Building
Pistachio Scale Models |
by Ondrej (Andy) Mitas
July 3rd, 2004
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"Pistachio Scale" is defined as any
model with a wingspan of 8 inches or less, or
a length of 6 inches or less (builder's choice).
These models have a high "cuteness factor"
which appeals to many free-flight modelers, and
also to R/C modelers using state-of-the-art lightweight
radio gear. Unfortunatey there are no kits, few
plans, and little information available for pistachio
scale, so building this type of model can seem
intimidating. This article provides some essential
information for pistachio construction, and I
hope it will instill confidence in those considering
such a project.
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Author's pistachio Se-5
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Plans for Pistachios
Currently, the best source of pistachio
scale plans is the collection of publications
from Hannan’s
Runway. Some of these are just larger plans
sized down (which I will discuss later), but
many actually include the correct wood dimensions
for building the 8” span model –
exactly what you want and need, with no guesswork
involved. I also have a few plans like this
of my own design on my
website. However, if the subject you are
interested in has no such plan available, you
must apply the compromises inherent to free-flight
design at a smaller scale.
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Working from scaled-down plans
If you reduce larger plans to
pistachio size you must consider what wood dimensions
to use. There are a few methods I suggest, one
of which is to simply use 1/32" square and
1/32" sheet throughout. In my experience
this is good for “small” pistachios
– airplanes with high aspect rations like
the Pilatus Turbo Porter that
build to very small 8” span models. On the
other hand a wide-chord airplane (like a Bleriot
IX, or Martinyside Semiquaver,
or most racing airplanes) would be extremely fragile
if made from 1/32”. In Europe, where wood
usually comes in millimetre measurements, the
plans often use 1x1 or 1mm sheet throughout. Because
it's slightly larger than 1/32” (which is
0.8 mm), using 1mm throughout is a good compromise.
Most of my designs have used 1/32” square
and sheet, no wing spar, and 1/32” sliced
ribs - in the style of an indoor duration model.
Sometimes I scaled down the wingspan to 6 or 7
inches to accomodate the frail wood pieces. As
you can imagine this led to very light models
(always under a gram), but lots of accidental
“crunches” from handling, not to mention
warping.
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Underside of Se5
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For my
model of the Se5, built from Walt
Mooney’s peanut plan, I tried a different
approach to scaling the wood sizes: 1/8" and
1/16” was converted to 1/20”; 1/20”
converted to 1/32”; and 1/32” parts
stayed the same. Predictably, this built into a
heavy airplane – 2.2 grams – but it
still flew well and was a lot easier to handle without
breaking. My experience with the Se5
suggests that any airplane that builds into a relatively
large 8” model is just as well off built from
1/20” square throughout, especially if it
saves you anxiety about building and handling.
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Author's Yak-2 twin
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One of my more successful designs, in terms of weight
and structural integrity, was the Yak-2
twin. The plan shows most of the wood as
1/20”. Built from 1mm square, mine weighs
a gram and is plenty strong.
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Underside of Yak-2 twin
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Covering
As a free-flight model becomes
smaller, the thickness (and therefore weight)
of the covering becomes larger relative to the
frame. As a result, weight of the covering is
extremely important in pistachio scale, and
the standard “Japanese tissue” (Esaki)
turns out to be unacceptably heavy (except in
the occasional case of something really large,
like a Fike E).
This leaves few options. Occasionally, one can
find a commercial tissue lighter than Esaki.
Then there is Japanese tissue made from pure
Gampi fiber, resulting in a
silky texture and, sometimes, less weight. For
more significant weight savings, it is necessary
to use condenser paper, an
extremely thin, non-porous paper covering. Airbrushed,
it becomes quite brittle, but I usually just
inkjet-print it and have no problems with strength.
It can be glued to the frame with liquid paper
glue. I used Gampi tissue for my Se5,
but otherwise, I use condenser paper almost
exclusively for pistachios.
The main drawback to condenser paper is its
persistent shrinkage. Even after being pre-shrunk
with water 4 or 5 times, it will continue to
shrink. The best insurance is to either pre-shrink
and wrinkle it, or to run it over with an iron
on the “steam” setting. The steam
will help to pre-shrink it, and the ironing
will impart tiny, unnoticable wrinkles that
will take up the eventual shrinkage.
You'll also notice that only the upper
surfaces of my pistachio models are
covered. This compromise saves a huge amount
of weight, and the models still retain good
scale appearance.
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Download Andy's original Pistachio Plans!