I did for my first mold was
to put a wooden master on a spherically tuned surface of paxolin (any
material that can be polished and does not absorb the resin will do) and
cover it with a 1 cm thick layer of Plastic Padding (polyester putty) and
then add a piece of plywood on the top to stop the Plastic Padding from
cracking. The boom and the bottom half of the mold have to be waxed and
coated with a layer of PVA release agent to make the halves come apart after
they have hardened. The Plastic Padding mold may be good for 10-30 booms
depending on the airfoil and how it is used.
A better and much more durable
mold can be made from epoxy and fiberglass. I started by measuring my best Kevlar
MTA in detail and making supports on a wooden base to fit the bottom of this
boom. Then I made a master from soft 2 mm aluminum and polished the top
surface, but pick any material you like that can be given a good surface
finish. Put at least 3 coats of mold-release wax on the master!
A thin (about
0.1 mm) sheet of plastic was glued to the bottom of the master and thin pieces
of wood were glued to the plastic, both about 1-2 cm oversize all around the
edges. A good idea is to make some sort of edges, so the resin does not drip
over the edge. The master with its "collar" is now glued to the
wooden base and thereby is firmly supported in the right tune.
Put a layer of
PVA mold release on the boom and in the "collar", let dry and apply
two thick coats of a good mold gel coat. After the gel coat the tedious work
of building up the strength of the mold begins. I chose to build up a 1 cm
thick layer of cheap fiberglass and epoxy, but I know some people make model
airscrew molds by just building up 2-3 cm epoxy/microballoon backing and
that's it. To ensure my mold wouldn't warp, a boom-shaped stiffener made from
3 cm square steel tubing was added with the last layers of epoxy/fiber.
Now
the mold was turned over, the wood, plastic and boomerang was taken out of
the mold and, after fixing some imperfections, the top half of the mold was
waxed several times.
Two layers of self-adhesive plastic were added to the
bottom of the master which was then, using the PVA mold release as glue, put
back in the top half of the mold. Put on PVA, add masking-tape edges and
repeat the building process once again. Let cure for 24 hours or more.
Do not
separate the two halves at this stage! You should first drill holes in the
stiffeners for the alignment pins.
Now open the mold, take the master out and
admire your work! The self-adhesive plastic will have made a 0.1-0.2 mm deep,
boomerang shaped, cavity in the bottom half that will keep the fibers from
sliding out of position while the mold is pressed together.
Using the mold
Usually when I make an MTA using my mold I start by waxing
the halves and painting them with PVA release agent which is left to dry. A
new mold should be waxed at least 3 times before first use.
Then I apply
epoxy to the mold using a brush and put down the fibers. The idea is to use
just enough epoxy to wet the fibers, but not more.
When the shells have cured
I mix micro-balloons and epoxy until it resembles whipped cream, fill the
halves and put them together. After the boom has cured, it is taken out of
the mold and the leading and trailing edges are trimmed and sanded (wet
sanding, beware of health hazards!).
Another way is to be careful with the
amount of epoxy while wetting the fibers and then let the shells cure about 3
hours, or 'till the epoxy is like thick syrup and then do the filling. This
way is the fastest and easiest way and the booms resemble the mold very well.
Tuning
[Jonas notes that heat tuning was only used on his very first prototypes. He has
had many questions from people over the years who thought that all his MTAs were
heat tuned. In fact, every MTA ever sold went straight from the mold to flight
testing and shipping.]
Most people look very funny when I tell them I tune my
plastic MTAs using hammers, nails and a heat gun. It's not as brutal as it
sounds.
To my surprise I found both the epoxy and polyester to be tunable
using a heat gun to heat the boom to about 60-80 C (a little too hot to hold)
while the boom is held close to the desired shape. Only heat the
"pull" side (top if reducing dihedral) or the shell might separate
from the core.
The hammers and nails are used to hold the boom in the right
shape. To add dihedral, place the boom upside down on a flat surface and hold
down the tip and elbow with weights (the hammers), put two big nails under
the arm and slide them toward the tip and the elbow until the desired dihedral
is achieved. Now heat the arm, applying most heat to the middle of the arm
and then gradually reduce it toward the tip and elbow. Do not place a single
rest under the middle of the arm, or you will end up with a spectacular,
V-shaped dihedral.
And?
Well, how did it work out? Can I mass produce record
breaking boomerangs, or did I waste a whole lot of time, but not so much
money on total failure? In my opinion it worked out pretty well. The booms
from the Plastic Padding mold came out pretty warped, but the new, fiberglass
one gives me some nice booms flying at least 30 seconds, clearly better than
my wooden MTAs; and these stay tuned!
This molding method doesn't lend itself
to any big production lines as the process of preparing fibers, molding,
curing and afterwork takes a long time. The maximum production should be one
boom a day.
In short, I think I have got what I was looking for; a
weather-resistant MTA, stronger than plywood booms, that stays in (or out of)
tune until retuned. I have also had a lot of fun experimenting with new
materials and techniques.