Grumman Ag-Cat Article & Plans
April 1961 American Modeler
visitor Joe S. ,of the great state of Texas, wrote asking me to
post this article on the Grumman Ag-Cat biplane crop duster. According
to Joe, "In the 70's I worked for Grumman as a design engineer located
at the Schweizer Aircraft plant at the Chemung County Airport
near Elmira New York. I actually grew up in Elmira. For Grumman,
I worked on the design of the Pratt & Whitney PT6-15AG engine
installation for the
G-164D Turbo Ag-Cat." It's easy to see why he has such an interest
in the airplane.
Grumman's "Ag-Cat" Proves Perfect Subject For Control Line Scale
By Paul J. Palanek
Wildcat, Hellcat, Panther, Cougar, Tiger and now, the Ag-Cat crop
dusting bipe by Grumman Aircraft. Like its predecessors it is a
fighter-a fighter of agricultural pests.
Big Ag·Cat with 450 hp engine totes 2,100·lb payloads using
Swathmaster chemical dispenser made by Transland Aircraft, Torrance,
Calif. 220 hp Ag·Cat takes 1,600· pounds.
Grumman agricultural bipe with dummy engine and scale prop.
AG·CAT BILL OF MATERIAL
Balsa unless otherwise specified)
One piece 1/2"
x 4" x 36" for Fuselage sides; (1) 1/4" x 2" x 36" for Fuselage
deck; (1) 1/4" x 3" x 36" for Fuselage formers; (3) 1/8" x 2"
x 36" for Wing ribs, fuselage bottom; (2) 1/4" x 1/4" x 36"
for Fuselage filler strip; (3) 1/4" x 1/8" x 36" for Wing spars;
(2) 3/4" x 3/4" x 36" for Wing leading edge; (2) 1/16" x 4"
x 36" for Wing bottom; (2) 1/16 x 3" x 36" for Wing top; (2)
1/16" x 2" x 36" for Wing top; 1" x 1-1/2" x 24" block for Wing
tips; 3/8" x 5/8" x 24" Hardwood for Engine mounts; 3/16" x
5" x 24" Plywood for Formers, bellcrank platform, gear reinforcements;
1/8" x 4" x 16" Plywood for "N" struts cabane struts; Scrap
block balsa for Dummy engine and scale prop.
Misc. items - 3" bellcrank; Acme fuel tank; wheels; 1/16"
and 1/32" dia. wire; nuts and bolts; sheet brass; solder; cement;
clear and colored dopes; Silkspan; stretch thread rigging; large
Veco horn; dummy pilot; decals; .29 engine; and 9-6 Tornado
ground up the Cat is designed for crop spraying or dusting. It incorporates
a number of unusual features. For one thing, the plane will take
half a dozen different engines in the 200 to the 450 h.p. range.
The choice is left to the buyer. Upper and lower wing panels are
interchangeable along with the ailerons. Rigging is simple, no internal
and a minimum of external wiring.
Both the air speed
indicator and engine tachometer are mounted on a separate panel
forward of the cockpit in the pilot's "flying line of vision". During
dusting or spraying operations pilots have little time to look inside
Specifications for the airplane are overall
length of 24' 8"; 35' 8" span; gross weight 3600 lbs; empty weight
2180 lbs; 29 cu. ft. (217 gals.) hopper Volume. Hopper load is 1000
lbs. The Ag-Cat has been designed to make a continuous 1.5-G turn
at dusting speed and at maximum gross weight.
model is designed around a 1" scale bringing the upper panel to
a 36"full span. The .29 power plant is housed behind a dummy 7 cylinder
radial engine; with a short nose moment all movements of control
are most responsive. The model floats in on a monostrut landing
gear and a spring leaf swivel tail wheel.
could begin with almost any part of the model. However, we prefer
to start with the fuselage since all members fasten to it one way
or another. The sides are 1/2" sheet balsa broken and shaped as
indicated. All formers aft of the firewall are 1/4" sheet balsa.
The firewall is 3/16" plywood. A 1/4" thick disk of sheet balsa
forms the nose of your Ag-Cat. The engine is mounted prone; be certain
the 3/8" x 5/8" hardwood motor bearers are secured to the fuselage
Fasten the 3" bellcrank to the plywood
platform as indicated. Secure a 1/16" dia. wire pushrod to the crank
with the tail end extending beyond the fuselage. The deck and nose
sections are built up from 1/4" sheet balsa laminations, shaped
as in the cross sections. Prior to fastening the lower 1/8" sheet
covering, fashion the landing gear from 3/32" dia. wire as shown.
Sandwich this between two sheets of 3/32" plywood, fasten to the
fuselage. At this point, place the Acme fuel tank in position in
former "B", secure with cement.
As all this is left
to dry proceed with the sheet balsa tail surfaces. Both the vertical
and horizontal members are fashioned from 1/4" medium grade sheet
balsa. A large Veco horn fastens the elevator halves and these are
joined to the stab with cloth hinges. These surfaces carry a full
radius on all their edges. Fasten the stab assembly to the fuselage
and the fin on top in a previously cut slot. Join the pushrod to
the Veco horn. Be certain these additions to the fuselage are cemented
At his point, the lower 1/8" sheet covering
is put in place. A hardwood wedge is inserted at the tail end of
the fuselage, this member carries the tail wheel assembly. Make
the necessary cutouts for cockpit, lower wing and proper fitting
of the engine. Before setting the fuselage aside, shape from 1/32"
sheet brass the dummy landing gear legs, softsolder to the gear
The wing panels are extremely simple, a minimum of
structure is employed. All ribs are 1/8" sheet balsa, the leading
edge is 3/4" sq. strip; there is no trailing edge. Start by laying
out the lower sheet covering of each panel. Shape the leading edge
with the proper dihedral at each tip, cement to the lower sheet
covering. When dry, add the ribs; follow with the 1/8" sq. leading
edge spars and 1/8" x 1/4" main spars. Allow this assembly to properly
With the panel sufficiently dry to work, sandpaper
the ribs blending in all rough spots also, feather edge the trailing
edge to receive the upper covering. The purpose of the spars is
to support the upper wing covering which is 2" and 3" wide sheet
balsa with the joining seams falling on the spars. The wing tips
are fashioned from balsa block, 1" x 1-1/2", cement these in place,
when dry shape to proper contour.
completed all structures and having brought them to proper finish
and contour, apply two coats of clear dope with a light sanding
after each application. Cover all wood surfaces with a light grade
Silk-span, doping all over as you proceed. When completed, apply
a second coat of clear dope allowing to dry hard at least ten hours.
When satisfied, apply four coats of balsa sanding sealer, sanding
after each brushing. All surfaces receive three coats of Aircraft
gray and two coats of Orange yellow trim. Be certain the lower wing
is fastened in place with lower sheet covering added, then proceed.
Allowing time for the above to dry, then add the plywood cabane
and "N" struts. Both are cut from three-ply plywood, take note of
the grain direction, this is important for maximum strength. Assemble
the struts between the double ribs, forcing the cabane struts into
the fuselage deck. Both wings are set at zero degree incidence.
This assembly must be done with care and proper fitting.
All rigging is stretch thread, gray in color. The wind screen
is .020 celluloid, the pilot is a painted plastic dummy cemented
in cockpit. The line guide is formed from .020 music wire, cloth
bind to the left wing "N" strut. Add antenna assembly, decal numerals,
cockpit liner, instrument fairing and other details now.
The drawing shows the components used in the assembly of the
tail wheel. Brass spring leaf is used for the strut with standard
sheet brass for the yoke. The yoke swivels in the strut with the
spring neutralizing the action. This entire assembly is fastened
to the fuselage with two # 3 wood screws.
Ag-Cat carried a Fox 35 power plant; however, a "29" should show
excellent results - the difference in weight being of little consequence.
A prop of 9-6 proportions will provide good performance. On the
field we use 75' lines with little fear of slack. The Cat has tremendous
area and, while under complete control, in theory we almost have
a free flight on our hands.
You will note photos and plans
show a hopper carried under the lower wing along with a 7 cylinder
radial engine in the nose. To add to the realism of the completed
model these items should be constructed and added. The hopper fashioned
from 1/16" sheet balsa is made a permanent fixture of the aircraft.
The dummy engine is for display only. In building, the crankcase
is hollowed to fit the shaft and washer of the mounted engine. The
scale prop is fastened to the engine with a modeler's pin. For the
radial, balsa is used throughout. Thread is wrapped around the cylinders
to simulate fins. Crankcase and rocker boxes are painted grey, pushrod
and cylinders are black. Remember, the dummy engine is removed for
Full size working drawings for the Grumman
Ag·Cat are part of Group Plan #461 from Hobby Helpers, 1543 Stillwell
Avenue, New York 61, N. Y. (S5c). At right young Ronnie Palanek,
12, prepares his Dad's model for flight. Dummy engine has been removed
and flying prop substituted for scale exhibition airscrew.
Grumman Ag-Cat Plans Sheet
<click for larger
Grumman Ag-Cat Plans Sheet
<click for larger
The AMA Plans Service offers a full-size
version of many of the plans show here at a very reasonable cost. They will scale the plans any size for you. It is always
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will be glad to send you my higher resolution version.
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