IN 2003 and 2004 I authored 10 installments
of the 31-part “From the Ground Up” series
that was published in MA. The sequence
of articles reads like a book and takes a
new (beginner) modeler from square one
up through his or her first RC model
flights.
As part of that series I came up with a
design I called the “Scratch-One,” which
was an ultra-simple trainer design that
was intended to be a first-time
construction project. The popular “From
the Ground Up” features are still posted
on the AMA Web site.
Shortly after the series was completed,
I decided to reduce the Scratch-One to a
size and weight that would permit it to be
flown indoors in areas as small as school
gymnasiums. I shrunk the Scratch-One to
65% of the original size, resulting in a wing
area of only 105 square inches. The total
weight came out to 2.7 ounces.
The total weight of all these components is merely 0.63 ounce.
The BSD Micro RC special cable (R) is essential if you plan to run a
brushless motor from this AR6400 brick.
Outlines of the various aircraft parts are transferred to the foam
sheet and then cut with an X-Acto knife.
A fuselage assembly fixture was made from 3/16 balsa sheet. It
follows the top view of the fuselage. Both Depron foam sides are
pinned to this fixture. Partial formers are added above and below
the fixture.
A small gymnasium is plenty of room in which to fly a Scratch Foamy. The
polyhedral wing helps it turn easily with only rudder input. Joe Cabana photo.
Airfoil curvature on wing panels results from rolling foam pieces over
a large-diameter length of PVC pipe. The outer wing panels have
carbon-fiber strips at the point of maximum camber. Polyhedral is
added to both wingtip panels; the center-section is flat.
Photos by the author except as noted
May 2010 31
From the
ground up,
this RC sport
design helps
beginners
go micro
05sig1.QXD_00MSTRPG.QXD 3/24/10 11:33 AM Page 31
3322 MMOODDEELLAAVVIAIATTIOIONN
Tail pieces are cut from 2mm Depron sheeting. After spacing the
elevator and rudder, hinge tape is pasted in place. Du-Bro Micro
Control Horns are used on both control surfaces.
Two center wing ribs fit snugly inside the two fuselage sides. Align
the wing with respect to tail surface, set in the proper incidence
angle, and apply CA cement. This is an easy process.
The forward nose section can be
covered and dressed with a
simulated cockpit area. Bob
made the black windows from
material from SR Batteries.
The complete power and RC s yst em is forward of the wing LE
with motor installe d. An ESC is taped to the side ; its wir es run
unde r the ba ttery tray . The ParkZ o n e AR 640 0 bri ck cont ains
receiver and servos.
Below: Bob decorated
his completed Scratch
Foamy with a light
misting of Krylon H2O
Latex spray paint
(blue and then red).
The process takes
little time, but
some color is
ne c e s s a r y—
otherwise you end up
with an all-white
model.
Type: RC sport park flyer
Skill level: Beginner
Wingspan: 29.5 inches
Wing area: 105 square inches
Length: 20 inches
Weight: 1.64 ounces
Wing loading: 2.2 ounces/square foot
Power system: Bob Selman AP03
(7500 Kv) 3-gram brushless outrunner motor,
GWS 4.5 x 3 propeller, LCD 1-gram/3-amp brushless ESC,
Full River single-cell, 250 mAh Li-Poly battery
Radio: Spektrum AR6400 2.4 GHz receiver/ESC/servo
module, Spektrum DX7 transmitter
Construction: 2mm Depron
Finish: Builder’s choice
Power loading: 85 watts/pound
Flight duration: 12-15 minutes with throttle management
Scratch Foamy
05sig1.QXD_00MSTRPG.QXD 3/24/10 11:56 AM Page 32
I called this new version the “Scratch-65.”
A full construction article, with plans,
appeared in the February 2005 MA.
As originally published, the Scratch-65
was constructed from conventional modeling
materials such as balsa sheets and sticks along
with iron-on covering. The radio and power
systems were basically what was popular five
years ago.
The radio, power system, and battery cost
roughly $200 back then. The motor was a
brushed and geared variety that was not
particularly efficient. The battery was a twocell
Li-Poly with a capacity of just 145 mAh.
Power loading was a mild 30 watts per pound,
but the little airplane flew well, mostly at full
throttle.
Now, five years later, I decided that with
so much improved technology in micro RC
systems, motors, batteries, and even
construction materials, this was the time to
revisit the Scratch-65. A new design resulted,
which I call the “Scratch Foamy.”
As the name implies, this version is
constructed entirely from 2mm Depron foam
sheeting. You can build a complete model
from a single sheet of this material that
measures 15 x 39 inches, at a cost of roughly
$3.
With the success of 2.4 GHz spread
spectrum technology, it was natural to try a
micro RC system. One of the best setups for
indoor/micro application is the ParkZone
AR6400 “brick” module, which is a receiver
with two linear output servos mounted on a
single PC board.
You can purchase this unit separately or,
as I did, buy a ParkZone Sukhoi RTF for
approximately $100. All you need to do is
bind the airborne setup to any Spektrum
DSM2-type transmitter.
After having a lot of fun flying the Sukhoi,
I removed the brick and installed it in the new
Scratch Foamy. A nice advantage of using 2.4
GHz is that the 34-inch-long receiver antenna
that was required five years ago on my 72
MHz RC system has been reduced to 2 or 3
inches.
I decided against a brushed/geared motor
and opted for a new 3-gram AP03 brushless
outrunner and a companion LCD 1-gram/3-
amp micro brushless ESC. I obtained the
motor and ESC from Bob Selman at BSD
Micro RC.
The best propeller for the AP03 has
proved to be a GWS 4.5 x 3 operating on a
single Full River 250 mAh Li-Poly cell. This
runs at 2.5 amps and 8.8 watts of input power.
The original Scratch-65 weighed 2.7
ounces, and the motor input power was only 5
watts. The revised design weighs only 1.64
ounces with a motor that operates at 8.8 watts
of input power. So the power loading
increased from 30 watts per pound (five years
ago) to 85 watts per pound. Instead of flying
at full throttle, I can fly the Scratch Foamy at
less than half throttle. That can provide 12-15
minutes of flying time on a charge. Keep in
mind that the brushless motor is more
efficient than the brushed variety and
certainly has a much longer service life, since
there is really nothing to wear out.
The 250 mAh Li-Poly replaced the heavier
145 mAh unit, permitting longer flight times.
Still another advantage is that the cost of the
power/RC system is less than it was five years
ago. The 2.4 GHz spread spectrum radio
offers essentially interference-free operation
with regard to other modelers.
This new version of the Scratch-65 is a
tremendous improvement in every respect.
CONSTRUCTION
I won’t go into every detail of the build
sequence, but I will highlight the important
areas.
I made copies of certain portions of the
plans and pasted them onto manila folders.
The various parts were cut out, and then I
traced the outlines onto Depron foam.
Cutting all of the Depron parts takes little
time using an X-Acto knife. You will need a
foam-friendly, medium-viscosity CA cement
and companion accelerator.
Wing: The airfoil shape of the three wing
panels is achieved by rolling the foam
sheeting over a large-diameter PVC pipe (3 or
4 inches in diameter). Slowly roll the sheeting
for a few minutes. It is neither necessary nor
desirable to apply heat to gain the airfoil
curvature.
A total of only six ribs will help maintain
the airfoil shape. Two ribs—located in the
center-section—will help attach the wing to
the fuselage later. Two more ribs are at the
poly joints, and the other two are placed at
each wingtip.
Since no spars were employed, I resorted
to .007-inch-thick x 1/4-inch-wide carbon-tape
reinforcement under both wingtip panels.
Cemented in place on the underside, that tape
provides a tad extra strength to these outer
wing panels.
Fuselage: I cut a fuselage assembly fixture
from 3/16 balsa to the outline of the top view.
Then I pinned the thin fuselage sides to this
fixture.
Partial formers were inserted above and
below the fixture. Once there were enough
formers in place, I removed the pins and
withdrew the fixture, leaving an aligned
fuselage.
Then I cut the vertical and horizontal
tailpieces. For hinges I used Blenderm tape,
which you can obtain at most drug stores. An
equivalent Electric Flyer Hinge Tape is
available from Du-Bro Products as item 916. I
installed Du-Bro Micro Control Horns, item
848, on both the rudder and elevator.
The control rods were made from .025-
inch-diameter carbon rods. At each rod end I
attached short lengths of .015-inch-diameter
wire with Z-bends made on the ends. I
attached these wire ends to the carbon rods,
using either heat-shrink tubing or thindiameter
aluminum tubing that could be
crimped with pliers. This technique allows
you to align control surfaces at the neutral
positions, before the final crimp of the tubing.
The ParkZone AR6400 brick was
mounted to a double layer of 2mm Depron
using double-stick tape. The LCD 1-gram/3-
amp brushless ESC was mounted to the
fuselage side, just behind the plywood
firewall, also using double-stick tape.
When mounting the AP03 motor to the
firewall, make sure that you drill a clearance
hole for the motor shaft that protrudes out the
rear of the power plant.
Your rudder servo will initially operate
from the left-hand transmitter control stick.
That’s because this brick was intended for
four-channel control with the aileron function
on the right stick.
To make the rudder work from the rightside
transmitter stick, evoke aileron/rudder
mix on your transmitter and set it for 100%.
As soon as you do that, your rudder will
operate from the right-side transmitter stick.
To operate the brushless motor, you will
need a special $13 cable that is available
from BSD Micro RC. You will also have to
reprogram the AR6400 receiver for brushless
operation, as outlined in the ParkZone User
Guide. It’s a simple process and takes only a
few seconds, but it’s necessary when you
switch from a brushed to a brushless motor.
Final Assembly:When everything is
working, such as the controls and motor,
install the wing to the fuselage. The two
center ribs were spaced so that they fit just
inside the fuselage and press against both
sides.
Make sure that the wing is aligned with
respect to the tail surfaces. Cement it in
place.
Finishing: To decorate the Scratch Foamy, I
misted on two colors of paint to dress up the
all-white foam sheeting. I used the Krylonbrand
H2O Latex spray paint.
I applied the blue on the wing and
stabilizer’s LEs. Then I sprayed the red, but it
didn’t quite cover the blue. This paint job
was quick to do and added little weight.
Flying: Final control throw ended up with
the rudder moving 3/8 inch either side of the
neutral position and 3/16 inch either side for
the elevator. Control throws were set using
the EPA on my DX7 transmitter. The balance
point is 11/4 inch back from the wing LE as
shown on the plans. My aircraft balanced
perfectly, requiring no added weights.
Flying, to date, has taken place in a
double-size high school gymnasium located
near Lake Ronkonkoma, Long Island, New
York. Ceiling height is approximately 20
feet.
Most of my flying has been at half
throttle, which says a lot for the 85-watt-perpound
power loading. Turns can be made
tightly with no resulting stalls. This is a greatflying
indoor sport model.
Anyone with basic RC flying skills
should be able to handle this airplane.
Although the Scratch Foamy is intended
primarily for indoor flying, it can also be
flown outdoors in extremely calm conditions.
So don’t rule out flying in the early
morning or evening during the summer
months. In a sense, that makes this design a
park flyer as well.
The ParkZone AR6400 brick (receiver with
two built-in servos and brushed-motor ESC)
has six-channel control capability. You can
fly aircraft such as the Sukhoi and P-51 using
full four-channel controls, and you could add
extra functions such as flaps and retracts.
This particular application required only
rudder, elevator, and motor control. But to
use a brushless motor, I had to go to this new
AR6400 brick. If you use a brushed/gear
motor, such as the ones supplied with the
Sukhoi and P-51, you could get away with
the ParkZone three-channel brick that is used
in the Ember and Vapor aircraft.
I’ve proved that this little 3-gram AP03
brushless outrunner, operating at roughly 9
watts of input power, is capable of flying
models weighing as much as 2 ounces and
possibly 2.5.
As such, it would be a perfect micro
power system that is capable of flying many
small Rubber Scale designs that companies
such as Dumas Products and Dare Designs
sell in kit form. You might want to consider
this for future indoor/micro projects.
You can get a lot of information about
indoor/micro RC from the RC Micro World
monthly online magazine. Not to sound
commercial, but I have a book/CD, “The
World of Indoor/Micro Radio Controlled
Model Aircraft,” that covers this entire
subject. It is available from the RC Micro
World publisher and the AMA museum store.
If you have any questions about this
design, please feel free to contact me. MA
Bob Aberle
[email protected]
Sources:
“From the Ground Up” series:
www.modelaircraft.org/mag/FTGU/titlespag
eftgu.htm
Carbon wing-reinforcing tape:
Aerospace Composite Products
(800) 811-2009
www.acp-composites.com
AP03-7500 Kv brushless 3-gram outrunner
motor, LCD 1-gram/3-amp brushless ESC,
Full River 250 mAh Li-Poly cell, special
interconnecting cable form ESC to brick,
carbon control rods:
Bob Selman Designs
(417) 358-9521
www.bsdmicrorc.com
Foam-safe CA, companion accelerator, GWS
propellers:
BP Hobbies
(732) 287-3933
www.bphobbies.com
Du-Bro
(800) 848-9411
www.dubro.com
Spektrum DX7 transmitter, AR6400 brick
removed from the RTF Sukhoi (but can be
purchased separately)
Horizon Hobby
(800) 338-4639
www.horizonhobby.com
Krylon
www.krylon.com
2mm Depron foam:
RCfoam
(404) 363-6680
www.rcfoam.com
RC Micro World
www.cloud9rc.com
Simulated black window material:
SR Batteries
(631) 286-0079
www.srbatteries.com
