Focus on Competition - 2011/04

April 2011 157
TECH STUFF, Part Two: To reiterate
what I ended with last month, the
importance of the care and size of wire for
servo leads cannot be overemphasized. I
don’t know how many times I’ve had to
explain to modelers that their servos either
didn’t work correctly or at all because of the
wire they used for the lead.
I get these looks of disbelief, but you
can’t feed a 4.8-volt servo 3.0 volts and
Technically Speaking ...
Greg Hahn
Technical Director
[email protected]
Focus on Competition
expect it to operate properly. And unless
you do things right, that could be all the
volts reaching that servo out on the aileron 3
or 4 feet from the receiver.
Servos can do some funny things when
they are starving for power, including
giving up the ghost. Some servos can
actually be destroyed by lack of proper
voltage, so pay close attention to the lead
wire you’re using; it can save your model.
Another issue with wire, especially now
since most of the wire we use is imported, is
that the metallurgy can often be
questionable. Instead of solid copper wire,
now it is often copper coated and no one
knows what the base metal is—so there can
easily be problems with compatibility with
itself and with other metals it comes in
contact with.
Soldering the wires together with either
a silver-based or lead-based solder can have
a big impact on the compatibility issues, so
I recommend the use of a silver-based
solder with a light acid flux. Often the use
of paste fluxes such as ruby fluid and others
will leave too much of the paste behind and
the flux will creep up the wire and cause
what I call the dreaded black wire
syndrome.
I know you all have seen it, so now you
have a name for it. When you cut a wire and
you see black wires instead of copper, you
have a problem and the conductivity of that
wire is greatly compromised. Replace it
immediately!
A BEC, known otherwise as “battery
eliminator circuit,” is a neat little thing that
is built into most electronic speed
controllers. The BEC, as I’ll call it as we go
along here, allows the radio system to
parasitically operate off the battery used for
the propulsion, therefore eliminating the
need for a separate battery to operate the
receiver and servos.
For many of the smaller foamies and
stick-and-tissue electrics, the weight savings
can be the difference between a flying
model and a static model. The problems
arise as the size and weight of the model go
up. The use of a BEC on a larger, more
complex, and heavier model can become a
safety issue in a hurry.
One characteristic of a BEC is that when
the propulsion battery loses power, so does
your radio system since it’s using the same
battery. Obviously when that battery will no
longer power your propeller, it also won’t
power your servos, so your ability to control
the direction and glide path goes out with it.
On a 6-ounce foamie that’s usually not a big
deal, but on a 10-pound sport airplane at 60
mph, that is a completely different animal.
I always recommend that any model
weighing more than 1 or 2 pounds should
have the control system on a separate
battery. The weight savings isn’t worth
compromising safety and control to the
extent that it becomes dangerous to others.
Spring is on the way! Be safe. MA
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