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Ever
on the lookout for another lightish model that could be flown
in only moderate lift, we (Mike Briggs and I) felt that a
Tucano of around 1:12 scale could be made to fill the bill (Tucano
– bill; bill – Tucano), using Selig 3021, fairly lifty yet
nimble, as the wing section. The Tucano is a pleasantly
proportioned aircraft, characterised by a curvaceous fuselage
that flows right up into the fin, a wing with a fairly high
taper-ratio (> 2:1), and a big clear canopy.
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Embraer
EMB-312 Tucano 3-view |
To
begin, we looked at the plans that were around, and found that
apart from only one being about the right size, none
adequately reproduced the essential character of the aircraft;
fuselage lines were too straight, fin fairings were too
rudimentary, and wings were anything but Tucano-shaped.
Nothing for it, then but to start from scratch. As it
happened, we had a 1:72 scale drawing that had been published
in a well-known full-size aircraft magazine, so we set about
scaling it up. For a change, it was of the original Embraer
version, not the Shorts one which is more often seen.
What
an abortion that drawing proved to be! On it were three side
elevations and three plan views; no two fuselage lengths were
the same, no two wingspans matched, and the fuselage was
asymmetrical in plan. Lines in one view were in different
places in others. If that was produced by a draughtsman, he
was only so called because he must have drunk a lot of ale. It
took a lot of checking and cross-correlation with other
sources to produce a drawing with which, eventually, we were
satisfied. The wing, of only 216 sq.in, was enlarged 10% in
span and chord to give an area of 260 sq.in, while retaining
its very evident taper, and moderate washout was incorporated
for safety at low speeds. The remainder of the aircraft was
drawn as closely as possible to 1:12 scale, with only minor
deviations where construction would have become unduly
difficult, to simplify the shapes of the control surfaces, and
to enlarge the ailerons. Aileron and elevator controls only
were felt to be sufficient, so a non-working rudder was
selected. A little more effort produced a working drawing from
which to build, and so I set about cutting and gluing.
Minor
details were amended during the construction process, but on
the whole it went together quickly and easily with only a few
days’ labour, the early work put into producing accurately
(well, fairly accurately) lofted formers paying off. An added
bonus was that the canopy that we produce to fit Andy
Conway’s Hawk fitted the Tucano even better, though with the
original front to the rear instead. The whole airframe, ready
for covering, weighed in at 14.1 oz.
The
next problem, as ever, was selecting a colour scheme. The
Embraer-built Tucano is flown by a small number of South
American air forces, and by the French Air Force in a slightly
different form. Most are finished in training schemes of white
with various shades of red, with the odd one in camouflage.
Not wanting particularly to go through the laborious process
of surface preparation and painting, the camouflage schemes
quickly got the order of the boot. Then I happened across a
photo of the aircraft in RAF colours of red and white, with
pale grey inner wing-panels, as used in the original
demonstrations by Embraer and Shorts, and I decided to go with
that. Solarfilm and Profilm were used, bringing the weight up
to 15.8 oz.
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Embraer
EMB-312 Tucano by Steve Griffiths
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Cockpit
fittings (limited to the major items visible from a distance)
and two pilots were shaped from white foam, surfaced, painted
and fitted, and the canopy attached with strips of
self-adhesive trim matching roughly the canopy framing.
National insignia were made up from the same material (in
other colours, obviously) and ejector seat and rescue markings
produced on my inkjet printer, on paper. A little work on
these produced quite thin, waterproof items which were
attached using acrylic varnish as an adhesive. Aerials were
cut and shaped from ply, covered and installed, and all
linkages fitted. Finally, not being able to put it off any
longer, it was time to set about the exhaust stubs that are
another prominent feature of this aircraft. Experimentation
with some plastic tube and a heat gun soon showed the way, and
two (handed) stubs were formed on a hard balsa plug, painted
in polished steel Rub’n’Buff, and epoxied into holes cut
in the fuselage sides.
By
this stage the weight had risen to only 16.9 oz, which I felt
was quite acceptable for a 40” span model with a quite bulky
fuselage, even if a little in excess of our target weight. The
Tucano’s wing area is 260 sq.in, NAD, or 1.81 sq.ft, and we
were aiming for a wing loading not in excess of 14 oz./sq.ft,
so an AUW of no more than 25 oz was targeted. The margin
within which r/c equipment and balance weight had to fit was
therefore no more than 8.1 oz. Adding the radio equipment,
using the heaviest 4x600 Rx pack I had, and using two
mini-servos and a small receiver, raised the weight to 23.9
oz. Not much left for nose weight, and some was definitely
needed. In fact, to bring the balance point forwards to 30%
MAC required just under 2.5 oz, so the weight for first test
flights is a little over 26 oz, with a wing loading of 14.6
oz./sq.ft.
Mike Briggs'
and Steve Griffiths' Tucanos |
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I
predict that I can save about an ounce by fitting a lighter
battery pack, and compensating the balance with a lesser
weight of lead further forwards. Besides this reduction, I
anticipate that the balance point will move backwards to
perhaps 33% or 35% MAC, saving a little more, so the weight
will be then more in line with initial estimates. Of course,
the ultimate test is to see how the thing performs, and as we
are unable to fly at the moment, because of restrictions
brought on by the foot and mouth outbreak, that may or may not
be this year. After proving flights, and any mods we feel are
necessary to bring performance to where we want it, we do
intend to make plans available, perhaps through one of the
national aeromodelling magazines.
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