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I have been meaning
to write an article on building an EZB for some time now,
but between
circumstances beyond my control and one of my cats eating the parts for
the last model I was going to make, have intervened. However, time has
passed and a request from a couple of members has prompted me to return
to the project. Once again, this is how I do it and it
works for me (most of the time anyway).
Big Hands
If you have never
made one of these models before, forget the old thoughts of “I can’t do
these models because the wood sizes are too small” etc, what a load of
old rubbish, I have size 10 hands and have little trouble, my trouble
comes mainly from the cats pocking
their noses and paws into things?. By following some of these tips I
have picked up over the years, maybe construction might become easier
and maybe some of you more experienced builders may pick up a new idea
or even have some feedback suggestions. So where do we start?
Tools and
Materials
If
this is your first model then a choice of one of the plans in my pack-up
is a good start. It pays to use Indoor quality wood, as
this is infinitely better than the stuff used on R/C models, but at a
price, compare the size against a 3in sheet. On the up side, you don’t
use much wood and many models can be built from a few sheets. Tools
needed are limited, a model knife with a supply of new blad es,
good 6” and 12” steel rule, tooth picks and/or glue
applicator and an additional light source, preferably with a
magnifier (this is good for everyday use as well) A vernier
calliper and or a micrometer may be useful. I only use Wet
& Dry paper for sanding these days; it gives a far superior finish
and use only balsa cement or PVA for construction, suitably thinned down
with
acetone or water respectively. Any joint made with balsa cement can be
loosened by applying acetone with a brush and repositioned; as the
solvent evaporates, the joint is remade? Avoid using super glues, they
may be fast, but they impregnate the wood fibres and make everything
brittle and sometimes difficult to repair.
Jigs and
Templates
Without a Jig, you
will not be able to build these models, wings, tail and the propeller
all need a jig
 or
template to build on. There are several ways to do these and I have had
success from them all. I use materials that are readily or easily
available; I have used cardboard (cornflake box, backboard of note-pad
etc) balsa, ply, acrylic, plasticard, laminates and depron. The fin and
tail plane will be built flat around a card template with notches cut
where any ribs if any are to be positioned. The idea of the notches is
to prevent the ribs from both fouling and gluing to the template. The
templates are given a protective coat of wax, by rubbing a candle around
the edges for the same reason. If the outline is curved or round, t hen
the strip wood is soaked in hot water for 10/15 minutes and anchored to
the template with a balsa block and pin (do not stick pins in any of the
structure). Pull some tension in the strip and wrap it around the
template and secure with another balsa block and pin, leave to dry
before gluing any pieces to it.
The wing jig can be
just a flat piece of card with rib
 notches
cut out and the wing built flat as per the tail. This time though, the
L/E and T/E spars are overlapped at the centre rib by about
10mm, so after covering, any dihedral and wash in/out can be set and a
neat scarf joint can be made at the centre before fitting the wing
posts. Alternatively, the more elaborate built up jig as shown on the
plans can be built. Time spent here can speed up and simplify wing
construction and also ensure proper orientation of the wing posts. A
template for the wing and tail ribs is cut to the required arc% and used
to make sliced ribs of the appropriate thickness.
Marking and
Cutting the Wood
Whether you use the
Vernier calliper, Micrometre or eyeball and steel rule to measure the
components, how to make a mark that is not wider than the piece you are
going to cut? To use a pencil, even a propelling pencil with a fine
0.5mm or less lead, ball pen or fine felt marker is out of
 the
question, all these give too thick a mark to be anywhere near accurate.
I use a scalpel blade and large felt tip marker together with a 6” steel
rule and the magnifier/lamp. With the wood positioned on the cutting
mat under the lamp (lots of light is good) set the steel rule with the
appropriate graduations at the woods edge and wipe the scalpel blade
through the tip of the felt marker and align to the dimension and stroke
the wood surface which leaves a very fine mark.
Align the steel rule
between the marks and make a light cut with the scalpel, several cuts
may be needed to separate the pieces, even on very thin sections of
wood. Cut wing and tail ribs in the same way, by moving the rib
template down the marks and slicing off the ribs. The main difference
between cutting out bits for an R/C model and an indoor model is there
is little in the way of tolerance. If the component does not fit, then
it does not fit, you cannot use any filler or thick glues to bridge
gaps, glue is the heavy stuff here. In addition, because of the thin
sections of wood used, a sharp blade is essential all the time, you must
cut the wood and not crush it, I get through several blades on every
model I make, but at least the joints fit and the minimum of that heavy
glue is required. The discarded blades are still good for R/C modelling
though!
Getting it
together
When using a flat
card template for the tail or wing, use scrap pieces of balsa to block
the spars
 etc
against the template and secure with pins. If using a wing jig, secure
the spars with tissue plasters, these are made from clear sticky tape
with a tissue centre strip a nd
then cut into 3/8”plasters. The tissue part safely holds the spar, wing
post or whatever securely in place dry and the sticky bits secure the
component to the jig. Cement the ribs in place to the front
spar with a small blob of balsa cement and allow the excess rib length
to
 overhang
the rear spar, allow to dry. If the design uses warren type ribs, then
bevel the rib to mate with the front spar and then as above. When all
has dried, using a fresh knife blade, (remember, we cut the wood, not
snap or crush it) trim the rear of the rib to fit against the rear spar
and secure with a small blob of cement. Wing and / or tail finished
ready to cover.
Motor stick
Wood for the motor
stick needs to be carefully selected; it must be straight, light and
rigid. About 5lb/cu.ft 1/8in quarter grain balsa seems best here to
counter the bending forces of an attached fully wound motor. Any
sideways bend
 gives
a lot of side thrust, either left or right and will pull the model out
of circuit trim for up to a minute until the motor torque winds off.
This usually shows as right side thrust and the model, trimmed for a
left/left circuit will exhibit a pronounced straight-line sideslip, then
gradually come on trim. Bending like a banana is the other problem;
this gives down thrust and kills the initial climb-out. Unfortunately,
both these distortions seem to apply themselves at the same time and
this affects the alignment of the wing posts, which introduces warping
of the wings and gives unwanted wingtip washin and washout, generally
resulting in aerobatics or a bunt. So beware; the motor stick can make
or break a successful model.
Tail boom
Because of the small
cross section of this component, this also needs to be light but fairly
rigid and made from straight-grained balsa. Apart from supporting the
tail-plane, the tail boom acts as a torsion bar allowing the tail-plane
to vary its tilt angle in bumpy conditions and help keep the model on
track. The tail-boom is usually attached to the motor stick with a bit
of up-sweep by a small blob of balsa cement and reinforced with a
rap-around piece of tissue soaked in cement which also supports the rear
motor hook.
Propeller
This is cut as per
the plan template, or use your own favourite if you already
 have
one. Use the template and cut the blanks from 10, 12 or 15 thou thick
4-5lb cu/ft quarter grain balsa. These are then soaked in hot water for
about 15 minutes and then positioned on a 3.5in diameter cylindrical
form at around a 15 degrees tilt and bound in place with strip rubber,
thin bandage or similar. The whole lot is then popped into the oven and
baked for 15 minutes
at gas mark 3 and then allowed to cool outside the oven. A jig will be
needed to fix the blades to the prop-spar to ensure that both blades
have identical pitch. The jig also ensures that the prop-shaft is
aligned vertically and true to the prop-spar. Suitable jigs are shown
on the plans in my plan pack.
Making the
prop spar and wing posts
These are fun! They
are made from 1/16 sq and rounded off. Wing posts first have the four
corners sanded flat to make a hexagonal shape and then again to make a
bi-hexagonal. Use wet and dry paper 400 grit on a sanding block, mine
are 6mm MDF of varying widths, 10, 15, 20, 30, 40 and 50mm attached with
vinyl double sided tape. Do not use the paper type tape, as this is the
devil to get off when you change the wet and dry, vinyl pulls away
cleanly. Then take a piece of wet and dry paper folded between finger
and thumb and lightly clamp the wing post whilst twisting
and pulling it through until all the knobbly bits feel
smooth. Never push the balsa; it will always snap. The result is a
near perfect round stick, just cut to length and fix to the wing spars.
The prop spar is
made in the same way; but this time the spar is cut to length first and
the centre is marked all round and also a centre line along all four
sides. This is done chippie style, holding a fine tip marker pen and
using the fingers as a marking gauge lightly draw from end to end.
These lines are used as a guide because the spar is now sanded to a
taper from the centre of the spar to the tip, making it 1/16th
sq – 1/32nd sq both ends. Then the same as above to make a
round tapered spar. This is easier to do than it sounds and the spar
and wing posts are remarkably strong.
Paper tubes
Paper tubes for the
fixing of the wing etc are dead easy to make and can be used on all
sorts of models. For EZBs, use a 1/16th drill or 16g piano
wire and rub it with a candle to help stop the balsa cement holding.
Cut a piece of Jap tissue wide enough to make the wing tubes and
a little more for trimming and by about 20mm and wrap it around the
drill/piano wire form and at the same time impregnating the tissue with
cement. When done, twist the form whilst squeegeeing out excess cement
between finger and thumb, then slide the tube off the form and leave to
cure overnight. By pushing a piece of wire or a drill back into the
tube, you can run a knife blade across it to cleanly cut off the length
required.
Covering
With this sort of
model, you must use either Condenser paper or a plastic
film such as Ultra Film, all available from the likes of Sams
Models or Flite Hook. These coverings a re
very light and durable provided you don’t have a tear in them,
particularly the Ultra Film. The method of covering the wing and
tail plane is the same for both types of covering, but the preparation
is different.
Condenser paper
comes all nice and smooth but needs to be pre-shrunk before fixing to
avoid airframe distortion. The way to do this is to sandwich the
Condenser paper between a sheet of modelspan tissue or the like and then
steam iron it. The result is a rather crinkled finish, but this works
well on the model, a sort of inbuilt turbulation system.
Ultra Film;
like all plastic films holds a lot of static and this needs to be
disposed off if we are to have any control over the material. To do
this, first roll off sufficient film onto the cutting board and trim off
with a steel rule and sharp blade, then gather the film into a ball and
scrunch it up. It is then easy to lay out on the board by blowing on
its surface and teasing with the fingers, surface tension then holds all
secure?
3M photo mount or
the like is used to adhere the covering to the airframe, but you need to
be careful. Make sure the room is well ventilated and you have lots of
old newspaper spread over the bench or floor; and it’s a good idea to
wear a facemask. Lay the wing, tail and rudder on the newspaper and
lightly spray the photo mount in the air over the frames (do not spray
directly on the structure, this is heavy stuff) and allow the particles
to settle on the components. Lift the wing and lay the T/E on the
covering material and lightly press down to grab (lightly I said) then
follow through by rolling the rest of the frame over to the L/E making
sure that all the ribs are also stuck to the covering. Trim off the
excess covering using a new blade and keep changing the blade as soon as
it feels like it not cutting, Condenser paper blunts blades quite fast.
On the Ultra Film; a soldering iron can be used to burn the excess away,
but make sure to trim only on the lower edges of the wing spars and
ribs; it is all too easy to burn a hole in the top covering if you have
a twitch?
The wing centre rib
will need to have some photo mount painted on over the covering applied
to the first wing half if you built on a wing jig so that the second
half can be secured to the centre rib. This is applied either with a
tooth pick or a small brush, the brush can be cleaned with lighter fuel
or cellulose thinner etc.
If you opted for the
flat card type jig, then the whole wing will be covered in one go. Once
covered and excess trimmed off, the wing can be jigged up with scrap
balsa or card etc to the required dihedral and any wash-in or wash-out
set. The wing spars are then cut with that new blade for a perfect
scarf joint and secured with a blob of cement. When all has dried, fix
the wing posts paying close attention to their alignment; if they are
out of true, the wing will take on an unwanted warp.
Tying motors
There are many ways
that people go about doing this; some are a bit complicated, some are
fairly simple, some lash with thread and some use super-glue. Me; I
have used many ways over the years, but for the last decade and a bit I
have used the following method with no knot failures that I can recall?
I take the length of rubber for the motor loop with about 3in extra
(something to hold on to) and double it over; the two ends are salivered
up by putting them in your mouth, this acts as lubricant to stop the
edges fraying and causing a breakage. Holing the two ends together, tie
an ordinary granny knot to give the correct loop length, lube up
again and tie the two loose ends together with another granny knot
to lock the first one. Dead simple and totally foolproof, trim off the
excess rubber close to the knot, the saliva evaporates; now use proper
rubber lube for a long life motor.
I hope that this
article will be of some use to anyone thinking of building an EZB model;
the durations attainable are very satisfying, especially when you break
the 10 minute mark. My plan pack is available to anyone wanting a copy;
just send me your mailing address. Happy winding.
Phil Morgan
(Indoor Comp Sec)
Email: rafmaa-indoor@rafmaa.co.uk
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