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Here is an ingenious method
for mechanically generating properly proportioned ribs for wings that are not rectangular
- including sing and double straight tapers and even elliptical planforms. Of course
today you can print out a perfectly dimensioned set of ribs to accommodate any planform
and root-to-tip thickness taper profile. In 1960 when this "'Simplex' Airfoil Templates"
article appeared in the 1960 Annual issue of Air Trails magazine, such
conveniences were in the purview of universities and government research facilities.
Even if you have no need of cutting ribs for tapered wings, it is worth your time
to read this brief article about the mathematical principle - logarithmic (aka equiangular)
spirals - behind the scheme. There are no formulas, so don't be scared off ;-)
"Simplex" Airfoil Templates
Stick two pins vertically into sheet rib stock from which the
ribs are to be cut. Separate the pins by a distance equal to the desired rib length.
Select appropriate template, engage front notch and rotate template upwards to press
against rear pin. Upper camber of rib can now be cut. Use straightedge to cut bottom
camber.
By Robert H. W. Annenberg
Now that more graceful radio-controlled models are being designed in ever-increasing
numbers, and U-control stunt models are going "pretty," an easy method for making
ribs of an elliptical or tapered wing planform is in demand. Here is a set of templates
- called Simplex Airfoils - each of which, when cut out of metal or plastic will
give a geometrically similar airfoil, regardless of the chord length to its trailing
edge. By suitably combining any pair of templates of different "percentage value,"
undercambered or double convex airfoils may be created in a wide range of thickness
and mean camber ratios.
The mathematical basis of the templates is a curve known as the "Logarithmic
( or Equiangular) Spiral." This has the unique property that portions subtended
by any chord line from the origin are geometrically similar, regardless of the length
of the chord. It turns out that the maximum "thickness" occurs at approximately
35% chord; very convenient for aeromodelers! The numbers on the templates refer
to the maximum ordinate height above the chord line drawn from the "notch" to any
point on the curve - in percent, of course.
 The full-size templates here are suitable
for ten-inch chord lengths or less, and should be carefully glued onto metal or
plastic with rubber cement before trimming to the final shape. Saw and file carefully
to the exact outline, including the leading edge notch which is exceedingly important.
Don't hurry, because mistakes on a master template show up on each wing rib made
from it!
To use the templates, place a pin at the desired leading edge position, push
the "notch" of the chosen thickness template against the pin, and rotate the template
until it coincides with the trailing edge. Then trace or cut out your Simplex Airfoil.
It should be noted that these templates tend to give a sharp leading edge. To allow
material for sanding a reasonable leading edge radius, simply "bias" the notch and
trailing edge crossover points about 1/16 or 1/8 inch above the true chord line.
Don't forget to allow for this leading edge bias in selecting your percentage, though.
Simplex templates are not only suited for cutting out ordinary-style ribs for
tapered planforms, but they are ideal for making "sliced" ribs for indoor models
and diagonal ribs, too. To find the proper "percentage value" for the diagonal rib,
simply multiply the basic airfoil thickness value by the ratio of the-average-chord-length-between-ribs
to the-diagonal-rib-length. (This ratio will always be less than one, so diagonal
ribs require a smaller percentage value template than the basic airfoil percentage
value.)
Before those frantic flying and re-building days arrive - prepare your Simplex
Airfoil templates for action!
Posted April 17, 2021
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