Radio Control Electrics-2004/12

98 MODEL AVIATION
Bob Kopski, 25 West End Dr., Lansdale PA 19446
RADIO CONTROL ELECTRICS
LVRCS President Lou Hayden recognizes 92-year-old club
member Ellis Grumer for many years of Electric accomplishment.
Mark Drogowitz’s potent Hobby Lobby Bonnie 20 ARF has AXI
“motovation” on 10 1.7s with 9 x 7 APC E-series propeller.
Master craftsman Ellis Grumer and Andy Kunz hold Ellis’s scratchbuilt,
great-flying Shoestring. Has geared MP Jet brushless.
THIS COLUMN BEGINS with a personal wish list of Egoodies,
shares some reader questions and experiences, and
describes one Dump’r “watch-out.”
This month’s photos are from the annual LVRCS (Lehigh
Valley Radio Control Society, in Easton PA) Electric Fly that
took place June 5-6. This well-established meet has been
growing for many years and is one of my favorites.
’Tis the season for wish lists, so it’s a good time to describe
some needs that would benefit many E-aeromodelers. I’m
hoping that at least some of these wishes can be realized by this
time next year!
Approximately a year ago I wrote that this hobby needs an
ESC with a “better BEC.” All commonly used ESC/BEC
products I’ve seen are limited to roughly 10 cells max for BEC
use. The actual limiting number is driven by several factors
including the ESC design, the number of servos in the system,
and even the kind of servos in use. Most of these speed controls
can be used with much higher cell counts when the BEC is not
used.
The BEC-driven cell-count limitation is because of heat
generated mostly during servo operation. At this time the
current demand is greatest, and this current multiplied by the
voltage difference on the BEC regulator device is power that
produces heat within the circuit. Linear regulator integrated
circuits are commonly used to power the radio stuff, so these
parts get hot. In fact, they can get disastrously hot.
I see a simple way around this—at least in principle. If the
BEC regulator were of switch mode design, there would be little
power loss (heat) associated with radio and servo operation. The
cell-count limitation would essentially disappear, so even the
largest Electrics could benefit from the availability of a BEC—
just as smaller models do now.
At roughly the time I covered this topic, I reported that at
least one manufacturer was planning such an ESC/BEC product.
This has not come to pass, so “switch-mode BEC” ESCs
continue to be on my wish list!
12sig4.QXD 10/25/04 8:50 am Page 98
December 2004 99
Rod (R) and Chris Maier with a mostly foam, MonoKote-covered
Sukhoi. Has AXI 280824, three-cell 2100 mAh Li-Poly pack.
Chris holds it steady! ARC (Almost Ready-to-Cover) Sukhoi by
the Swedish Extreme Team is available from eDogFight.com.
Another item I’d like to see also lies within the ESC domain:
ESCs with “max-out” programmability.
With Li-Poly batteries’ increasing popularity, modelers are
finding it harder to compose an optimum electric power system.
Because Li-Poly chemistry results in a nominal cell voltage of
3.7, each Li-Poly cell is the equivalent of slightly more than two
Ni-Cd or NiMH cells in terminal voltage. As a result, adding or
removing a cell within a pack has a much greater impact on
power-system makeup and operation; it is not nearly as easy to
fine-tune the overall battery/motor/propeller combination with
cell count.
Because of this, I’d like to see an ESC design with an
adjustable “equivalent maximum output” voltage; that is, I’d
like to see an ESC adjustment that would allow full stick to
actually mean, for example, 90% effective battery voltage. It
would be a sort of “ATV” (Adjustable Travel Volume) spelled
“AVT,” or pseudo Adjustable Voltage Travel on the throttle
stick.
Thus one could install that extra Li-Poly cell when only a
fractional cell would be desired and make the ESC act as though
only such a fractional cell actually existed. If properly executed,
this would mean no wasted resources and all stored energy
would be fully usable for flight.
The third wish on my list is for a simple line-operated Li-
Poly charger. Right now, as far as I know, all commonly used
Li-Poly chargers are powered from 12-volt sources. This classic
approach works fine but is not always as simple or convenient
as it could be. Since the normal highest charge rate is 1C and
since it’s often nice to do this in the shop, either a robust lineoperated
12-volt power supply or a car battery is needed.
A dedicated line-operated Li-Poly slow charger would be a
desirable electric accessory. I can imagine such a charger having
multiple outputs to accommodate several packs simultaneously.
Although it might take overnight to charge Li-Polys this way, it
typically takes that anyway to charge a transmitter. Therefore,
all rechargeables could be ready to go at the same time—no
disadvantage there.
Right now I do all of my Li-Poly charging in the shop (I
don’t bother with a charger in the field), but I need a separate
12-volt, line-operated power supply just to power the 12-voltinput
Li-Poly chargers. This is cumbersome at best.
This wish assumes that slow-charging Li-Poly batteries is
acceptable. Does anyone out there actually know if that’s true?
Reader mail is a constant source of topics for me to share with
everyone. What follows is an assorted collection of experiences
and advisories from that fountain.
As classic a no-no as it may be, one reader learned this the
hard way! Please remember that electrical soldering must never
be done with acid-core solder or acid fluxes! These soldering
products are intended for mechanical purposes only, such as
plumbing applications or perhaps some kind of sheet-metal
work. For electrical and electronic applications, use only rosincore
solders and rosin fluxes. Acid-core solder/acid flux will
eat electrical connections to death!
Generally, electrical and electronic soldering can be done
using only rosin-core solder, which has built-in flux; i.e., rosin
flux resides inside a central space throughout the length of the
solder. When heat is applied to melt the solder, hot flux is
released onto the work and prepares the surfaces for better
solder flow and adhesion.
The flux is intended to remove small amounts of surface
corrosion so that the solder can stick to a clean metal surface.
Even if the surface is bright and shiny, the flux is still a big
help.
Sometimes surface conditions and circumstances warrant
additional flux. Consider a multistrand copper wire (the
untinned kind) that has been lying around for a long time. The
individual copper strands may be dull or dark-colored and
difficult to polish bright because of their number and fineness.
It may be so bad that flux within solder is insufficient to clean
all strands adequately, which is where additional rosin flux can
be useful. (However, sometimes the condition is so bad that no
amount of flux will work.)
This flux can be in liquid or paste form, and I generally use
the latter. You simply dip the wire end in the paste; heat it with
the soldering iron tip to melt, flow, and activate the flux; and
then add the solder.
You can find such flux—make sure it is described for
electrical and electronic applications—in hardware stores such
as Sears and in places such as RadioShack and from
electronics suppliers such as Mouser.
There are many kinds of solder available, but you can’t go
wrong with common “60/40” rosin core. The numbers refer to
the mix of tin and lead that make up the solder. Another
variable is the solder diameter, and you can make some
judgment on that. I generally use 0.032- to 0.022-inch diameter
in my RC and electronic assembly pursuits, but smaller and
larger sizes are available.
Another reader question reminded me of a different
problem: sometimes a motor won’t start. This is often an
intermittent matter; sometimes the motor comes on okay and
12sig4.QXD 10/25/04 8:50 am Page 99
sometimes it doesn’t. I have seen several
“hidden” reasons that can account for this
in brushed motors, one of which is worn
brushes. It is common to find one or both
brushes worn so bad (short) that the brush
springs can no longer force them into
contact with the commutator.
By the time this condition manifests,
there is a good possibility that commutator
damage has occurred. For one thing,
reduced spring pressure (and weak brushcommutator
contact) can result in
increased heat in this area. This, in turn,
can accelerate wear of both surfaces.
Furthermore, by the time this happens,
the brush wire (shunt) has often emerged
on the inner (contact) surface of the brush
and has begun to wear a groove in the
commutator surface. It’s best to keep an
eye on brush wear and replace them early!
Another possible reason why a motor
won’t start is a variation of the preceding.
Sometimes brush dust clogs a metal brush
guide (the metal tube in which the brush
rides), and the brush may get stuck so that
the spring cannot push it against the
commutator. Removing and reinstalling
the brush will usually eliminate this
problem by knocking the clog loose.
However, if this sticking persists, chances
are that the brushes are well worn and
need replacing anyway.
Sometimes a commutator develops a
“dead segment.” This could be a single
section of the group of segments that has
lost connection to the armature windings.
I’ve seen a winding end break loose from
its cold-weld commutator tong, and I’ve
seen solder connections melt and fly off
and the connection fail. If the motor stops
(from a previous run) at a place where this
particular section falls under one brush or
the other, the next attempt will not start
the motor; the circuit is “open.”
In all of the preceding situations, the
motor might start if you move the
propeller a little or perhaps spin it, and
you may be tempted to fly with it
However, such a motor does need
attention; its performance is certainly
lowered. The problem will not go away by
itself and will surely happen again.
I’ve had a reader innocently ask where
the capacitors are when you are looking at
a brushless motor. Since a brushless motor
in fact has no brushes, it doesn’t need
brush capacitors! Along the same lines, all
brushless motors have three wires (or
other connections), and a classic brushed
motor has two.
Each brushless motor must have a
controller to run it. The controller
generates the three-phase drive signal
needed to make it spin. You cannot
connect any combination of the three
wires to a battery and make the motor run.
On the other hand, a brushed motor
will run without a controller; just connect
the two wires to a battery. A controller
(ESC) is still needed if rpm control is
desired. (In the earliest days of E-power,
when there were no ESCs, I charged the
battery, turned on the hard-wired motor
power switch, and then flew “full on” ’til
the battery drained. But that’s also how I
flew wet power in my earliest days—when
adjustable carburetors didn’t exist!)
I have learned about a possible problem
with the Dump’r (published in the October
2003 MA). It is physically possible for the
cell-count knob to mechanically slip on
the switch shaft and indicate the wrong
cell count. This all has to do with how
tight you make the knob setscrew.
If you have any doubts, repeat the final
checkout procedure within the article.
Dump’r is designed to safely discharge Ni-
Cd and NiMH packs, so the correct knob
indexing is quite important!
So concludes this last column of 2004. I
wish all aeromodelers and other readers a
happy holiday season and a great E-flyin’
new year! And please remember to enclose
an SASE with any correspondence for
which you’d like a reply. Everyone so doing
does get one. MA