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It's
always difficult to decide, when you're after a new model,
whether to go for something really attractive, or something
you actually have a chance of catching. Aircraft are no
different - you have to make a number of decisions before you
select a suitable prototype to replicate, so when Mike Briggs
and I decided to design one, we set out some criteria to which
it had to conform. It had to be easy to build, so flat
surfaces and, for possible other builders a minimum of
non-wood parts, were preferred. For economy's sake, it should
not need balsa longer than 36". Once built, it had to be
easy to launch, so a low-wing was not our first choice.
At 1:12 scale (our preference for single-engined
aircraft), the model should not span more than 50", and
the chosen aircraft should have some real character, with a
reasonable choice of variants and colour-schemes. The canopy
should not be massive, to ease the production of a moulding.
Finally, it should finish up with a wing-loading in the 10-14
oz/sq.ft. range, as our local slopes are a bit tame. After a
bit of book-worming, we came up with the Su-25 Frogfoot.
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Unfortunately,
we could find only one scale 3-view, and that was less than
3" span, so we started by enlarging it on a photocopier
to a reasonable size. From that, and the full-size
measurements given, we worked out the sizes at 1:12 scale, and
it all looked about right.
After an educated? guess at the
finished weight, we decided to enlarge the wing slightly,
increasing the span by about 4", the root chord by
1", and the tip chord by 1/2", to keep the original
proportions and give the desired wing-loading. The enlarged
wing was then positioned to maintain the ratio of front to
rear fuselage length, and avoid it looking wrong. As far as
possible, the side view was drawn to exact scale, but in plan
the fuselage and bulky engine nacelles were slimmed. Because
of the fuselage width, the front ends of the nacelles were
designed to knock off if the wing swung round, but this has
since proved to be an unnecessary complication, as has the
fitting of knock-off wing-tip pods.
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Having
decided to use Selig-Donovan 6060 as the wing section, three
section plots were produced: one each for the root and tip,
and a third for the position at which the nacelle sides would
come. Otherwise, with a highly-tapered wing and a wide
fuselage, use of the root section to draw the shape at the
nacelle edge would result in a very bad fit. To simplify
building, wing anhedral and tailplane dihedral were reduced by
setting out the wing lower surface and the tailplane upper
surface flat. A mini-servo in each wing was the selection for
aileron control, both to avoid difficulties in wing-mounting
and to provide the options of flapperons and pop-up aileron
brakes, while a snake was chosen to drive the all-moving tail.
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To
maximise structural integrity, we decided to have only one
hole in the fuselage, beneath the bolted-on wing, and to have
a tube, from the bulkhead at the front of that, leading down
towards the nose, in which the Rx and battery could be
positioned for balance, and to omit fuselage doublers. From
there, it was merely the simple task of drawing up the formers
and sheet parts and building it. The prototype has a foam
wing, and although the drawings show balsa tail-feathers and
fuselage top-deck, as we had the equipment we cut them from
foam (SD8020 for the tailplane and thinned SD8020 for the fin,
which is huge).
Finishing
was with Tamiya acrylic paints brushed onto Litespan covering,
with the edges at the colour-changes stippled with an
almost-dry brush to give the effect of overspray. All the
insignia were hand-cut from Solartrim, and panel-lines applied
with medium-grey fibre-tipped pen, the whole then being given
a thin coat of Cuprinol matt varnish. This has proved
reasonably durable, although the Litespan is a little prone to
bubbling in the scorching summer sun we experience (!),
possibly because I didn't have enough hands to apply it in the
recommended way, i.e. stretching it as it is heated, but
before it is ironed down. Various aerials, lights and other
paraphernalia were added to give the finishing touches,
cobbled up from bits of ply and glass-headed pins.
Initial
proving flights were a bit traumatic, due to the wing servos
not being fixed quite as firmly as I had thought. The net
result was that after the application of full up-aileron, the
servos moved in their boxes, leaving an appreciable amount of
up-aileron as a very effective lift-dumper. From launch, it
went off straight and true, and climbed away happily, but
after a couple of turns it went like a bat out of hell and
lost height very quickly. The first flight finished up flying
straight into a dry stone wall in dead ground below the slope
edge, with a very audible result. Fearing the worst, I went to
retrieve it, but all it had suffered was a slightly bruised
nose - who needs ply doublers? In the fierce deceleration, the
servos had re-seated, so there was no obvious cause. A couple
of flights more, a few terrified sheep, and the cause became
apparent: that was put right, and it now flies very well,
although tight loops are out of the question, the short tail
moment quite typically giving too much braking effect if full
up-elevator is held on.
Final
verdict? At 30 oz, it turned out a couple of ounces heavier
than planned, but this doesn't seem to be in any way
detrimental to performance. It's robust, compact, and does
what it was designed to do, so we're satisfied with it. Doing
it again, without the knock-off bits, it would be easier and
lighter, but who wants to build the same model twice?
Prototype
statistics:
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Length:
Wing
Span:
Wing
Area:
Weight:
Wing
Loading:
Wing
Section:
Tailplane/Fin
Section:
Controls:
Servos: |
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42 inches
44 inches plus tip pods
312 square inches (2.17 square feet)
30
ounces
13.8 ounces per square foot
Selig-Donovan SD6060
Selig-Donovan SD8020
Elevator and aileron
(also used as flapperons and brakes)
Three (two HS101, one
Futaba S148) |
Steve
Griffiths.
January 18th, 1997.
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