Free Flight Duration - 2004/06

142 MODEL AVIATION
IN A FREE FLIGHT model, weight is at a premium. There is no
room on the scale to overengineer each part with excess wood. No
matter what kind of adhesive you use for construction, it will add
weight. Sloppy joints filled with excess glue add weight and take
away strength. One often-overlooked glue joint on our models is
the one between the wing ribs and the trailing edge.
A butt joint is quick and easy to make, but it depends on an
accurate fit and an end-grain glue joint. The gluing surface is
limited to the rib cross-section where it meets the trailing edge. If
you simply apply adhesive to the end of the rib and put it in
position, the result will be a weak joint. The end grain of any
wood, especially a porous wood such as balsa, acts like a sponge to
suck the glue deep into the wood, leaving the joint starved for glue.
If you must use this type of joint, the best solution is to preglue
the end grain, as follows. After carefully fitting the part, coat the
end with adhesive, wipe off any excess, and wait a few minutes.
Carefully examine the end grain with a magnifying glass. If its
surface looks glossy, apply a final coat of adhesive, position the
part, and allow it to dry. If the end grain looks dry, apply a second
coat of adhesive, wipe away any excess, and let that dry.
Apply adhesive and position the part. Wipe off any excess
glue. A fillet of adhesive on either side of the rib will add little
strength but a great deal of weight. Sure it’s a lot of trouble, but
the joint will be much stronger than a butt joint that is not
preglued.
To get a stronger trailing edge joint, you need to increase the
gluing area. A popular method is to notch the trailing edge to
receive the ribs. Although cutting the notches can be a bit of
trouble, it actually speeds construction. Since the trailing-edge
notches help locate the ribs, there is less fiddling around trying to
line things up on the plans. Also, the notched joint requires a less
critical fit from front to back than a butt joint. If the rib is
trimmed a fraction of an inch short, it doesn’t significantly
weaken the joint.
For those reasons, a notched trailing edge or, better yet, a
notched leading edge and trailing edge is a good choice for a
beginner’s model, allowing stronger, more accurate construction
without the need for perfectly fitted parts.
The notches are surprisingly easy to make. You can do so with
a flat Swiss file or a piece of thin plywood with a strip of 120-grit
sandpaper along the edge. You can also use a fine-toothed
hacksaw blade; a single thickness makes a 1⁄32-inch-wide notch,
stack two for a 1⁄16-inch notch, and so on.
I carefully break off short lengths of an old hacksaw blade and
clamp them together. I glue a strip of scrap balsa on either side of
the blade stack to act as a depth gauge. The depth of the notches
isn’t critical; however, they should be deep enough to strengthen
the rib-to-trailing edge glue joint but not so deep that they
significantly weaken the trailing edge. For a small model with 1⁄32-
or 1⁄16-inch ribs, a depth of 1⁄8 inch should work. Go a bit deeper
for larger models—say, 3⁄16 or 1⁄4 inch for a model with 1⁄8-inchthick
ribs.
One variation on the notched trailing edge uses a trailing edge
laminated up from several pieces of wood. It might consist of a
Louis Joyner, 6 Saturday Rd., Mt. Pleasant SC 29464
FREE FLIGHT DURATION
These alternatives to simple butt joint take more effort but result in stronger, more durable joint between ribs and TE.
butt joint
notched trailing edge
triangular gussets
carbon fiber cap strips
06sig5QXD 3/24/04 11:46 am Page 142
piece of 1⁄8-inch square soft balsa, then a piece of 1⁄16 x 1⁄8 spruce
set vertically, then a piece of medium 1⁄8 x 3⁄8 balsa. Only the soft
strip of 1⁄8-inch square should be notched. The idea is that the strip
of spruce provides extra strength in bending and allows lighterweight
balsa to be used. An alternative is to substitute a vertical
strip of carbon fiber for the spruce.
Gussets are another way to strengthen the rib-to-trailing edge
joint. For this method, a simple butt joint is used, then the gussets
are fitted to strengthen the joint. The gussets are typically cut
from balsa that is the same thickness as the ribs or slightly
thinner; they do not all have to be the same thickness.
On a wing with 1⁄16-inch ribs, you could use 1⁄32-inch gussets
for most of the joints and 1⁄16-inch gussets at the more highly
stressed locations such as the center-section and dihedral-break
ribs.
Usually only one gusset per rib is used, but you can gusset
more highly stressed joints on both sides. Ideally, the gusset
should be aligned with the centerline of the trailing edge and the
rib; this prevents the covering from touching the gusset. This does
require you to slightly bevel the side of the gusset that attaches to
the trailing edge.
Gussets also help stiffen the ribs themselves and reduce the
tendency for the rib to bend if the covering is shrunk too tight.
(Ribs cut from thin balsa are especially prone to bowing as the
covering tightens across the wing chord. Using thicker, softer rib
material will also reduce the bowing, often with little or no weight
gain. Another way to reduce rib bowing is to attach the covering
to the top and bottom of every rib.)
For maximum strength, cut gussets so that the grain runs
parallel to the long side (hypotenuse) of the triangle. The easiest
way to mass-produce gussets is to cut them from a strip of balsa
using a right angle and alternating sides. Gussets do not need to be
equilateral triangles. It is often more effective for the side of the
gusset that attaches to the rib to be longer, to provide extra
stiffening.
Carbon-fiber capstrips provide strength for the rib-to-trailing
edge joint while stiffening the rib in bending. Thin, unidirectional
carbon is available in sheet form from several sources, such as
Bradley Model Products (E-mail: [email protected]) and
CST (www.cstsales.com).
It’s surprisingly easy to strip the carbon down to the required
width. As an alternative, you can order ready-to-use carbon caps
from Alexander Andriukov at http://home.pacbell.net/andriukov/
or Star Link at www.inland.net/~abrush.
With any type of carbon cap, it is important to lightly sand
each one to remove any mold release before gluing it in place. For
this to be effective, you must cap the top and bottom of every rib.
You generally apply the capstrips after you have assembled
and sanded the wing panel. Although some people use slow-
Bill Vanderbeek’s O&R .60-powered Herky uses triangular
gussets to strengthen the joints between the ribs and the TE.
Carbon rib caps on Ed Wiley’s F1B’s wing stiffen the balsa ribs
and tie the carbon-fiber D-box to the carbon-fiber TE.
Thin carbon strips strengthen Bob Mattes’ Power model’s
traditional balsa structure. Strips are also applied to bottom.
setting cyanoacrylate, I have had the best luck using a slow epoxy
resin. I apply a thin coat of epoxy to each capstrip, and then I
position each capstrip on the bottom of a rib. (I cut the capstrips
approximately a half inch long to provide a “handle” that
overhangs the trailing edge.)
The front of the capstrip should extend to the front edge of the
main spar for a model with a D-box or all the way to the leading
edge. After all the bottom caps are in place, I position the wing
panel on a form that matches the undercamber of the wing. (For a
flat-bottomed airfoil, this can, of course, be a flat surface such as
a piece of thick glass.) The form should be protected with plastic
wrap to keep the wing from sticking. I weight the wing panel with
a stack of magazines.
After the epoxy sets, I remove the wing from the form and
apply the upper capstrips. I carefully shim the wing panel to the
desired washin or washout and cover it
with plastic wrap. I drape a stack of
magazines over the wing panel to hold the
capstrips in place.
Be careful not to disturb the caps or
mash down the wing with too heavy a
stack of magazines. Allow the epoxy to
dry. You will be surprised by the gain in
stiffness that the carbon caps provide.
Seeing Better: For close work at the
building board, I often use a pair of
headband magnifiers. Mine has a flip-up
visor containing interchangeable plastic
lenses. When down, there is enough room
for the visor to fit over the front of my
regular glasses. Most hobby shops stock
these in a variety of magnifications; try to
find the one that will bring things into
focus at the working distance you find
comfortable. (Mine focus at roughly eight
inches.)
I’ve used the visor magnifiers at the
flying site for close work, such as hooking
up the timer lines, and for field repair, but
they are bulky and cumbersome for field
use. Recently I found a compact
alternative: clip-on magnifiers. They work
like the familiar clip-on sunglasses. A
padded spring clip attaches the magnifiers
to your regular glasses, and then you can
flip the magnifiers down for close work or
out of the way.
The clip-ons are intended for fly
fishermen, who often have to deal with
tying knots in monofilament while kneedeep
in a trout stream. The magnifiers
come in several different magnifications
and sell for approximately $12. The brand
name is Flip & Focus. They are available
in 1.50, 2.00, 2.50, and 3.00 diopters. If
you can’t find them at your local flyfishing
outfitter, you can order them from
www.flyfishusa.com.
If you don’t wear glasses, consider a
cap-mounted magnifier. It will clip onto
the bill of a ball cap and flip down when
needed.
A few years ago I switched from
reading glasses to blended trifocals. For
the most part, these do make life easier.
For one thing, my vision is better; for
another, I don’t have to fool with putting
on (and losing) reading glasses.
However, the blended trifocals do
cause a few problems, especially in the
transition areas of the lenses. Straight
lines don’t always appear straight. The old
technique of sighting down a model’s
fuselage to check wing washin or washout
simply doesn’t work.
Blaine Miller suggested an alternative
based on an old woodworking trick for
checking twists in a board. The solution,
which is actually more accurate regardless
of your vision, is to use two pieces of 1⁄4
square balsa strapped to the bottom of the
wing with rubber bands. The balsa strips
should be straight and at least twice the
wing chord; three times is better. Strap the
pieces on so that they are parallel to the
wing ribs, with an equal overhang at each
end.
To check the wing wash of the right
main panel, for example, position one
strip just inboard of the right dihedral
break and the other strip near the center
rib. Sight down the wing from the tip. The
balsa strips will, in effect, magnify any
washin or washout.
Building with the blended trifocals also
requires a bit of adjustment. Now instead
of trusting my vision, I use a straightedge
to make sure that things really are straight.
I position a straightedge along the plans
just in front of the leading edge, and then I
pin the leading edge down tight against the
straightedge. With my glasses, wing ribs
seemed to end up canted slightly to one
side or the other. Now I use two scrap
pieces of 3⁄8 square balsa—one on each
side of the rib—to make sure it is vertical
while the glue dries.
Another problem showed up when I
tried taking pictures while wearing the
blended trifocals. The only way I could
see to focus was to use the bottom third of
the glasses. The solution was to shoot
without them but use an eyepiece
correction lens fitted to the camera
eyepiece. These are available in various
diopters from most camera manufacturers.
You simply screw out the old eyepiece,
which is flat glass, and screw in the
correction lens.
The proper strength is probably
whatever your near-distance correction is
for your glasses. If possible, try several
different strengths until you find one that
puts the information (f-stop, shutter
speed numbers, etc.) that is visible in the
camera’s finder in focus. MA