Frequently Asked Questions-2011/12

LEASE WRITE IN with your questions;
that is the only way we can keep this column
format going. When referring to previously
published questions and answers (for followups),
always provide the number as a
reference.
Also note that references to addresses and
websites are placed at the end of this column
under “Sources.” A “Tips” feature is provided
in addition to frequently asked questions;
these hints are numbered in the same sequence
as the questions and answers.
Q530: “Many fliers would like to combine the
high-energy density of LiPo batteries for use
with conventional RC receivers and servos.
The average airborne RC system needs 4.8 to
6.0 volts. But LiPo battery packs come in
multiples of 3.7 volts per cell. One cell is not
enough, while two cells in series might be too
much. Can you comment on what these
receivers and servos can take (voltage-wise)?
Also, is there some kind of a voltage reducer
that I might purchase?”
A530: RC receivers and servos actually have
different voltage requirements. If you asked
the manufacturer of your particular system,
you would find out that a typical RC receiver
can work upward of 9 volts and even much
more.
The servo is a different story. The electric
motors found inside every servo are wound to
specific voltage levels to provide speed and
power at the highest efficiency within the
specified range.
A servo designed for 4.8 to 6.0 volts will
be “overdriven” if 7.4 volts is applied. When
this higher voltage is applied, the servo will
have greater speed and power, but will lose
efficiency, significantly decreasing the
motor’s lifespan and possibly taxing other
components. Older servos will not last long
using a two-cell LiPo battery pack.
Many manufacturers have begun to offer
servos with motor windings that are designed
to accept 6.0 to 7.4 volts in normal operation.
Hitec initially did this with its high-end
servos. The company now also offers highvoltage-
capable sport servos at average price
levels. Any of these servos can accept the
voltage from two LiPo cells without any
problem. Other manufacturers are following
suit.
If you have conventional and/or older
style servos, it is possible to purchase a
voltage reducer. This is a device that can
reduce 7.4 volts (and more) to a regulated 5.0
volts. A particularly good voltage reducer is
the Castle Creations Battery Eliminator
Circuits (CC BEC). This device has been
around for a while. It was originally
Also included in this column:
• Digital servos
• Solid sheet vs. open structure
• Cutting O-rings—follow-up
• Suggestion for Deans Ultra
Connectors
• Flight Journal magazine
Cutting O-rings from surgical tubing is easy
if you slip the tubing over a wood dowel. Just
roll the dowel while holding the knife blade
to the tubing.
The 0.4-ounce Castle Creations CC BEC
will reduce voltage sources as much as 25
volts to a regulated 4 to 9 volts (selectable)
and at currents up to 10 amps.
Several types of digital RC servos look the same,
but digital servos can offer much more than
analog servos. The author offers an excellent
reference source.
Left: Typically you insert the CC BEC between
your battery pack and RC receiver switch
harness. The BEC reduces voltage to a suitable
level for your servos.
December 2011 77
developed for the electric-power enthusiast
who required more current capability to
operate heavy-duty servos.
Speed control BECs are usually limited to
approximately 1 amp current. The CC BEC
can except voltages as high as 25 volts and can
produce a regulated 5.1 volts output at up to
10 amps. It is perfectly suited for a fueled pilot
who wants to use a two-cell (or more) LiPo
battery pack to power an airborne RC system
(receiver and servos). All you need to do is
insert the CC BEC between your battery pack
and your receiver switch harness.
The output voltage is selectable in 0.1-volt
increments between 4.9 and 9.0 volts. You
also have the option of setting the output
voltage at 5.5 or 5.8 volts to get your servos
working a little faster. It’s your choice!
The CC BEC weighs only 0.4 ounce (11
grams) and measures 1½-inches long x 5/8-
inch wide x 3/8-inch thick. It costs roughly
$25. This is your chance to use lightweight
LiPo batteries to power your airborne RC
system without the risk of burning anything
out. Also note that these LiPo batteries can be
fully recharged at the field in one hour or even
less if the battery already has some charge!
Q531: “Bob, please share your knowledge of
digital servos and their value to the average
sport flier.”
A531: This could be a long answer; it might
be appropriate for a dedicated article. Let me
boil it down to a few basic thoughts and
provide a good reference source for more
detailed information.
I asked Shawn Spiker of Hitec RCD to
give me his thoughts on this question because
his company supplies a wide variety of digital
servos. This is what Shawn had to say:
“Regarding the question of why we might
want to use a digital servo … it all depends
upon application and the level of performance
required from the airframe or from the pilot.
General sport-flying, trainer-type park flyers
up to .40-size aircraft typically do not need
digital servos.
“As the performance requirements increase
to Pattern or 3-D-type flying or to higher
speeds where increased control surface size,
control surface
loads, and aircraft
weight are factors,
then digital servos
will offer better
control.
“Why do digital
servos offer better
performance? An
analog servo is
trying to do what it
is commanded 30
times a second,
while a digital servo
is doing the same
thing at 300 times a
second.”
Digital servos
can offer increased
resolution, travel
The author constructed a stabilizer from sheet balsa and one with open structure to provide
a weight comparison. The open structure weighs 30% less than the solid sheet balsa.
This is the cover of a recent Flight Journal.
The magazine’s website is loaded with fullscale
aircraft articles, photos, PDF plans
files, and technical information.
Installing heavy-duty No. 10 gage wire to APP connectors can
prove difficult. An easier substitute is the popular Deans Ultra
connector.
speed, more precise centering, and can handle
larger control surface loads, but you don’t get
these features for nothing. Digital servos cost
more and they draw more current than the
traditional analog servo.
I found an excellent write-up on RC
servos that appears on the Airfield Models
website. This is provided free to all modelers
by Paul K. Johnson. I’ve listed Paul’s specific
site for RC servos in the “Sources” section.
It contains information on servo
specifications and properties. It discusses
analog vs. digital servos. At the end it has a
section on “Selecting a Servo” for your
application. This has always been a great
website for RC reference information. Keep it
on your list of favorite sites.
Thank you, Mr. Johnson, for your
valuable contribution to our model aviation
hobby.
Q532: “I’ve noted in many of your Old-Timer
and vintage aircraft designs that appear in the
RC Micro World [online magazine] that you
substitute solid sheet balsa for the stabilizer
and vertical fin instead of the original
open/covered structures. What is your
reasoning for doing that? Which is really
better?”
A532:My choice for using sheet balsa tail
surfaces is because it is simple to construct
and speeds up the building process. I can build
sheet balsa surfaces in a matter of minutes.
The difference between having an airfoil
shape or a flat surface seems to have little
effect on these typically small-size models.
It is important to note that solid sheet balsa
surfaces will weigh more than an open
structure. To prove this point, I recently
constructed a built-up stabilizer from sheet
balsa for my new 100-square-inch Bootstrap
vintage RC aircraft. That stabilizer and
elevator weighed 7.4 grams. I built the same
stabilizer using sticks instead of the solid
sheet balsa. The open structure weighed 5.2
grams or 30% less.
If you estimate that your aircraft may end
up tail-heavy, you want to use the lightest
possible tail structure. In my case, I didn’t
want to extend the nose length so I opted for
the open-structure stabilizer and fin. This is a
better choice than having to add dead weight
at the nose to balance the airplane. It will take
longer to build, but not that much longer!
Follow-up to Q489: In the March 2011 issue
of MA, I showed a technique on how to cut Orings
from surgical rubber tubing. AMA
member, Ron Ogren, had a better idea. The
included photo tells the story. He slips the
tubing over a short length of wood dowel.
Then, while holding the X-Acto knife to the
tubing, he simply rolls the dowel. It results in
a perfect cut or slice.
T533: I bought a high-capacity LiPo battery
pack. The claim was that the battery was
capable of 30 to 40C power loading. I was
impressed to see that the pack wiring included
No. 10-gage heavy-duty wire. That is the way
it should be!
But I ran into a problem when attempting
to install my favorite APP (aka Sermos)
connectors. The wire would not fit into the
30-amp pins. When I tried using the 45-amp
pins, I couldn’t get the pin to fit inside the
plastic housing.
To continue the installation, I removed
some of the wire strands until what was
remaining of the wire fit inside the APP
connector pin. Please don’t write in! That was
truly the wrong thing to do. I even needed
clear silicone seal at the wire/connector pin
terminating point.
But the proper solution was quickly found!
I switched to the popular Deans Ultra
connector and soldering the No. 10 wire to
these pins proved an easy job. Don’t forget to
use heat-shrink tubing at the pin solder joint.
I’ve offered several suggestions about
separating Deans Ultra connectors. I found
that using a heavy-gage No. 10 wire, you
can get a much better grip on the
connector. Separating the connector halves
is then easier and you run little risk of
pulling the wire off the pin.
T534: Flight Journal magazine is produced
by Air Age Media. It has been out for roughly
10 years now and covers full-scale aircraft—
sport, military, or experimental types. I looked
at the Flight Journal website and was
surprised to see the amount of information
available to the general public.
You can even download PDF files of
William Wylam or Willis Nye plans with
each issue. To find these plans, scroll down to
“From the Magazine,” then to “Online Free
Artwork.” New plans are added each month.
I’ve listed the main website in the “Sources”
section and I urge you to look it up. MA
Sources:
Castle Creations
(913) 390-6939
www.castlecreations.com
Airfield Models
www.airfieldmodels.com/index.html
Flight Journal
(800) 442-1871
www.flightjournal.com/ME2/Default.aspx