If It Flies ...-2007/12

A fun weekend with Civil Air Patrol Cadets
December 2007 117
If It Flies... Dean Pappas | [email protected]
Does it get any better than this? Cadet Maverick learned to “Do
some of that pilot stuff” in a hurry and soloed in just one weekend.
Cadets Dragon and Helmet watch the throttle-linkage
adjustment procedure described in the text, guided by
instructors Pat Monacelli (R) and one who is unidentified.
very few of us on the road at that hour,
and driving along I found myself thinking,
“If these people aren’t going to a modelairplane
contest, then what on earth are
they doing on the road at this hour?”
I’ll cut the fishermen out there a break,
but at dark-thirty on a weekend morning it
was obvious that this model-airplane thing
must be special. Normal people probably
IF IT FLIES, I’m interested in it! Sometimes
it’s easy to forget that fliers in other corners
of the far-flung Model Airplane Galaxy are
having a blast all the time because there just
aren’t enough hours in a day for me to try all
the things that sound like fun. I wonder about
others, though.
I can remember driving to a contest at 5
a.m. just a few Saturdays ago; there were
Also included in this column:
• Voltage is my friend
Cadets Red Dragon (Carlos Pineda), Nephew (Marvin Pineda), Romanian (Ryan Olson), Helmet (Esau Ozer), Spartan (Thomas Welch),
Raskle, Goose (Nick Tomei), and Maverick (Akil Romey) with instructors Nick Maggio, Armand Graziani, Bob Levanduski, Nobu
Iwasawa, Keith Zimmerly, and Seth Hunter.
think we toy-airplane types are special too!
The first thing that occurs to people who
are involved in an activity they think is the
greatest thing since sliced bread is to show it
off and then try to convert the world. This bit
of understandable psychology is a perfect
mate to the part of AMA’s mission statement
that describes how we are organized, in part,
to promote and educate with regard to
aeromodeling activities. You can find the
mission statement by clicking on the “About
AMA” tab on the Academy’s Web site.
I had the good fortune to be included in
such an event recently. AMA and the Civil
Air Patrol (CAP) have jointly created a pilot
program of model-flying-instruction
weekends. The youngsters—I mean
Cadets!—under the command of Captain
Ulric Gordon-Lewis were on a weekend-long
camp that was ambitiously aimed at getting
them soloed in one weekend.
It was an audacious plan, and several shifts
of us instructors barely managed to keep up
with these young men for two full days of
training flights. The Cadets’ energy and
enthusiasm were almost boundless. They slept
when they got home; after all, they are young!
This column includes a photo of the entire
group of Cadets and a caption that includes
their names and call signs. Yes, call signs!
There was even a Goose and a Maverick in
the group. Wait a minute, didn’t Goose die in
Top Gun? I’ll bet the young man in question
didn’t think that through.
Many of us went all weekend without
learning the Cadets’ real names. But for a
weekend steeped in fighter-pilot training (the
course included some basic aerobatics), call
signs were better than mere names.
When all was said and done, six of eight
Cadets did indeed solo, which still impresses
those of us who had a hand in the effort. For
half the weekend the winds and turbulence
threatened the impressive first-solo success
rate.
Members of the Washington’s Crossing
RC Flyers joined forces with their host—the
Mercer County RC Society—at the MCRCS
field. This lovely site hosts several large flyins
each year.
Several trainers were built for instruction.
They held up remarkably well despite
requiring substantial trimming adjustments,
complete fuel-system rebuilds, and radical
control-linkage adjustments, not to mention
being used for landing training!
Those maintenance and construction issues
served nicely as learning opportunities for the
Cadets, and they reminded me of the types of
problems airplanes, both kit-built and ARF,
tend to have when constructed by neophytes.
It was a veritable gold mine of “what’s useful”
to the beginner, and these pages should benefit
from the experience. Both the students and
instructors enjoyed a singularly productive
weekend, and we all welcome the Cadets to
the ranks of aeromodelers.
Voltage Is My Friend: I’m going to cover
something that is almost purely RC related.
A short while ago a flying buddy of mine
put together his first retractable-landinggear-
equipped model. I was not present for
the first flights, but those who were told me
that all two-and-a-half were beautiful. Yes,
two-and-a-half—a number that approaches
but does not quite equal three successful
flights.
When the remains were retrieved from a
considerable distance (Gulp!), the battery
was found to be better than two-thirds
discharged—all in two-and-a-half flights.
The landing gear was never even retracted
on the first flight.
The importance of this became clear only
after the crash. When the gear was retracted,
the servo stalled and could be heard grinding
against the mechanical stops. A stalled servo
such as this can kill the battery in just a
flight and a half!
Flight-surface linkages such as the rudder
ailerons and elevator can often bind at the
extremes of throw, and although stalling the
servo this way is to be avoided, the highcurrent
overloads produced are momentary
in nature. Three controls tend to stay in the
same place for long periods of time: the
retractable landing gear, flaps, and throttle.
The electronics in a servo draw only 10
or 20 milliamperes. As long as the motor
does not need to run to move the output arm,
four servos could run all day long from a
normal-sized flight-pack battery.
Moving quickly, without a substantial
load, a servo can consume roughly as much
as 1/10 amp, although some of the highperformance
digitals will consume even more.
Grinding against a hard mechanical stop
produces currents that are limited only by the
electronics in the servo.
Back in the old days a stalled servo would
consume close to an amp, and that can flatten
a battery in just a few flights. Long before the
battery is dead, however, the voltage at the
receiver and other servos will become
dangerously low, possibly causing
malfunction and loss of control. That’s
because the wiring from the battery has a
series of electrically resistive elements, none
of which is large but which add up.
High currents cause voltage drops in the
wires attached to the battery pack, the battery
connector, the wiring of the switch harness,
through the switch contacts, and through yet
another set of connectors into the receiver.
Roughly 10 milliohms here and 10 or so there,
and the situation can quickly add up to enough
resistance to rob the radio of one-quarter or a
half volt under the heavy load of a stalled
servo. This is in addition to the voltage droop
of the battery itself!
Try this sometime. Hook up a voltmeter to
the red and black leads of a servo extension
wire and plug it into a free servo connection
on your receiver. Grab a primary control
surface and push against the servo. Be
aggressive. How far did the voltage drop? It
might surprise you.
If you are flying a large, heavy, or fast
airplane, you might want to investigate ways
to reduce this voltage droop, for safety’s sake.
How? Yes, we can write about that someday. I
have a friend who hasn’t written for an
aeromodeling magazine for a while now, and
he is quite expert at this sort of thing. Hey, Jim
Oddino, are you out there? Have I got a job
for you!
As I mentioned in the first installment of
this column, there are experts with meaningful
contributions out there in every corner of the
hobby/sport.
The throttle is the first control that needs to
be checked. At least one model at the CAP
camp needed a radical throttle-linkage fix.
The throttle servo would jam against the
mechanical stops in the carburetor at both
high and low throttle! Not only would this
have killed the battery, but the engine would
have quit anywhere below roughly one-
quarter throttle stick. That wouldn’t do.
Most middle- and higher-price-range
radios these days have throw adjustments, or
Adjustable Travel Volume (ATV), on all the
primary controls, as well as throttle trims that
do not affect the servo position near the highthrottle
end of the stick. If you have one of
these radios, the straightforward procedure is
to put the throttle stick and trim in the middle,
adjust the length of the linkage so the
carburetor barrel is exactly half open, and then
adjust the ATVs until the throws are correct. I
will describe this in a couple of paragraphs.
The lowest-priced radios—that is, the ones
beginners are likely to buy when starting
out—often have nothing more than reversing
switches. Following is a note to the radio
manufacturers. In most cases the inexpensive
radios’ throttle trim potentiometers can be
rewired to at least be ineffective at wide-open
throttle. Why isn’t this done?
Back to the subject. We can deal with this;
it will just take a half-hour or so of fiddling. If
you are quite experienced, you can do it in
five minutes while making it look all too easy
for the gaggle of watching Cadets. (Face it;
we’re all show-offs at heart!)
The goal in adjusting the throttle linkage is
twofold. You want the carburetor to be fully
open with the throttle stick and the throttle
trim pushed up fully. While you want the
carburetor fully open, the servo and linkage
must not be jammed up against the stop built
into the carburetor.
The servo should be making no buzzing
noises. If you reach into the engine
compartment and gently pull on the linkage to
try to open the carburetor a little more, there
should be a tiny bit of movement left.
Meanwhile, back at the other end of the
throttle ...
With the throttle stick and trim pulled all
the way back, the carburetor barrel should be
closed, and when you push just the throttle
trim up the opening in the carburetor barrel
should be 1/16 inch or more. This means
somewhere within the range of the trim lever
you should be able to find a reasonable idle
rpm setting.
To achieve both of these requirements you
need to get the total amount of throw right and
adjust the length of the throttle pushrod. The
problem is that you don’t have a fine throw
adjustment.
Here’s what you do. First, remove the
screw holding the throttle-servo output arm to
the servo. With the radio turned on and the
throttle stick and trim centered, place the arm
on the splined servo output shaft so it makes a
right angle with the throttle linkage. Adjust
the linkage so that the carburetor barrel is
exactly halfway open.
Now check the throw by moving the stick
and trim. Do you have too little throw? If so,
move the linkage to a hole that is farther out
on the servo arm or to the next hole in on the
carburetor throttle arm. In general it is best to
keep the linkage hooked to the outermost hole
on the carburetor arm and adjust the throw at
the servo end.
Do you have way too much throw? If so,
move the linkage attachment inward on the
servo arm, one hole at a time. Your goal is to
pick the holes on the servo and carburetor
arms to get a little bit more throw than is
absolutely necessary to make the carburetor
barrel close and open completely.
Now comes the tricky part. Turn the servo
arm one spline in the direction that opens the
carburetor barrel, and then readjust the length
of the throttle linkage so that full stick plus
full trim just hits the stop. Pull the throttle
stick back and check the idle.
Can you change the barrel opening from
fully closed to a bit more than 1/16 inch open?
Then you are finished. If you can’t open the
carburetor barrel enough with the trim lever at
idle, move the servo arm one more spline in
the direction that opens the carburetor barrel
and readjust the linkage for full throttle again.
Offsetting the arm this way has two
benefits. It is a finer adjustment of throw than
you can get with the holes on the servo arm
and it reduces the effect of the trim lever on
the carburetor barrel at full throttle while
preserving the trim authority at idle. If you
get to the point where the servo arm is
pointed straight forward, in line with the
linkage, and you still have too much throw,
you need to move the linkage in one hole on
the servo.
That got awfully detailed in a hurry; maybe
next time I’ll write some more about trickyto-
set up control linkages. Control geometry is
a large subject that drags in the CL and some
FF fliers as well. That will be fun.
Until next time, go fly an airplane—any
kind of airplane! MA
Sources:
AMA
(765) 287-1256
www.modelaircraft.org