January 2003 59
ThE OT-40 is the modeling equivalent of what we called a
“sleeper” in the bad, old days of street racing: a four-door Chevy
Nova with a zillion-horsepower engine. The OT-40 looks like an
Old-Timer. It resembles many pre-World War I airplanes with an
open rear fuselage; exposed pilot; and shoulder-mounted, high-lift
wing.
You would expect the OT-40 to be great at flying low, slow,
large circles and not much else. Wrong! It does have great slowflight
behavior, lands at approximately 5 knots, and is forgiving, but
it climbs like a rocket, rolls fast, does outside maneuvers and
inverted flight as well as it flies upright, and will outrun most sport
airplanes. The secret is a fairly thick symmetrical wing with the
center of lift placed well forward and a blunt leading edge (LE)
paired with a sophisticated design combining strength and low
weight.
The design is elegant, using one part to do the work of several.
The brace for the wing dowel is the doubler for the spars. The wingbolt
mounting plate is the trailing-edge (TE) doubler. The pre-shaped
Frank Granelli
P r o d u c t R e v i e w
24 Old Middletown Rd., Rockaway NJ 07866
Four Pi OT-40
Pros:
• Many preformed parts make building fun.
• Outstanding flying qualities
cons:
• The instruction booklet could be more complete.
Don’t let the name and appearance fool you; this is a
performance airplane, capable of Pattern-like execution.
LEs are recessed, allowing the wing sheeting to fit flush behind the
LE, saving weight and sanding. The vertical fin is slotted inside the
stabilizer, eliminating braces.
The result is a rugged, 53+-inch-wingspan airplane that weighs 4
pounds, 10 ounces. Because it is so light, this is one of the few 40-size
models that will fly almost as well on a bushed .40 as it would on a
high-performance engine.
Building is easy; key parts are pre-shaped, sanded, and the right
size. Each subassembly is packaged together for easy identification.
The quality prefabrication and packaging are important because this
kit’s instructions will let you build the OT-40 or a boat—your choice!
60 M ODEL AVIATION
The plans help, but they have some unclear
areas. Keep the box top because that photo
will help you through several foggy sections.
With better instructions and a few photos,
the OT-40 would be a great first balsa kit for
the Almost Ready-to-Fly (ARF) generation. It
is easy and fun to build, performs well in the
air, and looks good at the field. I received
many compliments at my flying site
(normally a very tough house) that were
earned more by the airplane’s good looks than
by my building. I took credit anyway.
construction: The easiest part of this model
to construct is the wing. Building a kit has
advantages over an ARF. The instructions
recommend putting 5⁄16-inch blocks on the
plans and pinning the spars to these blocks.
This works, but it makes it harder to get a
straight wing.
I put two of the fuselage longerons
spanwise on the plans as rib levelers. The rear
of each longeron should be exactly 421⁄32
inches forward of the TE. The rib bottoms
rest on these braces.
Use the four longest 1⁄4 square balsa sticks
as wing spars that reach the wingtips. There
are also shorter, 1⁄4-inch sticks in the kit. If
you see any slight warps, pin the “bowed”
center down first then pin the raised ends.
Offset warps by installing the top spar with
bows as mirror images. Save the four softest
wing sheets for the LEs.
The ribs are exact matches for each other
and are packed into one block, as are the
riblets. While still in the “block,” position the
riblets, the wing ribs, and the 3⁄4-inch center
rib together so that the airfoils match. Draw a
line across the tops of all three, marking the
top.
Save two medium-weight ribs for the tips.
Install the heaviest ribs in the center and the
lighter ribs toward the tips. The two mediumweight
ribs are at the tip for strength. Weigh
the sheeting, putting the heaviest sides
inward. Put the heaviest of each rib pair and
sheeting on the side away from the muffler to
save extra tip-balance weight later. The
lighter the outboard wing sections, the faster
the roll will begin and the more precise the
stop will be.
The center riblet block has the wing holddown
dowel hole already drilled, so get this
hole centered. Don’t forget the dihedral brace.
Install the preformed LE, the bottom TE
sheeting, the preformed TE, the precut spar
webbings, the riblets, the center riblet, and the
pre-shaped wing-mounting plate. Notice all
the “preformeds” and “pre-shapeds”; the
manufacturer does so much work that this is a
fun kit to build.
The preformed LE has the sheeting slots
already cut. Install the LE sheets by wetting
them with an ammonia-water mixture then
installing. Since every part of the wing braces
another part, this is a strong wing. Finish
installing the precut center sheeting. Do not
forget to install the cockpit floor.
Make the decorative flying wire brace if
you want, but install it after the wing is
covered. Do the mount construction now if
you are going to use a machine gun.
The rear of each wingtip is braced with
The completed OT-40 airframe is rugged but light. It certainly has the old-time, openframework
appearance! This project is good for a change of pace.
On the field and finished this model harkens back to an earlier era, but that perception
changes at takeoff; this is really a high-performance airplane.
This is a builder’s kit, and it is complete. The key parts are packaged in units for ease of
identification, and the parts are cleanly cut.
Photos courtesy the author
balsa strips. I cut a slot in the center of the TE
with a razor saw instead and installed a 1⁄4-
inch strip of 1⁄64 plywood. Cut a 2- and a 1-
inch lightening hole in each wingtip to save tip
weight.
Move the entire aileron-servo installation
one bay inward to save tip weight. The kit has
no radio-installation instructions, so I installed
the two aileron servos “Pattern style.”
Brace the rib sides with 1⁄64 plywood and
install 1⁄4 x 3⁄8 hardwood servo rails on triangle
stock between the ribs high enough to allow
the servo arm to clear the outside of the wing.
The plywood cover plate should be lower than
the final wing height. Glue a 1⁄4-inch soft-balsa
plate onto the plywood cover plate and sand it
to match the wing’s outside contours.
The instructions are so silent about
building the tail that you might think the OT-
40 was a flying wing. The rear flying surfaces
are built from balsa sticks, saving weight but
presenting a few challenges.
Use the hardest 3⁄4-inch strip for both TEs.
Ensure that the stabilizer’s TE is straight as
the stabilizer’s installation aligns the vertical
fin. The center of the stabilizer has a short 1⁄4
balsa piece between the two 3⁄4-inch balsa
center pieces, leaving the stabilizer’s center
open to allow for installation of the fin. This
makes for a strong, light tail section.
Block-sand the elevator halves together to
ensure that they match. I cut the elevator TE
space provided for rudder deflection 3⁄4-inch
deeper to allow for extra rudder throw. I
highly recommend doing this. Use a
cardboard template to make sure the
stabilizer’s rounded corners match.
Examine the fuselage plans closely before
starting. The directions won’t help much here.
The front fuselage is constructed from various
subassemblies then sheeted for integrity. This
is a light and strong construction technique.
Align the two wing-saddle sections,
matching the curved saddles, then block-sand
the three straight sides to match. This ensures
that the wing sits true on the fuselage. Position
the saddle and wheel subassemblies on the
plans first, install the spruce longerons, and
connect the front subassemblies with the 1⁄4
balsa.
Use the trimmed spruce longerons to build
the crossmembers. Cut the longest pieces first.
If you cut one too short, you can use it to
make the next rearward piece. I didn’t waste
any, yet I needed an extra 12 inches. If this
happens to you, use the nonkit pieces as
crossmembers in the rear areas that will be
sheeted. The kit-supplied longerons are color
matched. Sheet the sides, making sure that
there is a right and a left.
From here, the fuselage is built just as if it
were “sheet sided” using longeron
crossmembers instead of formers. Make sure
that the firewall has the proper down- and
right-thrust angles for the higher-performance
engine that this airplane deserves.
I installed a 3⁄32 light-plywood floor under
the stabilizer for extra strength, put a small
piece of spruce triangle stock in front of the
stabilizer for smoother airflow, and installed a
rear bulkhead to seal in the rear and provide a
center pushrod brace. I used a Du-Bro .40-size
tail-wheel bracket on a 3⁄32 light-plywood base.
Install the rear fuselage sheeting only after
you are satisfied that everything “back there”
works well. Similarly, don’t enclose the front
top and bottom until everything up front is
operating. The front top is supposed to be
sheeted, but I strip-planked instead using 1⁄3-
inch strips. The fuselage nose uses false
formers and balsa sticks to “bulge” outward,
simulating the cowling. The box top comes in
handy here, as does careful plans studying.
The landing gear is made from two prebent
pieces soldered together with wire wrap. Use
full-length wire wrap on the outboard sides
since this area takes all the landing stress. The
3-inch Williams Bros. vintage wheels match
the airplane’s antique looks.
Make It goWhere You Want: Everything
about the OT-40 says “performance,” so I
used a SuperTigre G-45 turning an APC 11 x
6 propeller. This older SuperTigre is heavier
than many of today’s .45s, requiring 2 ounces
of tail weight. The perfect engine would be
one of the smaller-framed .50s, eliminating 2
ounces in back and 4 ounces up front. At 4
pounds, 4 ounces with a strong .50, vertical
would be even more incredible.
I installed my JR 652; this low-priced
computer radio allows the ailerons to act as
independent flaps or spoilers while
automatically providing elevator trim. The
ailerons can also be sequenced with elevator
input such as on a Control Line Stunt airplane.
Anyone for 10-foot-diameter loops? The OT-
40 required roughly 5% elevator/rudder
mixing to straighten the knife-edge.
Set the rudder to 70° each way; the
elevator to 18° low, 35° high rate; and the
ailerons at 12° low, 20° high. The center of
gravity was 1⁄8 inch in front of where it was on
the plans. Balancing the wingtips required one
11⁄2-inch finishing nail. Lateral balance is
important, but especially so for a wing
without dihedral.
Do I Have to Fly It? The editor said yes so to
the field we went, and many of my friends
wondered at the “slow old-timer.” Everyone
loved the looks, and I received many unearned
building compliments because the design
looked so great.
Still, it was an “Old-Timer,” so I said that I
was getting too old to keep up with highperformance
airplanes. I started the engine
while receiving some strange looks, but I
knew I had a real sleeper under me.
The 75-foot, half-throttle takeoff was
straight without rudder. No flying trim was
required. The takeoff went so well that I
pulled vertical from approximately 20 feet
high, hit the gas, and up it went. The model
leveled out at 500 feet with the last 100 feet of
vertical climb needing 50% right rudder.
The OT-40 came back across the top
inverted and rolling with barely a hint of
down-elevator. The inverted rolls were
axial—three in roughly five seconds—and
straight since little down-elevator was needed.
The OT-40 came down the back side in a Half
Square Loop with Two of Four Point Rolls,
exited inverted, and roared into center, doing a
large-diameter Outside Loop. Allowed to
unwind, this airplane is fast.
Inverted out of the loop and straight up
into a tall Stall Turn with Half Rolls, the OT-
40 needed throttle to complete the Stall
Turn; the rudder is a bit small. The extra
rudder throw came in handy. From the Stall
Turn it flew inverted and slow rolling across
center into an Outside Immelmann, inverted
across the top back past center, down into a
reverse Humpty Bump, with a Half Roll
down, inverted across the bottom loop, then
did a full Roll up. Not bad for an antique
“Old-Timer.”
I slowed the OT-40 to check out its slow
flight. This is where all that thick-airfoil
wing area and light weight really shows. In a
light wind the airplane nearly stopped
without a wing drop. The ailerons stayed
positive throughout, and the stall was nearly
impossible to induce. I couldn’t get a spin
without a great deal of power, and recovery
meant just lowering the throttle, stopping the
spin immediately.
On approach, airspeed/altitude
management was too easy, again because of
that wing. The OT-40 crawled to a
touchdown, dead center in the runway, and
stopped in approximately five feet. Final
approach was so slow, I thought the thing
would never land!
I did some computer-trimming stuff
throughout several flights then took off to
really try out the OT-40. Graceful slow rolls
were easy. Four Point Rolls from upright or
inverted were no problem but needed a great
deal of rudder. Extended knife-edge flight is
limited to 200 feet by the lack of fuselage
and rudder area.
The OT-40 required all 35° of elevator
plus a little power to do Snap Rolls, even
from inverted. Inverted Spins, even on high
rate, also required power, but then I
deployed the flaperons, crossed the controls,
applied full power, and the OT-40 actually
went into an inverted semi-Blender. “Old-
Timer,” huh?
I tried a quick hover, put in full down, hit
the flaperons and high throttle, and got an
acceptable Waterfall. The small rudder and
fuselage side area limit other 3-D maneuvers
that the OT-40 wasn’t designed for anyway,
but the Blender and Waterfall were
impressive.
The OT-40 is equally impressive in slow
flight. Even abused, it is gentle and honest.
In low-speed, full-elevator, vertically banked
turns the airplane slowly descends, turning in
a small-diameter circle without snapping.
Inverted stalls have to be forced. Inverted or
upright, keeping full elevator in the stall just
produces wing-level oscillations. Even a
pilot who is not far past the trainer stage will
have no problem flying this gentle but
surprising performer.
The OT-40 was easy and fun to build, looks
great on the ground, and is a surprising flying
machine when you let it unwind. It’s a great
sleeper. I’m keeping this one. MA
(Editor’s note: Since this review was
written, Four Pi has rewritten the instruction
book with more details for construction. You
can contact the company at 4944 N. Orange
Ave., Norridge IL 60706-3224; Tel./Fax:
(708) 457-2177. Order forms are available on
the Web site: www.fourpi.com.)