Control Line Aerobatics - 2009/09

him whether or not he thought that converting my glow-powered
Genesis Extreme to electric power was possible. He asked a few
pertinent questions about its weight and wing area and then did a
few quick calculations. He
determined that the airplane was a
perfect size and weight for such a
conversion.
Dean suggested an AXI
2826/10 motor and a 4000 mAh,
four-cell Li-Poly battery. There
were no great sequencers available
then, so we used a simple timer
that set the ESC to full throttle for
approximately six to six-and-ahalf
minutes. Then we chose a
propeller (pitch, number of blades,
and diameter) that flew the
Genesis at the desired lap time
and absorbed the 120 watts per
pound of all-up flying weight.
As I walked out to the handle,
the initial power burst caused by the fully charged battery was
“burning down,” and the battery was warming up internally. The
horsepower was outrageous up to the Horizontal Eight, and then as
I’M TAKING A
break from the
Project Hole Shot
series this month.
The model is ready
for finishing and
I’m contemplating
trying something new, so it is sitting for a while.
This break gives me the opportunity to write about something
that has become important to me and something that is about to
become significant in the CL Precision Aerobatics (Stunt) world:
electric power.
I’m now a total convert to electric power for my AMA and FAI
Stunt programs. (I’ll still fly Classic and Old Time with glow
engines.) However, I have not strived to be a “salesman” for
electric power in print. I figured that it would eventually display its
many merits without my help.
Besides, I’m not one of the pioneers of using electric power for
Stunt. That distinction belongs to Mike Palko, Walt Brownell, and a
handful of others. I have believed, up to this point, that it has been
theirs to promote.
If you follow various threads on Stunt online forums, you might
know that many pilots have made much progress in bringing
electric power to fruition as a
viable alternative to glow
power. You can type “electricpowered
CL Stunt” into the
subject bar on an Internet
search engine, and that will
bring up dozens of videos
showing the practicality and
success of using motors in our
event.
It is time to acknowledge
the advantages of this power
source and promote electric to
the masses. Until now, I did
not believe that the
technology and component
availability was such that the
average modeler could
achieve success without having a local “Guru” explain things.
Electric power for CL has fully arrived, and it deserves some ink.
Even though I had watched Mike Palko fly electric-powered
Stunt models for a few years, my eye-opening personal encounter
with it happened at the 2005 Nats. I was watching Walt Brownell
fly his ArcAngel model through a pattern. At that time I didn’t even
know that it was an electric model.
I was walking toward the circle in which he was flying. As I got
closer, I realized that the model was emitting no discernable noise,
but it was doing a fantastic job of going through the pattern in a
stiff wind. To that point, I had felt that electric power was
interesting but probably still a few years away from being truly
viable for top competition use.
Later that day, after the competition had concluded, I
approached Walt and did something I rarely do; I asked him if I
could fly the ArcAngel. Walt seemed pleased that I asked and said
“Sure!”
During the takeoff roll on that flight, I knew that my modelflying
paradigms had changed. The amazing amount of line tension,
the smoothness, and the model’s solid feel even before it broke
ground convinced me that practical electric power had arrived. The
rest of the flight confirmed my initial feelings.
On the trip home, I called my buddy, Dean Pappas, and asked
You won’t miss the “lovely” castor oil smell
[[email protected]]
Control Line Aerobatics Bob Hunt
Also included in this column:
• Advantages of electric power
for Stunt
The author converted his Genesis Extreme from glow to electric
using an AXI 2826/10 motor and Castle Creations Phoenix-45
ESC. Frank Fanelli photo.
Walt Brownell’s ArcAngel introduced Bob to modern electric
flight. It has an AXI 2826/10, Phoenix-45 ESC, Thunder Power
4000 mAh battery, prototype Zigras timer, and Graupner 11 x 6
GR/EP propeller.
Mike Palko is a pioneer of competition electric Stunt. He modified
the drawings of the Bob Hunt-designed Mustang to accept an
electric system. Mike placed eighth with it at the 2008 Nats. Palko
photo.
September 2009 137
09sig5.QXD 7/23/09 2:02 PM Page 137
slowed roughly one-10th of a second per lap
for the next few maneuvers.
Then as the battery voltage continued to
diminish, the lap times slowed again
approximately another one-10th and the line
tension dropped correspondingly—exactly
where I really needed it the most. Clearly
this was not an optimal setup. But let the
record show that it was good enough to
allow me to make the 2006 FAI team a
mere three weeks after having flown Walt’s
ArcAngel.
The next step in the program was to
stabilize the lap times and maneuvering
speeds, and improve the horsepower
everywhere. And this would require some
actual technology.
At this point, Dean convinced himself
that if we could govern the motor rpm to a
fixed value, even as the battery voltage
drooped and as the airplane climbed and
dived, we would have something that
roughly simulated a classic two-four Stunt
run.
An electric-model pioneer, Bob Boucher
of AstroFlight, suggested to Dean that we
try to optimize the helicopter rotor-speed
governor that was designed into the Castle
Creations Phoenix-series ESCs. This was a
bit of a leap, because helicopter rotor speed
controls are slow compared to what we
needed.
But sure enough, the required control
range was available in the Castle unit. Now
we would have to build a timer that
produced the “throttle setting” that would
yield the desired rpm and lap times.
Dean installed an RC receiver in my
Genesis Extreme, and I flew it with him
sitting at my feet in the center of the circle
with a transmitter. He adjusted the throttle
and made notes of the pulse-width numbers
so we could produce a “script” of throttle
pulse-width settings (as with a radioreceiver
output). Our initial “script” read
something similar to the following.
Run up to almost 9,000 rpm for several
seconds and then to a 4,000 rpm idle for 30
seconds. This would allow time to walk out
to the handle while saving battery capacity.
The next setting corresponded to our inflight
setting of roughly 9,000 rpm for fiveand-
a-half to six minutes.
Five seconds before the end of the run,
the script called for the rpm to drop to half
for approximately one second and then
recover to the 9,000 rpm setting for five
more seconds, so I could have a warning
and safely return to level flight before the
power shut off.
Now all we needed was a timer that
could be programmed to follow that script
with fine adjustability in every parameter.
Dean and I decided on the features we
wanted (okay, he told me what features we
wanted), and our good friend, Andy Kunz,
who is a gifted code writer, took on the task
of making our first-generation timer.
I’d like to tell you that this process
immediately yielded the ultimate device,
but that wouldn’t be the truth. There wasn’t
enough time to fully develop the system
before leaving for Spain the next year for
the World Championships. The results were
predictable; I didn’t do too well.
The good news is that a new model
timer resulted from all of our trials and
tribulations, and it has an abundance of neat
features. I’ll invite Dean to write about that
timer in a future column. Thanks to him for
staying up late and helping me hammer out
the preceding in proper order and
terminology.
What I really want to write about in this
column are the advantages of electric power
for Stunt flying and the reasons why you
should consider it. They are as follows.
• Noise (or lack thereof): The most obvious
advantage of electric power is the fact that
it emits far less noise than a glow engine. It
emits no combustive noise. Yes, there is
propeller noise, and that will vary from
minimal to almost imperceivable in our
application, depending on what rpm you are
running.
The real advantage is that you will be
able to fly at times and at places that were
never possible with glow power. The noise
issue is what really got me interested in
electric power to begin with.
I have a field one-quarter of a mile from
my house that is surrounded by homes. I
could now fly there at daybreak and no one
would have a clue I was there unless he or
she looked out the window! New fields will,
and have, opened up to many of us strictly
because of the lack of noise emission.
• No vibration: Motors simply don’t
vibrate. Your airframes will last much
longer, and they won’t have to be built as
robustly, thus allowing a significant
reduction in airframe weight in the bargain.
Also, your model will not soak up weight
from fuel residue.
• Constant CG location: In glow
applications, the full tank of fuel—which is
situated ahead of the model’s CG—burns
off as the flight progresses, effectively
shifting the CG aft. There is no CG shift
when using a motor! Your model will feel
the same to you throughout the flight.
Have you ever noticed how difficult it is
to perform Overhead Figure Eights in a
wind? The model does not want to track
properly throughout the maneuver and is
significantly “pushed” down on the sides.
That is mainly because the CG has
shifted so far aft by that point in the flight
that the model has no drive. I’ve noticed
that Vertical Eights, Hourglasses, Overhead
Figure Eights, and entries to Four Leaf
Clovers in the wind are vastly improved
with the constant-CG electric setup.
• Weight concentration: We have found
that an electric setup’s all-up weight is
virtually the same as that of a glow setup,
providing that you factor in the weight of
the fuel when considering the glow setup.
Fuel weighs approximately 1 ounce per
fluid ounce. But the concentration of that
weight is significant.
In the glow setup, the engine weighs
more than the equivalent motor of the same
power output range would weigh. In my
setup, the engine that was replaced by the
motor weighed approximately 10 ounces
with the header attached. The AXI 2826/10
weighs 6 ounces. Yes, the battery weighs
more than the full tank of fuel did, but,
again, the two setups’ total weights were
virtually the same.
The kicker is that the center of the mass
of the weight that is now balancing the
model at the desired CG location is much
closer to that location! The fulcrum is
shorter, thus making it easier to start that
weight moving and easier to stop it. Thank
you, Mr. Newton!
In fact, I was able to carry a CG location
in the electric model that was a bit farther
forward, therefore more groovy feeling, but
still have even more corner and lock than
with the glow engine and a farther-aft CG
location. It’s akin to having your cake,
eating it too, and then going out for a big
desert afterward!
• Vertical CG location adjustability: With
an electric setup, you can move the battery
up or down within its bay to affect the
vertical CG location. With a glow setup,
you have to place the tank in a position that
makes the engine run the same upright and
inverted. You cannot use that weight to
adjust the vertical CG for that reason and
because the weight in the tank (fuel) will
burn off gradually as the flight progresses
anyway.
You may not think this is a significant
plus, but it is. I’ve had many models that
flew with the outside tip slightly high and
assumed that it was a wing-warp issue. I
tried slight flap tweaking to fix the problem,
only to have other trim problems arise in the
process.
The Genesis Extreme was always
sensitive to roll trim, and I tried many
combinations of tweaks, tip weight, and
trim tabs before realizing that the real
problem was the vertical CG location. Now
I can simply move the battery (up in my
case) to fix the problem. And since the
battery weight doesn’t burn off, as fuel
does, the fix is permanent.
• Run consistency: Of all the pluses that
electric power offers, the ability to achieve
a consistent and repeatable run is perhaps
the most important. Once you attain the run
you desire, the system will repeat it for
hours, days, weeks, and months. It’s almost
boring!
The ability to have a consistent run
means that your practice time will be
optimized and your timing will be
sharpened to a fine point. Glow power
cannot even begin to approach the
consistency of electric power.
• Ability to use reverse-pitch propellers:
Kaz Minato came up with another major
advantage for electric CL fliers. He
discovered that reverse-pitch propellers
(pusher propellers) essentially give us
right rudder trim throughout the flight,
whereas standard-rotation propellers yield
left turn trim. Why? Spiral airflow.
Our normal-rotation propeller actually
produces spiral airflow that circles the
fuselage and pushes against our model’s
vertical fin’s left side, forcing the aft end
of the fuselage toward the outside of the
circle. This means that the nose is being
forced in toward the pilot. We have to
trim our models to stay at the end of the
lines.
With the reverse-pitch propeller, the
spiral airflow is pushing against the right
side of the vertical fin, causing the model
to want to turn right, thus yielding more
line tension.
Reversing the rotation of a typical
outrunner motor is a simple matter of
swapping any two wires coming from the
ESC to the motor. This allows you to
make A-B comparisons on back-to-back
flights using normal and reverse-pitch
propellers.
I was skeptical about this until I tried
it. Now I’m a firm believer. The overhead
and vertical tension have vastly improved,
and weren’t bad to begin with!
• Ancillary benefits: Electric power has
many not-so-obvious benefits. Your
clothes won’t smell of burnt castor oil
anymore, and neither will your car’s
backseat!
You can test-fly your models before
you paint them, because no fuel residue is
emitted. The worst you might have to do
is clean off a bit of chlorophyll from grass
cut by the propeller before you paint. This
advantage allows you to be sure that the
wing and stabilizer are in alignment
before finishing the airplane.
You can converse with your coach
from inside the circle without having to
raise your voice.
Finally, you can test-run your motor
inside your shop. There is one caveat here;
make sure your shop is clean before
starting your motor. You will be amazed
by how much air blast that propeller
generates!
That’s plenty for this time, but I will
expound on each benefit of electric power
for Stunt in the future.
Till next time, fly Stunt. MA
Sources:
AXI
Sladkovského 594
530 02 Pardubice
Czech Republic
www.modelmotors.cz
Castle Creations
(913) 390-6939
www.castlecreations.com