As with the fuselage, let’s start with
the elements that run the long axis of
Leading edge (LE): A stout element
at the front of the airfoil, the LE needs
to be meaty enough to allow it to
be shaped into a radius. This may be
a simple piece of square stock or a
lamination of several strips of wood,
depending on the airframe’s size and
Trailing edge (TE): The back edge of
the wing is often a wide element that
allows room for rib notches and tapering to a thin, aerodynamic
Spar: Similar to a keel, a spar is a stout, structural element
that normally runs the length of the wing without interruption.
Many designs use a main spar near the thickest part of the
wing’s airfoil, and a rear spar behind the thickest point.
Shear webs: The main spar is often made of an upper and a
lower spar. These two independent spars may be strong enough
for a light airframe, but they can be made much stronger by
converting them to a C-channel. A shear web ties the upper
and lower spars together and makes the face of the “C.” For
additional strength, shear webs on both sides of the spars can
form a box spar.
Dihedral brace: This is two shear webs joined to tie the
two wings together. Dihedral braces are normally made from
plywood or another strong material.
Ribs: Similar to fuselage formers, ribs are the airfoil profiles
that run perpendicular to the long elements previously
discussed. Some designs include half ribs between the full ribs
at the LE to improve the airfoil’s shape. Short riblets are often
used in ailerons, where the full rib is intersected by the aileron
Tail group: The fin, rudder, stabilizers, and elevators also have
LEs and TEs. For small models, the stock inside of the outlines is
often referred to as bracing.
In high school Spanish class you may have learned that
languages are mastered by speaking them, so let’s discuss balsa
construction. Many methods have been used throughout the
years, but stick frame, keel frame, and sheeted construction are
three of the most popular.
Stick Frame Construction
The oldest, lightest, and most elegant form of balsa
construction is stick frame. Ivan Pettigrew and Pat
Tritle are two designers known for building incredibly
lightweight structures using this method. The stick-frame fuselage is created from two identical flat
frameworks of longerons with crossbraces. The frames
are made from square balsa. When complete, the
frames become the sides of the fuselage when joined
with additional crossbracing.
The result is a boxy framework reminiscent of most
World War I aircraft. That’s not a coincidence because
most aircraft of that era were built the same way.
For airplanes with more shapely fuselages, formers
are attached to the latitudinal crossbraces. Stringers
are then secured to the formers to round out the
The tail parts are often started by laminating thin,
flexible strips of balsa into strong, but lightweight,
outlines. Balsa strip stock is used on each side of the
parting line of the elevator and rudder. Square balsa is
used for bracing between the laminated outlines and
the parting stock.
The wing is built from lightweight ribs and spars.
Keeping the focus on lightness, a few stringers might
be used to smooth the upper airfoil between the LE
and the main spar.
Stick frame construction begins with two mirrored structures
for the fuselage sides.
Sheeting is used sparingly for structural purposes in this
ultralight airframe. Pat Tritle’s 54-inch 310 twin came in at 35
ounces all-up weight.
48 Model Aviation APRIL 2015 www.ModelAviation.com