This article is from the Model Rockets FAQ, by Wolfram von Kiparski with numerous contributions by others.
Lots of them. The most important things to consider are to
build light, strong, and warp-free. Weight is the enemy of a
glider. A weak glider will break easily. A warped glider is
very difficult to make glide properly. All three of these
problems are hard to fix later.
Weight and strength are tradeoffs. Reducing one usually
reduces the other. Weight must be controlled when
building. This starts by selecting the right wood. Many kits
from big manufacturers have really awful wood. If it's bad,
replace it with better wood from a hobby shop or mail order
supplier. More on this later. Proper shaping not only improves
the airfoil, but removes excess weight. I prefer to build
light, and then reinforce the glider with composite materials,
thus minimizing weight and maximizing strength.
In order to keep surfaces straight and free of undesirable
warps, I recommend the use of a building board. A scrap of
kitchen counter, larger than the finished model is perfect for
this purpose. A scrap hollow core door can also make a large
workbench and building board. 2x4 ceiling tiles work well, and
you can pin plans to them easily, but they can be damaged
easily. The building board should have at least one straight
perpendicular edge. All planing, sanding, cutting, and gluing
is done on this work surface.
The flying surfaces of most gliders need to be airfoiled to
work best. Unlike other rocket parts, a glider wing needs a
non-symmetric airfoil. The standard fin airfoil shape, split
in half, is a good place to begin. To rapidly shape a wing
airfoil, use a device called a razor plane. Much like its big
brother used for carpentry, this tool shaves off wood
quickly. The difference is that it uses a razor blade or
equivalent to do so. Many different types are available. My
personal favorite is the David Combi. An inexpensive nylon one
is available from Master Airscrew. The cheap cast metal planes
that use double edge razor blades are usually of poor quality.
These and many other handy tools can be found in model
airplane catalogs. The SIG catalog in particular is an
excellent source of many materials needed to build and fly
gliders, including these two razor planes.
Once roughly shaped, a sanding block is needed to get
everything smooth. A 6" piece of 1x2 is perfect to wrap 1/6 of
a sheet of sandpaper around (or 1/3 of a sheet around a 12"
block). Use thumb tacks to hold the sheet in place. The
extruded aluminum sanding blocks are particularly nice for
airfoiling glider wings. Sanding across the grain removes wood
fast, sanding with the grain gives a nice final finish. Start
with 100 grit, and work down to 400. The stab and rudder are
similarly airfoiled, usually symmetrically. Note that Delta
wing and canard (stab in front, wing in rear, like some of the
Rutan aircraft designs) gliders often need different
airfoils. Consult the instructions or plans for your model.
In order to be stable in glide, your glider will need
dihedral. This is the upward tilting or curving of the
wings. Some designs use multiple joints, trihedral or
polyhedral. To do this, cut the wing in half (or thirds,
quarters, etc. as per the plan). A razor saw is the best tool
to do this, but a modeling knife and a straight edge will
do. Tilt each tip up the required amount on your building
board. Use a handy scrap or a piece of 1x2 to prop the wing
pieces up. Now bevel the root edges using a sanding block and
the edge of the building board so that they are once again
perpendicular to your work surface.
The two edges can now be glued together. Standard wood glues
can be used for this, either carpenters, CA, epoxy, or
Amberoid or Duco. I particularly like Amberoid or Duco cement
for gliders because it can be dissolved to remove parts that
end up misaligned. For extra strength poke several pinholes in
the edges to be joined before gluing.
The wing, stab, and rudder are now glued to the fuselage of
the glider. Take care to align the parts accurately. Typically
a design will call for a tilt in the wing or stab, in order to
make the glider gently turn in flight. This prevents very long
chases to retrieve your glider. Also designs will frequently
include a few degrees of negative incidence or decalage in the
stab. By putting the stab at a slight pitch angle to the wing,
it aids in the transition of the glider from boost to glide,
and prevents the "death dive" where the glider flys straight
down. This angle can be set by tapering the fuselage at the
attachment point, or by gluing a small block at the
appropriate end of the fuselage to support the stab.