Flying Information
Flight Trimming Secrets
By Mike Fortune
There is an old saying that goes like this:
"It's a small world" (we don't do Disney here!
-ED) and I can say that it certainly is! I have
enjoyed this hobby of flying radio controlled
aircraft for over 32 years. Not bad for a 70 year
old man, huh? Well, since I'm only 44, you can
see that I've been flying for over 2/3 of my life
and I have a lot of RC related stories ... like
this one: I make my living as a Lead Aircraft
Mechanic for the largest express freight
airline in the world. I recently had the
opportunity to travel to the Airbus
assembly factory in Tolousse, France.
The trip was for my employer, as
well as for A&Ps who work for
commercial airlines that have ordered
the A380 aircraft. The purpose was to
interface with the factory
representatives to discuss the
maintainability of the aircraft once it's
delivered. While we "students" were
on our breaks, I took to reading the
latest issue of the AMA's Model
Aviation Magazine ... the one that I
grabbed on the way out the door to the
airport. RC model aviation as a hobby
creates a bond all modelers share, and
that bond crosses any country's
boarders and many political barriers.
My copy of Model Aviation acted as an
ambassador, and through its exposure
on various breaks I attracted the
attentions of one of the design
engineers for the A380, who flys a
Raptor 30, and the chief engineer for
Malaysia Airlines who shares our hobby
with his son. It truly is a small world.
But I digress....
While visiting the Airbus A380
assembly factory, I was impressed with
the extensive tooling and fixturing used
to assemble a modern airliner. The one tool
that impressed me the most was the wing to
fuse alignment jig. Each wing half is so large
that it could hold over 100 automobiles on its
upper surface. Obviously, TLAR (That Looks
About Right) fixturing for the wing's alignment
would not be adequate enough. Airbus
developed a series of jacks and strain gages
that together were capable of accuracy to
1/10th of 1 degree. As the wing and fuse were
mated, each strain gage had to be within it's
tolerance before the wing was considered
mated and then properly joined. For a guy who
likes tools, fixtures and gadgets, (have anyone
in mind Mike? -ED) this would have to be as
close to heaven as he could have experienced!
Besides flying, one of the things I do for
enjoyment with this hobby is my business,
Fortune Model Products. While I never intend
to use my position with this magazine to
bring attention to my business, I have been
encouraged by Jeff to share my tooling
theories, thoughts and designs with the
readers of MHT. I started designing tools for
the RC model industry by first making them
for myself. I have always wanted to know
with confidence that if I crashed a model, I
could re-build it so it would fly the same, or
at least close to the
same as it was before
the crash. Some of
these tools were my
own unique designs
and some were
modifications I felt necessary to make an
existing tool or tool concept more versatile.
I also have designed tools for the 1:1 aerospace
industry that range from very small hand
tools to tooling that is larger the family ride ... You can see that I like fooling around with
this stuff.
One of the "fixture systems" I want to
show you in this issue is the one that will help
answer the oft pondered but seldom asked
question, "How long should this pushrod be?"
I know that probably does sound a little funny,
but there is actually a method that can be
applied to make the pushrod the correct length
the first time. In the pictures you will see
examples of a JR Vigor with the push/pull
CCPM control system pushrods and the single
throttle rod.
First the single push rod: I cannot think of
a single pushrod installation that has not been
subjected to the "Rule of 90 (degrees)"
regarding the relation of the servo output
arm to the thing pivoted. I'm sure there must
have been one, somewhere ... but I just don't
know where it is or why you would have ever
wanted to use it. With that said, were going
to review the pushrod "Rule of 90" and it's
effect on the servo arm and the thing it's moving.
How long should this pushrod be?
The pushrod length should be the same length
as is the length from the center of the servo
output shaft to the center of the bell crank or
lever pivot (for example, the throttle barrel
arm mounting screw-hole). Believe it or not ...
it's just that easy!
I can hear you asking...
OK, now that we know how long the PR is,
where does it fasten on the servo arm? And
then, how do we know it's really the right
dimension?
Here's what you do if there is only one
control run to measure, as in the case of a
throttle pushrod. It's easy: measure the throttle
arm from the mounting screw center to the
outermost hole and find a servo arm that
measures the same. There is an advantage to
this I'll explain. Now, measure from the center
of the servo output shaft to the center of the
throttle barrel pivot, where the arm screws
on, and size the pushrod to that same length:
if it was 112mm, then make the pushrod
112mm. Next set throttle stick to 50% - mid
stick. Now, install the pushrod on throttle arm
and servo. The throttle barrel arm should be
loosely affixed on the barrel at this point. Now
set the angle on the pushrod-to-throttleservo-
arm to 90 deg (see pics). If 90 deg is
not possible at that point, due to the spline
to arm arrangement in the servo arm, rotate
the throttle servo arm, and keep rechecking
until you get a 90. Once the pushrod is 90
deg to the servo arm side of the system,
secure the arm onto the servo and install it's
retaining screw. Next, set the carb opening
to 1/2 open throttle, align the throttle barrel
arm, as you did the servo arm, making sure
it is at 90 to the pushrod as well: 90-90! This
answers question number two and three: the lengths of both the arm hole/ball locations
as well as the right dimension. It's the
"Rule of 90" setup.
Why use a servo arm the same
length as the throttle arm? It keeps the
setting consistent (especially from one
model to the next). In other words, when
I look into my radio's throttle curve and
I see, for example, that I have the stick at
65 percent and the output is 75 percent,
I know then that the throttle is at 75
percent power or 75 percent throttle barrel
opening. And I know that is standard for
each model I have, or that I will build.
Keeping both the servo arm and the
throttle arm the same length keeps
everything linear. Linear servo response
is critical. Why? Because we fly right side
up and upside down and we want
everything to be the same, no matter
what side is up! Now, adjust throttle end
points, high and low equally, in the radio
as you typically would to obtain full open
and full closed carb setting.
With a dual push rod setup,
otherwise known as a push/pull setup, or
a closed loop configuration, both push
rods are required to be the exactly the
same length. This is easy to do. Make up
one rod measuring the same way as you
did above (servo output to bell crank
pivot). Place one push rod on the bell
crank and the other one on the servo.
Make up the other the same way, but here
is a trick for checking your work and
insuring that you have a 90-90 square
setup. Measure diagonally from one side
of the bell crank ball to the opposite side
ball on the servo arm. Then measure the
opposite diagonal: it should be the same
dimension. If so, snap on the remaining
pushrod and you're finished. If the distance
is not the same, it's most likely caused by
the ball links you're using not being 1/2
turn adjustable. (I sourced the FMP Ultra
Ball Link specifically to fix this problem.
The Ultra links are 1/2 turn adjustable).
Now, both pushrods are the same length
and the bell crank is 90 deg to the pushrod.
This is a "must do" to minimize the
interactions that naturally accompany any
eCCPM system. I hope that this article is of
help to you and that you understand the
humble pushrod a little better.
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