58 M ODEL AVIATION
The author gets ready to “dust” another field with his workhorse biplane. This
is a stable, slow, and majestic flier.
Find a field with soft, long grass for trimming purposes. You can adjust the
propeller pitch for the best flight profile.
n Dave Rees
An FAC Giant Scale
biplane for dusting
the competition
Grumman Ag-Cat
March 2001 59
Add as many details as you like, but watch the weight; it adds
up! Here’s the simulated steerable tailwheel assembly.
The author constructed this high-quality simulated engine from
nine Sig Manufacturing Pratt & Whitney cylinder kits.
Details make the difference. Note the hopper hatch, simulated
wing walk, flying wires, and instrumentation.
Type: FF FAC Giant Scale
Wingspan: 36 inches
Power: Rubber
Construction: Sheet and stick balsa
Flying weight: 3.59 ounces
Covering/finish: Tissue and dope
THe GIAnT SCAle event is relatively
new in the Flying Aces Club (FAC)
division. Not to be confused with Radio
Control Giant Scale, which is wellestablished,
FAC rules state that the class is
for rubber-powered Scale models with
wingspans of 42 inches or longer for singlewing
subjects and 36 inches for biplanes.
Nothing is more majestic than those
large models floating over the field at the
national meet in Geneseo NY. However, I
had dismissed the idea of making one after
looking at our minivan and the amount of
stuff we bring with us.
Even if taken apart, one Giant Scale
model would take the space of three normalsize
airplanes—that’s three events I couldn’t
enter! (I usually fly in too many events.)
I have been going through my Coconut
Scale period the last few seasons, and now I
am used to handling and transporting 36-
inch-span models. So I thought, maybe if I
pick the right biplane subject and use
Coconut technology, I can still get
everything in the minivan.
The Ag-Cat was the natural choice. If
you can transport a 36-inch-span model, you
can fly Giant Scale.
Crop-spraying is popular in my home state
of North Carolina. At work, nobody could
resist leaving his/her computer and/or cubicle
to look out the windows as a seemingly fearless
pilot showered the field next to our parking lot.
Nothing beats the sound of a big,
chugging radial engine! It’s quite a show!
When I began the research phase of this
project, I learned that the Ag-Cat was made
near Geneseo, in Elmira NY at the Schweizer
glider factory. Several thousand Ag-Cats were
made there from the 1950s until the 1990s.
The idea for the airplane was conceived
at the Grumman factory in Bethpage, Long
Grumman
Photos by David Franks Graphic Design by Carla Kunz
62 M ODEL AVIATION
Island NY near the end of World War II,
when the company began looking for
peacetime airplanes to manufacture.
The name Ag-Cat was an extension of
Grumman’s famous “cat” series: the
Wildcat, the Hellcat, the Tigercat, etc.
A decision was made not to manufacture
the Ag-Cat at Grumman facilities, and
Schweizer was chosen as the subcontractor.
The company had made subcomponents for
Grumman, and it had a good workforce of
excellent airplane craftsmen.
The first model produced—the G164—
was my choice for this project, because it fits
my image of how a sprayplane should look.
The open cockpit was enclosed when it
was learned that the low-pressure area
created during flight drew in the sprayed
materials, making the pilot look like a
snowman! I like open cockpits.
The G164 had the least bulk for the span
of all the types, while presenting a
reasonably long nose moment. One of the
Coconut concepts is low wing loading, as
achieved by minimum bulk.
The Ag-Cat is a simple airplane. It’s all one
color with few of the things judges like, such as
guns, insignia, or panel lines. The Scale score
must be boosted by one thing: the engine.
Fortunately, the 36-inch-span Ag-Cat is
almost one-inch-per-foot scale; therefore,
the Williams Bros. Pratt & Whitney cylinder
kits can be used to great advantage.
You will spend a few evenings
assembling all nine cylinders, but the result
is a very nice simulated engine. Don’t worry
too much about the weight; my biplanes
usually need nose ballast, probably because
I use big, long, heavy motors.
The completed Pratt & Whitney engine
weighed 23 grams, which is not bad for a
100-gram total airplane weight, less rubber.
The wing loading is very low when you
have 347 square inches to play with.
Your center of gravity (CG) will be in
the right place as long as you don’t deviate
too far from my component weights.
Are you ready for an article about
current lightweight airframes? Let’s go!
CONSTRUCTION
Start with wood selection. Forget about the
precut sticks; they are too heavy. Choose the
right sheets, then strip everything from them.
Not all structural members have the same
stresses, so use the lighter wood in places
where the loads are less.
I weigh each sheet of wood I use, which
really pays off in extra-light structures.
The fuselage longerons take the most
shock loads and torque from the motor when
wound, so I use eight- to 10-pound/cubicfoot
balsa for them. The cabane and
interwing struts also need that weight.
Next in strength are the fuselage
uprights’ crossmembers and diagonals and
wing leading and trailing edges; go with sixto
eight-pound balsa. The tail members also
fall in this category.
That’s basically it—six- to 10-pound is the
range. The blocks at the nose area can be
four-to six-pound, but they are hard to find.
Here are some sheet weights for these
grades: 1⁄16 x 3 x 36—101⁄2 grams, six
pounds/cubic foot; 3⁄32 x 3 x 36—21 grams,
eight pounds/cubic foot; 3⁄32 x 3 x 36—27
grams, 10 pounds/cubic foot; 1⁄8 x 3 x 36—281⁄2
grams, eight pounds/cubic foot.
Be careful of nonhomogenous sheets;
they can be too hard in one place and too
soft in another, but still give the correct
overall weights. Look for nice, even sheets.
The full-scale Ag-Cat was made essentially
like a model is constructed, so building up the
parts won’t present any challenges.
Fuselage: Build the sides, one on top of
the other, using Ambroid cement as an
adhesive. It’s plenty strong for a model
this size.
The fuselage has each bay stiffened with
diagonals, so it will not buckle under the
torque of a fully wound motor.
I have added a hard-point where you will
launch the airplane. Launching with a fully
wound motor can be tricky on a breezy day,
and I find myself scrunching the fuselage. A
little reinforcement is worth the weight
penalty.
The formers and stringers on the sides
and bottom, along with the 1⁄32 sheet areas
on top complete the fuselage structure.
Assemble a box of light 3⁄8 and 1/2 sheets, as
I show in dotted lines on the section view.
Use white glue for added strength and
toughness under impact.
Shape the outside and sand smooth, then
hollow to a 3⁄32-inch wall from the inside. I
use a very sharp gouge for this operation,
which results in a surprisingly light
structure.
I noted many of these component
weights on the plan, when I remembered to
weigh them.
Notice the slight downthrust angle so the
final downthrust won’t look so bad. Cement
the whole box to the front of the fuselage
frame with white glue.
landing gear on the Ag-Cat is a modeler’s
dream—no extra braces!
Bend up the .047 (3/64) wire over the
plan. The sandwich former is an important
structural member in an airplane this size, so
proceed carefully.
Position the two sheets, then squeeze
them in a vise to make dents for the wire.
Score them slightly deeper with the end of
the same wire.
Saturate the halves with ZAP® cement,
and quickly squeeze in the vise until cured.
No gaps, please. This will stay on the wire,
but you must mount it firmly.
Remove the 3⁄32 cross-stick where the
gear fits in the fuselage, and cement the
sandwich in its place. Add the big gussets,
to transfer the loads to the wing saddle.
Cement scraps of 1⁄20 sheet to the wire
legs, and cover with copy paper and white
glue. This will absorb a great deal of abuse
and not split off.
engine: Purchase nine packs of #20300 or
WB-203 engine cylinder kits for Pratt &
Whitney from Sig Manufacturing (401-7 S.
Front St., Montezuma IA 50171-0520).
They cost approximately $3 each.
Each pack comes with a small pattern
for making the crankcase, which I made
from balsa.
The rubber runs through the case in our
model, so make it with a one-inch-diameter
hole in the center. Make the case with the
grain running axially, so it won’t crush, and
face it with 1⁄32 plywood cemented with
ZAP®, to prevent splitting.
Now you can begin assembling the
cylinders. I moved the ignition-harness
assembly behind the pushrods and omitted the
intake manifolds, to save weight. These would
get knocked off easily during flight anyway.
The cement is critical. I used a styrene
solvent from Plastruct, available from HO train
stores. It leaves no residue and fuses the parts
together so they’re like a one-piece molding.
Cement each cylinder to the crankcase with
ZAP®. The center part of the crankcase is also
the removable nose plug, which you can make
normally using the grain running axially.
The quality of the engine is very good
when it’s completed.
I like balsa wheels because they’re
lighter, but try Dave Brown’s Lite Flite twoinch-
diameter wheels if you live in an area
where there is not much grass. The weight
penalty is small, but you will have to bush
the holes to fit the wire.
Wings: There are many wings to be built;
fortunately, all ribs are alike.
I start by making an accurate template of
the wing cross-section from aluminum
roofing flashing. The template can be laid
on a six- to eight-pound piece of 1⁄16 sheet,
and the ribs can be sliced off quickly. There
are 58 ribs, so the method must be fast.
Select and strip all the sticks for leading
edges (LE) and tailing edges (TE) and spars
from eight-pound wood, making sure there
are no hard spots or tendencies to bow;
those are sure ways to produce warps.
I design wings made in one piece rather
than glued on each side of the fuselage. This
removes all mismatch, and the wings are
much stronger when covered through the
center-section.
Pin down the LE and TE using a
straightedge, to avoid scallops. Glue in all
the lower ribs from 1⁄16 square balsa. Notice
that the lower wing has two pieces of 1⁄16 x
1⁄8 where the wing saddle fits.
While all this is drying, bend up the 1⁄32 x
1⁄16 bamboo wingtips over a hot soldering
iron. They are tricky to bend, because they
match the upper rib curvature as well as the
plan view. Cement the wingtips to the LE
and TE with ZAP®. Lay in the 1⁄16 sheet
spars and glue them to the ribs.
Put in the 1⁄20 sheet diagonals, and make sure
to glue them to the lower ribs and the two spars.
Cut several strips 5⁄16-inch wide, then when cut
to length, trim off a little height so each piece
will fit against the rear spar. These diagonals
stiffen the wings—particularly in torsion.
Slice off a pile of upper ribs from 1⁄16
sheet, and cement them all in place. A
March 2001 63
hypodermic syringe filled with Ambroid
helps make all these joints neat.
When making the lower wing, two pieces
of 1⁄16 x 1⁄8 fit the fuselage saddle. They must
be hot-bent, to match the rib upper curvature.
These pieces can be slightly harder—roughly
eight pounds/cubic foot. Remember to slant
the rib pairs to match the outer strut slant.
Both wings have one inch/foot dihedral
(approximately 5°). Cut through the spars,
LE, and TE; bevel them, and glue joints
with CyA. Applying 1⁄32 patches over the
spar joints help prevent breakage.
When dry, remove the wing from the
plan and repeat the process for the next
wing. While it is drying, sand the LE and
the TE into an airfoil shape. Be careful; it is
easy to knock a rib loose.
Stablilizer and Rudder: These are simple
to make, as was the full-scale aircraft’s tail.
Don’t worry about strength—two sets of
bracing wires will do an excellent job of
stiffening the assembly.
The tips are all hot-bent bamboo, just
like the wings.
Notice the 3⁄32- x 1/4-inch-wide strip in
the center of the stabilizer—the rudder will
mount to it.
When dry, sand all the edges round. Make
slots in the spars for the aluminum hinges using
a #11 X-Acto™ blade, as shown on the plan.
Propeller: This is a very important part of the
Ag-Cat’s performance. If you feel the
adjustable one I used is too complex, use one
of the 11- or 12-inch-diameter plastic versions.
I am a confirmed “tweaker”; nothing
bothers me more than not being able to
adjust things in the endless search for
better performance.
The adjustable-pitch propeller will allow
settings to suit the flying weather. For a calm
day, set the pitch high and the motor seems to
take forever to unwind. On a breezy day,
lower the pitch until it penetrates the wind.
You must get into the habit of inspecting
the propeller before launching, to make sure
it has not changed from the previous flight
and landing.
The blade dowels are moistened and
inserted into the tube at the start of the flying
session. They hold well for a day or two. If you
find a setting you think works best, glue it with
CyA. However, you will lose the ability to
change a broken blade at the field.
I always make a duplicate set of blades, in
case one is broken. To make a set, cut a piece of
1⁄32 plywood and a piece of 10-pound/cubic-foot
balsa for each blade. Soak the pieces in water
and wrap them around a glass gallon jug at 15°,
using strips of cloth to hold them in place.
Put the whole thing in the oven at 250°
for an hour, and let it cool slowly.
Remove the blades, and cement the balsa
ones on top of the plywood ones with ZAP®.
The balsa provides a softer medium to sand
into an airfoil curvature. Plane and sand all
the blades until they look good, then glue the
5/32 dowels into the slots with CyA.
Wrap the blade with fiveounce/
square-yard fiberglass, to
strengthen it for roughly the inner two
inches; this is where they break.
Paint the blades with clear dope several
times, to obtain a good finish, then spray on
a coat of silver. The dowels will be too tight
in the hub tubes at first, so scrape and sand
until you achieve a snug fit.
You may notice that I am a bobbin person;
if you are an S-hook or a crocket-hook type,
feel free to use what works best for you. The
same goes for latch-type freewheelers.
Covering: My philosophy is low-shrink;
that says it all. It allows you to build light
structures that don’t need to be strong
enough to withstand the shrinking tissue.
The airplane also lasts longer before
succumbing to warps—particularly in the
tail parts. This is important! It’s where all
your adjustments are made. You can’t
tolerate changes here.
Start by covering the tail. It is almost
flat, so you can do a good job of pulling the
tissue taut. I ask you not to shrink the tissue,
so do the best job you can.
Slit through the tissue for the
aluminum hinges, and install them in one
side of the stabilizer and rudder with
CyA. Before installing, make sure the
aluminum’s grain is lengthwise! (The
hinges will break if it’s crosswise!) Set
the tail aside for painting.
These were the easiest wings to cover I
64 M ODEL AVIATION
have seen in a while. There won’t be any
wrinkles the way the tips are made.
When both wings are covered, shrink the
tissue with alcohol until it’s taut. The
structure is torsionally resistant enough to
withstand the shrink forces.
The fuselage has no flat surfaces, so it will
keep you busy and demonstrate your covering
prowess. Pay particular attention around the
saddle for the lower wings. Attach the tissue
roughly 1⁄8-inch all around the wing cutout—
kind of a quirky contour. The fuselage also
gets the alcohol shrink treatment.
Anyone who has flown FAC events for a
while knows that the coloring in tissue fades
under ultraviolet light. In time, it loses the
brilliance so necessary for a good score from
the judges. I know the rules state that painted
and tissue colors are supposed to get the same
points. The brilliance factor is intangible.
For some time, I have used an overcoat
of colored dope to make the tissue colors
more brilliant and durable. Forget the clear
dope. The vehicle in color dope is the same
thing. Just spray on an even coat.
I have tried most colors in the AeroGloss
chart, and they all enhance the colors very
well. It’s like colored tissue—only better!
Control the shrinkage with a plasticizer
such as TCP, or distortion will result. I set a
weight target, so I don’t get out of control with
the amount of spraying I do. Six or seven
grams are all I allow on an airplane this size.
Final Assembly: As with all biplanes,
getting the wings on right is the hard part.
Here’s one way to do it.
The painted lower wing should slip into
the hole for it in the wing saddle. If it doesn’t,
trim the opening carefully until it fits snugly.
Check for perfect alignment in all directions,
then cement fast with white glue.
While this is drying, make two wingalignment
fixtures from 1/2-inch-thick blue
foam. Copy the wing positions from the plan
side view, and make a cutout for each wing.
The wing struts are first. They should have
been made and painted matching the plan.
Slit the tissue at the wing-attachment
points and slide the struts in place. Make the
same slits on the underside of the upper
wing, and push the wing onto the struts.
If things line up well, try the cabane
struts for fit. I used the foam fixtures near
the fuselage, to help keep the wings parallel
from the front view. This is a careful
process—don’t rush things.
When you are satisfied, squirt CyA in
each joint and it’s together forever. Notice
how strong the wings become when they are
joined and braced like this! Light wings can
be strong enough in a biplane!
The tail is the easy part. Cut the respective
slots, and cement fast with white glue when
the alignment is good. Note that the rudder
attaches to the center of the elevator.
CyA the hinges in the elevators and
rudder, so all is permanent.
That’s the sequence.
Details are the part I like best. Each modeler
can put on as much as he or she wants. Buy one
of the reference books, because there is a limit
to how much can be shown on a plan.
Flying: Use the rubber sizes listed if you
have controlled the weight as you have
built the model, and the weight is close to
the total shown.
As weight goes up, the motor can be
shortened to produce more power;
however, adding more rubber could
overload the carefully designed structure.
Proceed with caution.
You need a “soft” field—grass or high
weeds—until the Ag-Cat is flying safely, even
if it means driving some distance. You have
spent a great deal of time on this project, so
don’t lose it during trimming. The Ag-Cat flies
very slowly, which makes trimming much
smoother.
With a slightly taut motor, balance the
airplane with your fingers. Your fingers should
be close to the airplane’s CG position, at least
to start out. Ballast as needed. Crank in 300
turns, and see how it flies at a medium-pitch
position on the propeller.
Adjust flying surfaces as required.
Slowly increase the winds while correcting
bad characteristics.
As the Ag-Cat climbs high enough,
notice what happens in the glide phase of the
flight. Fine-tune the adjustments to optimize
the glide. My Ag-Cat climbs out at a steep
angle, but slowly, which really looks strange!
Try adjusting the propeller pitch last,
until you see which pitch works best and
gives the longest flight.
When adjusting the propeller, make sure
the blades are not twisted off the dowels.
The dowels really stay put in the hub. I
grab the dowels with a smooth-jawed pair
of pliers to turn them, rather than risk
putting the load on the blades.
I am sure you will like this big, slowflying
Ag-Cat. MA
Dave Rees
606 Walnut Creek Dr.
Goldsboro NC 27534
