When I review a Scale model, I
generally offer some history;
however, this is quite an
involved review, so I’m going to get
straight to it. I will ask that you read
the fl ying portion of this review fi rst
so you’ll understand how good this
ECOMRC Stinson SR-9 ARF is when
it’s fi nished.
Go ahead, jump to the fl ying portion!
I’ll wait …
Now that you know what the result
is, let’s start from the beginning. The
ECOMRC Stinson SR-9 ARF is a big
airplane and it’s double boxed for
protection during shipping. Unpacking
everything, the only damage I found was
on the lower tip of the windscreen, but
that was easily trimmed away.
The airframe is completely built-up
from balsa and laser-cut plywood. It is
stiff, but not overly heavy. The wings and
stabilizer are plug-in designs, and the
wings include fl aps.
Covering the airframe is a white,
iron-on fi lm with red trim and black
pinstripes that matches the color scheme
of the full-scale Stinson SR-9 (n-number
VH-UXL) that was used by the Vacuum
Oil Company back in 1936. A few
spots were slightly wrinkled, but a few
minutes of work with my covering iron
took care of them.
Matching the iron-on fi lm are the
prepainted fi berglass cowl and wheel
pants. The cowl features the Reliant’s
multitude of “bumps” around the
circumference. On the rear of the cowl,
there’s a cutout for exhaust and cooling.
The 16-page instruction manual
shows the steps in a series of drawings.
It’s slightly lacking in places, but an
experienced modeler should be able
to fi gure out the steps. Construction is
straightforward.
Rounding out the kit is the hardware,
which includes everything from the
thick aluminum landing gear halves, to
the fuel tank, the carbon-fi ber wing and
stabilizer tubes, the vacuum-formed
plastic windows and gear fairings,
wheels, pushrods, and stickers. There are
also bags with bolts, nuts, and smaller
pieces, as well as the pull-pull cable
hardware for the rudder. All threaded
hardware is metric.
The manual mentions that a sixchannel
system is needed. This is true
if you use Y harnesses for the ailerons
and special reversing Y harnesses for the
elevator halves and flap servos. However,
if you don’t want to use the Y harnesses,
you’ll need at least an eight-channel
radio that can handle three separate
mixes of master and slave servos.
This is an International Miniature
Aircraft Association-legal (IMAA)
aircraft, and if you want comply with
IMAA rules, you’ll need another channel
for the remote ignition cutoff.
Nothing in the manual covers what
types of servos are needed (standard,
high-torque, high-speed, etc.). Looking
at the list of recommended accessories
on the SR-9 page of Troy Built Models
(TBM) website, I found several servos
listed with torque ratings of 133 ounces
per square inch and higher. With the
large areas of the SR-9’s control surfaces,
I chose to use a combination of servos
I had on hand. Using a six-volt setup,
the lowest torque was the JR Sport
ST125MG servos at 142 ounce-inches.
Construction
If you choose to power the SR-9 with
a gasoline engine, be sure to have some
threadlock handy; nearly every step
where parts are bolted together requires
a drop or two. Any gasoline engine will
vibrate and quickly loosen even the
tightest of screws.
I normally don’t discuss the
construction of review models, but there
are a few problems areas that need to be
detailed. You can download the manual
from TBM’s website located in the
“Sources” section.
Construction starts by attaching the
ailerons and flaps to the wing
halves. Nylon “hinge-point”
hinges are used. Be sure to coat
the center section of each with
lithium grease, petroleum jelly,
or a suitable alternative to keep
the epoxy away from the pivot
point.
Install the aileron and flap
servos into the wing halves. The
servos mount to the back of
the preassembled servo covers.
Any standard-size servo easily
fits into the cutout; however,
because longer, slightly thicker,
heavy-duty servo arms are
needed, the servos cannot be
fitted with the arms in place.
Nor can the arms be fitted after
the servos are mounted.
I cut away the strip of
plywood between the two
mounting platforms, then
stiffened the remaining mounts
by wicking thin CA. After
it cures, the servo with the
horn attached could slide into
position.
Using a long, thin wire or a
string with a weight attached,
you can fish the servo wires and
extensions through the wing
halves. However, I found that I
had to trim a bit of the plywood
framing to allow the servos to
properly fit in the wings.
Once that’s finished, the
control hardware is attached.
The included control arms are epoxied
into the control surface. To keep them
aligned, insert the screw that holds in
the ball link through both control arms
and use it to hold the arms in position.
This keeps the holes aligned and makes
things easier.
The pushrods and ball links are heavyduty
parts, held to the servo by a wheel
collar. Be sure to grind a flat spot on the
pushrod for a secure fit. The ball links
can slide slightly between the control
horns. To take up this slack and have a
more precise link, cut two thin slices of
fuel tubing. Place them on either side of
the ball link to act as soft washers.
The elevator halves are next. Each
elevator half has its own servo mounted
inside. The servo fits well, but I had to
file away some of the wood so the servo
wire could exit without crimping.
When it was time to epoxy the
control horn pieces into the
elevators and the rudder, I ran
into the first major problem.
These horns are smaller than
those used on the wings, but
the precut slots are so big that
the horns are swimming inside
them.
I contacted TBM and we
decided to use the control
horns and fill the rest of the
opening with epoxy mixed with
microballoons. After trimming
the length of the horns to fit the
slots, everything was epoxied
into place and it fixed the
problem.
The manual shows that the rudder
should be epoxied into position next,
but I advise putting off this step.
A heavy-duty tail wheel is included,
which requires some assembly as well
as some measuring so everything works
properly. The aluminum plate that the
wire runs through had to be drilled out
to accept the 1/8-inch wire.
The prebent wire has extra length.
It’s designed to extend through the
outer cover, through an aluminum
mounting plate, through a plywood
plate, then through another hole
predrilled in a rib that is further up
into the rudder.
Once cut to length, this wire also
needs a few flats cut to match the
positions of the wheel collars. At this
point, everything can be bolted into
position except the tail wheel. Take a
few minutes to install the pull-pull
wires that steer this assembly.
After everything is mounted in
position, the rear cover, tail wheel, and
wheel collars are attached. Although
the manual doesn’t say to bolt on the
landing gear struts until later in the
build, now is a good time to do so. It
will give you a firmer foundation for
the next step.
This is why I mentioned postponing
attaching the rudder. During the engine
and cowl installation, you can stand
the fuselage vertically, setting it on the
fin. It provides a flat, solid surface. And
having the second landing gear struts in
place gives you support when propping
up the fuselage against a vertical
surface such as a wall or door.
TBM supplied an engine with the
review kit: the Pterodactyl PTE36R
36cc gas engine. It’s a rear intake,
rear exhaust, single-cylinder
engine and it mounts on four
aluminum standoffs.
To figure out the engine’s
mounting position, I skipped
forward in the manual and
epoxied the cowl ring into
place. This requires sanding
flat the area on the front of the
fuselage where the ring mounts.
Take a moment to wick in
some thin CA around each of
the blocks on the mounting
ring. A few of the parts were
moving around slightly on my
piece, so make sure nothing
falls off on yours. Be careful not to get
CA in any of the threads.
After the CA cured, I took some
time to position the cowl and get an
idea of how the engine would fit. The
cowl will take a bit of trimming to
allow the muffler and the front fins of
the head to clear.
The engine needed to be spaced
11/2 inches out from the firewall to
allow the cowl to fit properly. No
wood is included in the kit for this, so
I designed a spacer from plywood and
poplar.
You can download a diagram from
the www.ModelAviation.com website.
Paint it with clear polyurethane to
fuelproof it. To bolt the spacer to the
fuselage, I picked up a set of four 1/4-20
x 2 bolts and matching t-nuts.
Now is the time to finish the steps I
skipped. I mounted the rudder servo,
epoxied the rudder into place, and
installed the pull-pull cables. There’s
nothing wrong with the way these cables are installed, but
I had a hard time getting them right, especially the cables
coming forward from the tail wheel.
I’m not a surgeon, but I imagine trying to thread these
cables through the holes and tubes is like arthroscopic
surgery. I was using long needle-nose pliers at weird angles,
while reaching through the door and windscreen opening.
The rudder servo is mounted back in the passenger
compartment, which compounded the issue for me, but I
did manage to get it sorted out.
The throttle servo mounts slightly forward of the rudder
servo and has to operate a long throttle pushrod. A thin
metal pushrod is included in the kit, but in order to keep
electronic engine noise to a minimum, I substituted this for
a Du-Bro flexible pushrod system.
When I attached the throttle pushrod to the carburetor,
I took the time to add another pushrod that would let me
manually operate the choke through the cowl opening. A
simple z-bend went through the choke lever, and I rigged
up a bracket that would support the bracket from the front,
which was held in place by the engine’s mounting bolts.
I also substituted one other item: the fuel tank. There’s
nothing wrong with the supplied tank; it’s a standard blowmolded
tank. However, I received a RotoFlow fuel tank
from J&L Power Products and this looked to be the perfect
time to try it out.
The RotoFlow comes preassembled and the clunk rotates
around a central pickup line. All I had to do is hook up the
fuel and vent lines, and then strap it in position with the tie
wraps that came in the SR-9 kit.
The vent line wraps around the tank. This is a tip I picked
up in some of my work with gas-powered International
Miniature Aerobatic Club (IMAC) airplanes. The vent line
on a gas engine is not pressurized and vents to the outside.
Fuel can leak out of the vent if the airplane is in certain
positions such as a vertical downline.
To keep this from happening, the vent line is wrapped
along the side, the back, and back along the other side to
form a loop. It works well and only requires a bit more
tubing for a trouble-free tank installation.
Next the landing gear fairings, wheels, and wheel pants
are installed. If you attached the landing gear struts first,
make sure you put the landing gear fairings on first. They
are vacuum-formed pieces and there is a left and right so
pay attention. They must be trimmed and if you cut to the
molded-in trim line, the fit is perfect.
You’ll have to eyeball a cutout in each fairing to allow the
threaded spacers to protrude. These spacers are where the
wing struts bolt to the fuselage. Glue the fairings in position
and then attach the wheels.
When it came time to attach the wheels, another problem
occurred. There was no included hardware that was correct
for mounting the wheels. The wheels have a 6mm diameter
hole, yet the instructions called for a 5mm bolt to make up
the axle. None was included.
The remaining bolts were 3mm, which are too thin and
too short. I picked up a set of 1/4- 20 x 21/4 hex bolts and
matching nuts that worked. They are slightly larger in
diameter than the 6mm size, so the
wheels and the landing gear holes had
to be drilled larger.
There are no predrilled holes in the
wheel pants, so you’ll have to eyeball
that as well. There are provisions for a
smaller screw to be installed above the
axle bolt that will stop the wheel pants
from turning or spinning around the
axle.
With the SR-9 on its feet, I set about
installing the radio. There’s plenty of
room in the fuselage to mount nearly
any brand and size of radio you choose.
Extensions have to be run back to the
left and right elevator servos, as well as
up to the wing roots for the flap and
elevator servos on each side. To hold
these in place I picked up a set of Wire
Keeps. These small, die-cut foam pieces
have adhesive strips on the back and
make installations neater. They also keep
the extensions to the wing root in place,
and out of sight.
I installed a Smart-Fly remote ignition
cutoff. Quest makes these cutoffs and
they allow you to cut the ignition
from your transmitter using a spare
channel. It utilizes a fiber-optic system
so the ignition is opto-isolated from the
receiver.
The receiver, dual batteries, and the
Smart-Fly system are held in place with
hook-and-loop. I glued in a couple of
squares of 1/16-inch balsa sheet to go
under the receiver and batteries, giving
the hook-and-loopmaterial a surface area
on which to mount.
I included a plywood plate inside
the port cabin door that is not part of
the kit. I despise mounting switches on
the surface of the fuselage of any Scale
airplane, and make every attempt to hide
them. With such a large fuselage and
two functional doors, it seems like the
perfect place to hide both the receiver
power and ignition power switches. I
did a bit of measuring and made this
plate from some spare light plywood.
In hindsight, I should have also put the
Aluminum Fuel Dot on the plate as
well.
I originally set up the radio with a
master/slave combination for both
ailerons servos, both elevator servos,
and both flap servos. After extensive
experimenting with my 9503 radio, I
found that the elevator servos weren’t
matching in throw and speed. They
weren’t off too much, but it was enough
that it would have induced a bit of roll,
making trimming the SR-9 impossible.
No amount of transmitter programming
could entirely fix the problem and I
found that the flaps were worse. The
9503 had no midpoint flap adjustment
so both servos were way out of sync.
So, to solve this, I bought a couple of
reversing Y harnesses and installed
them, which solved the elevator and flap
problems.
When the radio was in place, I finished
the fuselage by installing the windows,
windscreen, and the cabin doors. With
the exception of the windscreen, all
the windows are glued in place. The
windscreen is held in place with a
number of smaller screws.
Several of the screws which are
installed above the tank area only thread
into a thin layer of balsa. Even wicking
a bit of thin CA will not give enough
material for the screws to firmly hold.
I cut a few 3/4-inch squares from some
1/8-inch light plywood. After I drilled
the holes for the windscreen, I could
then feel where these squares should
be applied. I used medium CA to hold
them in position. I redrilled the holes
to extend through the plywood so the
screws tightened nicely.
The instructions say to epoxy the
door hinges into place on the door and
the fuselage. If the hinges were epoxied,
the doors would only open roughly 60°.
I decided not epoxy the hinges to the
fuselage, but to hold each hinge in place
with a single screw (the same type that
holds the windscreen in position). This
will allow me to completely remove the
doors if a problem arises.
One last issue popped up during the
final assembly. The wing struts are held
in place with two screws: one for the
top, and one for the bottom. The bottom
screw threads into the spacer that
extends from the landing gear fairings,
but there were no screws included
that were short enough to allow me
to tighten the strut firmly. I had to
outsource those screws as well. They
need to be roughly 1/4-inch to 3/8-inch
long.
The assembly was finally finished.
Several problems were overcome;
however, when it’s finished, it really
looks great.
It’s hard to determine the exact
assembly time, but I’d estimate 15 to 20
hours. I spent a couple hours each night
for 2 weeks, but not all of it was actual
building. Some was waiting for the
epoxy to cure.
There was a good surprise: the CG
was spot-on, even with the twin receiver
batteries. The weight came in at 191/4
pounds, ready to fly.
One last note before I get to the
flying portion. Have you ever put
together a project such as a piece of
furniture, a child’s play set, etc., and
have a few screws and other hardware
left over? Our review SR-9 kit takes
this to another level. After doublechecking
to make sure I didn’t miss any
steps, I found myself with a tidy pile of
hardware: a handful of screws, nuts and
bolts, a few dozen washers of various
sizes, t-nuts, flat hinges, two sizes of
point hinges, and even an extra threaded
spacer.
These all can be used in future
projects, but it seems as though someone
at the factory scooped up a few handfuls
of miscellaneous hardware and put it in
the kit. Maybe this helps make up for
the hardware that wasn’t included.
You’ll probably need spares of the
smaller screws and washers that secure
the stabilizer halves and the wing struts.
Have extras so you won’t ruin a day’s
flying because of a missing screw.
Engine First Run
Before the first flight, I took the time
to start the break-in procedure on the
Pterodactyl PTE36R 36cc. A fuel-tooil
mixture between 20:1 and 40:1 is
called for, so I used a 30:1 mixture I
had on hand. The break-in propeller is
not specified in the manual, but TBM’s
website suggests an 18 x 8 propeller and
puts the break-in time at 5 gallons of
fuel.
With everything in place, it was time
to start flipping. It took roughly 25 to 30
flips to get the first ignition burp when
choked, then another 10 or so to bring
the engine to life. I ran the first tankful at
25% throttle with no adjustments to the
carburetor to lubricate everything.
During the next run, I adjusted
the carburetor so the top end was
approximately 7,800 rpm and idle was
roughly 2,400. Satisfied with this as a
starting point, it was time to put on the
cowl and see how the SR-9 flew.
Flying
It will take roughly 10 to 15 minutes
to bolt the wings and stabilizer into
place, attach the wing struts, and top off
the tank when assembling at the field.
The plug-in wings and stabilizer halves
are easy to install and have a good, firm
fit. The stabilizer halves are held in place
with a couple of screws.
The wings are secured by reaching
into the fuselage through the cabin
doors and threading a 6mm screw into
each wing root. You’ll probably want
to assemble it on a table to keep from
crawling under the wings.
Firing the PTE36R for the first flight,
I found that the idle was too high for
proper taxiing, but because the cowl was
already on and I knew that I could kill
the engine at any time with the Smart-
Fly ignition cutoff, I went with it as is.
The SR-9 took off in roughly 100
feet and had a good climb rate at only
three-fourths throttle. After making a
few passes for trim, I was fighting the
model because it was way too sensitive,
especially in pitch (elevator). Most of
the early turns looked like those a Pylon
racer would make. Any touch of the
elevator equated big changes in altitude.
The manual calls out a throw or
40mm each way; that’s a touch over 11/2
inches. The rudder was similarly touchy,
so I set up for a few practice approaches.
With the idle set as high as it was, the
Stinson would not land, even with flaps,
so I climbed up to a safe altitude, cut the
throttle, and glided in for an uneventful
landing.
It was time for reprogramming. I
cut the elevator and rudder throw
considerably (1 inch). I also added
exponential: 60% on the elevator and
45% on the rudder. I left the ailerons
alone.
I removed the cowl and adjusted the
idle setting, lowering it to approximately
2,000 rpm. The engine felt as though
it would quit if it was any lower, so I
bolted everything together and it was on
to flight two.
Although not quite perfect, the flight
was much more scalelike and I was able
to get a better feel for the model. You
might think it would fly like a giant
trainer, but it requires some attention,
especially in the turns where you’ll have
to coordinate the ailerons and rudder.
The full-scale Stinson is not an
aerobatic design, but this model will do
modest loops, stall turns, and barrel rolls,
and will spin easily with that big rudder.
Cruising around is where the Stinson
shines, and low-level passes are beautiful.
The 9503 has a three-position switch
for flaps, and I had the most fun at full
flaps. At this setting, the flaps were
deployed close to 60° and they slow
the SR-9 to a crawl. With the ample
power of the PTE36R, there’s plenty of
propeller blast over the control surfaces.
Landings are easier and slower using
flaps. I found it better to land with a
steeper approach, a little more power,
and then flare right roughly 2 feet off
the runway. It will settle, and normally
you can land without too much
bouncing. The gear and hard foam tires
don’t have much give, so it is very easy
to bounce a landing on pavement, but
the Stinson lands well on grass.
Conclusion
TBM’s Stinson SR-9 is a beautiful
airplane and faithful to its full-scale
counterpart. It’s a big model with a 100-
inch wingspan, but this size makes it
easy to work on and in. There are some
problems along the way, but it’s nothing
that an intermediate or experienced
modeler can’t overcome, and I described
how to dodge these problems in the
review!
In the air, the SR-9 looks the part.
Resist the urge to fly it at full throttle,
and please don’t shoehorn a 50cc (or
larger) engine in the cowl. It doesn’t
need it. The PTE36 is plenty of power
and if you fly the SR-9 as intended,
you’ll rarely need anything over half
throttle.
Edition: Model Aviation - 2012/10
Page Numbers: 46,47,48,49,50,51,52
Edition: Model Aviation - 2012/10
Page Numbers: 46,47,48,49,50,51,52
When I review a Scale model, I
generally offer some history;
however, this is quite an
involved review, so I’m going to get
straight to it. I will ask that you read
the fl ying portion of this review fi rst
so you’ll understand how good this
ECOMRC Stinson SR-9 ARF is when
it’s fi nished.
Go ahead, jump to the fl ying portion!
I’ll wait …
Now that you know what the result
is, let’s start from the beginning. The
ECOMRC Stinson SR-9 ARF is a big
airplane and it’s double boxed for
protection during shipping. Unpacking
everything, the only damage I found was
on the lower tip of the windscreen, but
that was easily trimmed away.
The airframe is completely built-up
from balsa and laser-cut plywood. It is
stiff, but not overly heavy. The wings and
stabilizer are plug-in designs, and the
wings include fl aps.
Covering the airframe is a white,
iron-on fi lm with red trim and black
pinstripes that matches the color scheme
of the full-scale Stinson SR-9 (n-number
VH-UXL) that was used by the Vacuum
Oil Company back in 1936. A few
spots were slightly wrinkled, but a few
minutes of work with my covering iron
took care of them.
Matching the iron-on fi lm are the
prepainted fi berglass cowl and wheel
pants. The cowl features the Reliant’s
multitude of “bumps” around the
circumference. On the rear of the cowl,
there’s a cutout for exhaust and cooling.
The 16-page instruction manual
shows the steps in a series of drawings.
It’s slightly lacking in places, but an
experienced modeler should be able
to fi gure out the steps. Construction is
straightforward.
Rounding out the kit is the hardware,
which includes everything from the
thick aluminum landing gear halves, to
the fuel tank, the carbon-fi ber wing and
stabilizer tubes, the vacuum-formed
plastic windows and gear fairings,
wheels, pushrods, and stickers. There are
also bags with bolts, nuts, and smaller
pieces, as well as the pull-pull cable
hardware for the rudder. All threaded
hardware is metric.
The manual mentions that a sixchannel
system is needed. This is true
if you use Y harnesses for the ailerons
and special reversing Y harnesses for the
elevator halves and flap servos. However,
if you don’t want to use the Y harnesses,
you’ll need at least an eight-channel
radio that can handle three separate
mixes of master and slave servos.
This is an International Miniature
Aircraft Association-legal (IMAA)
aircraft, and if you want comply with
IMAA rules, you’ll need another channel
for the remote ignition cutoff.
Nothing in the manual covers what
types of servos are needed (standard,
high-torque, high-speed, etc.). Looking
at the list of recommended accessories
on the SR-9 page of Troy Built Models
(TBM) website, I found several servos
listed with torque ratings of 133 ounces
per square inch and higher. With the
large areas of the SR-9’s control surfaces,
I chose to use a combination of servos
I had on hand. Using a six-volt setup,
the lowest torque was the JR Sport
ST125MG servos at 142 ounce-inches.
Construction
If you choose to power the SR-9 with
a gasoline engine, be sure to have some
threadlock handy; nearly every step
where parts are bolted together requires
a drop or two. Any gasoline engine will
vibrate and quickly loosen even the
tightest of screws.
I normally don’t discuss the
construction of review models, but there
are a few problems areas that need to be
detailed. You can download the manual
from TBM’s website located in the
“Sources” section.
Construction starts by attaching the
ailerons and flaps to the wing
halves. Nylon “hinge-point”
hinges are used. Be sure to coat
the center section of each with
lithium grease, petroleum jelly,
or a suitable alternative to keep
the epoxy away from the pivot
point.
Install the aileron and flap
servos into the wing halves. The
servos mount to the back of
the preassembled servo covers.
Any standard-size servo easily
fits into the cutout; however,
because longer, slightly thicker,
heavy-duty servo arms are
needed, the servos cannot be
fitted with the arms in place.
Nor can the arms be fitted after
the servos are mounted.
I cut away the strip of
plywood between the two
mounting platforms, then
stiffened the remaining mounts
by wicking thin CA. After
it cures, the servo with the
horn attached could slide into
position.
Using a long, thin wire or a
string with a weight attached,
you can fish the servo wires and
extensions through the wing
halves. However, I found that I
had to trim a bit of the plywood
framing to allow the servos to
properly fit in the wings.
Once that’s finished, the
control hardware is attached.
The included control arms are epoxied
into the control surface. To keep them
aligned, insert the screw that holds in
the ball link through both control arms
and use it to hold the arms in position.
This keeps the holes aligned and makes
things easier.
The pushrods and ball links are heavyduty
parts, held to the servo by a wheel
collar. Be sure to grind a flat spot on the
pushrod for a secure fit. The ball links
can slide slightly between the control
horns. To take up this slack and have a
more precise link, cut two thin slices of
fuel tubing. Place them on either side of
the ball link to act as soft washers.
The elevator halves are next. Each
elevator half has its own servo mounted
inside. The servo fits well, but I had to
file away some of the wood so the servo
wire could exit without crimping.
When it was time to epoxy the
control horn pieces into the
elevators and the rudder, I ran
into the first major problem.
These horns are smaller than
those used on the wings, but
the precut slots are so big that
the horns are swimming inside
them.
I contacted TBM and we
decided to use the control
horns and fill the rest of the
opening with epoxy mixed with
microballoons. After trimming
the length of the horns to fit the
slots, everything was epoxied
into place and it fixed the
problem.
The manual shows that the rudder
should be epoxied into position next,
but I advise putting off this step.
A heavy-duty tail wheel is included,
which requires some assembly as well
as some measuring so everything works
properly. The aluminum plate that the
wire runs through had to be drilled out
to accept the 1/8-inch wire.
The prebent wire has extra length.
It’s designed to extend through the
outer cover, through an aluminum
mounting plate, through a plywood
plate, then through another hole
predrilled in a rib that is further up
into the rudder.
Once cut to length, this wire also
needs a few flats cut to match the
positions of the wheel collars. At this
point, everything can be bolted into
position except the tail wheel. Take a
few minutes to install the pull-pull
wires that steer this assembly.
After everything is mounted in
position, the rear cover, tail wheel, and
wheel collars are attached. Although
the manual doesn’t say to bolt on the
landing gear struts until later in the
build, now is a good time to do so. It
will give you a firmer foundation for
the next step.
This is why I mentioned postponing
attaching the rudder. During the engine
and cowl installation, you can stand
the fuselage vertically, setting it on the
fin. It provides a flat, solid surface. And
having the second landing gear struts in
place gives you support when propping
up the fuselage against a vertical
surface such as a wall or door.
TBM supplied an engine with the
review kit: the Pterodactyl PTE36R
36cc gas engine. It’s a rear intake,
rear exhaust, single-cylinder
engine and it mounts on four
aluminum standoffs.
To figure out the engine’s
mounting position, I skipped
forward in the manual and
epoxied the cowl ring into
place. This requires sanding
flat the area on the front of the
fuselage where the ring mounts.
Take a moment to wick in
some thin CA around each of
the blocks on the mounting
ring. A few of the parts were
moving around slightly on my
piece, so make sure nothing
falls off on yours. Be careful not to get
CA in any of the threads.
After the CA cured, I took some
time to position the cowl and get an
idea of how the engine would fit. The
cowl will take a bit of trimming to
allow the muffler and the front fins of
the head to clear.
The engine needed to be spaced
11/2 inches out from the firewall to
allow the cowl to fit properly. No
wood is included in the kit for this, so
I designed a spacer from plywood and
poplar.
You can download a diagram from
the www.ModelAviation.com website.
Paint it with clear polyurethane to
fuelproof it. To bolt the spacer to the
fuselage, I picked up a set of four 1/4-20
x 2 bolts and matching t-nuts.
Now is the time to finish the steps I
skipped. I mounted the rudder servo,
epoxied the rudder into place, and
installed the pull-pull cables. There’s
nothing wrong with the way these cables are installed, but
I had a hard time getting them right, especially the cables
coming forward from the tail wheel.
I’m not a surgeon, but I imagine trying to thread these
cables through the holes and tubes is like arthroscopic
surgery. I was using long needle-nose pliers at weird angles,
while reaching through the door and windscreen opening.
The rudder servo is mounted back in the passenger
compartment, which compounded the issue for me, but I
did manage to get it sorted out.
The throttle servo mounts slightly forward of the rudder
servo and has to operate a long throttle pushrod. A thin
metal pushrod is included in the kit, but in order to keep
electronic engine noise to a minimum, I substituted this for
a Du-Bro flexible pushrod system.
When I attached the throttle pushrod to the carburetor,
I took the time to add another pushrod that would let me
manually operate the choke through the cowl opening. A
simple z-bend went through the choke lever, and I rigged
up a bracket that would support the bracket from the front,
which was held in place by the engine’s mounting bolts.
I also substituted one other item: the fuel tank. There’s
nothing wrong with the supplied tank; it’s a standard blowmolded
tank. However, I received a RotoFlow fuel tank
from J&L Power Products and this looked to be the perfect
time to try it out.
The RotoFlow comes preassembled and the clunk rotates
around a central pickup line. All I had to do is hook up the
fuel and vent lines, and then strap it in position with the tie
wraps that came in the SR-9 kit.
The vent line wraps around the tank. This is a tip I picked
up in some of my work with gas-powered International
Miniature Aerobatic Club (IMAC) airplanes. The vent line
on a gas engine is not pressurized and vents to the outside.
Fuel can leak out of the vent if the airplane is in certain
positions such as a vertical downline.
To keep this from happening, the vent line is wrapped
along the side, the back, and back along the other side to
form a loop. It works well and only requires a bit more
tubing for a trouble-free tank installation.
Next the landing gear fairings, wheels, and wheel pants
are installed. If you attached the landing gear struts first,
make sure you put the landing gear fairings on first. They
are vacuum-formed pieces and there is a left and right so
pay attention. They must be trimmed and if you cut to the
molded-in trim line, the fit is perfect.
You’ll have to eyeball a cutout in each fairing to allow the
threaded spacers to protrude. These spacers are where the
wing struts bolt to the fuselage. Glue the fairings in position
and then attach the wheels.
When it came time to attach the wheels, another problem
occurred. There was no included hardware that was correct
for mounting the wheels. The wheels have a 6mm diameter
hole, yet the instructions called for a 5mm bolt to make up
the axle. None was included.
The remaining bolts were 3mm, which are too thin and
too short. I picked up a set of 1/4- 20 x 21/4 hex bolts and
matching nuts that worked. They are slightly larger in
diameter than the 6mm size, so the
wheels and the landing gear holes had
to be drilled larger.
There are no predrilled holes in the
wheel pants, so you’ll have to eyeball
that as well. There are provisions for a
smaller screw to be installed above the
axle bolt that will stop the wheel pants
from turning or spinning around the
axle.
With the SR-9 on its feet, I set about
installing the radio. There’s plenty of
room in the fuselage to mount nearly
any brand and size of radio you choose.
Extensions have to be run back to the
left and right elevator servos, as well as
up to the wing roots for the flap and
elevator servos on each side. To hold
these in place I picked up a set of Wire
Keeps. These small, die-cut foam pieces
have adhesive strips on the back and
make installations neater. They also keep
the extensions to the wing root in place,
and out of sight.
I installed a Smart-Fly remote ignition
cutoff. Quest makes these cutoffs and
they allow you to cut the ignition
from your transmitter using a spare
channel. It utilizes a fiber-optic system
so the ignition is opto-isolated from the
receiver.
The receiver, dual batteries, and the
Smart-Fly system are held in place with
hook-and-loop. I glued in a couple of
squares of 1/16-inch balsa sheet to go
under the receiver and batteries, giving
the hook-and-loopmaterial a surface area
on which to mount.
I included a plywood plate inside
the port cabin door that is not part of
the kit. I despise mounting switches on
the surface of the fuselage of any Scale
airplane, and make every attempt to hide
them. With such a large fuselage and
two functional doors, it seems like the
perfect place to hide both the receiver
power and ignition power switches. I
did a bit of measuring and made this
plate from some spare light plywood.
In hindsight, I should have also put the
Aluminum Fuel Dot on the plate as
well.
I originally set up the radio with a
master/slave combination for both
ailerons servos, both elevator servos,
and both flap servos. After extensive
experimenting with my 9503 radio, I
found that the elevator servos weren’t
matching in throw and speed. They
weren’t off too much, but it was enough
that it would have induced a bit of roll,
making trimming the SR-9 impossible.
No amount of transmitter programming
could entirely fix the problem and I
found that the flaps were worse. The
9503 had no midpoint flap adjustment
so both servos were way out of sync.
So, to solve this, I bought a couple of
reversing Y harnesses and installed
them, which solved the elevator and flap
problems.
When the radio was in place, I finished
the fuselage by installing the windows,
windscreen, and the cabin doors. With
the exception of the windscreen, all
the windows are glued in place. The
windscreen is held in place with a
number of smaller screws.
Several of the screws which are
installed above the tank area only thread
into a thin layer of balsa. Even wicking
a bit of thin CA will not give enough
material for the screws to firmly hold.
I cut a few 3/4-inch squares from some
1/8-inch light plywood. After I drilled
the holes for the windscreen, I could
then feel where these squares should
be applied. I used medium CA to hold
them in position. I redrilled the holes
to extend through the plywood so the
screws tightened nicely.
The instructions say to epoxy the
door hinges into place on the door and
the fuselage. If the hinges were epoxied,
the doors would only open roughly 60°.
I decided not epoxy the hinges to the
fuselage, but to hold each hinge in place
with a single screw (the same type that
holds the windscreen in position). This
will allow me to completely remove the
doors if a problem arises.
One last issue popped up during the
final assembly. The wing struts are held
in place with two screws: one for the
top, and one for the bottom. The bottom
screw threads into the spacer that
extends from the landing gear fairings,
but there were no screws included
that were short enough to allow me
to tighten the strut firmly. I had to
outsource those screws as well. They
need to be roughly 1/4-inch to 3/8-inch
long.
The assembly was finally finished.
Several problems were overcome;
however, when it’s finished, it really
looks great.
It’s hard to determine the exact
assembly time, but I’d estimate 15 to 20
hours. I spent a couple hours each night
for 2 weeks, but not all of it was actual
building. Some was waiting for the
epoxy to cure.
There was a good surprise: the CG
was spot-on, even with the twin receiver
batteries. The weight came in at 191/4
pounds, ready to fly.
One last note before I get to the
flying portion. Have you ever put
together a project such as a piece of
furniture, a child’s play set, etc., and
have a few screws and other hardware
left over? Our review SR-9 kit takes
this to another level. After doublechecking
to make sure I didn’t miss any
steps, I found myself with a tidy pile of
hardware: a handful of screws, nuts and
bolts, a few dozen washers of various
sizes, t-nuts, flat hinges, two sizes of
point hinges, and even an extra threaded
spacer.
These all can be used in future
projects, but it seems as though someone
at the factory scooped up a few handfuls
of miscellaneous hardware and put it in
the kit. Maybe this helps make up for
the hardware that wasn’t included.
You’ll probably need spares of the
smaller screws and washers that secure
the stabilizer halves and the wing struts.
Have extras so you won’t ruin a day’s
flying because of a missing screw.
Engine First Run
Before the first flight, I took the time
to start the break-in procedure on the
Pterodactyl PTE36R 36cc. A fuel-tooil
mixture between 20:1 and 40:1 is
called for, so I used a 30:1 mixture I
had on hand. The break-in propeller is
not specified in the manual, but TBM’s
website suggests an 18 x 8 propeller and
puts the break-in time at 5 gallons of
fuel.
With everything in place, it was time
to start flipping. It took roughly 25 to 30
flips to get the first ignition burp when
choked, then another 10 or so to bring
the engine to life. I ran the first tankful at
25% throttle with no adjustments to the
carburetor to lubricate everything.
During the next run, I adjusted
the carburetor so the top end was
approximately 7,800 rpm and idle was
roughly 2,400. Satisfied with this as a
starting point, it was time to put on the
cowl and see how the SR-9 flew.
Flying
It will take roughly 10 to 15 minutes
to bolt the wings and stabilizer into
place, attach the wing struts, and top off
the tank when assembling at the field.
The plug-in wings and stabilizer halves
are easy to install and have a good, firm
fit. The stabilizer halves are held in place
with a couple of screws.
The wings are secured by reaching
into the fuselage through the cabin
doors and threading a 6mm screw into
each wing root. You’ll probably want
to assemble it on a table to keep from
crawling under the wings.
Firing the PTE36R for the first flight,
I found that the idle was too high for
proper taxiing, but because the cowl was
already on and I knew that I could kill
the engine at any time with the Smart-
Fly ignition cutoff, I went with it as is.
The SR-9 took off in roughly 100
feet and had a good climb rate at only
three-fourths throttle. After making a
few passes for trim, I was fighting the
model because it was way too sensitive,
especially in pitch (elevator). Most of
the early turns looked like those a Pylon
racer would make. Any touch of the
elevator equated big changes in altitude.
The manual calls out a throw or
40mm each way; that’s a touch over 11/2
inches. The rudder was similarly touchy,
so I set up for a few practice approaches.
With the idle set as high as it was, the
Stinson would not land, even with flaps,
so I climbed up to a safe altitude, cut the
throttle, and glided in for an uneventful
landing.
It was time for reprogramming. I
cut the elevator and rudder throw
considerably (1 inch). I also added
exponential: 60% on the elevator and
45% on the rudder. I left the ailerons
alone.
I removed the cowl and adjusted the
idle setting, lowering it to approximately
2,000 rpm. The engine felt as though
it would quit if it was any lower, so I
bolted everything together and it was on
to flight two.
Although not quite perfect, the flight
was much more scalelike and I was able
to get a better feel for the model. You
might think it would fly like a giant
trainer, but it requires some attention,
especially in the turns where you’ll have
to coordinate the ailerons and rudder.
The full-scale Stinson is not an
aerobatic design, but this model will do
modest loops, stall turns, and barrel rolls,
and will spin easily with that big rudder.
Cruising around is where the Stinson
shines, and low-level passes are beautiful.
The 9503 has a three-position switch
for flaps, and I had the most fun at full
flaps. At this setting, the flaps were
deployed close to 60° and they slow
the SR-9 to a crawl. With the ample
power of the PTE36R, there’s plenty of
propeller blast over the control surfaces.
Landings are easier and slower using
flaps. I found it better to land with a
steeper approach, a little more power,
and then flare right roughly 2 feet off
the runway. It will settle, and normally
you can land without too much
bouncing. The gear and hard foam tires
don’t have much give, so it is very easy
to bounce a landing on pavement, but
the Stinson lands well on grass.
Conclusion
TBM’s Stinson SR-9 is a beautiful
airplane and faithful to its full-scale
counterpart. It’s a big model with a 100-
inch wingspan, but this size makes it
easy to work on and in. There are some
problems along the way, but it’s nothing
that an intermediate or experienced
modeler can’t overcome, and I described
how to dodge these problems in the
review!
In the air, the SR-9 looks the part.
Resist the urge to fly it at full throttle,
and please don’t shoehorn a 50cc (or
larger) engine in the cowl. It doesn’t
need it. The PTE36 is plenty of power
and if you fly the SR-9 as intended,
you’ll rarely need anything over half
throttle.
Edition: Model Aviation - 2012/10
Page Numbers: 46,47,48,49,50,51,52
When I review a Scale model, I
generally offer some history;
however, this is quite an
involved review, so I’m going to get
straight to it. I will ask that you read
the fl ying portion of this review fi rst
so you’ll understand how good this
ECOMRC Stinson SR-9 ARF is when
it’s fi nished.
Go ahead, jump to the fl ying portion!
I’ll wait …
Now that you know what the result
is, let’s start from the beginning. The
ECOMRC Stinson SR-9 ARF is a big
airplane and it’s double boxed for
protection during shipping. Unpacking
everything, the only damage I found was
on the lower tip of the windscreen, but
that was easily trimmed away.
The airframe is completely built-up
from balsa and laser-cut plywood. It is
stiff, but not overly heavy. The wings and
stabilizer are plug-in designs, and the
wings include fl aps.
Covering the airframe is a white,
iron-on fi lm with red trim and black
pinstripes that matches the color scheme
of the full-scale Stinson SR-9 (n-number
VH-UXL) that was used by the Vacuum
Oil Company back in 1936. A few
spots were slightly wrinkled, but a few
minutes of work with my covering iron
took care of them.
Matching the iron-on fi lm are the
prepainted fi berglass cowl and wheel
pants. The cowl features the Reliant’s
multitude of “bumps” around the
circumference. On the rear of the cowl,
there’s a cutout for exhaust and cooling.
The 16-page instruction manual
shows the steps in a series of drawings.
It’s slightly lacking in places, but an
experienced modeler should be able
to fi gure out the steps. Construction is
straightforward.
Rounding out the kit is the hardware,
which includes everything from the
thick aluminum landing gear halves, to
the fuel tank, the carbon-fi ber wing and
stabilizer tubes, the vacuum-formed
plastic windows and gear fairings,
wheels, pushrods, and stickers. There are
also bags with bolts, nuts, and smaller
pieces, as well as the pull-pull cable
hardware for the rudder. All threaded
hardware is metric.
The manual mentions that a sixchannel
system is needed. This is true
if you use Y harnesses for the ailerons
and special reversing Y harnesses for the
elevator halves and flap servos. However,
if you don’t want to use the Y harnesses,
you’ll need at least an eight-channel
radio that can handle three separate
mixes of master and slave servos.
This is an International Miniature
Aircraft Association-legal (IMAA)
aircraft, and if you want comply with
IMAA rules, you’ll need another channel
for the remote ignition cutoff.
Nothing in the manual covers what
types of servos are needed (standard,
high-torque, high-speed, etc.). Looking
at the list of recommended accessories
on the SR-9 page of Troy Built Models
(TBM) website, I found several servos
listed with torque ratings of 133 ounces
per square inch and higher. With the
large areas of the SR-9’s control surfaces,
I chose to use a combination of servos
I had on hand. Using a six-volt setup,
the lowest torque was the JR Sport
ST125MG servos at 142 ounce-inches.
Construction
If you choose to power the SR-9 with
a gasoline engine, be sure to have some
threadlock handy; nearly every step
where parts are bolted together requires
a drop or two. Any gasoline engine will
vibrate and quickly loosen even the
tightest of screws.
I normally don’t discuss the
construction of review models, but there
are a few problems areas that need to be
detailed. You can download the manual
from TBM’s website located in the
“Sources” section.
Construction starts by attaching the
ailerons and flaps to the wing
halves. Nylon “hinge-point”
hinges are used. Be sure to coat
the center section of each with
lithium grease, petroleum jelly,
or a suitable alternative to keep
the epoxy away from the pivot
point.
Install the aileron and flap
servos into the wing halves. The
servos mount to the back of
the preassembled servo covers.
Any standard-size servo easily
fits into the cutout; however,
because longer, slightly thicker,
heavy-duty servo arms are
needed, the servos cannot be
fitted with the arms in place.
Nor can the arms be fitted after
the servos are mounted.
I cut away the strip of
plywood between the two
mounting platforms, then
stiffened the remaining mounts
by wicking thin CA. After
it cures, the servo with the
horn attached could slide into
position.
Using a long, thin wire or a
string with a weight attached,
you can fish the servo wires and
extensions through the wing
halves. However, I found that I
had to trim a bit of the plywood
framing to allow the servos to
properly fit in the wings.
Once that’s finished, the
control hardware is attached.
The included control arms are epoxied
into the control surface. To keep them
aligned, insert the screw that holds in
the ball link through both control arms
and use it to hold the arms in position.
This keeps the holes aligned and makes
things easier.
The pushrods and ball links are heavyduty
parts, held to the servo by a wheel
collar. Be sure to grind a flat spot on the
pushrod for a secure fit. The ball links
can slide slightly between the control
horns. To take up this slack and have a
more precise link, cut two thin slices of
fuel tubing. Place them on either side of
the ball link to act as soft washers.
The elevator halves are next. Each
elevator half has its own servo mounted
inside. The servo fits well, but I had to
file away some of the wood so the servo
wire could exit without crimping.
When it was time to epoxy the
control horn pieces into the
elevators and the rudder, I ran
into the first major problem.
These horns are smaller than
those used on the wings, but
the precut slots are so big that
the horns are swimming inside
them.
I contacted TBM and we
decided to use the control
horns and fill the rest of the
opening with epoxy mixed with
microballoons. After trimming
the length of the horns to fit the
slots, everything was epoxied
into place and it fixed the
problem.
The manual shows that the rudder
should be epoxied into position next,
but I advise putting off this step.
A heavy-duty tail wheel is included,
which requires some assembly as well
as some measuring so everything works
properly. The aluminum plate that the
wire runs through had to be drilled out
to accept the 1/8-inch wire.
The prebent wire has extra length.
It’s designed to extend through the
outer cover, through an aluminum
mounting plate, through a plywood
plate, then through another hole
predrilled in a rib that is further up
into the rudder.
Once cut to length, this wire also
needs a few flats cut to match the
positions of the wheel collars. At this
point, everything can be bolted into
position except the tail wheel. Take a
few minutes to install the pull-pull
wires that steer this assembly.
After everything is mounted in
position, the rear cover, tail wheel, and
wheel collars are attached. Although
the manual doesn’t say to bolt on the
landing gear struts until later in the
build, now is a good time to do so. It
will give you a firmer foundation for
the next step.
This is why I mentioned postponing
attaching the rudder. During the engine
and cowl installation, you can stand
the fuselage vertically, setting it on the
fin. It provides a flat, solid surface. And
having the second landing gear struts in
place gives you support when propping
up the fuselage against a vertical
surface such as a wall or door.
TBM supplied an engine with the
review kit: the Pterodactyl PTE36R
36cc gas engine. It’s a rear intake,
rear exhaust, single-cylinder
engine and it mounts on four
aluminum standoffs.
To figure out the engine’s
mounting position, I skipped
forward in the manual and
epoxied the cowl ring into
place. This requires sanding
flat the area on the front of the
fuselage where the ring mounts.
Take a moment to wick in
some thin CA around each of
the blocks on the mounting
ring. A few of the parts were
moving around slightly on my
piece, so make sure nothing
falls off on yours. Be careful not to get
CA in any of the threads.
After the CA cured, I took some
time to position the cowl and get an
idea of how the engine would fit. The
cowl will take a bit of trimming to
allow the muffler and the front fins of
the head to clear.
The engine needed to be spaced
11/2 inches out from the firewall to
allow the cowl to fit properly. No
wood is included in the kit for this, so
I designed a spacer from plywood and
poplar.
You can download a diagram from
the www.ModelAviation.com website.
Paint it with clear polyurethane to
fuelproof it. To bolt the spacer to the
fuselage, I picked up a set of four 1/4-20
x 2 bolts and matching t-nuts.
Now is the time to finish the steps I
skipped. I mounted the rudder servo,
epoxied the rudder into place, and
installed the pull-pull cables. There’s
nothing wrong with the way these cables are installed, but
I had a hard time getting them right, especially the cables
coming forward from the tail wheel.
I’m not a surgeon, but I imagine trying to thread these
cables through the holes and tubes is like arthroscopic
surgery. I was using long needle-nose pliers at weird angles,
while reaching through the door and windscreen opening.
The rudder servo is mounted back in the passenger
compartment, which compounded the issue for me, but I
did manage to get it sorted out.
The throttle servo mounts slightly forward of the rudder
servo and has to operate a long throttle pushrod. A thin
metal pushrod is included in the kit, but in order to keep
electronic engine noise to a minimum, I substituted this for
a Du-Bro flexible pushrod system.
When I attached the throttle pushrod to the carburetor,
I took the time to add another pushrod that would let me
manually operate the choke through the cowl opening. A
simple z-bend went through the choke lever, and I rigged
up a bracket that would support the bracket from the front,
which was held in place by the engine’s mounting bolts.
I also substituted one other item: the fuel tank. There’s
nothing wrong with the supplied tank; it’s a standard blowmolded
tank. However, I received a RotoFlow fuel tank
from J&L Power Products and this looked to be the perfect
time to try it out.
The RotoFlow comes preassembled and the clunk rotates
around a central pickup line. All I had to do is hook up the
fuel and vent lines, and then strap it in position with the tie
wraps that came in the SR-9 kit.
The vent line wraps around the tank. This is a tip I picked
up in some of my work with gas-powered International
Miniature Aerobatic Club (IMAC) airplanes. The vent line
on a gas engine is not pressurized and vents to the outside.
Fuel can leak out of the vent if the airplane is in certain
positions such as a vertical downline.
To keep this from happening, the vent line is wrapped
along the side, the back, and back along the other side to
form a loop. It works well and only requires a bit more
tubing for a trouble-free tank installation.
Next the landing gear fairings, wheels, and wheel pants
are installed. If you attached the landing gear struts first,
make sure you put the landing gear fairings on first. They
are vacuum-formed pieces and there is a left and right so
pay attention. They must be trimmed and if you cut to the
molded-in trim line, the fit is perfect.
You’ll have to eyeball a cutout in each fairing to allow the
threaded spacers to protrude. These spacers are where the
wing struts bolt to the fuselage. Glue the fairings in position
and then attach the wheels.
When it came time to attach the wheels, another problem
occurred. There was no included hardware that was correct
for mounting the wheels. The wheels have a 6mm diameter
hole, yet the instructions called for a 5mm bolt to make up
the axle. None was included.
The remaining bolts were 3mm, which are too thin and
too short. I picked up a set of 1/4- 20 x 21/4 hex bolts and
matching nuts that worked. They are slightly larger in
diameter than the 6mm size, so the
wheels and the landing gear holes had
to be drilled larger.
There are no predrilled holes in the
wheel pants, so you’ll have to eyeball
that as well. There are provisions for a
smaller screw to be installed above the
axle bolt that will stop the wheel pants
from turning or spinning around the
axle.
With the SR-9 on its feet, I set about
installing the radio. There’s plenty of
room in the fuselage to mount nearly
any brand and size of radio you choose.
Extensions have to be run back to the
left and right elevator servos, as well as
up to the wing roots for the flap and
elevator servos on each side. To hold
these in place I picked up a set of Wire
Keeps. These small, die-cut foam pieces
have adhesive strips on the back and
make installations neater. They also keep
the extensions to the wing root in place,
and out of sight.
I installed a Smart-Fly remote ignition
cutoff. Quest makes these cutoffs and
they allow you to cut the ignition
from your transmitter using a spare
channel. It utilizes a fiber-optic system
so the ignition is opto-isolated from the
receiver.
The receiver, dual batteries, and the
Smart-Fly system are held in place with
hook-and-loop. I glued in a couple of
squares of 1/16-inch balsa sheet to go
under the receiver and batteries, giving
the hook-and-loopmaterial a surface area
on which to mount.
I included a plywood plate inside
the port cabin door that is not part of
the kit. I despise mounting switches on
the surface of the fuselage of any Scale
airplane, and make every attempt to hide
them. With such a large fuselage and
two functional doors, it seems like the
perfect place to hide both the receiver
power and ignition power switches. I
did a bit of measuring and made this
plate from some spare light plywood.
In hindsight, I should have also put the
Aluminum Fuel Dot on the plate as
well.
I originally set up the radio with a
master/slave combination for both
ailerons servos, both elevator servos,
and both flap servos. After extensive
experimenting with my 9503 radio, I
found that the elevator servos weren’t
matching in throw and speed. They
weren’t off too much, but it was enough
that it would have induced a bit of roll,
making trimming the SR-9 impossible.
No amount of transmitter programming
could entirely fix the problem and I
found that the flaps were worse. The
9503 had no midpoint flap adjustment
so both servos were way out of sync.
So, to solve this, I bought a couple of
reversing Y harnesses and installed
them, which solved the elevator and flap
problems.
When the radio was in place, I finished
the fuselage by installing the windows,
windscreen, and the cabin doors. With
the exception of the windscreen, all
the windows are glued in place. The
windscreen is held in place with a
number of smaller screws.
Several of the screws which are
installed above the tank area only thread
into a thin layer of balsa. Even wicking
a bit of thin CA will not give enough
material for the screws to firmly hold.
I cut a few 3/4-inch squares from some
1/8-inch light plywood. After I drilled
the holes for the windscreen, I could
then feel where these squares should
be applied. I used medium CA to hold
them in position. I redrilled the holes
to extend through the plywood so the
screws tightened nicely.
The instructions say to epoxy the
door hinges into place on the door and
the fuselage. If the hinges were epoxied,
the doors would only open roughly 60°.
I decided not epoxy the hinges to the
fuselage, but to hold each hinge in place
with a single screw (the same type that
holds the windscreen in position). This
will allow me to completely remove the
doors if a problem arises.
One last issue popped up during the
final assembly. The wing struts are held
in place with two screws: one for the
top, and one for the bottom. The bottom
screw threads into the spacer that
extends from the landing gear fairings,
but there were no screws included
that were short enough to allow me
to tighten the strut firmly. I had to
outsource those screws as well. They
need to be roughly 1/4-inch to 3/8-inch
long.
The assembly was finally finished.
Several problems were overcome;
however, when it’s finished, it really
looks great.
It’s hard to determine the exact
assembly time, but I’d estimate 15 to 20
hours. I spent a couple hours each night
for 2 weeks, but not all of it was actual
building. Some was waiting for the
epoxy to cure.
There was a good surprise: the CG
was spot-on, even with the twin receiver
batteries. The weight came in at 191/4
pounds, ready to fly.
One last note before I get to the
flying portion. Have you ever put
together a project such as a piece of
furniture, a child’s play set, etc., and
have a few screws and other hardware
left over? Our review SR-9 kit takes
this to another level. After doublechecking
to make sure I didn’t miss any
steps, I found myself with a tidy pile of
hardware: a handful of screws, nuts and
bolts, a few dozen washers of various
sizes, t-nuts, flat hinges, two sizes of
point hinges, and even an extra threaded
spacer.
These all can be used in future
projects, but it seems as though someone
at the factory scooped up a few handfuls
of miscellaneous hardware and put it in
the kit. Maybe this helps make up for
the hardware that wasn’t included.
You’ll probably need spares of the
smaller screws and washers that secure
the stabilizer halves and the wing struts.
Have extras so you won’t ruin a day’s
flying because of a missing screw.
Engine First Run
Before the first flight, I took the time
to start the break-in procedure on the
Pterodactyl PTE36R 36cc. A fuel-tooil
mixture between 20:1 and 40:1 is
called for, so I used a 30:1 mixture I
had on hand. The break-in propeller is
not specified in the manual, but TBM’s
website suggests an 18 x 8 propeller and
puts the break-in time at 5 gallons of
fuel.
With everything in place, it was time
to start flipping. It took roughly 25 to 30
flips to get the first ignition burp when
choked, then another 10 or so to bring
the engine to life. I ran the first tankful at
25% throttle with no adjustments to the
carburetor to lubricate everything.
During the next run, I adjusted
the carburetor so the top end was
approximately 7,800 rpm and idle was
roughly 2,400. Satisfied with this as a
starting point, it was time to put on the
cowl and see how the SR-9 flew.
Flying
It will take roughly 10 to 15 minutes
to bolt the wings and stabilizer into
place, attach the wing struts, and top off
the tank when assembling at the field.
The plug-in wings and stabilizer halves
are easy to install and have a good, firm
fit. The stabilizer halves are held in place
with a couple of screws.
The wings are secured by reaching
into the fuselage through the cabin
doors and threading a 6mm screw into
each wing root. You’ll probably want
to assemble it on a table to keep from
crawling under the wings.
Firing the PTE36R for the first flight,
I found that the idle was too high for
proper taxiing, but because the cowl was
already on and I knew that I could kill
the engine at any time with the Smart-
Fly ignition cutoff, I went with it as is.
The SR-9 took off in roughly 100
feet and had a good climb rate at only
three-fourths throttle. After making a
few passes for trim, I was fighting the
model because it was way too sensitive,
especially in pitch (elevator). Most of
the early turns looked like those a Pylon
racer would make. Any touch of the
elevator equated big changes in altitude.
The manual calls out a throw or
40mm each way; that’s a touch over 11/2
inches. The rudder was similarly touchy,
so I set up for a few practice approaches.
With the idle set as high as it was, the
Stinson would not land, even with flaps,
so I climbed up to a safe altitude, cut the
throttle, and glided in for an uneventful
landing.
It was time for reprogramming. I
cut the elevator and rudder throw
considerably (1 inch). I also added
exponential: 60% on the elevator and
45% on the rudder. I left the ailerons
alone.
I removed the cowl and adjusted the
idle setting, lowering it to approximately
2,000 rpm. The engine felt as though
it would quit if it was any lower, so I
bolted everything together and it was on
to flight two.
Although not quite perfect, the flight
was much more scalelike and I was able
to get a better feel for the model. You
might think it would fly like a giant
trainer, but it requires some attention,
especially in the turns where you’ll have
to coordinate the ailerons and rudder.
The full-scale Stinson is not an
aerobatic design, but this model will do
modest loops, stall turns, and barrel rolls,
and will spin easily with that big rudder.
Cruising around is where the Stinson
shines, and low-level passes are beautiful.
The 9503 has a three-position switch
for flaps, and I had the most fun at full
flaps. At this setting, the flaps were
deployed close to 60° and they slow
the SR-9 to a crawl. With the ample
power of the PTE36R, there’s plenty of
propeller blast over the control surfaces.
Landings are easier and slower using
flaps. I found it better to land with a
steeper approach, a little more power,
and then flare right roughly 2 feet off
the runway. It will settle, and normally
you can land without too much
bouncing. The gear and hard foam tires
don’t have much give, so it is very easy
to bounce a landing on pavement, but
the Stinson lands well on grass.
Conclusion
TBM’s Stinson SR-9 is a beautiful
airplane and faithful to its full-scale
counterpart. It’s a big model with a 100-
inch wingspan, but this size makes it
easy to work on and in. There are some
problems along the way, but it’s nothing
that an intermediate or experienced
modeler can’t overcome, and I described
how to dodge these problems in the
review!
In the air, the SR-9 looks the part.
Resist the urge to fly it at full throttle,
and please don’t shoehorn a 50cc (or
larger) engine in the cowl. It doesn’t
need it. The PTE36 is plenty of power
and if you fly the SR-9 as intended,
you’ll rarely need anything over half
throttle.
Edition: Model Aviation - 2012/10
Page Numbers: 46,47,48,49,50,51,52
When I review a Scale model, I
generally offer some history;
however, this is quite an
involved review, so I’m going to get
straight to it. I will ask that you read
the fl ying portion of this review fi rst
so you’ll understand how good this
ECOMRC Stinson SR-9 ARF is when
it’s fi nished.
Go ahead, jump to the fl ying portion!
I’ll wait …
Now that you know what the result
is, let’s start from the beginning. The
ECOMRC Stinson SR-9 ARF is a big
airplane and it’s double boxed for
protection during shipping. Unpacking
everything, the only damage I found was
on the lower tip of the windscreen, but
that was easily trimmed away.
The airframe is completely built-up
from balsa and laser-cut plywood. It is
stiff, but not overly heavy. The wings and
stabilizer are plug-in designs, and the
wings include fl aps.
Covering the airframe is a white,
iron-on fi lm with red trim and black
pinstripes that matches the color scheme
of the full-scale Stinson SR-9 (n-number
VH-UXL) that was used by the Vacuum
Oil Company back in 1936. A few
spots were slightly wrinkled, but a few
minutes of work with my covering iron
took care of them.
Matching the iron-on fi lm are the
prepainted fi berglass cowl and wheel
pants. The cowl features the Reliant’s
multitude of “bumps” around the
circumference. On the rear of the cowl,
there’s a cutout for exhaust and cooling.
The 16-page instruction manual
shows the steps in a series of drawings.
It’s slightly lacking in places, but an
experienced modeler should be able
to fi gure out the steps. Construction is
straightforward.
Rounding out the kit is the hardware,
which includes everything from the
thick aluminum landing gear halves, to
the fuel tank, the carbon-fi ber wing and
stabilizer tubes, the vacuum-formed
plastic windows and gear fairings,
wheels, pushrods, and stickers. There are
also bags with bolts, nuts, and smaller
pieces, as well as the pull-pull cable
hardware for the rudder. All threaded
hardware is metric.
The manual mentions that a sixchannel
system is needed. This is true
if you use Y harnesses for the ailerons
and special reversing Y harnesses for the
elevator halves and flap servos. However,
if you don’t want to use the Y harnesses,
you’ll need at least an eight-channel
radio that can handle three separate
mixes of master and slave servos.
This is an International Miniature
Aircraft Association-legal (IMAA)
aircraft, and if you want comply with
IMAA rules, you’ll need another channel
for the remote ignition cutoff.
Nothing in the manual covers what
types of servos are needed (standard,
high-torque, high-speed, etc.). Looking
at the list of recommended accessories
on the SR-9 page of Troy Built Models
(TBM) website, I found several servos
listed with torque ratings of 133 ounces
per square inch and higher. With the
large areas of the SR-9’s control surfaces,
I chose to use a combination of servos
I had on hand. Using a six-volt setup,
the lowest torque was the JR Sport
ST125MG servos at 142 ounce-inches.
Construction
If you choose to power the SR-9 with
a gasoline engine, be sure to have some
threadlock handy; nearly every step
where parts are bolted together requires
a drop or two. Any gasoline engine will
vibrate and quickly loosen even the
tightest of screws.
I normally don’t discuss the
construction of review models, but there
are a few problems areas that need to be
detailed. You can download the manual
from TBM’s website located in the
“Sources” section.
Construction starts by attaching the
ailerons and flaps to the wing
halves. Nylon “hinge-point”
hinges are used. Be sure to coat
the center section of each with
lithium grease, petroleum jelly,
or a suitable alternative to keep
the epoxy away from the pivot
point.
Install the aileron and flap
servos into the wing halves. The
servos mount to the back of
the preassembled servo covers.
Any standard-size servo easily
fits into the cutout; however,
because longer, slightly thicker,
heavy-duty servo arms are
needed, the servos cannot be
fitted with the arms in place.
Nor can the arms be fitted after
the servos are mounted.
I cut away the strip of
plywood between the two
mounting platforms, then
stiffened the remaining mounts
by wicking thin CA. After
it cures, the servo with the
horn attached could slide into
position.
Using a long, thin wire or a
string with a weight attached,
you can fish the servo wires and
extensions through the wing
halves. However, I found that I
had to trim a bit of the plywood
framing to allow the servos to
properly fit in the wings.
Once that’s finished, the
control hardware is attached.
The included control arms are epoxied
into the control surface. To keep them
aligned, insert the screw that holds in
the ball link through both control arms
and use it to hold the arms in position.
This keeps the holes aligned and makes
things easier.
The pushrods and ball links are heavyduty
parts, held to the servo by a wheel
collar. Be sure to grind a flat spot on the
pushrod for a secure fit. The ball links
can slide slightly between the control
horns. To take up this slack and have a
more precise link, cut two thin slices of
fuel tubing. Place them on either side of
the ball link to act as soft washers.
The elevator halves are next. Each
elevator half has its own servo mounted
inside. The servo fits well, but I had to
file away some of the wood so the servo
wire could exit without crimping.
When it was time to epoxy the
control horn pieces into the
elevators and the rudder, I ran
into the first major problem.
These horns are smaller than
those used on the wings, but
the precut slots are so big that
the horns are swimming inside
them.
I contacted TBM and we
decided to use the control
horns and fill the rest of the
opening with epoxy mixed with
microballoons. After trimming
the length of the horns to fit the
slots, everything was epoxied
into place and it fixed the
problem.
The manual shows that the rudder
should be epoxied into position next,
but I advise putting off this step.
A heavy-duty tail wheel is included,
which requires some assembly as well
as some measuring so everything works
properly. The aluminum plate that the
wire runs through had to be drilled out
to accept the 1/8-inch wire.
The prebent wire has extra length.
It’s designed to extend through the
outer cover, through an aluminum
mounting plate, through a plywood
plate, then through another hole
predrilled in a rib that is further up
into the rudder.
Once cut to length, this wire also
needs a few flats cut to match the
positions of the wheel collars. At this
point, everything can be bolted into
position except the tail wheel. Take a
few minutes to install the pull-pull
wires that steer this assembly.
After everything is mounted in
position, the rear cover, tail wheel, and
wheel collars are attached. Although
the manual doesn’t say to bolt on the
landing gear struts until later in the
build, now is a good time to do so. It
will give you a firmer foundation for
the next step.
This is why I mentioned postponing
attaching the rudder. During the engine
and cowl installation, you can stand
the fuselage vertically, setting it on the
fin. It provides a flat, solid surface. And
having the second landing gear struts in
place gives you support when propping
up the fuselage against a vertical
surface such as a wall or door.
TBM supplied an engine with the
review kit: the Pterodactyl PTE36R
36cc gas engine. It’s a rear intake,
rear exhaust, single-cylinder
engine and it mounts on four
aluminum standoffs.
To figure out the engine’s
mounting position, I skipped
forward in the manual and
epoxied the cowl ring into
place. This requires sanding
flat the area on the front of the
fuselage where the ring mounts.
Take a moment to wick in
some thin CA around each of
the blocks on the mounting
ring. A few of the parts were
moving around slightly on my
piece, so make sure nothing
falls off on yours. Be careful not to get
CA in any of the threads.
After the CA cured, I took some
time to position the cowl and get an
idea of how the engine would fit. The
cowl will take a bit of trimming to
allow the muffler and the front fins of
the head to clear.
The engine needed to be spaced
11/2 inches out from the firewall to
allow the cowl to fit properly. No
wood is included in the kit for this, so
I designed a spacer from plywood and
poplar.
You can download a diagram from
the www.ModelAviation.com website.
Paint it with clear polyurethane to
fuelproof it. To bolt the spacer to the
fuselage, I picked up a set of four 1/4-20
x 2 bolts and matching t-nuts.
Now is the time to finish the steps I
skipped. I mounted the rudder servo,
epoxied the rudder into place, and
installed the pull-pull cables. There’s
nothing wrong with the way these cables are installed, but
I had a hard time getting them right, especially the cables
coming forward from the tail wheel.
I’m not a surgeon, but I imagine trying to thread these
cables through the holes and tubes is like arthroscopic
surgery. I was using long needle-nose pliers at weird angles,
while reaching through the door and windscreen opening.
The rudder servo is mounted back in the passenger
compartment, which compounded the issue for me, but I
did manage to get it sorted out.
The throttle servo mounts slightly forward of the rudder
servo and has to operate a long throttle pushrod. A thin
metal pushrod is included in the kit, but in order to keep
electronic engine noise to a minimum, I substituted this for
a Du-Bro flexible pushrod system.
When I attached the throttle pushrod to the carburetor,
I took the time to add another pushrod that would let me
manually operate the choke through the cowl opening. A
simple z-bend went through the choke lever, and I rigged
up a bracket that would support the bracket from the front,
which was held in place by the engine’s mounting bolts.
I also substituted one other item: the fuel tank. There’s
nothing wrong with the supplied tank; it’s a standard blowmolded
tank. However, I received a RotoFlow fuel tank
from J&L Power Products and this looked to be the perfect
time to try it out.
The RotoFlow comes preassembled and the clunk rotates
around a central pickup line. All I had to do is hook up the
fuel and vent lines, and then strap it in position with the tie
wraps that came in the SR-9 kit.
The vent line wraps around the tank. This is a tip I picked
up in some of my work with gas-powered International
Miniature Aerobatic Club (IMAC) airplanes. The vent line
on a gas engine is not pressurized and vents to the outside.
Fuel can leak out of the vent if the airplane is in certain
positions such as a vertical downline.
To keep this from happening, the vent line is wrapped
along the side, the back, and back along the other side to
form a loop. It works well and only requires a bit more
tubing for a trouble-free tank installation.
Next the landing gear fairings, wheels, and wheel pants
are installed. If you attached the landing gear struts first,
make sure you put the landing gear fairings on first. They
are vacuum-formed pieces and there is a left and right so
pay attention. They must be trimmed and if you cut to the
molded-in trim line, the fit is perfect.
You’ll have to eyeball a cutout in each fairing to allow the
threaded spacers to protrude. These spacers are where the
wing struts bolt to the fuselage. Glue the fairings in position
and then attach the wheels.
When it came time to attach the wheels, another problem
occurred. There was no included hardware that was correct
for mounting the wheels. The wheels have a 6mm diameter
hole, yet the instructions called for a 5mm bolt to make up
the axle. None was included.
The remaining bolts were 3mm, which are too thin and
too short. I picked up a set of 1/4- 20 x 21/4 hex bolts and
matching nuts that worked. They are slightly larger in
diameter than the 6mm size, so the
wheels and the landing gear holes had
to be drilled larger.
There are no predrilled holes in the
wheel pants, so you’ll have to eyeball
that as well. There are provisions for a
smaller screw to be installed above the
axle bolt that will stop the wheel pants
from turning or spinning around the
axle.
With the SR-9 on its feet, I set about
installing the radio. There’s plenty of
room in the fuselage to mount nearly
any brand and size of radio you choose.
Extensions have to be run back to the
left and right elevator servos, as well as
up to the wing roots for the flap and
elevator servos on each side. To hold
these in place I picked up a set of Wire
Keeps. These small, die-cut foam pieces
have adhesive strips on the back and
make installations neater. They also keep
the extensions to the wing root in place,
and out of sight.
I installed a Smart-Fly remote ignition
cutoff. Quest makes these cutoffs and
they allow you to cut the ignition
from your transmitter using a spare
channel. It utilizes a fiber-optic system
so the ignition is opto-isolated from the
receiver.
The receiver, dual batteries, and the
Smart-Fly system are held in place with
hook-and-loop. I glued in a couple of
squares of 1/16-inch balsa sheet to go
under the receiver and batteries, giving
the hook-and-loopmaterial a surface area
on which to mount.
I included a plywood plate inside
the port cabin door that is not part of
the kit. I despise mounting switches on
the surface of the fuselage of any Scale
airplane, and make every attempt to hide
them. With such a large fuselage and
two functional doors, it seems like the
perfect place to hide both the receiver
power and ignition power switches. I
did a bit of measuring and made this
plate from some spare light plywood.
In hindsight, I should have also put the
Aluminum Fuel Dot on the plate as
well.
I originally set up the radio with a
master/slave combination for both
ailerons servos, both elevator servos,
and both flap servos. After extensive
experimenting with my 9503 radio, I
found that the elevator servos weren’t
matching in throw and speed. They
weren’t off too much, but it was enough
that it would have induced a bit of roll,
making trimming the SR-9 impossible.
No amount of transmitter programming
could entirely fix the problem and I
found that the flaps were worse. The
9503 had no midpoint flap adjustment
so both servos were way out of sync.
So, to solve this, I bought a couple of
reversing Y harnesses and installed
them, which solved the elevator and flap
problems.
When the radio was in place, I finished
the fuselage by installing the windows,
windscreen, and the cabin doors. With
the exception of the windscreen, all
the windows are glued in place. The
windscreen is held in place with a
number of smaller screws.
Several of the screws which are
installed above the tank area only thread
into a thin layer of balsa. Even wicking
a bit of thin CA will not give enough
material for the screws to firmly hold.
I cut a few 3/4-inch squares from some
1/8-inch light plywood. After I drilled
the holes for the windscreen, I could
then feel where these squares should
be applied. I used medium CA to hold
them in position. I redrilled the holes
to extend through the plywood so the
screws tightened nicely.
The instructions say to epoxy the
door hinges into place on the door and
the fuselage. If the hinges were epoxied,
the doors would only open roughly 60°.
I decided not epoxy the hinges to the
fuselage, but to hold each hinge in place
with a single screw (the same type that
holds the windscreen in position). This
will allow me to completely remove the
doors if a problem arises.
One last issue popped up during the
final assembly. The wing struts are held
in place with two screws: one for the
top, and one for the bottom. The bottom
screw threads into the spacer that
extends from the landing gear fairings,
but there were no screws included
that were short enough to allow me
to tighten the strut firmly. I had to
outsource those screws as well. They
need to be roughly 1/4-inch to 3/8-inch
long.
The assembly was finally finished.
Several problems were overcome;
however, when it’s finished, it really
looks great.
It’s hard to determine the exact
assembly time, but I’d estimate 15 to 20
hours. I spent a couple hours each night
for 2 weeks, but not all of it was actual
building. Some was waiting for the
epoxy to cure.
There was a good surprise: the CG
was spot-on, even with the twin receiver
batteries. The weight came in at 191/4
pounds, ready to fly.
One last note before I get to the
flying portion. Have you ever put
together a project such as a piece of
furniture, a child’s play set, etc., and
have a few screws and other hardware
left over? Our review SR-9 kit takes
this to another level. After doublechecking
to make sure I didn’t miss any
steps, I found myself with a tidy pile of
hardware: a handful of screws, nuts and
bolts, a few dozen washers of various
sizes, t-nuts, flat hinges, two sizes of
point hinges, and even an extra threaded
spacer.
These all can be used in future
projects, but it seems as though someone
at the factory scooped up a few handfuls
of miscellaneous hardware and put it in
the kit. Maybe this helps make up for
the hardware that wasn’t included.
You’ll probably need spares of the
smaller screws and washers that secure
the stabilizer halves and the wing struts.
Have extras so you won’t ruin a day’s
flying because of a missing screw.
Engine First Run
Before the first flight, I took the time
to start the break-in procedure on the
Pterodactyl PTE36R 36cc. A fuel-tooil
mixture between 20:1 and 40:1 is
called for, so I used a 30:1 mixture I
had on hand. The break-in propeller is
not specified in the manual, but TBM’s
website suggests an 18 x 8 propeller and
puts the break-in time at 5 gallons of
fuel.
With everything in place, it was time
to start flipping. It took roughly 25 to 30
flips to get the first ignition burp when
choked, then another 10 or so to bring
the engine to life. I ran the first tankful at
25% throttle with no adjustments to the
carburetor to lubricate everything.
During the next run, I adjusted
the carburetor so the top end was
approximately 7,800 rpm and idle was
roughly 2,400. Satisfied with this as a
starting point, it was time to put on the
cowl and see how the SR-9 flew.
Flying
It will take roughly 10 to 15 minutes
to bolt the wings and stabilizer into
place, attach the wing struts, and top off
the tank when assembling at the field.
The plug-in wings and stabilizer halves
are easy to install and have a good, firm
fit. The stabilizer halves are held in place
with a couple of screws.
The wings are secured by reaching
into the fuselage through the cabin
doors and threading a 6mm screw into
each wing root. You’ll probably want
to assemble it on a table to keep from
crawling under the wings.
Firing the PTE36R for the first flight,
I found that the idle was too high for
proper taxiing, but because the cowl was
already on and I knew that I could kill
the engine at any time with the Smart-
Fly ignition cutoff, I went with it as is.
The SR-9 took off in roughly 100
feet and had a good climb rate at only
three-fourths throttle. After making a
few passes for trim, I was fighting the
model because it was way too sensitive,
especially in pitch (elevator). Most of
the early turns looked like those a Pylon
racer would make. Any touch of the
elevator equated big changes in altitude.
The manual calls out a throw or
40mm each way; that’s a touch over 11/2
inches. The rudder was similarly touchy,
so I set up for a few practice approaches.
With the idle set as high as it was, the
Stinson would not land, even with flaps,
so I climbed up to a safe altitude, cut the
throttle, and glided in for an uneventful
landing.
It was time for reprogramming. I
cut the elevator and rudder throw
considerably (1 inch). I also added
exponential: 60% on the elevator and
45% on the rudder. I left the ailerons
alone.
I removed the cowl and adjusted the
idle setting, lowering it to approximately
2,000 rpm. The engine felt as though
it would quit if it was any lower, so I
bolted everything together and it was on
to flight two.
Although not quite perfect, the flight
was much more scalelike and I was able
to get a better feel for the model. You
might think it would fly like a giant
trainer, but it requires some attention,
especially in the turns where you’ll have
to coordinate the ailerons and rudder.
The full-scale Stinson is not an
aerobatic design, but this model will do
modest loops, stall turns, and barrel rolls,
and will spin easily with that big rudder.
Cruising around is where the Stinson
shines, and low-level passes are beautiful.
The 9503 has a three-position switch
for flaps, and I had the most fun at full
flaps. At this setting, the flaps were
deployed close to 60° and they slow
the SR-9 to a crawl. With the ample
power of the PTE36R, there’s plenty of
propeller blast over the control surfaces.
Landings are easier and slower using
flaps. I found it better to land with a
steeper approach, a little more power,
and then flare right roughly 2 feet off
the runway. It will settle, and normally
you can land without too much
bouncing. The gear and hard foam tires
don’t have much give, so it is very easy
to bounce a landing on pavement, but
the Stinson lands well on grass.
Conclusion
TBM’s Stinson SR-9 is a beautiful
airplane and faithful to its full-scale
counterpart. It’s a big model with a 100-
inch wingspan, but this size makes it
easy to work on and in. There are some
problems along the way, but it’s nothing
that an intermediate or experienced
modeler can’t overcome, and I described
how to dodge these problems in the
review!
In the air, the SR-9 looks the part.
Resist the urge to fly it at full throttle,
and please don’t shoehorn a 50cc (or
larger) engine in the cowl. It doesn’t
need it. The PTE36 is plenty of power
and if you fly the SR-9 as intended,
you’ll rarely need anything over half
throttle.
Edition: Model Aviation - 2012/10
Page Numbers: 46,47,48,49,50,51,52
When I review a Scale model, I
generally offer some history;
however, this is quite an
involved review, so I’m going to get
straight to it. I will ask that you read
the fl ying portion of this review fi rst
so you’ll understand how good this
ECOMRC Stinson SR-9 ARF is when
it’s fi nished.
Go ahead, jump to the fl ying portion!
I’ll wait …
Now that you know what the result
is, let’s start from the beginning. The
ECOMRC Stinson SR-9 ARF is a big
airplane and it’s double boxed for
protection during shipping. Unpacking
everything, the only damage I found was
on the lower tip of the windscreen, but
that was easily trimmed away.
The airframe is completely built-up
from balsa and laser-cut plywood. It is
stiff, but not overly heavy. The wings and
stabilizer are plug-in designs, and the
wings include fl aps.
Covering the airframe is a white,
iron-on fi lm with red trim and black
pinstripes that matches the color scheme
of the full-scale Stinson SR-9 (n-number
VH-UXL) that was used by the Vacuum
Oil Company back in 1936. A few
spots were slightly wrinkled, but a few
minutes of work with my covering iron
took care of them.
Matching the iron-on fi lm are the
prepainted fi berglass cowl and wheel
pants. The cowl features the Reliant’s
multitude of “bumps” around the
circumference. On the rear of the cowl,
there’s a cutout for exhaust and cooling.
The 16-page instruction manual
shows the steps in a series of drawings.
It’s slightly lacking in places, but an
experienced modeler should be able
to fi gure out the steps. Construction is
straightforward.
Rounding out the kit is the hardware,
which includes everything from the
thick aluminum landing gear halves, to
the fuel tank, the carbon-fi ber wing and
stabilizer tubes, the vacuum-formed
plastic windows and gear fairings,
wheels, pushrods, and stickers. There are
also bags with bolts, nuts, and smaller
pieces, as well as the pull-pull cable
hardware for the rudder. All threaded
hardware is metric.
The manual mentions that a sixchannel
system is needed. This is true
if you use Y harnesses for the ailerons
and special reversing Y harnesses for the
elevator halves and flap servos. However,
if you don’t want to use the Y harnesses,
you’ll need at least an eight-channel
radio that can handle three separate
mixes of master and slave servos.
This is an International Miniature
Aircraft Association-legal (IMAA)
aircraft, and if you want comply with
IMAA rules, you’ll need another channel
for the remote ignition cutoff.
Nothing in the manual covers what
types of servos are needed (standard,
high-torque, high-speed, etc.). Looking
at the list of recommended accessories
on the SR-9 page of Troy Built Models
(TBM) website, I found several servos
listed with torque ratings of 133 ounces
per square inch and higher. With the
large areas of the SR-9’s control surfaces,
I chose to use a combination of servos
I had on hand. Using a six-volt setup,
the lowest torque was the JR Sport
ST125MG servos at 142 ounce-inches.
Construction
If you choose to power the SR-9 with
a gasoline engine, be sure to have some
threadlock handy; nearly every step
where parts are bolted together requires
a drop or two. Any gasoline engine will
vibrate and quickly loosen even the
tightest of screws.
I normally don’t discuss the
construction of review models, but there
are a few problems areas that need to be
detailed. You can download the manual
from TBM’s website located in the
“Sources” section.
Construction starts by attaching the
ailerons and flaps to the wing
halves. Nylon “hinge-point”
hinges are used. Be sure to coat
the center section of each with
lithium grease, petroleum jelly,
or a suitable alternative to keep
the epoxy away from the pivot
point.
Install the aileron and flap
servos into the wing halves. The
servos mount to the back of
the preassembled servo covers.
Any standard-size servo easily
fits into the cutout; however,
because longer, slightly thicker,
heavy-duty servo arms are
needed, the servos cannot be
fitted with the arms in place.
Nor can the arms be fitted after
the servos are mounted.
I cut away the strip of
plywood between the two
mounting platforms, then
stiffened the remaining mounts
by wicking thin CA. After
it cures, the servo with the
horn attached could slide into
position.
Using a long, thin wire or a
string with a weight attached,
you can fish the servo wires and
extensions through the wing
halves. However, I found that I
had to trim a bit of the plywood
framing to allow the servos to
properly fit in the wings.
Once that’s finished, the
control hardware is attached.
The included control arms are epoxied
into the control surface. To keep them
aligned, insert the screw that holds in
the ball link through both control arms
and use it to hold the arms in position.
This keeps the holes aligned and makes
things easier.
The pushrods and ball links are heavyduty
parts, held to the servo by a wheel
collar. Be sure to grind a flat spot on the
pushrod for a secure fit. The ball links
can slide slightly between the control
horns. To take up this slack and have a
more precise link, cut two thin slices of
fuel tubing. Place them on either side of
the ball link to act as soft washers.
The elevator halves are next. Each
elevator half has its own servo mounted
inside. The servo fits well, but I had to
file away some of the wood so the servo
wire could exit without crimping.
When it was time to epoxy the
control horn pieces into the
elevators and the rudder, I ran
into the first major problem.
These horns are smaller than
those used on the wings, but
the precut slots are so big that
the horns are swimming inside
them.
I contacted TBM and we
decided to use the control
horns and fill the rest of the
opening with epoxy mixed with
microballoons. After trimming
the length of the horns to fit the
slots, everything was epoxied
into place and it fixed the
problem.
The manual shows that the rudder
should be epoxied into position next,
but I advise putting off this step.
A heavy-duty tail wheel is included,
which requires some assembly as well
as some measuring so everything works
properly. The aluminum plate that the
wire runs through had to be drilled out
to accept the 1/8-inch wire.
The prebent wire has extra length.
It’s designed to extend through the
outer cover, through an aluminum
mounting plate, through a plywood
plate, then through another hole
predrilled in a rib that is further up
into the rudder.
Once cut to length, this wire also
needs a few flats cut to match the
positions of the wheel collars. At this
point, everything can be bolted into
position except the tail wheel. Take a
few minutes to install the pull-pull
wires that steer this assembly.
After everything is mounted in
position, the rear cover, tail wheel, and
wheel collars are attached. Although
the manual doesn’t say to bolt on the
landing gear struts until later in the
build, now is a good time to do so. It
will give you a firmer foundation for
the next step.
This is why I mentioned postponing
attaching the rudder. During the engine
and cowl installation, you can stand
the fuselage vertically, setting it on the
fin. It provides a flat, solid surface. And
having the second landing gear struts in
place gives you support when propping
up the fuselage against a vertical
surface such as a wall or door.
TBM supplied an engine with the
review kit: the Pterodactyl PTE36R
36cc gas engine. It’s a rear intake,
rear exhaust, single-cylinder
engine and it mounts on four
aluminum standoffs.
To figure out the engine’s
mounting position, I skipped
forward in the manual and
epoxied the cowl ring into
place. This requires sanding
flat the area on the front of the
fuselage where the ring mounts.
Take a moment to wick in
some thin CA around each of
the blocks on the mounting
ring. A few of the parts were
moving around slightly on my
piece, so make sure nothing
falls off on yours. Be careful not to get
CA in any of the threads.
After the CA cured, I took some
time to position the cowl and get an
idea of how the engine would fit. The
cowl will take a bit of trimming to
allow the muffler and the front fins of
the head to clear.
The engine needed to be spaced
11/2 inches out from the firewall to
allow the cowl to fit properly. No
wood is included in the kit for this, so
I designed a spacer from plywood and
poplar.
You can download a diagram from
the www.ModelAviation.com website.
Paint it with clear polyurethane to
fuelproof it. To bolt the spacer to the
fuselage, I picked up a set of four 1/4-20
x 2 bolts and matching t-nuts.
Now is the time to finish the steps I
skipped. I mounted the rudder servo,
epoxied the rudder into place, and
installed the pull-pull cables. There’s
nothing wrong with the way these cables are installed, but
I had a hard time getting them right, especially the cables
coming forward from the tail wheel.
I’m not a surgeon, but I imagine trying to thread these
cables through the holes and tubes is like arthroscopic
surgery. I was using long needle-nose pliers at weird angles,
while reaching through the door and windscreen opening.
The rudder servo is mounted back in the passenger
compartment, which compounded the issue for me, but I
did manage to get it sorted out.
The throttle servo mounts slightly forward of the rudder
servo and has to operate a long throttle pushrod. A thin
metal pushrod is included in the kit, but in order to keep
electronic engine noise to a minimum, I substituted this for
a Du-Bro flexible pushrod system.
When I attached the throttle pushrod to the carburetor,
I took the time to add another pushrod that would let me
manually operate the choke through the cowl opening. A
simple z-bend went through the choke lever, and I rigged
up a bracket that would support the bracket from the front,
which was held in place by the engine’s mounting bolts.
I also substituted one other item: the fuel tank. There’s
nothing wrong with the supplied tank; it’s a standard blowmolded
tank. However, I received a RotoFlow fuel tank
from J&L Power Products and this looked to be the perfect
time to try it out.
The RotoFlow comes preassembled and the clunk rotates
around a central pickup line. All I had to do is hook up the
fuel and vent lines, and then strap it in position with the tie
wraps that came in the SR-9 kit.
The vent line wraps around the tank. This is a tip I picked
up in some of my work with gas-powered International
Miniature Aerobatic Club (IMAC) airplanes. The vent line
on a gas engine is not pressurized and vents to the outside.
Fuel can leak out of the vent if the airplane is in certain
positions such as a vertical downline.
To keep this from happening, the vent line is wrapped
along the side, the back, and back along the other side to
form a loop. It works well and only requires a bit more
tubing for a trouble-free tank installation.
Next the landing gear fairings, wheels, and wheel pants
are installed. If you attached the landing gear struts first,
make sure you put the landing gear fairings on first. They
are vacuum-formed pieces and there is a left and right so
pay attention. They must be trimmed and if you cut to the
molded-in trim line, the fit is perfect.
You’ll have to eyeball a cutout in each fairing to allow the
threaded spacers to protrude. These spacers are where the
wing struts bolt to the fuselage. Glue the fairings in position
and then attach the wheels.
When it came time to attach the wheels, another problem
occurred. There was no included hardware that was correct
for mounting the wheels. The wheels have a 6mm diameter
hole, yet the instructions called for a 5mm bolt to make up
the axle. None was included.
The remaining bolts were 3mm, which are too thin and
too short. I picked up a set of 1/4- 20 x 21/4 hex bolts and
matching nuts that worked. They are slightly larger in
diameter than the 6mm size, so the
wheels and the landing gear holes had
to be drilled larger.
There are no predrilled holes in the
wheel pants, so you’ll have to eyeball
that as well. There are provisions for a
smaller screw to be installed above the
axle bolt that will stop the wheel pants
from turning or spinning around the
axle.
With the SR-9 on its feet, I set about
installing the radio. There’s plenty of
room in the fuselage to mount nearly
any brand and size of radio you choose.
Extensions have to be run back to the
left and right elevator servos, as well as
up to the wing roots for the flap and
elevator servos on each side. To hold
these in place I picked up a set of Wire
Keeps. These small, die-cut foam pieces
have adhesive strips on the back and
make installations neater. They also keep
the extensions to the wing root in place,
and out of sight.
I installed a Smart-Fly remote ignition
cutoff. Quest makes these cutoffs and
they allow you to cut the ignition
from your transmitter using a spare
channel. It utilizes a fiber-optic system
so the ignition is opto-isolated from the
receiver.
The receiver, dual batteries, and the
Smart-Fly system are held in place with
hook-and-loop. I glued in a couple of
squares of 1/16-inch balsa sheet to go
under the receiver and batteries, giving
the hook-and-loopmaterial a surface area
on which to mount.
I included a plywood plate inside
the port cabin door that is not part of
the kit. I despise mounting switches on
the surface of the fuselage of any Scale
airplane, and make every attempt to hide
them. With such a large fuselage and
two functional doors, it seems like the
perfect place to hide both the receiver
power and ignition power switches. I
did a bit of measuring and made this
plate from some spare light plywood.
In hindsight, I should have also put the
Aluminum Fuel Dot on the plate as
well.
I originally set up the radio with a
master/slave combination for both
ailerons servos, both elevator servos,
and both flap servos. After extensive
experimenting with my 9503 radio, I
found that the elevator servos weren’t
matching in throw and speed. They
weren’t off too much, but it was enough
that it would have induced a bit of roll,
making trimming the SR-9 impossible.
No amount of transmitter programming
could entirely fix the problem and I
found that the flaps were worse. The
9503 had no midpoint flap adjustment
so both servos were way out of sync.
So, to solve this, I bought a couple of
reversing Y harnesses and installed
them, which solved the elevator and flap
problems.
When the radio was in place, I finished
the fuselage by installing the windows,
windscreen, and the cabin doors. With
the exception of the windscreen, all
the windows are glued in place. The
windscreen is held in place with a
number of smaller screws.
Several of the screws which are
installed above the tank area only thread
into a thin layer of balsa. Even wicking
a bit of thin CA will not give enough
material for the screws to firmly hold.
I cut a few 3/4-inch squares from some
1/8-inch light plywood. After I drilled
the holes for the windscreen, I could
then feel where these squares should
be applied. I used medium CA to hold
them in position. I redrilled the holes
to extend through the plywood so the
screws tightened nicely.
The instructions say to epoxy the
door hinges into place on the door and
the fuselage. If the hinges were epoxied,
the doors would only open roughly 60°.
I decided not epoxy the hinges to the
fuselage, but to hold each hinge in place
with a single screw (the same type that
holds the windscreen in position). This
will allow me to completely remove the
doors if a problem arises.
One last issue popped up during the
final assembly. The wing struts are held
in place with two screws: one for the
top, and one for the bottom. The bottom
screw threads into the spacer that
extends from the landing gear fairings,
but there were no screws included
that were short enough to allow me
to tighten the strut firmly. I had to
outsource those screws as well. They
need to be roughly 1/4-inch to 3/8-inch
long.
The assembly was finally finished.
Several problems were overcome;
however, when it’s finished, it really
looks great.
It’s hard to determine the exact
assembly time, but I’d estimate 15 to 20
hours. I spent a couple hours each night
for 2 weeks, but not all of it was actual
building. Some was waiting for the
epoxy to cure.
There was a good surprise: the CG
was spot-on, even with the twin receiver
batteries. The weight came in at 191/4
pounds, ready to fly.
One last note before I get to the
flying portion. Have you ever put
together a project such as a piece of
furniture, a child’s play set, etc., and
have a few screws and other hardware
left over? Our review SR-9 kit takes
this to another level. After doublechecking
to make sure I didn’t miss any
steps, I found myself with a tidy pile of
hardware: a handful of screws, nuts and
bolts, a few dozen washers of various
sizes, t-nuts, flat hinges, two sizes of
point hinges, and even an extra threaded
spacer.
These all can be used in future
projects, but it seems as though someone
at the factory scooped up a few handfuls
of miscellaneous hardware and put it in
the kit. Maybe this helps make up for
the hardware that wasn’t included.
You’ll probably need spares of the
smaller screws and washers that secure
the stabilizer halves and the wing struts.
Have extras so you won’t ruin a day’s
flying because of a missing screw.
Engine First Run
Before the first flight, I took the time
to start the break-in procedure on the
Pterodactyl PTE36R 36cc. A fuel-tooil
mixture between 20:1 and 40:1 is
called for, so I used a 30:1 mixture I
had on hand. The break-in propeller is
not specified in the manual, but TBM’s
website suggests an 18 x 8 propeller and
puts the break-in time at 5 gallons of
fuel.
With everything in place, it was time
to start flipping. It took roughly 25 to 30
flips to get the first ignition burp when
choked, then another 10 or so to bring
the engine to life. I ran the first tankful at
25% throttle with no adjustments to the
carburetor to lubricate everything.
During the next run, I adjusted
the carburetor so the top end was
approximately 7,800 rpm and idle was
roughly 2,400. Satisfied with this as a
starting point, it was time to put on the
cowl and see how the SR-9 flew.
Flying
It will take roughly 10 to 15 minutes
to bolt the wings and stabilizer into
place, attach the wing struts, and top off
the tank when assembling at the field.
The plug-in wings and stabilizer halves
are easy to install and have a good, firm
fit. The stabilizer halves are held in place
with a couple of screws.
The wings are secured by reaching
into the fuselage through the cabin
doors and threading a 6mm screw into
each wing root. You’ll probably want
to assemble it on a table to keep from
crawling under the wings.
Firing the PTE36R for the first flight,
I found that the idle was too high for
proper taxiing, but because the cowl was
already on and I knew that I could kill
the engine at any time with the Smart-
Fly ignition cutoff, I went with it as is.
The SR-9 took off in roughly 100
feet and had a good climb rate at only
three-fourths throttle. After making a
few passes for trim, I was fighting the
model because it was way too sensitive,
especially in pitch (elevator). Most of
the early turns looked like those a Pylon
racer would make. Any touch of the
elevator equated big changes in altitude.
The manual calls out a throw or
40mm each way; that’s a touch over 11/2
inches. The rudder was similarly touchy,
so I set up for a few practice approaches.
With the idle set as high as it was, the
Stinson would not land, even with flaps,
so I climbed up to a safe altitude, cut the
throttle, and glided in for an uneventful
landing.
It was time for reprogramming. I
cut the elevator and rudder throw
considerably (1 inch). I also added
exponential: 60% on the elevator and
45% on the rudder. I left the ailerons
alone.
I removed the cowl and adjusted the
idle setting, lowering it to approximately
2,000 rpm. The engine felt as though
it would quit if it was any lower, so I
bolted everything together and it was on
to flight two.
Although not quite perfect, the flight
was much more scalelike and I was able
to get a better feel for the model. You
might think it would fly like a giant
trainer, but it requires some attention,
especially in the turns where you’ll have
to coordinate the ailerons and rudder.
The full-scale Stinson is not an
aerobatic design, but this model will do
modest loops, stall turns, and barrel rolls,
and will spin easily with that big rudder.
Cruising around is where the Stinson
shines, and low-level passes are beautiful.
The 9503 has a three-position switch
for flaps, and I had the most fun at full
flaps. At this setting, the flaps were
deployed close to 60° and they slow
the SR-9 to a crawl. With the ample
power of the PTE36R, there’s plenty of
propeller blast over the control surfaces.
Landings are easier and slower using
flaps. I found it better to land with a
steeper approach, a little more power,
and then flare right roughly 2 feet off
the runway. It will settle, and normally
you can land without too much
bouncing. The gear and hard foam tires
don’t have much give, so it is very easy
to bounce a landing on pavement, but
the Stinson lands well on grass.
Conclusion
TBM’s Stinson SR-9 is a beautiful
airplane and faithful to its full-scale
counterpart. It’s a big model with a 100-
inch wingspan, but this size makes it
easy to work on and in. There are some
problems along the way, but it’s nothing
that an intermediate or experienced
modeler can’t overcome, and I described
how to dodge these problems in the
review!
In the air, the SR-9 looks the part.
Resist the urge to fly it at full throttle,
and please don’t shoehorn a 50cc (or
larger) engine in the cowl. It doesn’t
need it. The PTE36 is plenty of power
and if you fly the SR-9 as intended,
you’ll rarely need anything over half
throttle.
Edition: Model Aviation - 2012/10
Page Numbers: 46,47,48,49,50,51,52
When I review a Scale model, I
generally offer some history;
however, this is quite an
involved review, so I’m going to get
straight to it. I will ask that you read
the fl ying portion of this review fi rst
so you’ll understand how good this
ECOMRC Stinson SR-9 ARF is when
it’s fi nished.
Go ahead, jump to the fl ying portion!
I’ll wait …
Now that you know what the result
is, let’s start from the beginning. The
ECOMRC Stinson SR-9 ARF is a big
airplane and it’s double boxed for
protection during shipping. Unpacking
everything, the only damage I found was
on the lower tip of the windscreen, but
that was easily trimmed away.
The airframe is completely built-up
from balsa and laser-cut plywood. It is
stiff, but not overly heavy. The wings and
stabilizer are plug-in designs, and the
wings include fl aps.
Covering the airframe is a white,
iron-on fi lm with red trim and black
pinstripes that matches the color scheme
of the full-scale Stinson SR-9 (n-number
VH-UXL) that was used by the Vacuum
Oil Company back in 1936. A few
spots were slightly wrinkled, but a few
minutes of work with my covering iron
took care of them.
Matching the iron-on fi lm are the
prepainted fi berglass cowl and wheel
pants. The cowl features the Reliant’s
multitude of “bumps” around the
circumference. On the rear of the cowl,
there’s a cutout for exhaust and cooling.
The 16-page instruction manual
shows the steps in a series of drawings.
It’s slightly lacking in places, but an
experienced modeler should be able
to fi gure out the steps. Construction is
straightforward.
Rounding out the kit is the hardware,
which includes everything from the
thick aluminum landing gear halves, to
the fuel tank, the carbon-fi ber wing and
stabilizer tubes, the vacuum-formed
plastic windows and gear fairings,
wheels, pushrods, and stickers. There are
also bags with bolts, nuts, and smaller
pieces, as well as the pull-pull cable
hardware for the rudder. All threaded
hardware is metric.
The manual mentions that a sixchannel
system is needed. This is true
if you use Y harnesses for the ailerons
and special reversing Y harnesses for the
elevator halves and flap servos. However,
if you don’t want to use the Y harnesses,
you’ll need at least an eight-channel
radio that can handle three separate
mixes of master and slave servos.
This is an International Miniature
Aircraft Association-legal (IMAA)
aircraft, and if you want comply with
IMAA rules, you’ll need another channel
for the remote ignition cutoff.
Nothing in the manual covers what
types of servos are needed (standard,
high-torque, high-speed, etc.). Looking
at the list of recommended accessories
on the SR-9 page of Troy Built Models
(TBM) website, I found several servos
listed with torque ratings of 133 ounces
per square inch and higher. With the
large areas of the SR-9’s control surfaces,
I chose to use a combination of servos
I had on hand. Using a six-volt setup,
the lowest torque was the JR Sport
ST125MG servos at 142 ounce-inches.
Construction
If you choose to power the SR-9 with
a gasoline engine, be sure to have some
threadlock handy; nearly every step
where parts are bolted together requires
a drop or two. Any gasoline engine will
vibrate and quickly loosen even the
tightest of screws.
I normally don’t discuss the
construction of review models, but there
are a few problems areas that need to be
detailed. You can download the manual
from TBM’s website located in the
“Sources” section.
Construction starts by attaching the
ailerons and flaps to the wing
halves. Nylon “hinge-point”
hinges are used. Be sure to coat
the center section of each with
lithium grease, petroleum jelly,
or a suitable alternative to keep
the epoxy away from the pivot
point.
Install the aileron and flap
servos into the wing halves. The
servos mount to the back of
the preassembled servo covers.
Any standard-size servo easily
fits into the cutout; however,
because longer, slightly thicker,
heavy-duty servo arms are
needed, the servos cannot be
fitted with the arms in place.
Nor can the arms be fitted after
the servos are mounted.
I cut away the strip of
plywood between the two
mounting platforms, then
stiffened the remaining mounts
by wicking thin CA. After
it cures, the servo with the
horn attached could slide into
position.
Using a long, thin wire or a
string with a weight attached,
you can fish the servo wires and
extensions through the wing
halves. However, I found that I
had to trim a bit of the plywood
framing to allow the servos to
properly fit in the wings.
Once that’s finished, the
control hardware is attached.
The included control arms are epoxied
into the control surface. To keep them
aligned, insert the screw that holds in
the ball link through both control arms
and use it to hold the arms in position.
This keeps the holes aligned and makes
things easier.
The pushrods and ball links are heavyduty
parts, held to the servo by a wheel
collar. Be sure to grind a flat spot on the
pushrod for a secure fit. The ball links
can slide slightly between the control
horns. To take up this slack and have a
more precise link, cut two thin slices of
fuel tubing. Place them on either side of
the ball link to act as soft washers.
The elevator halves are next. Each
elevator half has its own servo mounted
inside. The servo fits well, but I had to
file away some of the wood so the servo
wire could exit without crimping.
When it was time to epoxy the
control horn pieces into the
elevators and the rudder, I ran
into the first major problem.
These horns are smaller than
those used on the wings, but
the precut slots are so big that
the horns are swimming inside
them.
I contacted TBM and we
decided to use the control
horns and fill the rest of the
opening with epoxy mixed with
microballoons. After trimming
the length of the horns to fit the
slots, everything was epoxied
into place and it fixed the
problem.
The manual shows that the rudder
should be epoxied into position next,
but I advise putting off this step.
A heavy-duty tail wheel is included,
which requires some assembly as well
as some measuring so everything works
properly. The aluminum plate that the
wire runs through had to be drilled out
to accept the 1/8-inch wire.
The prebent wire has extra length.
It’s designed to extend through the
outer cover, through an aluminum
mounting plate, through a plywood
plate, then through another hole
predrilled in a rib that is further up
into the rudder.
Once cut to length, this wire also
needs a few flats cut to match the
positions of the wheel collars. At this
point, everything can be bolted into
position except the tail wheel. Take a
few minutes to install the pull-pull
wires that steer this assembly.
After everything is mounted in
position, the rear cover, tail wheel, and
wheel collars are attached. Although
the manual doesn’t say to bolt on the
landing gear struts until later in the
build, now is a good time to do so. It
will give you a firmer foundation for
the next step.
This is why I mentioned postponing
attaching the rudder. During the engine
and cowl installation, you can stand
the fuselage vertically, setting it on the
fin. It provides a flat, solid surface. And
having the second landing gear struts in
place gives you support when propping
up the fuselage against a vertical
surface such as a wall or door.
TBM supplied an engine with the
review kit: the Pterodactyl PTE36R
36cc gas engine. It’s a rear intake,
rear exhaust, single-cylinder
engine and it mounts on four
aluminum standoffs.
To figure out the engine’s
mounting position, I skipped
forward in the manual and
epoxied the cowl ring into
place. This requires sanding
flat the area on the front of the
fuselage where the ring mounts.
Take a moment to wick in
some thin CA around each of
the blocks on the mounting
ring. A few of the parts were
moving around slightly on my
piece, so make sure nothing
falls off on yours. Be careful not to get
CA in any of the threads.
After the CA cured, I took some
time to position the cowl and get an
idea of how the engine would fit. The
cowl will take a bit of trimming to
allow the muffler and the front fins of
the head to clear.
The engine needed to be spaced
11/2 inches out from the firewall to
allow the cowl to fit properly. No
wood is included in the kit for this, so
I designed a spacer from plywood and
poplar.
You can download a diagram from
the www.ModelAviation.com website.
Paint it with clear polyurethane to
fuelproof it. To bolt the spacer to the
fuselage, I picked up a set of four 1/4-20
x 2 bolts and matching t-nuts.
Now is the time to finish the steps I
skipped. I mounted the rudder servo,
epoxied the rudder into place, and
installed the pull-pull cables. There’s
nothing wrong with the way these cables are installed, but
I had a hard time getting them right, especially the cables
coming forward from the tail wheel.
I’m not a surgeon, but I imagine trying to thread these
cables through the holes and tubes is like arthroscopic
surgery. I was using long needle-nose pliers at weird angles,
while reaching through the door and windscreen opening.
The rudder servo is mounted back in the passenger
compartment, which compounded the issue for me, but I
did manage to get it sorted out.
The throttle servo mounts slightly forward of the rudder
servo and has to operate a long throttle pushrod. A thin
metal pushrod is included in the kit, but in order to keep
electronic engine noise to a minimum, I substituted this for
a Du-Bro flexible pushrod system.
When I attached the throttle pushrod to the carburetor,
I took the time to add another pushrod that would let me
manually operate the choke through the cowl opening. A
simple z-bend went through the choke lever, and I rigged
up a bracket that would support the bracket from the front,
which was held in place by the engine’s mounting bolts.
I also substituted one other item: the fuel tank. There’s
nothing wrong with the supplied tank; it’s a standard blowmolded
tank. However, I received a RotoFlow fuel tank
from J&L Power Products and this looked to be the perfect
time to try it out.
The RotoFlow comes preassembled and the clunk rotates
around a central pickup line. All I had to do is hook up the
fuel and vent lines, and then strap it in position with the tie
wraps that came in the SR-9 kit.
The vent line wraps around the tank. This is a tip I picked
up in some of my work with gas-powered International
Miniature Aerobatic Club (IMAC) airplanes. The vent line
on a gas engine is not pressurized and vents to the outside.
Fuel can leak out of the vent if the airplane is in certain
positions such as a vertical downline.
To keep this from happening, the vent line is wrapped
along the side, the back, and back along the other side to
form a loop. It works well and only requires a bit more
tubing for a trouble-free tank installation.
Next the landing gear fairings, wheels, and wheel pants
are installed. If you attached the landing gear struts first,
make sure you put the landing gear fairings on first. They
are vacuum-formed pieces and there is a left and right so
pay attention. They must be trimmed and if you cut to the
molded-in trim line, the fit is perfect.
You’ll have to eyeball a cutout in each fairing to allow the
threaded spacers to protrude. These spacers are where the
wing struts bolt to the fuselage. Glue the fairings in position
and then attach the wheels.
When it came time to attach the wheels, another problem
occurred. There was no included hardware that was correct
for mounting the wheels. The wheels have a 6mm diameter
hole, yet the instructions called for a 5mm bolt to make up
the axle. None was included.
The remaining bolts were 3mm, which are too thin and
too short. I picked up a set of 1/4- 20 x 21/4 hex bolts and
matching nuts that worked. They are slightly larger in
diameter than the 6mm size, so the
wheels and the landing gear holes had
to be drilled larger.
There are no predrilled holes in the
wheel pants, so you’ll have to eyeball
that as well. There are provisions for a
smaller screw to be installed above the
axle bolt that will stop the wheel pants
from turning or spinning around the
axle.
With the SR-9 on its feet, I set about
installing the radio. There’s plenty of
room in the fuselage to mount nearly
any brand and size of radio you choose.
Extensions have to be run back to the
left and right elevator servos, as well as
up to the wing roots for the flap and
elevator servos on each side. To hold
these in place I picked up a set of Wire
Keeps. These small, die-cut foam pieces
have adhesive strips on the back and
make installations neater. They also keep
the extensions to the wing root in place,
and out of sight.
I installed a Smart-Fly remote ignition
cutoff. Quest makes these cutoffs and
they allow you to cut the ignition
from your transmitter using a spare
channel. It utilizes a fiber-optic system
so the ignition is opto-isolated from the
receiver.
The receiver, dual batteries, and the
Smart-Fly system are held in place with
hook-and-loop. I glued in a couple of
squares of 1/16-inch balsa sheet to go
under the receiver and batteries, giving
the hook-and-loopmaterial a surface area
on which to mount.
I included a plywood plate inside
the port cabin door that is not part of
the kit. I despise mounting switches on
the surface of the fuselage of any Scale
airplane, and make every attempt to hide
them. With such a large fuselage and
two functional doors, it seems like the
perfect place to hide both the receiver
power and ignition power switches. I
did a bit of measuring and made this
plate from some spare light plywood.
In hindsight, I should have also put the
Aluminum Fuel Dot on the plate as
well.
I originally set up the radio with a
master/slave combination for both
ailerons servos, both elevator servos,
and both flap servos. After extensive
experimenting with my 9503 radio, I
found that the elevator servos weren’t
matching in throw and speed. They
weren’t off too much, but it was enough
that it would have induced a bit of roll,
making trimming the SR-9 impossible.
No amount of transmitter programming
could entirely fix the problem and I
found that the flaps were worse. The
9503 had no midpoint flap adjustment
so both servos were way out of sync.
So, to solve this, I bought a couple of
reversing Y harnesses and installed
them, which solved the elevator and flap
problems.
When the radio was in place, I finished
the fuselage by installing the windows,
windscreen, and the cabin doors. With
the exception of the windscreen, all
the windows are glued in place. The
windscreen is held in place with a
number of smaller screws.
Several of the screws which are
installed above the tank area only thread
into a thin layer of balsa. Even wicking
a bit of thin CA will not give enough
material for the screws to firmly hold.
I cut a few 3/4-inch squares from some
1/8-inch light plywood. After I drilled
the holes for the windscreen, I could
then feel where these squares should
be applied. I used medium CA to hold
them in position. I redrilled the holes
to extend through the plywood so the
screws tightened nicely.
The instructions say to epoxy the
door hinges into place on the door and
the fuselage. If the hinges were epoxied,
the doors would only open roughly 60°.
I decided not epoxy the hinges to the
fuselage, but to hold each hinge in place
with a single screw (the same type that
holds the windscreen in position). This
will allow me to completely remove the
doors if a problem arises.
One last issue popped up during the
final assembly. The wing struts are held
in place with two screws: one for the
top, and one for the bottom. The bottom
screw threads into the spacer that
extends from the landing gear fairings,
but there were no screws included
that were short enough to allow me
to tighten the strut firmly. I had to
outsource those screws as well. They
need to be roughly 1/4-inch to 3/8-inch
long.
The assembly was finally finished.
Several problems were overcome;
however, when it’s finished, it really
looks great.
It’s hard to determine the exact
assembly time, but I’d estimate 15 to 20
hours. I spent a couple hours each night
for 2 weeks, but not all of it was actual
building. Some was waiting for the
epoxy to cure.
There was a good surprise: the CG
was spot-on, even with the twin receiver
batteries. The weight came in at 191/4
pounds, ready to fly.
One last note before I get to the
flying portion. Have you ever put
together a project such as a piece of
furniture, a child’s play set, etc., and
have a few screws and other hardware
left over? Our review SR-9 kit takes
this to another level. After doublechecking
to make sure I didn’t miss any
steps, I found myself with a tidy pile of
hardware: a handful of screws, nuts and
bolts, a few dozen washers of various
sizes, t-nuts, flat hinges, two sizes of
point hinges, and even an extra threaded
spacer.
These all can be used in future
projects, but it seems as though someone
at the factory scooped up a few handfuls
of miscellaneous hardware and put it in
the kit. Maybe this helps make up for
the hardware that wasn’t included.
You’ll probably need spares of the
smaller screws and washers that secure
the stabilizer halves and the wing struts.
Have extras so you won’t ruin a day’s
flying because of a missing screw.
Engine First Run
Before the first flight, I took the time
to start the break-in procedure on the
Pterodactyl PTE36R 36cc. A fuel-tooil
mixture between 20:1 and 40:1 is
called for, so I used a 30:1 mixture I
had on hand. The break-in propeller is
not specified in the manual, but TBM’s
website suggests an 18 x 8 propeller and
puts the break-in time at 5 gallons of
fuel.
With everything in place, it was time
to start flipping. It took roughly 25 to 30
flips to get the first ignition burp when
choked, then another 10 or so to bring
the engine to life. I ran the first tankful at
25% throttle with no adjustments to the
carburetor to lubricate everything.
During the next run, I adjusted
the carburetor so the top end was
approximately 7,800 rpm and idle was
roughly 2,400. Satisfied with this as a
starting point, it was time to put on the
cowl and see how the SR-9 flew.
Flying
It will take roughly 10 to 15 minutes
to bolt the wings and stabilizer into
place, attach the wing struts, and top off
the tank when assembling at the field.
The plug-in wings and stabilizer halves
are easy to install and have a good, firm
fit. The stabilizer halves are held in place
with a couple of screws.
The wings are secured by reaching
into the fuselage through the cabin
doors and threading a 6mm screw into
each wing root. You’ll probably want
to assemble it on a table to keep from
crawling under the wings.
Firing the PTE36R for the first flight,
I found that the idle was too high for
proper taxiing, but because the cowl was
already on and I knew that I could kill
the engine at any time with the Smart-
Fly ignition cutoff, I went with it as is.
The SR-9 took off in roughly 100
feet and had a good climb rate at only
three-fourths throttle. After making a
few passes for trim, I was fighting the
model because it was way too sensitive,
especially in pitch (elevator). Most of
the early turns looked like those a Pylon
racer would make. Any touch of the
elevator equated big changes in altitude.
The manual calls out a throw or
40mm each way; that’s a touch over 11/2
inches. The rudder was similarly touchy,
so I set up for a few practice approaches.
With the idle set as high as it was, the
Stinson would not land, even with flaps,
so I climbed up to a safe altitude, cut the
throttle, and glided in for an uneventful
landing.
It was time for reprogramming. I
cut the elevator and rudder throw
considerably (1 inch). I also added
exponential: 60% on the elevator and
45% on the rudder. I left the ailerons
alone.
I removed the cowl and adjusted the
idle setting, lowering it to approximately
2,000 rpm. The engine felt as though
it would quit if it was any lower, so I
bolted everything together and it was on
to flight two.
Although not quite perfect, the flight
was much more scalelike and I was able
to get a better feel for the model. You
might think it would fly like a giant
trainer, but it requires some attention,
especially in the turns where you’ll have
to coordinate the ailerons and rudder.
The full-scale Stinson is not an
aerobatic design, but this model will do
modest loops, stall turns, and barrel rolls,
and will spin easily with that big rudder.
Cruising around is where the Stinson
shines, and low-level passes are beautiful.
The 9503 has a three-position switch
for flaps, and I had the most fun at full
flaps. At this setting, the flaps were
deployed close to 60° and they slow
the SR-9 to a crawl. With the ample
power of the PTE36R, there’s plenty of
propeller blast over the control surfaces.
Landings are easier and slower using
flaps. I found it better to land with a
steeper approach, a little more power,
and then flare right roughly 2 feet off
the runway. It will settle, and normally
you can land without too much
bouncing. The gear and hard foam tires
don’t have much give, so it is very easy
to bounce a landing on pavement, but
the Stinson lands well on grass.
Conclusion
TBM’s Stinson SR-9 is a beautiful
airplane and faithful to its full-scale
counterpart. It’s a big model with a 100-
inch wingspan, but this size makes it
easy to work on and in. There are some
problems along the way, but it’s nothing
that an intermediate or experienced
modeler can’t overcome, and I described
how to dodge these problems in the
review!
In the air, the SR-9 looks the part.
Resist the urge to fly it at full throttle,
and please don’t shoehorn a 50cc (or
larger) engine in the cowl. It doesn’t
need it. The PTE36 is plenty of power
and if you fly the SR-9 as intended,
you’ll rarely need anything over half
throttle.
Edition: Model Aviation - 2012/10
Page Numbers: 46,47,48,49,50,51,52
When I review a Scale model, I
generally offer some history;
however, this is quite an
involved review, so I’m going to get
straight to it. I will ask that you read
the fl ying portion of this review fi rst
so you’ll understand how good this
ECOMRC Stinson SR-9 ARF is when
it’s fi nished.
Go ahead, jump to the fl ying portion!
I’ll wait …
Now that you know what the result
is, let’s start from the beginning. The
ECOMRC Stinson SR-9 ARF is a big
airplane and it’s double boxed for
protection during shipping. Unpacking
everything, the only damage I found was
on the lower tip of the windscreen, but
that was easily trimmed away.
The airframe is completely built-up
from balsa and laser-cut plywood. It is
stiff, but not overly heavy. The wings and
stabilizer are plug-in designs, and the
wings include fl aps.
Covering the airframe is a white,
iron-on fi lm with red trim and black
pinstripes that matches the color scheme
of the full-scale Stinson SR-9 (n-number
VH-UXL) that was used by the Vacuum
Oil Company back in 1936. A few
spots were slightly wrinkled, but a few
minutes of work with my covering iron
took care of them.
Matching the iron-on fi lm are the
prepainted fi berglass cowl and wheel
pants. The cowl features the Reliant’s
multitude of “bumps” around the
circumference. On the rear of the cowl,
there’s a cutout for exhaust and cooling.
The 16-page instruction manual
shows the steps in a series of drawings.
It’s slightly lacking in places, but an
experienced modeler should be able
to fi gure out the steps. Construction is
straightforward.
Rounding out the kit is the hardware,
which includes everything from the
thick aluminum landing gear halves, to
the fuel tank, the carbon-fi ber wing and
stabilizer tubes, the vacuum-formed
plastic windows and gear fairings,
wheels, pushrods, and stickers. There are
also bags with bolts, nuts, and smaller
pieces, as well as the pull-pull cable
hardware for the rudder. All threaded
hardware is metric.
The manual mentions that a sixchannel
system is needed. This is true
if you use Y harnesses for the ailerons
and special reversing Y harnesses for the
elevator halves and flap servos. However,
if you don’t want to use the Y harnesses,
you’ll need at least an eight-channel
radio that can handle three separate
mixes of master and slave servos.
This is an International Miniature
Aircraft Association-legal (IMAA)
aircraft, and if you want comply with
IMAA rules, you’ll need another channel
for the remote ignition cutoff.
Nothing in the manual covers what
types of servos are needed (standard,
high-torque, high-speed, etc.). Looking
at the list of recommended accessories
on the SR-9 page of Troy Built Models
(TBM) website, I found several servos
listed with torque ratings of 133 ounces
per square inch and higher. With the
large areas of the SR-9’s control surfaces,
I chose to use a combination of servos
I had on hand. Using a six-volt setup,
the lowest torque was the JR Sport
ST125MG servos at 142 ounce-inches.
Construction
If you choose to power the SR-9 with
a gasoline engine, be sure to have some
threadlock handy; nearly every step
where parts are bolted together requires
a drop or two. Any gasoline engine will
vibrate and quickly loosen even the
tightest of screws.
I normally don’t discuss the
construction of review models, but there
are a few problems areas that need to be
detailed. You can download the manual
from TBM’s website located in the
“Sources” section.
Construction starts by attaching the
ailerons and flaps to the wing
halves. Nylon “hinge-point”
hinges are used. Be sure to coat
the center section of each with
lithium grease, petroleum jelly,
or a suitable alternative to keep
the epoxy away from the pivot
point.
Install the aileron and flap
servos into the wing halves. The
servos mount to the back of
the preassembled servo covers.
Any standard-size servo easily
fits into the cutout; however,
because longer, slightly thicker,
heavy-duty servo arms are
needed, the servos cannot be
fitted with the arms in place.
Nor can the arms be fitted after
the servos are mounted.
I cut away the strip of
plywood between the two
mounting platforms, then
stiffened the remaining mounts
by wicking thin CA. After
it cures, the servo with the
horn attached could slide into
position.
Using a long, thin wire or a
string with a weight attached,
you can fish the servo wires and
extensions through the wing
halves. However, I found that I
had to trim a bit of the plywood
framing to allow the servos to
properly fit in the wings.
Once that’s finished, the
control hardware is attached.
The included control arms are epoxied
into the control surface. To keep them
aligned, insert the screw that holds in
the ball link through both control arms
and use it to hold the arms in position.
This keeps the holes aligned and makes
things easier.
The pushrods and ball links are heavyduty
parts, held to the servo by a wheel
collar. Be sure to grind a flat spot on the
pushrod for a secure fit. The ball links
can slide slightly between the control
horns. To take up this slack and have a
more precise link, cut two thin slices of
fuel tubing. Place them on either side of
the ball link to act as soft washers.
The elevator halves are next. Each
elevator half has its own servo mounted
inside. The servo fits well, but I had to
file away some of the wood so the servo
wire could exit without crimping.
When it was time to epoxy the
control horn pieces into the
elevators and the rudder, I ran
into the first major problem.
These horns are smaller than
those used on the wings, but
the precut slots are so big that
the horns are swimming inside
them.
I contacted TBM and we
decided to use the control
horns and fill the rest of the
opening with epoxy mixed with
microballoons. After trimming
the length of the horns to fit the
slots, everything was epoxied
into place and it fixed the
problem.
The manual shows that the rudder
should be epoxied into position next,
but I advise putting off this step.
A heavy-duty tail wheel is included,
which requires some assembly as well
as some measuring so everything works
properly. The aluminum plate that the
wire runs through had to be drilled out
to accept the 1/8-inch wire.
The prebent wire has extra length.
It’s designed to extend through the
outer cover, through an aluminum
mounting plate, through a plywood
plate, then through another hole
predrilled in a rib that is further up
into the rudder.
Once cut to length, this wire also
needs a few flats cut to match the
positions of the wheel collars. At this
point, everything can be bolted into
position except the tail wheel. Take a
few minutes to install the pull-pull
wires that steer this assembly.
After everything is mounted in
position, the rear cover, tail wheel, and
wheel collars are attached. Although
the manual doesn’t say to bolt on the
landing gear struts until later in the
build, now is a good time to do so. It
will give you a firmer foundation for
the next step.
This is why I mentioned postponing
attaching the rudder. During the engine
and cowl installation, you can stand
the fuselage vertically, setting it on the
fin. It provides a flat, solid surface. And
having the second landing gear struts in
place gives you support when propping
up the fuselage against a vertical
surface such as a wall or door.
TBM supplied an engine with the
review kit: the Pterodactyl PTE36R
36cc gas engine. It’s a rear intake,
rear exhaust, single-cylinder
engine and it mounts on four
aluminum standoffs.
To figure out the engine’s
mounting position, I skipped
forward in the manual and
epoxied the cowl ring into
place. This requires sanding
flat the area on the front of the
fuselage where the ring mounts.
Take a moment to wick in
some thin CA around each of
the blocks on the mounting
ring. A few of the parts were
moving around slightly on my
piece, so make sure nothing
falls off on yours. Be careful not to get
CA in any of the threads.
After the CA cured, I took some
time to position the cowl and get an
idea of how the engine would fit. The
cowl will take a bit of trimming to
allow the muffler and the front fins of
the head to clear.
The engine needed to be spaced
11/2 inches out from the firewall to
allow the cowl to fit properly. No
wood is included in the kit for this, so
I designed a spacer from plywood and
poplar.
You can download a diagram from
the www.ModelAviation.com website.
Paint it with clear polyurethane to
fuelproof it. To bolt the spacer to the
fuselage, I picked up a set of four 1/4-20
x 2 bolts and matching t-nuts.
Now is the time to finish the steps I
skipped. I mounted the rudder servo,
epoxied the rudder into place, and
installed the pull-pull cables. There’s
nothing wrong with the way these cables are installed, but
I had a hard time getting them right, especially the cables
coming forward from the tail wheel.
I’m not a surgeon, but I imagine trying to thread these
cables through the holes and tubes is like arthroscopic
surgery. I was using long needle-nose pliers at weird angles,
while reaching through the door and windscreen opening.
The rudder servo is mounted back in the passenger
compartment, which compounded the issue for me, but I
did manage to get it sorted out.
The throttle servo mounts slightly forward of the rudder
servo and has to operate a long throttle pushrod. A thin
metal pushrod is included in the kit, but in order to keep
electronic engine noise to a minimum, I substituted this for
a Du-Bro flexible pushrod system.
When I attached the throttle pushrod to the carburetor,
I took the time to add another pushrod that would let me
manually operate the choke through the cowl opening. A
simple z-bend went through the choke lever, and I rigged
up a bracket that would support the bracket from the front,
which was held in place by the engine’s mounting bolts.
I also substituted one other item: the fuel tank. There’s
nothing wrong with the supplied tank; it’s a standard blowmolded
tank. However, I received a RotoFlow fuel tank
from J&L Power Products and this looked to be the perfect
time to try it out.
The RotoFlow comes preassembled and the clunk rotates
around a central pickup line. All I had to do is hook up the
fuel and vent lines, and then strap it in position with the tie
wraps that came in the SR-9 kit.
The vent line wraps around the tank. This is a tip I picked
up in some of my work with gas-powered International
Miniature Aerobatic Club (IMAC) airplanes. The vent line
on a gas engine is not pressurized and vents to the outside.
Fuel can leak out of the vent if the airplane is in certain
positions such as a vertical downline.
To keep this from happening, the vent line is wrapped
along the side, the back, and back along the other side to
form a loop. It works well and only requires a bit more
tubing for a trouble-free tank installation.
Next the landing gear fairings, wheels, and wheel pants
are installed. If you attached the landing gear struts first,
make sure you put the landing gear fairings on first. They
are vacuum-formed pieces and there is a left and right so
pay attention. They must be trimmed and if you cut to the
molded-in trim line, the fit is perfect.
You’ll have to eyeball a cutout in each fairing to allow the
threaded spacers to protrude. These spacers are where the
wing struts bolt to the fuselage. Glue the fairings in position
and then attach the wheels.
When it came time to attach the wheels, another problem
occurred. There was no included hardware that was correct
for mounting the wheels. The wheels have a 6mm diameter
hole, yet the instructions called for a 5mm bolt to make up
the axle. None was included.
The remaining bolts were 3mm, which are too thin and
too short. I picked up a set of 1/4- 20 x 21/4 hex bolts and
matching nuts that worked. They are slightly larger in
diameter than the 6mm size, so the
wheels and the landing gear holes had
to be drilled larger.
There are no predrilled holes in the
wheel pants, so you’ll have to eyeball
that as well. There are provisions for a
smaller screw to be installed above the
axle bolt that will stop the wheel pants
from turning or spinning around the
axle.
With the SR-9 on its feet, I set about
installing the radio. There’s plenty of
room in the fuselage to mount nearly
any brand and size of radio you choose.
Extensions have to be run back to the
left and right elevator servos, as well as
up to the wing roots for the flap and
elevator servos on each side. To hold
these in place I picked up a set of Wire
Keeps. These small, die-cut foam pieces
have adhesive strips on the back and
make installations neater. They also keep
the extensions to the wing root in place,
and out of sight.
I installed a Smart-Fly remote ignition
cutoff. Quest makes these cutoffs and
they allow you to cut the ignition
from your transmitter using a spare
channel. It utilizes a fiber-optic system
so the ignition is opto-isolated from the
receiver.
The receiver, dual batteries, and the
Smart-Fly system are held in place with
hook-and-loop. I glued in a couple of
squares of 1/16-inch balsa sheet to go
under the receiver and batteries, giving
the hook-and-loopmaterial a surface area
on which to mount.
I included a plywood plate inside
the port cabin door that is not part of
the kit. I despise mounting switches on
the surface of the fuselage of any Scale
airplane, and make every attempt to hide
them. With such a large fuselage and
two functional doors, it seems like the
perfect place to hide both the receiver
power and ignition power switches. I
did a bit of measuring and made this
plate from some spare light plywood.
In hindsight, I should have also put the
Aluminum Fuel Dot on the plate as
well.
I originally set up the radio with a
master/slave combination for both
ailerons servos, both elevator servos,
and both flap servos. After extensive
experimenting with my 9503 radio, I
found that the elevator servos weren’t
matching in throw and speed. They
weren’t off too much, but it was enough
that it would have induced a bit of roll,
making trimming the SR-9 impossible.
No amount of transmitter programming
could entirely fix the problem and I
found that the flaps were worse. The
9503 had no midpoint flap adjustment
so both servos were way out of sync.
So, to solve this, I bought a couple of
reversing Y harnesses and installed
them, which solved the elevator and flap
problems.
When the radio was in place, I finished
the fuselage by installing the windows,
windscreen, and the cabin doors. With
the exception of the windscreen, all
the windows are glued in place. The
windscreen is held in place with a
number of smaller screws.
Several of the screws which are
installed above the tank area only thread
into a thin layer of balsa. Even wicking
a bit of thin CA will not give enough
material for the screws to firmly hold.
I cut a few 3/4-inch squares from some
1/8-inch light plywood. After I drilled
the holes for the windscreen, I could
then feel where these squares should
be applied. I used medium CA to hold
them in position. I redrilled the holes
to extend through the plywood so the
screws tightened nicely.
The instructions say to epoxy the
door hinges into place on the door and
the fuselage. If the hinges were epoxied,
the doors would only open roughly 60°.
I decided not epoxy the hinges to the
fuselage, but to hold each hinge in place
with a single screw (the same type that
holds the windscreen in position). This
will allow me to completely remove the
doors if a problem arises.
One last issue popped up during the
final assembly. The wing struts are held
in place with two screws: one for the
top, and one for the bottom. The bottom
screw threads into the spacer that
extends from the landing gear fairings,
but there were no screws included
that were short enough to allow me
to tighten the strut firmly. I had to
outsource those screws as well. They
need to be roughly 1/4-inch to 3/8-inch
long.
The assembly was finally finished.
Several problems were overcome;
however, when it’s finished, it really
looks great.
It’s hard to determine the exact
assembly time, but I’d estimate 15 to 20
hours. I spent a couple hours each night
for 2 weeks, but not all of it was actual
building. Some was waiting for the
epoxy to cure.
There was a good surprise: the CG
was spot-on, even with the twin receiver
batteries. The weight came in at 191/4
pounds, ready to fly.
One last note before I get to the
flying portion. Have you ever put
together a project such as a piece of
furniture, a child’s play set, etc., and
have a few screws and other hardware
left over? Our review SR-9 kit takes
this to another level. After doublechecking
to make sure I didn’t miss any
steps, I found myself with a tidy pile of
hardware: a handful of screws, nuts and
bolts, a few dozen washers of various
sizes, t-nuts, flat hinges, two sizes of
point hinges, and even an extra threaded
spacer.
These all can be used in future
projects, but it seems as though someone
at the factory scooped up a few handfuls
of miscellaneous hardware and put it in
the kit. Maybe this helps make up for
the hardware that wasn’t included.
You’ll probably need spares of the
smaller screws and washers that secure
the stabilizer halves and the wing struts.
Have extras so you won’t ruin a day’s
flying because of a missing screw.
Engine First Run
Before the first flight, I took the time
to start the break-in procedure on the
Pterodactyl PTE36R 36cc. A fuel-tooil
mixture between 20:1 and 40:1 is
called for, so I used a 30:1 mixture I
had on hand. The break-in propeller is
not specified in the manual, but TBM’s
website suggests an 18 x 8 propeller and
puts the break-in time at 5 gallons of
fuel.
With everything in place, it was time
to start flipping. It took roughly 25 to 30
flips to get the first ignition burp when
choked, then another 10 or so to bring
the engine to life. I ran the first tankful at
25% throttle with no adjustments to the
carburetor to lubricate everything.
During the next run, I adjusted
the carburetor so the top end was
approximately 7,800 rpm and idle was
roughly 2,400. Satisfied with this as a
starting point, it was time to put on the
cowl and see how the SR-9 flew.
Flying
It will take roughly 10 to 15 minutes
to bolt the wings and stabilizer into
place, attach the wing struts, and top off
the tank when assembling at the field.
The plug-in wings and stabilizer halves
are easy to install and have a good, firm
fit. The stabilizer halves are held in place
with a couple of screws.
The wings are secured by reaching
into the fuselage through the cabin
doors and threading a 6mm screw into
each wing root. You’ll probably want
to assemble it on a table to keep from
crawling under the wings.
Firing the PTE36R for the first flight,
I found that the idle was too high for
proper taxiing, but because the cowl was
already on and I knew that I could kill
the engine at any time with the Smart-
Fly ignition cutoff, I went with it as is.
The SR-9 took off in roughly 100
feet and had a good climb rate at only
three-fourths throttle. After making a
few passes for trim, I was fighting the
model because it was way too sensitive,
especially in pitch (elevator). Most of
the early turns looked like those a Pylon
racer would make. Any touch of the
elevator equated big changes in altitude.
The manual calls out a throw or
40mm each way; that’s a touch over 11/2
inches. The rudder was similarly touchy,
so I set up for a few practice approaches.
With the idle set as high as it was, the
Stinson would not land, even with flaps,
so I climbed up to a safe altitude, cut the
throttle, and glided in for an uneventful
landing.
It was time for reprogramming. I
cut the elevator and rudder throw
considerably (1 inch). I also added
exponential: 60% on the elevator and
45% on the rudder. I left the ailerons
alone.
I removed the cowl and adjusted the
idle setting, lowering it to approximately
2,000 rpm. The engine felt as though
it would quit if it was any lower, so I
bolted everything together and it was on
to flight two.
Although not quite perfect, the flight
was much more scalelike and I was able
to get a better feel for the model. You
might think it would fly like a giant
trainer, but it requires some attention,
especially in the turns where you’ll have
to coordinate the ailerons and rudder.
The full-scale Stinson is not an
aerobatic design, but this model will do
modest loops, stall turns, and barrel rolls,
and will spin easily with that big rudder.
Cruising around is where the Stinson
shines, and low-level passes are beautiful.
The 9503 has a three-position switch
for flaps, and I had the most fun at full
flaps. At this setting, the flaps were
deployed close to 60° and they slow
the SR-9 to a crawl. With the ample
power of the PTE36R, there’s plenty of
propeller blast over the control surfaces.
Landings are easier and slower using
flaps. I found it better to land with a
steeper approach, a little more power,
and then flare right roughly 2 feet off
the runway. It will settle, and normally
you can land without too much
bouncing. The gear and hard foam tires
don’t have much give, so it is very easy
to bounce a landing on pavement, but
the Stinson lands well on grass.
Conclusion
TBM’s Stinson SR-9 is a beautiful
airplane and faithful to its full-scale
counterpart. It’s a big model with a 100-
inch wingspan, but this size makes it
easy to work on and in. There are some
problems along the way, but it’s nothing
that an intermediate or experienced
modeler can’t overcome, and I described
how to dodge these problems in the
review!
In the air, the SR-9 looks the part.
Resist the urge to fly it at full throttle,
and please don’t shoehorn a 50cc (or
larger) engine in the cowl. It doesn’t
need it. The PTE36 is plenty of power
and if you fly the SR-9 as intended,
you’ll rarely need anything over half
throttle.