Radio Control Slope Soaring - 2004/04

I FOUND A picture that brought back a flood of memories: flying
with Bob and Katie Martin at Soar Utah 1995. Bob was ahead of his
time—way ahead. At the time of the US bicentennial in 1976, he
designed and produced kits for the Coyote and the SR-7: the first
“energy planes” designed for the slope that I knew of in my early
slope-flying days.
Not many sailplanes in that period sported foam-core wing
construction and 1⁄64 plywood sheeting. These Slopers’ weight was a
little scary when all we ever read or heard was that gliders must be
light to fly well. The Coyote overcame the weight penalty with
wingspan and the SR-7 with wing area, and we learned that if you
keep the model moving, the airfoil will go to work for you and keep it
airborne, even in light lift.
Beyond this basic fact, we learned that the energy planes’ speed
potential could take us to a whole new level of excitement and permit
aerobatics we did not imagine when flying the floaters.
I was on my third Coyote by Soar Utah 1995 (“There’s no shame
in wearing out an airframe.”), and by then I had begun to modify them
during construction so that they would pack small for travel and
shipping. My third Coyote (still flying, by the way) has split wings and
removable tail parts, and it fits back into its original kit box. It has
traveled as airline checked luggage and been shipped easily and
cheaply by UPS and the USPS.
This column will explain techniques I have used many times to
build split wings and make tail parts dismountable, and a new method
I’ve tried to make a foamie-warbird wing removable.
Split-wing construction makes the longest part of the sailplane
shorter by half. This goes a long way toward making your model pack
small—particularly swept-wing designs. The objective is to install one
or more wing-joiner rods and make the joint strong enough to bear the
intended loads, but no heavier than necessary. The general principle is
to replace the original wing-joiner part with a wing rod that slips into
receiver tubes embedded in each wing half.
A common installation for me is using a pair of 1⁄4-inch-diameter
steel wing-joiner rods which slip into 9⁄32-inch brass tubing set into
each wing half. Sizing the wing-rod length and diameter is a design
decision, balancing weight, strength, and three ounces that the steel
rods and brass tubes weigh. To save weight you could opt for
aluminum or carbon-fiber rods and tubes.
Dave Garwood, 5 Birch Ln., Scotia NY 12302; E-mail: [email protected]
RADIO CONTROL SLOPE SOARING
Dave Garwood (L), Katie and Bob Martin at Soar Utah 1995, at
which Bob and Katie flew Dave’s Coyote. Joe Chovan photo.
The Coyote has a split-wing modification and bolt-on, removable
tail parts, and it fits into the original kit box.
These parts are used to make a split-wing modification to a foamcore
wing. The author explains how in the text.
A 1⁄4-20 tap is being used to cut threads in the plywood receiver
block glued in the EPP-foam fuselage.
136 MODEL AVIATION
Often, as in the Coyote, I use two joiner rods to spread out the
stress riser points created by embedding steel rods in foam wings.
Many Slope-glider wings have no spar; balsa skins provide the wing
stiffness. If you were to install a wing-joiner system on a spar-type
wing, you’d want to tie the joiner system to the spars.
Parameters for locating the wing-rod receiver tubes include placing
them as far apart as possible while considering the space needed for the
wing-mounting mechanism and allowing sufficient thickness in the
foam core to keep the receiver tubes below the surface of the foam.
Think about where the aileron servos will fit. When you’ve decided the
wing-joiner rod lengths and locations, you’re ready to install them in
the wing cores.
Mark the location on the cores and cut slots with a razor saw or
sharp knife. Cover the ends of the tubes with small pieces of masking
tape to keep epoxy out. Set the tubes in the slots cut in the cores,
watching to make sure that the ends don’t extend above or below the
slots past the surface of the foam cores.
Work carefully during these steps; alignment of these parts
determines not only the wing dihedral but the precise alignment of the
wing halves as they fit together. You want to avoid introducing a twist
into the wing halves.
Position the receiver tubes so that when dihedral is set up, the tube
ends are embedded below the surface of the foam. Insert plywood or
hardwood end-stop blocks. When all is carefully aligned, and with one
core raised to the proper dihedral angle, pour in a moderate amount of
epoxy and microballoon slurry to fix the receiver tubes in place. Note
that the receiver tubes are not cut in the center before they are epoxied
in place; you cut them later. That helps with the alignment of the tubes
in each wing half.
When the epoxy has cured, gently turn the cores over and apply
epoxy and microballoons to the bottom, and allow them to cure
thoroughly.
With no spar, it’s the wing skins that provide strength and stiffness
you want to tie the receiver tubes into the wing skins. You want a filler
material that’s tough enough—not any heavier than necessary and not
too tough to sand to a final shape to match the foam cores. I’ve come to
rely on drywall spackle for this.
The underside of an EPP-foam warbird shows three 1⁄4 plywood
blocks glued in place to receive three wing mount bolts.
Underside of EPP-foam wing mounted on EPP-foam fuselage
with three 1⁄4-20 nylon bolts. A 1⁄32 plywood strip keeps bolt heads
from pulling through wing.
April 2004 137
138 MODEL AVIATION
Cut the tubes at the wing centerline, and,
after sanding the cured spackle flush with the
surface of the foam cores, sheet them and add
leading-edge and trailing-edge stock as usual.
When you face off the center portions with a
root rib, don’t forget to mark where the brass
tubes are located so you can open them with a
small drill and a round file.
Dismountable tail parts make a huge
difference in what size box is needed to pack
a disassembled airplane. The main way I
make the horizontal stabilizer dismountable is
to look for a place I can install a pair of blind
nuts, sometimes called T-nuts, and mount the
stabilizer with a pair of machine screws—
most often 2-56 or 4-40 size.
If the fuselage has a flat place for the
horizontal stabilizer to rest, no sweat; just
mount the blind nuts on the underside of that
shelf. On a foamie I generally install a 1⁄16 or
1⁄32 plywood plate over the blind-nut location,
glue the plywood plate to the expandedpolypropylene
(EPP)-foam fuselage, and then
wrap it with strapping tape.
You may have to install “hard points” in
the stabilizer to withstand the clamping force
of tightening the screws, or you may be able
to use washers alone. Wanting to keep the tail
light, I think about using nylon bolts but am
fearful of having to extract a broken nylon
bolt part, so up to now I’ve used only steel
bolts.
To install a dismountable vertical
stabilizer in a foamie, I’ve seen guys cut the
slot in the fuselage accurately and hold the
vertical in place with friction. I generally go
ahead and glue vertical stabilizers in place on
foamies because permanently mounting this
part still allows the fuselage to fit back in the
original kit box.
For molded plastic fuselages, I prepare the
vertical to accept bolts coming through from
below. For my Coyote, the vertical fin has a
narrow plywood plate at its bottom, and
socket-head 4-40 bolts come upward through
the upper surface of the fuselage, through the
horizontal stabilizer, and into the blind nuts
embedded in the base of the vertical stabilizer.
Tightening two bolts holds the vertical and
horizontal stabilizers in place. I cut two small
circular holes in the lower side of the Dura-
Lene fuselage to be able to reach the bolts
with a hex wrench.
For fiberglass fuselages I use a technique I
learned while building a Charlie Richardson
Renegade: make the lower section of the
vertical fin from hard wood, slightly broader
than the balsa sheeting of the rest of the fin,
and tap it for 2-56 threads, and it accepts long
bolts that insert through the top and bottom of
the fuselage. Here also, tightening the bolts
mounts the vertical and clamps the horizontal
in place.
A removable wing, even a one-piece wing,
goes a long way toward making a foamie
warbird packable. I’ve seen it done with
strapping-tape straps, but they do not provide
as solid a mounting method as I would like.
New York Slope Dog Richard Loud has a
Dave’s Aircraft Works Foam 51 slope combat
fighter on which he mounts the wing with two
bolts. I asked him how he liked it, and he said
he thought three bolts would be better—less
likely to rip a mounting hard point out of the
EPP foam fuselage in a collision or cartwheel
landing. I fitted a three-bolt mount on my
Kawafoamie Ki61.
Designed around cheap, light, and sturdy
1⁄4-20 nylon bolts, these go through the wing
and into 1⁄4 plywood plates glued into cutouts
in the fuselage with Goop glue. The tap is a
handy tool for cutting threads in material.
After installing the plywood blocks, align
the wing in its final position and drill through
it and into the blocks. Tap the threads in the
blocks. Drill out the wing holes to clear the
bolt diameter. You can further harden the
threads with a dribble of thin cyanoacrylate
glue and retap the threads.
On a foamie wing you’ll need something
to keep the foam from being crushed as you
tighten the bolts. On my Kawafoamie I
mounted a 1⁄32 plywood plate to make a
backing for tightening all three bolts.
I learned another method of making hard
points for bolt-on wings from the new
Composite Systems Development
(www.rcglider.com) Slope Scale airplanes.
Drill out a large hole, maybe 3⁄4-inch diameter
or more, and fill it with epoxy and
microballoons or epoxy and milled fiberglass.
Drill the epoxy plug to allow the mounting
bolts to get through.
If you aren’t using taps in your shop
yet, give them a try. The modern world is
held together with threaded fasteners, and
adding threads wherever you want adds
versatility to your model construction and
modification. MA