Control Line Aerobatics - 2006/11

138 MODEL AVIATION
Nats/CL World Championships recap
[email protected]
Control Line Aerobatics P.T. Granderson
Also included in this column:
• Are you in control of your
model? Are you sure?
A complete control system featuring all components and different styles of handles. See
text for details. Notice the tip-weight box in the upper right.
Jim Aron with his personal stylization of the Nats-winning, Brett
Buck-designed Infiniti. It has PA .65 power. Jim checks pipe length
and makes sure there are no leaks.
Mike Sholtes fires up the SuperTigre .51 in a Top Flite Score.
Peter Parker (Mr. Inverted) holds. If you are ever in Northern CA,
visit Bill Osborne field in Alameda; there are great people there.
HOW MANY Californians does it take to
tweak a flap? That depends on your
definition of a flap. Just when you thought
it was back to business as usual and it was
safe to hit the snooze button, I’m back,
bridging the gap between the old and the
yet to be discovered.
By the time you read this, the Nats and
the World Championships will be history.
We know who the winners are and have
heard many of the stories.
Two kids made the headlines, one of
which is new National Champion Brett
Buck. He did it the only way it can be
done: through consistency, commitment,
and perseverance.
The same is true for the other kid—
Dave Fitzgerald—who, for possibly the
blink of an eye, might have been the
reigning World Champ. Dave led the
strong US team of Paul Walker, who
finished third, Bob Hunt, and Junior
member Chris Rud, who finished second
in that division.
Bill Werwage was the returning Senior
World Champ, and he finished 10th this
time. Robby Gruber was the defending
Junior Champ, and he finished in 19th
place overall. Congratulations to all for
their excellent performances under some
challenging conditions.
Are you in control? Since the first
machines were invented, we’ve been
designing and building ever more
complicated contraptions. Did you ever
wonder why machines are built and then
some of the greatest minds on the planet
spend massive amounts of energy and
brainpower trying to get them to behave?
It seems as though every time we figure
out how to do something, we come up with
a better way and progress is once again
defined by whether or not we can control
the better thing we just built to replace the
thing that worked pretty well in the first
place.
Take heart; control is possible. We have
the tools and we have the talent.
Let’s say you have a new airplane. The
engine is running and you have the handle
in your hand. Are you sure you’ve got
control? What will the model do when it
leaves the strict confines of the person
launching? What if blah, blah, blah, ad
infinitum?
The control system for CL Precision
Aerobatics (Stunt) airplanes is often talked
about, yet somehow shrouded in mystery.
11sig5.QXD 9/26/06 9:27 AM Page 138It is perhaps the most important part of the
airplane and it often receives far less
attention than it deserves.
In most instances the control system is
installed using a combination of theory,
hearsay, and best guess. It can be technical
and complicated with formulas, response
rates, ratios, resistance, etc.
Oops! That caused a huge spike in the
“Mind Numbing Analysis Meter.” Resist
the temptation to join in and don’t worry;
you won’t miss anything. The analysis is
and always will be there whenever you
want to join in. To better understand
control and simplify things, it makes sense
to look at it conceptually and then follow
with a flexible application.
Where does the first control input
originate? Most people would say it’s at
the handle. Actually control begins far in
advance of any handle movement. The
first input of control begins and ends in
your brain.
It starts with the selection, assembly,
and installation of all the parts of the
control system. It then moves to the center
of the circle where, contrary to popular
belief, the brain is still in control, sending
signals to your hand directing its
movement.
Let’s stop here for a minute. Your
brain is in control—not someone else’s
brain. What is good for you may not be
good for someone else. In reality, when
you decide you are in control of the
machine, the machine doesn’t have a
chance. It’s your airplane. You’re the
captain; take ownership and control.
The weakest part of the entire system is
what happens after your hand moves.
Paying close attention to how everything
is connected is the key. Make sure the
mechanical parts are of good quality and
installed while paying close attention to
solid fundamentals.
Rather than throwing together the
control system and embarking on the ever
popular “endless adjust-a-thon,” why not
take full advantage of some excellent
control-system components that are
readily available and install them to take
full advantage or their design?
No binding, sticking, or rattling! Keep
those lines clean.
Consider the bellcrank. It should be big
enough to have sufficient leverage for
control of the airplane without applying
excessive input pressure at the handle. For
our purposes most models with more than
400 square inches of wing area will be
well suited for a 4-inch bellcrank.
The next item to consider is the
pushrod(s). Flexing is your enemy. You
can minimize flex by making sure the
pushrods are straight.
If you use music wire attached to the
carbon rods, there should be only one
90° bend at each end for insertion into
the bellcrank and control horn. Never
make another bend in the wire. My
preference is to use carbon-fiber hollow
tubes for the main pushrod body and ball
links at both ends.
Consider the control horns, where thereis some controversy. On airplanes
weighing less than 65 ounces I have found
no advantage to using control horns with
1/8-inch-diameter wire; 3/32-inch-diameter
wire works well for most applications.
Control horns are usually bent 90° at
both ends for insertion into the control
surface (elevator and flap). The length of
the wire inserted into the control surface
should be kept relatively short.
I shorten the wire to 3/4 inch at the
insertion into the control surface and make
sure the attachment is heavily reinforced
by plywood. This will minimize leverage
at both ends of the horn and greatly reduce
its tendency to twist under loads. There’s
no need to get technical here; this is just
simple applied physics.
Although it is commonly stated and
accepted that the best flap-to-elevator ratio
is 1:1 (equal amount of flap and elevator
deflection), I have not found that to be the
case. Remember that we want a solid
foundation.
The geometry of the nonadjustable
parts is critical. In the case of a built-up
fuselage, most of the crucial components
will be internal and not easily accessible
without cutting into the airplane.
For many years I was right there with
everyone; I use the same parts and set
things up the same way. The result was
ultimately fighting for control of the
airplane most of the time.
You may want to consider making the
control response slower than you would
normally when determining the spacing of
the pushrod connection to the pivot hole of
the bellcrank.
Simply attach the pushrod closer to the
pivot, or center hole. This will allow the
speed of the system and response of the
airplane to be changed at the handle,
where it is easy to do.
Another commonly accepted practice is
to have “slop,” or loose fit, at the control
horn where the pushrod is inserted. As do
many procedures, this comes from the
early days of control systems when the
components would wear excessively and
airplanes would seem to become more
stable, especially in level flight.
Today we know that stability is more a
factor of alignment and CG, but that is a
topic for a future column. Slop is not a
good thing and can cause more problems
than it cures.
The control system is the primary
guidance system for flight. In CL our
system is relatively simple, consisting of a
bellcrank, pushrod(s), control horn(s),
leadouts, wingtip weight, lines, and
handle. What makes the system good is the
size, type, and geometry of its
components.
Beyond the actual components, in my
opinion the most important function is the
ability to tune the system for the specific
aircraft and your individual feel for flyingWith that goal in mind, it makes sense to
ensure that the major portion of tuning is
in the most accessible system components.
Leadout position must be adjustable.
The model pivots on the pitch and yaw
axis (up/down, in/out) where the leadouts
exit the wing. Although I have tried
individually adjustable leadouts, there is
no evidence that the additional work of
doing this is advantageous. Leadouts
should be spaced no more than 3/4 inch
apart.
The amount of tip weight is, and
always should be, adjustable.
We now have elevator control horns
(commonly called “sliders”) which allow
the ratio of flap-to-elevator movement
and the relative speed of movement to be
easily adjusted. I highly recommend their
use.
We also have pushrod couplings that
allow us to bias the neutral relationship
of flaps to elevator. We can easily change
substantive incidence and flight attitude
to compensate for alignment imperfections.
Handles can make a huge difference.
There are many variations, but most fall
into two categories, one of which is
cable-type handles that have a heavygauge
wire attached to the lines by a line
clip. The other kind is the hard-point
type, in which the control line attaches
directly to both ends of the handle using
only a line clip.
There is a huge difference in the feel
of the controls between the two handles.
If you have not tried both, I strongly urge
you to do so.
Also, no one size of handle fits all. I
make a concerted effort to contour the
handle to fit my hand comfortably. Part
of being in control is being comfortable.
Given the fact that our models are
constructed from wood and covered with
materials that constantly expand and
contract, the often touted “perfect
alignment” does not exist, or at best is
momentary.
Our airplanes are not static. Parts
continually move depending on
differences in temperature, atmosphere,
winds, and dynamic flight loads. A wellthought-
out control system that will let
you adapt to those changes will go a long
way toward achieving consistent,
predictable performance.
Other components of the control
system are the rudder, propeller, and
airfoil. These are personal preferences. I
like rudder offset, usually no more than
3°, and thin rudders with moderate area.
There’s some controversy about those
preferences, but they work for me.
Propellers are the biggest mystery of
all and can make a huge difference in the
flight and feel of your airplane. And the
subject of airfoils is like a deep hole
filled with Gila monsters! Here’s a hint:
don’t bet the farm on thick airfoils.
This Californian wishes you flat
bottoms and slippery lines. Rely on the
fact that something is working to validate the fact that it does work.