May 2001 97
This monTh I’ll feature some of Carl (Mac) McBurnett’s latest
airplanes. Mac manufactures composite gliders in San Antonio TX.
His Two-Meter Dartar was released roughly a year ago; in the fall
he worked with Pat Bowman to create a prototype airplane called the
DS-Demon, expressly for Dynamic Soaring; and the Texas Twister
discus-throw Hand-Launched Glider is Mac’s latest addition.
The Ds-Demon is the most exotic of the new airplanes. It was created
for the relatively new sport of Dynamic Soaring (DS), which is a way
of flying models through wind gradients to extract energy from them.
Imagine flying loops in a region where the air is moving faster up
high. The top of the loop is flown in the same direction as the fastermoving
air. Like the water wheel on an old mill, energy can be
extracted from the fluid.
A couple years ago Joe Wurts demonstrated that this could be
done on the downwind side of Parker Mountain (in Southern
Oregon) with a Radio Control (RC) glider.
When conditions are right, the airplane
can build up to an excess of 150 mph with
four or five circles. Pilots report a huge
adrenaline rush. The speed is addictive. If
the pilot keeps turning circles with the
airplane, it eventually reaches a speed at
which structural failure occurs.
To date, there has been no glider that
could DS indefinitely on a windy day at
Parker Mountain. F3B airplanes are the
natural candidates, but even they blow up
when pushed too hard. That is where the DSDemon
comes in.
Pat Bowman designed the Demon and Mac
built it. It has a 108-inch wingspan with an
RG15 airfoil. Flying weight is six-and-a-half
pounds, and most of the weight is carbon in
the wings. The model was computer-designed
for 60 positive and 25 negative Gs. It has been
flight-tested at Parker on several occasions.
One day with the Santa Ana winds
howling, the stabilizer bellcrank broke.
Mike Garton, 506 NE 6th St., Ankeny IA 50021; E-mail: [email protected]
RADIO CONTROL SOARING
Front the view of Twister. The thin airfoils and small fuselage diameter keep the frontal
area very low. The model utilizes an SD6063 airfoil.
The Texas Twister Hand-Launched Gl ider. It weighs
approximately 8.5 ounces, and is stressed for wing launching.
The Twister in flight. Because of its speed, it is recommended for
intermediate and expert pilots. Scott Miller photo.
Forces on the tail get extreme doing loops at 150 mph. The wings
and fuselage survived the mishap, and the Demon has come back
for more. Now the airplane uses a 1⁄4-inch carbon joiner rod just
for the stabilizers!
A Web link at the end of the column has pictures and “mpeg” video
clips of the Demon in action. While viewing the video clips on your
computer, make sure you turn on the sound to hear the airplane scream.
Right now the Demon is just a prototype. The model will go into
production after Pat and Mac are satisfied with the flight-test program.
There have been many technological advances in Formula 1 race
cars that have trickled down to production cars. The same thing
happens in RC soaring; F3B has driven development of fast molded
airplanes. Now Dynamic Soaring is the harshest test environment
available for Soaring.
Mac is applying some of the things he learned on the DS-Demon
project to his other production airplanes. Both of his new production
98 M ODEL AVIATION
models have a great deal of carbon and few
stress concentrations.
The Texas Twister: This 1.5-meter Hand-
Launched Glider (HLG) was designed for
discus-style launching. Right-handed
throwers grip the left wingtip with their
palm facing down. After spinning around
once, the airplane is released. Some pilots
choose to throw without the body spin, by
just twisting their trunks.
The Twister uses a highly thinned
SD6063 airfoil, which is one of the fastest
glider airfoils available. I would wager that
this airplane can beat 60-inch-span Slope
Racers when ballasted to the same weight.
The Twister has less than 2/3 the frontal area
of an RG15-equipped Sloper. With the
trilerons, it probably turns cleaner too.
The planform is double-taper with
moderate aspect ratio. Mac precuts skin
hinges, including trilerons for the
flaperons. I like trilerons and was happy to
see them on a production kit. Special high-density blue foam and
large carbon darts were required to stand up to the stresses of
discus-launching.
Many design-test iterations went into this airplane. Each time the
model broke, Mac would change the design a little, build a new
airplane, and continue the test program. The models finally stopped
breaking after five generations.
The fuselage is much like a long fiberglass tube reinforced
with carbon. Most of the cross-sections are circular. This fuselage
is stiffer than the pod-and-boom types; it is the stiffest HLG
fuselage I have seen. Strength is similar to that of the 1999
The Two-Meter Dartar from Texan Carl “Mac” McBurnett. It
weighs roughly 25 ounces and has extreme wing strength.
Servos are strapped to a Popsicle® stick and bolted to the bottom of the model’s
fuselage. This method is easy, lightweight, and avoids stress risers.
Tai l detai l of the Dartar. Spectra pul l-pul l l ines exit the
fuselage in front of the tail.
vintage Maple Leaf airplanes (very strong). Carbon pushrods are
standard for this kit.
The Twister’s V-tail is vacuum-bagged fiberglass over foam
with large doubled carbon darts. It mounts into a saddle on the
bottom of the fuselage. Mac tells me that the tail-fuselage joint is
critical on this airplane. The surface must be well scored. A twohour
epoxy is recommended.
To better visualize the forces applied to the tail of a discuslaunch
airplane, imagine sticking the tail of an HLG out the window
of your car at 55 mph. Then imagine turning its tail so it is not
aligned with the wind. You get the idea.
Advertised flying weight is 8-8.5 ounces with specific radio gear.
My Twister came in at 8.6 ounces using four CS10 servos, a Berg
receiver, and a four-cell 110 pack. The nose is long enough to
balance with any battery. Mac usually puts his servos under the
wing, but they can be moved way forward if necessary.
An up-elevator preset is used for launch. This airplane does not
need a rudder preset; it tracks pretty well without it. The Twister is
zippy in flight. It is intended for intermediate to expert pilots.
Most of the tricks John Roe taught me for a DJ Aerotech Wizard
(see the March column) apply to this airplane. I would not move the
center of gravity behind four inches from
the trailing edge. I originally tested mine
with a three-cell battery. I replaced it with
a four-cell pack to get faster servo speed.
There is a great deal of technique and
finesse in discus-throwing. Chris Oster has
been reporting consistent dead-air times of
one minute, 20 seconds with his Texas
Twister. Imagine how much sky this fast
HLG covers in one minute, 20 seconds!
You cannot expect to reach your full
potential the first few times out. It is
important to keep an open mind, study the
better throwers, experiment systematically,
and practice a lot.
My first day discus-throwing, the
throws were only roughly 45 feet, resulting
in 40-second flights. Then again, the 26
inches of December snow and winter gear
made spinning difficult.
Multiple sources tell me that once you
have the technique down, the discus
throws will be approximately 50% higher
than good javelin throws. If you are not
very good at the javelin throws, your
discus throws may be 100% higher. They
put less stress on the body, but more
stress on the airplane.
The Twister is the first commercially
available discus-launch airplane with
flaperons; as of my January deadline, it is
still the only one with flaperons. The
flaperons will be especially helpful in
turbulent conditions.
Five degrees of camber increases the
climb rate in a thermal. I was able to
climb out quickly, even over the snow
cover. Forty-five degrees slows it to a
crawl for landing. A couple degrees of
reflex can be used for the launch.
Once you get the discus technique
down, this airplane will be a serious
contender in HLG competition. Just when
I thought HLG technology had plateaued,
whammo! The bar gets raised a couple
more notches.
The Dartar Two-Meter is a much tamer
airplane than the Twister. Its low wing
loading and SA7035 airfoil make it very
friendly. Like the Twister, this Two-Meter
can be built in seven to 10 hours.
Wing control surfaces are precut with
skin hinges. The servo holes are
prerouted, including a hole for wiring.
The wing comes as halves. The builder
joins the two parts to make a one-piece
bolt-on wing. It is vacuum-bagged
carbon and fiberglass skins over foam.
This Two-Meter has more carbon in the
wing lay-up than any Two-Meter I have
ever heard of. There are six large pieces of
unidirectional carbon stacked on the top
skin. The first full-span piece starts at the
leading edge and ends at the controlsurface
cutouts. Each of the five additional
darts is approximately an inch narrower.
The bottom of the wing has roughly half as
much carbon. Most manufacturers would
make three or four airplanes with that much.
The wing structure has been prooftested
with full pedal-to-the-medal
May 2001 99
launches with a “real-balls” ball-bearing
winch in 35 mph winds.
The Two-Meter Dartar’s fuselage is
fiberglass with carbon reinforcement. It
has a shallow saddle in the top for the
wing and a slip-on nose cone. I replaced
the two stock metal wing bolts with nylon
ones. The wing should shear off in the
event of a hard crash, minimizing damage
to the fuselage and the wing.
Mac is known for using minimal
epoxy resin to make a part. My model’s
fuselage weighed 3.2 ounces before I
installed the radio gear. It is strong
enough for a decent pilot. Those pilots
who cartwheel often would be wise to
request a beefier lay-up from Mac.
The Two-Meter Dartar can be ordered
with a conventional or a V-tail. The tail
saddle is flat, to allow mounting the
conventional stabilizer. I used triangular
balsa to cradle the V-tail on my airplane.
The conventional and the V-tail are
made from fiberglass vacuum-bagged
over thin Spyder Foam. The cross-section
of the tails are almost flat plates. The top
and bottom are flat, but there is more
thickness at the leading edge. It is light,
and it seems to work fine.
The Two-Meter Dartar and the Twister
come complete with a big bag of hardware.
They even include twisted servo wire and
Deans connectors for the wing servos.
Spectra pull-pull lines are used to drive the
tail. When Mac told me the pulls-pulls were
working fine during the Two-Meter’s DS trials,
it convinced me to use them. Mac also
included carbon pushrods, but he warned that
they are heavier than the pull-pulls.
With a painted fuselage, larger-thanstock
battery, and extra wood in the tail, my
model came out at 27 ounces. This is 11
ounces lighter than my last full-house,
zoom-launching Two-Meter. The Dartar
Two-Meter would have weighed 25 ounces
if I had built it completely stock and used
the recommended radio gear.
In flight, the Dartar Two-Meter is quite a
floater. It can turn very tight and work the
lightest lift. Mac chose a fairly conservative
triple-taper planform with a touch of washout
so the model does not tip-stall, even during
slow camber-on circles.
This airplane is a blast to hand-launch.
It takes me less effort to catch a thermal
from a hand-launch than it does to set out
the hi-start. The slow speed makes
landings easy.
The Dartar must be ballasted for windy
days. Ballast is bolted into the fuselage
above the towhook.
If you have ever wished for a prefab
airplane that can climb out easily from a
hand-launch and do hard zoom-launches,
here it is. MA
sources:
Link to DS-Demon pictures and online
video clips:
www.bludartar.com/demon/demon.html
Blu Dartar Two-Meter and Texas Twister:
Carl McBurnett
7506 Legend Point Dr.
San Antonio TX 78244
(210) 662-9503
www.bludartar.com/
