138 MODEL AVIATION
TECH STUFF, Part One: This month I’m
going to change direction a little and
explain (in my layman’s terms) some
issues and my own tried-and-true methods
of combating common problems
pertaining to onboard radio setups.
Lately I’ve had some conversations in
the office with members and during some
of the FAA workgroup sessions about
common problems with servos, wire,
BECs, and so on. I’m often asked to
troubleshoot electrical problems sight
unseen, which can be challenging. I hope
Technically Speaking ...
Greg Hahn
Technical Director
[email protected]
Focus on Competition
I’ve helped at least some of the time.
Aircraft modeling requires many skill sets,
and you often need much more than
elementary knowledge to accomplish many of
the required tasks. Even in today’s ARF
environment you need to be fairly
knowledgeable in basic electricity.
You might not have to build and cover the
airframe, but you still have to get the radio
system installed cleanly and in working order.
The power system, if you’re using electric,
also needs to be the same carefully organized
installation.
Organization can’t be overemphasized;
having things in their places makes fixing
problems much easier. You haven’t had fun
until you’ve been asked to find a bad
connection in a well-built bird’s nest.
I like to centrally locate the receiver and
bring the wires in grouped in the proper
direction. I use many zip-ties to keep the
wires in their proper sequence and place.
If you have more than one control lead
grouped together, marking them with a
Sharpie will help you remember where they
go. This is extra important with complex
aircraft, because keeping 15 or 20 servo leads
in the right places can quickly turn into a
nightmare.
Wire is one of the most important and
overlooked aspects of radio setup. I dare say
that the majority of physical problems with
radios and power are directly caused by the
quality, size, and care of the wire used.
Your servo might be rated to deliver 300
in. oz. of torque at the specified voltage. But
if the wire running from the receiver to it isn’t
of adequate size or has too many mechanical
connectors for its length, that voltage will not
be there—and in extreme cases the lack of
needed voltage can cause damage to the
servo.
I won’t bore you with specifics, but the
voltage drop for each mechanical connection
added to the length per foot of the lead wire
can be amazing. Especially in larger models,
in which servo leads are routinely longer than
3 feet, the voltage drop is unbelievable and
will render your system inadequate.
Keep in mind that the wire lead that comes
on the servo from the factory is sized
adequately to be used at the factory length. If
you add to the factory lead with an extension
or hard solder an extension, the added wire
needs to be significantly larger to deliver the
proper voltage. I have witnessed voltage
drops of 1-1.5 over the length of 3-4 feet,
which severely limits the performance of your
system in all parameters.
To combat this situation I normally use
lead wire that is at least one size—and
preferably two sizes—larger than what comes
with the servo. Overkill in this circumstance
is not a bad thing, other than adding a little
weight.
Also keep the number of “mechanical”
connectors to a minimum. I usually hardsolder
the extensions rather than use a
connector, so that the only mechanical
connection is at the receiver or, at most, the
wing root and receiver.
I’ll continue with the second part next month.
Until then, be safe and have fun. MA
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03sig5.QXD_00MSTRPG.QXD 1/24/11 3:42 PM Page 138
Edition: Model Aviation - 2011/03
Page Numbers: 138