August 2007 43
The “kit-bashed” twin looks great from any angle. It appears as if it were designed and produced for twin-engine flying.
BY JIM FELDMANN
TWIN-ENGINE MODEL aircraft have a charm about them. The
sound of two engines running together is hypnotic, they can look and
fly like full-scale airplanes, and in some cases their performance is
better than a single-engine equivalent.
However, these aircraft have a scary reputation. Conventional
wisdom is that it is nearly impossible to get the engines running
correctly and that if one quits the airplane will instantly enter an
uncontrolled spin and crash.
The bad reputation undoubtedly comes from the heavily loaded and
often underpowered twin-engine warbirds we see at Scale meets. No
disrespect is intended; I am in awe of the craftsmanship these models
exhibit—especially twins and other multiengine airplanes.
But by the time the retracts and flaps are in; the fiberglassing,
painting, and detailing is done; and they get into the air, they can be
severely unforgiving. Hey, flying a heavy, single-engine Scale warbird
is no picnic either.
It doesn’t have to be that way. The Sport Utility Twin (SUT) is as
light as a single-engine trainer. It flies exactly like the single-engine
Hobbico Hobbistar 60 MK III ARF that formed the basis of this
conversion.
If you can tune one engine to run reliably, there is no reason why
you can’t tune two. Even if one engine sags or quits, the SUT won’t do
anything scary. It will fly fine on only one power plant.
The SUT is also a great utility airplane. You can use it to test
engines and radios, put a clear nose on it and use it as a cameracarrying
model, install a gyro or autopilot and see how that works, or
experiment with your transmitter’s mixing capabilities. If you mix the
throttles to the rudder, you get awesome flat spins and stall turns but it
messes up knife-edge flight.
I started with the excellent Hobbistar 60 MK III to keep this project
simple and inexpensive. It has a semisymmetrical airfoil and can be
aerobatic if you increase the control throws.
Adding the locating blocks and using foam tape on the wing saddle
makes the rubber-band-on wing secure, especially since the fuselage is
much lighter without an engine in it. If you are planning to get “wild
and crazy” with the SUT, consider switching to a bolt-on wing. That
conversion is not difficult. (For more information go to www.masport
aviator.com or read “Not Your Stock Hobbistar MK III” in the
September 2005 MA.)
I initially installed a pair of O.S. .40 LA engines because of their
low cost. Although they ran well, they were difficult to synchronize.
The engine choice is up to you.
The SUT flies exceptionally well on a couple of bushing engines,
but there is no reason why you couldn’t use higher-performance ballbearing
.40s. Then you could even use the SUT as a glider tug.
The nacelles are designed to hold 8- or 11-ounce Hayes fuel tanks.
My .40 LAs (and later, .40 FPs) will run at least 20 minutes on 11
ounces of fuel.
The plans and instructions show the installation of dual aileron
servos: one in each engine nacelle. This offers more precise aileron
control than the standard torque-rod setup and allows the use of
flaperon, spoileron, differential, elevator/flap, and flap/elevator mixing
capabilities of most computer radios.
However, two aileron servos are not required to fly the SUT. The
airplane will fly fine with the single-servo setup that comes in the kit.
How about other 60-size trainers? Do you have to use a Hobbistar?
No, but you might have to change the shape of the top of the nacelles
to match the airfoil on the bottom of your model’s wing, and you will
Convert an ARF advanced trainer into a twin-engine fun flier with double the potential.
08sig2.QXD 6/22/07 11:24 AM Page 43
Twin-
Engine
Setup Tips
Setting up a
twin-engine
model’s power
plants is far more
critical than setting
up a single-engine
model. Many twins
will suffer handling
problems if an engine sags or quits, especially on takeoff. The SUT
is extremely forgiving and will usually make it back in one piece on
one engine, but you might as well learn the right way now for that
higher-performance twin that is in your future.
Reliability—not peak performance—is the goal. You must tune
each engine separately for a steady and reliable idle, clean midrange,
and slightly rich full throttle.
It is unnecessary to synchronize the engines’ rpm at full throttle.
A difference of a few hundred rpm will be unnoticeable to you and
to the airplane in flight.
However, it is important that the engines be synchronized at idle.
When you pull the throttle back to idle, you want them to stay
running and respond to throttle-up at the same rate.
The best way to set up twin throttles is on separate channels. You
will need a transmitter that can mix a second channel with the
throttle channel so the throttle-trim and -cut functions will work on
both channels. On a Futaba 8U that requires the use of channels 3
and 8.
Since you have already mechanically set up the pushrods to
work the same and the engines are running their best, you can
use each channel’s ATV function to coordinate their idle rpm.
Once they are synchronized, normal throttle trim can be used as
needed and both engines should respond the same.
If your transmitter can’t handle separate channels, you can
use your regular throttle channel and a Y harness. In this case
you will have to adjust the length of the throttle pushrods to
synchronize the idle rpm.
Never try to synchronize the engine speeds, low or high, by
adjusting the needle valves. To make the engines run the same
speed you have to richen the faster one or lean the slower one.
Either way, that engine is not running at its individual best and
is much more likely to quit.
I shudder when I see someone trying to tune a twin’s
engines with both running. If you must have them synchronized
at high speed (they do sound better that way), adjust the highside
ATV to reduce the faster engine’s speed to match the
slower one. If you are using a Y harness, shave the slower
engine’s propeller. Sometimes just switching propellers
between engines helps.
Following are rules of twin-engine reliability.
1) Pad the fuel tanks well to prevent fuel frothing. (Fuel filters
are also a good idea.)
2) Adjust each engine separately for its best idle, cleanest
acceleration, and most reliable top end, and then leave it alone.
3) Match the idle speeds by adjusting the throttle pushrods’
length or, with separate channels, use ATV.
The cardinal rule of twin-engine survival is that if you do
lose an engine, keep
your model’s speed up
until you are in
position to make a
dead-stick landing.
Then throttle back to
idle and land it. MA
—Jim Feldmann
44 MODEL AVIATION
Photos by the author
All nacelle parts ready for assembly. The firewalls have been marked
and drilled for engine mounts and blind nuts have been installed.
Doublers are glued to the nacelle sides and then the firewall and
former F-2 are glued in place. Then the second side is added,
along with firewall brace, rear spreader, and hatch mounts.
The hatches are installed, 1/16 balsa rear sheeting is added, and then
the balsa sides are sanded down to match the hatches and sheeting.
The nacelles and rudder can be covered with any iron-on film.
A soldering pencil cuts the covering along the center of the
capstrip on the first rib out from the wing-bottom center
sheeting. A strip of masking tape is used to guide the cut.
08sig2.QXD 6/22/07 11:26 AM Page 44
August 2007 45
The nacelle is glued to the wing bottom,
and F-1 is glued to the back of the LE to
anchor the nacelle.
How F-1 and F-1A go together. Slots in F-1A
need to be wide enough to let F-1A rotate
to match the angle of the wing bottom.
A generous amount of epoxy bonds F-1A
to F-1, the bottom of the wing, and the
nacelle side doublers.
Engine installation is straightforward and
follows the same procedures as in the kit
manual.
Remove the heavy aluminum factory engine
mount and add a second nose-gear block.
Install the nose-wheel strut per the kit
manual.
The nose cover can be made from white
or clear .030 plastic or 1/64 plywood
covered with iron-on film. It is screwed to
the bottom and top of the firewall.
The throttle servo and optional aileron servo are screwed to
hardwood mounting rails, and then the rails are fitted and glued
into the nacelle.
The 11-ounce tank has been installed with foam rubber pads in
front of the wing, on both sides of the tank, and glued to the
hatch.
The proof is in the performance. It works
great!
The completed Sport Utility Twin taxies out for its maiden flight. It looks powerful,
doesn’t it?
08sig2.QXD 6/22/07 11:28 AM Page 45
46 MODEL AVIATION
Model type:
Pilot skill level:
Wingspan:
Wing area:
Length:
Weight:
Wing loading:
Engines:
Radio system:
Construction:
Covering/finish:
Street price:
Flight duration:
Hobbistar 60 MK III
Trainer ARF
Beginner/intermediate
71 inches
888 square inches
55 inches
7.0-8.0 pounds
18-21 ounces/square foot
.61 two-stroke
.91 four-stroke
Four channels minimum,
four standard servos
Built-up balsa and plywood
Iron-on film
$149.99
Exceeds 10 minutes
Sport Utility Twin
General-purpose ARF
Intermediate
71 inches
888 square inches
55 inches
8.5 pounds
22 ounces/square foot
.40-.46 two-stroke
.52-.61 four-stroke
Four channels minimum,
five or six standard servos
Built-up balsa and plywood
Iron-on film
Not applicable
Up to 30 minutes
Sport
Utility
Twin
This is, after all, a utility model, so it should be no surprise that it also works well on
floats. Takeoff power is certainly not an issue.
need to check the wing incidence and
downthrust to make sure the final angle of the
twin firewalls will match the angle of the
stock firewall in your kit.
Complete all the following steps before
you start the assembly instructions that come
with the kit. The first thing you need to do is
open the kit and become familiar with its
parts and pieces. Read through the assembly
instructions and then set them aside.
Remove any taped-on control surfaces.
Clean off the tape residue with mineral spirits
(paint thinner) and remove the covering from
the rudder. For all the rest of the parts use a
covering iron to seal the edges of the trim and
the joints where the covering overlaps. Use a
heat gun and the iron to eliminate any
wrinkles or bubbles, and firmly iron the rest
of the covering to the wood.
Cut the parts templates from the SUT
plans sheet. Lightly spray 3M Super 77 spray
adhesive on the back of the templates and let
it dry for roughly a minute. Stick the
templates to your wood. Cut out all the new
parts that are required.
Modifying the Rudder: Twins need more
rudder authority than single-engine airplanes
because there is no propeller blast over its
surface. The following steps will
approximately double the rudder’s size and
effectiveness.
Cut the kit’s rudder in two parts 1 inch
behind its LE. This will allow you to use the
original hinge slots. Glue the 1/4 balsa rudder
insert between the two pieces of the original
rudder, and add the new bottom piece.
Sand the outline to shape as shown on the
plans and sand both sides smooth. Round off
the top of the rudder to match the top of the
fin.
Assembling the Nacelles: Mark the
firewalls’ centerlines as shown on the plans.
Use the templates that come with the Great
Planes engine mounts to center, mark, and
drill the engine-mount holes. Install the blind
nuts and hold them in place with a bit of thin
cyanoacrylate.
Mark the position of the firewall and
former F-2 on the inside of the light-plywood
doublers. Use slow-drying cyanoacrylate to
attach the light-plywood doublers to the 1/8
balsa sheeting. Be absolutely sure you make
two left sides and two right sides.
Trim the edges of the balsa sides to match
the doublers. It is crucial to leave 1/8 inch of
balsa protruding past the bottom of the
doubler between the firewall and the rear end
of the nacelle.
Sand the top and bottom of the firewall to
the approximate angles shown of the plans,
and then glue the firewall and former F-2 to
one side of each nacelle. Use a triangle to
make sure they are 90° to each nacelle side.
Add the second side to each nacelle.
Recheck to make sure everything is
square. Add the light-plywood firewall brace
(F-4), the 1/4 square balsa rear spreader (F-3),
and the hatch mounts (F-5 and F-6).
Glue 1/4 balsa triangle reinforcements
behind the sides of the firewalls, and then add
08sig2.QXD 6/22/07 11:38 AM Page 46
August 2007 47
Kit, Engines, Hardware:
Quantity Item
One Hobbico Hobbistar 60 MK III ARF
Two 36-.46 two-stroke or .52-.72 four-stroke engines
Two Great Planes .40-.70 adjustable engine mounts
Two Propellers to suit
Two 21/2-inch spinners
Two 8-ounce regular or 11-ounce regular Hayes fuel tanks
Three feet Medium fuel line
One package Great Planes nylon nose-gear blocks for 5/32-inch-diameter wire
One Great Planes wingtip skid
Eight 6-32 x 1-inch machine screws
Eight 6-32 blind nuts
21 #2 x 1/4-inch sheet-metal screws
One 1/16 x 36-inch music wire
One 1/8 x 36-inch plastic antenna tube
One Extra servo to match your intended throttle servo
To use separate channels for the throttles:
Quantity Item
Four Extra 8- or 12-inch servo wire extensions
To use one channel for both throttles:
Quantity Item
One Y harness servo-wire extension
One Extra 8- or 12 inch servo-wire extension
For Optional Dual-Aileron-Servo Setup:
Quantity Item
One package Great Planes large nylon control horns
Four #2-56 x 3/4-inch machine screws
One Extra servo to match your intended aileron servo
To use separate channels for the ailerons:
Quantity Item
Three Extra 8- or 12-inch servo-wire extensions
To use one channel for both ailerons:
Quantity Item
One Y-harness servo-wire extension
Wood:
Quantity Item
One 1/8 x 12 x 48 light (poplar) plywood
One 1/4 x 3 x 6 aircraft (birch) plywood
One 1/16 x 6 x 12 aircraft (birch) plywood
One 1/64 x 6 x 12 plywood or .030 clear or white plastic
One 3/8 x 3/8 x 12 hardwood
Two 1/8 x 3 x 36 balsa sheet
One 1/4 x 3 x 12 balsa sheet
One 1/16 x 3 x 12 balsa sheet
One 3/8 x 3/8 x 6 balsa stick
One 1/4 x 1/4 x 6 balsa stick
One 1/4-inch triangle x 6-inch balsa stick MA
SPORT UTILITY TWIN
Bill of Materials
the 1/16 balsa front filler. Install the 1/16
plywood tank hatch and servo hatch, and
then fit and glue on the 1/16 balsa rear
sheeting.
Once the 1/8 balsa top sheeting is glued
in place, sand the bottom of the balsa
nacelle sides to match the installed hatches
and the rear sheeting. Round all the edges to
roughly match the radius of the fuselage
corners. Do not round the edge of the
nacelle where it will attach to the wing.
Cover the nacelles and the hatches with
matching or contrasting iron-on covering.
Cover the modified rudder and the nose
cover (if required).
Attaching the Nacelles to the Wings: Turn
one wing upside down. Make sure you are
working on the bottom of it. You can do one
wing and then the other or work on both at
the same time.
Locate the first rib out from the center
sheeting and cut the covering material in the
middle of the capstrip over that rib. Make
the cut from the center of the LE back to 1/2
inch from the wing TE. Run a piece of
masking tape along the wing as a guide for
the cut.
I like to use a pencil-type soldering iron
to burn through the covering. You do not
want to use a blade and cut into the wood.
Hold a nacelle up against the wing with
the outer edge just covering the burned edge
of the covering. Draw a line on the wing
along the inner edge of the nacelle. Use the
soldering pencil to cut the covering 1/16 inch
inside the line and across the front and rear.
Remove the covering inside the cut lines.
The covering in the next bay out will
wrinkle slightly; do not reshrink it yet. To
do so would bend the rib and could cause
the covering to pull away from the capstrip.
You can reshrink the covering with no
problems after the nacelle is glued on.
Cut a slot through the exposed sheeting
at the rear of the solid-wood LE so F-1 can
be glued in later. Make the slot at least 3/16
inch wide.
Hold the nacelle against the wing with
the top sheeting against the LE, and run thin
cyanoacrylate along the joint. Follow
immediately with thick cyanoacrylate to fill
any minor gaps. If all gaps are filled, the
cyanoacrylate’s strength is more than
adequate and much easier than trying to use
epoxy.
Fit F-1 into the slot. You will probably
have to shorten it slightly on the inner side
and then glue it firmly to the back of the
LE. Mix a batch of slow-cure epoxy and
glue F-1A on top of F-1. Be sure to firmly
glue F-1A to the nacelle side doublers as
well.
Use finishing resin or thinned epoxy to
coat the engine bay and the fuel-tank area of
the nacelles to protect them from fuel
residue. Cover the inside of the hatches too,
and make sure the fuel-proofing overlaps
and seals the covering’s edges.
Installing the Servos: Mount the throttle and
(optional) aileron servos to their 3/8-inch
08sig2.QXD 6/22/07 11:28 AM Page 47
48 MODEL AVIATION
Full-Size Plans Available—See Page 191
08sig2.QXD 6/22/07 11:51 AM Page 48
square hardwood mounts, sand the mounts to
the right length, and glue them in place as
shown on the plans. Run the servo wires out
through the existing holes in the ribs and the
kit’s aileron servo hole in the bottom center
sheeting.
If you are using dual aileron servos, cut off
the metal torque rods that are factory installed
in the wings and then attach the ailerons to the
wing using the hinges provided in the kit.
Install the aileron horn in line with the aileron
servo arm.
Line up the pushrod and drill a 1/8-inchdiameter
hole through the rear sheeting of the
nacelle where the pushrod will pass through.
Use the kit-provided aileron pushrods,
clevises, and snap connectors, and enlarge the
hole in the sheeting as needed for clearance.
Mounting the Engines: Drill two 1/4-inchdiameter
holes near the center of each firewall
for the fuel lines. Drill a 1/8-inch-diameter hole
for the throttle pushrod that is tight against the
right-side doubler, 11/4 inches down from the
top of the firewall.
Position the engines on their mounts with
the back of the spinner 4 inches from the back
of each mount. Screw the mounts to the
firewalls.
Install a section of plastic tubing from the
firewall through former F-2. Make a Z bend in
the end of the throttle pushrod, attach it to the
engine’s throttle arm, and run it through the
tube, back to the servo arm. This will take
some creative bending, but you can curve the
second pushrod to match the first, so at least
you don’t have to do it twice. Use one of the
kit-provided adjustable pushrod connectors to
attach the pushrod to the servo arm.
Hook the two throttle servos up to your
radio and adjust the pushrods so the
carburetor barrels begin to open at the same
point on the throttle stick. This is critical if
you will use a single channel to work both
throttles and it gives a good starting point,
even if you will use two separate channels.
Install each servo hatch cover with four #2
x 1/4-inch sheet-metal screws.
Installing the Fuel Tanks: Assemble the fuel
tanks and pad them well with 1/8-inch-thick
foam wing-saddle tape in the areas where they
might touch wood. Add a foam pad inside the
nacelle, over F-1A.
Attach fuel lines and install the tanks. Add
foam on both sides of the tank and between
the tank and the hatch. Do not skip the foam
tape and padding. If the tank touches any
wood, fuel frothing is likely and you will
never get the engines to run consistently.
Connect the fuel lines to your engines and
install the fuel-tank hatch covers with #2 x 1/4-
inch sheet-metal screws.
Installing the Nose Cover: The nose cover
can be made from plywood and covered with
iron-on film or made from white or clear
plastic. If it is plastic you should use a
fiberglass reinforcement at the top and bottom
ends to prevent cracking around the mounting
screws.
Locate the fuselage and remove that
monster of an engine mount that comes
with the kit; you won’t need it. Add a
second nylon nose-gear block 1 inch above
the one at the bottom of the firewall. The
necessary screws and blind nuts come with
the block.
Install the nose-wheel strut, steering
arm, and wheel collars as shown in the kit
instructions. Do not shorten the steering
arm as shown. That would make the
steering far too sensitive.
Instead, drill a new hole through the
firewall for the steering pushrod. This hole
should be aligned with the existing outer
hole of the steering arm.
Fit the notched end of the nose cover
over the nose-gear strut and use a #2 x 1/4-
inch sheet-metal screw to attach it to one
corner of the bottom of the firewall. Wrap
the cover tightly over the nose and use
three screws to attach it to the top of the
firewall. Go back and attach the other side
of the bottom with the last screw.
Adding the Tail Skid: With the wing
removed the SUT fuselage will drop down
onto its tail. To protect the tail end of the
fuselage from gravel rash, drill the two
required holes and glue in a nylon wingtip
skid.
Installing the Wing-Locating Blocks: Join
the wings by following the kit instructions.
Use plenty of epoxy on the dihedral joiner.
If you are using engines that are at the
top of the power range and are planning to
get wild and crazy with the SUT, you may
want to fiberglass the wing center joint.
That should be unnecessary in normal use.
Cut a piece of one of the 1/4 x 1/2 balsa
sticks provided in the kit to fit across the
inside of the fuselage, approximately 1/2
inch behind the front of the wing saddle.
Cut another piece to fit across roughly 1/2
inch in front of the rear of the wing saddle.
These locating blocks will prevent the
wing from shifting under load and
allowing the propellers to hit the side of
the fuselage.
Carefully mark the center of each block
and cut 80% of the way through at the
mark so the block will bend to match the
wing’s dihedral. Position the blocks on the
bottom of the wing approximately 1/2 inch
from the LE and TE, with the center marks
over the wing joint, and draw around them
with a pencil. Remove the covering from
under the pieces and then glue them in
place with thick cyanoacrylate.
You have just created a Sport Utility Twin
ARF! Go ahead and assemble it using the
kit instructions as needed.
The CG and control throws will be the
same as the kit’s except that you will need
more rudder travel. I set up the rudder with
the pushrod in the outer hole of the control
horn and the standard servo arm.