Radio Control Scale Aerobatics - 2010/04

116 MODEL AVIATION
Make the right choices for your Scale Aerobatics model
[[email protected]]
Radio Control Scale Aerobatics John Glezellis
Also included in this column:
• How many servos your
airplane needs
• Custom-fit servo mounts
• Switch-mounting tip
Before determining various locations in
which to mount a regulator, switch harness,
and receiver battery, plug all components
together and position them internally within
the airframe. When you make an opening
for the switch, ensure that it will reach the
voltage regulator, to avoid spending time
soldering extensions.
The manufacturer has specified a location
for the receiver. Most models have
predetermined locations for some
equipment.
More time spent in the shop working on your airplane generally means more
time flying something similar to this 35% Extra 260 at the field.
TO THE RELATIVE newcomer, building a
giant-scale aerobatic model can be
intimidating, especially if you are putting
together a large model for the first time.
On today’s competition scene, many ARF
airplanes exist. However, not all of them are
designed to use the same radio equipment
and/or engine. With technology constantly
evolving for the better, you might have to
modify certain areas of your model throughout
the build process.
You might learn that a given airplane has
rudder-servo cutouts that are too large for the
servo of your choice. Or a specific location for
a switch harness or a battery to mount may not
exist.
This month, I will write about how you can
overcome these minor dilemmas and discuss a
few tricks of the trade that I am certain will
benefit you.
As I’ve mentioned in this column, behind
every great pilot is a properly built and
programmed airplane. Now let’s get started.
Decisions, Decisions … :When constructing
your model, start by deciding what engine,
exhaust, servos, batteries, and receiver you
want to use. To do so, I first turn to the
instruction manual that was included with an
aircraft, to see what equipment was used.
If you are unsure of what servos (and how
many) to install, I have some advice for you.
For radio equipment, I highly recommend
digital servos.
Years ago, it was uncommon to see a
standard-size servo with 400 inch-ounce or
more torque. However, look at the following
servos that are currently on the market and their
specifications.
• JR 8711: 403 inch-ounce torque on 6 volts
• JR 8711HV: 480 inch-ounce on 7.4 volts
• Futaba S9157: 425 inch-ounce. on 6 volts
• Hitec HS-7950TH: 403 inch-ounce on 6 volts
and 486 inch-ounce on 7.4 volts
If you are into precision flying only, I
recommend that you use a regulated voltage of
5.9 or similar. At this voltage, I like the speed
of the servo, and the servo has more than
enough torque for my applications, which are
primarily 40% aerobatic airplanes.
Choosing the right servo is critical for a few
reasons. You need the correct amount of servo
torque for a given application so you don’t
experience what is commonly referred to as
“blow back.” This term is used to describe how
much deflection a control surface has on the
ground vs. in the air.
You might obtain a value of 40° controlsurface
deflection on the ground. However,
once airborne and after a load is present on the
control surface, you might have only 30°
control-surface deflection. Blow back is most
commonly seen on the rudder control surface
and/or ailerons.
Eliminating blow back is crucial. The key
to success is consistency, and a pilot will be
inconsistent when flying if his or her airplane
flies inconsistently. With the recent
development (see the preceding list) of some
extremely high-torque servos, I recommend the
following.
• 85cc model: One servo per aileron, one servo
per elevator half, and one servo on the rudder.
• 100cc model: Two servos per aileron half,
one servo per elevator half, and two servos on
the rudder.
• All-composite 150cc-200cc model: Two
servos per aileron half, two servos per elevator
half, and two servos on the rudder for a pushpull
setup or three to four servos on the rudder
when using a pull-pull system.
• Built-up 150cc-200cc model: Three servos
per aileron for a built-up aileron or two servos
for a foam-sheeted aileron, one servo per
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After making proper measurements for
the switch harness, use a sharp #11 X-Acto
blade and a ruler to cut the opening for the
switch. Then drill for switch-harness bolts.
A 1/8 plywood support is temporarily
fastened to the bottom of a rudder servo
tray. This model was designed for large
servos, so its tray had to be modified to
accept standard-size servos. On top of the
support, light plywood that is the same
thickness as the existing tray will be glued
so that the servo will sit flush.
After the rudder tray is modified to accept
the new servos, drill the servo mounting
hole locations with a 1/16 drill bit. Thread a
servo screw in each location and remove
the screw.
Once all servo screws have been threaded
and removed, place a drop of thin CA in
each hole location to harden the threads.
finished, you will need to add a reinforcement
piece of 1/8 light plywood to support the
underside of the glue joint.
5. Cut a strip of 1/8 light plywood that
measures 1 inch or so in width and roughly 3
inches in length.
6. Glue the reinforcement strip from step 5
on the underside of the rudder tray. Center the
rudder servo in the opening and drill 1/16-inch
pilot holes so that it is ready to mount.
7. Thread a servo screw in each hole,
remove it, and apply a drop of thin CA to
harden the threads.
8. Repeat the preceding step for the other
servos.
Measure Twice; Cut Once: When making
necessary cutouts for switch harnesses, batterypack
locations, etc., start by selecting all of the
equipment you will use in your model. Plug the
batteries to their corresponding voltage
regulator (if one is being used), plug the
regulator to the corresponding switch harness,
and plug the regulator to the receiver (if using a
Spektrum VR6010 regulator).
The receiver location is predetermined in
most airplanes. Check your aircraft’s CG in
relation to the instruction manual, and move
the batteries, regulators, and receiver
accordingly. Once you find the proper CG, you
know where you will need to secure all of your
equipment.
Once you have secured the batteries and
voltage regulators, you will know where to cut
for the switch harnesses. Make sure that you do
not mount the switch in an area that will be
covered.
I have seen pilots working on fuselages
without the wings nearby. Then they mount the
switches, only to find out that they will
interfere with the wing panels when plugged
in.
To avoid this, insert the right and left wing
panel in the fuselage. Mark the location of the
root rib with a felt-tip marker. Remove the
wing panels, measure for the switch harness,
and cut away.
I have shared a few “tricks of the trade”
regarding equipment selection and airframe
elevator half, and two servos on the rudder for
a push-pull setup or three to four servos on the
rudder when using a pull-pull system.
It is important to not only choose a servo
with the proper amount of torque, but also to
choose a servo that centers properly and has a
good gear train. If your servo doesn’t center
properly, please make sure that the linkage to
which it is attached can move freely.
If any binding exists, try to find the source
of the problem. If no binding exists and the
servo still does not center properly, send it to
the manufacturer to be serviced.
In the past I have experimented with a few
servos from various manufacturers. All in all, I
was most impressed with the centering of the
JR 8711 and 8711HV.
In the ARFs I have built, all wing and tail
surfaces have been designed for standard-size
servos. However, not all rudder setups are
designed for standard-size servos.
Some airplanes that were designed a few
years ago might be set up for servos that are in
the 1/4-scale size category or larger. Those have
been replaced with much higher-torque servos
that are much smaller in size, but they were the
best option during their time period.
Several of those models are still in
production, but manufacturers don’t always
keep up with modern technology. If your
aircraft needs to be modified to accept smaller
servos, don’t worry.
Following is a step-by-step description of
what I did on my 35% Katana to make it accept
JR8711 servos on an existing rudder tray that
was made for larger servos.
1. Measure the length of the servo opening
and compare it to the servo.
2. On light plywood that is the same
thickness as the rudder servo tray, use a pencil
to mark the length of the difference noted in
step 1.
3. Measure the width of the servo opening
in the rudder servo tray. Mark this
measurement on the piece of light plywood
from step 2. Cut out this piece.
4. Use medium CA to glue the piece of
light plywood into the servo tray. Once
modification to accept different servos,
switches, etc. Always take your time when
constructing an aircraft.
I have learned that more time spent in the
shop building generally translates to less time
spent at the field working on a model. After all,
time spent at the flying field should be for only
two things: socializing and flying.
Until next time, fly hard! MA
Sources:
Futaba
(800) 637-7660
www.futaba-rc.com
Hitec RCD
(858) 748-6948
www.hitecrcd.com
JR
(877) 504-0233
www.jrradios.com
Spektrum
(800) 338-4639
www.spektrumrc.com
International Miniature Aerobatic Club
www.mini-iac.com
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