Photos by the author except as noted
24 MODEL AVIATION
by Mike Palko
I HAD MY first experience with electric flight when I was 10
years old. I broke my plastic CL RTF model after just a few flights,
and after that my dad and I decided to build an electric-powered RC
trainer together. I thought it was the coolest thing ever, because, as
with my RC cars, I just had to charge the batteries and turn the
model on to fly. But its flight performance wasn’t too successful,
and I quickly lost interest in it.
Shortly after that I was introduced to a club called the Philly
Fliers. I became interested in CL flying and haven’t looked back. It
wasn’t until 1997, at the age of 17, that I became interested in
electric flight again, and I built my first electric-powered Precision
Aerobatics (Stunt) model.
I had seen several electric CL models fly, but none of them
performed with enough authority or duration to be competitive. I’m
not sure what it was, but I was drawn to electric power and thought
it would be fun and challenging to see how well I could get an
electric Stunt model to perform.
I had no idea where to start, so I built a Sig Twister and used an
05 can motor with a seven-cell battery pack to get a base point. As I
worked on the project, I realized that I was not only developing a
power system that was new to CL, but one that showed some real
advantages in competition.
Some of the most important benefits were consistent motor runs,
no CG shift during flight as the fuel burned off, and never having
another over-run or an engine that wouldn’t start. In addition,
motors produce little vibration and leave no residue to soak into the
balsa and eventually ruin an otherwise good Stunt model.
There are also drawbacks to electric power, such as power-toweight
ratio, cost, and safety. But as technology advances, electric
power will continue to approach the performance of internalcombustion
engines, and the safety issues will be addressed.
I worked with the Twister on and off until 2003, when I reached
a point where I felt I was close to having a competitive Stunt
model. At that point I needed to take the next step and design a new
airplane from the ground up. The Twister flew great for an electricpowered
CL airplane—better than most people had ever seen such
a model fly—but it was still far from what I really wanted.
It was a profile, and it didn’t have the drive to get through high
winds. It was extremely close on run time, so if I missed the wind
and had to take an extra lap or two or got blown out of a maneuver,
it didn’t have the battery capacity to get through to the end of the
flight. The Twister was built so light that it was to the point of
being weak. I needed a new approach.
Power System: I felt that the Silencer would be the answer to the
problems I mentioned. Its design would incorporate a stronger,
more aerodynamic airframe and a new power system.
The biggest performance gain would be in the battery pack. I
switched from a Sanyo 10-cell 2600 mAh NiMH pack to a Thunder
Power (Gen2) 4S2P 4200 mAh Li-Poly pack. This is really two
2S2P packs wired in series because a 4S2P is not available off the
shelf.
This change alone would increase the battery capacity by roughly
two-thirds, increase the voltage by 2.8 volts, and drop the weight by
approximately 8 ounces! It would allow me to run a higher voltage
with a lower amp draw and maintain the same, if not increase the
setup’s, output in watts. It’s safer and more efficient to run lower
current with higher voltage because there is less heat buildup, which
also increases the life of the motor and the battery.
I also switched to a motor that was capable of turning a lowerpitch
propeller at a higher rpm. I replaced the AXI 2820/10 that I
had been using with a Plettenberg Orbit 15-18, which can handle
more voltage and higher current levels. I have found it to be more
An electric revolution in action. This is the most successful electric-powered CL Stunt model to date. John Glatthorn photo.
It’s hard to tell that the model is electric—until you realize that
there is no smoke trail from burnt fuel! Will Hubin photo.
Competitive electric-powered CL Precision Aerobatics becomes a reality
March 2005 25
With cowl and battery cover removed, it’s easy to see the system
components’ placement. Good airflow through this area is a must!
Power is the Plettenberg Orbit 15-18 motor, two Thunder Power
2S2P Li-Poly battery packs wired in series, a Castle Creations
Phoenix-45 ESC, and a Sergio Zigras timer.
Mike’s model has all of the normal flight-trim features, such as
this adjustable leadout guide. Neat workmanship!
The structure had to be light and strong. Mike used the Lost Foam
wing-building system to construct the rigid Warren truss-type wing.
efficient than the AXI by 8-10%. This would give me more usable
capacity from the battery, thus extending my flight times.
The AXI and the Orbit are brushless outrunners, or rotating can
motors. This means that the entire motor case spins, creating a power
plant that cools itself slightly and is capable of turning a large propeller
without using a gearbox because of its higher torque output.
This was important to me because a gearbox adds weight, creates
noise, reduces the strength of the power train—because it may strip or
wear out—and adds cost and complexity to the setup. This does not
mean that direct drive is the only way to go, but there were more
negatives than positives in using a gearbox for my application.
I had been using a Castle Creations Phoenix-35 ESC, which is
capable of handling 35 amps continuously. I knew I would be pulling
close to 35 amps, so I changed to a Phoenix-45 to increase the safety
margin. The Orbit runs at full throttle the entire flight, so the ESC has to
be able to handle the current and heat buildup for five to seven minutes
at a time.
However, an ESC does more than control the motor’s speed. The
Castle Creations ESC controls eight parameters of the motor, including
cutoff voltage and throttle type. These two factors are particularly
important because they will benefit the CL flier the most.
The cutoff-voltage feature is a must with Li-Poly batteries. With a 4S
battery pack, you need to set the cutoff voltage to 12. Li-Poly cells
should never be discharged to less than 3 volts per cell under load;
otherwise, permanent damage or fire could result. If the timer would
ever fail, the ESC would turn off the motor when the battery pack
reached 12 volts, saving the pack and airplane from possible damage.
With the cowling on, the model’s nose looks like a normal
“engined” Stunter. Note the cooling holes in the nose ring.
26 MODEL AVIATION
Does Mike look proud? He should! He has elevated the
performance of electric-powered models to a competitive level in
CL Stunt. Look for more from him! Hubin photo.
Dan Banjock launches Mike’s Silencer during 2004 Nats Advanced
competition. Mike finished a credible sixth. Hubin photo.
The sleek, clean model gives up nothing in aesthetics—or
performance—to glow-powered models of the same size and type.
The throttle type can be chosen from four settings. I have learned
that the high-rpm governor mode works best for our purposes. This
helps hold the motor at a constant rpm, preventing whip-up and
keeping a uniform speed throughout a maneuver, much like a tunedpipe
setup.
However, the ESC will not work without something telling it what
to do. To control the ESC, I am using a timer that Sergio Zigras
designed and built. The run time is adjustable from five to seven
minutes by turning a small speed potentiometer. The timer plugs into
the ESC and, when turned on, gives it a signal to arm and then slowly
ramps to full power. No external power is needed to run the timer. It
uses power from the Li-Poly pack.
As for the power system’s weight, the motor weighs 6.21 ounces,
the ESC weighs 1.06 ounces, the timer is negligible at .07 ounce, and
the battery weighs 14.18 ounces. This is a total of 21.52 ounces
including all connectors.
It seems heavy, but with attention to detail and the lack of plywood
doublers, engine beams, and crutch, the bare airframe’s weight can be
lowered dramatically to compensate.
CONSTRUCTION
Keep the overall construction as light as possible. I weighed each
part as I built the Silencer, looking for areas where I could save
Type: CL Precision Aerobatics
Wingspan: 52 inches
Power: Plettenberg Orbit 15-18 motor
Flying weight: 44 ounces
Construction: Balsa and plywood
Covering/finish: Light-grade silkspan with Sig Litecoat
and Brodak modeling dopes
weight. Weight is the biggest concern with
electric power because you are “behind” from
the start; however, with the right wood
selection and a light finish, you can keep the
weight to a minimum.
Wing: The wing is built up and utilizes a
Warren truss-type ribbing scheme. The main
ribs are angled, and there are half ribs
between the opposing sets of ribs at the LE to
support the sheeting. The model has a 52-inch
wingspan and 510 square inches of wing area,
including the flaps. The wing panels are of
equal length.
I built the wing using Bob Hunt’s Lost
Foam wing-building system because it is one
of, if not the most, accurate ways to construct
a wing. It allows you to build the structure
extremely light and maintain its integrity, and
it provides the easiest and most accurate way
to produce the Warren-truss rib sets.
Using the Lost Foam method, you mark
the desired rib locations on the front and rear
of a foam blank that is cut to the planform of
the wing. Cut and sand the core, and mark the
rib locations chordwise on it using a ballpoint
pen. Also mark the spar location on the core
on the top and the bottom.
The rib locations are then accurately
scribed into the lower cradle half from which
the core was cut. This cradle is as accurate a
negative shape as the core is a positive shape,
and it can be used as a building fixture. The
core is sliced vertically at each rib station,
yielding perfectly accurate rib templates from
which balsa ribs can be generated.
Bob has produced a two-video set about
the Lost Foam wing-building system, and it
includes all of the information about how to
cut and prepare your own fixture sets. His
company—Robin’s View Productions—sells
the videos and offers a cutting service. He can
supply complete Lost Foam fixture sets for
this model and hundreds of others.
To help keep weight to a minimum, I used
4- to 6-pound, contest-grade wood
throughout. The flaps are made from 1⁄4-inch
straight C-grain balsa, with the grain
following the TE to help reduce the chance of
warps. The outboard flap is 1⁄8-inch wider at
the tip than the inboard flap is, to help the
inboard and outboard wing panels lift equally
in a turn.
The fuselage blankets the outboard wing
because the model is flying in a circle and is
angled somewhat tangent to the path of flight.
Therefore, the outboard wing and flap have
less effective area. The outboard flap’s larger
area helps the wing turn flat and without a
rolling tendency, even though there is less
airflow over it.
Tailplane: The stabilizer is 3⁄8-inch thick and
is built using a Warren truss-style
construction. The LE and TE are made from
1⁄4 x 3⁄8 balsa. I laminated the forward face of
the stabilizer TE with .008-inch carbon fiber
over the full span, and I used a double layer in
the center-section for added stiffness. The tips
are soft balsa, carved to shape and hollowed.
The elevators are 5⁄16-inch thick, and I built
them using a sheet of 1⁄16 balsa that was
shimmed 1⁄8 inch off the building board. The
LE and TE were glued to the 1⁄16 sheet, as
were the ribs. Then I flipped the elevator over
and glued the bottom ribs in place.
I capped the inside root edges of the
elevators with hard balsa to support the
elevator horn, and I capped the tips with soft
balsa and carved them to shape. The elevators
were then sanded and tapered to 1⁄8 inch at the
TE. Once they were completed, I went back
and removed the wood between each rib to
reduce the weight even further.
Fuselage: The fuselage is built with 1⁄16 Cgrain
sides. On the inside I doped on .5-ounce
carbon-fiber mat as a replacement for the
doublers. The motor mount is 1⁄8 aircraft
plywood with three holes drilled and lightly
countersunk in each side to allow for motor
cooling.
The motor mount sits roughly 1⁄8 inch
behind the front edge of the fuselage. The
overhanging fuselage sides act as a small
scoop to help guide air into the cooling holes.
I put a fillet of Aeropoxy Lite on the inside
and outside of the motor mount glue joint for
added strength and to help smooth the
airflow.
I covered the nose section with .75-ounce
fiberglass, making sure to wrap around the
front of the nose to reinforce the motor-mount
joint. I also reinforced the inside motor-mount
joint with .75-ounce fiberglass.
Final Assembly: Install the battery tray after
the wing is joined to the fuselage. Once the
wing is in place, its lower center-section needs
to be removed for battery clearance. During
wing construction, the lower bellcrank mount
must be sunk into the wing 5⁄8 inch so you
don’t sacrifice strength. Otherwise, this
section of the bellcrank mount would be
removed to provide clearance for the battery
tray.
I made the battery tray from three layers of
1⁄16 balsa with alternating grain, to make balsa
plywood. I laminated each layer together with
epoxy and .5-ounce carbon-fiber mat. I
epoxied this tray directly to the landing-gear
mount, the wing, and the lower bellcrank
mount, tying everything together. I used
Aeropoxy Lite to make fillets inside the
battery compartment and around the wing to
help blend and reinforce the joints.
I removed the lower wing center-section to
position the battery pack as close to the
airplane’s centerline as possible, in an effort
to keep the vertical CG in the proper location.
If the 14-ounce battery was placed too far
from the intended vertical CG, you could end
up with an airplane that would rock and roll as
speed changes were made during flight or
cause the outboard wing to fly high or low in
level flight, resulting in poor performance.
The battery pack tucked high into the
fuselage also allowed me to have a fuselage
with minimal side area. I wanted a model that
would fly well in light or heavy winds.
Airplanes with large fuselages or vertical
surfaces are usually affected more by the wind
or tend to “weather vane” during flight.
So far this design has proven to work
extremely well. Its first real test was at the
2004 Nats, where I flew it in winds exceeding
20 mph, gusting at times to more than 30
mph. This was an extreme case, and in winds
that high it’s difficult to get any airplane to
perform well. The Silencer made it through
the wind slowly at times, but I was able to
complete the pattern and land it safely.
Flying: The ready-to-fly weight came in at 44
ounces. This gives the Silencer a wing loading
of 11.59 ounces per square foot of wing area,
which is close to that of glow-powered Stunt
models. Performance so far has been better
than expected. As of this writing I have put
only 25 flights on the Silencer, so I need to do
more trim work to get it dialed in, but the
potential is surely there.
The propeller is turning out to be one of
the most important areas of trimming. First
flights yielded lap times in the mid- to lowfour-
second range with a 10 x 5 APC-E
propeller. The model is being flown on 19-
strand, .015 x 60-foot, eyelet-to-eyelet control
lines.
So far the best propeller for this model has
been a Graupner CAM 11 x 4 two-blade,
repitched to 11 x 3.8. On the same 60-foot
lines, I am now turning 5.1- to 5.2-second lap
times.
At launch the motor is pulling 32-34 amps
and spinning the 11 x 3.8 propeller roughly
11,800 rpm. This equates to approximately
450 watts in, or .6 horsepower, and roughly
382 watts out to the propeller, or .51 shaft
horsepower.
After the first flights, the battery
temperature was 100° and the motor
temperature was 140°, measured at the
windings. The motor temperature has to be
measured at the windings because the motor
case spins and cools more than the windings,
giving you a false reading. The Li-Poly
batteries should never exceed 140° during
discharge, and I have been told that brushless
motors can handle as much as roughly 200°
safely.
History: September 7, 2003, I competed in
the Bergen County CL contest and finished
with 497.5 points. This put me in eighth place
out of 17 entrants in Expert with my electric
Twister.
The following summer at the 2004 AMA
Nats, I flew the Silencer to sixth place out of
37 entrants. I also received the James A. Hunt
Technical Innovation Award for my
accomplishment.
Thanks: This project has turned out to be
more fun and rewarding than I ever
imagined it would be. I thank Castle
Creations and Thunder Power batteries for
their fantastic customer service, along with
everyone who has helped or supported me
throughout this project. Without them, it
wouldn’t be where it is today.
wouldn’t be where it is today.
I look forward to the future technology of
electric power and what it will bring us. I
know I will continue to enjoy developing
new electric-powered models, and I hope
you will too. MA
Mike Palko
121 N. 4th St.
Telford PA 18969
[email protected]
Sources:
Gen2 4S2P 4200 mAh Li-Poly pack:
Thunder Power
4720 W. University Ave.
Las Vegas NV 89103
(702) 228-8883
www.thunderpower-batteries.com
Plettenberg Orbit 15-18 motor:
ICARE
381 Joseph-Huet
Boucherville, Quebec, J4B 2C5 Canada
(450) 449-9094
www.icare-rc.com
Phoenix-45 ESC:
Castle Creations
402 E. Pendleton Ave.
Wellsville KS 66092
(785) 883-4519
www.castlecreations.com
Timer mentioned in text:
Sergio Zigras
171 Arundel Rd.
Paramus NJ 07652
Lost-Foam wing-building system fixtures,
video sets:
Robin’s View Productions
Box 68
Stockertown PA 18083
(610) 746-0106
Edition: Model Aviation - 2005/03
Page Numbers: 24,25,26,27,28,30.32
Edition: Model Aviation - 2005/03
Page Numbers: 24,25,26,27,28,30.32
Photos by the author except as noted
24 MODEL AVIATION
by Mike Palko
I HAD MY first experience with electric flight when I was 10
years old. I broke my plastic CL RTF model after just a few flights,
and after that my dad and I decided to build an electric-powered RC
trainer together. I thought it was the coolest thing ever, because, as
with my RC cars, I just had to charge the batteries and turn the
model on to fly. But its flight performance wasn’t too successful,
and I quickly lost interest in it.
Shortly after that I was introduced to a club called the Philly
Fliers. I became interested in CL flying and haven’t looked back. It
wasn’t until 1997, at the age of 17, that I became interested in
electric flight again, and I built my first electric-powered Precision
Aerobatics (Stunt) model.
I had seen several electric CL models fly, but none of them
performed with enough authority or duration to be competitive. I’m
not sure what it was, but I was drawn to electric power and thought
it would be fun and challenging to see how well I could get an
electric Stunt model to perform.
I had no idea where to start, so I built a Sig Twister and used an
05 can motor with a seven-cell battery pack to get a base point. As I
worked on the project, I realized that I was not only developing a
power system that was new to CL, but one that showed some real
advantages in competition.
Some of the most important benefits were consistent motor runs,
no CG shift during flight as the fuel burned off, and never having
another over-run or an engine that wouldn’t start. In addition,
motors produce little vibration and leave no residue to soak into the
balsa and eventually ruin an otherwise good Stunt model.
There are also drawbacks to electric power, such as power-toweight
ratio, cost, and safety. But as technology advances, electric
power will continue to approach the performance of internalcombustion
engines, and the safety issues will be addressed.
I worked with the Twister on and off until 2003, when I reached
a point where I felt I was close to having a competitive Stunt
model. At that point I needed to take the next step and design a new
airplane from the ground up. The Twister flew great for an electricpowered
CL airplane—better than most people had ever seen such
a model fly—but it was still far from what I really wanted.
It was a profile, and it didn’t have the drive to get through high
winds. It was extremely close on run time, so if I missed the wind
and had to take an extra lap or two or got blown out of a maneuver,
it didn’t have the battery capacity to get through to the end of the
flight. The Twister was built so light that it was to the point of
being weak. I needed a new approach.
Power System: I felt that the Silencer would be the answer to the
problems I mentioned. Its design would incorporate a stronger,
more aerodynamic airframe and a new power system.
The biggest performance gain would be in the battery pack. I
switched from a Sanyo 10-cell 2600 mAh NiMH pack to a Thunder
Power (Gen2) 4S2P 4200 mAh Li-Poly pack. This is really two
2S2P packs wired in series because a 4S2P is not available off the
shelf.
This change alone would increase the battery capacity by roughly
two-thirds, increase the voltage by 2.8 volts, and drop the weight by
approximately 8 ounces! It would allow me to run a higher voltage
with a lower amp draw and maintain the same, if not increase the
setup’s, output in watts. It’s safer and more efficient to run lower
current with higher voltage because there is less heat buildup, which
also increases the life of the motor and the battery.
I also switched to a motor that was capable of turning a lowerpitch
propeller at a higher rpm. I replaced the AXI 2820/10 that I
had been using with a Plettenberg Orbit 15-18, which can handle
more voltage and higher current levels. I have found it to be more
An electric revolution in action. This is the most successful electric-powered CL Stunt model to date. John Glatthorn photo.
It’s hard to tell that the model is electric—until you realize that
there is no smoke trail from burnt fuel! Will Hubin photo.
Competitive electric-powered CL Precision Aerobatics becomes a reality
March 2005 25
With cowl and battery cover removed, it’s easy to see the system
components’ placement. Good airflow through this area is a must!
Power is the Plettenberg Orbit 15-18 motor, two Thunder Power
2S2P Li-Poly battery packs wired in series, a Castle Creations
Phoenix-45 ESC, and a Sergio Zigras timer.
Mike’s model has all of the normal flight-trim features, such as
this adjustable leadout guide. Neat workmanship!
The structure had to be light and strong. Mike used the Lost Foam
wing-building system to construct the rigid Warren truss-type wing.
efficient than the AXI by 8-10%. This would give me more usable
capacity from the battery, thus extending my flight times.
The AXI and the Orbit are brushless outrunners, or rotating can
motors. This means that the entire motor case spins, creating a power
plant that cools itself slightly and is capable of turning a large propeller
without using a gearbox because of its higher torque output.
This was important to me because a gearbox adds weight, creates
noise, reduces the strength of the power train—because it may strip or
wear out—and adds cost and complexity to the setup. This does not
mean that direct drive is the only way to go, but there were more
negatives than positives in using a gearbox for my application.
I had been using a Castle Creations Phoenix-35 ESC, which is
capable of handling 35 amps continuously. I knew I would be pulling
close to 35 amps, so I changed to a Phoenix-45 to increase the safety
margin. The Orbit runs at full throttle the entire flight, so the ESC has to
be able to handle the current and heat buildup for five to seven minutes
at a time.
However, an ESC does more than control the motor’s speed. The
Castle Creations ESC controls eight parameters of the motor, including
cutoff voltage and throttle type. These two factors are particularly
important because they will benefit the CL flier the most.
The cutoff-voltage feature is a must with Li-Poly batteries. With a 4S
battery pack, you need to set the cutoff voltage to 12. Li-Poly cells
should never be discharged to less than 3 volts per cell under load;
otherwise, permanent damage or fire could result. If the timer would
ever fail, the ESC would turn off the motor when the battery pack
reached 12 volts, saving the pack and airplane from possible damage.
With the cowling on, the model’s nose looks like a normal
“engined” Stunter. Note the cooling holes in the nose ring.
26 MODEL AVIATION
Does Mike look proud? He should! He has elevated the
performance of electric-powered models to a competitive level in
CL Stunt. Look for more from him! Hubin photo.
Dan Banjock launches Mike’s Silencer during 2004 Nats Advanced
competition. Mike finished a credible sixth. Hubin photo.
The sleek, clean model gives up nothing in aesthetics—or
performance—to glow-powered models of the same size and type.
The throttle type can be chosen from four settings. I have learned
that the high-rpm governor mode works best for our purposes. This
helps hold the motor at a constant rpm, preventing whip-up and
keeping a uniform speed throughout a maneuver, much like a tunedpipe
setup.
However, the ESC will not work without something telling it what
to do. To control the ESC, I am using a timer that Sergio Zigras
designed and built. The run time is adjustable from five to seven
minutes by turning a small speed potentiometer. The timer plugs into
the ESC and, when turned on, gives it a signal to arm and then slowly
ramps to full power. No external power is needed to run the timer. It
uses power from the Li-Poly pack.
As for the power system’s weight, the motor weighs 6.21 ounces,
the ESC weighs 1.06 ounces, the timer is negligible at .07 ounce, and
the battery weighs 14.18 ounces. This is a total of 21.52 ounces
including all connectors.
It seems heavy, but with attention to detail and the lack of plywood
doublers, engine beams, and crutch, the bare airframe’s weight can be
lowered dramatically to compensate.
CONSTRUCTION
Keep the overall construction as light as possible. I weighed each
part as I built the Silencer, looking for areas where I could save
Type: CL Precision Aerobatics
Wingspan: 52 inches
Power: Plettenberg Orbit 15-18 motor
Flying weight: 44 ounces
Construction: Balsa and plywood
Covering/finish: Light-grade silkspan with Sig Litecoat
and Brodak modeling dopes
weight. Weight is the biggest concern with
electric power because you are “behind” from
the start; however, with the right wood
selection and a light finish, you can keep the
weight to a minimum.
Wing: The wing is built up and utilizes a
Warren truss-type ribbing scheme. The main
ribs are angled, and there are half ribs
between the opposing sets of ribs at the LE to
support the sheeting. The model has a 52-inch
wingspan and 510 square inches of wing area,
including the flaps. The wing panels are of
equal length.
I built the wing using Bob Hunt’s Lost
Foam wing-building system because it is one
of, if not the most, accurate ways to construct
a wing. It allows you to build the structure
extremely light and maintain its integrity, and
it provides the easiest and most accurate way
to produce the Warren-truss rib sets.
Using the Lost Foam method, you mark
the desired rib locations on the front and rear
of a foam blank that is cut to the planform of
the wing. Cut and sand the core, and mark the
rib locations chordwise on it using a ballpoint
pen. Also mark the spar location on the core
on the top and the bottom.
The rib locations are then accurately
scribed into the lower cradle half from which
the core was cut. This cradle is as accurate a
negative shape as the core is a positive shape,
and it can be used as a building fixture. The
core is sliced vertically at each rib station,
yielding perfectly accurate rib templates from
which balsa ribs can be generated.
Bob has produced a two-video set about
the Lost Foam wing-building system, and it
includes all of the information about how to
cut and prepare your own fixture sets. His
company—Robin’s View Productions—sells
the videos and offers a cutting service. He can
supply complete Lost Foam fixture sets for
this model and hundreds of others.
To help keep weight to a minimum, I used
4- to 6-pound, contest-grade wood
throughout. The flaps are made from 1⁄4-inch
straight C-grain balsa, with the grain
following the TE to help reduce the chance of
warps. The outboard flap is 1⁄8-inch wider at
the tip than the inboard flap is, to help the
inboard and outboard wing panels lift equally
in a turn.
The fuselage blankets the outboard wing
because the model is flying in a circle and is
angled somewhat tangent to the path of flight.
Therefore, the outboard wing and flap have
less effective area. The outboard flap’s larger
area helps the wing turn flat and without a
rolling tendency, even though there is less
airflow over it.
Tailplane: The stabilizer is 3⁄8-inch thick and
is built using a Warren truss-style
construction. The LE and TE are made from
1⁄4 x 3⁄8 balsa. I laminated the forward face of
the stabilizer TE with .008-inch carbon fiber
over the full span, and I used a double layer in
the center-section for added stiffness. The tips
are soft balsa, carved to shape and hollowed.
The elevators are 5⁄16-inch thick, and I built
them using a sheet of 1⁄16 balsa that was
shimmed 1⁄8 inch off the building board. The
LE and TE were glued to the 1⁄16 sheet, as
were the ribs. Then I flipped the elevator over
and glued the bottom ribs in place.
I capped the inside root edges of the
elevators with hard balsa to support the
elevator horn, and I capped the tips with soft
balsa and carved them to shape. The elevators
were then sanded and tapered to 1⁄8 inch at the
TE. Once they were completed, I went back
and removed the wood between each rib to
reduce the weight even further.
Fuselage: The fuselage is built with 1⁄16 Cgrain
sides. On the inside I doped on .5-ounce
carbon-fiber mat as a replacement for the
doublers. The motor mount is 1⁄8 aircraft
plywood with three holes drilled and lightly
countersunk in each side to allow for motor
cooling.
The motor mount sits roughly 1⁄8 inch
behind the front edge of the fuselage. The
overhanging fuselage sides act as a small
scoop to help guide air into the cooling holes.
I put a fillet of Aeropoxy Lite on the inside
and outside of the motor mount glue joint for
added strength and to help smooth the
airflow.
I covered the nose section with .75-ounce
fiberglass, making sure to wrap around the
front of the nose to reinforce the motor-mount
joint. I also reinforced the inside motor-mount
joint with .75-ounce fiberglass.
Final Assembly: Install the battery tray after
the wing is joined to the fuselage. Once the
wing is in place, its lower center-section needs
to be removed for battery clearance. During
wing construction, the lower bellcrank mount
must be sunk into the wing 5⁄8 inch so you
don’t sacrifice strength. Otherwise, this
section of the bellcrank mount would be
removed to provide clearance for the battery
tray.
I made the battery tray from three layers of
1⁄16 balsa with alternating grain, to make balsa
plywood. I laminated each layer together with
epoxy and .5-ounce carbon-fiber mat. I
epoxied this tray directly to the landing-gear
mount, the wing, and the lower bellcrank
mount, tying everything together. I used
Aeropoxy Lite to make fillets inside the
battery compartment and around the wing to
help blend and reinforce the joints.
I removed the lower wing center-section to
position the battery pack as close to the
airplane’s centerline as possible, in an effort
to keep the vertical CG in the proper location.
If the 14-ounce battery was placed too far
from the intended vertical CG, you could end
up with an airplane that would rock and roll as
speed changes were made during flight or
cause the outboard wing to fly high or low in
level flight, resulting in poor performance.
The battery pack tucked high into the
fuselage also allowed me to have a fuselage
with minimal side area. I wanted a model that
would fly well in light or heavy winds.
Airplanes with large fuselages or vertical
surfaces are usually affected more by the wind
or tend to “weather vane” during flight.
So far this design has proven to work
extremely well. Its first real test was at the
2004 Nats, where I flew it in winds exceeding
20 mph, gusting at times to more than 30
mph. This was an extreme case, and in winds
that high it’s difficult to get any airplane to
perform well. The Silencer made it through
the wind slowly at times, but I was able to
complete the pattern and land it safely.
Flying: The ready-to-fly weight came in at 44
ounces. This gives the Silencer a wing loading
of 11.59 ounces per square foot of wing area,
which is close to that of glow-powered Stunt
models. Performance so far has been better
than expected. As of this writing I have put
only 25 flights on the Silencer, so I need to do
more trim work to get it dialed in, but the
potential is surely there.
The propeller is turning out to be one of
the most important areas of trimming. First
flights yielded lap times in the mid- to lowfour-
second range with a 10 x 5 APC-E
propeller. The model is being flown on 19-
strand, .015 x 60-foot, eyelet-to-eyelet control
lines.
So far the best propeller for this model has
been a Graupner CAM 11 x 4 two-blade,
repitched to 11 x 3.8. On the same 60-foot
lines, I am now turning 5.1- to 5.2-second lap
times.
At launch the motor is pulling 32-34 amps
and spinning the 11 x 3.8 propeller roughly
11,800 rpm. This equates to approximately
450 watts in, or .6 horsepower, and roughly
382 watts out to the propeller, or .51 shaft
horsepower.
After the first flights, the battery
temperature was 100° and the motor
temperature was 140°, measured at the
windings. The motor temperature has to be
measured at the windings because the motor
case spins and cools more than the windings,
giving you a false reading. The Li-Poly
batteries should never exceed 140° during
discharge, and I have been told that brushless
motors can handle as much as roughly 200°
safely.
History: September 7, 2003, I competed in
the Bergen County CL contest and finished
with 497.5 points. This put me in eighth place
out of 17 entrants in Expert with my electric
Twister.
The following summer at the 2004 AMA
Nats, I flew the Silencer to sixth place out of
37 entrants. I also received the James A. Hunt
Technical Innovation Award for my
accomplishment.
Thanks: This project has turned out to be
more fun and rewarding than I ever
imagined it would be. I thank Castle
Creations and Thunder Power batteries for
their fantastic customer service, along with
everyone who has helped or supported me
throughout this project. Without them, it
wouldn’t be where it is today.
wouldn’t be where it is today.
I look forward to the future technology of
electric power and what it will bring us. I
know I will continue to enjoy developing
new electric-powered models, and I hope
you will too. MA
Mike Palko
121 N. 4th St.
Telford PA 18969
[email protected]
Sources:
Gen2 4S2P 4200 mAh Li-Poly pack:
Thunder Power
4720 W. University Ave.
Las Vegas NV 89103
(702) 228-8883
www.thunderpower-batteries.com
Plettenberg Orbit 15-18 motor:
ICARE
381 Joseph-Huet
Boucherville, Quebec, J4B 2C5 Canada
(450) 449-9094
www.icare-rc.com
Phoenix-45 ESC:
Castle Creations
402 E. Pendleton Ave.
Wellsville KS 66092
(785) 883-4519
www.castlecreations.com
Timer mentioned in text:
Sergio Zigras
171 Arundel Rd.
Paramus NJ 07652
Lost-Foam wing-building system fixtures,
video sets:
Robin’s View Productions
Box 68
Stockertown PA 18083
(610) 746-0106
Edition: Model Aviation - 2005/03
Page Numbers: 24,25,26,27,28,30.32
Photos by the author except as noted
24 MODEL AVIATION
by Mike Palko
I HAD MY first experience with electric flight when I was 10
years old. I broke my plastic CL RTF model after just a few flights,
and after that my dad and I decided to build an electric-powered RC
trainer together. I thought it was the coolest thing ever, because, as
with my RC cars, I just had to charge the batteries and turn the
model on to fly. But its flight performance wasn’t too successful,
and I quickly lost interest in it.
Shortly after that I was introduced to a club called the Philly
Fliers. I became interested in CL flying and haven’t looked back. It
wasn’t until 1997, at the age of 17, that I became interested in
electric flight again, and I built my first electric-powered Precision
Aerobatics (Stunt) model.
I had seen several electric CL models fly, but none of them
performed with enough authority or duration to be competitive. I’m
not sure what it was, but I was drawn to electric power and thought
it would be fun and challenging to see how well I could get an
electric Stunt model to perform.
I had no idea where to start, so I built a Sig Twister and used an
05 can motor with a seven-cell battery pack to get a base point. As I
worked on the project, I realized that I was not only developing a
power system that was new to CL, but one that showed some real
advantages in competition.
Some of the most important benefits were consistent motor runs,
no CG shift during flight as the fuel burned off, and never having
another over-run or an engine that wouldn’t start. In addition,
motors produce little vibration and leave no residue to soak into the
balsa and eventually ruin an otherwise good Stunt model.
There are also drawbacks to electric power, such as power-toweight
ratio, cost, and safety. But as technology advances, electric
power will continue to approach the performance of internalcombustion
engines, and the safety issues will be addressed.
I worked with the Twister on and off until 2003, when I reached
a point where I felt I was close to having a competitive Stunt
model. At that point I needed to take the next step and design a new
airplane from the ground up. The Twister flew great for an electricpowered
CL airplane—better than most people had ever seen such
a model fly—but it was still far from what I really wanted.
It was a profile, and it didn’t have the drive to get through high
winds. It was extremely close on run time, so if I missed the wind
and had to take an extra lap or two or got blown out of a maneuver,
it didn’t have the battery capacity to get through to the end of the
flight. The Twister was built so light that it was to the point of
being weak. I needed a new approach.
Power System: I felt that the Silencer would be the answer to the
problems I mentioned. Its design would incorporate a stronger,
more aerodynamic airframe and a new power system.
The biggest performance gain would be in the battery pack. I
switched from a Sanyo 10-cell 2600 mAh NiMH pack to a Thunder
Power (Gen2) 4S2P 4200 mAh Li-Poly pack. This is really two
2S2P packs wired in series because a 4S2P is not available off the
shelf.
This change alone would increase the battery capacity by roughly
two-thirds, increase the voltage by 2.8 volts, and drop the weight by
approximately 8 ounces! It would allow me to run a higher voltage
with a lower amp draw and maintain the same, if not increase the
setup’s, output in watts. It’s safer and more efficient to run lower
current with higher voltage because there is less heat buildup, which
also increases the life of the motor and the battery.
I also switched to a motor that was capable of turning a lowerpitch
propeller at a higher rpm. I replaced the AXI 2820/10 that I
had been using with a Plettenberg Orbit 15-18, which can handle
more voltage and higher current levels. I have found it to be more
An electric revolution in action. This is the most successful electric-powered CL Stunt model to date. John Glatthorn photo.
It’s hard to tell that the model is electric—until you realize that
there is no smoke trail from burnt fuel! Will Hubin photo.
Competitive electric-powered CL Precision Aerobatics becomes a reality
March 2005 25
With cowl and battery cover removed, it’s easy to see the system
components’ placement. Good airflow through this area is a must!
Power is the Plettenberg Orbit 15-18 motor, two Thunder Power
2S2P Li-Poly battery packs wired in series, a Castle Creations
Phoenix-45 ESC, and a Sergio Zigras timer.
Mike’s model has all of the normal flight-trim features, such as
this adjustable leadout guide. Neat workmanship!
The structure had to be light and strong. Mike used the Lost Foam
wing-building system to construct the rigid Warren truss-type wing.
efficient than the AXI by 8-10%. This would give me more usable
capacity from the battery, thus extending my flight times.
The AXI and the Orbit are brushless outrunners, or rotating can
motors. This means that the entire motor case spins, creating a power
plant that cools itself slightly and is capable of turning a large propeller
without using a gearbox because of its higher torque output.
This was important to me because a gearbox adds weight, creates
noise, reduces the strength of the power train—because it may strip or
wear out—and adds cost and complexity to the setup. This does not
mean that direct drive is the only way to go, but there were more
negatives than positives in using a gearbox for my application.
I had been using a Castle Creations Phoenix-35 ESC, which is
capable of handling 35 amps continuously. I knew I would be pulling
close to 35 amps, so I changed to a Phoenix-45 to increase the safety
margin. The Orbit runs at full throttle the entire flight, so the ESC has to
be able to handle the current and heat buildup for five to seven minutes
at a time.
However, an ESC does more than control the motor’s speed. The
Castle Creations ESC controls eight parameters of the motor, including
cutoff voltage and throttle type. These two factors are particularly
important because they will benefit the CL flier the most.
The cutoff-voltage feature is a must with Li-Poly batteries. With a 4S
battery pack, you need to set the cutoff voltage to 12. Li-Poly cells
should never be discharged to less than 3 volts per cell under load;
otherwise, permanent damage or fire could result. If the timer would
ever fail, the ESC would turn off the motor when the battery pack
reached 12 volts, saving the pack and airplane from possible damage.
With the cowling on, the model’s nose looks like a normal
“engined” Stunter. Note the cooling holes in the nose ring.
26 MODEL AVIATION
Does Mike look proud? He should! He has elevated the
performance of electric-powered models to a competitive level in
CL Stunt. Look for more from him! Hubin photo.
Dan Banjock launches Mike’s Silencer during 2004 Nats Advanced
competition. Mike finished a credible sixth. Hubin photo.
The sleek, clean model gives up nothing in aesthetics—or
performance—to glow-powered models of the same size and type.
The throttle type can be chosen from four settings. I have learned
that the high-rpm governor mode works best for our purposes. This
helps hold the motor at a constant rpm, preventing whip-up and
keeping a uniform speed throughout a maneuver, much like a tunedpipe
setup.
However, the ESC will not work without something telling it what
to do. To control the ESC, I am using a timer that Sergio Zigras
designed and built. The run time is adjustable from five to seven
minutes by turning a small speed potentiometer. The timer plugs into
the ESC and, when turned on, gives it a signal to arm and then slowly
ramps to full power. No external power is needed to run the timer. It
uses power from the Li-Poly pack.
As for the power system’s weight, the motor weighs 6.21 ounces,
the ESC weighs 1.06 ounces, the timer is negligible at .07 ounce, and
the battery weighs 14.18 ounces. This is a total of 21.52 ounces
including all connectors.
It seems heavy, but with attention to detail and the lack of plywood
doublers, engine beams, and crutch, the bare airframe’s weight can be
lowered dramatically to compensate.
CONSTRUCTION
Keep the overall construction as light as possible. I weighed each
part as I built the Silencer, looking for areas where I could save
Type: CL Precision Aerobatics
Wingspan: 52 inches
Power: Plettenberg Orbit 15-18 motor
Flying weight: 44 ounces
Construction: Balsa and plywood
Covering/finish: Light-grade silkspan with Sig Litecoat
and Brodak modeling dopes
weight. Weight is the biggest concern with
electric power because you are “behind” from
the start; however, with the right wood
selection and a light finish, you can keep the
weight to a minimum.
Wing: The wing is built up and utilizes a
Warren truss-type ribbing scheme. The main
ribs are angled, and there are half ribs
between the opposing sets of ribs at the LE to
support the sheeting. The model has a 52-inch
wingspan and 510 square inches of wing area,
including the flaps. The wing panels are of
equal length.
I built the wing using Bob Hunt’s Lost
Foam wing-building system because it is one
of, if not the most, accurate ways to construct
a wing. It allows you to build the structure
extremely light and maintain its integrity, and
it provides the easiest and most accurate way
to produce the Warren-truss rib sets.
Using the Lost Foam method, you mark
the desired rib locations on the front and rear
of a foam blank that is cut to the planform of
the wing. Cut and sand the core, and mark the
rib locations chordwise on it using a ballpoint
pen. Also mark the spar location on the core
on the top and the bottom.
The rib locations are then accurately
scribed into the lower cradle half from which
the core was cut. This cradle is as accurate a
negative shape as the core is a positive shape,
and it can be used as a building fixture. The
core is sliced vertically at each rib station,
yielding perfectly accurate rib templates from
which balsa ribs can be generated.
Bob has produced a two-video set about
the Lost Foam wing-building system, and it
includes all of the information about how to
cut and prepare your own fixture sets. His
company—Robin’s View Productions—sells
the videos and offers a cutting service. He can
supply complete Lost Foam fixture sets for
this model and hundreds of others.
To help keep weight to a minimum, I used
4- to 6-pound, contest-grade wood
throughout. The flaps are made from 1⁄4-inch
straight C-grain balsa, with the grain
following the TE to help reduce the chance of
warps. The outboard flap is 1⁄8-inch wider at
the tip than the inboard flap is, to help the
inboard and outboard wing panels lift equally
in a turn.
The fuselage blankets the outboard wing
because the model is flying in a circle and is
angled somewhat tangent to the path of flight.
Therefore, the outboard wing and flap have
less effective area. The outboard flap’s larger
area helps the wing turn flat and without a
rolling tendency, even though there is less
airflow over it.
Tailplane: The stabilizer is 3⁄8-inch thick and
is built using a Warren truss-style
construction. The LE and TE are made from
1⁄4 x 3⁄8 balsa. I laminated the forward face of
the stabilizer TE with .008-inch carbon fiber
over the full span, and I used a double layer in
the center-section for added stiffness. The tips
are soft balsa, carved to shape and hollowed.
The elevators are 5⁄16-inch thick, and I built
them using a sheet of 1⁄16 balsa that was
shimmed 1⁄8 inch off the building board. The
LE and TE were glued to the 1⁄16 sheet, as
were the ribs. Then I flipped the elevator over
and glued the bottom ribs in place.
I capped the inside root edges of the
elevators with hard balsa to support the
elevator horn, and I capped the tips with soft
balsa and carved them to shape. The elevators
were then sanded and tapered to 1⁄8 inch at the
TE. Once they were completed, I went back
and removed the wood between each rib to
reduce the weight even further.
Fuselage: The fuselage is built with 1⁄16 Cgrain
sides. On the inside I doped on .5-ounce
carbon-fiber mat as a replacement for the
doublers. The motor mount is 1⁄8 aircraft
plywood with three holes drilled and lightly
countersunk in each side to allow for motor
cooling.
The motor mount sits roughly 1⁄8 inch
behind the front edge of the fuselage. The
overhanging fuselage sides act as a small
scoop to help guide air into the cooling holes.
I put a fillet of Aeropoxy Lite on the inside
and outside of the motor mount glue joint for
added strength and to help smooth the
airflow.
I covered the nose section with .75-ounce
fiberglass, making sure to wrap around the
front of the nose to reinforce the motor-mount
joint. I also reinforced the inside motor-mount
joint with .75-ounce fiberglass.
Final Assembly: Install the battery tray after
the wing is joined to the fuselage. Once the
wing is in place, its lower center-section needs
to be removed for battery clearance. During
wing construction, the lower bellcrank mount
must be sunk into the wing 5⁄8 inch so you
don’t sacrifice strength. Otherwise, this
section of the bellcrank mount would be
removed to provide clearance for the battery
tray.
I made the battery tray from three layers of
1⁄16 balsa with alternating grain, to make balsa
plywood. I laminated each layer together with
epoxy and .5-ounce carbon-fiber mat. I
epoxied this tray directly to the landing-gear
mount, the wing, and the lower bellcrank
mount, tying everything together. I used
Aeropoxy Lite to make fillets inside the
battery compartment and around the wing to
help blend and reinforce the joints.
I removed the lower wing center-section to
position the battery pack as close to the
airplane’s centerline as possible, in an effort
to keep the vertical CG in the proper location.
If the 14-ounce battery was placed too far
from the intended vertical CG, you could end
up with an airplane that would rock and roll as
speed changes were made during flight or
cause the outboard wing to fly high or low in
level flight, resulting in poor performance.
The battery pack tucked high into the
fuselage also allowed me to have a fuselage
with minimal side area. I wanted a model that
would fly well in light or heavy winds.
Airplanes with large fuselages or vertical
surfaces are usually affected more by the wind
or tend to “weather vane” during flight.
So far this design has proven to work
extremely well. Its first real test was at the
2004 Nats, where I flew it in winds exceeding
20 mph, gusting at times to more than 30
mph. This was an extreme case, and in winds
that high it’s difficult to get any airplane to
perform well. The Silencer made it through
the wind slowly at times, but I was able to
complete the pattern and land it safely.
Flying: The ready-to-fly weight came in at 44
ounces. This gives the Silencer a wing loading
of 11.59 ounces per square foot of wing area,
which is close to that of glow-powered Stunt
models. Performance so far has been better
than expected. As of this writing I have put
only 25 flights on the Silencer, so I need to do
more trim work to get it dialed in, but the
potential is surely there.
The propeller is turning out to be one of
the most important areas of trimming. First
flights yielded lap times in the mid- to lowfour-
second range with a 10 x 5 APC-E
propeller. The model is being flown on 19-
strand, .015 x 60-foot, eyelet-to-eyelet control
lines.
So far the best propeller for this model has
been a Graupner CAM 11 x 4 two-blade,
repitched to 11 x 3.8. On the same 60-foot
lines, I am now turning 5.1- to 5.2-second lap
times.
At launch the motor is pulling 32-34 amps
and spinning the 11 x 3.8 propeller roughly
11,800 rpm. This equates to approximately
450 watts in, or .6 horsepower, and roughly
382 watts out to the propeller, or .51 shaft
horsepower.
After the first flights, the battery
temperature was 100° and the motor
temperature was 140°, measured at the
windings. The motor temperature has to be
measured at the windings because the motor
case spins and cools more than the windings,
giving you a false reading. The Li-Poly
batteries should never exceed 140° during
discharge, and I have been told that brushless
motors can handle as much as roughly 200°
safely.
History: September 7, 2003, I competed in
the Bergen County CL contest and finished
with 497.5 points. This put me in eighth place
out of 17 entrants in Expert with my electric
Twister.
The following summer at the 2004 AMA
Nats, I flew the Silencer to sixth place out of
37 entrants. I also received the James A. Hunt
Technical Innovation Award for my
accomplishment.
Thanks: This project has turned out to be
more fun and rewarding than I ever
imagined it would be. I thank Castle
Creations and Thunder Power batteries for
their fantastic customer service, along with
everyone who has helped or supported me
throughout this project. Without them, it
wouldn’t be where it is today.
wouldn’t be where it is today.
I look forward to the future technology of
electric power and what it will bring us. I
know I will continue to enjoy developing
new electric-powered models, and I hope
you will too. MA
Mike Palko
121 N. 4th St.
Telford PA 18969
[email protected]
Sources:
Gen2 4S2P 4200 mAh Li-Poly pack:
Thunder Power
4720 W. University Ave.
Las Vegas NV 89103
(702) 228-8883
www.thunderpower-batteries.com
Plettenberg Orbit 15-18 motor:
ICARE
381 Joseph-Huet
Boucherville, Quebec, J4B 2C5 Canada
(450) 449-9094
www.icare-rc.com
Phoenix-45 ESC:
Castle Creations
402 E. Pendleton Ave.
Wellsville KS 66092
(785) 883-4519
www.castlecreations.com
Timer mentioned in text:
Sergio Zigras
171 Arundel Rd.
Paramus NJ 07652
Lost-Foam wing-building system fixtures,
video sets:
Robin’s View Productions
Box 68
Stockertown PA 18083
(610) 746-0106
Edition: Model Aviation - 2005/03
Page Numbers: 24,25,26,27,28,30.32
Photos by the author except as noted
24 MODEL AVIATION
by Mike Palko
I HAD MY first experience with electric flight when I was 10
years old. I broke my plastic CL RTF model after just a few flights,
and after that my dad and I decided to build an electric-powered RC
trainer together. I thought it was the coolest thing ever, because, as
with my RC cars, I just had to charge the batteries and turn the
model on to fly. But its flight performance wasn’t too successful,
and I quickly lost interest in it.
Shortly after that I was introduced to a club called the Philly
Fliers. I became interested in CL flying and haven’t looked back. It
wasn’t until 1997, at the age of 17, that I became interested in
electric flight again, and I built my first electric-powered Precision
Aerobatics (Stunt) model.
I had seen several electric CL models fly, but none of them
performed with enough authority or duration to be competitive. I’m
not sure what it was, but I was drawn to electric power and thought
it would be fun and challenging to see how well I could get an
electric Stunt model to perform.
I had no idea where to start, so I built a Sig Twister and used an
05 can motor with a seven-cell battery pack to get a base point. As I
worked on the project, I realized that I was not only developing a
power system that was new to CL, but one that showed some real
advantages in competition.
Some of the most important benefits were consistent motor runs,
no CG shift during flight as the fuel burned off, and never having
another over-run or an engine that wouldn’t start. In addition,
motors produce little vibration and leave no residue to soak into the
balsa and eventually ruin an otherwise good Stunt model.
There are also drawbacks to electric power, such as power-toweight
ratio, cost, and safety. But as technology advances, electric
power will continue to approach the performance of internalcombustion
engines, and the safety issues will be addressed.
I worked with the Twister on and off until 2003, when I reached
a point where I felt I was close to having a competitive Stunt
model. At that point I needed to take the next step and design a new
airplane from the ground up. The Twister flew great for an electricpowered
CL airplane—better than most people had ever seen such
a model fly—but it was still far from what I really wanted.
It was a profile, and it didn’t have the drive to get through high
winds. It was extremely close on run time, so if I missed the wind
and had to take an extra lap or two or got blown out of a maneuver,
it didn’t have the battery capacity to get through to the end of the
flight. The Twister was built so light that it was to the point of
being weak. I needed a new approach.
Power System: I felt that the Silencer would be the answer to the
problems I mentioned. Its design would incorporate a stronger,
more aerodynamic airframe and a new power system.
The biggest performance gain would be in the battery pack. I
switched from a Sanyo 10-cell 2600 mAh NiMH pack to a Thunder
Power (Gen2) 4S2P 4200 mAh Li-Poly pack. This is really two
2S2P packs wired in series because a 4S2P is not available off the
shelf.
This change alone would increase the battery capacity by roughly
two-thirds, increase the voltage by 2.8 volts, and drop the weight by
approximately 8 ounces! It would allow me to run a higher voltage
with a lower amp draw and maintain the same, if not increase the
setup’s, output in watts. It’s safer and more efficient to run lower
current with higher voltage because there is less heat buildup, which
also increases the life of the motor and the battery.
I also switched to a motor that was capable of turning a lowerpitch
propeller at a higher rpm. I replaced the AXI 2820/10 that I
had been using with a Plettenberg Orbit 15-18, which can handle
more voltage and higher current levels. I have found it to be more
An electric revolution in action. This is the most successful electric-powered CL Stunt model to date. John Glatthorn photo.
It’s hard to tell that the model is electric—until you realize that
there is no smoke trail from burnt fuel! Will Hubin photo.
Competitive electric-powered CL Precision Aerobatics becomes a reality
March 2005 25
With cowl and battery cover removed, it’s easy to see the system
components’ placement. Good airflow through this area is a must!
Power is the Plettenberg Orbit 15-18 motor, two Thunder Power
2S2P Li-Poly battery packs wired in series, a Castle Creations
Phoenix-45 ESC, and a Sergio Zigras timer.
Mike’s model has all of the normal flight-trim features, such as
this adjustable leadout guide. Neat workmanship!
The structure had to be light and strong. Mike used the Lost Foam
wing-building system to construct the rigid Warren truss-type wing.
efficient than the AXI by 8-10%. This would give me more usable
capacity from the battery, thus extending my flight times.
The AXI and the Orbit are brushless outrunners, or rotating can
motors. This means that the entire motor case spins, creating a power
plant that cools itself slightly and is capable of turning a large propeller
without using a gearbox because of its higher torque output.
This was important to me because a gearbox adds weight, creates
noise, reduces the strength of the power train—because it may strip or
wear out—and adds cost and complexity to the setup. This does not
mean that direct drive is the only way to go, but there were more
negatives than positives in using a gearbox for my application.
I had been using a Castle Creations Phoenix-35 ESC, which is
capable of handling 35 amps continuously. I knew I would be pulling
close to 35 amps, so I changed to a Phoenix-45 to increase the safety
margin. The Orbit runs at full throttle the entire flight, so the ESC has to
be able to handle the current and heat buildup for five to seven minutes
at a time.
However, an ESC does more than control the motor’s speed. The
Castle Creations ESC controls eight parameters of the motor, including
cutoff voltage and throttle type. These two factors are particularly
important because they will benefit the CL flier the most.
The cutoff-voltage feature is a must with Li-Poly batteries. With a 4S
battery pack, you need to set the cutoff voltage to 12. Li-Poly cells
should never be discharged to less than 3 volts per cell under load;
otherwise, permanent damage or fire could result. If the timer would
ever fail, the ESC would turn off the motor when the battery pack
reached 12 volts, saving the pack and airplane from possible damage.
With the cowling on, the model’s nose looks like a normal
“engined” Stunter. Note the cooling holes in the nose ring.
26 MODEL AVIATION
Does Mike look proud? He should! He has elevated the
performance of electric-powered models to a competitive level in
CL Stunt. Look for more from him! Hubin photo.
Dan Banjock launches Mike’s Silencer during 2004 Nats Advanced
competition. Mike finished a credible sixth. Hubin photo.
The sleek, clean model gives up nothing in aesthetics—or
performance—to glow-powered models of the same size and type.
The throttle type can be chosen from four settings. I have learned
that the high-rpm governor mode works best for our purposes. This
helps hold the motor at a constant rpm, preventing whip-up and
keeping a uniform speed throughout a maneuver, much like a tunedpipe
setup.
However, the ESC will not work without something telling it what
to do. To control the ESC, I am using a timer that Sergio Zigras
designed and built. The run time is adjustable from five to seven
minutes by turning a small speed potentiometer. The timer plugs into
the ESC and, when turned on, gives it a signal to arm and then slowly
ramps to full power. No external power is needed to run the timer. It
uses power from the Li-Poly pack.
As for the power system’s weight, the motor weighs 6.21 ounces,
the ESC weighs 1.06 ounces, the timer is negligible at .07 ounce, and
the battery weighs 14.18 ounces. This is a total of 21.52 ounces
including all connectors.
It seems heavy, but with attention to detail and the lack of plywood
doublers, engine beams, and crutch, the bare airframe’s weight can be
lowered dramatically to compensate.
CONSTRUCTION
Keep the overall construction as light as possible. I weighed each
part as I built the Silencer, looking for areas where I could save
Type: CL Precision Aerobatics
Wingspan: 52 inches
Power: Plettenberg Orbit 15-18 motor
Flying weight: 44 ounces
Construction: Balsa and plywood
Covering/finish: Light-grade silkspan with Sig Litecoat
and Brodak modeling dopes
weight. Weight is the biggest concern with
electric power because you are “behind” from
the start; however, with the right wood
selection and a light finish, you can keep the
weight to a minimum.
Wing: The wing is built up and utilizes a
Warren truss-type ribbing scheme. The main
ribs are angled, and there are half ribs
between the opposing sets of ribs at the LE to
support the sheeting. The model has a 52-inch
wingspan and 510 square inches of wing area,
including the flaps. The wing panels are of
equal length.
I built the wing using Bob Hunt’s Lost
Foam wing-building system because it is one
of, if not the most, accurate ways to construct
a wing. It allows you to build the structure
extremely light and maintain its integrity, and
it provides the easiest and most accurate way
to produce the Warren-truss rib sets.
Using the Lost Foam method, you mark
the desired rib locations on the front and rear
of a foam blank that is cut to the planform of
the wing. Cut and sand the core, and mark the
rib locations chordwise on it using a ballpoint
pen. Also mark the spar location on the core
on the top and the bottom.
The rib locations are then accurately
scribed into the lower cradle half from which
the core was cut. This cradle is as accurate a
negative shape as the core is a positive shape,
and it can be used as a building fixture. The
core is sliced vertically at each rib station,
yielding perfectly accurate rib templates from
which balsa ribs can be generated.
Bob has produced a two-video set about
the Lost Foam wing-building system, and it
includes all of the information about how to
cut and prepare your own fixture sets. His
company—Robin’s View Productions—sells
the videos and offers a cutting service. He can
supply complete Lost Foam fixture sets for
this model and hundreds of others.
To help keep weight to a minimum, I used
4- to 6-pound, contest-grade wood
throughout. The flaps are made from 1⁄4-inch
straight C-grain balsa, with the grain
following the TE to help reduce the chance of
warps. The outboard flap is 1⁄8-inch wider at
the tip than the inboard flap is, to help the
inboard and outboard wing panels lift equally
in a turn.
The fuselage blankets the outboard wing
because the model is flying in a circle and is
angled somewhat tangent to the path of flight.
Therefore, the outboard wing and flap have
less effective area. The outboard flap’s larger
area helps the wing turn flat and without a
rolling tendency, even though there is less
airflow over it.
Tailplane: The stabilizer is 3⁄8-inch thick and
is built using a Warren truss-style
construction. The LE and TE are made from
1⁄4 x 3⁄8 balsa. I laminated the forward face of
the stabilizer TE with .008-inch carbon fiber
over the full span, and I used a double layer in
the center-section for added stiffness. The tips
are soft balsa, carved to shape and hollowed.
The elevators are 5⁄16-inch thick, and I built
them using a sheet of 1⁄16 balsa that was
shimmed 1⁄8 inch off the building board. The
LE and TE were glued to the 1⁄16 sheet, as
were the ribs. Then I flipped the elevator over
and glued the bottom ribs in place.
I capped the inside root edges of the
elevators with hard balsa to support the
elevator horn, and I capped the tips with soft
balsa and carved them to shape. The elevators
were then sanded and tapered to 1⁄8 inch at the
TE. Once they were completed, I went back
and removed the wood between each rib to
reduce the weight even further.
Fuselage: The fuselage is built with 1⁄16 Cgrain
sides. On the inside I doped on .5-ounce
carbon-fiber mat as a replacement for the
doublers. The motor mount is 1⁄8 aircraft
plywood with three holes drilled and lightly
countersunk in each side to allow for motor
cooling.
The motor mount sits roughly 1⁄8 inch
behind the front edge of the fuselage. The
overhanging fuselage sides act as a small
scoop to help guide air into the cooling holes.
I put a fillet of Aeropoxy Lite on the inside
and outside of the motor mount glue joint for
added strength and to help smooth the
airflow.
I covered the nose section with .75-ounce
fiberglass, making sure to wrap around the
front of the nose to reinforce the motor-mount
joint. I also reinforced the inside motor-mount
joint with .75-ounce fiberglass.
Final Assembly: Install the battery tray after
the wing is joined to the fuselage. Once the
wing is in place, its lower center-section needs
to be removed for battery clearance. During
wing construction, the lower bellcrank mount
must be sunk into the wing 5⁄8 inch so you
don’t sacrifice strength. Otherwise, this
section of the bellcrank mount would be
removed to provide clearance for the battery
tray.
I made the battery tray from three layers of
1⁄16 balsa with alternating grain, to make balsa
plywood. I laminated each layer together with
epoxy and .5-ounce carbon-fiber mat. I
epoxied this tray directly to the landing-gear
mount, the wing, and the lower bellcrank
mount, tying everything together. I used
Aeropoxy Lite to make fillets inside the
battery compartment and around the wing to
help blend and reinforce the joints.
I removed the lower wing center-section to
position the battery pack as close to the
airplane’s centerline as possible, in an effort
to keep the vertical CG in the proper location.
If the 14-ounce battery was placed too far
from the intended vertical CG, you could end
up with an airplane that would rock and roll as
speed changes were made during flight or
cause the outboard wing to fly high or low in
level flight, resulting in poor performance.
The battery pack tucked high into the
fuselage also allowed me to have a fuselage
with minimal side area. I wanted a model that
would fly well in light or heavy winds.
Airplanes with large fuselages or vertical
surfaces are usually affected more by the wind
or tend to “weather vane” during flight.
So far this design has proven to work
extremely well. Its first real test was at the
2004 Nats, where I flew it in winds exceeding
20 mph, gusting at times to more than 30
mph. This was an extreme case, and in winds
that high it’s difficult to get any airplane to
perform well. The Silencer made it through
the wind slowly at times, but I was able to
complete the pattern and land it safely.
Flying: The ready-to-fly weight came in at 44
ounces. This gives the Silencer a wing loading
of 11.59 ounces per square foot of wing area,
which is close to that of glow-powered Stunt
models. Performance so far has been better
than expected. As of this writing I have put
only 25 flights on the Silencer, so I need to do
more trim work to get it dialed in, but the
potential is surely there.
The propeller is turning out to be one of
the most important areas of trimming. First
flights yielded lap times in the mid- to lowfour-
second range with a 10 x 5 APC-E
propeller. The model is being flown on 19-
strand, .015 x 60-foot, eyelet-to-eyelet control
lines.
So far the best propeller for this model has
been a Graupner CAM 11 x 4 two-blade,
repitched to 11 x 3.8. On the same 60-foot
lines, I am now turning 5.1- to 5.2-second lap
times.
At launch the motor is pulling 32-34 amps
and spinning the 11 x 3.8 propeller roughly
11,800 rpm. This equates to approximately
450 watts in, or .6 horsepower, and roughly
382 watts out to the propeller, or .51 shaft
horsepower.
After the first flights, the battery
temperature was 100° and the motor
temperature was 140°, measured at the
windings. The motor temperature has to be
measured at the windings because the motor
case spins and cools more than the windings,
giving you a false reading. The Li-Poly
batteries should never exceed 140° during
discharge, and I have been told that brushless
motors can handle as much as roughly 200°
safely.
History: September 7, 2003, I competed in
the Bergen County CL contest and finished
with 497.5 points. This put me in eighth place
out of 17 entrants in Expert with my electric
Twister.
The following summer at the 2004 AMA
Nats, I flew the Silencer to sixth place out of
37 entrants. I also received the James A. Hunt
Technical Innovation Award for my
accomplishment.
Thanks: This project has turned out to be
more fun and rewarding than I ever
imagined it would be. I thank Castle
Creations and Thunder Power batteries for
their fantastic customer service, along with
everyone who has helped or supported me
throughout this project. Without them, it
wouldn’t be where it is today.
wouldn’t be where it is today.
I look forward to the future technology of
electric power and what it will bring us. I
know I will continue to enjoy developing
new electric-powered models, and I hope
you will too. MA
Mike Palko
121 N. 4th St.
Telford PA 18969
[email protected]
Sources:
Gen2 4S2P 4200 mAh Li-Poly pack:
Thunder Power
4720 W. University Ave.
Las Vegas NV 89103
(702) 228-8883
www.thunderpower-batteries.com
Plettenberg Orbit 15-18 motor:
ICARE
381 Joseph-Huet
Boucherville, Quebec, J4B 2C5 Canada
(450) 449-9094
www.icare-rc.com
Phoenix-45 ESC:
Castle Creations
402 E. Pendleton Ave.
Wellsville KS 66092
(785) 883-4519
www.castlecreations.com
Timer mentioned in text:
Sergio Zigras
171 Arundel Rd.
Paramus NJ 07652
Lost-Foam wing-building system fixtures,
video sets:
Robin’s View Productions
Box 68
Stockertown PA 18083
(610) 746-0106
Edition: Model Aviation - 2005/03
Page Numbers: 24,25,26,27,28,30.32
Photos by the author except as noted
24 MODEL AVIATION
by Mike Palko
I HAD MY first experience with electric flight when I was 10
years old. I broke my plastic CL RTF model after just a few flights,
and after that my dad and I decided to build an electric-powered RC
trainer together. I thought it was the coolest thing ever, because, as
with my RC cars, I just had to charge the batteries and turn the
model on to fly. But its flight performance wasn’t too successful,
and I quickly lost interest in it.
Shortly after that I was introduced to a club called the Philly
Fliers. I became interested in CL flying and haven’t looked back. It
wasn’t until 1997, at the age of 17, that I became interested in
electric flight again, and I built my first electric-powered Precision
Aerobatics (Stunt) model.
I had seen several electric CL models fly, but none of them
performed with enough authority or duration to be competitive. I’m
not sure what it was, but I was drawn to electric power and thought
it would be fun and challenging to see how well I could get an
electric Stunt model to perform.
I had no idea where to start, so I built a Sig Twister and used an
05 can motor with a seven-cell battery pack to get a base point. As I
worked on the project, I realized that I was not only developing a
power system that was new to CL, but one that showed some real
advantages in competition.
Some of the most important benefits were consistent motor runs,
no CG shift during flight as the fuel burned off, and never having
another over-run or an engine that wouldn’t start. In addition,
motors produce little vibration and leave no residue to soak into the
balsa and eventually ruin an otherwise good Stunt model.
There are also drawbacks to electric power, such as power-toweight
ratio, cost, and safety. But as technology advances, electric
power will continue to approach the performance of internalcombustion
engines, and the safety issues will be addressed.
I worked with the Twister on and off until 2003, when I reached
a point where I felt I was close to having a competitive Stunt
model. At that point I needed to take the next step and design a new
airplane from the ground up. The Twister flew great for an electricpowered
CL airplane—better than most people had ever seen such
a model fly—but it was still far from what I really wanted.
It was a profile, and it didn’t have the drive to get through high
winds. It was extremely close on run time, so if I missed the wind
and had to take an extra lap or two or got blown out of a maneuver,
it didn’t have the battery capacity to get through to the end of the
flight. The Twister was built so light that it was to the point of
being weak. I needed a new approach.
Power System: I felt that the Silencer would be the answer to the
problems I mentioned. Its design would incorporate a stronger,
more aerodynamic airframe and a new power system.
The biggest performance gain would be in the battery pack. I
switched from a Sanyo 10-cell 2600 mAh NiMH pack to a Thunder
Power (Gen2) 4S2P 4200 mAh Li-Poly pack. This is really two
2S2P packs wired in series because a 4S2P is not available off the
shelf.
This change alone would increase the battery capacity by roughly
two-thirds, increase the voltage by 2.8 volts, and drop the weight by
approximately 8 ounces! It would allow me to run a higher voltage
with a lower amp draw and maintain the same, if not increase the
setup’s, output in watts. It’s safer and more efficient to run lower
current with higher voltage because there is less heat buildup, which
also increases the life of the motor and the battery.
I also switched to a motor that was capable of turning a lowerpitch
propeller at a higher rpm. I replaced the AXI 2820/10 that I
had been using with a Plettenberg Orbit 15-18, which can handle
more voltage and higher current levels. I have found it to be more
An electric revolution in action. This is the most successful electric-powered CL Stunt model to date. John Glatthorn photo.
It’s hard to tell that the model is electric—until you realize that
there is no smoke trail from burnt fuel! Will Hubin photo.
Competitive electric-powered CL Precision Aerobatics becomes a reality
March 2005 25
With cowl and battery cover removed, it’s easy to see the system
components’ placement. Good airflow through this area is a must!
Power is the Plettenberg Orbit 15-18 motor, two Thunder Power
2S2P Li-Poly battery packs wired in series, a Castle Creations
Phoenix-45 ESC, and a Sergio Zigras timer.
Mike’s model has all of the normal flight-trim features, such as
this adjustable leadout guide. Neat workmanship!
The structure had to be light and strong. Mike used the Lost Foam
wing-building system to construct the rigid Warren truss-type wing.
efficient than the AXI by 8-10%. This would give me more usable
capacity from the battery, thus extending my flight times.
The AXI and the Orbit are brushless outrunners, or rotating can
motors. This means that the entire motor case spins, creating a power
plant that cools itself slightly and is capable of turning a large propeller
without using a gearbox because of its higher torque output.
This was important to me because a gearbox adds weight, creates
noise, reduces the strength of the power train—because it may strip or
wear out—and adds cost and complexity to the setup. This does not
mean that direct drive is the only way to go, but there were more
negatives than positives in using a gearbox for my application.
I had been using a Castle Creations Phoenix-35 ESC, which is
capable of handling 35 amps continuously. I knew I would be pulling
close to 35 amps, so I changed to a Phoenix-45 to increase the safety
margin. The Orbit runs at full throttle the entire flight, so the ESC has to
be able to handle the current and heat buildup for five to seven minutes
at a time.
However, an ESC does more than control the motor’s speed. The
Castle Creations ESC controls eight parameters of the motor, including
cutoff voltage and throttle type. These two factors are particularly
important because they will benefit the CL flier the most.
The cutoff-voltage feature is a must with Li-Poly batteries. With a 4S
battery pack, you need to set the cutoff voltage to 12. Li-Poly cells
should never be discharged to less than 3 volts per cell under load;
otherwise, permanent damage or fire could result. If the timer would
ever fail, the ESC would turn off the motor when the battery pack
reached 12 volts, saving the pack and airplane from possible damage.
With the cowling on, the model’s nose looks like a normal
“engined” Stunter. Note the cooling holes in the nose ring.
26 MODEL AVIATION
Does Mike look proud? He should! He has elevated the
performance of electric-powered models to a competitive level in
CL Stunt. Look for more from him! Hubin photo.
Dan Banjock launches Mike’s Silencer during 2004 Nats Advanced
competition. Mike finished a credible sixth. Hubin photo.
The sleek, clean model gives up nothing in aesthetics—or
performance—to glow-powered models of the same size and type.
The throttle type can be chosen from four settings. I have learned
that the high-rpm governor mode works best for our purposes. This
helps hold the motor at a constant rpm, preventing whip-up and
keeping a uniform speed throughout a maneuver, much like a tunedpipe
setup.
However, the ESC will not work without something telling it what
to do. To control the ESC, I am using a timer that Sergio Zigras
designed and built. The run time is adjustable from five to seven
minutes by turning a small speed potentiometer. The timer plugs into
the ESC and, when turned on, gives it a signal to arm and then slowly
ramps to full power. No external power is needed to run the timer. It
uses power from the Li-Poly pack.
As for the power system’s weight, the motor weighs 6.21 ounces,
the ESC weighs 1.06 ounces, the timer is negligible at .07 ounce, and
the battery weighs 14.18 ounces. This is a total of 21.52 ounces
including all connectors.
It seems heavy, but with attention to detail and the lack of plywood
doublers, engine beams, and crutch, the bare airframe’s weight can be
lowered dramatically to compensate.
CONSTRUCTION
Keep the overall construction as light as possible. I weighed each
part as I built the Silencer, looking for areas where I could save
Type: CL Precision Aerobatics
Wingspan: 52 inches
Power: Plettenberg Orbit 15-18 motor
Flying weight: 44 ounces
Construction: Balsa and plywood
Covering/finish: Light-grade silkspan with Sig Litecoat
and Brodak modeling dopes
weight. Weight is the biggest concern with
electric power because you are “behind” from
the start; however, with the right wood
selection and a light finish, you can keep the
weight to a minimum.
Wing: The wing is built up and utilizes a
Warren truss-type ribbing scheme. The main
ribs are angled, and there are half ribs
between the opposing sets of ribs at the LE to
support the sheeting. The model has a 52-inch
wingspan and 510 square inches of wing area,
including the flaps. The wing panels are of
equal length.
I built the wing using Bob Hunt’s Lost
Foam wing-building system because it is one
of, if not the most, accurate ways to construct
a wing. It allows you to build the structure
extremely light and maintain its integrity, and
it provides the easiest and most accurate way
to produce the Warren-truss rib sets.
Using the Lost Foam method, you mark
the desired rib locations on the front and rear
of a foam blank that is cut to the planform of
the wing. Cut and sand the core, and mark the
rib locations chordwise on it using a ballpoint
pen. Also mark the spar location on the core
on the top and the bottom.
The rib locations are then accurately
scribed into the lower cradle half from which
the core was cut. This cradle is as accurate a
negative shape as the core is a positive shape,
and it can be used as a building fixture. The
core is sliced vertically at each rib station,
yielding perfectly accurate rib templates from
which balsa ribs can be generated.
Bob has produced a two-video set about
the Lost Foam wing-building system, and it
includes all of the information about how to
cut and prepare your own fixture sets. His
company—Robin’s View Productions—sells
the videos and offers a cutting service. He can
supply complete Lost Foam fixture sets for
this model and hundreds of others.
To help keep weight to a minimum, I used
4- to 6-pound, contest-grade wood
throughout. The flaps are made from 1⁄4-inch
straight C-grain balsa, with the grain
following the TE to help reduce the chance of
warps. The outboard flap is 1⁄8-inch wider at
the tip than the inboard flap is, to help the
inboard and outboard wing panels lift equally
in a turn.
The fuselage blankets the outboard wing
because the model is flying in a circle and is
angled somewhat tangent to the path of flight.
Therefore, the outboard wing and flap have
less effective area. The outboard flap’s larger
area helps the wing turn flat and without a
rolling tendency, even though there is less
airflow over it.
Tailplane: The stabilizer is 3⁄8-inch thick and
is built using a Warren truss-style
construction. The LE and TE are made from
1⁄4 x 3⁄8 balsa. I laminated the forward face of
the stabilizer TE with .008-inch carbon fiber
over the full span, and I used a double layer in
the center-section for added stiffness. The tips
are soft balsa, carved to shape and hollowed.
The elevators are 5⁄16-inch thick, and I built
them using a sheet of 1⁄16 balsa that was
shimmed 1⁄8 inch off the building board. The
LE and TE were glued to the 1⁄16 sheet, as
were the ribs. Then I flipped the elevator over
and glued the bottom ribs in place.
I capped the inside root edges of the
elevators with hard balsa to support the
elevator horn, and I capped the tips with soft
balsa and carved them to shape. The elevators
were then sanded and tapered to 1⁄8 inch at the
TE. Once they were completed, I went back
and removed the wood between each rib to
reduce the weight even further.
Fuselage: The fuselage is built with 1⁄16 Cgrain
sides. On the inside I doped on .5-ounce
carbon-fiber mat as a replacement for the
doublers. The motor mount is 1⁄8 aircraft
plywood with three holes drilled and lightly
countersunk in each side to allow for motor
cooling.
The motor mount sits roughly 1⁄8 inch
behind the front edge of the fuselage. The
overhanging fuselage sides act as a small
scoop to help guide air into the cooling holes.
I put a fillet of Aeropoxy Lite on the inside
and outside of the motor mount glue joint for
added strength and to help smooth the
airflow.
I covered the nose section with .75-ounce
fiberglass, making sure to wrap around the
front of the nose to reinforce the motor-mount
joint. I also reinforced the inside motor-mount
joint with .75-ounce fiberglass.
Final Assembly: Install the battery tray after
the wing is joined to the fuselage. Once the
wing is in place, its lower center-section needs
to be removed for battery clearance. During
wing construction, the lower bellcrank mount
must be sunk into the wing 5⁄8 inch so you
don’t sacrifice strength. Otherwise, this
section of the bellcrank mount would be
removed to provide clearance for the battery
tray.
I made the battery tray from three layers of
1⁄16 balsa with alternating grain, to make balsa
plywood. I laminated each layer together with
epoxy and .5-ounce carbon-fiber mat. I
epoxied this tray directly to the landing-gear
mount, the wing, and the lower bellcrank
mount, tying everything together. I used
Aeropoxy Lite to make fillets inside the
battery compartment and around the wing to
help blend and reinforce the joints.
I removed the lower wing center-section to
position the battery pack as close to the
airplane’s centerline as possible, in an effort
to keep the vertical CG in the proper location.
If the 14-ounce battery was placed too far
from the intended vertical CG, you could end
up with an airplane that would rock and roll as
speed changes were made during flight or
cause the outboard wing to fly high or low in
level flight, resulting in poor performance.
The battery pack tucked high into the
fuselage also allowed me to have a fuselage
with minimal side area. I wanted a model that
would fly well in light or heavy winds.
Airplanes with large fuselages or vertical
surfaces are usually affected more by the wind
or tend to “weather vane” during flight.
So far this design has proven to work
extremely well. Its first real test was at the
2004 Nats, where I flew it in winds exceeding
20 mph, gusting at times to more than 30
mph. This was an extreme case, and in winds
that high it’s difficult to get any airplane to
perform well. The Silencer made it through
the wind slowly at times, but I was able to
complete the pattern and land it safely.
Flying: The ready-to-fly weight came in at 44
ounces. This gives the Silencer a wing loading
of 11.59 ounces per square foot of wing area,
which is close to that of glow-powered Stunt
models. Performance so far has been better
than expected. As of this writing I have put
only 25 flights on the Silencer, so I need to do
more trim work to get it dialed in, but the
potential is surely there.
The propeller is turning out to be one of
the most important areas of trimming. First
flights yielded lap times in the mid- to lowfour-
second range with a 10 x 5 APC-E
propeller. The model is being flown on 19-
strand, .015 x 60-foot, eyelet-to-eyelet control
lines.
So far the best propeller for this model has
been a Graupner CAM 11 x 4 two-blade,
repitched to 11 x 3.8. On the same 60-foot
lines, I am now turning 5.1- to 5.2-second lap
times.
At launch the motor is pulling 32-34 amps
and spinning the 11 x 3.8 propeller roughly
11,800 rpm. This equates to approximately
450 watts in, or .6 horsepower, and roughly
382 watts out to the propeller, or .51 shaft
horsepower.
After the first flights, the battery
temperature was 100° and the motor
temperature was 140°, measured at the
windings. The motor temperature has to be
measured at the windings because the motor
case spins and cools more than the windings,
giving you a false reading. The Li-Poly
batteries should never exceed 140° during
discharge, and I have been told that brushless
motors can handle as much as roughly 200°
safely.
History: September 7, 2003, I competed in
the Bergen County CL contest and finished
with 497.5 points. This put me in eighth place
out of 17 entrants in Expert with my electric
Twister.
The following summer at the 2004 AMA
Nats, I flew the Silencer to sixth place out of
37 entrants. I also received the James A. Hunt
Technical Innovation Award for my
accomplishment.
Thanks: This project has turned out to be
more fun and rewarding than I ever
imagined it would be. I thank Castle
Creations and Thunder Power batteries for
their fantastic customer service, along with
everyone who has helped or supported me
throughout this project. Without them, it
wouldn’t be where it is today.
wouldn’t be where it is today.
I look forward to the future technology of
electric power and what it will bring us. I
know I will continue to enjoy developing
new electric-powered models, and I hope
you will too. MA
Mike Palko
121 N. 4th St.
Telford PA 18969
[email protected]
Sources:
Gen2 4S2P 4200 mAh Li-Poly pack:
Thunder Power
4720 W. University Ave.
Las Vegas NV 89103
(702) 228-8883
www.thunderpower-batteries.com
Plettenberg Orbit 15-18 motor:
ICARE
381 Joseph-Huet
Boucherville, Quebec, J4B 2C5 Canada
(450) 449-9094
www.icare-rc.com
Phoenix-45 ESC:
Castle Creations
402 E. Pendleton Ave.
Wellsville KS 66092
(785) 883-4519
www.castlecreations.com
Timer mentioned in text:
Sergio Zigras
171 Arundel Rd.
Paramus NJ 07652
Lost-Foam wing-building system fixtures,
video sets:
Robin’s View Productions
Box 68
Stockertown PA 18083
(610) 746-0106
Edition: Model Aviation - 2005/03
Page Numbers: 24,25,26,27,28,30.32
Photos by the author except as noted
24 MODEL AVIATION
by Mike Palko
I HAD MY first experience with electric flight when I was 10
years old. I broke my plastic CL RTF model after just a few flights,
and after that my dad and I decided to build an electric-powered RC
trainer together. I thought it was the coolest thing ever, because, as
with my RC cars, I just had to charge the batteries and turn the
model on to fly. But its flight performance wasn’t too successful,
and I quickly lost interest in it.
Shortly after that I was introduced to a club called the Philly
Fliers. I became interested in CL flying and haven’t looked back. It
wasn’t until 1997, at the age of 17, that I became interested in
electric flight again, and I built my first electric-powered Precision
Aerobatics (Stunt) model.
I had seen several electric CL models fly, but none of them
performed with enough authority or duration to be competitive. I’m
not sure what it was, but I was drawn to electric power and thought
it would be fun and challenging to see how well I could get an
electric Stunt model to perform.
I had no idea where to start, so I built a Sig Twister and used an
05 can motor with a seven-cell battery pack to get a base point. As I
worked on the project, I realized that I was not only developing a
power system that was new to CL, but one that showed some real
advantages in competition.
Some of the most important benefits were consistent motor runs,
no CG shift during flight as the fuel burned off, and never having
another over-run or an engine that wouldn’t start. In addition,
motors produce little vibration and leave no residue to soak into the
balsa and eventually ruin an otherwise good Stunt model.
There are also drawbacks to electric power, such as power-toweight
ratio, cost, and safety. But as technology advances, electric
power will continue to approach the performance of internalcombustion
engines, and the safety issues will be addressed.
I worked with the Twister on and off until 2003, when I reached
a point where I felt I was close to having a competitive Stunt
model. At that point I needed to take the next step and design a new
airplane from the ground up. The Twister flew great for an electricpowered
CL airplane—better than most people had ever seen such
a model fly—but it was still far from what I really wanted.
It was a profile, and it didn’t have the drive to get through high
winds. It was extremely close on run time, so if I missed the wind
and had to take an extra lap or two or got blown out of a maneuver,
it didn’t have the battery capacity to get through to the end of the
flight. The Twister was built so light that it was to the point of
being weak. I needed a new approach.
Power System: I felt that the Silencer would be the answer to the
problems I mentioned. Its design would incorporate a stronger,
more aerodynamic airframe and a new power system.
The biggest performance gain would be in the battery pack. I
switched from a Sanyo 10-cell 2600 mAh NiMH pack to a Thunder
Power (Gen2) 4S2P 4200 mAh Li-Poly pack. This is really two
2S2P packs wired in series because a 4S2P is not available off the
shelf.
This change alone would increase the battery capacity by roughly
two-thirds, increase the voltage by 2.8 volts, and drop the weight by
approximately 8 ounces! It would allow me to run a higher voltage
with a lower amp draw and maintain the same, if not increase the
setup’s, output in watts. It’s safer and more efficient to run lower
current with higher voltage because there is less heat buildup, which
also increases the life of the motor and the battery.
I also switched to a motor that was capable of turning a lowerpitch
propeller at a higher rpm. I replaced the AXI 2820/10 that I
had been using with a Plettenberg Orbit 15-18, which can handle
more voltage and higher current levels. I have found it to be more
An electric revolution in action. This is the most successful electric-powered CL Stunt model to date. John Glatthorn photo.
It’s hard to tell that the model is electric—until you realize that
there is no smoke trail from burnt fuel! Will Hubin photo.
Competitive electric-powered CL Precision Aerobatics becomes a reality
March 2005 25
With cowl and battery cover removed, it’s easy to see the system
components’ placement. Good airflow through this area is a must!
Power is the Plettenberg Orbit 15-18 motor, two Thunder Power
2S2P Li-Poly battery packs wired in series, a Castle Creations
Phoenix-45 ESC, and a Sergio Zigras timer.
Mike’s model has all of the normal flight-trim features, such as
this adjustable leadout guide. Neat workmanship!
The structure had to be light and strong. Mike used the Lost Foam
wing-building system to construct the rigid Warren truss-type wing.
efficient than the AXI by 8-10%. This would give me more usable
capacity from the battery, thus extending my flight times.
The AXI and the Orbit are brushless outrunners, or rotating can
motors. This means that the entire motor case spins, creating a power
plant that cools itself slightly and is capable of turning a large propeller
without using a gearbox because of its higher torque output.
This was important to me because a gearbox adds weight, creates
noise, reduces the strength of the power train—because it may strip or
wear out—and adds cost and complexity to the setup. This does not
mean that direct drive is the only way to go, but there were more
negatives than positives in using a gearbox for my application.
I had been using a Castle Creations Phoenix-35 ESC, which is
capable of handling 35 amps continuously. I knew I would be pulling
close to 35 amps, so I changed to a Phoenix-45 to increase the safety
margin. The Orbit runs at full throttle the entire flight, so the ESC has to
be able to handle the current and heat buildup for five to seven minutes
at a time.
However, an ESC does more than control the motor’s speed. The
Castle Creations ESC controls eight parameters of the motor, including
cutoff voltage and throttle type. These two factors are particularly
important because they will benefit the CL flier the most.
The cutoff-voltage feature is a must with Li-Poly batteries. With a 4S
battery pack, you need to set the cutoff voltage to 12. Li-Poly cells
should never be discharged to less than 3 volts per cell under load;
otherwise, permanent damage or fire could result. If the timer would
ever fail, the ESC would turn off the motor when the battery pack
reached 12 volts, saving the pack and airplane from possible damage.
With the cowling on, the model’s nose looks like a normal
“engined” Stunter. Note the cooling holes in the nose ring.
26 MODEL AVIATION
Does Mike look proud? He should! He has elevated the
performance of electric-powered models to a competitive level in
CL Stunt. Look for more from him! Hubin photo.
Dan Banjock launches Mike’s Silencer during 2004 Nats Advanced
competition. Mike finished a credible sixth. Hubin photo.
The sleek, clean model gives up nothing in aesthetics—or
performance—to glow-powered models of the same size and type.
The throttle type can be chosen from four settings. I have learned
that the high-rpm governor mode works best for our purposes. This
helps hold the motor at a constant rpm, preventing whip-up and
keeping a uniform speed throughout a maneuver, much like a tunedpipe
setup.
However, the ESC will not work without something telling it what
to do. To control the ESC, I am using a timer that Sergio Zigras
designed and built. The run time is adjustable from five to seven
minutes by turning a small speed potentiometer. The timer plugs into
the ESC and, when turned on, gives it a signal to arm and then slowly
ramps to full power. No external power is needed to run the timer. It
uses power from the Li-Poly pack.
As for the power system’s weight, the motor weighs 6.21 ounces,
the ESC weighs 1.06 ounces, the timer is negligible at .07 ounce, and
the battery weighs 14.18 ounces. This is a total of 21.52 ounces
including all connectors.
It seems heavy, but with attention to detail and the lack of plywood
doublers, engine beams, and crutch, the bare airframe’s weight can be
lowered dramatically to compensate.
CONSTRUCTION
Keep the overall construction as light as possible. I weighed each
part as I built the Silencer, looking for areas where I could save
Type: CL Precision Aerobatics
Wingspan: 52 inches
Power: Plettenberg Orbit 15-18 motor
Flying weight: 44 ounces
Construction: Balsa and plywood
Covering/finish: Light-grade silkspan with Sig Litecoat
and Brodak modeling dopes
weight. Weight is the biggest concern with
electric power because you are “behind” from
the start; however, with the right wood
selection and a light finish, you can keep the
weight to a minimum.
Wing: The wing is built up and utilizes a
Warren truss-type ribbing scheme. The main
ribs are angled, and there are half ribs
between the opposing sets of ribs at the LE to
support the sheeting. The model has a 52-inch
wingspan and 510 square inches of wing area,
including the flaps. The wing panels are of
equal length.
I built the wing using Bob Hunt’s Lost
Foam wing-building system because it is one
of, if not the most, accurate ways to construct
a wing. It allows you to build the structure
extremely light and maintain its integrity, and
it provides the easiest and most accurate way
to produce the Warren-truss rib sets.
Using the Lost Foam method, you mark
the desired rib locations on the front and rear
of a foam blank that is cut to the planform of
the wing. Cut and sand the core, and mark the
rib locations chordwise on it using a ballpoint
pen. Also mark the spar location on the core
on the top and the bottom.
The rib locations are then accurately
scribed into the lower cradle half from which
the core was cut. This cradle is as accurate a
negative shape as the core is a positive shape,
and it can be used as a building fixture. The
core is sliced vertically at each rib station,
yielding perfectly accurate rib templates from
which balsa ribs can be generated.
Bob has produced a two-video set about
the Lost Foam wing-building system, and it
includes all of the information about how to
cut and prepare your own fixture sets. His
company—Robin’s View Productions—sells
the videos and offers a cutting service. He can
supply complete Lost Foam fixture sets for
this model and hundreds of others.
To help keep weight to a minimum, I used
4- to 6-pound, contest-grade wood
throughout. The flaps are made from 1⁄4-inch
straight C-grain balsa, with the grain
following the TE to help reduce the chance of
warps. The outboard flap is 1⁄8-inch wider at
the tip than the inboard flap is, to help the
inboard and outboard wing panels lift equally
in a turn.
The fuselage blankets the outboard wing
because the model is flying in a circle and is
angled somewhat tangent to the path of flight.
Therefore, the outboard wing and flap have
less effective area. The outboard flap’s larger
area helps the wing turn flat and without a
rolling tendency, even though there is less
airflow over it.
Tailplane: The stabilizer is 3⁄8-inch thick and
is built using a Warren truss-style
construction. The LE and TE are made from
1⁄4 x 3⁄8 balsa. I laminated the forward face of
the stabilizer TE with .008-inch carbon fiber
over the full span, and I used a double layer in
the center-section for added stiffness. The tips
are soft balsa, carved to shape and hollowed.
The elevators are 5⁄16-inch thick, and I built
them using a sheet of 1⁄16 balsa that was
shimmed 1⁄8 inch off the building board. The
LE and TE were glued to the 1⁄16 sheet, as
were the ribs. Then I flipped the elevator over
and glued the bottom ribs in place.
I capped the inside root edges of the
elevators with hard balsa to support the
elevator horn, and I capped the tips with soft
balsa and carved them to shape. The elevators
were then sanded and tapered to 1⁄8 inch at the
TE. Once they were completed, I went back
and removed the wood between each rib to
reduce the weight even further.
Fuselage: The fuselage is built with 1⁄16 Cgrain
sides. On the inside I doped on .5-ounce
carbon-fiber mat as a replacement for the
doublers. The motor mount is 1⁄8 aircraft
plywood with three holes drilled and lightly
countersunk in each side to allow for motor
cooling.
The motor mount sits roughly 1⁄8 inch
behind the front edge of the fuselage. The
overhanging fuselage sides act as a small
scoop to help guide air into the cooling holes.
I put a fillet of Aeropoxy Lite on the inside
and outside of the motor mount glue joint for
added strength and to help smooth the
airflow.
I covered the nose section with .75-ounce
fiberglass, making sure to wrap around the
front of the nose to reinforce the motor-mount
joint. I also reinforced the inside motor-mount
joint with .75-ounce fiberglass.
Final Assembly: Install the battery tray after
the wing is joined to the fuselage. Once the
wing is in place, its lower center-section needs
to be removed for battery clearance. During
wing construction, the lower bellcrank mount
must be sunk into the wing 5⁄8 inch so you
don’t sacrifice strength. Otherwise, this
section of the bellcrank mount would be
removed to provide clearance for the battery
tray.
I made the battery tray from three layers of
1⁄16 balsa with alternating grain, to make balsa
plywood. I laminated each layer together with
epoxy and .5-ounce carbon-fiber mat. I
epoxied this tray directly to the landing-gear
mount, the wing, and the lower bellcrank
mount, tying everything together. I used
Aeropoxy Lite to make fillets inside the
battery compartment and around the wing to
help blend and reinforce the joints.
I removed the lower wing center-section to
position the battery pack as close to the
airplane’s centerline as possible, in an effort
to keep the vertical CG in the proper location.
If the 14-ounce battery was placed too far
from the intended vertical CG, you could end
up with an airplane that would rock and roll as
speed changes were made during flight or
cause the outboard wing to fly high or low in
level flight, resulting in poor performance.
The battery pack tucked high into the
fuselage also allowed me to have a fuselage
with minimal side area. I wanted a model that
would fly well in light or heavy winds.
Airplanes with large fuselages or vertical
surfaces are usually affected more by the wind
or tend to “weather vane” during flight.
So far this design has proven to work
extremely well. Its first real test was at the
2004 Nats, where I flew it in winds exceeding
20 mph, gusting at times to more than 30
mph. This was an extreme case, and in winds
that high it’s difficult to get any airplane to
perform well. The Silencer made it through
the wind slowly at times, but I was able to
complete the pattern and land it safely.
Flying: The ready-to-fly weight came in at 44
ounces. This gives the Silencer a wing loading
of 11.59 ounces per square foot of wing area,
which is close to that of glow-powered Stunt
models. Performance so far has been better
than expected. As of this writing I have put
only 25 flights on the Silencer, so I need to do
more trim work to get it dialed in, but the
potential is surely there.
The propeller is turning out to be one of
the most important areas of trimming. First
flights yielded lap times in the mid- to lowfour-
second range with a 10 x 5 APC-E
propeller. The model is being flown on 19-
strand, .015 x 60-foot, eyelet-to-eyelet control
lines.
So far the best propeller for this model has
been a Graupner CAM 11 x 4 two-blade,
repitched to 11 x 3.8. On the same 60-foot
lines, I am now turning 5.1- to 5.2-second lap
times.
At launch the motor is pulling 32-34 amps
and spinning the 11 x 3.8 propeller roughly
11,800 rpm. This equates to approximately
450 watts in, or .6 horsepower, and roughly
382 watts out to the propeller, or .51 shaft
horsepower.
After the first flights, the battery
temperature was 100° and the motor
temperature was 140°, measured at the
windings. The motor temperature has to be
measured at the windings because the motor
case spins and cools more than the windings,
giving you a false reading. The Li-Poly
batteries should never exceed 140° during
discharge, and I have been told that brushless
motors can handle as much as roughly 200°
safely.
History: September 7, 2003, I competed in
the Bergen County CL contest and finished
with 497.5 points. This put me in eighth place
out of 17 entrants in Expert with my electric
Twister.
The following summer at the 2004 AMA
Nats, I flew the Silencer to sixth place out of
37 entrants. I also received the James A. Hunt
Technical Innovation Award for my
accomplishment.
Thanks: This project has turned out to be
more fun and rewarding than I ever
imagined it would be. I thank Castle
Creations and Thunder Power batteries for
their fantastic customer service, along with
everyone who has helped or supported me
throughout this project. Without them, it
wouldn’t be where it is today.
wouldn’t be where it is today.
I look forward to the future technology of
electric power and what it will bring us. I
know I will continue to enjoy developing
new electric-powered models, and I hope
you will too. MA
Mike Palko
121 N. 4th St.
Telford PA 18969
[email protected]
Sources:
Gen2 4S2P 4200 mAh Li-Poly pack:
Thunder Power
4720 W. University Ave.
Las Vegas NV 89103
(702) 228-8883
www.thunderpower-batteries.com
Plettenberg Orbit 15-18 motor:
ICARE
381 Joseph-Huet
Boucherville, Quebec, J4B 2C5 Canada
(450) 449-9094
www.icare-rc.com
Phoenix-45 ESC:
Castle Creations
402 E. Pendleton Ave.
Wellsville KS 66092
(785) 883-4519
www.castlecreations.com
Timer mentioned in text:
Sergio Zigras
171 Arundel Rd.
Paramus NJ 07652
Lost-Foam wing-building system fixtures,
video sets:
Robin’s View Productions
Box 68
Stockertown PA 18083
(610) 746-0106
Edition: Model Aviation - 2005/03
Page Numbers: 24,25,26,27,28,30.32
Photos by the author except as noted
24 MODEL AVIATION
by Mike Palko
I HAD MY first experience with electric flight when I was 10
years old. I broke my plastic CL RTF model after just a few flights,
and after that my dad and I decided to build an electric-powered RC
trainer together. I thought it was the coolest thing ever, because, as
with my RC cars, I just had to charge the batteries and turn the
model on to fly. But its flight performance wasn’t too successful,
and I quickly lost interest in it.
Shortly after that I was introduced to a club called the Philly
Fliers. I became interested in CL flying and haven’t looked back. It
wasn’t until 1997, at the age of 17, that I became interested in
electric flight again, and I built my first electric-powered Precision
Aerobatics (Stunt) model.
I had seen several electric CL models fly, but none of them
performed with enough authority or duration to be competitive. I’m
not sure what it was, but I was drawn to electric power and thought
it would be fun and challenging to see how well I could get an
electric Stunt model to perform.
I had no idea where to start, so I built a Sig Twister and used an
05 can motor with a seven-cell battery pack to get a base point. As I
worked on the project, I realized that I was not only developing a
power system that was new to CL, but one that showed some real
advantages in competition.
Some of the most important benefits were consistent motor runs,
no CG shift during flight as the fuel burned off, and never having
another over-run or an engine that wouldn’t start. In addition,
motors produce little vibration and leave no residue to soak into the
balsa and eventually ruin an otherwise good Stunt model.
There are also drawbacks to electric power, such as power-toweight
ratio, cost, and safety. But as technology advances, electric
power will continue to approach the performance of internalcombustion
engines, and the safety issues will be addressed.
I worked with the Twister on and off until 2003, when I reached
a point where I felt I was close to having a competitive Stunt
model. At that point I needed to take the next step and design a new
airplane from the ground up. The Twister flew great for an electricpowered
CL airplane—better than most people had ever seen such
a model fly—but it was still far from what I really wanted.
It was a profile, and it didn’t have the drive to get through high
winds. It was extremely close on run time, so if I missed the wind
and had to take an extra lap or two or got blown out of a maneuver,
it didn’t have the battery capacity to get through to the end of the
flight. The Twister was built so light that it was to the point of
being weak. I needed a new approach.
Power System: I felt that the Silencer would be the answer to the
problems I mentioned. Its design would incorporate a stronger,
more aerodynamic airframe and a new power system.
The biggest performance gain would be in the battery pack. I
switched from a Sanyo 10-cell 2600 mAh NiMH pack to a Thunder
Power (Gen2) 4S2P 4200 mAh Li-Poly pack. This is really two
2S2P packs wired in series because a 4S2P is not available off the
shelf.
This change alone would increase the battery capacity by roughly
two-thirds, increase the voltage by 2.8 volts, and drop the weight by
approximately 8 ounces! It would allow me to run a higher voltage
with a lower amp draw and maintain the same, if not increase the
setup’s, output in watts. It’s safer and more efficient to run lower
current with higher voltage because there is less heat buildup, which
also increases the life of the motor and the battery.
I also switched to a motor that was capable of turning a lowerpitch
propeller at a higher rpm. I replaced the AXI 2820/10 that I
had been using with a Plettenberg Orbit 15-18, which can handle
more voltage and higher current levels. I have found it to be more
An electric revolution in action. This is the most successful electric-powered CL Stunt model to date. John Glatthorn photo.
It’s hard to tell that the model is electric—until you realize that
there is no smoke trail from burnt fuel! Will Hubin photo.
Competitive electric-powered CL Precision Aerobatics becomes a reality
March 2005 25
With cowl and battery cover removed, it’s easy to see the system
components’ placement. Good airflow through this area is a must!
Power is the Plettenberg Orbit 15-18 motor, two Thunder Power
2S2P Li-Poly battery packs wired in series, a Castle Creations
Phoenix-45 ESC, and a Sergio Zigras timer.
Mike’s model has all of the normal flight-trim features, such as
this adjustable leadout guide. Neat workmanship!
The structure had to be light and strong. Mike used the Lost Foam
wing-building system to construct the rigid Warren truss-type wing.
efficient than the AXI by 8-10%. This would give me more usable
capacity from the battery, thus extending my flight times.
The AXI and the Orbit are brushless outrunners, or rotating can
motors. This means that the entire motor case spins, creating a power
plant that cools itself slightly and is capable of turning a large propeller
without using a gearbox because of its higher torque output.
This was important to me because a gearbox adds weight, creates
noise, reduces the strength of the power train—because it may strip or
wear out—and adds cost and complexity to the setup. This does not
mean that direct drive is the only way to go, but there were more
negatives than positives in using a gearbox for my application.
I had been using a Castle Creations Phoenix-35 ESC, which is
capable of handling 35 amps continuously. I knew I would be pulling
close to 35 amps, so I changed to a Phoenix-45 to increase the safety
margin. The Orbit runs at full throttle the entire flight, so the ESC has to
be able to handle the current and heat buildup for five to seven minutes
at a time.
However, an ESC does more than control the motor’s speed. The
Castle Creations ESC controls eight parameters of the motor, including
cutoff voltage and throttle type. These two factors are particularly
important because they will benefit the CL flier the most.
The cutoff-voltage feature is a must with Li-Poly batteries. With a 4S
battery pack, you need to set the cutoff voltage to 12. Li-Poly cells
should never be discharged to less than 3 volts per cell under load;
otherwise, permanent damage or fire could result. If the timer would
ever fail, the ESC would turn off the motor when the battery pack
reached 12 volts, saving the pack and airplane from possible damage.
With the cowling on, the model’s nose looks like a normal
“engined” Stunter. Note the cooling holes in the nose ring.
26 MODEL AVIATION
Does Mike look proud? He should! He has elevated the
performance of electric-powered models to a competitive level in
CL Stunt. Look for more from him! Hubin photo.
Dan Banjock launches Mike’s Silencer during 2004 Nats Advanced
competition. Mike finished a credible sixth. Hubin photo.
The sleek, clean model gives up nothing in aesthetics—or
performance—to glow-powered models of the same size and type.
The throttle type can be chosen from four settings. I have learned
that the high-rpm governor mode works best for our purposes. This
helps hold the motor at a constant rpm, preventing whip-up and
keeping a uniform speed throughout a maneuver, much like a tunedpipe
setup.
However, the ESC will not work without something telling it what
to do. To control the ESC, I am using a timer that Sergio Zigras
designed and built. The run time is adjustable from five to seven
minutes by turning a small speed potentiometer. The timer plugs into
the ESC and, when turned on, gives it a signal to arm and then slowly
ramps to full power. No external power is needed to run the timer. It
uses power from the Li-Poly pack.
As for the power system’s weight, the motor weighs 6.21 ounces,
the ESC weighs 1.06 ounces, the timer is negligible at .07 ounce, and
the battery weighs 14.18 ounces. This is a total of 21.52 ounces
including all connectors.
It seems heavy, but with attention to detail and the lack of plywood
doublers, engine beams, and crutch, the bare airframe’s weight can be
lowered dramatically to compensate.
CONSTRUCTION
Keep the overall construction as light as possible. I weighed each
part as I built the Silencer, looking for areas where I could save
Type: CL Precision Aerobatics
Wingspan: 52 inches
Power: Plettenberg Orbit 15-18 motor
Flying weight: 44 ounces
Construction: Balsa and plywood
Covering/finish: Light-grade silkspan with Sig Litecoat
and Brodak modeling dopes
weight. Weight is the biggest concern with
electric power because you are “behind” from
the start; however, with the right wood
selection and a light finish, you can keep the
weight to a minimum.
Wing: The wing is built up and utilizes a
Warren truss-type ribbing scheme. The main
ribs are angled, and there are half ribs
between the opposing sets of ribs at the LE to
support the sheeting. The model has a 52-inch
wingspan and 510 square inches of wing area,
including the flaps. The wing panels are of
equal length.
I built the wing using Bob Hunt’s Lost
Foam wing-building system because it is one
of, if not the most, accurate ways to construct
a wing. It allows you to build the structure
extremely light and maintain its integrity, and
it provides the easiest and most accurate way
to produce the Warren-truss rib sets.
Using the Lost Foam method, you mark
the desired rib locations on the front and rear
of a foam blank that is cut to the planform of
the wing. Cut and sand the core, and mark the
rib locations chordwise on it using a ballpoint
pen. Also mark the spar location on the core
on the top and the bottom.
The rib locations are then accurately
scribed into the lower cradle half from which
the core was cut. This cradle is as accurate a
negative shape as the core is a positive shape,
and it can be used as a building fixture. The
core is sliced vertically at each rib station,
yielding perfectly accurate rib templates from
which balsa ribs can be generated.
Bob has produced a two-video set about
the Lost Foam wing-building system, and it
includes all of the information about how to
cut and prepare your own fixture sets. His
company—Robin’s View Productions—sells
the videos and offers a cutting service. He can
supply complete Lost Foam fixture sets for
this model and hundreds of others.
To help keep weight to a minimum, I used
4- to 6-pound, contest-grade wood
throughout. The flaps are made from 1⁄4-inch
straight C-grain balsa, with the grain
following the TE to help reduce the chance of
warps. The outboard flap is 1⁄8-inch wider at
the tip than the inboard flap is, to help the
inboard and outboard wing panels lift equally
in a turn.
The fuselage blankets the outboard wing
because the model is flying in a circle and is
angled somewhat tangent to the path of flight.
Therefore, the outboard wing and flap have
less effective area. The outboard flap’s larger
area helps the wing turn flat and without a
rolling tendency, even though there is less
airflow over it.
Tailplane: The stabilizer is 3⁄8-inch thick and
is built using a Warren truss-style
construction. The LE and TE are made from
1⁄4 x 3⁄8 balsa. I laminated the forward face of
the stabilizer TE with .008-inch carbon fiber
over the full span, and I used a double layer in
the center-section for added stiffness. The tips
are soft balsa, carved to shape and hollowed.
The elevators are 5⁄16-inch thick, and I built
them using a sheet of 1⁄16 balsa that was
shimmed 1⁄8 inch off the building board. The
LE and TE were glued to the 1⁄16 sheet, as
were the ribs. Then I flipped the elevator over
and glued the bottom ribs in place.
I capped the inside root edges of the
elevators with hard balsa to support the
elevator horn, and I capped the tips with soft
balsa and carved them to shape. The elevators
were then sanded and tapered to 1⁄8 inch at the
TE. Once they were completed, I went back
and removed the wood between each rib to
reduce the weight even further.
Fuselage: The fuselage is built with 1⁄16 Cgrain
sides. On the inside I doped on .5-ounce
carbon-fiber mat as a replacement for the
doublers. The motor mount is 1⁄8 aircraft
plywood with three holes drilled and lightly
countersunk in each side to allow for motor
cooling.
The motor mount sits roughly 1⁄8 inch
behind the front edge of the fuselage. The
overhanging fuselage sides act as a small
scoop to help guide air into the cooling holes.
I put a fillet of Aeropoxy Lite on the inside
and outside of the motor mount glue joint for
added strength and to help smooth the
airflow.
I covered the nose section with .75-ounce
fiberglass, making sure to wrap around the
front of the nose to reinforce the motor-mount
joint. I also reinforced the inside motor-mount
joint with .75-ounce fiberglass.
Final Assembly: Install the battery tray after
the wing is joined to the fuselage. Once the
wing is in place, its lower center-section needs
to be removed for battery clearance. During
wing construction, the lower bellcrank mount
must be sunk into the wing 5⁄8 inch so you
don’t sacrifice strength. Otherwise, this
section of the bellcrank mount would be
removed to provide clearance for the battery
tray.
I made the battery tray from three layers of
1⁄16 balsa with alternating grain, to make balsa
plywood. I laminated each layer together with
epoxy and .5-ounce carbon-fiber mat. I
epoxied this tray directly to the landing-gear
mount, the wing, and the lower bellcrank
mount, tying everything together. I used
Aeropoxy Lite to make fillets inside the
battery compartment and around the wing to
help blend and reinforce the joints.
I removed the lower wing center-section to
position the battery pack as close to the
airplane’s centerline as possible, in an effort
to keep the vertical CG in the proper location.
If the 14-ounce battery was placed too far
from the intended vertical CG, you could end
up with an airplane that would rock and roll as
speed changes were made during flight or
cause the outboard wing to fly high or low in
level flight, resulting in poor performance.
The battery pack tucked high into the
fuselage also allowed me to have a fuselage
with minimal side area. I wanted a model that
would fly well in light or heavy winds.
Airplanes with large fuselages or vertical
surfaces are usually affected more by the wind
or tend to “weather vane” during flight.
So far this design has proven to work
extremely well. Its first real test was at the
2004 Nats, where I flew it in winds exceeding
20 mph, gusting at times to more than 30
mph. This was an extreme case, and in winds
that high it’s difficult to get any airplane to
perform well. The Silencer made it through
the wind slowly at times, but I was able to
complete the pattern and land it safely.
Flying: The ready-to-fly weight came in at 44
ounces. This gives the Silencer a wing loading
of 11.59 ounces per square foot of wing area,
which is close to that of glow-powered Stunt
models. Performance so far has been better
than expected. As of this writing I have put
only 25 flights on the Silencer, so I need to do
more trim work to get it dialed in, but the
potential is surely there.
The propeller is turning out to be one of
the most important areas of trimming. First
flights yielded lap times in the mid- to lowfour-
second range with a 10 x 5 APC-E
propeller. The model is being flown on 19-
strand, .015 x 60-foot, eyelet-to-eyelet control
lines.
So far the best propeller for this model has
been a Graupner CAM 11 x 4 two-blade,
repitched to 11 x 3.8. On the same 60-foot
lines, I am now turning 5.1- to 5.2-second lap
times.
At launch the motor is pulling 32-34 amps
and spinning the 11 x 3.8 propeller roughly
11,800 rpm. This equates to approximately
450 watts in, or .6 horsepower, and roughly
382 watts out to the propeller, or .51 shaft
horsepower.
After the first flights, the battery
temperature was 100° and the motor
temperature was 140°, measured at the
windings. The motor temperature has to be
measured at the windings because the motor
case spins and cools more than the windings,
giving you a false reading. The Li-Poly
batteries should never exceed 140° during
discharge, and I have been told that brushless
motors can handle as much as roughly 200°
safely.
History: September 7, 2003, I competed in
the Bergen County CL contest and finished
with 497.5 points. This put me in eighth place
out of 17 entrants in Expert with my electric
Twister.
The following summer at the 2004 AMA
Nats, I flew the Silencer to sixth place out of
37 entrants. I also received the James A. Hunt
Technical Innovation Award for my
accomplishment.
Thanks: This project has turned out to be
more fun and rewarding than I ever
imagined it would be. I thank Castle
Creations and Thunder Power batteries for
their fantastic customer service, along with
everyone who has helped or supported me
throughout this project. Without them, it
wouldn’t be where it is today.
wouldn’t be where it is today.
I look forward to the future technology of
electric power and what it will bring us. I
know I will continue to enjoy developing
new electric-powered models, and I hope
you will too. MA
Mike Palko
121 N. 4th St.
Telford PA 18969
[email protected]
Sources:
Gen2 4S2P 4200 mAh Li-Poly pack:
Thunder Power
4720 W. University Ave.
Las Vegas NV 89103
(702) 228-8883
www.thunderpower-batteries.com
Plettenberg Orbit 15-18 motor:
ICARE
381 Joseph-Huet
Boucherville, Quebec, J4B 2C5 Canada
(450) 449-9094
www.icare-rc.com
Phoenix-45 ESC:
Castle Creations
402 E. Pendleton Ave.
Wellsville KS 66092
(785) 883-4519
www.castlecreations.com
Timer mentioned in text:
Sergio Zigras
171 Arundel Rd.
Paramus NJ 07652
Lost-Foam wing-building system fixtures,
video sets:
Robin’s View Productions
Box 68
Stockertown PA 18083
(610) 746-0106