Sccrattcch-65
Electric-powered RC indoor
sailplane/trainer showcases new
miniature Cirrus Micro Joule components
A FEATURE THAT appeared in my MA “From the Ground Up”
installments was a construction article for a model that an RC
beginner could build strictly from raw materials without the aid of a
kit. We call this technique “building from scratch.”
The airplane I designed expressly for this feature was the
Scratch-One. It was presented in two parts, in the November 2003
and January 2004 issues. From all the photos that were sent in, many
of you built this little RC electric-powered training model with
excellent success.
Jumping ahead a little, I wrote another feature article titled
“Introduction to Indoor RC” (July 2004 MA). Right after it was
published, the people at Global Hobby Distributors sent in samples
of their newly updated Cirrus Micro Joule miniature RC-system
components.
This equipment was first made available well more than a year
ago. More recently, based on field experience, several modifications
were made to further enhance this micro RC equipment’s
performance. I was provided a Micro Joule receiver, three
microservos, and two types of tiny electric motor speed controllers.
My usual routine would be to photograph these components,
operate them, and report on them using a standard product-review
format. The thought came to mind that there aren’t very many RTF
or ARF models available for RC aircraft that weigh only
approximately 2.5 ounces. If I could design a simple model that the
average indoor RC beginner could easily construct, I could be
providing a specific application for these new Cirrus Micro Joule
components.
With the Scratch-One’s success so noted, I thought it might be
fun to scale it down to a size that would accept the Cirrus Micro
Joule equipment, a tiny electric motor, and several equally tiny Li-
Poly batteries, all in an aircraft nearing that 2.5-ounce total.
The result is this “Scratch-65”: a Scratch-One at 65% of its
original size. The wingspan worked out to roughly 30 inches, and the
area is slightly more than 100 square inches.
So settle in and read my showcase of the new Cirrus Micro Joule
miniature RC components, and then read on for the full (and simple)
construction article for the little Scratch-65. It has proven to be an
excellent flier and another perfect RC trainer.
Global Hobby Cirrus Micro Joule RC System: My sample airbornesystem
components consisted of the Cirrus Micro Joule four-channel
miniature FM receiver, two Cirrus Micro Joule CS-3
ultramicroservos, and two new 5-amp-rated ESCs: the S5A1 and the
S5A2. These items can be purchased separately.
Micro JST connectors with white plastic housing are used on
these RC components. There really isn’t any established
convention yet for the pin wiring on these tiny connectors. On
these Cirrus types, the center pin has a red wire for battery
positive. Gray wires are used for the two “outboard” pins that
contain the battery negative and the servo signal wire.
When you are using all Cirrus Micro Joule components, there
will be no problem with connector-pin polarity. But if you mix and
match from other manufacturers, you had best check the pin polarity
before you turn the power on for the first time.
The connections from the ESC to the motor and the battery pack
use the larger, red, plastic JST connectors, which adhere to the
industry-accepted polarity convention (pin No. 1 black negative and
pin No. 2 red positive).
The Micro Joule four-channel-function receiver has automatic
shift select, so it can basically be operated by any brand of RC FM
transmitter. Voltage range is claimed to be 3.5 to 7.0. The weight is
slightly more than 3 grams (or .11 ounce). Dimensions are 1.11
inches long, 0.31 inch wide, and 0.33 inch high. The crystal clearly
sticks out farther than this height dimension.
The antenna wire is 34 inches in length. I used the full-length
antenna to obtain a baseline check on the receiver’s range capability.
Later I will substitute an E-Cubed RC short antenna and report on
the effect of the radio reception.
This is a single-conversion receiver. The specifications simply
state that it is “narrow band.” I resorted to a couple of simple field
tests to verify that claim. I was able to put the Scratch-65 almost out
of sight (on a calm day!) and was able to maintain perfect control.
Then I had a club member set up his synthesized RC transmitter
one channel away from mine (that is only 20 kHz). I flew the model
down the length of the runway, allowing it to get much closer to the
other transmitter than to my own. Both transmitter antennas were
fully extended. I did this on several occasions and noticed no
interference of any kind.
Tom Hunt about to launch the Scratch-65, with Bob behind the
camera. This shows you how small the model really is.
February 2005 31
by Bob Aberle
Photos by the author and Mark Lanterman
A comparison of the Speed 400-powered Scratch-One and the
smaller Scratch-65. Both are easy to build and fun to fly!
It’s hard to tell the Scratch-65’s size in this flight shot. These small
models are stable and fly in much the same manner as their larger
cousins.
Global Hobby Cirrus Micro Joule RC micro components (bottom to
top): four-channel receiver, S5A2 ESC, two CS-3 ultramicroservos.
Total weight of this equipment is .36 ounce!
Cirrus Micro Joule four-channel receiver with dime for size
reference. Crystal must be purchased separately. Receiver
weighs 3 grams, or .11 ounce!
Different view of equipment in above right photo (bottom to top):
CS-3 ultramicro proportional servo with its separate cable, fourchannel
receiver with coiled receiver antenna wire at left, and one
of the two new 5-amp ESCs.
During an entire morning at my flying field, I made several longduration
flights. While I did this, many of my clubmates flew on a
variety of other 72 MHz RC channels. This Micro Joule receiver
never got hit! Keep in mind that this is not a laboratory test; it is
merely a practical test based on everyday-type flying. The receiver’s
street price is $39.99, and the required crystal is another $12.95.
The little CS-3 servos turned out to be lighter than the claim. I
measured one at 2.1 grams, so the two I used came to 4.2 grams, or
.15 ounce.
The cable going from the servo to the receiver is a separate item.
You plug one end into the servo and the other end into the receiver.
The length of the cable is slightly less than 3 inches. Other cable
lengths and cables that adapt to other equipment can be purchased
separately.
Voltage requirement for these servos is 3.0-7.0. At 4.8 volts, the
claim is for a 14 inch-ounce output torque. Transit time at 4.8 volts
is .18 second for 60° rotation.
The first issue of these tiny servos tended to be noisy, and the
neutral centering was observed to be inaccurate; the servo never
returned to exactly the same neutral position. This new version of
the Micro Joule servo seemed quieter. The neutral-return accuracy
also seems better. I can’t say it’s “dead on,” but it’s certainly
acceptable for anything except a high-speed, highly maneuverable
aircraft.
I don’t expect to see many airplanes in this category weighing
just 2.5 ounces! The street price for an individual Micro Joule
servo is $49.99, which is impressive for a 2.1-gram, fully
proportional-feedback servo.
32 MODEL AVIATION
Closer look at four-channel FM single-conversion receiver. It will
auto-select proper FM shift to work with any existing RC FM
transmitter.
Closer look at CS-3 servo. Cable is a separate item. You can
purchase longer cables or even cable adapters that will allow you
to use these servos with other RC components.
S5A1 ESC intended for use with one Li-Poly battery or two to
three Ni-Cd/NiMH cells. Red plastic JST connectors go to
battery and motor.
Peak Electronics unit is Bob’s favorite charger for smaller Li-Poly
batteries. It can handle one, two, or three Li-Poly cells at discrete
100-1500 mA charge currents.
Type: RC indoor sailplane/trainer
Wingspan: 30 inches
Power: GWS LPS B2C-C motor with 6.2:1 gear drive
Flying weight: 2.69 ounces
Construction: Balsa and plywood
Covering/finish: Dupli-Color spray enamel and Solite
Scratch-65
The first-issue Micro Joule ESC had a BEC regulator set for a
low 3.0 volts. Most of us felt this was done to accommodate
possibly a problem in either the receiver or servos. The regulated
voltage is now back up to the more appropriate 5.0-volt level. (I
verified this point!)
The cutoff-voltage level on the first Micro Joule ESC was set
low. It definitely wasn’t conducive to using Li-Poly batteries, where
it is important not to discharge to less than 2.5-3.0 volts per cell.
To counter this problem, Global has introduced two new types of
ESCs. Model S5A1 is intended for use with one Li-Poly battery or
two to three Ni-Cd/Nymph cells. The cutoff is set at 2.7 volts.
Continuous current rating is 5 amps, which is more than enough for
this application. It weighs 1.9 grams (.067 ounce) and has a street
price of $29.99.
The model S5A2 ESC is intended for use with two or three Li-
Poly batteries or three to six Ni-Cd/NiMH cells. This is the one I
used on the Scratch-65. It weighs 2.78 grams (.10 ounce) and
measures .74 inch long, .31 inch wide, and .25 inch high. That
weight includes one micro JST connector that goes to the throttle
port on the receiver, along with male and female red JST connectors
that go to the battery and motor. The BEC regulated voltage is again
5.0 volts, and the continuous current rating is 5 amps.
The voltage cutoff on this particular ESC is automatically set by
February 2005 33
All wood parts necessary to build Scratch-65. Almost entire model
is constructed from full 1⁄16 x 3 x 36-inch balsa sheet and roughly
half sheet of 1⁄32 balsa.
Wing under construction. LE is 1⁄8-inch-diameter hardwood dowel.
Center-panel spar is 1⁄16 x 1⁄4 spruce for added strength. There are
no wing ribs—only bent sticks of balsa.
Fuselage sides assembled with two formers and spacer sticks.
All tail-assembly pieces are made from 1⁄16 balsa sheet.
Wing panel ready to be joined, with fuselage sides with stiffeners
in place. Fuselage sides are made from 1⁄16 balsa sheet.
Scratch-65 in Action
In early November I attended and flew in what is probably
the biggest indoor RC Electric fly in the USA—the JR Indoor
Electric Festival—which was held in Columbus, Ohio. A
complete report on this event will be published soon in MA.
At one point I put my little Scratch-65 (all 2.6 ounces of it!)
up for a flight. After a minute or two of flying around, I
realized that many other modelers were flying along with me.
My helper made a count, and including mine there were 12
models in the air at one time. I continued to fly the Scratch-65
for more than 10 minutes. It covered most of the inflated dome
area from side to side and floor to ceiling.
Only after I landed did it dawn on me that I had not
experienced a single glitch with my radio system. So when you
ask me how well this Micro Joule equipment performs, my
answer is “outstanding.” MA
—Bob Aberle
the internal software. It will establish the cutoff voltage at 72% of a
fully charged battery. So if you hook up a fully charged two-cell Li-
Poly battery, the voltage might be roughly 7.4 volts. This ESC will
then set the cutoff voltage to 72% of that, or 5.3 volts (2.67 volts per
cell).
I tried monitoring this claimed auto cutoff. The first time I did not
have a fully charged battery pack; therefore, the auto-select voltage
set itself too low! When I made sure the pack was fully charged, the
voltage was set correctly.
The only thing that might bother me is that on occasion I will fly
out a half charge, land, take a rest, and then fly the remainder of the
charge. When I turn it on the second time (with only a half-charged
battery), will the ESC set the voltage too low?
The motor speed controlling was linear from full throttle down to
idle speeds. Street price is identical to the other ESC, at $29.99.
From what I have written, there have been notable improvements
in the Micro Joule equipment. The receiver range and selectivity is
excellent. Servo resolution seems adequate for almost any kind of
flying. The regulated voltage from the BEC is now in line with what
we normally expect, and the voltage cutoff point is now appropriate
for one, two, and three Li-Poly battery cells.
I received filtering capacitors with both ESCs and was told to put
them on my motor, but my GWS already had several capacitors
installed. I chose not to use the extra capacitors, and everything
worked perfectly.
The street price for this receiver, crystal, two servos, and your
choice of ESC is $178.15. If you want to go to full four-channel
control, you will need one more servo at $49.99.
34 MODEL AVIATION
Fully assembled Scratch-65 waiting for coat of spray paint and
wing covering. Nothing to be afraid of trying here.
Two CS-3 servos mounted on top of fuselage, out in the open,
just aft of wing TE. Servos are affixed with 1⁄16-inch double-stick
mounting tape.
GWS LPS B2C-C motor ready to slip on end of 1⁄4-inch-diameter
hardwood dowel mount. Dowel passes through plywood former
F1 and balsa former F2.
CONSTRUCTION
Since the plans for the Scratch-One were made with a computer
graphics system, all I had to do was specify the size reduction and I
had my new set of plans. A 50% decrease would have been
convenient, but it clearly resulted in a model that was too small. I
finally ended up at 65%, which yielded a wingspan of
approximately 30 inches and an exact wing area of 105 square
inches.
Adding the weight of the Cirrus Micro Joule RC components
(0.36 ounce), the motor and propeller (0.49 ounce), and the two-cell
FMA Direct Kokam 145 mAh Li-Poly battery (0.35 ounce) resulted
in a (nonaircraft) weight of 1.2 ounces. I usually use a figure
similar to two times the equipment weight to get a feel for the
model’s total weight. In this case that was 2.4 ounces. I guessed
that an even 2.5 would be okay. In the final analysis, it came to
2.69 ounces—not bad!
The next trick was to select the appropriate wood sizes to fit the
reduced-size aircraft. I chose 1⁄16 balsa for the fuselage sides and tail
pieces. The fuselage top and bottom was 1⁄32 balsa.
As with the Scratch-One, the wing had sticks instead of ribs to
make things easy for the beginner. The bottom stick was made from
1⁄16 balsa, and the top was made from 1⁄32 balsa. A single spar
seemed adequate for this size of model.
For the center panel I used 1⁄16 x 1⁄4 spruce for the spar, to
provide a bit of extra strength. The balsa tip-panel spars were made
from balsa and tapered from 1⁄4 inch in height to 1⁄8 inch at the tip.
As I did with the Scratch-One, I mounted the two tiny Micro
Joule servos on top of the fuselage, just aft of the wing TE. This
makes installing the control rods a snap. Because of the Scratch-
65’s small size, I selected .015-inch-diameter wire for the control
rods.
Then I inserted the wire into small-diameter Teflon tubing that I
purchased from House of Balsa. The tubing was intended for use
with cyanoacrylate cement bottles and is designated as catalog item
NEC-04 (6 feet of cyanoacrylate micro tubing).
On the Scratch-One, I placed the big battery pack on the
underside of the fuselage, centered on the model’s CG, so that the
battery could be accessed without removing the wing after every
flight. Modelers liked that feature!
With the lightweight two-cell FMA/Kokam 145 mAh Li-Poly
pack, I needed the weight as far forward as possible to maintain the
proper balance. I decided to make a small compartment on top, just
behind the firewall (F-1). The pack drops into this compartment,
fitting flush with the top of the fuselage. I used two small magnets
from Forcefield Inc. to hold the battery pack in place.
Choosing a motor was easy. I wanted the smallest available
GWS that would operate on two Li-Poly cells (roughly 7.4 volts).
The best and only choice proved to be the GWS LPS-series B2C-C
motor with a 6.2:1 gear reduction ratio. The best propeller for this
setup proved to be a GWS orange 6 x 5.
I mounted the motor by slipping it onto the end of a 1⁄4-inchdiameter
hardwood dowel that is passed through formers F-1 and F-
2. You may have to sand that dowel to get it to fit in the motor’s
mounting hole. Once it is in place, drill a small hole through the
motor casing and into the dowel. Insert a No. 2 sheet-metal screw
to anchor it in place.
The two-pin male connector exiting the motor will plug
perfectly into the motor connector located on the Micro Joule ESC.
If you ever need to replace a motor, you can easily do it at the
flying field since no soldering is necessary.
Hinging and Radio Installation: I made the rudder and elevator
hinges from strips of clear iron-on Doculam covering material
(available from Modelair Tech). I used Z-bends on the ends of the
wires attaching to the servo output arms. At the control-surface
ends I used Du-Bro micro E-Z connectors and control horns.
I affixed both servos, the receiver, and the ESC to the fuselage
with thin (1⁄16-inch) double-stick foam-rubber tape. I installed the
radio before I painted the model.
Covering and Finishing: I gave the entire fuselage and tail
assembly a light spray coat of paint—just enough to seal the wood,
February 2005 35
GWS LPS B2C-C motor in place on 1⁄4-inch dowel. No. 2 sheet-metal
screw firmly anchors motor so it can’t move, yet is easy to replace
when necessary.
Upper front portion of Scratch-65 showing motor and compartment
for two Kokam 145 mAh Li-Poly battery pack. Battery is held in
place with help of two Forcefield Inc. tiny magnet pairs.
Looking inside RC compartment located under wing, you can see
four-channel receiver affixed to one fuselage side with help of
double-stick tape.
One of the two Du-Bro micro E-Z connectors. Here the
connection is to the rudder horn, which is also a Du-Bro
product.
addition, it employs 1⁄8-inch dowel for the
wing LE, a strip of 3⁄32 x 1⁄4-inch balsa for
the TE, and a short length of 1⁄16 x 1⁄4-inch
spruce for the center-panel spar.
It was so little material that I chose not
to bother my buddy Craig Wagner at
Aerocraft Inc. for a “bag of balsa,” as I did
for the Scratch-One.
Motor Parameters: The GWS B2C-C
seemed happiest using the GWS 6 x 5
orange propeller. Motor current at full
charge and full throttle measured 600 mA,
7.0 volts, and 5 watts. That worked out to
1.86 watts per ounce, which is more than
enough for this powered sailplane trainertype
aircraft. The rpm was measured at
4,200.
At the 600 mA motor current and with
145 mAh Kokam Li-Poly batteries, the
estimated motor run time is 15 minutes. At
half throttle, this amount of time can be
greatly extended. Best of all is the fact that
FMA Direct sells the two-cell battery pack
for only $14.95.
Flying: This proved to be the best part. My
goal was to provide a simple and fast
application for the new miniature Cirrus
but not even enough to actually cover the
wood grain, which kind of “bleeds”
through.
Before spraying, cover the open RC
compartment, the servos, the battery, and
the motor area with masking tape. For a
spray, I took Don Stackhouse’s (of DJ
Aerotech) suggestion and bought a can of
Dupli-Color Import Auto Spray
(www.duplicolor.com). I used shade 88-
01517 Yellow, which I found at a local
Wal-Mart. It has a strong odor, so it is best
to apply it outside on a calm day. Drying
time is quite short.
The wing structure before covering
weighed 0.49 ounce. For it I purchased
Transparent Red Solite covering from Balsa
Products. After covering, the wing weighed
0.67 ounce; Solite added only 0.18 ounce to
the total weight.
When it was all finished, my Scratch-65
weighed 2.69 ounces, which yielded a wing
loading of only 3.7 ounces per square foot.
Balance was exactly as shown on the
plans—just slightly forward of the main
(and only) wing spar.
This entire aircraft was built from a
single sheet of 1⁄16 x 3 x 36-inch balsa and
roughly a half sheet of 1⁄32 balsa. In
Dupli-Color spray paint that Bob used to
mist a single coat of yellow on raw
fuselage and tail balsa surfaces.
36 MODEL AVIATION
Micro Joule RC airborne equipment. These
components worked flawlessly in a regular
flying-field environment with considerable
and varied activity going on all around.
The Scratch-65 took to the air on its first
flight, requiring no trim whatsoever. It flew
straight and level, with a slight gentle climb.
The power was enough that I could quickly
reduce the throttle to half and still maintain
the model’s altitude. Doing that slowed the
model to a point where almost anybody can
fly it. Better yet, it will stay up for an
extremely long time before needing a
recharge.
I have flown the Scratch-65 in winds
estimated at 5-7 mph with no difficulty. I
haven’t had a chance to fly it indoors, but I
suspect that at reduced throttle settings it
could easily be flown in a double-size
gymnasium facility.
Final control throws were measured at 3⁄8
inch either side for the rudder and 5⁄16 inch
either side of neutral for the elevator.
This fun aircraft can be enjoyed by rank
beginners as well as regular sport fliers. If
you haven’t tried flying a diminutive model
that still uses conventional proportional
servos, consider the Scratch-65. Please share
your photos and experiences with me! MA
Bob Aberle
[email protected]
Edition: Model Aviation - 2005/02
Page Numbers: 31,32,33,34,35,36,37,38
Edition: Model Aviation - 2005/02
Page Numbers: 31,32,33,34,35,36,37,38
Sccrattcch-65
Electric-powered RC indoor
sailplane/trainer showcases new
miniature Cirrus Micro Joule components
A FEATURE THAT appeared in my MA “From the Ground Up”
installments was a construction article for a model that an RC
beginner could build strictly from raw materials without the aid of a
kit. We call this technique “building from scratch.”
The airplane I designed expressly for this feature was the
Scratch-One. It was presented in two parts, in the November 2003
and January 2004 issues. From all the photos that were sent in, many
of you built this little RC electric-powered training model with
excellent success.
Jumping ahead a little, I wrote another feature article titled
“Introduction to Indoor RC” (July 2004 MA). Right after it was
published, the people at Global Hobby Distributors sent in samples
of their newly updated Cirrus Micro Joule miniature RC-system
components.
This equipment was first made available well more than a year
ago. More recently, based on field experience, several modifications
were made to further enhance this micro RC equipment’s
performance. I was provided a Micro Joule receiver, three
microservos, and two types of tiny electric motor speed controllers.
My usual routine would be to photograph these components,
operate them, and report on them using a standard product-review
format. The thought came to mind that there aren’t very many RTF
or ARF models available for RC aircraft that weigh only
approximately 2.5 ounces. If I could design a simple model that the
average indoor RC beginner could easily construct, I could be
providing a specific application for these new Cirrus Micro Joule
components.
With the Scratch-One’s success so noted, I thought it might be
fun to scale it down to a size that would accept the Cirrus Micro
Joule equipment, a tiny electric motor, and several equally tiny Li-
Poly batteries, all in an aircraft nearing that 2.5-ounce total.
The result is this “Scratch-65”: a Scratch-One at 65% of its
original size. The wingspan worked out to roughly 30 inches, and the
area is slightly more than 100 square inches.
So settle in and read my showcase of the new Cirrus Micro Joule
miniature RC components, and then read on for the full (and simple)
construction article for the little Scratch-65. It has proven to be an
excellent flier and another perfect RC trainer.
Global Hobby Cirrus Micro Joule RC System: My sample airbornesystem
components consisted of the Cirrus Micro Joule four-channel
miniature FM receiver, two Cirrus Micro Joule CS-3
ultramicroservos, and two new 5-amp-rated ESCs: the S5A1 and the
S5A2. These items can be purchased separately.
Micro JST connectors with white plastic housing are used on
these RC components. There really isn’t any established
convention yet for the pin wiring on these tiny connectors. On
these Cirrus types, the center pin has a red wire for battery
positive. Gray wires are used for the two “outboard” pins that
contain the battery negative and the servo signal wire.
When you are using all Cirrus Micro Joule components, there
will be no problem with connector-pin polarity. But if you mix and
match from other manufacturers, you had best check the pin polarity
before you turn the power on for the first time.
The connections from the ESC to the motor and the battery pack
use the larger, red, plastic JST connectors, which adhere to the
industry-accepted polarity convention (pin No. 1 black negative and
pin No. 2 red positive).
The Micro Joule four-channel-function receiver has automatic
shift select, so it can basically be operated by any brand of RC FM
transmitter. Voltage range is claimed to be 3.5 to 7.0. The weight is
slightly more than 3 grams (or .11 ounce). Dimensions are 1.11
inches long, 0.31 inch wide, and 0.33 inch high. The crystal clearly
sticks out farther than this height dimension.
The antenna wire is 34 inches in length. I used the full-length
antenna to obtain a baseline check on the receiver’s range capability.
Later I will substitute an E-Cubed RC short antenna and report on
the effect of the radio reception.
This is a single-conversion receiver. The specifications simply
state that it is “narrow band.” I resorted to a couple of simple field
tests to verify that claim. I was able to put the Scratch-65 almost out
of sight (on a calm day!) and was able to maintain perfect control.
Then I had a club member set up his synthesized RC transmitter
one channel away from mine (that is only 20 kHz). I flew the model
down the length of the runway, allowing it to get much closer to the
other transmitter than to my own. Both transmitter antennas were
fully extended. I did this on several occasions and noticed no
interference of any kind.
Tom Hunt about to launch the Scratch-65, with Bob behind the
camera. This shows you how small the model really is.
February 2005 31
by Bob Aberle
Photos by the author and Mark Lanterman
A comparison of the Speed 400-powered Scratch-One and the
smaller Scratch-65. Both are easy to build and fun to fly!
It’s hard to tell the Scratch-65’s size in this flight shot. These small
models are stable and fly in much the same manner as their larger
cousins.
Global Hobby Cirrus Micro Joule RC micro components (bottom to
top): four-channel receiver, S5A2 ESC, two CS-3 ultramicroservos.
Total weight of this equipment is .36 ounce!
Cirrus Micro Joule four-channel receiver with dime for size
reference. Crystal must be purchased separately. Receiver
weighs 3 grams, or .11 ounce!
Different view of equipment in above right photo (bottom to top):
CS-3 ultramicro proportional servo with its separate cable, fourchannel
receiver with coiled receiver antenna wire at left, and one
of the two new 5-amp ESCs.
During an entire morning at my flying field, I made several longduration
flights. While I did this, many of my clubmates flew on a
variety of other 72 MHz RC channels. This Micro Joule receiver
never got hit! Keep in mind that this is not a laboratory test; it is
merely a practical test based on everyday-type flying. The receiver’s
street price is $39.99, and the required crystal is another $12.95.
The little CS-3 servos turned out to be lighter than the claim. I
measured one at 2.1 grams, so the two I used came to 4.2 grams, or
.15 ounce.
The cable going from the servo to the receiver is a separate item.
You plug one end into the servo and the other end into the receiver.
The length of the cable is slightly less than 3 inches. Other cable
lengths and cables that adapt to other equipment can be purchased
separately.
Voltage requirement for these servos is 3.0-7.0. At 4.8 volts, the
claim is for a 14 inch-ounce output torque. Transit time at 4.8 volts
is .18 second for 60° rotation.
The first issue of these tiny servos tended to be noisy, and the
neutral centering was observed to be inaccurate; the servo never
returned to exactly the same neutral position. This new version of
the Micro Joule servo seemed quieter. The neutral-return accuracy
also seems better. I can’t say it’s “dead on,” but it’s certainly
acceptable for anything except a high-speed, highly maneuverable
aircraft.
I don’t expect to see many airplanes in this category weighing
just 2.5 ounces! The street price for an individual Micro Joule
servo is $49.99, which is impressive for a 2.1-gram, fully
proportional-feedback servo.
32 MODEL AVIATION
Closer look at four-channel FM single-conversion receiver. It will
auto-select proper FM shift to work with any existing RC FM
transmitter.
Closer look at CS-3 servo. Cable is a separate item. You can
purchase longer cables or even cable adapters that will allow you
to use these servos with other RC components.
S5A1 ESC intended for use with one Li-Poly battery or two to
three Ni-Cd/NiMH cells. Red plastic JST connectors go to
battery and motor.
Peak Electronics unit is Bob’s favorite charger for smaller Li-Poly
batteries. It can handle one, two, or three Li-Poly cells at discrete
100-1500 mA charge currents.
Type: RC indoor sailplane/trainer
Wingspan: 30 inches
Power: GWS LPS B2C-C motor with 6.2:1 gear drive
Flying weight: 2.69 ounces
Construction: Balsa and plywood
Covering/finish: Dupli-Color spray enamel and Solite
Scratch-65
The first-issue Micro Joule ESC had a BEC regulator set for a
low 3.0 volts. Most of us felt this was done to accommodate
possibly a problem in either the receiver or servos. The regulated
voltage is now back up to the more appropriate 5.0-volt level. (I
verified this point!)
The cutoff-voltage level on the first Micro Joule ESC was set
low. It definitely wasn’t conducive to using Li-Poly batteries, where
it is important not to discharge to less than 2.5-3.0 volts per cell.
To counter this problem, Global has introduced two new types of
ESCs. Model S5A1 is intended for use with one Li-Poly battery or
two to three Ni-Cd/Nymph cells. The cutoff is set at 2.7 volts.
Continuous current rating is 5 amps, which is more than enough for
this application. It weighs 1.9 grams (.067 ounce) and has a street
price of $29.99.
The model S5A2 ESC is intended for use with two or three Li-
Poly batteries or three to six Ni-Cd/NiMH cells. This is the one I
used on the Scratch-65. It weighs 2.78 grams (.10 ounce) and
measures .74 inch long, .31 inch wide, and .25 inch high. That
weight includes one micro JST connector that goes to the throttle
port on the receiver, along with male and female red JST connectors
that go to the battery and motor. The BEC regulated voltage is again
5.0 volts, and the continuous current rating is 5 amps.
The voltage cutoff on this particular ESC is automatically set by
February 2005 33
All wood parts necessary to build Scratch-65. Almost entire model
is constructed from full 1⁄16 x 3 x 36-inch balsa sheet and roughly
half sheet of 1⁄32 balsa.
Wing under construction. LE is 1⁄8-inch-diameter hardwood dowel.
Center-panel spar is 1⁄16 x 1⁄4 spruce for added strength. There are
no wing ribs—only bent sticks of balsa.
Fuselage sides assembled with two formers and spacer sticks.
All tail-assembly pieces are made from 1⁄16 balsa sheet.
Wing panel ready to be joined, with fuselage sides with stiffeners
in place. Fuselage sides are made from 1⁄16 balsa sheet.
Scratch-65 in Action
In early November I attended and flew in what is probably
the biggest indoor RC Electric fly in the USA—the JR Indoor
Electric Festival—which was held in Columbus, Ohio. A
complete report on this event will be published soon in MA.
At one point I put my little Scratch-65 (all 2.6 ounces of it!)
up for a flight. After a minute or two of flying around, I
realized that many other modelers were flying along with me.
My helper made a count, and including mine there were 12
models in the air at one time. I continued to fly the Scratch-65
for more than 10 minutes. It covered most of the inflated dome
area from side to side and floor to ceiling.
Only after I landed did it dawn on me that I had not
experienced a single glitch with my radio system. So when you
ask me how well this Micro Joule equipment performs, my
answer is “outstanding.” MA
—Bob Aberle
the internal software. It will establish the cutoff voltage at 72% of a
fully charged battery. So if you hook up a fully charged two-cell Li-
Poly battery, the voltage might be roughly 7.4 volts. This ESC will
then set the cutoff voltage to 72% of that, or 5.3 volts (2.67 volts per
cell).
I tried monitoring this claimed auto cutoff. The first time I did not
have a fully charged battery pack; therefore, the auto-select voltage
set itself too low! When I made sure the pack was fully charged, the
voltage was set correctly.
The only thing that might bother me is that on occasion I will fly
out a half charge, land, take a rest, and then fly the remainder of the
charge. When I turn it on the second time (with only a half-charged
battery), will the ESC set the voltage too low?
The motor speed controlling was linear from full throttle down to
idle speeds. Street price is identical to the other ESC, at $29.99.
From what I have written, there have been notable improvements
in the Micro Joule equipment. The receiver range and selectivity is
excellent. Servo resolution seems adequate for almost any kind of
flying. The regulated voltage from the BEC is now in line with what
we normally expect, and the voltage cutoff point is now appropriate
for one, two, and three Li-Poly battery cells.
I received filtering capacitors with both ESCs and was told to put
them on my motor, but my GWS already had several capacitors
installed. I chose not to use the extra capacitors, and everything
worked perfectly.
The street price for this receiver, crystal, two servos, and your
choice of ESC is $178.15. If you want to go to full four-channel
control, you will need one more servo at $49.99.
34 MODEL AVIATION
Fully assembled Scratch-65 waiting for coat of spray paint and
wing covering. Nothing to be afraid of trying here.
Two CS-3 servos mounted on top of fuselage, out in the open,
just aft of wing TE. Servos are affixed with 1⁄16-inch double-stick
mounting tape.
GWS LPS B2C-C motor ready to slip on end of 1⁄4-inch-diameter
hardwood dowel mount. Dowel passes through plywood former
F1 and balsa former F2.
CONSTRUCTION
Since the plans for the Scratch-One were made with a computer
graphics system, all I had to do was specify the size reduction and I
had my new set of plans. A 50% decrease would have been
convenient, but it clearly resulted in a model that was too small. I
finally ended up at 65%, which yielded a wingspan of
approximately 30 inches and an exact wing area of 105 square
inches.
Adding the weight of the Cirrus Micro Joule RC components
(0.36 ounce), the motor and propeller (0.49 ounce), and the two-cell
FMA Direct Kokam 145 mAh Li-Poly battery (0.35 ounce) resulted
in a (nonaircraft) weight of 1.2 ounces. I usually use a figure
similar to two times the equipment weight to get a feel for the
model’s total weight. In this case that was 2.4 ounces. I guessed
that an even 2.5 would be okay. In the final analysis, it came to
2.69 ounces—not bad!
The next trick was to select the appropriate wood sizes to fit the
reduced-size aircraft. I chose 1⁄16 balsa for the fuselage sides and tail
pieces. The fuselage top and bottom was 1⁄32 balsa.
As with the Scratch-One, the wing had sticks instead of ribs to
make things easy for the beginner. The bottom stick was made from
1⁄16 balsa, and the top was made from 1⁄32 balsa. A single spar
seemed adequate for this size of model.
For the center panel I used 1⁄16 x 1⁄4 spruce for the spar, to
provide a bit of extra strength. The balsa tip-panel spars were made
from balsa and tapered from 1⁄4 inch in height to 1⁄8 inch at the tip.
As I did with the Scratch-One, I mounted the two tiny Micro
Joule servos on top of the fuselage, just aft of the wing TE. This
makes installing the control rods a snap. Because of the Scratch-
65’s small size, I selected .015-inch-diameter wire for the control
rods.
Then I inserted the wire into small-diameter Teflon tubing that I
purchased from House of Balsa. The tubing was intended for use
with cyanoacrylate cement bottles and is designated as catalog item
NEC-04 (6 feet of cyanoacrylate micro tubing).
On the Scratch-One, I placed the big battery pack on the
underside of the fuselage, centered on the model’s CG, so that the
battery could be accessed without removing the wing after every
flight. Modelers liked that feature!
With the lightweight two-cell FMA/Kokam 145 mAh Li-Poly
pack, I needed the weight as far forward as possible to maintain the
proper balance. I decided to make a small compartment on top, just
behind the firewall (F-1). The pack drops into this compartment,
fitting flush with the top of the fuselage. I used two small magnets
from Forcefield Inc. to hold the battery pack in place.
Choosing a motor was easy. I wanted the smallest available
GWS that would operate on two Li-Poly cells (roughly 7.4 volts).
The best and only choice proved to be the GWS LPS-series B2C-C
motor with a 6.2:1 gear reduction ratio. The best propeller for this
setup proved to be a GWS orange 6 x 5.
I mounted the motor by slipping it onto the end of a 1⁄4-inchdiameter
hardwood dowel that is passed through formers F-1 and F-
2. You may have to sand that dowel to get it to fit in the motor’s
mounting hole. Once it is in place, drill a small hole through the
motor casing and into the dowel. Insert a No. 2 sheet-metal screw
to anchor it in place.
The two-pin male connector exiting the motor will plug
perfectly into the motor connector located on the Micro Joule ESC.
If you ever need to replace a motor, you can easily do it at the
flying field since no soldering is necessary.
Hinging and Radio Installation: I made the rudder and elevator
hinges from strips of clear iron-on Doculam covering material
(available from Modelair Tech). I used Z-bends on the ends of the
wires attaching to the servo output arms. At the control-surface
ends I used Du-Bro micro E-Z connectors and control horns.
I affixed both servos, the receiver, and the ESC to the fuselage
with thin (1⁄16-inch) double-stick foam-rubber tape. I installed the
radio before I painted the model.
Covering and Finishing: I gave the entire fuselage and tail
assembly a light spray coat of paint—just enough to seal the wood,
February 2005 35
GWS LPS B2C-C motor in place on 1⁄4-inch dowel. No. 2 sheet-metal
screw firmly anchors motor so it can’t move, yet is easy to replace
when necessary.
Upper front portion of Scratch-65 showing motor and compartment
for two Kokam 145 mAh Li-Poly battery pack. Battery is held in
place with help of two Forcefield Inc. tiny magnet pairs.
Looking inside RC compartment located under wing, you can see
four-channel receiver affixed to one fuselage side with help of
double-stick tape.
One of the two Du-Bro micro E-Z connectors. Here the
connection is to the rudder horn, which is also a Du-Bro
product.
addition, it employs 1⁄8-inch dowel for the
wing LE, a strip of 3⁄32 x 1⁄4-inch balsa for
the TE, and a short length of 1⁄16 x 1⁄4-inch
spruce for the center-panel spar.
It was so little material that I chose not
to bother my buddy Craig Wagner at
Aerocraft Inc. for a “bag of balsa,” as I did
for the Scratch-One.
Motor Parameters: The GWS B2C-C
seemed happiest using the GWS 6 x 5
orange propeller. Motor current at full
charge and full throttle measured 600 mA,
7.0 volts, and 5 watts. That worked out to
1.86 watts per ounce, which is more than
enough for this powered sailplane trainertype
aircraft. The rpm was measured at
4,200.
At the 600 mA motor current and with
145 mAh Kokam Li-Poly batteries, the
estimated motor run time is 15 minutes. At
half throttle, this amount of time can be
greatly extended. Best of all is the fact that
FMA Direct sells the two-cell battery pack
for only $14.95.
Flying: This proved to be the best part. My
goal was to provide a simple and fast
application for the new miniature Cirrus
but not even enough to actually cover the
wood grain, which kind of “bleeds”
through.
Before spraying, cover the open RC
compartment, the servos, the battery, and
the motor area with masking tape. For a
spray, I took Don Stackhouse’s (of DJ
Aerotech) suggestion and bought a can of
Dupli-Color Import Auto Spray
(www.duplicolor.com). I used shade 88-
01517 Yellow, which I found at a local
Wal-Mart. It has a strong odor, so it is best
to apply it outside on a calm day. Drying
time is quite short.
The wing structure before covering
weighed 0.49 ounce. For it I purchased
Transparent Red Solite covering from Balsa
Products. After covering, the wing weighed
0.67 ounce; Solite added only 0.18 ounce to
the total weight.
When it was all finished, my Scratch-65
weighed 2.69 ounces, which yielded a wing
loading of only 3.7 ounces per square foot.
Balance was exactly as shown on the
plans—just slightly forward of the main
(and only) wing spar.
This entire aircraft was built from a
single sheet of 1⁄16 x 3 x 36-inch balsa and
roughly a half sheet of 1⁄32 balsa. In
Dupli-Color spray paint that Bob used to
mist a single coat of yellow on raw
fuselage and tail balsa surfaces.
36 MODEL AVIATION
Micro Joule RC airborne equipment. These
components worked flawlessly in a regular
flying-field environment with considerable
and varied activity going on all around.
The Scratch-65 took to the air on its first
flight, requiring no trim whatsoever. It flew
straight and level, with a slight gentle climb.
The power was enough that I could quickly
reduce the throttle to half and still maintain
the model’s altitude. Doing that slowed the
model to a point where almost anybody can
fly it. Better yet, it will stay up for an
extremely long time before needing a
recharge.
I have flown the Scratch-65 in winds
estimated at 5-7 mph with no difficulty. I
haven’t had a chance to fly it indoors, but I
suspect that at reduced throttle settings it
could easily be flown in a double-size
gymnasium facility.
Final control throws were measured at 3⁄8
inch either side for the rudder and 5⁄16 inch
either side of neutral for the elevator.
This fun aircraft can be enjoyed by rank
beginners as well as regular sport fliers. If
you haven’t tried flying a diminutive model
that still uses conventional proportional
servos, consider the Scratch-65. Please share
your photos and experiences with me! MA
Bob Aberle
[email protected]
Edition: Model Aviation - 2005/02
Page Numbers: 31,32,33,34,35,36,37,38
Sccrattcch-65
Electric-powered RC indoor
sailplane/trainer showcases new
miniature Cirrus Micro Joule components
A FEATURE THAT appeared in my MA “From the Ground Up”
installments was a construction article for a model that an RC
beginner could build strictly from raw materials without the aid of a
kit. We call this technique “building from scratch.”
The airplane I designed expressly for this feature was the
Scratch-One. It was presented in two parts, in the November 2003
and January 2004 issues. From all the photos that were sent in, many
of you built this little RC electric-powered training model with
excellent success.
Jumping ahead a little, I wrote another feature article titled
“Introduction to Indoor RC” (July 2004 MA). Right after it was
published, the people at Global Hobby Distributors sent in samples
of their newly updated Cirrus Micro Joule miniature RC-system
components.
This equipment was first made available well more than a year
ago. More recently, based on field experience, several modifications
were made to further enhance this micro RC equipment’s
performance. I was provided a Micro Joule receiver, three
microservos, and two types of tiny electric motor speed controllers.
My usual routine would be to photograph these components,
operate them, and report on them using a standard product-review
format. The thought came to mind that there aren’t very many RTF
or ARF models available for RC aircraft that weigh only
approximately 2.5 ounces. If I could design a simple model that the
average indoor RC beginner could easily construct, I could be
providing a specific application for these new Cirrus Micro Joule
components.
With the Scratch-One’s success so noted, I thought it might be
fun to scale it down to a size that would accept the Cirrus Micro
Joule equipment, a tiny electric motor, and several equally tiny Li-
Poly batteries, all in an aircraft nearing that 2.5-ounce total.
The result is this “Scratch-65”: a Scratch-One at 65% of its
original size. The wingspan worked out to roughly 30 inches, and the
area is slightly more than 100 square inches.
So settle in and read my showcase of the new Cirrus Micro Joule
miniature RC components, and then read on for the full (and simple)
construction article for the little Scratch-65. It has proven to be an
excellent flier and another perfect RC trainer.
Global Hobby Cirrus Micro Joule RC System: My sample airbornesystem
components consisted of the Cirrus Micro Joule four-channel
miniature FM receiver, two Cirrus Micro Joule CS-3
ultramicroservos, and two new 5-amp-rated ESCs: the S5A1 and the
S5A2. These items can be purchased separately.
Micro JST connectors with white plastic housing are used on
these RC components. There really isn’t any established
convention yet for the pin wiring on these tiny connectors. On
these Cirrus types, the center pin has a red wire for battery
positive. Gray wires are used for the two “outboard” pins that
contain the battery negative and the servo signal wire.
When you are using all Cirrus Micro Joule components, there
will be no problem with connector-pin polarity. But if you mix and
match from other manufacturers, you had best check the pin polarity
before you turn the power on for the first time.
The connections from the ESC to the motor and the battery pack
use the larger, red, plastic JST connectors, which adhere to the
industry-accepted polarity convention (pin No. 1 black negative and
pin No. 2 red positive).
The Micro Joule four-channel-function receiver has automatic
shift select, so it can basically be operated by any brand of RC FM
transmitter. Voltage range is claimed to be 3.5 to 7.0. The weight is
slightly more than 3 grams (or .11 ounce). Dimensions are 1.11
inches long, 0.31 inch wide, and 0.33 inch high. The crystal clearly
sticks out farther than this height dimension.
The antenna wire is 34 inches in length. I used the full-length
antenna to obtain a baseline check on the receiver’s range capability.
Later I will substitute an E-Cubed RC short antenna and report on
the effect of the radio reception.
This is a single-conversion receiver. The specifications simply
state that it is “narrow band.” I resorted to a couple of simple field
tests to verify that claim. I was able to put the Scratch-65 almost out
of sight (on a calm day!) and was able to maintain perfect control.
Then I had a club member set up his synthesized RC transmitter
one channel away from mine (that is only 20 kHz). I flew the model
down the length of the runway, allowing it to get much closer to the
other transmitter than to my own. Both transmitter antennas were
fully extended. I did this on several occasions and noticed no
interference of any kind.
Tom Hunt about to launch the Scratch-65, with Bob behind the
camera. This shows you how small the model really is.
February 2005 31
by Bob Aberle
Photos by the author and Mark Lanterman
A comparison of the Speed 400-powered Scratch-One and the
smaller Scratch-65. Both are easy to build and fun to fly!
It’s hard to tell the Scratch-65’s size in this flight shot. These small
models are stable and fly in much the same manner as their larger
cousins.
Global Hobby Cirrus Micro Joule RC micro components (bottom to
top): four-channel receiver, S5A2 ESC, two CS-3 ultramicroservos.
Total weight of this equipment is .36 ounce!
Cirrus Micro Joule four-channel receiver with dime for size
reference. Crystal must be purchased separately. Receiver
weighs 3 grams, or .11 ounce!
Different view of equipment in above right photo (bottom to top):
CS-3 ultramicro proportional servo with its separate cable, fourchannel
receiver with coiled receiver antenna wire at left, and one
of the two new 5-amp ESCs.
During an entire morning at my flying field, I made several longduration
flights. While I did this, many of my clubmates flew on a
variety of other 72 MHz RC channels. This Micro Joule receiver
never got hit! Keep in mind that this is not a laboratory test; it is
merely a practical test based on everyday-type flying. The receiver’s
street price is $39.99, and the required crystal is another $12.95.
The little CS-3 servos turned out to be lighter than the claim. I
measured one at 2.1 grams, so the two I used came to 4.2 grams, or
.15 ounce.
The cable going from the servo to the receiver is a separate item.
You plug one end into the servo and the other end into the receiver.
The length of the cable is slightly less than 3 inches. Other cable
lengths and cables that adapt to other equipment can be purchased
separately.
Voltage requirement for these servos is 3.0-7.0. At 4.8 volts, the
claim is for a 14 inch-ounce output torque. Transit time at 4.8 volts
is .18 second for 60° rotation.
The first issue of these tiny servos tended to be noisy, and the
neutral centering was observed to be inaccurate; the servo never
returned to exactly the same neutral position. This new version of
the Micro Joule servo seemed quieter. The neutral-return accuracy
also seems better. I can’t say it’s “dead on,” but it’s certainly
acceptable for anything except a high-speed, highly maneuverable
aircraft.
I don’t expect to see many airplanes in this category weighing
just 2.5 ounces! The street price for an individual Micro Joule
servo is $49.99, which is impressive for a 2.1-gram, fully
proportional-feedback servo.
32 MODEL AVIATION
Closer look at four-channel FM single-conversion receiver. It will
auto-select proper FM shift to work with any existing RC FM
transmitter.
Closer look at CS-3 servo. Cable is a separate item. You can
purchase longer cables or even cable adapters that will allow you
to use these servos with other RC components.
S5A1 ESC intended for use with one Li-Poly battery or two to
three Ni-Cd/NiMH cells. Red plastic JST connectors go to
battery and motor.
Peak Electronics unit is Bob’s favorite charger for smaller Li-Poly
batteries. It can handle one, two, or three Li-Poly cells at discrete
100-1500 mA charge currents.
Type: RC indoor sailplane/trainer
Wingspan: 30 inches
Power: GWS LPS B2C-C motor with 6.2:1 gear drive
Flying weight: 2.69 ounces
Construction: Balsa and plywood
Covering/finish: Dupli-Color spray enamel and Solite
Scratch-65
The first-issue Micro Joule ESC had a BEC regulator set for a
low 3.0 volts. Most of us felt this was done to accommodate
possibly a problem in either the receiver or servos. The regulated
voltage is now back up to the more appropriate 5.0-volt level. (I
verified this point!)
The cutoff-voltage level on the first Micro Joule ESC was set
low. It definitely wasn’t conducive to using Li-Poly batteries, where
it is important not to discharge to less than 2.5-3.0 volts per cell.
To counter this problem, Global has introduced two new types of
ESCs. Model S5A1 is intended for use with one Li-Poly battery or
two to three Ni-Cd/Nymph cells. The cutoff is set at 2.7 volts.
Continuous current rating is 5 amps, which is more than enough for
this application. It weighs 1.9 grams (.067 ounce) and has a street
price of $29.99.
The model S5A2 ESC is intended for use with two or three Li-
Poly batteries or three to six Ni-Cd/NiMH cells. This is the one I
used on the Scratch-65. It weighs 2.78 grams (.10 ounce) and
measures .74 inch long, .31 inch wide, and .25 inch high. That
weight includes one micro JST connector that goes to the throttle
port on the receiver, along with male and female red JST connectors
that go to the battery and motor. The BEC regulated voltage is again
5.0 volts, and the continuous current rating is 5 amps.
The voltage cutoff on this particular ESC is automatically set by
February 2005 33
All wood parts necessary to build Scratch-65. Almost entire model
is constructed from full 1⁄16 x 3 x 36-inch balsa sheet and roughly
half sheet of 1⁄32 balsa.
Wing under construction. LE is 1⁄8-inch-diameter hardwood dowel.
Center-panel spar is 1⁄16 x 1⁄4 spruce for added strength. There are
no wing ribs—only bent sticks of balsa.
Fuselage sides assembled with two formers and spacer sticks.
All tail-assembly pieces are made from 1⁄16 balsa sheet.
Wing panel ready to be joined, with fuselage sides with stiffeners
in place. Fuselage sides are made from 1⁄16 balsa sheet.
Scratch-65 in Action
In early November I attended and flew in what is probably
the biggest indoor RC Electric fly in the USA—the JR Indoor
Electric Festival—which was held in Columbus, Ohio. A
complete report on this event will be published soon in MA.
At one point I put my little Scratch-65 (all 2.6 ounces of it!)
up for a flight. After a minute or two of flying around, I
realized that many other modelers were flying along with me.
My helper made a count, and including mine there were 12
models in the air at one time. I continued to fly the Scratch-65
for more than 10 minutes. It covered most of the inflated dome
area from side to side and floor to ceiling.
Only after I landed did it dawn on me that I had not
experienced a single glitch with my radio system. So when you
ask me how well this Micro Joule equipment performs, my
answer is “outstanding.” MA
—Bob Aberle
the internal software. It will establish the cutoff voltage at 72% of a
fully charged battery. So if you hook up a fully charged two-cell Li-
Poly battery, the voltage might be roughly 7.4 volts. This ESC will
then set the cutoff voltage to 72% of that, or 5.3 volts (2.67 volts per
cell).
I tried monitoring this claimed auto cutoff. The first time I did not
have a fully charged battery pack; therefore, the auto-select voltage
set itself too low! When I made sure the pack was fully charged, the
voltage was set correctly.
The only thing that might bother me is that on occasion I will fly
out a half charge, land, take a rest, and then fly the remainder of the
charge. When I turn it on the second time (with only a half-charged
battery), will the ESC set the voltage too low?
The motor speed controlling was linear from full throttle down to
idle speeds. Street price is identical to the other ESC, at $29.99.
From what I have written, there have been notable improvements
in the Micro Joule equipment. The receiver range and selectivity is
excellent. Servo resolution seems adequate for almost any kind of
flying. The regulated voltage from the BEC is now in line with what
we normally expect, and the voltage cutoff point is now appropriate
for one, two, and three Li-Poly battery cells.
I received filtering capacitors with both ESCs and was told to put
them on my motor, but my GWS already had several capacitors
installed. I chose not to use the extra capacitors, and everything
worked perfectly.
The street price for this receiver, crystal, two servos, and your
choice of ESC is $178.15. If you want to go to full four-channel
control, you will need one more servo at $49.99.
34 MODEL AVIATION
Fully assembled Scratch-65 waiting for coat of spray paint and
wing covering. Nothing to be afraid of trying here.
Two CS-3 servos mounted on top of fuselage, out in the open,
just aft of wing TE. Servos are affixed with 1⁄16-inch double-stick
mounting tape.
GWS LPS B2C-C motor ready to slip on end of 1⁄4-inch-diameter
hardwood dowel mount. Dowel passes through plywood former
F1 and balsa former F2.
CONSTRUCTION
Since the plans for the Scratch-One were made with a computer
graphics system, all I had to do was specify the size reduction and I
had my new set of plans. A 50% decrease would have been
convenient, but it clearly resulted in a model that was too small. I
finally ended up at 65%, which yielded a wingspan of
approximately 30 inches and an exact wing area of 105 square
inches.
Adding the weight of the Cirrus Micro Joule RC components
(0.36 ounce), the motor and propeller (0.49 ounce), and the two-cell
FMA Direct Kokam 145 mAh Li-Poly battery (0.35 ounce) resulted
in a (nonaircraft) weight of 1.2 ounces. I usually use a figure
similar to two times the equipment weight to get a feel for the
model’s total weight. In this case that was 2.4 ounces. I guessed
that an even 2.5 would be okay. In the final analysis, it came to
2.69 ounces—not bad!
The next trick was to select the appropriate wood sizes to fit the
reduced-size aircraft. I chose 1⁄16 balsa for the fuselage sides and tail
pieces. The fuselage top and bottom was 1⁄32 balsa.
As with the Scratch-One, the wing had sticks instead of ribs to
make things easy for the beginner. The bottom stick was made from
1⁄16 balsa, and the top was made from 1⁄32 balsa. A single spar
seemed adequate for this size of model.
For the center panel I used 1⁄16 x 1⁄4 spruce for the spar, to
provide a bit of extra strength. The balsa tip-panel spars were made
from balsa and tapered from 1⁄4 inch in height to 1⁄8 inch at the tip.
As I did with the Scratch-One, I mounted the two tiny Micro
Joule servos on top of the fuselage, just aft of the wing TE. This
makes installing the control rods a snap. Because of the Scratch-
65’s small size, I selected .015-inch-diameter wire for the control
rods.
Then I inserted the wire into small-diameter Teflon tubing that I
purchased from House of Balsa. The tubing was intended for use
with cyanoacrylate cement bottles and is designated as catalog item
NEC-04 (6 feet of cyanoacrylate micro tubing).
On the Scratch-One, I placed the big battery pack on the
underside of the fuselage, centered on the model’s CG, so that the
battery could be accessed without removing the wing after every
flight. Modelers liked that feature!
With the lightweight two-cell FMA/Kokam 145 mAh Li-Poly
pack, I needed the weight as far forward as possible to maintain the
proper balance. I decided to make a small compartment on top, just
behind the firewall (F-1). The pack drops into this compartment,
fitting flush with the top of the fuselage. I used two small magnets
from Forcefield Inc. to hold the battery pack in place.
Choosing a motor was easy. I wanted the smallest available
GWS that would operate on two Li-Poly cells (roughly 7.4 volts).
The best and only choice proved to be the GWS LPS-series B2C-C
motor with a 6.2:1 gear reduction ratio. The best propeller for this
setup proved to be a GWS orange 6 x 5.
I mounted the motor by slipping it onto the end of a 1⁄4-inchdiameter
hardwood dowel that is passed through formers F-1 and F-
2. You may have to sand that dowel to get it to fit in the motor’s
mounting hole. Once it is in place, drill a small hole through the
motor casing and into the dowel. Insert a No. 2 sheet-metal screw
to anchor it in place.
The two-pin male connector exiting the motor will plug
perfectly into the motor connector located on the Micro Joule ESC.
If you ever need to replace a motor, you can easily do it at the
flying field since no soldering is necessary.
Hinging and Radio Installation: I made the rudder and elevator
hinges from strips of clear iron-on Doculam covering material
(available from Modelair Tech). I used Z-bends on the ends of the
wires attaching to the servo output arms. At the control-surface
ends I used Du-Bro micro E-Z connectors and control horns.
I affixed both servos, the receiver, and the ESC to the fuselage
with thin (1⁄16-inch) double-stick foam-rubber tape. I installed the
radio before I painted the model.
Covering and Finishing: I gave the entire fuselage and tail
assembly a light spray coat of paint—just enough to seal the wood,
February 2005 35
GWS LPS B2C-C motor in place on 1⁄4-inch dowel. No. 2 sheet-metal
screw firmly anchors motor so it can’t move, yet is easy to replace
when necessary.
Upper front portion of Scratch-65 showing motor and compartment
for two Kokam 145 mAh Li-Poly battery pack. Battery is held in
place with help of two Forcefield Inc. tiny magnet pairs.
Looking inside RC compartment located under wing, you can see
four-channel receiver affixed to one fuselage side with help of
double-stick tape.
One of the two Du-Bro micro E-Z connectors. Here the
connection is to the rudder horn, which is also a Du-Bro
product.
addition, it employs 1⁄8-inch dowel for the
wing LE, a strip of 3⁄32 x 1⁄4-inch balsa for
the TE, and a short length of 1⁄16 x 1⁄4-inch
spruce for the center-panel spar.
It was so little material that I chose not
to bother my buddy Craig Wagner at
Aerocraft Inc. for a “bag of balsa,” as I did
for the Scratch-One.
Motor Parameters: The GWS B2C-C
seemed happiest using the GWS 6 x 5
orange propeller. Motor current at full
charge and full throttle measured 600 mA,
7.0 volts, and 5 watts. That worked out to
1.86 watts per ounce, which is more than
enough for this powered sailplane trainertype
aircraft. The rpm was measured at
4,200.
At the 600 mA motor current and with
145 mAh Kokam Li-Poly batteries, the
estimated motor run time is 15 minutes. At
half throttle, this amount of time can be
greatly extended. Best of all is the fact that
FMA Direct sells the two-cell battery pack
for only $14.95.
Flying: This proved to be the best part. My
goal was to provide a simple and fast
application for the new miniature Cirrus
but not even enough to actually cover the
wood grain, which kind of “bleeds”
through.
Before spraying, cover the open RC
compartment, the servos, the battery, and
the motor area with masking tape. For a
spray, I took Don Stackhouse’s (of DJ
Aerotech) suggestion and bought a can of
Dupli-Color Import Auto Spray
(www.duplicolor.com). I used shade 88-
01517 Yellow, which I found at a local
Wal-Mart. It has a strong odor, so it is best
to apply it outside on a calm day. Drying
time is quite short.
The wing structure before covering
weighed 0.49 ounce. For it I purchased
Transparent Red Solite covering from Balsa
Products. After covering, the wing weighed
0.67 ounce; Solite added only 0.18 ounce to
the total weight.
When it was all finished, my Scratch-65
weighed 2.69 ounces, which yielded a wing
loading of only 3.7 ounces per square foot.
Balance was exactly as shown on the
plans—just slightly forward of the main
(and only) wing spar.
This entire aircraft was built from a
single sheet of 1⁄16 x 3 x 36-inch balsa and
roughly a half sheet of 1⁄32 balsa. In
Dupli-Color spray paint that Bob used to
mist a single coat of yellow on raw
fuselage and tail balsa surfaces.
36 MODEL AVIATION
Micro Joule RC airborne equipment. These
components worked flawlessly in a regular
flying-field environment with considerable
and varied activity going on all around.
The Scratch-65 took to the air on its first
flight, requiring no trim whatsoever. It flew
straight and level, with a slight gentle climb.
The power was enough that I could quickly
reduce the throttle to half and still maintain
the model’s altitude. Doing that slowed the
model to a point where almost anybody can
fly it. Better yet, it will stay up for an
extremely long time before needing a
recharge.
I have flown the Scratch-65 in winds
estimated at 5-7 mph with no difficulty. I
haven’t had a chance to fly it indoors, but I
suspect that at reduced throttle settings it
could easily be flown in a double-size
gymnasium facility.
Final control throws were measured at 3⁄8
inch either side for the rudder and 5⁄16 inch
either side of neutral for the elevator.
This fun aircraft can be enjoyed by rank
beginners as well as regular sport fliers. If
you haven’t tried flying a diminutive model
that still uses conventional proportional
servos, consider the Scratch-65. Please share
your photos and experiences with me! MA
Bob Aberle
[email protected]
Edition: Model Aviation - 2005/02
Page Numbers: 31,32,33,34,35,36,37,38
Sccrattcch-65
Electric-powered RC indoor
sailplane/trainer showcases new
miniature Cirrus Micro Joule components
A FEATURE THAT appeared in my MA “From the Ground Up”
installments was a construction article for a model that an RC
beginner could build strictly from raw materials without the aid of a
kit. We call this technique “building from scratch.”
The airplane I designed expressly for this feature was the
Scratch-One. It was presented in two parts, in the November 2003
and January 2004 issues. From all the photos that were sent in, many
of you built this little RC electric-powered training model with
excellent success.
Jumping ahead a little, I wrote another feature article titled
“Introduction to Indoor RC” (July 2004 MA). Right after it was
published, the people at Global Hobby Distributors sent in samples
of their newly updated Cirrus Micro Joule miniature RC-system
components.
This equipment was first made available well more than a year
ago. More recently, based on field experience, several modifications
were made to further enhance this micro RC equipment’s
performance. I was provided a Micro Joule receiver, three
microservos, and two types of tiny electric motor speed controllers.
My usual routine would be to photograph these components,
operate them, and report on them using a standard product-review
format. The thought came to mind that there aren’t very many RTF
or ARF models available for RC aircraft that weigh only
approximately 2.5 ounces. If I could design a simple model that the
average indoor RC beginner could easily construct, I could be
providing a specific application for these new Cirrus Micro Joule
components.
With the Scratch-One’s success so noted, I thought it might be
fun to scale it down to a size that would accept the Cirrus Micro
Joule equipment, a tiny electric motor, and several equally tiny Li-
Poly batteries, all in an aircraft nearing that 2.5-ounce total.
The result is this “Scratch-65”: a Scratch-One at 65% of its
original size. The wingspan worked out to roughly 30 inches, and the
area is slightly more than 100 square inches.
So settle in and read my showcase of the new Cirrus Micro Joule
miniature RC components, and then read on for the full (and simple)
construction article for the little Scratch-65. It has proven to be an
excellent flier and another perfect RC trainer.
Global Hobby Cirrus Micro Joule RC System: My sample airbornesystem
components consisted of the Cirrus Micro Joule four-channel
miniature FM receiver, two Cirrus Micro Joule CS-3
ultramicroservos, and two new 5-amp-rated ESCs: the S5A1 and the
S5A2. These items can be purchased separately.
Micro JST connectors with white plastic housing are used on
these RC components. There really isn’t any established
convention yet for the pin wiring on these tiny connectors. On
these Cirrus types, the center pin has a red wire for battery
positive. Gray wires are used for the two “outboard” pins that
contain the battery negative and the servo signal wire.
When you are using all Cirrus Micro Joule components, there
will be no problem with connector-pin polarity. But if you mix and
match from other manufacturers, you had best check the pin polarity
before you turn the power on for the first time.
The connections from the ESC to the motor and the battery pack
use the larger, red, plastic JST connectors, which adhere to the
industry-accepted polarity convention (pin No. 1 black negative and
pin No. 2 red positive).
The Micro Joule four-channel-function receiver has automatic
shift select, so it can basically be operated by any brand of RC FM
transmitter. Voltage range is claimed to be 3.5 to 7.0. The weight is
slightly more than 3 grams (or .11 ounce). Dimensions are 1.11
inches long, 0.31 inch wide, and 0.33 inch high. The crystal clearly
sticks out farther than this height dimension.
The antenna wire is 34 inches in length. I used the full-length
antenna to obtain a baseline check on the receiver’s range capability.
Later I will substitute an E-Cubed RC short antenna and report on
the effect of the radio reception.
This is a single-conversion receiver. The specifications simply
state that it is “narrow band.” I resorted to a couple of simple field
tests to verify that claim. I was able to put the Scratch-65 almost out
of sight (on a calm day!) and was able to maintain perfect control.
Then I had a club member set up his synthesized RC transmitter
one channel away from mine (that is only 20 kHz). I flew the model
down the length of the runway, allowing it to get much closer to the
other transmitter than to my own. Both transmitter antennas were
fully extended. I did this on several occasions and noticed no
interference of any kind.
Tom Hunt about to launch the Scratch-65, with Bob behind the
camera. This shows you how small the model really is.
February 2005 31
by Bob Aberle
Photos by the author and Mark Lanterman
A comparison of the Speed 400-powered Scratch-One and the
smaller Scratch-65. Both are easy to build and fun to fly!
It’s hard to tell the Scratch-65’s size in this flight shot. These small
models are stable and fly in much the same manner as their larger
cousins.
Global Hobby Cirrus Micro Joule RC micro components (bottom to
top): four-channel receiver, S5A2 ESC, two CS-3 ultramicroservos.
Total weight of this equipment is .36 ounce!
Cirrus Micro Joule four-channel receiver with dime for size
reference. Crystal must be purchased separately. Receiver
weighs 3 grams, or .11 ounce!
Different view of equipment in above right photo (bottom to top):
CS-3 ultramicro proportional servo with its separate cable, fourchannel
receiver with coiled receiver antenna wire at left, and one
of the two new 5-amp ESCs.
During an entire morning at my flying field, I made several longduration
flights. While I did this, many of my clubmates flew on a
variety of other 72 MHz RC channels. This Micro Joule receiver
never got hit! Keep in mind that this is not a laboratory test; it is
merely a practical test based on everyday-type flying. The receiver’s
street price is $39.99, and the required crystal is another $12.95.
The little CS-3 servos turned out to be lighter than the claim. I
measured one at 2.1 grams, so the two I used came to 4.2 grams, or
.15 ounce.
The cable going from the servo to the receiver is a separate item.
You plug one end into the servo and the other end into the receiver.
The length of the cable is slightly less than 3 inches. Other cable
lengths and cables that adapt to other equipment can be purchased
separately.
Voltage requirement for these servos is 3.0-7.0. At 4.8 volts, the
claim is for a 14 inch-ounce output torque. Transit time at 4.8 volts
is .18 second for 60° rotation.
The first issue of these tiny servos tended to be noisy, and the
neutral centering was observed to be inaccurate; the servo never
returned to exactly the same neutral position. This new version of
the Micro Joule servo seemed quieter. The neutral-return accuracy
also seems better. I can’t say it’s “dead on,” but it’s certainly
acceptable for anything except a high-speed, highly maneuverable
aircraft.
I don’t expect to see many airplanes in this category weighing
just 2.5 ounces! The street price for an individual Micro Joule
servo is $49.99, which is impressive for a 2.1-gram, fully
proportional-feedback servo.
32 MODEL AVIATION
Closer look at four-channel FM single-conversion receiver. It will
auto-select proper FM shift to work with any existing RC FM
transmitter.
Closer look at CS-3 servo. Cable is a separate item. You can
purchase longer cables or even cable adapters that will allow you
to use these servos with other RC components.
S5A1 ESC intended for use with one Li-Poly battery or two to
three Ni-Cd/NiMH cells. Red plastic JST connectors go to
battery and motor.
Peak Electronics unit is Bob’s favorite charger for smaller Li-Poly
batteries. It can handle one, two, or three Li-Poly cells at discrete
100-1500 mA charge currents.
Type: RC indoor sailplane/trainer
Wingspan: 30 inches
Power: GWS LPS B2C-C motor with 6.2:1 gear drive
Flying weight: 2.69 ounces
Construction: Balsa and plywood
Covering/finish: Dupli-Color spray enamel and Solite
Scratch-65
The first-issue Micro Joule ESC had a BEC regulator set for a
low 3.0 volts. Most of us felt this was done to accommodate
possibly a problem in either the receiver or servos. The regulated
voltage is now back up to the more appropriate 5.0-volt level. (I
verified this point!)
The cutoff-voltage level on the first Micro Joule ESC was set
low. It definitely wasn’t conducive to using Li-Poly batteries, where
it is important not to discharge to less than 2.5-3.0 volts per cell.
To counter this problem, Global has introduced two new types of
ESCs. Model S5A1 is intended for use with one Li-Poly battery or
two to three Ni-Cd/Nymph cells. The cutoff is set at 2.7 volts.
Continuous current rating is 5 amps, which is more than enough for
this application. It weighs 1.9 grams (.067 ounce) and has a street
price of $29.99.
The model S5A2 ESC is intended for use with two or three Li-
Poly batteries or three to six Ni-Cd/NiMH cells. This is the one I
used on the Scratch-65. It weighs 2.78 grams (.10 ounce) and
measures .74 inch long, .31 inch wide, and .25 inch high. That
weight includes one micro JST connector that goes to the throttle
port on the receiver, along with male and female red JST connectors
that go to the battery and motor. The BEC regulated voltage is again
5.0 volts, and the continuous current rating is 5 amps.
The voltage cutoff on this particular ESC is automatically set by
February 2005 33
All wood parts necessary to build Scratch-65. Almost entire model
is constructed from full 1⁄16 x 3 x 36-inch balsa sheet and roughly
half sheet of 1⁄32 balsa.
Wing under construction. LE is 1⁄8-inch-diameter hardwood dowel.
Center-panel spar is 1⁄16 x 1⁄4 spruce for added strength. There are
no wing ribs—only bent sticks of balsa.
Fuselage sides assembled with two formers and spacer sticks.
All tail-assembly pieces are made from 1⁄16 balsa sheet.
Wing panel ready to be joined, with fuselage sides with stiffeners
in place. Fuselage sides are made from 1⁄16 balsa sheet.
Scratch-65 in Action
In early November I attended and flew in what is probably
the biggest indoor RC Electric fly in the USA—the JR Indoor
Electric Festival—which was held in Columbus, Ohio. A
complete report on this event will be published soon in MA.
At one point I put my little Scratch-65 (all 2.6 ounces of it!)
up for a flight. After a minute or two of flying around, I
realized that many other modelers were flying along with me.
My helper made a count, and including mine there were 12
models in the air at one time. I continued to fly the Scratch-65
for more than 10 minutes. It covered most of the inflated dome
area from side to side and floor to ceiling.
Only after I landed did it dawn on me that I had not
experienced a single glitch with my radio system. So when you
ask me how well this Micro Joule equipment performs, my
answer is “outstanding.” MA
—Bob Aberle
the internal software. It will establish the cutoff voltage at 72% of a
fully charged battery. So if you hook up a fully charged two-cell Li-
Poly battery, the voltage might be roughly 7.4 volts. This ESC will
then set the cutoff voltage to 72% of that, or 5.3 volts (2.67 volts per
cell).
I tried monitoring this claimed auto cutoff. The first time I did not
have a fully charged battery pack; therefore, the auto-select voltage
set itself too low! When I made sure the pack was fully charged, the
voltage was set correctly.
The only thing that might bother me is that on occasion I will fly
out a half charge, land, take a rest, and then fly the remainder of the
charge. When I turn it on the second time (with only a half-charged
battery), will the ESC set the voltage too low?
The motor speed controlling was linear from full throttle down to
idle speeds. Street price is identical to the other ESC, at $29.99.
From what I have written, there have been notable improvements
in the Micro Joule equipment. The receiver range and selectivity is
excellent. Servo resolution seems adequate for almost any kind of
flying. The regulated voltage from the BEC is now in line with what
we normally expect, and the voltage cutoff point is now appropriate
for one, two, and three Li-Poly battery cells.
I received filtering capacitors with both ESCs and was told to put
them on my motor, but my GWS already had several capacitors
installed. I chose not to use the extra capacitors, and everything
worked perfectly.
The street price for this receiver, crystal, two servos, and your
choice of ESC is $178.15. If you want to go to full four-channel
control, you will need one more servo at $49.99.
34 MODEL AVIATION
Fully assembled Scratch-65 waiting for coat of spray paint and
wing covering. Nothing to be afraid of trying here.
Two CS-3 servos mounted on top of fuselage, out in the open,
just aft of wing TE. Servos are affixed with 1⁄16-inch double-stick
mounting tape.
GWS LPS B2C-C motor ready to slip on end of 1⁄4-inch-diameter
hardwood dowel mount. Dowel passes through plywood former
F1 and balsa former F2.
CONSTRUCTION
Since the plans for the Scratch-One were made with a computer
graphics system, all I had to do was specify the size reduction and I
had my new set of plans. A 50% decrease would have been
convenient, but it clearly resulted in a model that was too small. I
finally ended up at 65%, which yielded a wingspan of
approximately 30 inches and an exact wing area of 105 square
inches.
Adding the weight of the Cirrus Micro Joule RC components
(0.36 ounce), the motor and propeller (0.49 ounce), and the two-cell
FMA Direct Kokam 145 mAh Li-Poly battery (0.35 ounce) resulted
in a (nonaircraft) weight of 1.2 ounces. I usually use a figure
similar to two times the equipment weight to get a feel for the
model’s total weight. In this case that was 2.4 ounces. I guessed
that an even 2.5 would be okay. In the final analysis, it came to
2.69 ounces—not bad!
The next trick was to select the appropriate wood sizes to fit the
reduced-size aircraft. I chose 1⁄16 balsa for the fuselage sides and tail
pieces. The fuselage top and bottom was 1⁄32 balsa.
As with the Scratch-One, the wing had sticks instead of ribs to
make things easy for the beginner. The bottom stick was made from
1⁄16 balsa, and the top was made from 1⁄32 balsa. A single spar
seemed adequate for this size of model.
For the center panel I used 1⁄16 x 1⁄4 spruce for the spar, to
provide a bit of extra strength. The balsa tip-panel spars were made
from balsa and tapered from 1⁄4 inch in height to 1⁄8 inch at the tip.
As I did with the Scratch-One, I mounted the two tiny Micro
Joule servos on top of the fuselage, just aft of the wing TE. This
makes installing the control rods a snap. Because of the Scratch-
65’s small size, I selected .015-inch-diameter wire for the control
rods.
Then I inserted the wire into small-diameter Teflon tubing that I
purchased from House of Balsa. The tubing was intended for use
with cyanoacrylate cement bottles and is designated as catalog item
NEC-04 (6 feet of cyanoacrylate micro tubing).
On the Scratch-One, I placed the big battery pack on the
underside of the fuselage, centered on the model’s CG, so that the
battery could be accessed without removing the wing after every
flight. Modelers liked that feature!
With the lightweight two-cell FMA/Kokam 145 mAh Li-Poly
pack, I needed the weight as far forward as possible to maintain the
proper balance. I decided to make a small compartment on top, just
behind the firewall (F-1). The pack drops into this compartment,
fitting flush with the top of the fuselage. I used two small magnets
from Forcefield Inc. to hold the battery pack in place.
Choosing a motor was easy. I wanted the smallest available
GWS that would operate on two Li-Poly cells (roughly 7.4 volts).
The best and only choice proved to be the GWS LPS-series B2C-C
motor with a 6.2:1 gear reduction ratio. The best propeller for this
setup proved to be a GWS orange 6 x 5.
I mounted the motor by slipping it onto the end of a 1⁄4-inchdiameter
hardwood dowel that is passed through formers F-1 and F-
2. You may have to sand that dowel to get it to fit in the motor’s
mounting hole. Once it is in place, drill a small hole through the
motor casing and into the dowel. Insert a No. 2 sheet-metal screw
to anchor it in place.
The two-pin male connector exiting the motor will plug
perfectly into the motor connector located on the Micro Joule ESC.
If you ever need to replace a motor, you can easily do it at the
flying field since no soldering is necessary.
Hinging and Radio Installation: I made the rudder and elevator
hinges from strips of clear iron-on Doculam covering material
(available from Modelair Tech). I used Z-bends on the ends of the
wires attaching to the servo output arms. At the control-surface
ends I used Du-Bro micro E-Z connectors and control horns.
I affixed both servos, the receiver, and the ESC to the fuselage
with thin (1⁄16-inch) double-stick foam-rubber tape. I installed the
radio before I painted the model.
Covering and Finishing: I gave the entire fuselage and tail
assembly a light spray coat of paint—just enough to seal the wood,
February 2005 35
GWS LPS B2C-C motor in place on 1⁄4-inch dowel. No. 2 sheet-metal
screw firmly anchors motor so it can’t move, yet is easy to replace
when necessary.
Upper front portion of Scratch-65 showing motor and compartment
for two Kokam 145 mAh Li-Poly battery pack. Battery is held in
place with help of two Forcefield Inc. tiny magnet pairs.
Looking inside RC compartment located under wing, you can see
four-channel receiver affixed to one fuselage side with help of
double-stick tape.
One of the two Du-Bro micro E-Z connectors. Here the
connection is to the rudder horn, which is also a Du-Bro
product.
addition, it employs 1⁄8-inch dowel for the
wing LE, a strip of 3⁄32 x 1⁄4-inch balsa for
the TE, and a short length of 1⁄16 x 1⁄4-inch
spruce for the center-panel spar.
It was so little material that I chose not
to bother my buddy Craig Wagner at
Aerocraft Inc. for a “bag of balsa,” as I did
for the Scratch-One.
Motor Parameters: The GWS B2C-C
seemed happiest using the GWS 6 x 5
orange propeller. Motor current at full
charge and full throttle measured 600 mA,
7.0 volts, and 5 watts. That worked out to
1.86 watts per ounce, which is more than
enough for this powered sailplane trainertype
aircraft. The rpm was measured at
4,200.
At the 600 mA motor current and with
145 mAh Kokam Li-Poly batteries, the
estimated motor run time is 15 minutes. At
half throttle, this amount of time can be
greatly extended. Best of all is the fact that
FMA Direct sells the two-cell battery pack
for only $14.95.
Flying: This proved to be the best part. My
goal was to provide a simple and fast
application for the new miniature Cirrus
but not even enough to actually cover the
wood grain, which kind of “bleeds”
through.
Before spraying, cover the open RC
compartment, the servos, the battery, and
the motor area with masking tape. For a
spray, I took Don Stackhouse’s (of DJ
Aerotech) suggestion and bought a can of
Dupli-Color Import Auto Spray
(www.duplicolor.com). I used shade 88-
01517 Yellow, which I found at a local
Wal-Mart. It has a strong odor, so it is best
to apply it outside on a calm day. Drying
time is quite short.
The wing structure before covering
weighed 0.49 ounce. For it I purchased
Transparent Red Solite covering from Balsa
Products. After covering, the wing weighed
0.67 ounce; Solite added only 0.18 ounce to
the total weight.
When it was all finished, my Scratch-65
weighed 2.69 ounces, which yielded a wing
loading of only 3.7 ounces per square foot.
Balance was exactly as shown on the
plans—just slightly forward of the main
(and only) wing spar.
This entire aircraft was built from a
single sheet of 1⁄16 x 3 x 36-inch balsa and
roughly a half sheet of 1⁄32 balsa. In
Dupli-Color spray paint that Bob used to
mist a single coat of yellow on raw
fuselage and tail balsa surfaces.
36 MODEL AVIATION
Micro Joule RC airborne equipment. These
components worked flawlessly in a regular
flying-field environment with considerable
and varied activity going on all around.
The Scratch-65 took to the air on its first
flight, requiring no trim whatsoever. It flew
straight and level, with a slight gentle climb.
The power was enough that I could quickly
reduce the throttle to half and still maintain
the model’s altitude. Doing that slowed the
model to a point where almost anybody can
fly it. Better yet, it will stay up for an
extremely long time before needing a
recharge.
I have flown the Scratch-65 in winds
estimated at 5-7 mph with no difficulty. I
haven’t had a chance to fly it indoors, but I
suspect that at reduced throttle settings it
could easily be flown in a double-size
gymnasium facility.
Final control throws were measured at 3⁄8
inch either side for the rudder and 5⁄16 inch
either side of neutral for the elevator.
This fun aircraft can be enjoyed by rank
beginners as well as regular sport fliers. If
you haven’t tried flying a diminutive model
that still uses conventional proportional
servos, consider the Scratch-65. Please share
your photos and experiences with me! MA
Bob Aberle
[email protected]
Edition: Model Aviation - 2005/02
Page Numbers: 31,32,33,34,35,36,37,38
Sccrattcch-65
Electric-powered RC indoor
sailplane/trainer showcases new
miniature Cirrus Micro Joule components
A FEATURE THAT appeared in my MA “From the Ground Up”
installments was a construction article for a model that an RC
beginner could build strictly from raw materials without the aid of a
kit. We call this technique “building from scratch.”
The airplane I designed expressly for this feature was the
Scratch-One. It was presented in two parts, in the November 2003
and January 2004 issues. From all the photos that were sent in, many
of you built this little RC electric-powered training model with
excellent success.
Jumping ahead a little, I wrote another feature article titled
“Introduction to Indoor RC” (July 2004 MA). Right after it was
published, the people at Global Hobby Distributors sent in samples
of their newly updated Cirrus Micro Joule miniature RC-system
components.
This equipment was first made available well more than a year
ago. More recently, based on field experience, several modifications
were made to further enhance this micro RC equipment’s
performance. I was provided a Micro Joule receiver, three
microservos, and two types of tiny electric motor speed controllers.
My usual routine would be to photograph these components,
operate them, and report on them using a standard product-review
format. The thought came to mind that there aren’t very many RTF
or ARF models available for RC aircraft that weigh only
approximately 2.5 ounces. If I could design a simple model that the
average indoor RC beginner could easily construct, I could be
providing a specific application for these new Cirrus Micro Joule
components.
With the Scratch-One’s success so noted, I thought it might be
fun to scale it down to a size that would accept the Cirrus Micro
Joule equipment, a tiny electric motor, and several equally tiny Li-
Poly batteries, all in an aircraft nearing that 2.5-ounce total.
The result is this “Scratch-65”: a Scratch-One at 65% of its
original size. The wingspan worked out to roughly 30 inches, and the
area is slightly more than 100 square inches.
So settle in and read my showcase of the new Cirrus Micro Joule
miniature RC components, and then read on for the full (and simple)
construction article for the little Scratch-65. It has proven to be an
excellent flier and another perfect RC trainer.
Global Hobby Cirrus Micro Joule RC System: My sample airbornesystem
components consisted of the Cirrus Micro Joule four-channel
miniature FM receiver, two Cirrus Micro Joule CS-3
ultramicroservos, and two new 5-amp-rated ESCs: the S5A1 and the
S5A2. These items can be purchased separately.
Micro JST connectors with white plastic housing are used on
these RC components. There really isn’t any established
convention yet for the pin wiring on these tiny connectors. On
these Cirrus types, the center pin has a red wire for battery
positive. Gray wires are used for the two “outboard” pins that
contain the battery negative and the servo signal wire.
When you are using all Cirrus Micro Joule components, there
will be no problem with connector-pin polarity. But if you mix and
match from other manufacturers, you had best check the pin polarity
before you turn the power on for the first time.
The connections from the ESC to the motor and the battery pack
use the larger, red, plastic JST connectors, which adhere to the
industry-accepted polarity convention (pin No. 1 black negative and
pin No. 2 red positive).
The Micro Joule four-channel-function receiver has automatic
shift select, so it can basically be operated by any brand of RC FM
transmitter. Voltage range is claimed to be 3.5 to 7.0. The weight is
slightly more than 3 grams (or .11 ounce). Dimensions are 1.11
inches long, 0.31 inch wide, and 0.33 inch high. The crystal clearly
sticks out farther than this height dimension.
The antenna wire is 34 inches in length. I used the full-length
antenna to obtain a baseline check on the receiver’s range capability.
Later I will substitute an E-Cubed RC short antenna and report on
the effect of the radio reception.
This is a single-conversion receiver. The specifications simply
state that it is “narrow band.” I resorted to a couple of simple field
tests to verify that claim. I was able to put the Scratch-65 almost out
of sight (on a calm day!) and was able to maintain perfect control.
Then I had a club member set up his synthesized RC transmitter
one channel away from mine (that is only 20 kHz). I flew the model
down the length of the runway, allowing it to get much closer to the
other transmitter than to my own. Both transmitter antennas were
fully extended. I did this on several occasions and noticed no
interference of any kind.
Tom Hunt about to launch the Scratch-65, with Bob behind the
camera. This shows you how small the model really is.
February 2005 31
by Bob Aberle
Photos by the author and Mark Lanterman
A comparison of the Speed 400-powered Scratch-One and the
smaller Scratch-65. Both are easy to build and fun to fly!
It’s hard to tell the Scratch-65’s size in this flight shot. These small
models are stable and fly in much the same manner as their larger
cousins.
Global Hobby Cirrus Micro Joule RC micro components (bottom to
top): four-channel receiver, S5A2 ESC, two CS-3 ultramicroservos.
Total weight of this equipment is .36 ounce!
Cirrus Micro Joule four-channel receiver with dime for size
reference. Crystal must be purchased separately. Receiver
weighs 3 grams, or .11 ounce!
Different view of equipment in above right photo (bottom to top):
CS-3 ultramicro proportional servo with its separate cable, fourchannel
receiver with coiled receiver antenna wire at left, and one
of the two new 5-amp ESCs.
During an entire morning at my flying field, I made several longduration
flights. While I did this, many of my clubmates flew on a
variety of other 72 MHz RC channels. This Micro Joule receiver
never got hit! Keep in mind that this is not a laboratory test; it is
merely a practical test based on everyday-type flying. The receiver’s
street price is $39.99, and the required crystal is another $12.95.
The little CS-3 servos turned out to be lighter than the claim. I
measured one at 2.1 grams, so the two I used came to 4.2 grams, or
.15 ounce.
The cable going from the servo to the receiver is a separate item.
You plug one end into the servo and the other end into the receiver.
The length of the cable is slightly less than 3 inches. Other cable
lengths and cables that adapt to other equipment can be purchased
separately.
Voltage requirement for these servos is 3.0-7.0. At 4.8 volts, the
claim is for a 14 inch-ounce output torque. Transit time at 4.8 volts
is .18 second for 60° rotation.
The first issue of these tiny servos tended to be noisy, and the
neutral centering was observed to be inaccurate; the servo never
returned to exactly the same neutral position. This new version of
the Micro Joule servo seemed quieter. The neutral-return accuracy
also seems better. I can’t say it’s “dead on,” but it’s certainly
acceptable for anything except a high-speed, highly maneuverable
aircraft.
I don’t expect to see many airplanes in this category weighing
just 2.5 ounces! The street price for an individual Micro Joule
servo is $49.99, which is impressive for a 2.1-gram, fully
proportional-feedback servo.
32 MODEL AVIATION
Closer look at four-channel FM single-conversion receiver. It will
auto-select proper FM shift to work with any existing RC FM
transmitter.
Closer look at CS-3 servo. Cable is a separate item. You can
purchase longer cables or even cable adapters that will allow you
to use these servos with other RC components.
S5A1 ESC intended for use with one Li-Poly battery or two to
three Ni-Cd/NiMH cells. Red plastic JST connectors go to
battery and motor.
Peak Electronics unit is Bob’s favorite charger for smaller Li-Poly
batteries. It can handle one, two, or three Li-Poly cells at discrete
100-1500 mA charge currents.
Type: RC indoor sailplane/trainer
Wingspan: 30 inches
Power: GWS LPS B2C-C motor with 6.2:1 gear drive
Flying weight: 2.69 ounces
Construction: Balsa and plywood
Covering/finish: Dupli-Color spray enamel and Solite
Scratch-65
The first-issue Micro Joule ESC had a BEC regulator set for a
low 3.0 volts. Most of us felt this was done to accommodate
possibly a problem in either the receiver or servos. The regulated
voltage is now back up to the more appropriate 5.0-volt level. (I
verified this point!)
The cutoff-voltage level on the first Micro Joule ESC was set
low. It definitely wasn’t conducive to using Li-Poly batteries, where
it is important not to discharge to less than 2.5-3.0 volts per cell.
To counter this problem, Global has introduced two new types of
ESCs. Model S5A1 is intended for use with one Li-Poly battery or
two to three Ni-Cd/Nymph cells. The cutoff is set at 2.7 volts.
Continuous current rating is 5 amps, which is more than enough for
this application. It weighs 1.9 grams (.067 ounce) and has a street
price of $29.99.
The model S5A2 ESC is intended for use with two or three Li-
Poly batteries or three to six Ni-Cd/NiMH cells. This is the one I
used on the Scratch-65. It weighs 2.78 grams (.10 ounce) and
measures .74 inch long, .31 inch wide, and .25 inch high. That
weight includes one micro JST connector that goes to the throttle
port on the receiver, along with male and female red JST connectors
that go to the battery and motor. The BEC regulated voltage is again
5.0 volts, and the continuous current rating is 5 amps.
The voltage cutoff on this particular ESC is automatically set by
February 2005 33
All wood parts necessary to build Scratch-65. Almost entire model
is constructed from full 1⁄16 x 3 x 36-inch balsa sheet and roughly
half sheet of 1⁄32 balsa.
Wing under construction. LE is 1⁄8-inch-diameter hardwood dowel.
Center-panel spar is 1⁄16 x 1⁄4 spruce for added strength. There are
no wing ribs—only bent sticks of balsa.
Fuselage sides assembled with two formers and spacer sticks.
All tail-assembly pieces are made from 1⁄16 balsa sheet.
Wing panel ready to be joined, with fuselage sides with stiffeners
in place. Fuselage sides are made from 1⁄16 balsa sheet.
Scratch-65 in Action
In early November I attended and flew in what is probably
the biggest indoor RC Electric fly in the USA—the JR Indoor
Electric Festival—which was held in Columbus, Ohio. A
complete report on this event will be published soon in MA.
At one point I put my little Scratch-65 (all 2.6 ounces of it!)
up for a flight. After a minute or two of flying around, I
realized that many other modelers were flying along with me.
My helper made a count, and including mine there were 12
models in the air at one time. I continued to fly the Scratch-65
for more than 10 minutes. It covered most of the inflated dome
area from side to side and floor to ceiling.
Only after I landed did it dawn on me that I had not
experienced a single glitch with my radio system. So when you
ask me how well this Micro Joule equipment performs, my
answer is “outstanding.” MA
—Bob Aberle
the internal software. It will establish the cutoff voltage at 72% of a
fully charged battery. So if you hook up a fully charged two-cell Li-
Poly battery, the voltage might be roughly 7.4 volts. This ESC will
then set the cutoff voltage to 72% of that, or 5.3 volts (2.67 volts per
cell).
I tried monitoring this claimed auto cutoff. The first time I did not
have a fully charged battery pack; therefore, the auto-select voltage
set itself too low! When I made sure the pack was fully charged, the
voltage was set correctly.
The only thing that might bother me is that on occasion I will fly
out a half charge, land, take a rest, and then fly the remainder of the
charge. When I turn it on the second time (with only a half-charged
battery), will the ESC set the voltage too low?
The motor speed controlling was linear from full throttle down to
idle speeds. Street price is identical to the other ESC, at $29.99.
From what I have written, there have been notable improvements
in the Micro Joule equipment. The receiver range and selectivity is
excellent. Servo resolution seems adequate for almost any kind of
flying. The regulated voltage from the BEC is now in line with what
we normally expect, and the voltage cutoff point is now appropriate
for one, two, and three Li-Poly battery cells.
I received filtering capacitors with both ESCs and was told to put
them on my motor, but my GWS already had several capacitors
installed. I chose not to use the extra capacitors, and everything
worked perfectly.
The street price for this receiver, crystal, two servos, and your
choice of ESC is $178.15. If you want to go to full four-channel
control, you will need one more servo at $49.99.
34 MODEL AVIATION
Fully assembled Scratch-65 waiting for coat of spray paint and
wing covering. Nothing to be afraid of trying here.
Two CS-3 servos mounted on top of fuselage, out in the open,
just aft of wing TE. Servos are affixed with 1⁄16-inch double-stick
mounting tape.
GWS LPS B2C-C motor ready to slip on end of 1⁄4-inch-diameter
hardwood dowel mount. Dowel passes through plywood former
F1 and balsa former F2.
CONSTRUCTION
Since the plans for the Scratch-One were made with a computer
graphics system, all I had to do was specify the size reduction and I
had my new set of plans. A 50% decrease would have been
convenient, but it clearly resulted in a model that was too small. I
finally ended up at 65%, which yielded a wingspan of
approximately 30 inches and an exact wing area of 105 square
inches.
Adding the weight of the Cirrus Micro Joule RC components
(0.36 ounce), the motor and propeller (0.49 ounce), and the two-cell
FMA Direct Kokam 145 mAh Li-Poly battery (0.35 ounce) resulted
in a (nonaircraft) weight of 1.2 ounces. I usually use a figure
similar to two times the equipment weight to get a feel for the
model’s total weight. In this case that was 2.4 ounces. I guessed
that an even 2.5 would be okay. In the final analysis, it came to
2.69 ounces—not bad!
The next trick was to select the appropriate wood sizes to fit the
reduced-size aircraft. I chose 1⁄16 balsa for the fuselage sides and tail
pieces. The fuselage top and bottom was 1⁄32 balsa.
As with the Scratch-One, the wing had sticks instead of ribs to
make things easy for the beginner. The bottom stick was made from
1⁄16 balsa, and the top was made from 1⁄32 balsa. A single spar
seemed adequate for this size of model.
For the center panel I used 1⁄16 x 1⁄4 spruce for the spar, to
provide a bit of extra strength. The balsa tip-panel spars were made
from balsa and tapered from 1⁄4 inch in height to 1⁄8 inch at the tip.
As I did with the Scratch-One, I mounted the two tiny Micro
Joule servos on top of the fuselage, just aft of the wing TE. This
makes installing the control rods a snap. Because of the Scratch-
65’s small size, I selected .015-inch-diameter wire for the control
rods.
Then I inserted the wire into small-diameter Teflon tubing that I
purchased from House of Balsa. The tubing was intended for use
with cyanoacrylate cement bottles and is designated as catalog item
NEC-04 (6 feet of cyanoacrylate micro tubing).
On the Scratch-One, I placed the big battery pack on the
underside of the fuselage, centered on the model’s CG, so that the
battery could be accessed without removing the wing after every
flight. Modelers liked that feature!
With the lightweight two-cell FMA/Kokam 145 mAh Li-Poly
pack, I needed the weight as far forward as possible to maintain the
proper balance. I decided to make a small compartment on top, just
behind the firewall (F-1). The pack drops into this compartment,
fitting flush with the top of the fuselage. I used two small magnets
from Forcefield Inc. to hold the battery pack in place.
Choosing a motor was easy. I wanted the smallest available
GWS that would operate on two Li-Poly cells (roughly 7.4 volts).
The best and only choice proved to be the GWS LPS-series B2C-C
motor with a 6.2:1 gear reduction ratio. The best propeller for this
setup proved to be a GWS orange 6 x 5.
I mounted the motor by slipping it onto the end of a 1⁄4-inchdiameter
hardwood dowel that is passed through formers F-1 and F-
2. You may have to sand that dowel to get it to fit in the motor’s
mounting hole. Once it is in place, drill a small hole through the
motor casing and into the dowel. Insert a No. 2 sheet-metal screw
to anchor it in place.
The two-pin male connector exiting the motor will plug
perfectly into the motor connector located on the Micro Joule ESC.
If you ever need to replace a motor, you can easily do it at the
flying field since no soldering is necessary.
Hinging and Radio Installation: I made the rudder and elevator
hinges from strips of clear iron-on Doculam covering material
(available from Modelair Tech). I used Z-bends on the ends of the
wires attaching to the servo output arms. At the control-surface
ends I used Du-Bro micro E-Z connectors and control horns.
I affixed both servos, the receiver, and the ESC to the fuselage
with thin (1⁄16-inch) double-stick foam-rubber tape. I installed the
radio before I painted the model.
Covering and Finishing: I gave the entire fuselage and tail
assembly a light spray coat of paint—just enough to seal the wood,
February 2005 35
GWS LPS B2C-C motor in place on 1⁄4-inch dowel. No. 2 sheet-metal
screw firmly anchors motor so it can’t move, yet is easy to replace
when necessary.
Upper front portion of Scratch-65 showing motor and compartment
for two Kokam 145 mAh Li-Poly battery pack. Battery is held in
place with help of two Forcefield Inc. tiny magnet pairs.
Looking inside RC compartment located under wing, you can see
four-channel receiver affixed to one fuselage side with help of
double-stick tape.
One of the two Du-Bro micro E-Z connectors. Here the
connection is to the rudder horn, which is also a Du-Bro
product.
addition, it employs 1⁄8-inch dowel for the
wing LE, a strip of 3⁄32 x 1⁄4-inch balsa for
the TE, and a short length of 1⁄16 x 1⁄4-inch
spruce for the center-panel spar.
It was so little material that I chose not
to bother my buddy Craig Wagner at
Aerocraft Inc. for a “bag of balsa,” as I did
for the Scratch-One.
Motor Parameters: The GWS B2C-C
seemed happiest using the GWS 6 x 5
orange propeller. Motor current at full
charge and full throttle measured 600 mA,
7.0 volts, and 5 watts. That worked out to
1.86 watts per ounce, which is more than
enough for this powered sailplane trainertype
aircraft. The rpm was measured at
4,200.
At the 600 mA motor current and with
145 mAh Kokam Li-Poly batteries, the
estimated motor run time is 15 minutes. At
half throttle, this amount of time can be
greatly extended. Best of all is the fact that
FMA Direct sells the two-cell battery pack
for only $14.95.
Flying: This proved to be the best part. My
goal was to provide a simple and fast
application for the new miniature Cirrus
but not even enough to actually cover the
wood grain, which kind of “bleeds”
through.
Before spraying, cover the open RC
compartment, the servos, the battery, and
the motor area with masking tape. For a
spray, I took Don Stackhouse’s (of DJ
Aerotech) suggestion and bought a can of
Dupli-Color Import Auto Spray
(www.duplicolor.com). I used shade 88-
01517 Yellow, which I found at a local
Wal-Mart. It has a strong odor, so it is best
to apply it outside on a calm day. Drying
time is quite short.
The wing structure before covering
weighed 0.49 ounce. For it I purchased
Transparent Red Solite covering from Balsa
Products. After covering, the wing weighed
0.67 ounce; Solite added only 0.18 ounce to
the total weight.
When it was all finished, my Scratch-65
weighed 2.69 ounces, which yielded a wing
loading of only 3.7 ounces per square foot.
Balance was exactly as shown on the
plans—just slightly forward of the main
(and only) wing spar.
This entire aircraft was built from a
single sheet of 1⁄16 x 3 x 36-inch balsa and
roughly a half sheet of 1⁄32 balsa. In
Dupli-Color spray paint that Bob used to
mist a single coat of yellow on raw
fuselage and tail balsa surfaces.
36 MODEL AVIATION
Micro Joule RC airborne equipment. These
components worked flawlessly in a regular
flying-field environment with considerable
and varied activity going on all around.
The Scratch-65 took to the air on its first
flight, requiring no trim whatsoever. It flew
straight and level, with a slight gentle climb.
The power was enough that I could quickly
reduce the throttle to half and still maintain
the model’s altitude. Doing that slowed the
model to a point where almost anybody can
fly it. Better yet, it will stay up for an
extremely long time before needing a
recharge.
I have flown the Scratch-65 in winds
estimated at 5-7 mph with no difficulty. I
haven’t had a chance to fly it indoors, but I
suspect that at reduced throttle settings it
could easily be flown in a double-size
gymnasium facility.
Final control throws were measured at 3⁄8
inch either side for the rudder and 5⁄16 inch
either side of neutral for the elevator.
This fun aircraft can be enjoyed by rank
beginners as well as regular sport fliers. If
you haven’t tried flying a diminutive model
that still uses conventional proportional
servos, consider the Scratch-65. Please share
your photos and experiences with me! MA
Bob Aberle
[email protected]
Edition: Model Aviation - 2005/02
Page Numbers: 31,32,33,34,35,36,37,38
Sccrattcch-65
Electric-powered RC indoor
sailplane/trainer showcases new
miniature Cirrus Micro Joule components
A FEATURE THAT appeared in my MA “From the Ground Up”
installments was a construction article for a model that an RC
beginner could build strictly from raw materials without the aid of a
kit. We call this technique “building from scratch.”
The airplane I designed expressly for this feature was the
Scratch-One. It was presented in two parts, in the November 2003
and January 2004 issues. From all the photos that were sent in, many
of you built this little RC electric-powered training model with
excellent success.
Jumping ahead a little, I wrote another feature article titled
“Introduction to Indoor RC” (July 2004 MA). Right after it was
published, the people at Global Hobby Distributors sent in samples
of their newly updated Cirrus Micro Joule miniature RC-system
components.
This equipment was first made available well more than a year
ago. More recently, based on field experience, several modifications
were made to further enhance this micro RC equipment’s
performance. I was provided a Micro Joule receiver, three
microservos, and two types of tiny electric motor speed controllers.
My usual routine would be to photograph these components,
operate them, and report on them using a standard product-review
format. The thought came to mind that there aren’t very many RTF
or ARF models available for RC aircraft that weigh only
approximately 2.5 ounces. If I could design a simple model that the
average indoor RC beginner could easily construct, I could be
providing a specific application for these new Cirrus Micro Joule
components.
With the Scratch-One’s success so noted, I thought it might be
fun to scale it down to a size that would accept the Cirrus Micro
Joule equipment, a tiny electric motor, and several equally tiny Li-
Poly batteries, all in an aircraft nearing that 2.5-ounce total.
The result is this “Scratch-65”: a Scratch-One at 65% of its
original size. The wingspan worked out to roughly 30 inches, and the
area is slightly more than 100 square inches.
So settle in and read my showcase of the new Cirrus Micro Joule
miniature RC components, and then read on for the full (and simple)
construction article for the little Scratch-65. It has proven to be an
excellent flier and another perfect RC trainer.
Global Hobby Cirrus Micro Joule RC System: My sample airbornesystem
components consisted of the Cirrus Micro Joule four-channel
miniature FM receiver, two Cirrus Micro Joule CS-3
ultramicroservos, and two new 5-amp-rated ESCs: the S5A1 and the
S5A2. These items can be purchased separately.
Micro JST connectors with white plastic housing are used on
these RC components. There really isn’t any established
convention yet for the pin wiring on these tiny connectors. On
these Cirrus types, the center pin has a red wire for battery
positive. Gray wires are used for the two “outboard” pins that
contain the battery negative and the servo signal wire.
When you are using all Cirrus Micro Joule components, there
will be no problem with connector-pin polarity. But if you mix and
match from other manufacturers, you had best check the pin polarity
before you turn the power on for the first time.
The connections from the ESC to the motor and the battery pack
use the larger, red, plastic JST connectors, which adhere to the
industry-accepted polarity convention (pin No. 1 black negative and
pin No. 2 red positive).
The Micro Joule four-channel-function receiver has automatic
shift select, so it can basically be operated by any brand of RC FM
transmitter. Voltage range is claimed to be 3.5 to 7.0. The weight is
slightly more than 3 grams (or .11 ounce). Dimensions are 1.11
inches long, 0.31 inch wide, and 0.33 inch high. The crystal clearly
sticks out farther than this height dimension.
The antenna wire is 34 inches in length. I used the full-length
antenna to obtain a baseline check on the receiver’s range capability.
Later I will substitute an E-Cubed RC short antenna and report on
the effect of the radio reception.
This is a single-conversion receiver. The specifications simply
state that it is “narrow band.” I resorted to a couple of simple field
tests to verify that claim. I was able to put the Scratch-65 almost out
of sight (on a calm day!) and was able to maintain perfect control.
Then I had a club member set up his synthesized RC transmitter
one channel away from mine (that is only 20 kHz). I flew the model
down the length of the runway, allowing it to get much closer to the
other transmitter than to my own. Both transmitter antennas were
fully extended. I did this on several occasions and noticed no
interference of any kind.
Tom Hunt about to launch the Scratch-65, with Bob behind the
camera. This shows you how small the model really is.
February 2005 31
by Bob Aberle
Photos by the author and Mark Lanterman
A comparison of the Speed 400-powered Scratch-One and the
smaller Scratch-65. Both are easy to build and fun to fly!
It’s hard to tell the Scratch-65’s size in this flight shot. These small
models are stable and fly in much the same manner as their larger
cousins.
Global Hobby Cirrus Micro Joule RC micro components (bottom to
top): four-channel receiver, S5A2 ESC, two CS-3 ultramicroservos.
Total weight of this equipment is .36 ounce!
Cirrus Micro Joule four-channel receiver with dime for size
reference. Crystal must be purchased separately. Receiver
weighs 3 grams, or .11 ounce!
Different view of equipment in above right photo (bottom to top):
CS-3 ultramicro proportional servo with its separate cable, fourchannel
receiver with coiled receiver antenna wire at left, and one
of the two new 5-amp ESCs.
During an entire morning at my flying field, I made several longduration
flights. While I did this, many of my clubmates flew on a
variety of other 72 MHz RC channels. This Micro Joule receiver
never got hit! Keep in mind that this is not a laboratory test; it is
merely a practical test based on everyday-type flying. The receiver’s
street price is $39.99, and the required crystal is another $12.95.
The little CS-3 servos turned out to be lighter than the claim. I
measured one at 2.1 grams, so the two I used came to 4.2 grams, or
.15 ounce.
The cable going from the servo to the receiver is a separate item.
You plug one end into the servo and the other end into the receiver.
The length of the cable is slightly less than 3 inches. Other cable
lengths and cables that adapt to other equipment can be purchased
separately.
Voltage requirement for these servos is 3.0-7.0. At 4.8 volts, the
claim is for a 14 inch-ounce output torque. Transit time at 4.8 volts
is .18 second for 60° rotation.
The first issue of these tiny servos tended to be noisy, and the
neutral centering was observed to be inaccurate; the servo never
returned to exactly the same neutral position. This new version of
the Micro Joule servo seemed quieter. The neutral-return accuracy
also seems better. I can’t say it’s “dead on,” but it’s certainly
acceptable for anything except a high-speed, highly maneuverable
aircraft.
I don’t expect to see many airplanes in this category weighing
just 2.5 ounces! The street price for an individual Micro Joule
servo is $49.99, which is impressive for a 2.1-gram, fully
proportional-feedback servo.
32 MODEL AVIATION
Closer look at four-channel FM single-conversion receiver. It will
auto-select proper FM shift to work with any existing RC FM
transmitter.
Closer look at CS-3 servo. Cable is a separate item. You can
purchase longer cables or even cable adapters that will allow you
to use these servos with other RC components.
S5A1 ESC intended for use with one Li-Poly battery or two to
three Ni-Cd/NiMH cells. Red plastic JST connectors go to
battery and motor.
Peak Electronics unit is Bob’s favorite charger for smaller Li-Poly
batteries. It can handle one, two, or three Li-Poly cells at discrete
100-1500 mA charge currents.
Type: RC indoor sailplane/trainer
Wingspan: 30 inches
Power: GWS LPS B2C-C motor with 6.2:1 gear drive
Flying weight: 2.69 ounces
Construction: Balsa and plywood
Covering/finish: Dupli-Color spray enamel and Solite
Scratch-65
The first-issue Micro Joule ESC had a BEC regulator set for a
low 3.0 volts. Most of us felt this was done to accommodate
possibly a problem in either the receiver or servos. The regulated
voltage is now back up to the more appropriate 5.0-volt level. (I
verified this point!)
The cutoff-voltage level on the first Micro Joule ESC was set
low. It definitely wasn’t conducive to using Li-Poly batteries, where
it is important not to discharge to less than 2.5-3.0 volts per cell.
To counter this problem, Global has introduced two new types of
ESCs. Model S5A1 is intended for use with one Li-Poly battery or
two to three Ni-Cd/Nymph cells. The cutoff is set at 2.7 volts.
Continuous current rating is 5 amps, which is more than enough for
this application. It weighs 1.9 grams (.067 ounce) and has a street
price of $29.99.
The model S5A2 ESC is intended for use with two or three Li-
Poly batteries or three to six Ni-Cd/NiMH cells. This is the one I
used on the Scratch-65. It weighs 2.78 grams (.10 ounce) and
measures .74 inch long, .31 inch wide, and .25 inch high. That
weight includes one micro JST connector that goes to the throttle
port on the receiver, along with male and female red JST connectors
that go to the battery and motor. The BEC regulated voltage is again
5.0 volts, and the continuous current rating is 5 amps.
The voltage cutoff on this particular ESC is automatically set by
February 2005 33
All wood parts necessary to build Scratch-65. Almost entire model
is constructed from full 1⁄16 x 3 x 36-inch balsa sheet and roughly
half sheet of 1⁄32 balsa.
Wing under construction. LE is 1⁄8-inch-diameter hardwood dowel.
Center-panel spar is 1⁄16 x 1⁄4 spruce for added strength. There are
no wing ribs—only bent sticks of balsa.
Fuselage sides assembled with two formers and spacer sticks.
All tail-assembly pieces are made from 1⁄16 balsa sheet.
Wing panel ready to be joined, with fuselage sides with stiffeners
in place. Fuselage sides are made from 1⁄16 balsa sheet.
Scratch-65 in Action
In early November I attended and flew in what is probably
the biggest indoor RC Electric fly in the USA—the JR Indoor
Electric Festival—which was held in Columbus, Ohio. A
complete report on this event will be published soon in MA.
At one point I put my little Scratch-65 (all 2.6 ounces of it!)
up for a flight. After a minute or two of flying around, I
realized that many other modelers were flying along with me.
My helper made a count, and including mine there were 12
models in the air at one time. I continued to fly the Scratch-65
for more than 10 minutes. It covered most of the inflated dome
area from side to side and floor to ceiling.
Only after I landed did it dawn on me that I had not
experienced a single glitch with my radio system. So when you
ask me how well this Micro Joule equipment performs, my
answer is “outstanding.” MA
—Bob Aberle
the internal software. It will establish the cutoff voltage at 72% of a
fully charged battery. So if you hook up a fully charged two-cell Li-
Poly battery, the voltage might be roughly 7.4 volts. This ESC will
then set the cutoff voltage to 72% of that, or 5.3 volts (2.67 volts per
cell).
I tried monitoring this claimed auto cutoff. The first time I did not
have a fully charged battery pack; therefore, the auto-select voltage
set itself too low! When I made sure the pack was fully charged, the
voltage was set correctly.
The only thing that might bother me is that on occasion I will fly
out a half charge, land, take a rest, and then fly the remainder of the
charge. When I turn it on the second time (with only a half-charged
battery), will the ESC set the voltage too low?
The motor speed controlling was linear from full throttle down to
idle speeds. Street price is identical to the other ESC, at $29.99.
From what I have written, there have been notable improvements
in the Micro Joule equipment. The receiver range and selectivity is
excellent. Servo resolution seems adequate for almost any kind of
flying. The regulated voltage from the BEC is now in line with what
we normally expect, and the voltage cutoff point is now appropriate
for one, two, and three Li-Poly battery cells.
I received filtering capacitors with both ESCs and was told to put
them on my motor, but my GWS already had several capacitors
installed. I chose not to use the extra capacitors, and everything
worked perfectly.
The street price for this receiver, crystal, two servos, and your
choice of ESC is $178.15. If you want to go to full four-channel
control, you will need one more servo at $49.99.
34 MODEL AVIATION
Fully assembled Scratch-65 waiting for coat of spray paint and
wing covering. Nothing to be afraid of trying here.
Two CS-3 servos mounted on top of fuselage, out in the open,
just aft of wing TE. Servos are affixed with 1⁄16-inch double-stick
mounting tape.
GWS LPS B2C-C motor ready to slip on end of 1⁄4-inch-diameter
hardwood dowel mount. Dowel passes through plywood former
F1 and balsa former F2.
CONSTRUCTION
Since the plans for the Scratch-One were made with a computer
graphics system, all I had to do was specify the size reduction and I
had my new set of plans. A 50% decrease would have been
convenient, but it clearly resulted in a model that was too small. I
finally ended up at 65%, which yielded a wingspan of
approximately 30 inches and an exact wing area of 105 square
inches.
Adding the weight of the Cirrus Micro Joule RC components
(0.36 ounce), the motor and propeller (0.49 ounce), and the two-cell
FMA Direct Kokam 145 mAh Li-Poly battery (0.35 ounce) resulted
in a (nonaircraft) weight of 1.2 ounces. I usually use a figure
similar to two times the equipment weight to get a feel for the
model’s total weight. In this case that was 2.4 ounces. I guessed
that an even 2.5 would be okay. In the final analysis, it came to
2.69 ounces—not bad!
The next trick was to select the appropriate wood sizes to fit the
reduced-size aircraft. I chose 1⁄16 balsa for the fuselage sides and tail
pieces. The fuselage top and bottom was 1⁄32 balsa.
As with the Scratch-One, the wing had sticks instead of ribs to
make things easy for the beginner. The bottom stick was made from
1⁄16 balsa, and the top was made from 1⁄32 balsa. A single spar
seemed adequate for this size of model.
For the center panel I used 1⁄16 x 1⁄4 spruce for the spar, to
provide a bit of extra strength. The balsa tip-panel spars were made
from balsa and tapered from 1⁄4 inch in height to 1⁄8 inch at the tip.
As I did with the Scratch-One, I mounted the two tiny Micro
Joule servos on top of the fuselage, just aft of the wing TE. This
makes installing the control rods a snap. Because of the Scratch-
65’s small size, I selected .015-inch-diameter wire for the control
rods.
Then I inserted the wire into small-diameter Teflon tubing that I
purchased from House of Balsa. The tubing was intended for use
with cyanoacrylate cement bottles and is designated as catalog item
NEC-04 (6 feet of cyanoacrylate micro tubing).
On the Scratch-One, I placed the big battery pack on the
underside of the fuselage, centered on the model’s CG, so that the
battery could be accessed without removing the wing after every
flight. Modelers liked that feature!
With the lightweight two-cell FMA/Kokam 145 mAh Li-Poly
pack, I needed the weight as far forward as possible to maintain the
proper balance. I decided to make a small compartment on top, just
behind the firewall (F-1). The pack drops into this compartment,
fitting flush with the top of the fuselage. I used two small magnets
from Forcefield Inc. to hold the battery pack in place.
Choosing a motor was easy. I wanted the smallest available
GWS that would operate on two Li-Poly cells (roughly 7.4 volts).
The best and only choice proved to be the GWS LPS-series B2C-C
motor with a 6.2:1 gear reduction ratio. The best propeller for this
setup proved to be a GWS orange 6 x 5.
I mounted the motor by slipping it onto the end of a 1⁄4-inchdiameter
hardwood dowel that is passed through formers F-1 and F-
2. You may have to sand that dowel to get it to fit in the motor’s
mounting hole. Once it is in place, drill a small hole through the
motor casing and into the dowel. Insert a No. 2 sheet-metal screw
to anchor it in place.
The two-pin male connector exiting the motor will plug
perfectly into the motor connector located on the Micro Joule ESC.
If you ever need to replace a motor, you can easily do it at the
flying field since no soldering is necessary.
Hinging and Radio Installation: I made the rudder and elevator
hinges from strips of clear iron-on Doculam covering material
(available from Modelair Tech). I used Z-bends on the ends of the
wires attaching to the servo output arms. At the control-surface
ends I used Du-Bro micro E-Z connectors and control horns.
I affixed both servos, the receiver, and the ESC to the fuselage
with thin (1⁄16-inch) double-stick foam-rubber tape. I installed the
radio before I painted the model.
Covering and Finishing: I gave the entire fuselage and tail
assembly a light spray coat of paint—just enough to seal the wood,
February 2005 35
GWS LPS B2C-C motor in place on 1⁄4-inch dowel. No. 2 sheet-metal
screw firmly anchors motor so it can’t move, yet is easy to replace
when necessary.
Upper front portion of Scratch-65 showing motor and compartment
for two Kokam 145 mAh Li-Poly battery pack. Battery is held in
place with help of two Forcefield Inc. tiny magnet pairs.
Looking inside RC compartment located under wing, you can see
four-channel receiver affixed to one fuselage side with help of
double-stick tape.
One of the two Du-Bro micro E-Z connectors. Here the
connection is to the rudder horn, which is also a Du-Bro
product.
addition, it employs 1⁄8-inch dowel for the
wing LE, a strip of 3⁄32 x 1⁄4-inch balsa for
the TE, and a short length of 1⁄16 x 1⁄4-inch
spruce for the center-panel spar.
It was so little material that I chose not
to bother my buddy Craig Wagner at
Aerocraft Inc. for a “bag of balsa,” as I did
for the Scratch-One.
Motor Parameters: The GWS B2C-C
seemed happiest using the GWS 6 x 5
orange propeller. Motor current at full
charge and full throttle measured 600 mA,
7.0 volts, and 5 watts. That worked out to
1.86 watts per ounce, which is more than
enough for this powered sailplane trainertype
aircraft. The rpm was measured at
4,200.
At the 600 mA motor current and with
145 mAh Kokam Li-Poly batteries, the
estimated motor run time is 15 minutes. At
half throttle, this amount of time can be
greatly extended. Best of all is the fact that
FMA Direct sells the two-cell battery pack
for only $14.95.
Flying: This proved to be the best part. My
goal was to provide a simple and fast
application for the new miniature Cirrus
but not even enough to actually cover the
wood grain, which kind of “bleeds”
through.
Before spraying, cover the open RC
compartment, the servos, the battery, and
the motor area with masking tape. For a
spray, I took Don Stackhouse’s (of DJ
Aerotech) suggestion and bought a can of
Dupli-Color Import Auto Spray
(www.duplicolor.com). I used shade 88-
01517 Yellow, which I found at a local
Wal-Mart. It has a strong odor, so it is best
to apply it outside on a calm day. Drying
time is quite short.
The wing structure before covering
weighed 0.49 ounce. For it I purchased
Transparent Red Solite covering from Balsa
Products. After covering, the wing weighed
0.67 ounce; Solite added only 0.18 ounce to
the total weight.
When it was all finished, my Scratch-65
weighed 2.69 ounces, which yielded a wing
loading of only 3.7 ounces per square foot.
Balance was exactly as shown on the
plans—just slightly forward of the main
(and only) wing spar.
This entire aircraft was built from a
single sheet of 1⁄16 x 3 x 36-inch balsa and
roughly a half sheet of 1⁄32 balsa. In
Dupli-Color spray paint that Bob used to
mist a single coat of yellow on raw
fuselage and tail balsa surfaces.
36 MODEL AVIATION
Micro Joule RC airborne equipment. These
components worked flawlessly in a regular
flying-field environment with considerable
and varied activity going on all around.
The Scratch-65 took to the air on its first
flight, requiring no trim whatsoever. It flew
straight and level, with a slight gentle climb.
The power was enough that I could quickly
reduce the throttle to half and still maintain
the model’s altitude. Doing that slowed the
model to a point where almost anybody can
fly it. Better yet, it will stay up for an
extremely long time before needing a
recharge.
I have flown the Scratch-65 in winds
estimated at 5-7 mph with no difficulty. I
haven’t had a chance to fly it indoors, but I
suspect that at reduced throttle settings it
could easily be flown in a double-size
gymnasium facility.
Final control throws were measured at 3⁄8
inch either side for the rudder and 5⁄16 inch
either side of neutral for the elevator.
This fun aircraft can be enjoyed by rank
beginners as well as regular sport fliers. If
you haven’t tried flying a diminutive model
that still uses conventional proportional
servos, consider the Scratch-65. Please share
your photos and experiences with me! MA
Bob Aberle
[email protected]
Edition: Model Aviation - 2005/02
Page Numbers: 31,32,33,34,35,36,37,38
Sccrattcch-65
Electric-powered RC indoor
sailplane/trainer showcases new
miniature Cirrus Micro Joule components
A FEATURE THAT appeared in my MA “From the Ground Up”
installments was a construction article for a model that an RC
beginner could build strictly from raw materials without the aid of a
kit. We call this technique “building from scratch.”
The airplane I designed expressly for this feature was the
Scratch-One. It was presented in two parts, in the November 2003
and January 2004 issues. From all the photos that were sent in, many
of you built this little RC electric-powered training model with
excellent success.
Jumping ahead a little, I wrote another feature article titled
“Introduction to Indoor RC” (July 2004 MA). Right after it was
published, the people at Global Hobby Distributors sent in samples
of their newly updated Cirrus Micro Joule miniature RC-system
components.
This equipment was first made available well more than a year
ago. More recently, based on field experience, several modifications
were made to further enhance this micro RC equipment’s
performance. I was provided a Micro Joule receiver, three
microservos, and two types of tiny electric motor speed controllers.
My usual routine would be to photograph these components,
operate them, and report on them using a standard product-review
format. The thought came to mind that there aren’t very many RTF
or ARF models available for RC aircraft that weigh only
approximately 2.5 ounces. If I could design a simple model that the
average indoor RC beginner could easily construct, I could be
providing a specific application for these new Cirrus Micro Joule
components.
With the Scratch-One’s success so noted, I thought it might be
fun to scale it down to a size that would accept the Cirrus Micro
Joule equipment, a tiny electric motor, and several equally tiny Li-
Poly batteries, all in an aircraft nearing that 2.5-ounce total.
The result is this “Scratch-65”: a Scratch-One at 65% of its
original size. The wingspan worked out to roughly 30 inches, and the
area is slightly more than 100 square inches.
So settle in and read my showcase of the new Cirrus Micro Joule
miniature RC components, and then read on for the full (and simple)
construction article for the little Scratch-65. It has proven to be an
excellent flier and another perfect RC trainer.
Global Hobby Cirrus Micro Joule RC System: My sample airbornesystem
components consisted of the Cirrus Micro Joule four-channel
miniature FM receiver, two Cirrus Micro Joule CS-3
ultramicroservos, and two new 5-amp-rated ESCs: the S5A1 and the
S5A2. These items can be purchased separately.
Micro JST connectors with white plastic housing are used on
these RC components. There really isn’t any established
convention yet for the pin wiring on these tiny connectors. On
these Cirrus types, the center pin has a red wire for battery
positive. Gray wires are used for the two “outboard” pins that
contain the battery negative and the servo signal wire.
When you are using all Cirrus Micro Joule components, there
will be no problem with connector-pin polarity. But if you mix and
match from other manufacturers, you had best check the pin polarity
before you turn the power on for the first time.
The connections from the ESC to the motor and the battery pack
use the larger, red, plastic JST connectors, which adhere to the
industry-accepted polarity convention (pin No. 1 black negative and
pin No. 2 red positive).
The Micro Joule four-channel-function receiver has automatic
shift select, so it can basically be operated by any brand of RC FM
transmitter. Voltage range is claimed to be 3.5 to 7.0. The weight is
slightly more than 3 grams (or .11 ounce). Dimensions are 1.11
inches long, 0.31 inch wide, and 0.33 inch high. The crystal clearly
sticks out farther than this height dimension.
The antenna wire is 34 inches in length. I used the full-length
antenna to obtain a baseline check on the receiver’s range capability.
Later I will substitute an E-Cubed RC short antenna and report on
the effect of the radio reception.
This is a single-conversion receiver. The specifications simply
state that it is “narrow band.” I resorted to a couple of simple field
tests to verify that claim. I was able to put the Scratch-65 almost out
of sight (on a calm day!) and was able to maintain perfect control.
Then I had a club member set up his synthesized RC transmitter
one channel away from mine (that is only 20 kHz). I flew the model
down the length of the runway, allowing it to get much closer to the
other transmitter than to my own. Both transmitter antennas were
fully extended. I did this on several occasions and noticed no
interference of any kind.
Tom Hunt about to launch the Scratch-65, with Bob behind the
camera. This shows you how small the model really is.
February 2005 31
by Bob Aberle
Photos by the author and Mark Lanterman
A comparison of the Speed 400-powered Scratch-One and the
smaller Scratch-65. Both are easy to build and fun to fly!
It’s hard to tell the Scratch-65’s size in this flight shot. These small
models are stable and fly in much the same manner as their larger
cousins.
Global Hobby Cirrus Micro Joule RC micro components (bottom to
top): four-channel receiver, S5A2 ESC, two CS-3 ultramicroservos.
Total weight of this equipment is .36 ounce!
Cirrus Micro Joule four-channel receiver with dime for size
reference. Crystal must be purchased separately. Receiver
weighs 3 grams, or .11 ounce!
Different view of equipment in above right photo (bottom to top):
CS-3 ultramicro proportional servo with its separate cable, fourchannel
receiver with coiled receiver antenna wire at left, and one
of the two new 5-amp ESCs.
During an entire morning at my flying field, I made several longduration
flights. While I did this, many of my clubmates flew on a
variety of other 72 MHz RC channels. This Micro Joule receiver
never got hit! Keep in mind that this is not a laboratory test; it is
merely a practical test based on everyday-type flying. The receiver’s
street price is $39.99, and the required crystal is another $12.95.
The little CS-3 servos turned out to be lighter than the claim. I
measured one at 2.1 grams, so the two I used came to 4.2 grams, or
.15 ounce.
The cable going from the servo to the receiver is a separate item.
You plug one end into the servo and the other end into the receiver.
The length of the cable is slightly less than 3 inches. Other cable
lengths and cables that adapt to other equipment can be purchased
separately.
Voltage requirement for these servos is 3.0-7.0. At 4.8 volts, the
claim is for a 14 inch-ounce output torque. Transit time at 4.8 volts
is .18 second for 60° rotation.
The first issue of these tiny servos tended to be noisy, and the
neutral centering was observed to be inaccurate; the servo never
returned to exactly the same neutral position. This new version of
the Micro Joule servo seemed quieter. The neutral-return accuracy
also seems better. I can’t say it’s “dead on,” but it’s certainly
acceptable for anything except a high-speed, highly maneuverable
aircraft.
I don’t expect to see many airplanes in this category weighing
just 2.5 ounces! The street price for an individual Micro Joule
servo is $49.99, which is impressive for a 2.1-gram, fully
proportional-feedback servo.
32 MODEL AVIATION
Closer look at four-channel FM single-conversion receiver. It will
auto-select proper FM shift to work with any existing RC FM
transmitter.
Closer look at CS-3 servo. Cable is a separate item. You can
purchase longer cables or even cable adapters that will allow you
to use these servos with other RC components.
S5A1 ESC intended for use with one Li-Poly battery or two to
three Ni-Cd/NiMH cells. Red plastic JST connectors go to
battery and motor.
Peak Electronics unit is Bob’s favorite charger for smaller Li-Poly
batteries. It can handle one, two, or three Li-Poly cells at discrete
100-1500 mA charge currents.
Type: RC indoor sailplane/trainer
Wingspan: 30 inches
Power: GWS LPS B2C-C motor with 6.2:1 gear drive
Flying weight: 2.69 ounces
Construction: Balsa and plywood
Covering/finish: Dupli-Color spray enamel and Solite
Scratch-65
The first-issue Micro Joule ESC had a BEC regulator set for a
low 3.0 volts. Most of us felt this was done to accommodate
possibly a problem in either the receiver or servos. The regulated
voltage is now back up to the more appropriate 5.0-volt level. (I
verified this point!)
The cutoff-voltage level on the first Micro Joule ESC was set
low. It definitely wasn’t conducive to using Li-Poly batteries, where
it is important not to discharge to less than 2.5-3.0 volts per cell.
To counter this problem, Global has introduced two new types of
ESCs. Model S5A1 is intended for use with one Li-Poly battery or
two to three Ni-Cd/Nymph cells. The cutoff is set at 2.7 volts.
Continuous current rating is 5 amps, which is more than enough for
this application. It weighs 1.9 grams (.067 ounce) and has a street
price of $29.99.
The model S5A2 ESC is intended for use with two or three Li-
Poly batteries or three to six Ni-Cd/NiMH cells. This is the one I
used on the Scratch-65. It weighs 2.78 grams (.10 ounce) and
measures .74 inch long, .31 inch wide, and .25 inch high. That
weight includes one micro JST connector that goes to the throttle
port on the receiver, along with male and female red JST connectors
that go to the battery and motor. The BEC regulated voltage is again
5.0 volts, and the continuous current rating is 5 amps.
The voltage cutoff on this particular ESC is automatically set by
February 2005 33
All wood parts necessary to build Scratch-65. Almost entire model
is constructed from full 1⁄16 x 3 x 36-inch balsa sheet and roughly
half sheet of 1⁄32 balsa.
Wing under construction. LE is 1⁄8-inch-diameter hardwood dowel.
Center-panel spar is 1⁄16 x 1⁄4 spruce for added strength. There are
no wing ribs—only bent sticks of balsa.
Fuselage sides assembled with two formers and spacer sticks.
All tail-assembly pieces are made from 1⁄16 balsa sheet.
Wing panel ready to be joined, with fuselage sides with stiffeners
in place. Fuselage sides are made from 1⁄16 balsa sheet.
Scratch-65 in Action
In early November I attended and flew in what is probably
the biggest indoor RC Electric fly in the USA—the JR Indoor
Electric Festival—which was held in Columbus, Ohio. A
complete report on this event will be published soon in MA.
At one point I put my little Scratch-65 (all 2.6 ounces of it!)
up for a flight. After a minute or two of flying around, I
realized that many other modelers were flying along with me.
My helper made a count, and including mine there were 12
models in the air at one time. I continued to fly the Scratch-65
for more than 10 minutes. It covered most of the inflated dome
area from side to side and floor to ceiling.
Only after I landed did it dawn on me that I had not
experienced a single glitch with my radio system. So when you
ask me how well this Micro Joule equipment performs, my
answer is “outstanding.” MA
—Bob Aberle
the internal software. It will establish the cutoff voltage at 72% of a
fully charged battery. So if you hook up a fully charged two-cell Li-
Poly battery, the voltage might be roughly 7.4 volts. This ESC will
then set the cutoff voltage to 72% of that, or 5.3 volts (2.67 volts per
cell).
I tried monitoring this claimed auto cutoff. The first time I did not
have a fully charged battery pack; therefore, the auto-select voltage
set itself too low! When I made sure the pack was fully charged, the
voltage was set correctly.
The only thing that might bother me is that on occasion I will fly
out a half charge, land, take a rest, and then fly the remainder of the
charge. When I turn it on the second time (with only a half-charged
battery), will the ESC set the voltage too low?
The motor speed controlling was linear from full throttle down to
idle speeds. Street price is identical to the other ESC, at $29.99.
From what I have written, there have been notable improvements
in the Micro Joule equipment. The receiver range and selectivity is
excellent. Servo resolution seems adequate for almost any kind of
flying. The regulated voltage from the BEC is now in line with what
we normally expect, and the voltage cutoff point is now appropriate
for one, two, and three Li-Poly battery cells.
I received filtering capacitors with both ESCs and was told to put
them on my motor, but my GWS already had several capacitors
installed. I chose not to use the extra capacitors, and everything
worked perfectly.
The street price for this receiver, crystal, two servos, and your
choice of ESC is $178.15. If you want to go to full four-channel
control, you will need one more servo at $49.99.
34 MODEL AVIATION
Fully assembled Scratch-65 waiting for coat of spray paint and
wing covering. Nothing to be afraid of trying here.
Two CS-3 servos mounted on top of fuselage, out in the open,
just aft of wing TE. Servos are affixed with 1⁄16-inch double-stick
mounting tape.
GWS LPS B2C-C motor ready to slip on end of 1⁄4-inch-diameter
hardwood dowel mount. Dowel passes through plywood former
F1 and balsa former F2.
CONSTRUCTION
Since the plans for the Scratch-One were made with a computer
graphics system, all I had to do was specify the size reduction and I
had my new set of plans. A 50% decrease would have been
convenient, but it clearly resulted in a model that was too small. I
finally ended up at 65%, which yielded a wingspan of
approximately 30 inches and an exact wing area of 105 square
inches.
Adding the weight of the Cirrus Micro Joule RC components
(0.36 ounce), the motor and propeller (0.49 ounce), and the two-cell
FMA Direct Kokam 145 mAh Li-Poly battery (0.35 ounce) resulted
in a (nonaircraft) weight of 1.2 ounces. I usually use a figure
similar to two times the equipment weight to get a feel for the
model’s total weight. In this case that was 2.4 ounces. I guessed
that an even 2.5 would be okay. In the final analysis, it came to
2.69 ounces—not bad!
The next trick was to select the appropriate wood sizes to fit the
reduced-size aircraft. I chose 1⁄16 balsa for the fuselage sides and tail
pieces. The fuselage top and bottom was 1⁄32 balsa.
As with the Scratch-One, the wing had sticks instead of ribs to
make things easy for the beginner. The bottom stick was made from
1⁄16 balsa, and the top was made from 1⁄32 balsa. A single spar
seemed adequate for this size of model.
For the center panel I used 1⁄16 x 1⁄4 spruce for the spar, to
provide a bit of extra strength. The balsa tip-panel spars were made
from balsa and tapered from 1⁄4 inch in height to 1⁄8 inch at the tip.
As I did with the Scratch-One, I mounted the two tiny Micro
Joule servos on top of the fuselage, just aft of the wing TE. This
makes installing the control rods a snap. Because of the Scratch-
65’s small size, I selected .015-inch-diameter wire for the control
rods.
Then I inserted the wire into small-diameter Teflon tubing that I
purchased from House of Balsa. The tubing was intended for use
with cyanoacrylate cement bottles and is designated as catalog item
NEC-04 (6 feet of cyanoacrylate micro tubing).
On the Scratch-One, I placed the big battery pack on the
underside of the fuselage, centered on the model’s CG, so that the
battery could be accessed without removing the wing after every
flight. Modelers liked that feature!
With the lightweight two-cell FMA/Kokam 145 mAh Li-Poly
pack, I needed the weight as far forward as possible to maintain the
proper balance. I decided to make a small compartment on top, just
behind the firewall (F-1). The pack drops into this compartment,
fitting flush with the top of the fuselage. I used two small magnets
from Forcefield Inc. to hold the battery pack in place.
Choosing a motor was easy. I wanted the smallest available
GWS that would operate on two Li-Poly cells (roughly 7.4 volts).
The best and only choice proved to be the GWS LPS-series B2C-C
motor with a 6.2:1 gear reduction ratio. The best propeller for this
setup proved to be a GWS orange 6 x 5.
I mounted the motor by slipping it onto the end of a 1⁄4-inchdiameter
hardwood dowel that is passed through formers F-1 and F-
2. You may have to sand that dowel to get it to fit in the motor’s
mounting hole. Once it is in place, drill a small hole through the
motor casing and into the dowel. Insert a No. 2 sheet-metal screw
to anchor it in place.
The two-pin male connector exiting the motor will plug
perfectly into the motor connector located on the Micro Joule ESC.
If you ever need to replace a motor, you can easily do it at the
flying field since no soldering is necessary.
Hinging and Radio Installation: I made the rudder and elevator
hinges from strips of clear iron-on Doculam covering material
(available from Modelair Tech). I used Z-bends on the ends of the
wires attaching to the servo output arms. At the control-surface
ends I used Du-Bro micro E-Z connectors and control horns.
I affixed both servos, the receiver, and the ESC to the fuselage
with thin (1⁄16-inch) double-stick foam-rubber tape. I installed the
radio before I painted the model.
Covering and Finishing: I gave the entire fuselage and tail
assembly a light spray coat of paint—just enough to seal the wood,
February 2005 35
GWS LPS B2C-C motor in place on 1⁄4-inch dowel. No. 2 sheet-metal
screw firmly anchors motor so it can’t move, yet is easy to replace
when necessary.
Upper front portion of Scratch-65 showing motor and compartment
for two Kokam 145 mAh Li-Poly battery pack. Battery is held in
place with help of two Forcefield Inc. tiny magnet pairs.
Looking inside RC compartment located under wing, you can see
four-channel receiver affixed to one fuselage side with help of
double-stick tape.
One of the two Du-Bro micro E-Z connectors. Here the
connection is to the rudder horn, which is also a Du-Bro
product.
addition, it employs 1⁄8-inch dowel for the
wing LE, a strip of 3⁄32 x 1⁄4-inch balsa for
the TE, and a short length of 1⁄16 x 1⁄4-inch
spruce for the center-panel spar.
It was so little material that I chose not
to bother my buddy Craig Wagner at
Aerocraft Inc. for a “bag of balsa,” as I did
for the Scratch-One.
Motor Parameters: The GWS B2C-C
seemed happiest using the GWS 6 x 5
orange propeller. Motor current at full
charge and full throttle measured 600 mA,
7.0 volts, and 5 watts. That worked out to
1.86 watts per ounce, which is more than
enough for this powered sailplane trainertype
aircraft. The rpm was measured at
4,200.
At the 600 mA motor current and with
145 mAh Kokam Li-Poly batteries, the
estimated motor run time is 15 minutes. At
half throttle, this amount of time can be
greatly extended. Best of all is the fact that
FMA Direct sells the two-cell battery pack
for only $14.95.
Flying: This proved to be the best part. My
goal was to provide a simple and fast
application for the new miniature Cirrus
but not even enough to actually cover the
wood grain, which kind of “bleeds”
through.
Before spraying, cover the open RC
compartment, the servos, the battery, and
the motor area with masking tape. For a
spray, I took Don Stackhouse’s (of DJ
Aerotech) suggestion and bought a can of
Dupli-Color Import Auto Spray
(www.duplicolor.com). I used shade 88-
01517 Yellow, which I found at a local
Wal-Mart. It has a strong odor, so it is best
to apply it outside on a calm day. Drying
time is quite short.
The wing structure before covering
weighed 0.49 ounce. For it I purchased
Transparent Red Solite covering from Balsa
Products. After covering, the wing weighed
0.67 ounce; Solite added only 0.18 ounce to
the total weight.
When it was all finished, my Scratch-65
weighed 2.69 ounces, which yielded a wing
loading of only 3.7 ounces per square foot.
Balance was exactly as shown on the
plans—just slightly forward of the main
(and only) wing spar.
This entire aircraft was built from a
single sheet of 1⁄16 x 3 x 36-inch balsa and
roughly a half sheet of 1⁄32 balsa. In
Dupli-Color spray paint that Bob used to
mist a single coat of yellow on raw
fuselage and tail balsa surfaces.
36 MODEL AVIATION
Micro Joule RC airborne equipment. These
components worked flawlessly in a regular
flying-field environment with considerable
and varied activity going on all around.
The Scratch-65 took to the air on its first
flight, requiring no trim whatsoever. It flew
straight and level, with a slight gentle climb.
The power was enough that I could quickly
reduce the throttle to half and still maintain
the model’s altitude. Doing that slowed the
model to a point where almost anybody can
fly it. Better yet, it will stay up for an
extremely long time before needing a
recharge.
I have flown the Scratch-65 in winds
estimated at 5-7 mph with no difficulty. I
haven’t had a chance to fly it indoors, but I
suspect that at reduced throttle settings it
could easily be flown in a double-size
gymnasium facility.
Final control throws were measured at 3⁄8
inch either side for the rudder and 5⁄16 inch
either side of neutral for the elevator.
This fun aircraft can be enjoyed by rank
beginners as well as regular sport fliers. If
you haven’t tried flying a diminutive model
that still uses conventional proportional
servos, consider the Scratch-65. Please share
your photos and experiences with me! MA
Bob Aberle
[email protected]
Edition: Model Aviation - 2005/02
Page Numbers: 31,32,33,34,35,36,37,38
Sccrattcch-65
Electric-powered RC indoor
sailplane/trainer showcases new
miniature Cirrus Micro Joule components
A FEATURE THAT appeared in my MA “From the Ground Up”
installments was a construction article for a model that an RC
beginner could build strictly from raw materials without the aid of a
kit. We call this technique “building from scratch.”
The airplane I designed expressly for this feature was the
Scratch-One. It was presented in two parts, in the November 2003
and January 2004 issues. From all the photos that were sent in, many
of you built this little RC electric-powered training model with
excellent success.
Jumping ahead a little, I wrote another feature article titled
“Introduction to Indoor RC” (July 2004 MA). Right after it was
published, the people at Global Hobby Distributors sent in samples
of their newly updated Cirrus Micro Joule miniature RC-system
components.
This equipment was first made available well more than a year
ago. More recently, based on field experience, several modifications
were made to further enhance this micro RC equipment’s
performance. I was provided a Micro Joule receiver, three
microservos, and two types of tiny electric motor speed controllers.
My usual routine would be to photograph these components,
operate them, and report on them using a standard product-review
format. The thought came to mind that there aren’t very many RTF
or ARF models available for RC aircraft that weigh only
approximately 2.5 ounces. If I could design a simple model that the
average indoor RC beginner could easily construct, I could be
providing a specific application for these new Cirrus Micro Joule
components.
With the Scratch-One’s success so noted, I thought it might be
fun to scale it down to a size that would accept the Cirrus Micro
Joule equipment, a tiny electric motor, and several equally tiny Li-
Poly batteries, all in an aircraft nearing that 2.5-ounce total.
The result is this “Scratch-65”: a Scratch-One at 65% of its
original size. The wingspan worked out to roughly 30 inches, and the
area is slightly more than 100 square inches.
So settle in and read my showcase of the new Cirrus Micro Joule
miniature RC components, and then read on for the full (and simple)
construction article for the little Scratch-65. It has proven to be an
excellent flier and another perfect RC trainer.
Global Hobby Cirrus Micro Joule RC System: My sample airbornesystem
components consisted of the Cirrus Micro Joule four-channel
miniature FM receiver, two Cirrus Micro Joule CS-3
ultramicroservos, and two new 5-amp-rated ESCs: the S5A1 and the
S5A2. These items can be purchased separately.
Micro JST connectors with white plastic housing are used on
these RC components. There really isn’t any established
convention yet for the pin wiring on these tiny connectors. On
these Cirrus types, the center pin has a red wire for battery
positive. Gray wires are used for the two “outboard” pins that
contain the battery negative and the servo signal wire.
When you are using all Cirrus Micro Joule components, there
will be no problem with connector-pin polarity. But if you mix and
match from other manufacturers, you had best check the pin polarity
before you turn the power on for the first time.
The connections from the ESC to the motor and the battery pack
use the larger, red, plastic JST connectors, which adhere to the
industry-accepted polarity convention (pin No. 1 black negative and
pin No. 2 red positive).
The Micro Joule four-channel-function receiver has automatic
shift select, so it can basically be operated by any brand of RC FM
transmitter. Voltage range is claimed to be 3.5 to 7.0. The weight is
slightly more than 3 grams (or .11 ounce). Dimensions are 1.11
inches long, 0.31 inch wide, and 0.33 inch high. The crystal clearly
sticks out farther than this height dimension.
The antenna wire is 34 inches in length. I used the full-length
antenna to obtain a baseline check on the receiver’s range capability.
Later I will substitute an E-Cubed RC short antenna and report on
the effect of the radio reception.
This is a single-conversion receiver. The specifications simply
state that it is “narrow band.” I resorted to a couple of simple field
tests to verify that claim. I was able to put the Scratch-65 almost out
of sight (on a calm day!) and was able to maintain perfect control.
Then I had a club member set up his synthesized RC transmitter
one channel away from mine (that is only 20 kHz). I flew the model
down the length of the runway, allowing it to get much closer to the
other transmitter than to my own. Both transmitter antennas were
fully extended. I did this on several occasions and noticed no
interference of any kind.
Tom Hunt about to launch the Scratch-65, with Bob behind the
camera. This shows you how small the model really is.
February 2005 31
by Bob Aberle
Photos by the author and Mark Lanterman
A comparison of the Speed 400-powered Scratch-One and the
smaller Scratch-65. Both are easy to build and fun to fly!
It’s hard to tell the Scratch-65’s size in this flight shot. These small
models are stable and fly in much the same manner as their larger
cousins.
Global Hobby Cirrus Micro Joule RC micro components (bottom to
top): four-channel receiver, S5A2 ESC, two CS-3 ultramicroservos.
Total weight of this equipment is .36 ounce!
Cirrus Micro Joule four-channel receiver with dime for size
reference. Crystal must be purchased separately. Receiver
weighs 3 grams, or .11 ounce!
Different view of equipment in above right photo (bottom to top):
CS-3 ultramicro proportional servo with its separate cable, fourchannel
receiver with coiled receiver antenna wire at left, and one
of the two new 5-amp ESCs.
During an entire morning at my flying field, I made several longduration
flights. While I did this, many of my clubmates flew on a
variety of other 72 MHz RC channels. This Micro Joule receiver
never got hit! Keep in mind that this is not a laboratory test; it is
merely a practical test based on everyday-type flying. The receiver’s
street price is $39.99, and the required crystal is another $12.95.
The little CS-3 servos turned out to be lighter than the claim. I
measured one at 2.1 grams, so the two I used came to 4.2 grams, or
.15 ounce.
The cable going from the servo to the receiver is a separate item.
You plug one end into the servo and the other end into the receiver.
The length of the cable is slightly less than 3 inches. Other cable
lengths and cables that adapt to other equipment can be purchased
separately.
Voltage requirement for these servos is 3.0-7.0. At 4.8 volts, the
claim is for a 14 inch-ounce output torque. Transit time at 4.8 volts
is .18 second for 60° rotation.
The first issue of these tiny servos tended to be noisy, and the
neutral centering was observed to be inaccurate; the servo never
returned to exactly the same neutral position. This new version of
the Micro Joule servo seemed quieter. The neutral-return accuracy
also seems better. I can’t say it’s “dead on,” but it’s certainly
acceptable for anything except a high-speed, highly maneuverable
aircraft.
I don’t expect to see many airplanes in this category weighing
just 2.5 ounces! The street price for an individual Micro Joule
servo is $49.99, which is impressive for a 2.1-gram, fully
proportional-feedback servo.
32 MODEL AVIATION
Closer look at four-channel FM single-conversion receiver. It will
auto-select proper FM shift to work with any existing RC FM
transmitter.
Closer look at CS-3 servo. Cable is a separate item. You can
purchase longer cables or even cable adapters that will allow you
to use these servos with other RC components.
S5A1 ESC intended for use with one Li-Poly battery or two to
three Ni-Cd/NiMH cells. Red plastic JST connectors go to
battery and motor.
Peak Electronics unit is Bob’s favorite charger for smaller Li-Poly
batteries. It can handle one, two, or three Li-Poly cells at discrete
100-1500 mA charge currents.
Type: RC indoor sailplane/trainer
Wingspan: 30 inches
Power: GWS LPS B2C-C motor with 6.2:1 gear drive
Flying weight: 2.69 ounces
Construction: Balsa and plywood
Covering/finish: Dupli-Color spray enamel and Solite
Scratch-65
The first-issue Micro Joule ESC had a BEC regulator set for a
low 3.0 volts. Most of us felt this was done to accommodate
possibly a problem in either the receiver or servos. The regulated
voltage is now back up to the more appropriate 5.0-volt level. (I
verified this point!)
The cutoff-voltage level on the first Micro Joule ESC was set
low. It definitely wasn’t conducive to using Li-Poly batteries, where
it is important not to discharge to less than 2.5-3.0 volts per cell.
To counter this problem, Global has introduced two new types of
ESCs. Model S5A1 is intended for use with one Li-Poly battery or
two to three Ni-Cd/Nymph cells. The cutoff is set at 2.7 volts.
Continuous current rating is 5 amps, which is more than enough for
this application. It weighs 1.9 grams (.067 ounce) and has a street
price of $29.99.
The model S5A2 ESC is intended for use with two or three Li-
Poly batteries or three to six Ni-Cd/NiMH cells. This is the one I
used on the Scratch-65. It weighs 2.78 grams (.10 ounce) and
measures .74 inch long, .31 inch wide, and .25 inch high. That
weight includes one micro JST connector that goes to the throttle
port on the receiver, along with male and female red JST connectors
that go to the battery and motor. The BEC regulated voltage is again
5.0 volts, and the continuous current rating is 5 amps.
The voltage cutoff on this particular ESC is automatically set by
February 2005 33
All wood parts necessary to build Scratch-65. Almost entire model
is constructed from full 1⁄16 x 3 x 36-inch balsa sheet and roughly
half sheet of 1⁄32 balsa.
Wing under construction. LE is 1⁄8-inch-diameter hardwood dowel.
Center-panel spar is 1⁄16 x 1⁄4 spruce for added strength. There are
no wing ribs—only bent sticks of balsa.
Fuselage sides assembled with two formers and spacer sticks.
All tail-assembly pieces are made from 1⁄16 balsa sheet.
Wing panel ready to be joined, with fuselage sides with stiffeners
in place. Fuselage sides are made from 1⁄16 balsa sheet.
Scratch-65 in Action
In early November I attended and flew in what is probably
the biggest indoor RC Electric fly in the USA—the JR Indoor
Electric Festival—which was held in Columbus, Ohio. A
complete report on this event will be published soon in MA.
At one point I put my little Scratch-65 (all 2.6 ounces of it!)
up for a flight. After a minute or two of flying around, I
realized that many other modelers were flying along with me.
My helper made a count, and including mine there were 12
models in the air at one time. I continued to fly the Scratch-65
for more than 10 minutes. It covered most of the inflated dome
area from side to side and floor to ceiling.
Only after I landed did it dawn on me that I had not
experienced a single glitch with my radio system. So when you
ask me how well this Micro Joule equipment performs, my
answer is “outstanding.” MA
—Bob Aberle
the internal software. It will establish the cutoff voltage at 72% of a
fully charged battery. So if you hook up a fully charged two-cell Li-
Poly battery, the voltage might be roughly 7.4 volts. This ESC will
then set the cutoff voltage to 72% of that, or 5.3 volts (2.67 volts per
cell).
I tried monitoring this claimed auto cutoff. The first time I did not
have a fully charged battery pack; therefore, the auto-select voltage
set itself too low! When I made sure the pack was fully charged, the
voltage was set correctly.
The only thing that might bother me is that on occasion I will fly
out a half charge, land, take a rest, and then fly the remainder of the
charge. When I turn it on the second time (with only a half-charged
battery), will the ESC set the voltage too low?
The motor speed controlling was linear from full throttle down to
idle speeds. Street price is identical to the other ESC, at $29.99.
From what I have written, there have been notable improvements
in the Micro Joule equipment. The receiver range and selectivity is
excellent. Servo resolution seems adequate for almost any kind of
flying. The regulated voltage from the BEC is now in line with what
we normally expect, and the voltage cutoff point is now appropriate
for one, two, and three Li-Poly battery cells.
I received filtering capacitors with both ESCs and was told to put
them on my motor, but my GWS already had several capacitors
installed. I chose not to use the extra capacitors, and everything
worked perfectly.
The street price for this receiver, crystal, two servos, and your
choice of ESC is $178.15. If you want to go to full four-channel
control, you will need one more servo at $49.99.
34 MODEL AVIATION
Fully assembled Scratch-65 waiting for coat of spray paint and
wing covering. Nothing to be afraid of trying here.
Two CS-3 servos mounted on top of fuselage, out in the open,
just aft of wing TE. Servos are affixed with 1⁄16-inch double-stick
mounting tape.
GWS LPS B2C-C motor ready to slip on end of 1⁄4-inch-diameter
hardwood dowel mount. Dowel passes through plywood former
F1 and balsa former F2.
CONSTRUCTION
Since the plans for the Scratch-One were made with a computer
graphics system, all I had to do was specify the size reduction and I
had my new set of plans. A 50% decrease would have been
convenient, but it clearly resulted in a model that was too small. I
finally ended up at 65%, which yielded a wingspan of
approximately 30 inches and an exact wing area of 105 square
inches.
Adding the weight of the Cirrus Micro Joule RC components
(0.36 ounce), the motor and propeller (0.49 ounce), and the two-cell
FMA Direct Kokam 145 mAh Li-Poly battery (0.35 ounce) resulted
in a (nonaircraft) weight of 1.2 ounces. I usually use a figure
similar to two times the equipment weight to get a feel for the
model’s total weight. In this case that was 2.4 ounces. I guessed
that an even 2.5 would be okay. In the final analysis, it came to
2.69 ounces—not bad!
The next trick was to select the appropriate wood sizes to fit the
reduced-size aircraft. I chose 1⁄16 balsa for the fuselage sides and tail
pieces. The fuselage top and bottom was 1⁄32 balsa.
As with the Scratch-One, the wing had sticks instead of ribs to
make things easy for the beginner. The bottom stick was made from
1⁄16 balsa, and the top was made from 1⁄32 balsa. A single spar
seemed adequate for this size of model.
For the center panel I used 1⁄16 x 1⁄4 spruce for the spar, to
provide a bit of extra strength. The balsa tip-panel spars were made
from balsa and tapered from 1⁄4 inch in height to 1⁄8 inch at the tip.
As I did with the Scratch-One, I mounted the two tiny Micro
Joule servos on top of the fuselage, just aft of the wing TE. This
makes installing the control rods a snap. Because of the Scratch-
65’s small size, I selected .015-inch-diameter wire for the control
rods.
Then I inserted the wire into small-diameter Teflon tubing that I
purchased from House of Balsa. The tubing was intended for use
with cyanoacrylate cement bottles and is designated as catalog item
NEC-04 (6 feet of cyanoacrylate micro tubing).
On the Scratch-One, I placed the big battery pack on the
underside of the fuselage, centered on the model’s CG, so that the
battery could be accessed without removing the wing after every
flight. Modelers liked that feature!
With the lightweight two-cell FMA/Kokam 145 mAh Li-Poly
pack, I needed the weight as far forward as possible to maintain the
proper balance. I decided to make a small compartment on top, just
behind the firewall (F-1). The pack drops into this compartment,
fitting flush with the top of the fuselage. I used two small magnets
from Forcefield Inc. to hold the battery pack in place.
Choosing a motor was easy. I wanted the smallest available
GWS that would operate on two Li-Poly cells (roughly 7.4 volts).
The best and only choice proved to be the GWS LPS-series B2C-C
motor with a 6.2:1 gear reduction ratio. The best propeller for this
setup proved to be a GWS orange 6 x 5.
I mounted the motor by slipping it onto the end of a 1⁄4-inchdiameter
hardwood dowel that is passed through formers F-1 and F-
2. You may have to sand that dowel to get it to fit in the motor’s
mounting hole. Once it is in place, drill a small hole through the
motor casing and into the dowel. Insert a No. 2 sheet-metal screw
to anchor it in place.
The two-pin male connector exiting the motor will plug
perfectly into the motor connector located on the Micro Joule ESC.
If you ever need to replace a motor, you can easily do it at the
flying field since no soldering is necessary.
Hinging and Radio Installation: I made the rudder and elevator
hinges from strips of clear iron-on Doculam covering material
(available from Modelair Tech). I used Z-bends on the ends of the
wires attaching to the servo output arms. At the control-surface
ends I used Du-Bro micro E-Z connectors and control horns.
I affixed both servos, the receiver, and the ESC to the fuselage
with thin (1⁄16-inch) double-stick foam-rubber tape. I installed the
radio before I painted the model.
Covering and Finishing: I gave the entire fuselage and tail
assembly a light spray coat of paint—just enough to seal the wood,
February 2005 35
GWS LPS B2C-C motor in place on 1⁄4-inch dowel. No. 2 sheet-metal
screw firmly anchors motor so it can’t move, yet is easy to replace
when necessary.
Upper front portion of Scratch-65 showing motor and compartment
for two Kokam 145 mAh Li-Poly battery pack. Battery is held in
place with help of two Forcefield Inc. tiny magnet pairs.
Looking inside RC compartment located under wing, you can see
four-channel receiver affixed to one fuselage side with help of
double-stick tape.
One of the two Du-Bro micro E-Z connectors. Here the
connection is to the rudder horn, which is also a Du-Bro
product.
addition, it employs 1⁄8-inch dowel for the
wing LE, a strip of 3⁄32 x 1⁄4-inch balsa for
the TE, and a short length of 1⁄16 x 1⁄4-inch
spruce for the center-panel spar.
It was so little material that I chose not
to bother my buddy Craig Wagner at
Aerocraft Inc. for a “bag of balsa,” as I did
for the Scratch-One.
Motor Parameters: The GWS B2C-C
seemed happiest using the GWS 6 x 5
orange propeller. Motor current at full
charge and full throttle measured 600 mA,
7.0 volts, and 5 watts. That worked out to
1.86 watts per ounce, which is more than
enough for this powered sailplane trainertype
aircraft. The rpm was measured at
4,200.
At the 600 mA motor current and with
145 mAh Kokam Li-Poly batteries, the
estimated motor run time is 15 minutes. At
half throttle, this amount of time can be
greatly extended. Best of all is the fact that
FMA Direct sells the two-cell battery pack
for only $14.95.
Flying: This proved to be the best part. My
goal was to provide a simple and fast
application for the new miniature Cirrus
but not even enough to actually cover the
wood grain, which kind of “bleeds”
through.
Before spraying, cover the open RC
compartment, the servos, the battery, and
the motor area with masking tape. For a
spray, I took Don Stackhouse’s (of DJ
Aerotech) suggestion and bought a can of
Dupli-Color Import Auto Spray
(www.duplicolor.com). I used shade 88-
01517 Yellow, which I found at a local
Wal-Mart. It has a strong odor, so it is best
to apply it outside on a calm day. Drying
time is quite short.
The wing structure before covering
weighed 0.49 ounce. For it I purchased
Transparent Red Solite covering from Balsa
Products. After covering, the wing weighed
0.67 ounce; Solite added only 0.18 ounce to
the total weight.
When it was all finished, my Scratch-65
weighed 2.69 ounces, which yielded a wing
loading of only 3.7 ounces per square foot.
Balance was exactly as shown on the
plans—just slightly forward of the main
(and only) wing spar.
This entire aircraft was built from a
single sheet of 1⁄16 x 3 x 36-inch balsa and
roughly a half sheet of 1⁄32 balsa. In
Dupli-Color spray paint that Bob used to
mist a single coat of yellow on raw
fuselage and tail balsa surfaces.
36 MODEL AVIATION
Micro Joule RC airborne equipment. These
components worked flawlessly in a regular
flying-field environment with considerable
and varied activity going on all around.
The Scratch-65 took to the air on its first
flight, requiring no trim whatsoever. It flew
straight and level, with a slight gentle climb.
The power was enough that I could quickly
reduce the throttle to half and still maintain
the model’s altitude. Doing that slowed the
model to a point where almost anybody can
fly it. Better yet, it will stay up for an
extremely long time before needing a
recharge.
I have flown the Scratch-65 in winds
estimated at 5-7 mph with no difficulty. I
haven’t had a chance to fly it indoors, but I
suspect that at reduced throttle settings it
could easily be flown in a double-size
gymnasium facility.
Final control throws were measured at 3⁄8
inch either side for the rudder and 5⁄16 inch
either side of neutral for the elevator.
This fun aircraft can be enjoyed by rank
beginners as well as regular sport fliers. If
you haven’t tried flying a diminutive model
that still uses conventional proportional
servos, consider the Scratch-65. Please share
your photos and experiences with me! MA
Bob Aberle
[email protected]