The Earl Stahl Wildcat is a FF classic, but with only a 19-inch
wingspan, it might be smaller than some builders are looking for.
Let’s start by working in inches.
There are 12 inches to a foot. With
this knowledge, we can convert the
full-scale wingspan into the same units
as the plans. We need to convert the
37-foot part of the wingspan to inches.
That extra 4-inch part of the wingspan
is already in the right units, so we’ll add
that bit on after the feet are converted.
Here is how the conversion looks
Full-scale Hawk 75 wingspan = ( 37
feet x 12 inches) + 4 inches =
Now that the full-scale units match
the model units, all we need to do is find
the ratio of the big airplane to the little
airplane. We do this by dividing the full-scale wingspan by the model wingspan:
Ratio of full-scale aircraft to model =
448 inches / 60 inches = 7. 47.
So, the scale is 1/7.47. This just
happens to be 7% more than 1/7 scale
and 7% less than 1/8 scale. There’s not
a lot of 1/7-scale stuff out there, but
1/8 scale is pretty common. Is that a
problem when we are looking for a
pilot? Perhaps, but when you consider
that full-scale Hawk pilots ranged in
size from somewhere between 5 feet, 6
inches and 6 feet, 3 inches (coincidently
a variation of +/-7%), we can probably
make a 1/8-scale pilot work.
A note on units: It doesn’t matter
if the units are in feet, meters, or
furlongs. The same
First convert as
necessary to get
both the full-scale
and the model into
the same units, and
then find the ratio.
Scaling Plans Up
scaling challenge is
changing the size
of the project. For
Digital printing has made this a simple
thing. As an example, let’s adjust that
aforementioned 19-inch Stahl Wildcat
to an even 30 inches. The ratio of the
30-inch model to the original 19-inch
one is 1. 58. All we need to do is print
our plans 158% larger.
Most of us don’t have a large-format
printer of our own, but many printing
resources do. These include copy centers,
local blueprint shops, and online printing
services. Most can change the scale of
your printout if you explain your goals.
The AMA Plans Service can also provide
plans that are larger or smaller than
For larger plans, blueprint shops can
be your best bet. They often have larger
machines than copy centers have and
they are more focused on technical
drawings rather than flyers. My local
shop costs slightly more, but its service is
excellent. The 36 x 48 plans for my last
project were less than $10 a copy.
Even if you don’t have a large-format
printer at home, you can still print your
own plans on a standard printer. This
is done by tiling the file. Tiling simply
breaks the large page down into standard
letter-size sheets. The sheets are then
taped together to make full-size plans.
I use Adobe Acrobat Pro for this, but
there are several tools available online.
Some file types scale more easily than
others. CAD formats such as DWG and
DXF are designed to be scaled up and
down. An important benefit of these
formats is that their line widths and
resolution don’t degrade as the scale is
blown up. A PDF file works nearly as
well, with high levels of resolution, but
the line width will vary with the scale.
Graphics files such as JPEG and TIFF
are a little trickier. The resolution of
these formats degrades as the scale goes
up. That means that a letter-size sheet
gets mighty fuzzy by the time it is blown
up to big plans.
Scaling a Design Up or Down
Now that we know that changing the
size of the drawing is simple, we can
make any size model from any plans,
right? Well, not exactly. There are a few
things to work out first.
The first thing to consider is the
thickness of the materials you will be
working with. When the scale changes, so
does the thickness of the stock.
To get this section started, here is a list
of standard wood thicknesses in inches.
As a side note, please be aware that the
actual thickness of balsa varies.
Some designs and scale combinations
work better than others. For instance, the
45-inch Bf 109D that was in the October
2016 issue of Model Aviation, began life
as a 30-inch model. We find the ratio of
the two by dividing the new project from
the old one:
Ratio of new model to original = 45
inches / 30 inches = 1. 5.
Bringing the 30-inch model up to 45
inches meant that everything needed
to get bigger by a factor of 1. 5. The 30-
inch model was built up from 1/16 and 1/8
wood. So what sizes of wood do we need
for the 45-inch model?
1/16 x 1. 5 = .063 x 1. 5 = .095 and 1/8 x
1. 5 = . 125 x 1. 5 = .188
26 Model Aviation JULY 2017 www.ModelAviation.com