FAQ: frequently asked questions - 2004/03

I have to admit that
including those four
words was an error on
my part and most unfortunately
changed the sentence’s entire
meaning. It is no wonder my reader
friend was confused. The sentence
should have read: “You can’t recharge
a battery at the trickle charge rate.”
You only use trickle charge
(which is a low level) to maintain the
charge in a battery that has already
been fully charged at C/10 overnight
or at a fast charge rate (2C or 3C), but
fast charging is generally only used at
the flying field between flights. You
can’t charge a battery at the trickle
rate.
THE FIRST ROUND of input
regarding the “From the Ground Up”
series ended with “Advanced RC
Systems,” which was published in the
February 2004 Model Aviation. During
the almost-one-year period of this series,
Bob Hunt and I have received many
letters from readers that
contain detailed followup
questions and
concerns.
The volume of these
letters makes it
impossible to answer
every one personally. I
try to respond to as many as I can, but
there is never enough time in the day.
The alternative is to sort through and
select the more important questions and
then, as I am about to do, provide an
article addressing the most frequently
asked questions (or FAQs, as I’ll call
them!).
This will be my first time doing this,
and as a result the questions have been
edited to save a great deal of space.
Many of you write pages and pages to
ask one question. I won’t identify any
readers by name or location (my
choice).
The subject matter will be random;
questions could be about Radio Control
(RC) equipment, electric power, model
building, model flying, etc. In this first
FAQ article, most of the questions
involve batteries, battery testing, and
battery charging. It is the major source
of concern and causes the most
problems for the average RC flier.
I’m going to number each question
sequentially. It is hoped to eventually
expand on this FAQ concept and
provide a search capability on the AMA
Web site so you can seek out specific
subject matter. Here we go!
FA

Q
78 MODEL AVIATION
frequently asked questions by Bob Aberle
Q1: In my “Battery Basics” article (in the
October 2003 Model Aviation) I wrote
the following: “The advantage of trickle
charging is that the batteries are
available the moment you want to fly.
However, you can only maintain a charge
level at trickle; you can’t recharge a
battery that has been used at the trickle
charge rate.”
A reader was confused by the words
“that has been used.” He went on to
point out that after reading this, he
assumed that after returning from flying
he should only put the battery on trickle
charge!
A1:
Q2: “Several years ago I saw an article that gave tips on how to rejuvenate old Ni-Cd
cells,” recalled one reader. “I can’t find that article today. Is there a technique that
you are aware of that would bring old batteries back to functionality?”
I recall the article but didn’t bother trying to look it up. The
following answer reflects my thinking on this subject.
When a pack goes down (indicates lower-than-normal rated
capacity), it is usually because one or two cells in a multiple-cell pack is/are
starting to go bad. So the first problem is to identify the bad cells. To open a pack
and search for the bad cells is a time-consuming process. After they are finally
identified, you either substitute new cells or, as you suggested, try a technique to
rejuvenate them.
There might be some tricks to revitalize those cells, but I wonder how long the
process will last. When you are finished with any such process, you may still have
a battery pack with a variety of cell characteristics.
The bottom line: Do you really think it is worth spending the time? When a
pack starts to lose capacity noticeably, I prefer to discard it in favor of a new pack.
When I consider the cost of a new one compared to the time it takes to repair an
old one, I’d rather be out flying with that new pack.
A2:
The reader is correct in his observation that motor identification can be
frustrating when dealing with electric power, but you have to look back to the
beginnings of fueled engines for model use.
In the early days it wasn’t obvious what a .29-displacement engine could fly. It was
an educational process that eventually got everyone tuned into relating engine
displacement into classes and then into model weight (which indirectly relates to size).
The same is happening in the world of electric power. The power of the motor expressed
in watts is the key factor.
Most of us who are
involved with electricpowered
flight today own
a good meter that reads
motor current (in
amperes), motor voltage,
and the product of the
two, which is motor
power (expressed in
watts). The AstroFlight
Digital Meter (Model
101) provides all of these
parameter measurements
and is a must-own item.
By experience we
have learned that dividing
the motor power in watts
by the model’s total
weight results in the term
“watts/ounce.” On the
smaller electric-powered
models such as a Speed
400 (and down), we
know from experience
that the average sport
model needs roughly 2.0
watts/ounce to fly in a
normal manner. A
lightweight, lightly
loaded powered sailplane
might need only 1.25
watts/ounce. More
energetic models, for
aerobatics and heavier
wing loadings, need 3.0
watts/ounce and more.
My Scratch-One
design from this series
had a 5.32 watts/ounce
figure which indicated that the recommended Speed 400 motor had more than enough
power. That model can fly at half throttle and lower because it has that extra power. As
you get into larger electric-powered models, the parameter changes to watts/pound, but
the concept is the same.
This may seem complicated at first, but the numbers and the form of identification
will get easier with experience. Motor manufacturers are being encouraged to provide
power ratings (wattage) for every motor.
Published reports (such as aircraft product reviews and construction articles) are
beginning to provide watts/ounce and watts/pound as references. If you can’t find this
information, ask the manufacturer or supplier. It should, by its own experience, be able to
point you to the correct model application for each motor.
Yes, you will have
to remove the battery.
I’m forced to do this
with almost every brand of RC
transmitter in my inventory.
However, that isn’t the end of
the world because most RC
transmitters have an easy-access
hatch cover at the rear of the case.
You open the cover and can usually
remove the battery pack with no
difficulty.
Most transmitter battery packs
also have a connector (provided by
thoughtful manufacturers) that
allows you to disconnect the battery
pack from the circuit. The trick then
is to obtain a mating connector that
will allow you to attach the battery
pack to your discharge testing
device (or cycler).
Many RC manufacturers will sell
you this necessary mating cable.
Battery suppliers such as Batteries
America and Radical RC also sell a
variety of transmitter battery cables
and connectors. Construct these
adapters once, and you are set for
the future.
March 2004 79
Q3: An old-time glow-fuel pilot mentioned that for years he referred to engines by their
classes, such as Class 1⁄2A, Class A, etc. Each class had a specific engine
displacement; Class A was understood to be .10-.199 cubic-inch displacement, a .19-
powered model was supposed to weigh approximately 19 ounces, and so on.
He went on to write that with Electrics the identification of motor size to model size
is difficult, to say the least. He cited examples of Speed 280, Speed 400, etc. How do
you decide on application?
Popular AstroFlight digital readout meter provides such
parameters as current, voltage, wattage (power),
amount of charge put into battery (in ampere-hours
[Ah]), or discharge taken out of battery (in Ah). Battery
is attached to right-side connectors; load or motor is
attached to left side.
Close-up of AstroFlight digital meter shows LCD [Liquid
Crystal Display] readout: “0.7A” is 0.7 amps current,
“9.8V” is voltage, “7W” is 7 watts (power); this is
actually the product of amps multiplied by volts. At
lower right, “0.004AH” means .004 Ah of charge has
just been put into battery. Figure “0.004AH” is same as
4 mAh.
A3:
Q4: “I tried using my ACE Power
Pacer cycler to check my Futaba
transmitter battery and found it
wouldn’t work,” wrote one reader.
“Didn’t I read something to the
effect that most RC transmitter
battery circuits contain a blocking
diode that prevents you from
measuring the battery voltage at the
charging jack? Should I remove the
battery from the transmitter for
testing purposes?”
A4:
RC transmitter’s rear hatch cover
has been opened and battery pack
has been removed. Battery
connector is still plugged into
transmitter. Simply unplug
connector, find similar mating half,
and attach battery pack to tester or
cycler. When finished, plug pack
back in and replace hatch cover.
Photos courtesy the author
80 MODEL AVIATION
The term “channel” or “channel number” actually has two meanings when
it comes to the RC hobby. In 1982 the Federal Communications Commission
granted a series of frequencies on 72 and 75 MHz (megahertz) for RC use.
Each frequency was identified by a series of five digits, such as 72.010 MHz.
From the start it was realized that trying to memorize frequency numbers would be
impossible, so borrowing from the television industry, AMA suggested that a channel
number be assigned to each authorized RC frequency. For RC-aircraft purposes, channel
numbers were
assigned from 11
up to 60 (a total of
50). On 75 MHz,
additional
frequencies
(therefore,
additional channel
numbers) were
assigned for the use
of surface vehicles
such as RC cars
and boats.
When you are
at a flying field and
are asked what
channel you are
operating on, the
answer will be
“I’m on 32,” “I’m
on 56,” and so on.
The channel
number is an easy
way to identify the
authorized RC
frequencies.
Separate from
this definition is the
term “channel
functions” or
“number of
channel functions”
when it comes to
RC transmitters
and receivers.
When we flew in
the early 1950s our
control was usually
limited to rudder.
Back then we
called that “single
channel” control.
Later, when
technology
provided for more
functions, we said
that RC systems
had “multichannel” capability. Each control function, such as rudder, elevator, aileron, and
throttle, was called a “channel function.” A “four-channel receiver” provided four controls
for a model aircraft.
When the reader saw the decal indicating “FM 7 Channel Receiver,” it meant that the
receiver could impart seven control functions to the aircraft. This seven-channel reference
has absolutely nothing to do with the channel number (two digits) displayed on the flag
that hangs from a transmitter’s antenna.
Q6: “I own a Hobbico Accu-Cycle
which has output charge rates of 50
and 125 mA [milliamperes],” wrote
a reader. “Recently I purchased an
1800 mAh [milliampere-hour]-
capacity four-cell receiver battery. I
read what you said about always
charging at the C/10 rate (which
would be 180 mA in this example)
but wondered if I could charge this
new pack at the 125 mA and just
settle for less than the rated
capacity?”
You probably could
get away with it, but I
would be uncomfortable
with that kind of charging in the
long run. It would almost be like
saying that the battery was never
fully charged but is that all right?
I would prefer that you buy a
variable output charger, such as the
ACE DDVC, set it for 180 mA,
and let it stay on charge overnight,
or at least for 10 hours. Then you
would have maximum battery
capacity available, and the battery
chemistry would be happy. The
result would be a long-lasting
battery pack.
I receive many questions
similar to this one; they all involve
making do with equipment
(chargers) on hand. Many
alternatives will probably work,
but I recommend the technically
correct approach.
A6:
ACE DDVC has two outputs that can be
adjusted from 0 to roughly 300 mA
current and up to total of 10 battery
cells. Bob has this battery set to 180 mA,
which is what you would normally
charge 1800 mAh batteries for at C/10
rate overnight. That is not an 1800 mAh
pack in the photo.
Q5: “I wonder about the term ‘channel number,’” one reader wrote. “I see references
to the channel number flags on the transmitter antenna, but then I noticed a receiver
label that states ‘FM 7 Channel Receiver.’ Why is this?”
A5:
Receiver label reads “FM 7 Channel Receiver,” meaning that it
has seven-channel control-function capability, such as
aileron, rudder, elevator, throttle, flaps, gear retract, and
bomb drop. It does not mean that this receiver operates on
Channel 07!
Flag on transmitter antenna indicates that it is operating on
Channel 51, which is the same as saying it operates on FCCapproved
frequency of 72.810 MHz. The Channel 51 and seven
“7 Channel” indication on receiver at right have different
meanings.
March 2004 81
That is a good and
sensible question!
Through the years the
Ni-Cd battery cell’s nominal voltage
has been identified as 1.2. I guess it
seemed easy way back. It just stuck,
and today we refer to a four-cell pack
as having 4.8 volts, an eight-cell pack
as having 9.6 volts, and so on.
The fact is that a fully charged Ni-
Cd cell can reach almost 1.4 volts, so
a fully charged four-cell Ni-Cd pack
might go as high as 5.6 volts.
Considering that fact and depending
on that pack’s rated capacity, it might
take one or two hours to get the
voltage down to 4.8, at which time I
recommend that you recharge it
before continuing to fly. In this
regard, the nominal voltage is the
minimum voltage before the need to
recharge.
This is strictly a point of
identification that started years ago
and carried through to today. I’m
sorry, but I can’t do anything to
correct this somewhat confusing
terminology.
Q7: “I’m concerned about your
statement that any Ni-Cd cell will
have a nominal (average) voltage of
1.2,” commented one reader. “Then
you went on in the testing
discussion to say that when a fourcell
receiver pack gets down to
roughly 4.8 volts, it’s time to
recharge. I question how could the
cell have an average of 1.2 volts or
4.8 volts (for four cells) and still
need recharging?”
A7:
This has been my first attempt at
providing FAQs for the readers’
benefit. This concept will become a
regular monthly column appearing in
Model Aviation. It will also be
available on the AMA Web site along
with the reprints of the entire “From
the Ground Up” series.
Although this first set of
question/answers was almost entirely
about electric power and batteryrelated
items, I welcome your
questions on any aspect of our hobby.
If I can’t answer your inquiries, I’ll
find an expert who can! I also value
your suggestions and criticism of what
we are attempting to do in this series.
Please share your thoughts!
Direct your questions and
suggestions to Bob Hunt at
[email protected] or Box 68,
Stockertown PA 18083. MA