August 2007 17
BY DICK SARPOLUS
TWIN-ENGINE airplanes are fun, larger airplanes are fun,
and I’ve been enjoying electric-powered flying, so here’s
the Lotsa Watts. It’s a big, double-motored aerobatic sport
flier that is easy to build and fly.
Twin-engine flying is fun to the point where through the years
I’ve had many of these types of models and continue to consider
them for future projects. I started this design because I wanted a
bigger electric-powered airplane, but I didn’t want to spring for a
huge motor, ESC, and battery pack.
I already had a good 45-size electric setup, so figured I’d
duplicate that system and the power of two 45s would enable me
to fly an 82-inch-wingspan aircraft with lively performance.
Those two propellers would make for extra excitement and fun. It
worked out well.
I wanted to make an airplane that would span more than 80
inches—the IMAA’s (International Miniature Aircraft
Association’s) definition of a “big model”—so it would be
eligible for its fly-ins. I figured two glow .45s would do the job,
so I compared them to the electric components. I estimated that a
glow .45, muffler, engine mount, throttle servo, and fuel tank with
the fuel would weigh at least 36 ounces.
The electric motor I wanted to use, roughly equivalent to a
glow .45, with an ESC and five-cell Li-Poly battery pack would
weigh approximately 32 ounces. I could go electric with no
problem and saw no reason to try really hard for an extra-light
airframe.
Not so many years ago, to go electric and get the performance
of a glow engine meant building an exceptionally light airframe.
With today’s electric-power technology, there’s no longer a
weight penalty in this size range.
With the absence of engine vibration, an airframe structure for
electric power can actually be a bit lighter. If you really like the
glow engines’ sound and oil, throw a pair of .40s or .45s on the
Lotsa Watts for some noisy fun.
The author holds his man-size design. It is easy to construct
and has pleasing lines both on the ground and in the air.
DSC0012
Rack up kilowatthours
at the field with
this RC sport twin
08sig1.QXD 6/22/07 12:42 PM Page 17
18 MODEL AVIATION
Dick’s friend Ray Borden produced these laser-cut ribs for the
project, but you can cut them with a band saw or a scroll saw.
The engine nacelle being fitted to the wing panel.
The wing panels can be built flat on a workbench. Put a piece of
waxed paper over the plans and then assemble the parts. This
ensures a warp-free wing!
A bottom view of the completed airframe shows the nacelle with
its bottom hatch in place.
Left: The fuselage’s and two nacelles’ basic assembly
reveals the simple structure of sheet-balsa sides, plywood
doublers, and plywood bulkheads.
Above: The bottom nacelle hatch cover is removed.
Notice the uncomplicated and effective latch system.
Photos courtesy the author
08sig1.QXD 6/22/07 11:56 AM Page 18
August 2007 19
A side view reveals the fuselage’s straight lines. This makes
construction straightforward but yields a pleasant profile.
The bottom of the unfinished airframe reveals the exceptionally
light structure. There is not too much sheeting!
The power system: two Welgard brushless outrunners; APC 12 x 8E
propellers; Du-Bro aluminum spinner nuts; Castle Creations Phoenix-80
ESCs; Thunder Power five-cell, 3850 mAh Li-Poly packs.
Motors are mounted on 1-inch nylon standoffs. The ESC
is held to the compartment floor with hook-and-loop
fastener. Next to the airframe is the removable battery
floor; it is screwed in place above the ESC.
Twin Engine Dos!
20p9 two-line cutline goes here
When I rounded up pictures of the twin-engine aircraft projects
I’ve done through the years, I was surprised to count 17 designs of
my own and two I built from kits. I like twin-engine airplanes.
Very few of my twins were true Scale projects. I’ve always
liked aerobatic capability in models, so most of my twins have
been stand-way-off scale or original aerobatic designs.
I have some tips for those of you who are going to tackle twinengine
projects.
• Use broken in, well adjusted, reliable engines.
• Electric power reduces much of the apprehension when it comes
to twin-engine flying.
• Lightweight models with plenty of power are the way to safety if
one engine dies.
If there’s any question about a twin’s single-engine flight
capability, cut back the other engine and put down the nose for a
landing if one power plant dies. Don’t try to stretch the approach.
I go after several things in a twin’s design. They are:
• A fairly light wing loading.
• Plenty of power for the airplane’s size.
• Keeping the engines as close together as reasonably possible.
• The use of a large amount of fin and rudder area.
Most of my aerobatic twins are comfortable flying on one
engine—either one. I’ve flown some on one power plant through
loops, rolls, turns in either direction, and landing go-arounds, with
no problems. Knowing that can be done reduces the worry about
one engine dying.
Regardless of the risks, expense, and extra work involved in
twin-engine projects, they’re so neat and so much fun I’m sure
they will always be a popular part of our hobby. It’s the twinengine
sound! MA
—Dick Sarpolus
Smooth, steady inverted flight is a cinch for
this model. It tracks straight and true
through all aerobatic maneuvers.
08sig1.QXD 6/22/07 12:11 PM Page 19
I laid out a wing planform with an 82-
inch span and a wing area of 1,066 square
inches. The wing is mildly tapered, with
strip ailerons and a semisymmetrical
airfoil.
I like Bruce Tharpe’s designs, so I
borrowed his Four-Star-type airfoil and
wing construction. There is no LE planking
or capstrips; this is simpler construction.
And with the straight line on the bottom of
the airfoil from the spar to the TE, the wing
panels could easily be built flat on the
workbench surface.
I made the fuselage 60 inches long with
generous-sized tail surfaces. I located the
nacelles on the wing to provide clearance
for 12-inch propellers and room for larger
propellers if desired.
I like a tricycle landing-gear setup on a
model such as this, but a tail-dragger gear
arrangement is more practical since I fly
from several grass fields that can be fairly
rough. The plans show both setups.
The basic airframe came out at
approximately 4 pounds. With everything
in it except the two battery packs, it
weighed 7.5 pounds. Ready to fly, the
prototype Lotsa Watts weighed roughly
9.75 pounds. I have to remind myself that it
is equivalent to a glow-powered model with
the fuel tanks filled—and that’s not the way
we normally give the weight of a glowpowered
airplane.
My model’s weight works out to a wing
loading of approximately 21 ounces per
square foot. That’s a nice figure, again
based on an airplane with “full fuel tanks.”
I used a separate 1200 mAh Ni-Cd
battery pack for the radio. Any ESC that is
handling the high voltage of five Li-Poly
cells will not typically provide a BEC radio
output. I could have used a separate BEC to
provide power for the radio, but I thought it
was easy enough to just use a standard 4.8-
volt radio battery.
I’d call this airplane a sport machine.
It’s certainly not aimed at RC Aerobatics
competition or 3-D flying, but it will do
anything aerobatic I can think of to try.
It has a great power-to-weight ratio and
a light wing loading, and it will slow down
for relaxed, easy flying. I use dual rates;
low rate for my easy flying and high rate
when I want to try wilder stuff.
This airframe is easy to build, so it
shouldn’t worry you if you want to try
scratch-building. A band saw or scroll saw
is needed since you do have to cut out all
the wing ribs, fuselage bulkheads, etc.
Sure, it’s more work than building from a
kit or an ARF, but I consider it a fun part of
the hobby.
If this model looks interesting to you
and you like to spend time in the workshop
making some wood chips and sawdust, go
for it!
This is the first of my designs for which
I didn’t have to cut my own wing ribs. My
friend Ray Borden, who did such a nice job
with his CAD equipment digitizing my
Type: RC sport
Wingspan: 82 inches
Weight: 7.5 pounds without batteries
Flying weight: 9.75 pounds ready to fly
Wing area: 1,266 square inches
Length: 60 inches
Wing loading (ready to fly): 21 ounces/square foot
Motors: Welgard C50/55/06 brushless outrunner
Propellers: APC 12 x 8E
Power: Castle Creations Phoenix 50- to 80-amp ESC, two five-cell 3850 mAh Li-
Poly batteries
Draw: 42 amps/motor
Output: 750 watts/motor
Watts/pound: 150
Radio: Four channels minimum with four standard servos
Construction: Built-up balsa and plywood
Covering/finish: MonoKote or similar film
This airplane is majestic in flight. It has the performance of a Pattern-type design but is
almost as docile as a trainer!
20 MODEL AVIATION
08sig1.QXD 6/22/07 12:16 PM Page 20
August 2007 21
Full-Size Plans Available—See Page 191
08sig1.QXD 6/22/07 12:17 PM Page 21
22 MODEL AVIATION
08sig1.QXD 6/22/07 12:46 PM Page 22
penciled plans, has a laser cutter. Since he
had my plans in CAD, he did more work
and laser-cut a set of ribs for me. With
some work you can certainly saw-cut your
own set of wing ribs, as I’ve done many
times.
Ray also has a vinyl cutter, so he made
the graphic airplane lettering for me. It’s
nice to have a friend like that.
I’ll cover the specific equipment I used
that worked so well for me. However,
there are certainly many hardware choices
on the market, which makes it nice for us
modelers.
I used the Welgard C50/55/06 brushless
outrunner motors from BP Hobbies. They
look to be very well made, and after using
one in a Nitro Models Bobcat I knew it
had plenty of power. I also knew Howard
at BP Hobbies would answer many of my
dumb questions about electric power
equipment; he has given me good advice
before.
The only thing I don’t like about
Welgard motors is their huge propeller
shaft. They are unusually large, and it’s a
pain reaming and/or drilling out propeller
hubs for use on the motors.
Bob Zambelli, a modeling friend with
machine-shop capability, turned down the
prop-shaft adapters and gave them 5/16-24
threads. I used Du-Bro aluminum spinner
nuts in front of the propellers.
I mounted the motors with 8-32 bolts
through 1-inch nylon spacers I found at
The Home Depot and am using APC 12 x
8E propellers. I’m happy with the
performance but will be trying other sizes
for comparison.
For ESCs to control the motors I went
with Castle Creations’ Phoenix-80s. Their
80-amp capacity is overkill, as I figured
each motor would draw less than 50 amps,
but the 60-amp units were not in stock at
the time, and these ESCs are so small and
light that the weight difference was
negligible. Also, I had the Castle Link
USB Programming Kit so I could easily
adjust any programmable settings on the
ESC with my computer.
I planned on five-cell Li-Poly battery
packs, and I knew that at the higher
voltage the ESC would not provide a BEC
output for the radio system. Rather than
use a separate BEC I decided to employ a
separate Ni-Cd pack for the radio system;
weight wasn’t an issue in this airplane.
For the Li-Poly batteries I went with
five-cell, 3850 mAh Thunder Power packs.
They are rated at 25C continuous
discharge, 30C sustained discharge, 50C
max short-burst discharge rates, and I
knew I’d be operating them much more
conservatively.
Thunder Power has been around for a
while and has a good reputation, and I
could count on these batteries. With the
power available from the two motors I
don’t fly continuously at full throttle
anyway, and the batteries would have
plenty of capacity for my flying habits.
I like Deans Ultra connectors, and
because they plug in so tightly I drill a 1/16-
inch-diameter hole in each connector and
use a pair of snap ring pliers from Sears to
help disconnect them. This works very
well. I had a setup of Blue Bird servos on
hand to use—item BMS-631MG from BP
Hobbies with 69 ounce-inch of torque—
and they have been working fine.
I’ve had good results with Berg
receivers, so got one of the new 7P
models. I don’t profess to understand the
True Digital Signal Processing and
Transmitter Signal Recognition features,
but they sound good to me. This is the
smallest receiver I’ve used in something
other than a park flyer-size airplane, but
Castle Creations says it’s full range and I
believe it. It works fine!
CONSTRUCTION
I suggest that you make a kit of the
parts before you begin construction. I
don’t have a wood list because I try to
keep a good balsa and plywood supply on
hand for building from scratch.
Lone Star Balsa is where I buy my
wood. After making a rough estimate of
the wood I think I’ll need, I’ll order more
than that. It will be needed down the line
for repairs or the next project.
I get a second copy of the plans or
make copies of the part drawings on the
24 MODEL AVIATION
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08sig1.QXD 6/22/07 12:17 PM Page 24
plans so I can cut the paper patterns to
trace around with a ballpoint pen onto the
balsa or plywood. It may be an oldfashioned
method, but it works.
I cut everything out with a band saw or
scroll saw. If the cutting is inaccurate,
work with a sanding block will get the
parts in shape. The accuracy is up to you.
Wing: I use waxed paper over the plans on
a hopefully flat, smooth worktable. With
this airplane’s wing section, the bottom of
the ribs from the lower spar to the TE is
straight. So with the lower spar, lower TE
sheeting, and center-section bottom
sheeting positioned over the plans, you can
locate, pin, and glue the ribs in place.
Oh, and you’ll have to make your own
tapered TE stock. I use a small, sharp
block plane and a sanding block to get it to
shape. Glue the TE and upper TE sheeting
in place.
Before you can add the upper spar,
you’ll have to cut the vertical-grain spar
webbing to fit. This webbing is a pain to
add—each piece is different because the
wing is tapered—but it adds a great deal of
strength. I cut and block-sand each piece to
fit.
Add the top spar, the LE, and the front
spar. The top and bottom LE sheeting and
the top center-section sheeting are added
after the wing panels are removed from the
building surface.
Plywood rib doublers are used to accept
the grooved landing-gear blocks for the
trike gear or the tail-dragger setup. Put the
cardboard tubes in place through the ribs
for the aileron-servo leads and the ESC
leads before adding the upper sheeting.
And you’ll have to cut a vertical slot
between the spars in the two inner ribs so
the plywood dihedral joiner can be
inserted.
Glue in place the plywood cap ribs on
the wingtips. I wrapped the center joint
with 4-inch-wide fiberglass cloth and
epoxy, scraping off the excess epoxy for a
smooth appearance. Glue a plywood
mounting tab in place through the LE and
center ribs to position the wing in the
fuselage.
The ailerons are made from 3/8 balsa
sheet. Their LEs are planed and blocksanded
to the beveled shape, and their TEs
are sanded round.
Tail Surfaces: The horizontal stabilizer,
elevators, fin, and rudder are built over the
plans from 3/8 balsa stock. Use whatever
type of glue you prefer. I do much of the
building with five-minute epoxy because it
seems that I’m always in a hurry. Bevel to
shape the elevator and rudder LEs, and
sand the outer edges of the surfaces to be
round.
The two elevators are joined by a bent
piece of 1/8-inch-diameter music wire
epoxied in place. On a model this size I
prefer the pinned nylon hinges, and I use
the Du-Bro tools to cut the slots for them. I
fit all the hinges and epoxy them in place
later, after the airplane is covered.
Fuselage and Nacelle: Start building the
fuselage by gluing the plywood doublers to
the balsa side pieces. I put lightening holes
in the plywood doublers with a hole saw in
a small drill press. The wing-saddle pieces,
stabilizer saddle doubler, and lower edge
strips also need to be glued to the fuselage
sides before assembly.
Glue bulkheads F3, F4, and F5 to one
fuselage side and then add the other side.
For easy alignment the fuselage sides are
parallel above the wing. The nose and tail
sections can be pulled in, and the
remaining bulkheads can be added.
The upper rear side pieces are cut
oversize, and their lower edges are beveled
to mate with the main fuselage sides. When
they are glued in place, I block-sand their
upper edges flat in line with the bulkhead
tops and then I add the 1/4 balsa top piece
and sand it smooth and round.
The forward-fuselage top sheet is
curved over the bulkhead tops and glued in
place. The fuselage bottom sheeting is
added later, after the tail-surface linkages
are in place.
I made a removable hatch in the
forward-fuselage bottom section for access
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Engine Required: 2-Stroke .60 - .75
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Wing Span: 59.75 in.
Engine Required: 2-Stroke .40 - .53
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08sig1.QXD 6/22/07 12:17 PM Page 26
to the radio battery pack and nose-gear
steering (if tricycle gear is used). If a large
balsa block is unavailable, you can make
the nose block by laminating thinner
material together.
The nacelles are built like the fuselage.
Start by gluing the plywood doublers to
the balsa sides. Join the sides with the
plywood motor-mounting bulkhead and the
rear bulkhead. Add the balsa top pieces
and round them off.
It’s easier to drill the motor-mounting
holes in the plywood bulkhead before
assembly. Epoxy in place the balsa motorcowling
side pieces and round their edges.
The nacelles fit in place on the wing
panels, between the two plywood partial
ribs, and are epoxied in place.
The nacelle motor-mount bulkheads
have spaces top and bottom to admit
cooling air to pass over the ESCs and Li-
Poly batteries. The cooling air can exit
through holes in the rear upper nacelle
sheeting and the opening in the lower rear
of the nacelle.
I have removable plywood hatches on
the bottom of the nacelles for access to the
ESCs and the Li-Poly battery packs. The
hatches have spring-loaded, quick-release
latches for convenience.
The ESCs are mounted to the floor of
the compartment with hook-and-loop
fastener and have removable plywood
plates screwed in place to accept the Li-
Poly battery packs, which are also held in
place with hook-and-loop fastener. There’s
plenty of room for cooling air to flow
through.
I mounted the motors with 1-inch nylon
standoffs; I felt that would leave room for
any other type of motor setup I might want
to try. The motors are pretty much in the
open for easy access and good cooling.
The landing gear is bent from 3/16-inchdiameter
music wire, and I use a heavyduty
K&S bender to do the job. You’ll
have to decide while building the airframe
if you want a trike-gear setup or the taildragger
setup I used.
I went with 3-inch light foam wheels to
handle grass fields. I like the leaf-springtype
tail-wheel assemblies, with the
steering arm connected to the rudder by
small coil springs.
Final Assembly: Fit the fuselage to the
wing, align it, and bolt it in place. The
bulkhead at the wing LE can be sanded or
shimmed for a good fit of the wingmounting
tab, and the fuselage wingsaddle
opening can be sanded for a good
fit to the wing. With the wing held in
place, I drill through the wing into the
fuselage mounting plate for the tapped
holes to accept the nylon wing-mounting
bolts.
The horizontal stabilizer is added and
glued into the fuselage once its alignment
with the wing is confirmed. The last step is
to add the vertical fin. With all control
surfaces hinged and in place, add the
control horns and linkages from the servos.
I like fiberglass-tube pushrods or the
flexible nylon-tube linkages. Plywood
aileron servo mounts are glued in place
between the closely spaced ribs in the
wing panels.
I used a plastic canopy from Sig
Manufacturing—the one from its Four-Star
60. I covered my model with MonoKote.
I’m used to working with it and doing the
heating and tugging necessary for a decent
job.
For final balancing of the airplane I
positioned the Li-Poly battery packs
toward the rear of the space in the nacelles
and positioned the radio battery just ahead
of the wing. I have the balance point
almost on the bottom wing-spar location,
at the fuselage side. As I get more flight
time on the airplane I may move the
balance point for finer tuning, to get the
response that makes me comfortable.
Then I set the control-surface throws.
The rudder has as much throw as it can get
without interfering with the elevators. The
elevators, measured at the TE, have
approximately 2 inches of throw; low rate
is set at 50%. The ailerons have roughly 1
28 MODEL AVIATION
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08sig1.QXD 6/22/07 12:17 PM Page 28
the excitement of a twin, with electrics
and without the worry of one motor
stopping at a bad time. Good stuff! MA
Dick Sarpolus
[email protected]
Sources:
Welgard motors:
BP Hobbies
140 Ethel Rd. W. Suite J
Piscataway NJ 08854
(732) 287-3933
www.bphobbies.com
Phoenix ESC, Berg 7P receiver:
Castle Creations
235 S. Kansas Ave.
Olathe KS 66061
(913) 390-6939
www.castlecreations.com
Li-Poly batteries:
Thunder Power
4720 W. University Ave.
Las Vegas NV 89103
(702) 228-8883
www.thunderpower-batteries.com
Ultra connectors:
W.S. Deans Co.
10875 Portal Dr.
Los Alamitos CA 90720
(714) 828-6494
www.wsdeans.com
Balsa, building materials:
Lone Star Balsa
115 Industrial
Lancaster TX 75134
(972) 218-9663
www.lonestar-models.com
Hardware, accessories:
Du-Bro
Box 815
Wauconda IL 60084
(800) 848-9411
http://dubro.com
Music wire, tools:
K&S Engineering:
6917 W. 59th St.
Chicago IL 60638
(773) 586-8503
www.ksmetals.com
Clear canopy:
Sig Manufacturing
Box 520
Montezuma IA 50171
(641) 623-5154
www.sigmfg.com
Top Flite MonoKote covering:
Great Planes Model Distributors