December 2005 — Vol. 22, No. 12 - RC Soaring - RCSoaring.com

Ignorance 101. A misnomer. Function
adjustments versus individual servo
adjustments .
Flashback to the slope: I fly more, but give
up on the flaperons. That’s my second
mistake. I could have adjusted endpoints
on the spot, with my JR 388S. But I was
carrying a false assumption.
Okay, I've been playing with these toys for
a few years, and I got fooled for the whole
time. I figured “endpoint adjustment”
meant “endpoint adjustment.”
How dumb! I figured “endpoint
adjustment” was a limiter. I figured that if I
set the endpoint adjustment at, say, fifty
percent, it would move as before till the
transmitter stick was about half way over,
and then it would stop, even if I pushed
the stick further. Wrong.
Well, I had a hint, because “endpoint
adjustment” is also called “travel adjust,”
in my JR 388S “manual.” Not enough of a
clue for me. My “manual” says, “offers
precise servo control deflection in either
direction of operation.” Pretty vague, as in
“meaningless marketing hype.”
“Endpoint adjustment” does affect
endpoints. But it does so in the same
manner as dual rates, exponential, and
differential affect endpoints. They are all
rate adjustments.
Technically, they adjust the proportion of
servo movement signal to transmitter stick
movement. That’s why our radios are
called, “proportional control.
Even counting fancy mixes, our radios
only know one trick: All they know how
to do is to change the proportion of servo
movement to stick movement. When the
stick hits its stops, it can’t tell the servo to
move further, so that does determine an
“endpoint.”
Q: Yeah, right, buddy. The radios do more
than proportion: What about trim
adjustments? A: Even servo centering and
trim adjustments are just the transmitter
fibbing about how much the stick is
deflected. On many transmitters, the trim
tabs mechanically rotate the stick
potentiometers. It’s kind of like the
butcher putting his thumb on the scale.
Q: So if they’re all rate adjustments, and
they all affect endpoints, what’s special
about the one labeled, “endpoint
adjustment?” A: Dual rate, exponential,
differential, and trim tab adjustments are
all aimed at functions, such as ailerons.
They may affect more than one servo, or
not. Endpoint and sub-trim adjustments
target individual servos. That’s the whole
difference.
The good news about “endpoint”
adjustments, but trim mechanically
first
The good news is that “endpoint”
adjustments work great! Unless you don’t
have them, or, as Bill Kuhlman said,
“Unless you’re using a Y-harness.”
(A Y-harness is one step simpler than an
electronic mixer.)
Even if your flaperon, spoileron, or elevon
(etc.) servos are mismatched in response
rate, if you set the endpoints for equal
deflection, they will be balanced across
their range. You can indeed, “set ’em an’
forget ’em.” Well, provided you did
everything else right.
If those wing servo arms aren’t centered at
the same angle to the wing, you’ll induce
unequal mechanical differential that you’ll
never get rid of.
Additionally, remember to make sure your
servo arm is installed so that it won’t hit its
internal mechanical stop before it moves
as far as you want it to. That’s another
reason to trim right mechanically before
you fine-tune electronically.
Keeping the suspense in programming
our radios
But please don’t worry that you now
possess the secret and esoteric knowledge
that will take the suspense out of
programming your radio.
Transmitter manuals (and manuals in
general) will always give us
half-intelligible directions, so guys like me
will still start writing articles like this one
recounting mistakes and
misinterpretations.
And even if you do “endpoints” correctly,
you can take trust in this: Being creative,
we’ll always find some other way to mess
things up.
December 2005 37