April 2001 93
this MOnth’s Soaring column will
cover three topics. I’ll review the
International Hand Launch Glider Festival
2000 video, then I’ll describe a high-tech
scratch-built Hand-Launched Glider
(HLG) by Matt Gewain.
Last, I’ll explain the good and the bad
of switch-jacks. These clever devices
perform the function of a power switch
and a charging jack for the flight pack.
People either love them or hate them. I
will attempt to explain why.
the international Hand Launch Glider
Festival is held each June in San Diego CA.
It is hosted by the Torrey Pines Gulls club.
Last year, Paul Naton of Radio Carbon Art
took footage for a video—International
Handlaunch 2000.
Paul is the creator of the classic Endless
Lift and Endless Lift 2 tapes. His Radio
Carbon Art videos are a level above the
home-brew types. They are digitally edited
and they have sound tracks.
Paul has a Web site and he takes credit
cards via phone or secure Web order.
Everything about the production and
distribution is very professional.
The Hand Launch Glider Festival 2000
video is a “must buy” if you fly HLG. You
get to see highlights of the largest and most
competitive HLG contest in the world
without having to attend it.
Joe Wurts talks about his new HLG
designs in the video. You also get to see
sidearm launches and midairs. Some of the
footage was taken with a microphone on the
pilot, so you know what he and his helper
are doing during the round.
I give this video five stars for each
category: quality, content, and value. The
source is listed at the end of the column.
A couple photos show Matt Gewain’s
scratch-built HLG. Matt owns Composite
Structures Technology (CST) in
Tehachapi, California. His model could be
considered a flagship of the high-tech
materials he sells.
The thing that first caught my eye on
Matt’s HLG was the way it glows blaze
orange in flight. The built-up wing structure
was covered with light polyester tissue, then
Matt put fluorescent orange pigment in
some dope and brushed it on. The covering
was shrunk with heat after doping.
When the sun shown through the wing, it
glowed. Because the light could shine
through, it was much more striking than an
opaque surface.
Closer inspection of the HLG revealed
that it has a high-tech structure with a lot of
carbon. The model is very strong, and it
weighs eight ounces ready-to-fly. The ribs
are balsa capped with 0.007-inch carbon.
The leading edge, trailing edge, and wingtips
are .050-inch-diameter carbon rods.
A small piece of 1⁄16 balsa was added on
top of the carbon leading edge. This balsa
piece increased the leading edge radius
without the weight of a larger piece of
carbon. The wing spar is a 0.25-inch insidediameter
carbon tube.
The fuselage of Matt’s HLG used a
homemade molded Kevlar™ pod. The
tailboom had a shiny metallic appearance. It
was an aluminum, carbon, and aluminum (alc-
al) composite tube.
I asked Matt why the tube manufacturer used
aluminum instead of carbon. Matt’s education is
in aerospace, and he had a good answer.
Mike Garton, 506 NE 6th St., Ankeny IA 50021; E-mail: [email protected]
RADIO CONTROL SOARING
Matt Gewain’s (Tehachapi, California) high-tech built-up Hand-Launched Glider has
carbon, Kevlar™, aluminum, and a little balsa for the ribs. Text has details.
A carbon-fiber tube serves as a throwing grip on Matt’s HLG. Note the large-diameter
tailboom and carbon-capped wing ribs.
94 M ODEL AVIATION
The thin layers of aluminum met the
manufacturer’s strength criteria at a lower
weight than the available carbon weaves. The
carbon fiber in the tailboom was sandwiched
between layers of very thin aluminum. All
the carbon fibers are unidirectional and run
lengthwise in the tube.
This gives the tube good resistance to axial
and bending loads, but very little resistance to
torsion or crushing loads. For torsional
rigidity, the tailboom needs material that is
stiff at 45° angles to the axis of the tailboom.
Material stiffness perpendicular to the axis of
the tailboom would resist crushing loads.
Aluminum is equally strong in all
directions. The thin layers give the tailboom
enough torsional rigidity, and they have some
resistance to crushing.
Since seeing Matt’s HLG, I have obtained
one of the aluminum-carbon-aluminum
tailbooms to play with. The diameter of the
tubes (.56 inch tapering to .27 inch) are
slightly larger than those of kite spars or
arrowshafts. The al-c-al tailboom is stiffer
under bending loads because of its larger
diameter. This would help maintain tracking
during high-speed launches.
These al-c-al tailbooms are being used in
some of the high-performance prefabricated
HLGs. They are roughly the same weight as
common spirally-wrapped carbon tubes (of
smaller diameter).
I do not recommend the al-c-al tailbooms
for beginners; they are more prone to
breaking where they have been bumped.
Unlike the spirally-wrapped carbon
tubes, the al-c-al tailboom feels squishy in
my hand. It gives when I squeeze it
between two fingers. Rough handling or
hangar rash could give one of these
tailbooms a stress concentration. The more
common spirally-wrapped tubes are much
more durable from this standpoint.
Matt’s Web site has pictures of the
HLG wing structure before covering. Also
included is more of a description of how
he built it. All the exotic materials Matt
used to build the wing are available from
CST. The contact and address are at the
end of the column.
before i explain why some people have
problems with switch-jacks and some do
not, I’ll describe how they function.
The female plug of the switch-jack is
installed flush with the outside of the glider.
Inside are two leads—one going to the battery
and one going to the receiver. The female
plug has a shunt switch inside it. When the
male jack is inserted into the female plug, it
turns off the power to the receiver. If you pull
the male jack out, the flight pack is turned on.
The male jack has a 12-inch red ribbon
coming out of it, to provide a visual cue that
the plug is in the airplane and the power is
off. The male jack also has a lead coming
out of it, for charging.
The advantages of a switch-jack are:
• You do not need to open your glider to
turn it on.
• It is lighter weight than a switch and
charge jack. The standard Hollyday Switch-
Jack weighs five grams, and the Micro
Switch-Jack weighs one-and-a-half grams.
• The jack and ribbon provide a visible sign
indicating the state of the switch. You
virtually eliminate the problem of launching
with the radio off.
• You do not need to open the glider to charge it.
• It is better-looking and cleaner than a
normal external switch/charge jack.
The disadvantages of a switch-jack depend
on whom you talk to and how they implemented
it. Many respected glider authorities have used
switch-jacks for years with no problems. Other
glider pilots have had problems with shorts,
fires, and intermittent contacts.
Why the different stories?
It boils down to getting the right parts,
then implementing the system correctly. The
majority of the power and phono jacks on the
market are not reliable for this application, but
a few can work when set up properly.
Even those who love the switch-jacks
will tell you they are not suitable for
power airplanes. Vibration and grease
could cause problems. These fliers also
admit that a self-wiping contact, mil-spec
blade switch would be safer.
Do I trust a switch-jack for my
expensive gliders?
Yes, but only if I buy it from Hollyday
Designs. There is quite a difference in
quality between these jacks and the common
consumer-grade jacks. I have yet to hear of a
Hollyday unit failing.
While researching this topic, I found a
whole list of things that can and do go
wrong with the inexpensive consumer-grade
phono jacks sold in retail stores. If you try to
make your own, be prepared to do a lot of
research and development.
Just because the jack fits in the plug and
the circuit powers up properly on your
bench does not mean it is reliable or safe.
One of the problems is that the jacks and
plugs you buy in the store have few or no
specifications on the box. If you have to ask,
you are probably not going to like the answer.
The contacts on the consumer-grade plugs are
of unstated metallurgy. The contacts on the
Hollyday Switch-Jacks are plated copper alloy.
Most companies don’t list the power
rating of the contacts, but it is important. If
you use too much power for the contacts,
they will develop pits and eventually
become intermittent. This could cause
power loss during flight.
Hollyday’s standard jack was rated for
five amps continuous at 12 volts DC
resistive. Hollyday’s mini switch-jack is
rated for one amp continuous at 12 volts DC
resistive. The amperage rating would go up
if the manufacturer took the time to rate
them at six volts.
To be conservative, Rich Hollyday
recommends the mini jack only for twoservo
HLGs. His standard Switch-Jack is
good enough for full-house contest gliders.
Some of the readily available plugs
quickly suffer from metal fatigue on their
exposed leaf springs. Hollyday’s are rated for
10,000 insertion/withdrawal cycles minimum.
There is variability in the length of the
inexpensive male jacks. The jack can short
Hollyday Designs’ Switch-Jack replaces a switch and a charging
jack. The wooden square with the hole is for mounting.
This is the Micro Switch-Jack. It weighs 1.3 grams, and is
suitable for Hand-Launched Gliders with two servos.
if it is not long enough. This could
happen as the airplane jostles around in
your car on the way to the field.
The female end of the system needs
more insulation than the standard phonoplug
part. This will help prevent shorts
in the event of a crash. Darwin Barrie
(one of the proponents) insulates the
critical areas with epoxy.
When compressed in a crash, an
uninsulated phono plug will short and cause a
“meltdown.” Some can also rotate relative to
their enclosures, causing a short. The Hollyday
plugs are enclosed, protected with heat-shrink,
and do not rotate inside their housings.
I don’t mean to sound like an
infomercial for Hollyday. However, I do
think Rich has figured out the right way
to implement this concept. The fact that
other people have had failures with cheap
phono plugs does not invalidate the idea.
You can find more information,
specifications, pictures, and instructions
on the Hollyday Web site (address at the
end of the column). MA
sources:
International Hand Launch Glider
Festival 2001 (June 3-4) information:
Torrey Pines Gulls
www.TorreyPinesGulls.org/
Radio Carbon Art (IHLGF 2000 video):
Box 2311
Corvallis OR 97339-2311
(541) 752-9661, 10 a.m.-6 p.m. Monday-
Friday (Pacific time)
www.radiocarbonart.com
More information on Matt Gewain’s
HLG:
www.cstsales.com/CarbonRodApp.htm
Switch-Jacks:
Hollyday Designs
4709 Randolph Ct.
Raleigh NC 27606
(919) 832-0199, 9 a.m.-5 p.m. Monday-
Friday (Eastern time)
www.hollyday.com/
