The G-Fast - 2010/12

I had a Moki 1.80 engine and no airplane to put
it in. Then I saw some photos of English 1930s
racing aircraft, and I wanted to try building a wing
with Bob Hunt’s Lost Foam construction technique.
The G-FAST is the result.
It’s an easy-to-build, 90-inch-wingspan IMAA (International
Miniature Aircraft Association)-legal model with a nice, thick fully
symmetrical airfoil over a tapered planform that takes up
approximately 1,400 square inches of wing area. The fuselage is 68
inches long, and the flying weight is 13 pounds, which equals
roughly a 21-ounce-per-square-foot wing loading.
“Easy” is the word, and it’s easy to fly yet fully aerobatic—but
not 3-D. It’s the kind of large airplane I like for Sunday, or anytime,
flying.
If you’re into scratch-building, making sawdust and wood chips,
or if you need an aircraft this size that doesn’t have to be exact scale
or do that 3-D stuff, take a look at my G-FAST.
Moki engines are made in Hungary and have enjoyed a good
reputation for quality, power, and reliability for many years. Because
of whatever business reasons, they are now labeled “Mark” engines
in the US, but they’re still the same old Mokis.
I knew that my Moki 1.80 was comfortable turning an 18 x 8
propeller, and I liked the Bisson muffler and an old JTEC castaluminum
mount I had for it, so I laid out this new model design
around that. Any power plant that can handle the G-FAST
specifications could be used.
A 90-inch wing will just fit in my minivan, so I went for a
simpler one-piece wing rather than plug-in wing panels with an
aluminum-tube joiner. If you go for plug-in wings, you could raise
the wing position on the fuselage for a different appearance.
Hey, if you’re working from plans, you can make any design
changes you want; it’s your airplane. That is a big part of the fun of
this hobby.
The G-FAST aerodynamic layout is conventional. It uses typical
giant scale-recommended hardware and standard construction
techniques and materials, except for Bob Hunt’s Lost Foam wing
construction, which you should try.
The Lost Foam method is an easy, accurate way to scratch-build
a built-up wing structure. I comment on it in some detail in an
accompanying sidebar.
I know that ARFs are extremely popular today, and for many
good reasons. But if you enjoy your workshop building sessions the
way I do, this project will certainly provide you with building fun.
I’ll review the construction procedures I follow. But if you’re an
old hand at it, you’ll simply need to obtain a copy of the plans and
do it your own way, at your own speed.
CONSTRUCTION
I use paper patterns to mark the wood for all parts cutting. Then I
cut a set of plans or trace the necessary parts and cut tracings for the
patterns. I draw around the paper patterns with a ballpoint pen and
cut the parts with my band saw or scroll saw.
I cut all the parts up front, make my own kit, and start from
there, or I cut the parts I need as construction progresses; any of
those methods works. I generally have a decent stock of plywood
28 MODEL AVIATION
All cut balsa and plywood parts needed for fuselage construction are shown. Today’s modeler has to pay through the nose for good
material, but we build for the fun, not the price.
The tail surfaces are
simple structures
built over the plans.
The stabilizer and fin are
solid balsa. Heavy-duty
leaf-style hinges are the
strongest option.
The completed basic fuselage assembly. Top and
bottom sheeting will be added next. The firewall is
pinned at the sides with screws.
The author had no balsa
blocks large enough for the
wheel pants, so they are made
from smaller blocks epoxied
together and then shaped.
This method works just fine.
The built-up construction is conventional;
it’s neither extremely light nor heavy. This is a
rigid structure with average moments, tapered wing layout,
and thick symmetrical airfoil—a tail-dragger with no
separate cowl around the engine.
12sig1x_00MSTRPG.QXD 10/21/10 11:36 AM Page 28
December 2010 29
The cutout is large enough through which to pull out the Moki
1.80. It is mounted sideways, so that the standard muffler tucks
in tightly against the fuselage. Balsa blocks are faired into the
spinner.
The plastic canopy was cut from a Sig Four-Star 120. A Williams
Brothers plastic pilot is finished in honor of the project creator.
Small screws hold these parts in place.
Make sure you can transport a 90-inch
one-piece wing. Employing fiberglass
reinforcement over the wing saddle area
is smart. Notice mounting plates under
the aileron control horns.
Right: The author was eager to get the
whole model together, to see how it
looked. A leaf-spring tail wheel adds
cushion. A heavy-duty horn on the
elevator offers security.
Static photos by the author. Flight photos by Lou McGuire
12sig1x_00MSTRPG.QXD 10/21/10 11:40 AM Page 29
This DVD set provides the how-to-doit
description and instructions for this
technique. It is available from Robin’s
View Productions and Airborne Media.
Foam b u ilding fixtures for the two
w i n g p a n e l s , wi t h r i b an d s pa r
positions marked. The built-up wing
s t r uc tu re, sp a r s , a nd r ibs w ill b e
assembled in these fixtures.
The wing core itself is cut apart at riblocation
p ositions, ready t o be used as
patterns for balsa ribs. Because ribs will
be cut from these foam patterns, they will
fit perfectly in the foam fixture.
The balsa ribs match the foam patterns.
They will fit perfectly into the fixture for
wing assembly. The way t h is works is
brilliant!
The loo se foam p iece is the LE buck ,
m ad e to fo r m th e L E she e tin g. Th e
sheeting is soaked, bent around the buck,
and bound with an Ace bandage until dry.
A short test piece of balsa sheeting has
been formed to illustrate the function
of the process. The LE sheeting will fit
perfectly in place over the wing ribs.
The wing assembles into the cradle. The
lower spar, lower TE planking, and first
few ribs a re shown p ositioned in the
fixture.
All ribs, upper spar, and TE planking are in
place with slow-cure adhesive. Verticalg
r a in spar w e bbing i s also i n p l ac e
between spars.
Bob Hunt developed the Lost Foam Wing
Building System in his efforts to fabricate
extremely accurate, lightweight wings for toplevel
CL Precision Aerobatics competition.
Foam-core wings have been used in model
aircraft for more than 30 years, and their
advantages of accurate, quick, and easy
construction are widely known and accepted.
Many aeromodelers prefer the more traditional
built-up ribs/spars/sheeting method of making
wings; they believe that the built-up wing
structures require more exacting work to result
in an accurate completed wing.
I see Bob’s unique construction technique
as combining foam and built-up methods to
make it easier to scratch-build a great wing
structure. I’ll briefly describe the procedure.
But to see it in detail, you’ll need to purchase
the set of two instructional DVDs.
To begin, cut a top-quality foam core for the
wing that will be built. This core is cut from
oversize, accurately squared-off foam blocks,
which will serve as a building fixture for the
built-up structure. The parts are cut from
heavier, denser foam than is normally used for
wing cores, to better serve as patterns and the
building fixtures.
Then the foam core is cut into pieces at the
rib-station locations. Those core sections are
used as patterns to make the balsa ribs. Since
the patterns are from the foam core itself, the
ribs will fit exactly into the foam building
fixtures.
The building fixtures ensure that the builtup
structure will be accurate and straight, with
no bends, twists, or warps. Extremely light
wood can be used, as desired, because the
building fixture holds everything in place as the
structure is assembled. The finished assembly
will be straight and rigid when you remove it
from the fixture.
There are other possible variations. You can
make the ribs to be positioned diagonally in the
structure. Again, since the rib patterns are the
cut core, they will fit perfectly in the building
fixture.
And then for the last step. Bob cuts a foam
LE “buck”—a precisely shaped LE section—to
be used for preforming the LE planking wood
before it’s glued in place on the wing structure.
The big advantage here is that the LE shape is
complete when the planking is adhered in
place. The radius—the curve of the LE—is
exactly as it should be from the root to the tip of
the wing.
There are no oversize wood pieces on the
LE to be planed and sanded to shape, and no
chance for inaccuracies in shaping the LE.
These details are crucial when you want a trueflying
aircraft.
Lost Foam Building System
Even the dihedral angle can be cut into
the foam building fixtures, so that the
win g panels can be t ig ht ly and eas ily
joined.
12sig1x_00MSTRPG.QXD 10/21/10 11:43 AM Page 30
As an RC sport/aerobatic
machine that is not intended for
competitive flying, the G-FAST
demands neither accurate nor
lightweight construction; it’s a
large, fun model. However, the
structure should be built accurately
so it will fly well, and light weight
is desirable for good flying.
I asked Bob for his help so that
I could try to build a wing his way,
and he let me spend a full day in
his workshop to see what it took to
fabricate the necessary foam
fixtures. Back in my workshop I
took photos so you can see how the
technique works.
And now I can state that it does
work. I ended up with a nicely
built, straight wing with an
accurately shaped LE. I knew the
wing structure was straight as I
removed it from the building
fixtures.
The Lost Foam technique is a
way to continually turn out accurate
wings; those foam patterns and building
fixtures can be reused many times. This
is handy if you know you’ll be building
more of a particular wing.
I’ve been cutting foam cores for 30
years or so, but I’d hesitate to try that
after seeing Bob’s precise setup to make
the Lost Foam fixtures. The foam pieces
have to be cut precisely for this system to
work.
For those who like the technique but
don’t want to cut foam, Bob produces the
required foam components on a custom
basis for any size and type of wing.
Therefore, anyone who wants can use
this building method. MA
—Dick Sarpolus
December 2010 31
close
The author looks
small next to his GFAST.
The racing
looks give this sport
model character and
appeal that reminds
us of why building
from scratch is so
much fun.
Sources:
Bob Hunt’s Robin’s View Productions
Box 68
Stockertown PA 18083
[email protected]
Airborne Media
(513) 755-7494
www.airbornemedia.com
Type: Large RC sport/aerobatic
Wingspan: 90 inches
Wing area: 1,450 square inches
Weight: 13 pounds
Wing loading: 21 ounces/square foot
Length: 67 inches
Engine used: Moki 1.80
Propeller: 18 x 8 Zinger
Construction: Standard built-up balsa and plywood
Wing: Bob Hunt’s Lost Foam construction method is
an option
Covering/finish: UltraCote
Other: 16-ounce fuel tank; four-channel radio with
five servos; 100 ounce-inch servos on ailerons,
elevator, and rudder; 3-inch aluminum spinner;
aluminum landing gear and plastic canopy from Sig;
31/2-inch wheels; leaf-spring tail wheel assembly
The
The Moki 1.80 is a dynamite-running glow
engine that purrs on zero-nitro fuel (cheap).
UltraCote covering offers a smooth finish
that is durable and fuelproof.
12sig1x_00MSTRPG.QXD 10/21/10 11:48 AM Page 31
balsa on hand, so I examine the plans and order more
wood than I think I’ll need. The wood probably won’t go
to waste; you’ll need it for repairs and for constructing
your next project.
I’ve had good luck with wood from several mail-order
suppliers, and I’m glad to see that Lone Star Balsa is back
in business, again furnishing us modelers with the balsa,
plywood, and basswood that we need.
Wing: You have a choice of three construction methods
here. Cut conventional foam cores to be sheeted with 3/32
balsa in the usual manner; use the Lost Foam procedures
(see sidebar); or cut the rib patterns, trace them on the 1/8
balsa, cut them with a band or scroll saw, and then build
the wing structure as usual.
Working on a good flat surface and placing waxed
paper over the plans for their protection, lay the lower
main spar in place as the first step, holding it in place with
lead weights.
Put the ribs in place next, over their positions on the
plans. The building feet at the TE of each rib puts it in the
correct location, and I hold those in place with T-pins
pushed into the plywood building surface.
Vertical-grain spar webbing is next, and yes it’s a pain.
With the G-FAST tapered wing, each spar webbing piece
must be cut/sanded to fit.
Add the top spar, the short aileron-section spars, and
the LE, and apply the LE and TE sheeting. With this
much completed, you can remove the wing panel from the
building board.
Trim the building feet off of the ribs, and add the
opposite-side LE and TE sheeting. The wing structure will
still be slightly flexible until you do this, so be sure to
check the alignment and not build a twist or warp into the
structure. Cardboard tubes for the aileron cables are glued
into the ribs before the wing halves are joined.
Cut the ailerons free from the built-up wing and trim
them so that the 1/8 balsa can be added to the wing panel
and the 1/2 balsa LE can be added to the aileron. The
inboard ribs on each wing panel must be cut between the
spars so that the plywood dihedral brace/wing joiner can
be adhered in place. I add center-section planking after
joining the wing halves.
For additional strength I add heavy fiberglass cloth
with epoxy wrapped around the wing center joint. Cut the
root ribs for the plywood wing mounting tab, and cut a
slot through the LE for the tab.
Finish the wing by adding the wingtips and capstrips.
Control Surfaces: Build the horizontal stabilizer,
elevators, and rudder structures over the plans, and sheet
the stabilizer. The vertical fin is solid sheet balsa.
I use 5-minute epoxy for most of this work, to provide
strength and keep the assembly moving rapidly. I use
large Klett-type nylon pin hinges on all control surfaces,
but there are many good giant scale-suitable hinges on the
market.
Fuselage: This structure accounts for most of the work,
but it’s straightforward.
Epoxy the 1/16 plywood doublers to the 1/8 balsa
fuselage sides, along with the wing saddle doublers, tailsection
doublers, and strips along the lower rear edges.
Since the fuselage sides are parallel from the firewall
to the wing TE position, I glue the first four bulkheads in
place on one fuselage side, ensuring that they are
perpendicular to the side. Then I adhere the other fuselage
side to the four bulkheads.
Pulling the rear ends of the sides together, glue the rear
bulkheads in place. Add the top front sheeting and the rear
turtledeck side sheeting.
Sand the rear sheeting flush with the tops of the
12sig2.QXD_00MSTRPG.QXD 10/21/10 10:39 AM Page 33
bulkheads. Add the thicker top piece and
plane/sand to shape. Don’t add the bottom
rear sheeting until the tail surfaces and
pushrod linkages are in place.
Because of the large, heavy engines used
in a model this size, I employ a 3/8 plywood
firewall, usually epoxying together a piece
of 1/8 and 1/4 plywood. I also drill two
screws, on each side of the firewall, through
the plywood fuselage doublers and into the
firewall sides.
Add the plywood wing mount plate and
landing gear mount plate. Both of those
plywood parts can be backed with an
additional piece of 1/4 plywood, to provide
more wood depth for the 1/4-20 tapped holes
for the nylon bolts. Add the plywood at the
end of the fuselage for the tail wheel mount.
With the wing bolted in place, add the
horizontal stabilizer, lining it up with the
wing. Then add the vertical fin, lining it up
with the horizontal stabilizer and the wing. I
work through the open bottom of the
fuselage, cutting holes in bulkheads to clear
the elevator and rudder pushrods.
I l ike carbo n-fib er pushrods, w ith 4-40
hardwa re. For control h orn mountings I
recess a 1/4 plywood mounting plate in to
the control surface, ep oxyin g it i n place.
Then the bo ttom fuselage planking can be
added.
I l ike whee l pants on a mode l such as
this. Rather than use the typical moldedfiberglass
par ts, I made a pair from
plywood and balsa. I left the outside of the
pant open around the wheel, for an old-time
appearance.
I mounted the big Moki 1.80 sideways
and glued balsa blocks in place around the
engine so that they could be shaped to fair
into the spinner with a simulated air inlet
below the spinner. I almost went for a
fiberglass round cowl to provide the radialengine
look, and I think a round-cowl
version would make for a good-looking
airplane.
Rather than try to find 6061-T6
aluminum to make my landing gear, I
bought a Sig Four-Star 120 landing gear. I
also used a Four-Star 120 plastic canopy,
trimmed a bit to fit the G-FAST.
A metal leaf-spring tail wheel setup was
used, coupled with small springs to the
rudder for steering. A 16-ounce fuel tank
works for me; if you like long flights, go for
a 20-ounce Du-Bro tank.
Finishing: I used UltraCote to cover the
model in a trim scheme that resembles those
I’ve seen in pictures of 1930s racing aircraft.
I also applied computer-cut vinyl
registration numbers in the English and
European style, for the foreign flavor. “GFAST”
seemed to be an appropriate
registration and provided the name for this
project.
Flying: Despite the name and styling, this is
not a racer; I expected the thick wing to keep
the speed down and make for easy flying and
full aerobatic capability. The G-FAST has
those traits, but that powerful Moki makes it
quite fast at full throttle.
I have an 18 x 8 Zinger propeller on the
engine, which I run on FAI fuel; that is, no
nitro. It’s designed for no-nitro fuel, with its
high compression, and that also keeps fuel
costs down.
Because of the conservative design
approach, I had no concerns before the test
flights. I adjusted the control throws a bit, to
suit my flying habits, and got the comfortable
flying machine I wanted. This stuff is fun! MA
Dick Sarpolus
[email protected]

I had a Moki 1.80 engine and no airplane to put
it in. Then I saw some photos of English 1930s
racing aircraft, and I wanted to try building a wing
with Bob Hunt’s Lost Foam construction technique.
The G-FAST is the result.
It’s an easy-to-build, 90-inch-wingspan IMAA (International
Miniature Aircraft Association)-legal model with a nice, thick fully
symmetrical airfoil over a tapered planform that takes up
approximately 1,400 square inches of wing area. The fuselage is 68
inches long, and the flying weight is 13 pounds, which equals
roughly a 21-ounce-per-square-foot wing loading.
“Easy” is the word, and it’s easy to fly yet fully aerobatic—but
not 3-D. It’s the kind of large airplane I like for Sunday, or anytime,
flying.
If you’re into scratch-building, making sawdust and wood chips,
or if you need an aircraft this size that doesn’t have to be exact scale
or do that 3-D stuff, take a look at my G-FAST.
Moki engines are made in Hungary and have enjoyed a good
reputation for quality, power, and reliability for many years. Because
of whatever business reasons, they are now labeled “Mark” engines
in the US, but they’re still the same old Mokis.
I knew that my Moki 1.80 was comfortable turning an 18 x 8
propeller, and I liked the Bisson muffler and an old JTEC castaluminum
mount I had for it, so I laid out this new model design
around that. Any power plant that can handle the G-FAST
specifications could be used.
A 90-inch wing will just fit in my minivan, so I went for a
simpler one-piece wing rather than plug-in wing panels with an
aluminum-tube joiner. If you go for plug-in wings, you could raise
the wing position on the fuselage for a different appearance.
Hey, if you’re working from plans, you can make any design
changes you want; it’s your airplane. That is a big part of the fun of
this hobby.
The G-FAST aerodynamic layout is conventional. It uses typical
giant scale-recommended hardware and standard construction
techniques and materials, except for Bob Hunt’s Lost Foam wing
construction, which you should try.
The Lost Foam method is an easy, accurate way to scratch-build
a built-up wing structure. I comment on it in some detail in an
accompanying sidebar.
I know that ARFs are extremely popular today, and for many
good reasons. But if you enjoy your workshop building sessions the
way I do, this project will certainly provide you with building fun.
I’ll review the construction procedures I follow. But if you’re an
old hand at it, you’ll simply need to obtain a copy of the plans and
do it your own way, at your own speed.
CONSTRUCTION
I use paper patterns to mark the wood for all parts cutting. Then I
cut a set of plans or trace the necessary parts and cut tracings for the
patterns. I draw around the paper patterns with a ballpoint pen and
cut the parts with my band saw or scroll saw.
I cut all the parts up front, make my own kit, and start from
there, or I cut the parts I need as construction progresses; any of
those methods works. I generally have a decent stock of plywood
28 MODEL AVIATION
All cut balsa and plywood parts needed for fuselage construction are shown. Today’s modeler has to pay through the nose for good
material, but we build for the fun, not the price.
The tail surfaces are
simple structures
built over the plans.
The stabilizer and fin are
solid balsa. Heavy-duty
leaf-style hinges are the
strongest option.
The completed basic fuselage assembly. Top and
bottom sheeting will be added next. The firewall is
pinned at the sides with screws.
The author had no balsa
blocks large enough for the
wheel pants, so they are made
from smaller blocks epoxied
together and then shaped.
This method works just fine.
The built-up construction is conventional;
it’s neither extremely light nor heavy. This is a
rigid structure with average moments, tapered wing layout,
and thick symmetrical airfoil—a tail-dragger with no
separate cowl around the engine.
12sig1x_00MSTRPG.QXD 10/21/10 11:36 AM Page 28
December 2010 29
The cutout is large enough through which to pull out the Moki
1.80. It is mounted sideways, so that the standard muffler tucks
in tightly against the fuselage. Balsa blocks are faired into the
spinner.
The plastic canopy was cut from a Sig Four-Star 120. A Williams
Brothers plastic pilot is finished in honor of the project creator.
Small screws hold these parts in place.
Make sure you can transport a 90-inch
one-piece wing. Employing fiberglass
reinforcement over the wing saddle area
is smart. Notice mounting plates under
the aileron control horns.
Right: The author was eager to get the
whole model together, to see how it
looked. A leaf-spring tail wheel adds
cushion. A heavy-duty horn on the
elevator offers security.
Static photos by the author. Flight photos by Lou McGuire
12sig1x_00MSTRPG.QXD 10/21/10 11:40 AM Page 29
This DVD set provides the how-to-doit
description and instructions for this
technique. It is available from Robin’s
View Productions and Airborne Media.
Foam b u ilding fixtures for the two
w i n g p a n e l s , wi t h r i b an d s pa r
positions marked. The built-up wing
s t r uc tu re, sp a r s , a nd r ibs w ill b e
assembled in these fixtures.
The wing core itself is cut apart at riblocation
p ositions, ready t o be used as
patterns for balsa ribs. Because ribs will
be cut from these foam patterns, they will
fit perfectly in the foam fixture.
The balsa ribs match the foam patterns.
They will fit perfectly into the fixture for
wing assembly. The way t h is works is
brilliant!
The loo se foam p iece is the LE buck ,
m ad e to fo r m th e L E she e tin g. Th e
sheeting is soaked, bent around the buck,
and bound with an Ace bandage until dry.
A short test piece of balsa sheeting has
been formed to illustrate the function
of the process. The LE sheeting will fit
perfectly in place over the wing ribs.
The wing assembles into the cradle. The
lower spar, lower TE planking, and first
few ribs a re shown p ositioned in the
fixture.
All ribs, upper spar, and TE planking are in
place with slow-cure adhesive. Verticalg
r a in spar w e bbing i s also i n p l ac e
between spars.
Bob Hunt developed the Lost Foam Wing
Building System in his efforts to fabricate
extremely accurate, lightweight wings for toplevel
CL Precision Aerobatics competition.
Foam-core wings have been used in model
aircraft for more than 30 years, and their
advantages of accurate, quick, and easy
construction are widely known and accepted.
Many aeromodelers prefer the more traditional
built-up ribs/spars/sheeting method of making
wings; they believe that the built-up wing
structures require more exacting work to result
in an accurate completed wing.
I see Bob’s unique construction technique
as combining foam and built-up methods to
make it easier to scratch-build a great wing
structure. I’ll briefly describe the procedure.
But to see it in detail, you’ll need to purchase
the set of two instructional DVDs.
To begin, cut a top-quality foam core for the
wing that will be built. This core is cut from
oversize, accurately squared-off foam blocks,
which will serve as a building fixture for the
built-up structure. The parts are cut from
heavier, denser foam than is normally used for
wing cores, to better serve as patterns and the
building fixtures.
Then the foam core is cut into pieces at the
rib-station locations. Those core sections are
used as patterns to make the balsa ribs. Since
the patterns are from the foam core itself, the
ribs will fit exactly into the foam building
fixtures.
The building fixtures ensure that the builtup
structure will be accurate and straight, with
no bends, twists, or warps. Extremely light
wood can be used, as desired, because the
building fixture holds everything in place as the
structure is assembled. The finished assembly
will be straight and rigid when you remove it
from the fixture.
There are other possible variations. You can
make the ribs to be positioned diagonally in the
structure. Again, since the rib patterns are the
cut core, they will fit perfectly in the building
fixture.
And then for the last step. Bob cuts a foam
LE “buck”—a precisely shaped LE section—to
be used for preforming the LE planking wood
before it’s glued in place on the wing structure.
The big advantage here is that the LE shape is
complete when the planking is adhered in
place. The radius—the curve of the LE—is
exactly as it should be from the root to the tip of
the wing.
There are no oversize wood pieces on the
LE to be planed and sanded to shape, and no
chance for inaccuracies in shaping the LE.
These details are crucial when you want a trueflying
aircraft.
Lost Foam Building System
Even the dihedral angle can be cut into
the foam building fixtures, so that the
win g panels can be t ig ht ly and eas ily
joined.
12sig1x_00MSTRPG.QXD 10/21/10 11:43 AM Page 30
As an RC sport/aerobatic
machine that is not intended for
competitive flying, the G-FAST
demands neither accurate nor
lightweight construction; it’s a
large, fun model. However, the
structure should be built accurately
so it will fly well, and light weight
is desirable for good flying.
I asked Bob for his help so that
I could try to build a wing his way,
and he let me spend a full day in
his workshop to see what it took to
fabricate the necessary foam
fixtures. Back in my workshop I
took photos so you can see how the
technique works.
And now I can state that it does
work. I ended up with a nicely
built, straight wing with an
accurately shaped LE. I knew the
wing structure was straight as I
removed it from the building
fixtures.
The Lost Foam technique is a
way to continually turn out accurate
wings; those foam patterns and building
fixtures can be reused many times. This
is handy if you know you’ll be building
more of a particular wing.
I’ve been cutting foam cores for 30
years or so, but I’d hesitate to try that
after seeing Bob’s precise setup to make
the Lost Foam fixtures. The foam pieces
have to be cut precisely for this system to
work.
For those who like the technique but
don’t want to cut foam, Bob produces the
required foam components on a custom
basis for any size and type of wing.
Therefore, anyone who wants can use
this building method. MA
—Dick Sarpolus
December 2010 31
close
The author looks
small next to his GFAST.
The racing
looks give this sport
model character and
appeal that reminds
us of why building
from scratch is so
much fun.
Sources:
Bob Hunt’s Robin’s View Productions
Box 68
Stockertown PA 18083
[email protected]
Airborne Media
(513) 755-7494
www.airbornemedia.com
Type: Large RC sport/aerobatic
Wingspan: 90 inches
Wing area: 1,450 square inches
Weight: 13 pounds
Wing loading: 21 ounces/square foot
Length: 67 inches
Engine used: Moki 1.80
Propeller: 18 x 8 Zinger
Construction: Standard built-up balsa and plywood
Wing: Bob Hunt’s Lost Foam construction method is
an option
Covering/finish: UltraCote
Other: 16-ounce fuel tank; four-channel radio with
five servos; 100 ounce-inch servos on ailerons,
elevator, and rudder; 3-inch aluminum spinner;
aluminum landing gear and plastic canopy from Sig;
31/2-inch wheels; leaf-spring tail wheel assembly
The
The Moki 1.80 is a dynamite-running glow
engine that purrs on zero-nitro fuel (cheap).
UltraCote covering offers a smooth finish
that is durable and fuelproof.
12sig1x_00MSTRPG.QXD 10/21/10 11:48 AM Page 31
balsa on hand, so I examine the plans and order more
wood than I think I’ll need. The wood probably won’t go
to waste; you’ll need it for repairs and for constructing
your next project.
I’ve had good luck with wood from several mail-order
suppliers, and I’m glad to see that Lone Star Balsa is back
in business, again furnishing us modelers with the balsa,
plywood, and basswood that we need.
Wing: You have a choice of three construction methods
here. Cut conventional foam cores to be sheeted with 3/32
balsa in the usual manner; use the Lost Foam procedures
(see sidebar); or cut the rib patterns, trace them on the 1/8
balsa, cut them with a band or scroll saw, and then build
the wing structure as usual.
Working on a good flat surface and placing waxed
paper over the plans for their protection, lay the lower
main spar in place as the first step, holding it in place with
lead weights.
Put the ribs in place next, over their positions on the
plans. The building feet at the TE of each rib puts it in the
correct location, and I hold those in place with T-pins
pushed into the plywood building surface.
Vertical-grain spar webbing is next, and yes it’s a pain.
With the G-FAST tapered wing, each spar webbing piece
must be cut/sanded to fit.
Add the top spar, the short aileron-section spars, and
the LE, and apply the LE and TE sheeting. With this
much completed, you can remove the wing panel from the
building board.
Trim the building feet off of the ribs, and add the
opposite-side LE and TE sheeting. The wing structure will
still be slightly flexible until you do this, so be sure to
check the alignment and not build a twist or warp into the
structure. Cardboard tubes for the aileron cables are glued
into the ribs before the wing halves are joined.
Cut the ailerons free from the built-up wing and trim
them so that the 1/8 balsa can be added to the wing panel
and the 1/2 balsa LE can be added to the aileron. The
inboard ribs on each wing panel must be cut between the
spars so that the plywood dihedral brace/wing joiner can
be adhered in place. I add center-section planking after
joining the wing halves.
For additional strength I add heavy fiberglass cloth
with epoxy wrapped around the wing center joint. Cut the
root ribs for the plywood wing mounting tab, and cut a
slot through the LE for the tab.
Finish the wing by adding the wingtips and capstrips.
Control Surfaces: Build the horizontal stabilizer,
elevators, and rudder structures over the plans, and sheet
the stabilizer. The vertical fin is solid sheet balsa.
I use 5-minute epoxy for most of this work, to provide
strength and keep the assembly moving rapidly. I use
large Klett-type nylon pin hinges on all control surfaces,
but there are many good giant scale-suitable hinges on the
market.
Fuselage: This structure accounts for most of the work,
but it’s straightforward.
Epoxy the 1/16 plywood doublers to the 1/8 balsa
fuselage sides, along with the wing saddle doublers, tailsection
doublers, and strips along the lower rear edges.
Since the fuselage sides are parallel from the firewall
to the wing TE position, I glue the first four bulkheads in
place on one fuselage side, ensuring that they are
perpendicular to the side. Then I adhere the other fuselage
side to the four bulkheads.
Pulling the rear ends of the sides together, glue the rear
bulkheads in place. Add the top front sheeting and the rear
turtledeck side sheeting.
Sand the rear sheeting flush with the tops of the
12sig2.QXD_00MSTRPG.QXD 10/21/10 10:39 AM Page 33
bulkheads. Add the thicker top piece and
plane/sand to shape. Don’t add the bottom
rear sheeting until the tail surfaces and
pushrod linkages are in place.
Because of the large, heavy engines used
in a model this size, I employ a 3/8 plywood
firewall, usually epoxying together a piece
of 1/8 and 1/4 plywood. I also drill two
screws, on each side of the firewall, through
the plywood fuselage doublers and into the
firewall sides.
Add the plywood wing mount plate and
landing gear mount plate. Both of those
plywood parts can be backed with an
additional piece of 1/4 plywood, to provide
more wood depth for the 1/4-20 tapped holes
for the nylon bolts. Add the plywood at the
end of the fuselage for the tail wheel mount.
With the wing bolted in place, add the
horizontal stabilizer, lining it up with the
wing. Then add the vertical fin, lining it up
with the horizontal stabilizer and the wing. I
work through the open bottom of the
fuselage, cutting holes in bulkheads to clear
the elevator and rudder pushrods.
I l ike carbo n-fib er pushrods, w ith 4-40
hardwa re. For control h orn mountings I
recess a 1/4 plywood mounting plate in to
the control surface, ep oxyin g it i n place.
Then the bo ttom fuselage planking can be
added.
I l ike whee l pants on a mode l such as
this. Rather than use the typical moldedfiberglass
par ts, I made a pair from
plywood and balsa. I left the outside of the
pant open around the wheel, for an old-time
appearance.
I mounted the big Moki 1.80 sideways
and glued balsa blocks in place around the
engine so that they could be shaped to fair
into the spinner with a simulated air inlet
below the spinner. I almost went for a
fiberglass round cowl to provide the radialengine
look, and I think a round-cowl
version would make for a good-looking
airplane.
Rather than try to find 6061-T6
aluminum to make my landing gear, I
bought a Sig Four-Star 120 landing gear. I
also used a Four-Star 120 plastic canopy,
trimmed a bit to fit the G-FAST.
A metal leaf-spring tail wheel setup was
used, coupled with small springs to the
rudder for steering. A 16-ounce fuel tank
works for me; if you like long flights, go for
a 20-ounce Du-Bro tank.
Finishing: I used UltraCote to cover the
model in a trim scheme that resembles those
I’ve seen in pictures of 1930s racing aircraft.
I also applied computer-cut vinyl
registration numbers in the English and
European style, for the foreign flavor. “GFAST”
seemed to be an appropriate
registration and provided the name for this
project.
Flying: Despite the name and styling, this is
not a racer; I expected the thick wing to keep
the speed down and make for easy flying and
full aerobatic capability. The G-FAST has
those traits, but that powerful Moki makes it
quite fast at full throttle.
I have an 18 x 8 Zinger propeller on the
engine, which I run on FAI fuel; that is, no
nitro. It’s designed for no-nitro fuel, with its
high compression, and that also keeps fuel
costs down.
Because of the conservative design
approach, I had no concerns before the test
flights. I adjusted the control throws a bit, to
suit my flying habits, and got the comfortable
flying machine I wanted. This stuff is fun! MA
Dick Sarpolus
[email protected]

I had a Moki 1.80 engine and no airplane to put
it in. Then I saw some photos of English 1930s
racing aircraft, and I wanted to try building a wing
with Bob Hunt’s Lost Foam construction technique.
The G-FAST is the result.
It’s an easy-to-build, 90-inch-wingspan IMAA (International
Miniature Aircraft Association)-legal model with a nice, thick fully
symmetrical airfoil over a tapered planform that takes up
approximately 1,400 square inches of wing area. The fuselage is 68
inches long, and the flying weight is 13 pounds, which equals
roughly a 21-ounce-per-square-foot wing loading.
“Easy” is the word, and it’s easy to fly yet fully aerobatic—but
not 3-D. It’s the kind of large airplane I like for Sunday, or anytime,
flying.
If you’re into scratch-building, making sawdust and wood chips,
or if you need an aircraft this size that doesn’t have to be exact scale
or do that 3-D stuff, take a look at my G-FAST.
Moki engines are made in Hungary and have enjoyed a good
reputation for quality, power, and reliability for many years. Because
of whatever business reasons, they are now labeled “Mark” engines
in the US, but they’re still the same old Mokis.
I knew that my Moki 1.80 was comfortable turning an 18 x 8
propeller, and I liked the Bisson muffler and an old JTEC castaluminum
mount I had for it, so I laid out this new model design
around that. Any power plant that can handle the G-FAST
specifications could be used.
A 90-inch wing will just fit in my minivan, so I went for a
simpler one-piece wing rather than plug-in wing panels with an
aluminum-tube joiner. If you go for plug-in wings, you could raise
the wing position on the fuselage for a different appearance.
Hey, if you’re working from plans, you can make any design
changes you want; it’s your airplane. That is a big part of the fun of
this hobby.
The G-FAST aerodynamic layout is conventional. It uses typical
giant scale-recommended hardware and standard construction
techniques and materials, except for Bob Hunt’s Lost Foam wing
construction, which you should try.
The Lost Foam method is an easy, accurate way to scratch-build
a built-up wing structure. I comment on it in some detail in an
accompanying sidebar.
I know that ARFs are extremely popular today, and for many
good reasons. But if you enjoy your workshop building sessions the
way I do, this project will certainly provide you with building fun.
I’ll review the construction procedures I follow. But if you’re an
old hand at it, you’ll simply need to obtain a copy of the plans and
do it your own way, at your own speed.
CONSTRUCTION
I use paper patterns to mark the wood for all parts cutting. Then I
cut a set of plans or trace the necessary parts and cut tracings for the
patterns. I draw around the paper patterns with a ballpoint pen and
cut the parts with my band saw or scroll saw.
I cut all the parts up front, make my own kit, and start from
there, or I cut the parts I need as construction progresses; any of
those methods works. I generally have a decent stock of plywood
28 MODEL AVIATION
All cut balsa and plywood parts needed for fuselage construction are shown. Today’s modeler has to pay through the nose for good
material, but we build for the fun, not the price.
The tail surfaces are
simple structures
built over the plans.
The stabilizer and fin are
solid balsa. Heavy-duty
leaf-style hinges are the
strongest option.
The completed basic fuselage assembly. Top and
bottom sheeting will be added next. The firewall is
pinned at the sides with screws.
The author had no balsa
blocks large enough for the
wheel pants, so they are made
from smaller blocks epoxied
together and then shaped.
This method works just fine.
The built-up construction is conventional;
it’s neither extremely light nor heavy. This is a
rigid structure with average moments, tapered wing layout,
and thick symmetrical airfoil—a tail-dragger with no
separate cowl around the engine.
12sig1x_00MSTRPG.QXD 10/21/10 11:36 AM Page 28
December 2010 29
The cutout is large enough through which to pull out the Moki
1.80. It is mounted sideways, so that the standard muffler tucks
in tightly against the fuselage. Balsa blocks are faired into the
spinner.
The plastic canopy was cut from a Sig Four-Star 120. A Williams
Brothers plastic pilot is finished in honor of the project creator.
Small screws hold these parts in place.
Make sure you can transport a 90-inch
one-piece wing. Employing fiberglass
reinforcement over the wing saddle area
is smart. Notice mounting plates under
the aileron control horns.
Right: The author was eager to get the
whole model together, to see how it
looked. A leaf-spring tail wheel adds
cushion. A heavy-duty horn on the
elevator offers security.
Static photos by the author. Flight photos by Lou McGuire
12sig1x_00MSTRPG.QXD 10/21/10 11:40 AM Page 29
This DVD set provides the how-to-doit
description and instructions for this
technique. It is available from Robin’s
View Productions and Airborne Media.
Foam b u ilding fixtures for the two
w i n g p a n e l s , wi t h r i b an d s pa r
positions marked. The built-up wing
s t r uc tu re, sp a r s , a nd r ibs w ill b e
assembled in these fixtures.
The wing core itself is cut apart at riblocation
p ositions, ready t o be used as
patterns for balsa ribs. Because ribs will
be cut from these foam patterns, they will
fit perfectly in the foam fixture.
The balsa ribs match the foam patterns.
They will fit perfectly into the fixture for
wing assembly. The way t h is works is
brilliant!
The loo se foam p iece is the LE buck ,
m ad e to fo r m th e L E she e tin g. Th e
sheeting is soaked, bent around the buck,
and bound with an Ace bandage until dry.
A short test piece of balsa sheeting has
been formed to illustrate the function
of the process. The LE sheeting will fit
perfectly in place over the wing ribs.
The wing assembles into the cradle. The
lower spar, lower TE planking, and first
few ribs a re shown p ositioned in the
fixture.
All ribs, upper spar, and TE planking are in
place with slow-cure adhesive. Verticalg
r a in spar w e bbing i s also i n p l ac e
between spars.
Bob Hunt developed the Lost Foam Wing
Building System in his efforts to fabricate
extremely accurate, lightweight wings for toplevel
CL Precision Aerobatics competition.
Foam-core wings have been used in model
aircraft for more than 30 years, and their
advantages of accurate, quick, and easy
construction are widely known and accepted.
Many aeromodelers prefer the more traditional
built-up ribs/spars/sheeting method of making
wings; they believe that the built-up wing
structures require more exacting work to result
in an accurate completed wing.
I see Bob’s unique construction technique
as combining foam and built-up methods to
make it easier to scratch-build a great wing
structure. I’ll briefly describe the procedure.
But to see it in detail, you’ll need to purchase
the set of two instructional DVDs.
To begin, cut a top-quality foam core for the
wing that will be built. This core is cut from
oversize, accurately squared-off foam blocks,
which will serve as a building fixture for the
built-up structure. The parts are cut from
heavier, denser foam than is normally used for
wing cores, to better serve as patterns and the
building fixtures.
Then the foam core is cut into pieces at the
rib-station locations. Those core sections are
used as patterns to make the balsa ribs. Since
the patterns are from the foam core itself, the
ribs will fit exactly into the foam building
fixtures.
The building fixtures ensure that the builtup
structure will be accurate and straight, with
no bends, twists, or warps. Extremely light
wood can be used, as desired, because the
building fixture holds everything in place as the
structure is assembled. The finished assembly
will be straight and rigid when you remove it
from the fixture.
There are other possible variations. You can
make the ribs to be positioned diagonally in the
structure. Again, since the rib patterns are the
cut core, they will fit perfectly in the building
fixture.
And then for the last step. Bob cuts a foam
LE “buck”—a precisely shaped LE section—to
be used for preforming the LE planking wood
before it’s glued in place on the wing structure.
The big advantage here is that the LE shape is
complete when the planking is adhered in
place. The radius—the curve of the LE—is
exactly as it should be from the root to the tip of
the wing.
There are no oversize wood pieces on the
LE to be planed and sanded to shape, and no
chance for inaccuracies in shaping the LE.
These details are crucial when you want a trueflying
aircraft.
Lost Foam Building System
Even the dihedral angle can be cut into
the foam building fixtures, so that the
win g panels can be t ig ht ly and eas ily
joined.
12sig1x_00MSTRPG.QXD 10/21/10 11:43 AM Page 30
As an RC sport/aerobatic
machine that is not intended for
competitive flying, the G-FAST
demands neither accurate nor
lightweight construction; it’s a
large, fun model. However, the
structure should be built accurately
so it will fly well, and light weight
is desirable for good flying.
I asked Bob for his help so that
I could try to build a wing his way,
and he let me spend a full day in
his workshop to see what it took to
fabricate the necessary foam
fixtures. Back in my workshop I
took photos so you can see how the
technique works.
And now I can state that it does
work. I ended up with a nicely
built, straight wing with an
accurately shaped LE. I knew the
wing structure was straight as I
removed it from the building
fixtures.
The Lost Foam technique is a
way to continually turn out accurate
wings; those foam patterns and building
fixtures can be reused many times. This
is handy if you know you’ll be building
more of a particular wing.
I’ve been cutting foam cores for 30
years or so, but I’d hesitate to try that
after seeing Bob’s precise setup to make
the Lost Foam fixtures. The foam pieces
have to be cut precisely for this system to
work.
For those who like the technique but
don’t want to cut foam, Bob produces the
required foam components on a custom
basis for any size and type of wing.
Therefore, anyone who wants can use
this building method. MA
—Dick Sarpolus
December 2010 31
close
The author looks
small next to his GFAST.
The racing
looks give this sport
model character and
appeal that reminds
us of why building
from scratch is so
much fun.
Sources:
Bob Hunt’s Robin’s View Productions
Box 68
Stockertown PA 18083
[email protected]
Airborne Media
(513) 755-7494
www.airbornemedia.com
Type: Large RC sport/aerobatic
Wingspan: 90 inches
Wing area: 1,450 square inches
Weight: 13 pounds
Wing loading: 21 ounces/square foot
Length: 67 inches
Engine used: Moki 1.80
Propeller: 18 x 8 Zinger
Construction: Standard built-up balsa and plywood
Wing: Bob Hunt’s Lost Foam construction method is
an option
Covering/finish: UltraCote
Other: 16-ounce fuel tank; four-channel radio with
five servos; 100 ounce-inch servos on ailerons,
elevator, and rudder; 3-inch aluminum spinner;
aluminum landing gear and plastic canopy from Sig;
31/2-inch wheels; leaf-spring tail wheel assembly
The
The Moki 1.80 is a dynamite-running glow
engine that purrs on zero-nitro fuel (cheap).
UltraCote covering offers a smooth finish
that is durable and fuelproof.
12sig1x_00MSTRPG.QXD 10/21/10 11:48 AM Page 31
balsa on hand, so I examine the plans and order more
wood than I think I’ll need. The wood probably won’t go
to waste; you’ll need it for repairs and for constructing
your next project.
I’ve had good luck with wood from several mail-order
suppliers, and I’m glad to see that Lone Star Balsa is back
in business, again furnishing us modelers with the balsa,
plywood, and basswood that we need.
Wing: You have a choice of three construction methods
here. Cut conventional foam cores to be sheeted with 3/32
balsa in the usual manner; use the Lost Foam procedures
(see sidebar); or cut the rib patterns, trace them on the 1/8
balsa, cut them with a band or scroll saw, and then build
the wing structure as usual.
Working on a good flat surface and placing waxed
paper over the plans for their protection, lay the lower
main spar in place as the first step, holding it in place with
lead weights.
Put the ribs in place next, over their positions on the
plans. The building feet at the TE of each rib puts it in the
correct location, and I hold those in place with T-pins
pushed into the plywood building surface.
Vertical-grain spar webbing is next, and yes it’s a pain.
With the G-FAST tapered wing, each spar webbing piece
must be cut/sanded to fit.
Add the top spar, the short aileron-section spars, and
the LE, and apply the LE and TE sheeting. With this
much completed, you can remove the wing panel from the
building board.
Trim the building feet off of the ribs, and add the
opposite-side LE and TE sheeting. The wing structure will
still be slightly flexible until you do this, so be sure to
check the alignment and not build a twist or warp into the
structure. Cardboard tubes for the aileron cables are glued
into the ribs before the wing halves are joined.
Cut the ailerons free from the built-up wing and trim
them so that the 1/8 balsa can be added to the wing panel
and the 1/2 balsa LE can be added to the aileron. The
inboard ribs on each wing panel must be cut between the
spars so that the plywood dihedral brace/wing joiner can
be adhered in place. I add center-section planking after
joining the wing halves.
For additional strength I add heavy fiberglass cloth
with epoxy wrapped around the wing center joint. Cut the
root ribs for the plywood wing mounting tab, and cut a
slot through the LE for the tab.
Finish the wing by adding the wingtips and capstrips.
Control Surfaces: Build the horizontal stabilizer,
elevators, and rudder structures over the plans, and sheet
the stabilizer. The vertical fin is solid sheet balsa.
I use 5-minute epoxy for most of this work, to provide
strength and keep the assembly moving rapidly. I use
large Klett-type nylon pin hinges on all control surfaces,
but there are many good giant scale-suitable hinges on the
market.
Fuselage: This structure accounts for most of the work,
but it’s straightforward.
Epoxy the 1/16 plywood doublers to the 1/8 balsa
fuselage sides, along with the wing saddle doublers, tailsection
doublers, and strips along the lower rear edges.
Since the fuselage sides are parallel from the firewall
to the wing TE position, I glue the first four bulkheads in
place on one fuselage side, ensuring that they are
perpendicular to the side. Then I adhere the other fuselage
side to the four bulkheads.
Pulling the rear ends of the sides together, glue the rear
bulkheads in place. Add the top front sheeting and the rear
turtledeck side sheeting.
Sand the rear sheeting flush with the tops of the
12sig2.QXD_00MSTRPG.QXD 10/21/10 10:39 AM Page 33
bulkheads. Add the thicker top piece and
plane/sand to shape. Don’t add the bottom
rear sheeting until the tail surfaces and
pushrod linkages are in place.
Because of the large, heavy engines used
in a model this size, I employ a 3/8 plywood
firewall, usually epoxying together a piece
of 1/8 and 1/4 plywood. I also drill two
screws, on each side of the firewall, through
the plywood fuselage doublers and into the
firewall sides.
Add the plywood wing mount plate and
landing gear mount plate. Both of those
plywood parts can be backed with an
additional piece of 1/4 plywood, to provide
more wood depth for the 1/4-20 tapped holes
for the nylon bolts. Add the plywood at the
end of the fuselage for the tail wheel mount.
With the wing bolted in place, add the
horizontal stabilizer, lining it up with the
wing. Then add the vertical fin, lining it up
with the horizontal stabilizer and the wing. I
work through the open bottom of the
fuselage, cutting holes in bulkheads to clear
the elevator and rudder pushrods.
I l ike carbo n-fib er pushrods, w ith 4-40
hardwa re. For control h orn mountings I
recess a 1/4 plywood mounting plate in to
the control surface, ep oxyin g it i n place.
Then the bo ttom fuselage planking can be
added.
I l ike whee l pants on a mode l such as
this. Rather than use the typical moldedfiberglass
par ts, I made a pair from
plywood and balsa. I left the outside of the
pant open around the wheel, for an old-time
appearance.
I mounted the big Moki 1.80 sideways
and glued balsa blocks in place around the
engine so that they could be shaped to fair
into the spinner with a simulated air inlet
below the spinner. I almost went for a
fiberglass round cowl to provide the radialengine
look, and I think a round-cowl
version would make for a good-looking
airplane.
Rather than try to find 6061-T6
aluminum to make my landing gear, I
bought a Sig Four-Star 120 landing gear. I
also used a Four-Star 120 plastic canopy,
trimmed a bit to fit the G-FAST.
A metal leaf-spring tail wheel setup was
used, coupled with small springs to the
rudder for steering. A 16-ounce fuel tank
works for me; if you like long flights, go for
a 20-ounce Du-Bro tank.
Finishing: I used UltraCote to cover the
model in a trim scheme that resembles those
I’ve seen in pictures of 1930s racing aircraft.
I also applied computer-cut vinyl
registration numbers in the English and
European style, for the foreign flavor. “GFAST”
seemed to be an appropriate
registration and provided the name for this
project.
Flying: Despite the name and styling, this is
not a racer; I expected the thick wing to keep
the speed down and make for easy flying and
full aerobatic capability. The G-FAST has
those traits, but that powerful Moki makes it
quite fast at full throttle.
I have an 18 x 8 Zinger propeller on the
engine, which I run on FAI fuel; that is, no
nitro. It’s designed for no-nitro fuel, with its
high compression, and that also keeps fuel
costs down.
Because of the conservative design
approach, I had no concerns before the test
flights. I adjusted the control throws a bit, to
suit my flying habits, and got the comfortable
flying machine I wanted. This stuff is fun! MA
Dick Sarpolus
[email protected]

I had a Moki 1.80 engine and no airplane to put
it in. Then I saw some photos of English 1930s
racing aircraft, and I wanted to try building a wing
with Bob Hunt’s Lost Foam construction technique.
The G-FAST is the result.
It’s an easy-to-build, 90-inch-wingspan IMAA (International
Miniature Aircraft Association)-legal model with a nice, thick fully
symmetrical airfoil over a tapered planform that takes up
approximately 1,400 square inches of wing area. The fuselage is 68
inches long, and the flying weight is 13 pounds, which equals
roughly a 21-ounce-per-square-foot wing loading.
“Easy” is the word, and it’s easy to fly yet fully aerobatic—but
not 3-D. It’s the kind of large airplane I like for Sunday, or anytime,
flying.
If you’re into scratch-building, making sawdust and wood chips,
or if you need an aircraft this size that doesn’t have to be exact scale
or do that 3-D stuff, take a look at my G-FAST.
Moki engines are made in Hungary and have enjoyed a good
reputation for quality, power, and reliability for many years. Because
of whatever business reasons, they are now labeled “Mark” engines
in the US, but they’re still the same old Mokis.
I knew that my Moki 1.80 was comfortable turning an 18 x 8
propeller, and I liked the Bisson muffler and an old JTEC castaluminum
mount I had for it, so I laid out this new model design
around that. Any power plant that can handle the G-FAST
specifications could be used.
A 90-inch wing will just fit in my minivan, so I went for a
simpler one-piece wing rather than plug-in wing panels with an
aluminum-tube joiner. If you go for plug-in wings, you could raise
the wing position on the fuselage for a different appearance.
Hey, if you’re working from plans, you can make any design
changes you want; it’s your airplane. That is a big part of the fun of
this hobby.
The G-FAST aerodynamic layout is conventional. It uses typical
giant scale-recommended hardware and standard construction
techniques and materials, except for Bob Hunt’s Lost Foam wing
construction, which you should try.
The Lost Foam method is an easy, accurate way to scratch-build
a built-up wing structure. I comment on it in some detail in an
accompanying sidebar.
I know that ARFs are extremely popular today, and for many
good reasons. But if you enjoy your workshop building sessions the
way I do, this project will certainly provide you with building fun.
I’ll review the construction procedures I follow. But if you’re an
old hand at it, you’ll simply need to obtain a copy of the plans and
do it your own way, at your own speed.
CONSTRUCTION
I use paper patterns to mark the wood for all parts cutting. Then I
cut a set of plans or trace the necessary parts and cut tracings for the
patterns. I draw around the paper patterns with a ballpoint pen and
cut the parts with my band saw or scroll saw.
I cut all the parts up front, make my own kit, and start from
there, or I cut the parts I need as construction progresses; any of
those methods works. I generally have a decent stock of plywood
28 MODEL AVIATION
All cut balsa and plywood parts needed for fuselage construction are shown. Today’s modeler has to pay through the nose for good
material, but we build for the fun, not the price.
The tail surfaces are
simple structures
built over the plans.
The stabilizer and fin are
solid balsa. Heavy-duty
leaf-style hinges are the
strongest option.
The completed basic fuselage assembly. Top and
bottom sheeting will be added next. The firewall is
pinned at the sides with screws.
The author had no balsa
blocks large enough for the
wheel pants, so they are made
from smaller blocks epoxied
together and then shaped.
This method works just fine.
The built-up construction is conventional;
it’s neither extremely light nor heavy. This is a
rigid structure with average moments, tapered wing layout,
and thick symmetrical airfoil—a tail-dragger with no
separate cowl around the engine.
12sig1x_00MSTRPG.QXD 10/21/10 11:36 AM Page 28
December 2010 29
The cutout is large enough through which to pull out the Moki
1.80. It is mounted sideways, so that the standard muffler tucks
in tightly against the fuselage. Balsa blocks are faired into the
spinner.
The plastic canopy was cut from a Sig Four-Star 120. A Williams
Brothers plastic pilot is finished in honor of the project creator.
Small screws hold these parts in place.
Make sure you can transport a 90-inch
one-piece wing. Employing fiberglass
reinforcement over the wing saddle area
is smart. Notice mounting plates under
the aileron control horns.
Right: The author was eager to get the
whole model together, to see how it
looked. A leaf-spring tail wheel adds
cushion. A heavy-duty horn on the
elevator offers security.
Static photos by the author. Flight photos by Lou McGuire
12sig1x_00MSTRPG.QXD 10/21/10 11:40 AM Page 29
This DVD set provides the how-to-doit
description and instructions for this
technique. It is available from Robin’s
View Productions and Airborne Media.
Foam b u ilding fixtures for the two
w i n g p a n e l s , wi t h r i b an d s pa r
positions marked. The built-up wing
s t r uc tu re, sp a r s , a nd r ibs w ill b e
assembled in these fixtures.
The wing core itself is cut apart at riblocation
p ositions, ready t o be used as
patterns for balsa ribs. Because ribs will
be cut from these foam patterns, they will
fit perfectly in the foam fixture.
The balsa ribs match the foam patterns.
They will fit perfectly into the fixture for
wing assembly. The way t h is works is
brilliant!
The loo se foam p iece is the LE buck ,
m ad e to fo r m th e L E she e tin g. Th e
sheeting is soaked, bent around the buck,
and bound with an Ace bandage until dry.
A short test piece of balsa sheeting has
been formed to illustrate the function
of the process. The LE sheeting will fit
perfectly in place over the wing ribs.
The wing assembles into the cradle. The
lower spar, lower TE planking, and first
few ribs a re shown p ositioned in the
fixture.
All ribs, upper spar, and TE planking are in
place with slow-cure adhesive. Verticalg
r a in spar w e bbing i s also i n p l ac e
between spars.
Bob Hunt developed the Lost Foam Wing
Building System in his efforts to fabricate
extremely accurate, lightweight wings for toplevel
CL Precision Aerobatics competition.
Foam-core wings have been used in model
aircraft for more than 30 years, and their
advantages of accurate, quick, and easy
construction are widely known and accepted.
Many aeromodelers prefer the more traditional
built-up ribs/spars/sheeting method of making
wings; they believe that the built-up wing
structures require more exacting work to result
in an accurate completed wing.
I see Bob’s unique construction technique
as combining foam and built-up methods to
make it easier to scratch-build a great wing
structure. I’ll briefly describe the procedure.
But to see it in detail, you’ll need to purchase
the set of two instructional DVDs.
To begin, cut a top-quality foam core for the
wing that will be built. This core is cut from
oversize, accurately squared-off foam blocks,
which will serve as a building fixture for the
built-up structure. The parts are cut from
heavier, denser foam than is normally used for
wing cores, to better serve as patterns and the
building fixtures.
Then the foam core is cut into pieces at the
rib-station locations. Those core sections are
used as patterns to make the balsa ribs. Since
the patterns are from the foam core itself, the
ribs will fit exactly into the foam building
fixtures.
The building fixtures ensure that the builtup
structure will be accurate and straight, with
no bends, twists, or warps. Extremely light
wood can be used, as desired, because the
building fixture holds everything in place as the
structure is assembled. The finished assembly
will be straight and rigid when you remove it
from the fixture.
There are other possible variations. You can
make the ribs to be positioned diagonally in the
structure. Again, since the rib patterns are the
cut core, they will fit perfectly in the building
fixture.
And then for the last step. Bob cuts a foam
LE “buck”—a precisely shaped LE section—to
be used for preforming the LE planking wood
before it’s glued in place on the wing structure.
The big advantage here is that the LE shape is
complete when the planking is adhered in
place. The radius—the curve of the LE—is
exactly as it should be from the root to the tip of
the wing.
There are no oversize wood pieces on the
LE to be planed and sanded to shape, and no
chance for inaccuracies in shaping the LE.
These details are crucial when you want a trueflying
aircraft.
Lost Foam Building System
Even the dihedral angle can be cut into
the foam building fixtures, so that the
win g panels can be t ig ht ly and eas ily
joined.
12sig1x_00MSTRPG.QXD 10/21/10 11:43 AM Page 30
As an RC sport/aerobatic
machine that is not intended for
competitive flying, the G-FAST
demands neither accurate nor
lightweight construction; it’s a
large, fun model. However, the
structure should be built accurately
so it will fly well, and light weight
is desirable for good flying.
I asked Bob for his help so that
I could try to build a wing his way,
and he let me spend a full day in
his workshop to see what it took to
fabricate the necessary foam
fixtures. Back in my workshop I
took photos so you can see how the
technique works.
And now I can state that it does
work. I ended up with a nicely
built, straight wing with an
accurately shaped LE. I knew the
wing structure was straight as I
removed it from the building
fixtures.
The Lost Foam technique is a
way to continually turn out accurate
wings; those foam patterns and building
fixtures can be reused many times. This
is handy if you know you’ll be building
more of a particular wing.
I’ve been cutting foam cores for 30
years or so, but I’d hesitate to try that
after seeing Bob’s precise setup to make
the Lost Foam fixtures. The foam pieces
have to be cut precisely for this system to
work.
For those who like the technique but
don’t want to cut foam, Bob produces the
required foam components on a custom
basis for any size and type of wing.
Therefore, anyone who wants can use
this building method. MA
—Dick Sarpolus
December 2010 31
close
The author looks
small next to his GFAST.
The racing
looks give this sport
model character and
appeal that reminds
us of why building
from scratch is so
much fun.
Sources:
Bob Hunt’s Robin’s View Productions
Box 68
Stockertown PA 18083
[email protected]
Airborne Media
(513) 755-7494
www.airbornemedia.com
Type: Large RC sport/aerobatic
Wingspan: 90 inches
Wing area: 1,450 square inches
Weight: 13 pounds
Wing loading: 21 ounces/square foot
Length: 67 inches
Engine used: Moki 1.80
Propeller: 18 x 8 Zinger
Construction: Standard built-up balsa and plywood
Wing: Bob Hunt’s Lost Foam construction method is
an option
Covering/finish: UltraCote
Other: 16-ounce fuel tank; four-channel radio with
five servos; 100 ounce-inch servos on ailerons,
elevator, and rudder; 3-inch aluminum spinner;
aluminum landing gear and plastic canopy from Sig;
31/2-inch wheels; leaf-spring tail wheel assembly
The
The Moki 1.80 is a dynamite-running glow
engine that purrs on zero-nitro fuel (cheap).
UltraCote covering offers a smooth finish
that is durable and fuelproof.
12sig1x_00MSTRPG.QXD 10/21/10 11:48 AM Page 31
balsa on hand, so I examine the plans and order more
wood than I think I’ll need. The wood probably won’t go
to waste; you’ll need it for repairs and for constructing
your next project.
I’ve had good luck with wood from several mail-order
suppliers, and I’m glad to see that Lone Star Balsa is back
in business, again furnishing us modelers with the balsa,
plywood, and basswood that we need.
Wing: You have a choice of three construction methods
here. Cut conventional foam cores to be sheeted with 3/32
balsa in the usual manner; use the Lost Foam procedures
(see sidebar); or cut the rib patterns, trace them on the 1/8
balsa, cut them with a band or scroll saw, and then build
the wing structure as usual.
Working on a good flat surface and placing waxed
paper over the plans for their protection, lay the lower
main spar in place as the first step, holding it in place with
lead weights.
Put the ribs in place next, over their positions on the
plans. The building feet at the TE of each rib puts it in the
correct location, and I hold those in place with T-pins
pushed into the plywood building surface.
Vertical-grain spar webbing is next, and yes it’s a pain.
With the G-FAST tapered wing, each spar webbing piece
must be cut/sanded to fit.
Add the top spar, the short aileron-section spars, and
the LE, and apply the LE and TE sheeting. With this
much completed, you can remove the wing panel from the
building board.
Trim the building feet off of the ribs, and add the
opposite-side LE and TE sheeting. The wing structure will
still be slightly flexible until you do this, so be sure to
check the alignment and not build a twist or warp into the
structure. Cardboard tubes for the aileron cables are glued
into the ribs before the wing halves are joined.
Cut the ailerons free from the built-up wing and trim
them so that the 1/8 balsa can be added to the wing panel
and the 1/2 balsa LE can be added to the aileron. The
inboard ribs on each wing panel must be cut between the
spars so that the plywood dihedral brace/wing joiner can
be adhered in place. I add center-section planking after
joining the wing halves.
For additional strength I add heavy fiberglass cloth
with epoxy wrapped around the wing center joint. Cut the
root ribs for the plywood wing mounting tab, and cut a
slot through the LE for the tab.
Finish the wing by adding the wingtips and capstrips.
Control Surfaces: Build the horizontal stabilizer,
elevators, and rudder structures over the plans, and sheet
the stabilizer. The vertical fin is solid sheet balsa.
I use 5-minute epoxy for most of this work, to provide
strength and keep the assembly moving rapidly. I use
large Klett-type nylon pin hinges on all control surfaces,
but there are many good giant scale-suitable hinges on the
market.
Fuselage: This structure accounts for most of the work,
but it’s straightforward.
Epoxy the 1/16 plywood doublers to the 1/8 balsa
fuselage sides, along with the wing saddle doublers, tailsection
doublers, and strips along the lower rear edges.
Since the fuselage sides are parallel from the firewall
to the wing TE position, I glue the first four bulkheads in
place on one fuselage side, ensuring that they are
perpendicular to the side. Then I adhere the other fuselage
side to the four bulkheads.
Pulling the rear ends of the sides together, glue the rear
bulkheads in place. Add the top front sheeting and the rear
turtledeck side sheeting.
Sand the rear sheeting flush with the tops of the
12sig2.QXD_00MSTRPG.QXD 10/21/10 10:39 AM Page 33
bulkheads. Add the thicker top piece and
plane/sand to shape. Don’t add the bottom
rear sheeting until the tail surfaces and
pushrod linkages are in place.
Because of the large, heavy engines used
in a model this size, I employ a 3/8 plywood
firewall, usually epoxying together a piece
of 1/8 and 1/4 plywood. I also drill two
screws, on each side of the firewall, through
the plywood fuselage doublers and into the
firewall sides.
Add the plywood wing mount plate and
landing gear mount plate. Both of those
plywood parts can be backed with an
additional piece of 1/4 plywood, to provide
more wood depth for the 1/4-20 tapped holes
for the nylon bolts. Add the plywood at the
end of the fuselage for the tail wheel mount.
With the wing bolted in place, add the
horizontal stabilizer, lining it up with the
wing. Then add the vertical fin, lining it up
with the horizontal stabilizer and the wing. I
work through the open bottom of the
fuselage, cutting holes in bulkheads to clear
the elevator and rudder pushrods.
I l ike carbo n-fib er pushrods, w ith 4-40
hardwa re. For control h orn mountings I
recess a 1/4 plywood mounting plate in to
the control surface, ep oxyin g it i n place.
Then the bo ttom fuselage planking can be
added.
I l ike whee l pants on a mode l such as
this. Rather than use the typical moldedfiberglass
par ts, I made a pair from
plywood and balsa. I left the outside of the
pant open around the wheel, for an old-time
appearance.
I mounted the big Moki 1.80 sideways
and glued balsa blocks in place around the
engine so that they could be shaped to fair
into the spinner with a simulated air inlet
below the spinner. I almost went for a
fiberglass round cowl to provide the radialengine
look, and I think a round-cowl
version would make for a good-looking
airplane.
Rather than try to find 6061-T6
aluminum to make my landing gear, I
bought a Sig Four-Star 120 landing gear. I
also used a Four-Star 120 plastic canopy,
trimmed a bit to fit the G-FAST.
A metal leaf-spring tail wheel setup was
used, coupled with small springs to the
rudder for steering. A 16-ounce fuel tank
works for me; if you like long flights, go for
a 20-ounce Du-Bro tank.
Finishing: I used UltraCote to cover the
model in a trim scheme that resembles those
I’ve seen in pictures of 1930s racing aircraft.
I also applied computer-cut vinyl
registration numbers in the English and
European style, for the foreign flavor. “GFAST”
seemed to be an appropriate
registration and provided the name for this
project.
Flying: Despite the name and styling, this is
not a racer; I expected the thick wing to keep
the speed down and make for easy flying and
full aerobatic capability. The G-FAST has
those traits, but that powerful Moki makes it
quite fast at full throttle.
I have an 18 x 8 Zinger propeller on the
engine, which I run on FAI fuel; that is, no
nitro. It’s designed for no-nitro fuel, with its
high compression, and that also keeps fuel
costs down.
Because of the conservative design
approach, I had no concerns before the test
flights. I adjusted the control throws a bit, to
suit my flying habits, and got the comfortable
flying machine I wanted. This stuff is fun! MA
Dick Sarpolus
[email protected]