The background to this kit review was the disastrous
loss of a brand new high-tech A2 whilst I was trimming it in France last
year. Such a setback necessitated a quick replacement. Whilst searching
through various catalogues, I came across a Lithuanian company, W Hobby,
who market a range of products which are used in modern F/F contest machinery
and, in addition, manufacture a small number of kits. One of these kits,
the 'Sija', provided a possible solution to my problem. This kit is unusual
in that it utilises tubular carbon fibre wing spars which are claimed
to be excellent in resisting bending and twisting loads. Furthermore,
this form of wing construction was claimed to be much simpler, and consequently
faster, to build than those based on carbon D-boxes. The makers also claimed
that the finished wing was exceptionally crash resistant.
WHAT YOU GET
The kit is extensively prefabricated and remarkably complete. It included
all the necessary hardware, including two-function timer, circle towhook
with rudder delay mechanism and an excellent 6.0 mm steel wing joiner. The
model makes extensive use of carbon fibre for the fuselage shells, tailboom,
tubular wing spars, trailing edge and rib caps. The wing and tail ribs
were very accurately cut and the quality of the balsa excellent, as was
that used for other components. The clearly drawn, full-size plan, together
with parts list and brief building instructions, were all conveniently
written in English.
Epoxy or cyanoacrylate (CA) adhesives were recommended for building the
model. Regular readers will already be aware of the techniques employed
when using carbon components and the need to ensure that no traces of
release agents (used in moulding) or grease (ex-fingers) are present when
bonding such materials. As a precaution I rubbed down all carbon parts
with 800 grade wet and dry and then wiped them with a tissue soaked in
acetone (or thinners) immediately prior to gluing.
BUILDING
The wing structure is based on tubular carbon spars of 9 mm diameter for
the inner panels and 7 mm for the tips. The recommended method for building
the wing panels was unusual in that the pre-cut ribs and riblets were
slid along the spar and spaced according to the plan. Once all the ribs
were parallel and in contact with the building board they were fixed into
place using thin CA. The excellent fit of the ribs on the spars made for
a really sound joint. The pre-shaped hard balsa LE was glued to the ribs
using PVA and, when dry, the carbon TE was added using CA. During the
construction it is recommended that appropriate packing for the undercamber
be used to support the ribs which are very thin at their rear. With care
this may not be necessary but probably is advisable. I was fortunate to
have a cambered building board which suited the Averianov wing section
and this made construction exceptionally fast and easy. Now we include
blue foam wing building jig into kit.
The most tedious part of the wing construction was adding the carbon rib
caps (0.2 x 1.5 mm). These were pre-cut overlength and it was essential
to overlap the caps at least 3 mm over the LE and TE in order to ensure
a strong wing. Medium viscosity CA was used to attach the caps and I found
it helpful to hold them in place with small tabs of masking tape. Thick
balsa ( 10 mm) ribs were used at the end of each panel and appropriately
wide rib caps were provided. The inner panel spars were reinforced with
an additional tubular spar which was a snug fit inside the main spar and
extended approximately two thirds the span of the panel.- The reinforcing
spar is no longer used, as the structure of the main spar is different,
- it has more carbon layers inside at root end (marked red or white).
In view of the towing loads likely to be applied to the centre panels,
these reinforcing spars were epoxied in place as were the short glass
fiber wing joiner tubes.
Wing tips were constructed in a similar manner but were built around a
7 mm spar tube. Because
of the taper on the wing tips it was necessary to taper the undercamber
packing accordingly. The plan did not indicate any wing warps but I chose
to build in approximately 3 mm of washout on each tip as a precaution.
It should be noted that, with these high tech structures, it is very difficult,
if not impossible, to change the warps once built.
Whilst building the tip panels I noted a serious draughting error on the
otherwise excellent plan. The LE and TE tapers on the port tip were reversed
on the starboard side. Needless to say, both panels were built with the
same LE (and TE) taper! The mistake is already eliminated, thanks to
Phil. An unusual feature of the model was that the tip panels were
joined using a hard aluminium dihedral brace epoxied inside the wing spars
and the spar ends were wrapped with Kevlar thread to reinforce this highly
stressed area.
The resulting wing was very light -l30 gm and remarkably stiff. White
polvester tissue was supplied
in the kit and this was used for covering
the centre panels. This was applied dry using thinned PVA and wrinkles
removed hy warming with a hair dryer as water shrinking has little effect
on polyester tissue. The tips were used for testing samples of a new heavyweight
tissue, Star Span, kindly supplied by 5 star Products of Liverpool. This
material (20 gm per sq. metre)
looks similar to heavy Modelspan but the wide range
of colours is more intense. In the dry state it is rather stiff but handles
beautifully when lightly sprayed with water and has excelent wet strength
and shrinkage.
The finished wing was given four coats of 50% dope and tipped the scales
at just 150 gm - remarkably light given the stiffness.
The lightweight version of Star Span ( 10 gm per sq. metre) was used to
cover the tailplane which was of conventional balsa construction and
went together quickly using thc pre-cut ribs. The resulting tail weighed
9 gm after two coats of thin dope.
FUSELAGE
The high degree of prefabrication made fuselage construction very straightforward.
The fuselage uses the W Hobby multifunction cartridge. The basis, of the
cartridge is a 0.5mm steel spine which carries the two
function clockwork timer, circle tow hook. rudder delay and timer start
mechanisms. The cartridge, which bolts directly into the fuselage, also
serves to strengthen the front of the fuselage and carries the considerable
towing loads.
The pod was built by epoxying the two carbon shells together and the pre-painted
carbon tail boom epoxied onto a spigot which was moulded on the pod. The
multifunctional Cartridge system is no longer used on Sija, as was too
complicated for unexperienced builders. Now we offer semi plastic fuselage
pod with the same features, but more convenient in building. It has a
wooden frame, where all machinery is set and Carbon shell that covers
bottom and sides of fuselage pod. The fuselage was completed by adding
pre-built fin and machined aluminium tail mount, having first removed
enamel from the boom to enable a decent joint to be made. Finally, the
pod was sprayed to match the tailboom. A minor problem was experienced
here as the aerosol spray primer did not take to the carbon surface, despite
thorough flatting prior to spraying. I suspect that the problem was due
to contamination of the carbon shells by silicone release agent used in
the moulding. It took several attempts before an acceptable finish could
be obtained.
The model required 35 gm of lead to bring the CG to the position shown
on the plan (52%) and the completed model was one gram over the minimum
- 410 gm weight required for FAI contests.
FLYING
Altogether the model was very satisfying to
build but the proof of the pudding is in the flying. The cartridge system
employed greatly simplified trimming since it allowed independent adjustment
of timer start, straight tow, circle tow, glide circle, zoom launch and
rudder delay settings. The latter function maintains the rudder in a straight
position for one or two seconds following launch, thereby eliminating
any tendency for the model to spin following a high speed launch. Trimming
proved to be very straightforward and the glide and towing characteristics
are free from vices. So far, it has not been necessary to use the rudder
delay facility. Perhaps when I become more confident and launch the model
harder it will be needed...
Overall the kit offers exceptional value and has provided me with a model
which excellent potential which I look to using in competition. |
