May 2007 65
Plane Talk: ElectriFly Siren EP Hotliner
BY PETER OESTERWINTER
THERE HAS BEEN a debate in some RC
circles recently about whether or not the
Great Planes Siren is a true “hotliner.” Some
people have even affectionately referred to
it as a “warmliner” since its performance is
somewhere between that of a true hotliner
and electric-powered sailplane.
So what exactly is a hotliner, anyway?
It’s an electric-powered sailplane that
uses an extremely powerful motor and highoutput
battery to achieve a rapid vertical
climb to altitude; you could even call it
“extreme climb.” After reaching its apex the
motor is shut down, the propeller folds
back, and the model flies as does any RC
sailplane—only faster because of its higher
wing loading and clean aerodynamics.
The wow factor is huge with these types
of radio-controlled aircraft because of their
extreme climb capability, high speed, and
aerobatic performance. Climbs should be
limited to 10- to 20-second bursts since
some of these motors can draw in upward of
80 amps. That’s nearly five times the
amperage of a household circuit breaker—
an amazing amount of power
So the question remains. Is the Siren a
true hotliner or a warmliner? It can be
answered by an individual’s choice of
power. When fitted with the right brushless
motor-and-battery combination, the Siren
will deliver true hotliner performance and
thrills to match.
Above: The fuselage is carbon fiber that is painted in the
mold. The wings and stabilizer are fully sheeted wood
construction with film covering.
Four-year-old Joshua Oesterwinter sizes up his father’s new
Great Planes Siren electric-powered hotliner.
The Kontronik motor system’s high output
allows the Siren to reach high altitudes in the
blink of an eye. It can cruise and fly like a sport model or thermal like a typical sailplane.
A high-tech RC sailplane that
doubles as a rocket ship
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66 MODEL AVIATION
The Siren’s clean lines allow it to stretch its glide for landing. The
APC folding propeller system keeps the blades tucked away safely.
An air vent on top of the fuselage, aft of the wing mounting bolt,
allows heat from the motor, ESC, and battery to escape.
The three-piece wing is joined with epoxy and supported with two
laminated light-plywood dihedral braces.
The author added an extra layer of 1/16 balsa to the wing rather
than attach the servo directly to the upper balsa wing sheeting.
Great Planes suggests several power
options, with one for every pocketbook.
They range from a brushed motor and Ni-Cd
pack to a brushless motor and NiMH pack,
all the way to a brushless motor and Li-Poly
pack. Performance will vary based on what
option is chosen.
The Siren in this review was run with
power chosen from the middle of the range
of options. I used a Kontronik Fun 480
package for power with a 10-cell, 2200 mAh
NiMH pack by Diversity Model Aircraft.
The Fun 480 package features a
powerful brushless 480-33 motor coupled to
a 4.2:1 planetary gearbox and the Jazz 40-6-
18 40-amp ESC. Swinging an APC 13 x 7
propeller, this combination provided
hotliner performance.
The Great Planes Siren offers the RC
enthusiast an excellent, low-priced way to
get into this spectacular branch of electricpowered
RC flight. This airplane’s
engineering and quality is first-rate, and the
look of the carbon-fiber fuselage is
stunning.
The Siren is well packed and has a low
parts count. Most of the work is done,
allowing for quick final assembly.
The model comes with five major
components: a carbon-fiber fuselage that is
prepainted in a smart-looking white-onblack
(carbon fiber is left visible) scheme
with red trim, the stabilizer, the center wing
panel, and two outer wing panels. The top of
the wings is white and the bottom is bright
red. Wing construction is traditional wood
with D-tube construction, covered in a heatsensitive
film.
All parts are individually bagged and
arrived in flawless condition. The control
surfaces are prehinged, but the hinge tape
does require careful trimming with a sharp,
new #11 blade to remove some bits that
overlapped.
The instruction manual is complete,
detailed, and well documented, with only a
few areas that are a bit difficult to
understand. A complete hardware package is
included.
Construction: As with an ARF, check all
the seams where the covering may need to
be resealed. The Siren’s construction begins
with the outer wing panels. Five precut 1/16
plywood plates are stacked together and
glued to form the wing joiner. This is a
messy procedure, and wearing disposable
gloves is highly recommended.
Aligning all five pieces and keeping
them in place while the slippery epoxy set
proved to be challenging, so I strayed from
the instructions telling the builder to glue
them together before proceeding.
Notice that the thicker half of the joiner
plate goes into the outer wing panel. Once
cured, test-fit the joiners in their respective
openings in the inner wing panel. Some
sanding will most likely be required.
I glued the plywood plates together per
the instructions, but then I immediately
glued the thicker end of the plates into the
outer wing panel. This automatically aligned
all five plates, and all that was left was to
tightly wrap the opposite end in rubber
bands while the epoxy set.
Rubber bands held the plates together
much better than a clamp because they
provided uniform tension. Either way, keep
plenty of paper towels and denatured
alcohol around for cleanup.
Before joining the outer panels to the
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May 2007 67
The 10-cell Zapped 2200 mAh battery from Diversity Model Aircraft is a tight fit but is
the perfect complement to the Kontronik Jazz motor system.
Plastic servo covers, painted to match, conceal the servo and aileron linkage.
Masking tape is applied around the edge of
the wing-panel glue joints. Excess epoxy is
removed along with the tape when the
structure is cured.
The supplied pushrod should be bent to the shape shown and
installed with the horizontal stabilizer.
A carbon-fiber hatch cover hides the elevator servo mounted into
the vertical stabilizer.
section, remove the covering from the
servo bay and draw the guide string into
the servo bay, which is taped to the centersection.
I was a bit confused here because
the instructions were to “install a 24-inch
extension through the wing centersection.”
Nothing more.
I believe that meant the guide string—
not the servo extension since the servo
extension leads need to be drawn from the
servo bays into the center-section. Since
only one guide string was provided, I
routed the servo extension through the
servo bay in the outer panel, attached it to
the guide string, and carefully pulled it
through the wing center-section.
If you plan to use extensions with
connectors, don’t; cut them off. The
cutouts in the ribs for the servo extensions
are oval, and it would be nearly impossible
to guide the connectors through them.
Instead, splice the servo leads together by
soldering the wires and sealing them with
heat-shrink tubing.
With the servo extensions successfully
installed and pulled through the wing, glue
the outer panels to the wing center-section.
An easy way to keep the epoxy from
oozing onto the wing panels is to wrap
low-tack masking tape around the panels
just outside the glue joint. This way, if the
epoxy oozes it will only get on the tape
and will be easy to remove.
The instructions call for the builder to
glue the servos directly into the servo
bays. An excellent tip is provided—to
wrap the servo in heat shrink, preventing it
from being ruined by the epoxy—but I felt
uncomfortable gluing wood to plastic in
such a large and fast airplane. This issue
will be addressed later with the elevator,
with greater emphasis on why this is a
personal safety issue.
The 1/16-inch top sheeting was made
from light-grade balsa. If the glued-in
servos needed to be removed for any
reason, doing so would surely take the
balsa sheeting along with the servo.
Instead I created a servo tray using 1/16
light plywood and 1/4 x 1/2 spruce, which
allows for easy extraction of the servos for
repair or adjustment.
I cut the tray to 17/8 x 11/2 inches,
which fits perfectly inside the servo wells.
Photos by the author; flight photos by Mario Meise
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68 MODEL AVIATION
Motor: Kontronik Fun 480-33 motor
with 4.2:1 gear ratio
Battery: Diversity Model Aircraft 2200
mAh, 10-cell NiMH
Propeller: Folding APC 13 x 7
Radio system: Futaba 6XAS
transmitter, Futaba FP-R127DF receiver,
three Futaba S3107 servos, three 24-inch
extensions, one Y harness, Jazz 40-6-18
40-amp speed controller
Ready-to-fly weight: 3.2 pounds, with
battery
Flight duration: Five to 30 minutes
(depending on flying style)
Test-Model Details
Model type: Electric-powered ARF
Pilot skill level: Intermediate pilots
Wingspan: 79 inches
Wing area: 792 square inches
Length: 38.5 inches
Weight: 32.5 ounces, minus battery
Wing loading: 9.1 ounces per square
foot, minus battery
Recommended power: Speed 600
brushed direct-drive system (minimum)
Radio: Three channels (minimum), three
microservos
Construction: Carbon-fiber molded
fuselage, built-up wooden wing and
horizontal stabilizer
Covering/finish: Painted-in-the-mold
fuselage, iron-on film for wing and
stabilizer
Price: $149.99
Specifications
+
• Excellent quality, parts fit, and finish.
• Rapid assembly.
• Excellent instructions.
• Wide choice of power options.
• Superb flight performance. -•
Recommended servo-mounting method
is not secure enough on an aircraft this
large.
Pluses and Minuses
Notching the spruce to tuck under the
lower sheeting allowed the servo tray to
slide directly into place.
A minor drawback to this method is that
the rear of the servo protrudes from the
servo bay just less than 1/16 inch. However,
the provided servo cover hides that well.
That is a fair compromise to make the
servo removable and avoid the safety
concerns.
Installing the stabilizer was a breeze,
and it was a first-time perfect fit. The top
half of the control horn was cut off to allow
for clearance because the opening on top of
the vertical stabilizer was not very large. I
needed to use a rotary tool with a grinder to
open the area enough to allow for the
control horn and pushrod to travel.
When mounting the control horn to the
elevator, do so with the stabilizer attached
to the fin. This will allow you to center the
control horn to the opening where there’s
not much room.
The next step is to install the elevator
servo. Here we revisit the issue of gluing
the servo directly to the aircraft. This
technique has become popular recently
with small park flyers, which are often
made from foam and are relatively slow
and light. The method works well in those
cases even though it can leave the servo
case messy and scarred for life.
However, the Siren is not a park flyer; it
is a high-performance hotliner that is
capable of reaching speeds approaching
100 mph. Furthermore, gluing a small,
rigid servo to a carbon-fiber fuselage that
can and will flex like plastic and fiberglass
promotes glue-joint failure. Because of my
experience I believe this installation
method isn’t the best idea for this model.
To stiffen the mounting area I cut two
servo rails from 1/4 x 1/2 spruce to fit inside
the vertical stabilizer, flush with the servo.
The support of the rails prevents flexing. I
mounted the elevator servo to these rails
with servo screws and glued the entire
assembly (yes, the servo too) into the
cutout. Prepare the pushrod and set the
servo first; once the glue sets there will be
no way to mechanically adjust the servo, so
be sure it has been centered with the radio.
Through the opposite side of the fin I
installed a servo screw into each spruce rail
to keep the entire assembly secure and in
place.
A circular carbon-fiber servo cover is
provided, which is taped over the opening.
To make it look cleaner I examined the
weave direction of the carbon fiber in the
cover and positioned it to match the weave
of the carbon fiber in the fuselage.
Motor installation is the final step in
preparing the Siren for flight. The ESC is
connected to the motor, and then, holding
the motor by the leads, guide the motor to
the mount. Once the motor is aligned
properly, install the two motor mounting
screws with threadlock compound.
Once the APC folding propeller and Great
Planes spinner are installed, the Siren is
almost ready!
The battery location will vary
depending on what the builder chooses. To
say the 10-cell, 2200 mAh NiMH battery
used was a tight squeeze would be an
understatement. The pack was built with
the cells arranged side by side. A pack
assembled with the cells touching end to
end would probably come out narrower in
width and fit better in the provided area.
Using this battery required removing the
plywood frame inside the fuselage.
Removal of that frame was
frighteningly easy. Flexing the fuselage
only a bit caused it to just “pop” out, glue
and all. This is a reminder of why I chose
to modify the elevator-servo installation.
With the frame removed, sliding the
battery all the way forward provided a
perfect CG balance. Hook-and-loop
fastener held the battery in place. Great
Planes provided a wedge-shaped balsa
block to help lift the battery over the motor
leads and ease sliding the battery into
place, but I didn’t use it.
Flying: Flight time came quickly because
of the short assembly time of less than five
hours.
With the recommended control throws
dialed in—1/2 inch up and down on the
elevator and ailerons—I set differential in
the ailerons to reduce the adverse yaw
generated by the lifting aileron. I reduced
the down throw by 50%, which I felt was
important for such a large model without a
working rudder.
I performed a thorough check of all
functions before tossing the Siren into a 10
mph headwind. I set the throttle at slightly
less than 50% upon launch to avoid any
surprises. Once level flight was obtained
and trim was adjusted, going to full throttle
unleashed a winged model rocket. The
Siren easily climbed at almost a 90° angle.
Fearing the model would reach the
stratosphere, I shut down the motor after
roughly 15 seconds. It looked to be
approximately 700 feet high.
Trimming the Siren for the best glide
required just a few clicks of down-trim.
Once a proper glide angle was established,
I was able to explore the airplane’s full
flight envelope. First I like to determine
how the airplane will behave at slow flight
and stall, so that if there are any bad habits
I will be ready for them when it comes time
to land.
The Siren showed no bad habits and
needed to be coaxed into a full stall.
Entering this maneuver took quite an effort
and a high angle of attack. Stall break and
recovery was uneventful and required a
small amount of down-elevator for the
latter.
Hauling back on the elevator from level
flight made the Siren mush around the sky.
The adverse yaw is noticeable when flying
at lower speeds, and the Siren longed for a
rudder.
That was enough slow flight; the Siren
was designed for quick climbs and efficient
cruise, which is where it truly stood out.
The Kontronics Fun 480 brushless motor
package delivered incredible performance
with the 10-cell NiMH battery.
Climbs were startling, at almost 90° in
no wind. Once at altitude the Siren cruised
with a moderate rate of descent in search of
thermals.
I performed the test flights in late winter,
so not much thermal activity was found. I
am confident that the Siren is capable of
thermaling in extremely light lift. Its clean
05sig3.QXD 3/23/07 10:48 AM Page 68
lines make it an extraordinarily efficient
airplane, and the energy retention is
excellent.
With a dive from altitude, huge, elegant
loops are possible, and the sailplane will
find itself at an altitude almost exactly
where it started with no help from the
motor. Rolls are slow and graceful, but, as I
mentioned, a rudder would have helped
make them more axial.
Inverted flight required little downelevator,
but I felt that the elevator throw
was a bit touchy while the Siren was
inverted. Given more time I would have
reduced the elevator down-throw.
One of my favorite maneuvers—a dive
from altitude, roll to inverted on the way
down, and inverted pass right off the deck
along the flying field and then climb back to
altitude—is easy and spectacular with the
Siren. Don’t try this at home unless you are
experienced; the speeds reached going
downhill can approach 100 mph.
Once I was finished showing off and
leaving the nitro fliers standing with their
mouths wide open in awe, it was time to
bring the Siren home. Since it behaved so
well in slow flight, setting up for landing
was as normal as for any high-performance
sailplane.
Allow plenty of distance for final
because the Siren has an excellent glide
ratio and can easily overshoot its target.
Using a computer radio to dial in air brakes
will help make spot landings much easier.
So what do we have here? A hotliner? A
warmliner? An electric-powered
sailplane? The Siren has the ability to
match all those descriptions, depending on
the power option you choose.
Great Planes is now offering a versatile
and well-engineered airplane with true
hotliner ability, with a modest price tag.
The quality and ease of building are firstrate,
and flight performance is equally
good. I give the Siren high marks and
recommend it to anyone who wants to step
into the world of high-performance
electric flying. MA
Peter C. Oesterwinter
[email protected]
Manufacturer/Distributor:
Great Planes Model Distributors
Box 9021
Champaign IL 61826
(800) 637-7660
(217) 398-3630
[email protected]
www.electrifly.com
Card Catalog—Other Articles About
This Product:
RC Groups:
www.rcgroups.com/forums/showthread.php
?t=356191
Fly RC:
June 2005 issue, page 100
Model Airplane News:
June 2006 issue
Quiet Flyer:
October, June 2005 issue, page 32
Items Used In Review:
APC propeller:
www.apcprop.com
2200 mAh 12-volt battery:
www.flydma.com
70 MODEL AVIATION
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