A Construction article,
but why?
Don't most people these days buy ARF models?
And that is fine if you only want the standard
type models, but if you would like something a
little different then the only way to get that
is either to build it yourself or pay to have it
built. Even if you only fly ARF models, you will
at some time have repairs or upgrades to carry
out on these models and if you do not have any
experience with building models you will have to
learn on the run or get assistance from someone
who knows how to fix your problem. But if you have
at least built one model "Off the Plan"
then you will have some idea of what is needed.
Off the
plan building is also very satisfying because
you start with a pile of wood and you sculpt it
into your latest plane that you can be proud
of
and want to show your skills to the other members
of your club. Anyway back to this construction
job of the Pushy Cat. It should go without mention
that when you receive a plan you should not start
building until you have studied the plan thoroughly
and worked out your own plan of attack. After
studying the plan carefully I decided to start
by building
the hardest part first, the wing. Before starting
to build I usually kit the plan. This means that
I cut out all the parts that I can before I start
assembly because this ensures that I have enough
balsa and ply to complete the main construction
and also makes the actual building process proceed
more smoothly and quickly.
The Wing
This wing is built in three sections and is also
built upside down over the plan. The first job
is to cut out the ribs accurately. You may have
a favourite way of transferring the rib outline
onto your balsa, but I used the old and tried
method of tracing each rib onto tracing paper
and then transferred this to the balsa. This
method has the benefit of not damaging the plans
with pins being pushed through to mark the outline
and leaving the plan a perforated piece of paper
that will rip more easily along these lines.
As this is a tapered wing, each rib has a different
profile so make sure that the tabs (to assist
with building) at the rear of each rib are cut
accurately so the wing will end up being built
straight.
I chose to build the centre section
first because this was the more complicated section.
With the
fact that the wing is built upside down it is not
possible to put the wing joiners (J1 and J2), for
the outer panels, in place until this section is
removed from the building board. I started with
pinning down the main spars for the two halves
and then trial fitting the ribs in place before
gluing to ensure that all is aligned correctly
and that the wing joiner (J3) slides through the
slots in the ribs. IMPORTANT -
I use plastic wrap over my plans to protect them
during all building
because this stops you gluing parts of your model
to the plan. I found that ribs W1 to W3 for both
sides of the centre section needed to be slid over
the joiner before fitting the ribs in place on
the top spar (which is the spar pinned down on
the board). Then the ribs were pinned in place,
ensuring they were at 90o to the building board
and then the 6.5 mm balsa "trailing edge" spar
is pinned in place at the rear of the ribs and
then all the joints were glued and left to set.
Remember that ribs W3 and W6 are made from ply
to support the undercarriage and also to give added
strength to the wing joiner and as such should
be glued with a better adhesive. I used epoxy for
these joints. The leading edge is then pinned in
place and glued. Next, the top (bottom - remember
the wing is upside down) spars are fitted (I joined
them with an opposite mitre joint to spar pinned
down) and undercarriage blocks are added and epoxied
in place.
You will note that there are some balsa
blocks in this centre section that need to be
fitted before
any sheeting is applied. These are for the dowels,
the wing bolt holes and where the exhaust exits
through the wing. This last one I chose to use
a balsa tube rather than a block to help reduce
any extra weight and also so I could place this
in at a later time to suit the location of my
exhaust. The centre section then has the sheer
webbing glued
in place between the top and bottom spars and
then it is sheeted with 1.5mm balsa before removing
from the building board.
Once you have the wing
off the building board it is possible to insert
and glue the joiners for
the outer panels and sheet the entire section
with 1.5mm balsa and trim off the building tabs
on the
ribs.
Now you can build the two outer panels, again
upside down over the plan. This is fairly standard
construction except ensuring that rib 6a is set
at the angle shown on the dihedral guide to allow
the outer panels to have the correct dihedral.
Again I pinned the top spar to the plan and built
the wing over the plan. The plan shows wing tips
are made from balsa block but I chose to make them
by laminating pieces of 6.5mm balsa. This is usually
cheaper to make and also most of us who have scrap
pieces of balsa lying around will often find enough
to make these wing tips without having to buy extra
block balsa that can be expensive. Also, it is
necessary to build the ailerons and the built up
trailing edges for the wing panels and attach the
trailing edge sections to the wing panels.
Once
these panels are completed it is possible to join
the outer wing panels to the centre section,
ensuring that the correct amount of dihedral is
achieved for each outer panel. The dihedral is
shown on the plan and I chose to use a 32mm spacer
under the rib W12. Do not forget that there are
two servos in each of the outer wing panel, one
for the aileron and one for the inverted V tail
and these servo bays need to set up and you need
to ensure that you can get the servo leads through
the wing to connect them.
The wing is now basically
finished except for the final shaping of the leading
edge, the fitting
of the dowels, the drilling of the holes for the
wing hold down bolts and the cutting the hole for
the exhaust. Now I put the wing aside so I could
build the fuselage.
The Fuselage
This is fairly straight forward except that I believe
a building jig is a great aid to building the fuselage.
If you donÕt have one, they are not hard
to make and you can use it for years to come. I
started by gluing the fuselage doublers to the
fuselage sides, ensuring that you make one left
and one right side. It is easy to make two of the
one side. I would suggest that when you glue these
doublers in place, it would be a good idea to slightly
curve the sides towards the ply doubler by placing
spacers under them and clamping them down. You
will end up with two curved fuselage sides bending
toward the ply. I did not do this and found it
difficult to bend the fuselage sides down to F1
and F2 even with a jig.
Next I glued F3, F3a, F4
and F5 in place along with the spruce longerons.
All this was done in
a home made jig as seen in the accompanying photo.
Once these were clamped in place I inserted F1
and F2 (with the nose wheel bracket attached)
and clamped the jig in tight to let the glue set.
I
used aliphatic resin for F1 to F4 and epoxy resin
on F5 because this is the firewall and needs
extra strength to cope with engine vibration. I
set this
aside to dry for 24 hours and then I was able
to fit and shape the top and top sides of the fuselage
with 9.5mm balsa. Once removed from the jig I
was
able to glue in place and shape the bottom sheeting
of 9.5mm balsa. The nose block was them carved
and sanded to the desired shape and glued in
place.
Now
comes the cockpit area which is basically the
canopy with a floor and back and front wall installed.
This may sound simple but let me tell you it
is
not as easy as it seems. The floor of the cockpit
is 3mm balsa and I found that it was necessary
to glue strips along each side to give you something
to glue the canopy onto. Before gluing the canopy
in place you need to think about whether you
want to have a pilot in there or not and if you
want
to paint the inside of the cockpit. To get the
cockpit area to mate to the fuselage properly
I used plastic food wrap over the fuselage and
when
I was ready to glue the canopy to the cockpit
floor and ends, I fitted it in place on the fuselage
and clamped it there with rubber bands and allowed
it to dry.
Probably the most difficult part of
building the fuselage was the installation of
the engine
mount
and making the cowl fit around the engine.
This plan has the engine laying over at 45o from
inverted
to allow the muffler to go through the former
F5 and have the exhaust exit down through the
wing.
Some engines will not allow you to do this
with the standard muffler. I used my OS 46 FX but
I had to modify the muffler so that it would
clear
the wing. Modifying the face of the exhaust
flange
to give me the correct angle did this. You
may choose to use a Pitts style muffler or what
ever,
this is your choice.
Be aware that installation
of the fuel tank is another area where you need
to be careful
because
this is a pusher set up and the front of
the tank needs to be facing the front of the fuselage
and
not to the engine.
With this being a pusher
I knew that I had to keep as much weight up front
as possible
and
so mounted
the servos just behind the nose block with
the battery pack between them and still had
to add
nose weight (more on this later).
The Booms
and Tail Feathers
I started with the tail feathers because
these are a little different to the norm.
I built
the two halves separately and then joined
them using
the ply joiners (TJ). This all sounds quite
simple but it is important to have this
all done accurately.
The two halves were built
over the plan individually in a similar manner
to that
used building
the wing. I pinned the bottom spar to
the plan
and glued
in place the ribs and attached the 6.5mm
balsa leading and trailing edges. Then
the top spar
was glued in place and the tail centre
(TC) was fitted
and shaped. The top 1.5mm balsa sheeting
was then glued in place with aliphatic
glue and
this was
left to dry over night. Once dry it was
removed from the building board and the
bottom sheeting
was glued on. The two sections were then
test fitted together until they were
at the correct
angle and
alignment and then they were glued together.
The trailing edge of the centre section
was built up
from 12.5mm balsa and sanded to the desired
shape. Next the ruddervators were built
over the plan,
making sure they were kept warp free.
The
booms were built individually over the plan.
This was really a simple matter
of
building as
if they are small fuselages and as
such needed to be kept straight and true.
I was tempted
to build them on their side, but decided
that this
would not be the best way to have them
both come out true down the centre
line of the
boom. Whilst
building the booms you need to look
at how you will install your snake controls
for
the ruddervators.
Once the booms were finished I measured
off the plans their centrelines and
marked these
on the
building board and attached the booms
to the building board in preparation
of fitting
the
inverted V
tail. This process needs to be done
with patience and care and not rushed so that
the correct
alignment is achieved. It entailed
quite a bit of trial
fitting and sanding and then the final
gluing in place
and then the fillets are glued in place
and the whole set up sanded to shape.
This configuration
of booms and inverted V tail are then
removed from the board and test fitted
and checked
for
alignment
before gluing to the wing. I needed
to enlarge the holes in ribs 6 and 6a to
allow a better
angle for the snake to pass through
and down the booms.
The inverted V tail presented
its own problems for set up. First, I was going
to set this
up as a V tail with ruddervators
and assign rudder
to
an extra channel for the steerable
nose wheel. But my radio had problems
because
I could
get the elevators to work in the
right direction but the
rudder control would be wrong and
if I reversed the settings the rudder
would be right but
the elevator would be wrong, so I
opted to have just
elevator on the V tail and use the
rudder channel
for the nose wheel.
Finishing
I chose to use the method I am comfortable with
and that is with doping, painting with spray
enamel and heat shrink covering for the wings.
The entire airframe was filled and sanded before
doping. I gave it two coats of dope, sanding
between coats, and then I painted the entire
area to be painted white and then masked off
the parts needed before painting the colour.
When
all assembled with the 6 servos (2 for ailerons,
2 for ruddervators, 1 for throttle and 1 for
steerable nose wheel) are installed and the engine
is fitted
it is time to balance the model. The balance
point needs to be 11.5cm from the leading edge
and to
achieve this I had to add 12oz of lead to the nose.
To the Field
The model attracted a lot of attention from all
corners because of the unusual design of a pusher
with twin tail booms and inverted V tail.
The
first trip to the field was an eventful one.
After all the setting up and tests completed
it
was time to fly. The motor fired up and I taxied
out to the strip and lined up and gassed her up
and she was in the air within about 10 meters and
after I had turned to the down wind leg the motor
cut and I called out Òdead stickÓ,
but one of our helicopter pilots, who was not standing
in the pilots area, did not hear me and as I turned
to land across the strip he flew in front of me
so I had to turn away from him causing the plane
to loose too much air speed and it hit the deck
fairly hard. Breaking the tail off and one wing
outer panel. The reason for the dead stick I believe
was two fold in that not really thinking I put
the tank in with the front of the tank to the motor,
but I should have had the front of the tank to
the front of the plane. The second reason was sufficient
cooling and I cannot emphasise too much that the
motor in a pusher configuration especially needs
good cooling particularly for running up the engine
in the pits as you do not have any prop wash over
the motor to cool it. Two weeks later with it repaired
and with the tank in the correct way around I was
back at the strip and this trip I was able to fly
a couple of flights to test Pushy Cat out. It is
quite a docile flyer because of the large wing
area, but it does handle well with no real vices
that I could detect. It again was airborne very
quickly and it was fun to fly doing loops, rolls
etc. With the ailerons set on low rates (5mm either
way) the roll rate was quite slow and even on high
rate (10mm either way) the roll rate was not too
fast. When coming in to land this plane just wants
to keep on flying because of the semi-symmetrical
aerofoil and large wing area.
