Radio Control Slope Soaring - 2010/02

110 MODEL AVIATION
because it ran out of onboard electrical
power—a “preventable” crash, which makes
for greater gnashing of teeth and more
colorful cussing during the postmortem
review.
In addition to replacing the receiver
battery pack, I needed to fix or replace three
stripped servos and repair a cracked spar.
Since you might have models that need
similar repairs, I’ll document the steps I used
to get this well-loved sailplane back to flying condition.
It’s not hard to diagnose a cracked spar; hold the wing tight to the
bench and gently lift the tip. If instead of flexing in a broad curve
along the span, the flex is concentrated at a point, that point is where
spar strength has failed or is otherwise compromised.
Start by removing the covering and the strapping tape, to get a
look at the wing’s parts. Chances are, you’ll know in very few
minutes where the fracture is.
The next step is to design a technique with which to restore the
spar’s strength. Keeping the weight of the repair low will be a plus.
For this Ka-6 fix, the method I used was to splint the spar with
spruce, using wood pieces that extended 3 inches on either side of the
break.
To have room to install the splints, remove some EPP foam with
either a rotary tool or by hand with a blade and long-nose pliers. I
used a Dremel Roto Tool with a cylindrical steel cutter bit to hollow
out a channel adjacent to the spar. Don’t be afraid to dig in here;
you’ll make it smooth again later.
I shaped the splints to fit close to the spar and glued them in with
polyurethane glue. (Gorilla Glue is one brand, and Elmer’s
Ultimate Polyurethane Glue is another.) That adhesive makes an
incredibly strong bond between porous materials, and it expands
slightly, helping to fill in the space left from grinding out wing
foam to get to the break.
[[email protected]]
Radio Control Slope Soaring Dave Garwood
Foam-wing cracked-spar repair
Also included in this column:
• Easy servo-gear replacement
• Where the slopes are in 2010
• The JART is here
Joe Chovan (Syracuse NY) and Dave Garwood (Albany NY) fly Leading Edge Gliders 60-
inch Lockheed P-80s over Wilson Lake at the 2008 Midwest Slope Challenge. Alex Paul
photo.
David Day (Crosby TX) launches his Dave’s Aircraft Works 120-
inch-span Ka-6E foamie over Wilson Lake from the “main hill” at
Lucas Park during the 2009 Midwest Slope Challenge. Paul photo.
The author’s Ka-6E foam-wing spar fix in glue-curing stage.
Cracked spar and sub-TE will be splinted. Water jug presses on
glued parts; sandbags keep wing flat against bench so it’s straight
after repair.
Ka-6E wing’s stress riser points determine where parts will break
when stressed. First is at end of the sub-LE; second is on sub-TE
where glued-on TE stock ends and aileron cutout begins.
I’M STILL REPAIRING my favorite long-span slope cruiser: a
Dave’s Aircraft Works 120-inch-span Schleicher Ka-6E EPP foamie.
It looks great in the sky, and it flies so smoothly.
You may remember it from a previous mention. I crashed,
02sig4.QXD_00MSTRPG.QXD 12/21/09 9:44 AM Page 110
I let the glue joints cure overnight with the wing weighted and flat
on the workbench. After using polyurethane glue, clean up with
mineral spirits—in a properly ventilated area, of course.
The next day, grind or sand off the excess glue and fill the
remaining cavities where necessary, using foaming glue or spackle.
In addition to the cracked spar I had a cracked sub-TE, so I cut
another slot, fashioned another splint, and glued it in and let it cure
during the second night. Finish by filling voids where they remain,
retaping and sanding smooth, and finally re-covering.
The spar and sub-TE are now stronger than the originals at the
fracture points, and the places where they will break in the next crash
have been moved a few inches. That is because I moved the stress
riser points, or the places where reinforcing strength ends suddenly,
and that generally defines where the break point will be when the
structure is overstressed.
It’s common to stiffen EPP-foam wings with spruce spars running
parallel to the span. And since more strength and toughness are needed
close to the fuselage, and lighter tips enhance roll quickness, a
conventional design is to fit “spar doublers” along part of the spar’s
span, usually in the center portion of the wingspan.
If the spar doublers have square ends, they create a classic stress
riser and basically define where the spar will break in a “catch a tip”
landing.
The foam-wing kit builder can spread this stress concentration point
and make the wing more flexible by tapering the spar doubler part over
9-12 inches of its span at each end.
Then the wing can flex more, which reduces the chance of its
breaking in a specific spot. An additional advantage is that the wing’s
outer portions will be a bit lighter and the model will roll slightly faster.
112 MODEL AVIATION
hold the event at Cape Blanco in Port Orford,
Oregon.
4. Kiona Publishing/RC Sport Flyer
Magazine will hold the Alpine Soaring
Adventure at Wallowa Lake in Joseph,
Oregon. It is scheduled for July 1-4.
5. Soar Utah will probably take place
during Labor Day weekend. The
Intermountain Silent Flyers will host this
event at Point of the Mountain in Sandy,
Utah.
A New Kit: Reed Sherman at JARTWorld
will be shipping a 70-inch-span short kit that
features a molded-fiberglass fuselage with
spiderfoam wing and stabilizer cores. The
fuselage is molded by Jeff Fukushima
(Vortech Models), who is a legendary slope
sailplane designer.
This design adds a new airframe size to the
sleek JART line, which started as a free plans
set and spread worldwide. The model has been
built from more combinations of materials,
including wood, EPP foam, fiberglass, carbon,
Kevlar, and by more builders than any other
slope sailplane that I can recall. The JART is
its own phenomenon. MA
Sources:
Do-it-yourself servo-repair article:
http://findarticles.com/p/articles/mi_qa3819/i
s_200006/ai_n8892199/?tag=content;col1
South Bay Soaring Society
www.sbss.org
Midwest Slope Challenge
www.midwestslopechallenge.com
Southern Oregon Slopiens
www.clubsos.itgo.com
RC Sport Flyer Magazine
http://rc-sf.com
Soar Utah
www.soarutah.org
JARTWorld
www.jartworld.com
League of Silent Flight
www.silentflight.org
Stripped servo gears are often a
consequence of a crash, and replacing or
repairing the servo is part of the restoration
process. Lucky for us, it’s not a tough job;
that’s why they sell replacement gear sets.
You can identify stripped gears in a servo
by its “cricking” noise when powered up and
the transmitter stick is moved, plus you may
notice irregularities in the way the linked
control surface moves. On a badly stripped
set of servo gears, the control arm will turn
freely with the servo powered off.
Start by gathering tools and materials
needed to perform the job: small-point
screwdrivers, tweezers, the replacement gear
set, and servo-gear grease. What, you forgot
to order servo-gear grease when you ordered
the gear set? I’ve used lithium grease from
the auto-parts store with no detectable bad
effects.
Don’t leave grease out of the repair;
lubricating the gears not only reduces gear
wear, but it also reduces gear noise. Working
over a low-bounce surface, such as a terry
cloth towel, and working in a well-lit area
makes it easy to find small dropped parts.
To remove the servo-case top cover, take
off any servo arms and loosen the four long
screws at the bottom corners of the case. A
bit of tape can be applied to keep the bottom
cover in place; you’ll save time by not
having to fit that back. A gentle wiggle and a
pull lifts off the top cover.
There might be a close fit around the
main shaft. The end of a servo arm can be
pressed down on this gear while you lift the
top case; this is often the smoothest way to
facilitate removal.
Look carefully at the gears’ arrangement.
Notice how they mesh and fit. Carefully
remove the gears from the servo body, and
remove any bearings from the gears that
might be present. In a pinch, a similar servo
can be used as a reference. Recording the
gear layout in a digital picture may save a
headache come reassembly time.
Removal order will likely be obvious or
easily learned by experiment. Sometimes two
gears are removed together, as on the JR 341.
After removing the gears, examine them for
missing teeth. Some broken teeth will be
easy to see, and some could require a
magnifying glass. Remove the damaged teeth
with tweezers or a tissue; you don’t want them
to jam the new gears.
Lay out the removed gears in order as a
reference for reassembly, but don’t reinstall
used gears. Use the new gears from the
replacement set.
Assemble the gears in the opposite order in
which you disassembled them. Refer to your
spare servo, notes, or pictures if needed.
Lightly lubricate the pin shafts before you
slide gears over them.
After assembly, lubricate the gear teeth. A
toothpick works as a precision grease
applicator. I use commercial servo-gear grease
when I have it, and white lithium grease in a
tube when I don’t. Use a light touch with the
grease, but work to assure complete coverage.
Once the moving parts have been greased
and all pins and gears are in their correct
locations, slide the case together, gently
tighten the screws, and inspect for a good fit.
Do not force the case together; when
assembled properly, it should slide smoothly
into place. Don’t overtighten the case screws;
it can cause the gears to bind.
When reassembly is complete, plug the
servo into a receiver, turn on the transmitter,
and check for proper operation. This is a good
time to install the servo arm in the neutral
position. Your first gear swap might take a
half-hour, but it soon becomes a 10-minute
job and you’ll have the satisfaction of
restoring the servo to its original function.
Slope Soaring Events for 2010: Following is
the list as of the time of this writing, in early
November. Some Soaring clubs have yet to
complete their planning meetings, and I’ll
update the list for the April 2010 issue.
1. The Los Banos Scale sailplane event is
scheduled for April 16-18. The host club is the
South Bay Soaring Society, and the location
will be the Los Banos reservoir in Los Banos,
California.
2. The dates for the Midwest Slope
Challenge are May 15-18, and it will take
place at Wilson Lake Reservoir in Lucas,
Kansas. The Wings Over Wilson flying club
is the host.
3. The Cape Blanco Slope Fest is
tentatively set for the weekend of August
21-22. The Southern Oregon Slopiens will
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