March 2001 93
thE CoMMoN thread in this column is
that all the stuff is related (at least
tangentially) to DJ Aerotech products.
I’ll detail John Roe’s contest setup for the
DJ Aerotech Wizard. It made a huge
difference on my airplane that had been set
up in a more standard manner. This is a good
lesson, even if you don’t have a Wizard.
I’ll show neat modifications for wings,
demonstrated on a Wizard.
Then I’ll give some insight into why DJ
Aerotech’s new Spectre-series gliders fly
well with slender wings.
DJ Aerotech has been producing composite
Hand-Launched Gliders (HLGs) for many
years. I had a Monarch D-Lite and I loved it.
The Monarch had an excellent glide ratio
and forgiving handling. This polyhedral
airplane was easy to fly, with wellcoordinated
thermal turns. It was also highperformance
for its time.
After reading the marketing hype, I
Mike Garton, 506 NE 6th St., Ankeny IA 50021; E-mail: [email protected]
RADIO CONTROL SOARING
Joe Hahn’s Spectre VR “poly” heads for the clouds. Prototype weighs 5.1 ounces!
Joe mugs for a photo with his prototype Spectre 120. It has a
high aspect ratio, thin airfoils, and weighs 46 ounces.
Author’s modified Wizard wing has RDS, Trilerons, two-piece
retrofit. RDS pocket under address label on flaperon surface.
94 M ODEL AVIATION
bought a DJ Aerotech Wizard. It was
billed as a higher-performance model. I
measure the root airfoil thickness at
approximately 6.5%. The airfoil has very
little camber.
I read the directions, and set my Wizard
up with the recommended center of gravity
(CG) and control throws. First impressions
showed a very fast airplane, with a
somewhat higher-than-average sink rate. It
had no tip-stall tendency.
I often play “last man down” with my
local flying buddies. We all throw at the
same time, and the last man’s model down
wins the flight and gets a point. The first
guy to get 10 points wins. Playing this
game late in the evening will teach you to
fly smoothly.
My throwing arm is pretty strong, so I often
get a height advantage on launch. My Wizard
would start out higher than my buddies’
models and sink at a slightly greater rate.
Detail of modified Wizard wingtip. The
Trileron is the triangle-shaped control
surface that moves with the flaperons.
The RDS flaperon linkage is completely internal. The black line perpendicular to the
torque rod is where the author inserted a thin plywood bearing.
It quickly became evident that the
Wizard was also losing a lot of energy in
the turns compared to the other HLGs.
Wide, shallow turns helped, but any turn
still lost more energy than a turn of the
other HLGs.
It soon became obvious that the “drag
bucket” was very narrow. A narrow drag
bucket means the stock airfoil will produce
high drag if you get out of its (small)
preferred range of angle of attack. This is
to be expected with thin airfoils.
After many frustrating flying sessions,
I E-mailed John Roe. He made the flyoff
with a Wizard at the International Hand
Launch Glider Festival in 1999. I’ll
paraphrase his suggestions. (He assumes
the pilot is experienced.)
John likes to fly his Wizard with the
CG roughly 3⁄16-inch behind the aft limit
marked on the plans. The tail no longer
produces a down force, so the effective
wing loading goes down slightly. This
lowers the sink rate a small amount, and
it makes the model less stable in pitch.
The unstable condition causes it to react
more to lift.
Because of the rearward CG, John’s
airplane tucks (nose-down) at high speed.
He uses an up-elevator preset to launch.
The preset has enough up-elevator to make
the model pitch upward very quickly.
John uses a spring-loaded toggle
switch on the Airtronics Stylus, which
can be actuated by the index finger on
the hand holding the transmitter. He
activates and holds the up-elevator
preset before launch.
The airplane pitches up to an
approximately vertical climb after it leaves
John’s hand, then the preset is released.
The model ends up almost directly
overhead at the end of its climb. The upelevator
preset is held for roughly the first
half-second of launch.
Joe Wurts was a pioneer of this launch
method, and most top HLG pilots use it. I
launch approximately 10% higher when I
use this technique.
After adjusting the CG and elevator
trim to his liking, John uses roughly 3⁄16-
inch of down-flaperon for the minimum
sink condition. This is not intuitive; most
airplanes have their lowest sink rate with
flaps in neutral. Camber for minimum sink
is Trick #1 for the Wizard.
I told John I didn’t think I could turn
my model tight without losing half the
airspeed. He said he often pirouetted on a
wingtip in small thermals. Through
discussion, I learned that John uses 1⁄4 inch
of down-camber mixed into the elevator.
So when he pulls back on the elevator to
turn, he gets additional camber.
Lowering the flaperons to add camber
to a wing effectively moves the drag
bucket to a high coefficient of lift (Cl).
Another way to think of it is that the wing
can produce a higher C[sub]l, while
staying within its preferred angle of
attack range.
Trick #2: Mix camber into the up-elevator.
March 2001 95
John sets his aileron differential for
3⁄4-inch up and 1⁄4-inch down. He puts a
“massive” amount of rudder mix into the
ailerons.
Also, the V-tail reverse differential is
the most that the Stylus will allow (20%).
This means his model’s V-tail will give
more down-ruddervator than upruddervator
in response to rudder inputs
on the transmitter.
That’s the setup. John Roe’s suggestions
made my Wizard fly better, but it is still not
my model of choice. DJ Aerotech
discontinued the Wizard, but many of them
are still lurking in basements. Pull one out
and try these tricks.
one of my flying buddies got a Wizard the
same summer I did. We observed that our
flying skills improved a noticeable amount
in a short period of time.
In hindsight, the Wizard required
constant attention to angle-of-attack
control. I usually didn’t think much about
angle-of-attack control.
This particular airplane provided very
productive flying practice; it made me a
better pilot with all my gliders.
Eastern Iowa Soaring Society president
Jim Porter observed that the pilots who fly
full-house gliders 99% of the time can
generally fly Rudder-Elevator-Spoiler
gliders better than the pilots who fly RES
100% of the time.
An interesting idea is to practice at a
higher intensity level than you use in
competition. In particular, practice with a
model that is slightly unstable and has an
airfoil with a narrow drag bucket. That’s
another reason to get the Wizard out of the
basement. Most F3B airplanes would also
fit in this category.
The photos show a modified Wizard
wing. I used it to test a light version of
Harley Michaelis’ Rotary Driver System
(RDS). More information about RDS is
available on the Web site listed at the
end of the article and in my September
1999 column.
If you have not been to the RDS Web
site for a while, it is worth another look.
Now there are detailed instructions for
96 M ODEL AVIATION
retrofitting in molded wings, skin-hinged
wings, and spoiler methods.
The pockets for the HLG RDS were
made from 1⁄16 contest balsa, faced on one
side with 1⁄64 plywood. The plywood faces
the inside of the pocket. This lightweight
laminate replaced the heavier Formica™
(kitchen countertop material) I use for
larger airplanes.
The torque rods were 1⁄16-inch-diameter
stainless-steel welding rod.
The experiment was successful. The
light pockets fit in the thin wing, and they
held up fine. The two RDS linkages added
just 0.15 ounces to the weight of a stock
Wizard wing. I used the old “customspline”
method of joining the coupler to
the small output shaft of the CS-10s.
I have an identical Wizard wing set,
except that it has normal external linkages.
I also have stick time on stock Wizards, to
ground my comparisons.
Can I tell the difference in flight?
Yes. The RDS wing gets roughly 10%
longer flights in dead air. It definitely
launches a little higher and seems to have
a slightly lower sink rate. I did not have
the patience to make a large statistical
test, but I alternated wings enough to
become a believer.
Wind-tunnel testing of external
linkages showed that they add
approximately 10% to the drag of a typical
glider at all speeds. (A Web link is given
at the end of this column.)
I theorize that external linkages make
even more of a difference on a small
airplane, because the external linkage is
larger relative to the small wing.
Another modification I played with was
putting Robertson Trilerons on the end of
the Wizard flaperons.
Trilerons are small triangular-shaped
control surfaces at the end of the ailerons
(or flaperons). They were invented by
Cody and Jerry Robertson. I give a link
to Darwin Barrie’s Web site at the end of
the column.
The Trilerons usually have a “follower”
built into them, so they move with the
control surface next to them. My followers
are 7 mil Mylar™ on the top and bottom.
The Mylar™ is taped to the flaperons, and
extends roughly an inch onto the Trileron.
The Trileron has a pocket of Scotch™ tape
on it, to trap the Mylar™.
To construct the pocket, I used a large
piece of Scotch™ tape placed over a smaller
piece of upside down Scotch™ tape.
The Mylar™ sliding in the pockets
allows the flaperons to travel through 45°
up without any trouble. I gave the
flaperons this much possible travel so I
could experiment with spoileron mixing to
dethermalize the Wizard.
In hindsight, getting the Wizard to
come down was never much of a
problem. To be fair, the Wizard could fly
in winds that made the other airplanes
fly backward.
The Trilerons should have less drag in
turning flight. I have not been able to
measure the impact of Trilerons; they do
have a positive effect, but I have not
figured out how to quantify it.
Although it is easy to measure
hangtime in dead air by flying one huge
circle, the Trilerons should help most in
medium to small circles. When flying
small circles, the pilot and his stick inputs
are not repeatable enough for comparisons.
There is one more tangible benefit of
the Trilerons: they greatly reduce the
stress concentration that occurs at the
end of the aileron.
Models in the new Spectre series by DJ
Aerotech have high-aspect-ratio wings. I
calculated the aspect ratio of their wings
using span and area specifications from
the DJ Aerotech Web site, and they are
as follows: HLG, 12.4; VR HLG, 15.8;
Two-Meter, 11.8; 100 inches, 14.9; and
120 inches, 19.2.
Many aerodynamicists (including
myself) had shown with computer
simulations that the optimum aspect ratio
for a HLG is roughly eight, a Two-Meter
is close to 10, and a Three-Meter is
approximately 14.
So why would a bright guy like Don
Stackhouse design gliders with these
higher aspect ratios?
The optimum aspect ratio calculations
of the past often assumed an airfoil like
Film provided
Envelope 96
March 2001 97
March 2001 97
an SD7037, which is 9.2% thick.
Airfoils of this thickness develop a great
deal of drag when the Reynolds numbers
get below 60k.
In simpler terms, thick airfoils do not
work well on small chords and/or light
gliders. Free Flight guys have known this
for decades.
Little wind-tunnel data exists for
airfoils in the 5%-7% thickness range, and
there are two probable reasons. First, it
used to be believed that glider wings could
not be built strong (and still be light) at
these thicknesses. We glider fliers did not
ask Selig for thinner airfoils in the 1980s.
Second, there are technical difficulties
in obtaining repeatable wind-tunnel results
below Reynold’s numbers of 40k; the
flows are very unsteady. Audible noise
and other minute disturbances can affect
the results.
Pilots of light HLGs often notice that
their models have greater hang time in
slightly turbulent air. This is because the
airfoil is getting naturally turbulated.
With good engineering and a lot of
experimentation, Don has come up with
airfoils that are happy on small, light
airplanes. If we had wind-tunnel data on
5%-7% airfoils going down to the 30k
range, I am pretty sure the airplane
simulations would show higher optimum
aspect ratios.
The lowest aspect ratio (11.8) Spectre
is the Two-Meter. I speculate that it
needed more wing area to carry up heavy
winch line at contests. Unlimited airplanes
(with more wing area) don’t mind the
weight of the line, and HLGs don’t have
this problem.
In my analysis, the Spectre series
maintains good penetration by having lowcamber
airfoils and a moderate wing
loading. The models get a low sink rate
from a low span loading.
The Spectres also maintain energy well
in the turns because of the high aspect ratio.
As with the Wizard, a little camber mixed
into the elevator may be a good thing.
I have seen big and small Spectres fly in
competition. Like the Monarch, they seem
to combine high performance with friendly
handling. I can understand why DJ
Aerotech discontinued the Monarch and the
Wizard in favor of these new toys. MA
Sources:
DJ Aerotech
719 Fisk St.
Piqua OH 45356
(937) 773-6772
www.bright.net/~djwerks/
Rotary Driver System (RDS) information:
www.proptwisters.org/rds2/
Drag of external linkages information:
http://beadec1.ea.bs.dlr.de/Airfoils/linkage.htm
Robertson Trileron information:
www.users.qwest.net/~slickraft/tritext.htm
