The Battery Clinic - 2009/09

SINCE I’VE TAKEN the quiet, clean path, I’ve
modified a dozen or more glow-powered models for
electric flight. It was simple; replace the glow
components with motor, ESC, and battery. Check for the
correct CG location and then fly. That was, until I
inherited an antique design: Gilbert Morris’s 1941
Kerswap.
Bob Strub, of the On Top of the World RC Club in
Ocala, Florida, who is a master modeler, built the
Kerswap. It was set up for electric, lacking only the
motor, ESC, and receiver.
I had what appeared to be a good choice for the
motor: a 1588 Kv Maitian brushless that I won in a pilots’ drawing
at a local electric meet. Combined with an inexpensive Welgard 18-
amp controller and an FMA Direct 2S 2100 Li-Poly battery, I had
the Kerswap ready for its maiden flight.
Hitec HS-55 servos with an FMA Direct M5 receiver drove
rudder and a half elevator. I was concerned that with only a half
elevator, the model would have a rolling tendency. I was unprepared
for the way the Kerswap responded to rudder commands.
Probably because of the pylon and polyhedral, I tended to way
overcontrol. Small inputs and patience were the only things required
to avoid what seemed, at first, to be erratic action. I merely had to
add a tiny amount of rudder and then wait for the airplane to do its
thing. I noted no adverse effects with the half elevator.
I would be interested to hear from others who have electrified
one of these pylon-configuration old-time FF models, remembering
that they were designed to climb nearly vertical under power and
then transition into a nice, flat glide.
Throttle-Lock Fixes: In response to my request for information, in
the January 2009 issue, about programming a transmitter to have a
throttle lock, I have several responses that cover the Airtronics
Stylus, Spektrum DX7, Spektrum DX6, and Futaba 9ZAP. The
setups are detailed and would take up a whole column by
themselves.
If you are interested in this information, send me an e-mail (or
letter via snail mail) and I’ll send the details. Thanks to the AMA
members who responded: Alan Buckner, Bill Inman, and Luis
Alvirez.
The Ugly Truth About Battery Ratings: Frank Donnelly wrote:
“I have a NiMH, 4.8 volt receiver pack that’s rated for
1000mAh. It’s very small since it’s comprised of Sanyo AAA cells.
I could purchase a NiMH, 4.8 pack with the same mAh rating in
larger cells such as AA all the way up to C.
The physical size of the pack would get larger
and it would weigh more but electrically it
should still supply 1000 mA for 1 hour.
“But something just doesn’t make sense to
me. How could such a small pack have the
same capacity? If it truly does, why wouldn’t
we use these small packs in large aircraft? For
that matter why would physically larger
batteries even be built if a small one can
provide the same capacity at the same
voltage?”
In some instances, the rating game has
been assumed by marketing types rather than
engineers. When the engineers had pushed the
capacity as far as they dared without
significantly compromising the battery’s life,
the marketing wonks
were unsatisfied and started
playing games using “nominal” vs.
“minimum” for capacity ratings.
That gave their products a perceived
advantage over the competition, until the
other companies started the same game.
Product failures or poor performance is
usually hung on the engineering or
manufacturing people—not on marketing or
sales.
The AAA cell rated at 1000 mAh is what
we used to call a “cram job” in the business.
Cramming this much into a cell that size
required other design parameters to be
compromised. Along with those concessions
was the cell’s service life.
The separator was thinner, winding
pressures were increased, and the winding
mandrel was smaller—all of which
contributed to early shorting. The can
thickness was decreased, which
compromised the sealing of the cell. So
whenever you see a cell that seems to have an
abnormal amount of capacity for its size, you
know that compromises were made.
A NiMH AA cell with a normal capacity
would be roughly 1000 mAh. A C-size cell at
1000 mAh is probably from the culls of a
production lot that didn’t make anywhere near
the capacity that would be expected or had
some other capacity-debilitating problem
caused by poor process control.
Rather than scrapping the cells, they were
pedaled to a secondary market where product
quality was not of prime interest. Products are
usually no-name brands made in China.
Then cells are directed to the consumer
that might be C size but are actually AA
cells inside a plastic shell, to make them
into a C or even D size.
B-29 Crashed and Burned: Our club
experimenter is way ahead of us when it
comes to taking on challenges. The most
recent was a B-29 built from a Guillow’s
kit that was intended for display rather
than flying. Then John Castronover
thrilled us with its first flight.
The second flight was even more
thrilling if you are into ugly. John’s model
was on final approach after a nice flight,
when it snapped into the ground from
close to 10 feet up.
Club members were quick to run to the
crash site, where the B-29 was consumed
by flames. A member with forethought
had grabbed his small fire extinguisher
and put out the fire, but not before the B-
29, two of the four brushless motors, and
four ESCs were destroyed.
Forensic analysis indicated that it was
one of the ESCs—not the battery—that
had shorted, which caused the fire. John’s
prognostication was that one ESC failed as
he applied power to stretch the landing,
allowing one motor (outboard) to stop,
resulting in the snap into the ground.
Powered-Glider Wing Flutter: I
encountered a perplexing issue. On the
first test flight of a Jolly, a powered glider
from Northeast Sailplane Products that
was set up with the recommended motor—
an AXI 2820/10—it developed a serious
wing flutter under full power. I noticed
this immediately after launch, as full
power was applied and it came up to
speed.
The first suspect was the ailerons, but
checking those showed that there was no
slop and that the gap was sealed. On a
subsequent flight, I used less power (it had
way more than needed, as it turns out).
That flight was great and required little, if
any, trimming. Then I took it up to full
power and was rewarded with more
violent flutter. Enough of that.
I learned later, according to Northeast
Sailplane Products’ Web site, that
performance was fine with a seven- to
eight-cell Ni-Cd/NiMH battery; I used a
3S 2500 Li-Poly. Dropping back to a 2S
Li-Poly pack fixed the problem.
Using a small drill bit (.080 inch) I
grabbed from the tool drawer to align a
motor’s mounting holes to the firewall, I
inadvertently snapped off the drill bit.
Where did the piece go? I found out when
I tried to turn the propeller to assure that
there was no interference with the
outrunner.
The drill bit was sucked in and attached
to the magnets in the motor. My only
choice was to take it apart and remove the
obstruction.
There is a tiny (very tiny) snap ring on
the shaft as it exits the housing. Using my
finest needle-nose pliers (my small snap
ring pliers were too big), I managed to
pull off the snap ring and carefully set it
aside. The motor’s outer casing was easy
to remove, and there, nestled between the
magnets, was the rogue part of the drill
bit.
With the stray piece removed, I
reassembled the motor; the last step was to
reinstall the snap ring. Sproing! It went
into that never-never land of the shop
where things disappear forever.
Oh well. I wouldn’t need it if the motor
were mounted so that the shaft were used
for the propeller attachment, rather than
reversing the motor and using the AXI
propeller adapter system.
Replacement snap rings are not
available from the distributor, but I got
some from McMaster-Carr. They come
100 in a package; if you need one or more,
let me know.
Is balancing a gimmick to sell balancing
chargers? A Li-Poly battery vendor claims
that it is. His batteries are matched and
never need balancing.
Considering that Li-Poly incidents
have all but disappeared coincident with
the introduction of balancing to the hobby,
you have to wonder.
Steve Anthony of Hangtimes Hobbies,
a quality pack vendor, wrote (and I agree
100%):
“We’ve learned (some the hard way)
that cells have individual characteristics.
After a few rapid charge and discharge
routines the small normal differences in
capacity and impedance can easily become
significant enough to force one cell in the
pack into an overcharge state before the
other cells come to full voltage.
“While it’s true that some of the
Pacific Rim manufacturers have different
production or assembly shops, one huge
issue is that virtually all of them share the
same raw material suppliers. Debris and
impurities in separator, tolerances on
materials, alloys, grading, etc., all done
outside of the control of the cell
manufacturer. Cells cannot be the same.
“Assuming the vendor is matching
cells at assembly, it’s still a pack. Cells on
the outside of the pack will see a different
operating environment, temp buildup,
cooling rates, physical contact with its
containment, handling pressure on the
outer cells jackets. A few hours into
service, it’s no longer matched.
“To proclaim this technology as not
requiring the common sense expedient of
using a balancing charger is, in my humble
opinion, ludicrous and dangerous.” MA
Sources:
Red Scholefield
12219 NW 9th Ln.
Newberry FL 32669
www.hangtimes.com/redsbatteryclinic.html