Free Flight Duration-2011/11

New construction materials for gliders
[[email protected]]
Free Flight Duration Louis Joyner
Also included in this column:
• Composite wing gliders at the Nats
MATERIAL CHANGES: For at least 80
years, balsa has been the material of choice
for hand-launch gliders. But, just as the event
has dramatically changed from the traditional
javelin launch to tip launch, construction
materials and techniques are also starting to
change.
Javelin-launch gliders typically measured
16 to 22 inches in wingspan and weighed
roughly an ounce. Models were also
considered somewhat expendable. Serious
fliers produced large batches of gliders
knowing that some would be broken, some
wouldn’t fly well, and others (always the good
ones) would be lost in thermals. Because it is
readily available and easily worked using
simple tools, balsa was the perfect material
choice for these disposable models.
However, the new tip-launch gliders are
much larger, typically with a wingspan from
30 inches up to the maximum of 1 meter
(39.37 inches). The weight is triple that of a
javelin-launch glider. With the nearly
universal use of a DT, flyaways are greatly
reduced. Consequently the tip-launch gliders
last longer, allowing more time and money
to be invested in construction. This also
means that models can be accurately
constructed.
Although balsa is still a viable choice for
larger tip-launch models, some modelers are
exploring other options for wing construction.
Although the choices vary in the materials
and building techniques used, they all utilize a
stressed-skin structure.
Unlike a solid balsa wing, a stressed-skin
structure uses two different materials, each to
its best advantage. The top and bottom surface
“skins” provide the bending and torsional
strength; the lightweight core provides the
airfoil shape and keeps the top and bottom
skins apart under bending loads.
Stressed-skin structures are nothing new in
modeling. Many RC models utilize a foam
core covered with thin-sheet balsa or even
heavy plastic film. Others use fiberglass,
aramid, or carbon fabric to handle thinner
airfoils and higher flight loads. In FF, wings
sheeted top and bottom with thin balsa have
been used for years. Closely spaced balsa ribs
provide the core function.
More recently, thin, hard aluminum foil,
fiberglass cloth, or carbon fabric have
provided the stiff outer skin, either over a
fully sheeted wing or a foam core. In
principle, even a traditional stick-and-tissue
wing is a stressed-skin structure, with the taut
tissue adding both bending and torsional
strength to a lightweight balsa framework
core.
The Core: For tip-launch gliders, the most
popular core choices are insulation foam,
Spyder Foam, and Rohacell foam. Pink and
blue insulation foam is available at most
home stores in 4 x 8-foot sheets in a variety of
thickness.
Although inexpensive and readily
available, these are not the best choices
among insulation foams, because the
compressive strength is quite low—typically
15 to 25 pounds per square inch (psi). For a
core material, compressive strength is one of
the most important attributes.
Specialized insulating foam, designed for
certain commercial applications, has at least
twice the compressive strength. Dow
Styrofoam Highload 60 insulation, has a
compressive strength of 60 psi and a weight
of 2.3 pounds per cubic foot (pcf). The sheet
size for the pale blue material is 2 x 8 feet in
both 2-inch and 3-inch thickness.
Dow also offers slightly lighter Highload
40 with a 40 psi compressive strength in both
2 x 8 and 4 x 8 sheets and 2- and 3-inch
thickness. Cost for a 2 x 8 sheet of 2-inch
material should be less than $25, provided
you can find a local dealer that stocks it. (In
my town, the Highload foam is a special order
item with a three-pallet minimum; I didn’t
bother to ask the price.)
Owens Corning offers a similar product.
Foamular is pink, high-compressive-strength
Narrow carbon-fiber spars on the top and bottom stiffen
Jim Buxton’s glider wing. Each dihedral joint is
reinforced with carbon tow.
Jim Buxton’s tip-launch
glider sports an insulation
foam wing, skinned with
two layers of lightweight
glass cloth. Its projected
wingspan is slightly less than
the 1-meter maximum.
126 MODEL AVIATION
rigid foam. Foamular 600 has a 60 psi
compressive strength and a weight of 2.2 pcf;
Foamular 400 has a strength rating of 40 psi
and weighs 1.8 pcf.
Spyder Foam, also known as surfboard
foam, is extruded so that its maximum
compressive strength is in thickness, just
where it is needed for a wing core. Spyder
Foam has similar strength and weight
characteristics to Highload 60, with a weight
of 2.3 pcf and a compressive strength of 60
psi. Formerly available only in white, Spyder
Foam now comes in blue.
The Composites Store (CST) stocks
Spyder Foam in 1.75-inch thickness; a 12 x
36-inch sheet is $15.95 plus shipping; a 24 x
36-inch sheet is also available for $27.95, but
there is a $20 packing charge and an oversize
shipping charge for the wider sheet.
According to Gail Gewain of CST, the
manufacturer discontinued production of
Spyder Foam in late July. When the current
stock is gone, it will no longer be available.
The foams I’ve mentioned are extruded
polystyrene foam (EPF). The next foam
choice is Rohacell, a white, closed-cell acrylic
foam. Unlike polystyrene, Rohacell cannot be
hot-wired. It must be machined or sanded to
shape.
Rohacell industrial grade (IG) foam comes
in three weights: IG 31 (2 pcf) with a
compressive strength of 57 psi; IG 51 (3.2 pcf
and 128 psi); and IG 71 (4.7 pcf and 213 psi).
Rohacell is widely used in the industry as a
core material for such things as wind turbine
blades, helicopter rotors, and even hockey
sticks.
Rohacell IG 31 is available from CST in
3.0mm, ¼ inch, and ½ inch thickness, and in a
range of sheet sizes. IG 51 comes in the
widest thickness range: 1.0mm, 1.5mm,
2.0mm, 3.0mm, and ¼ inch. IG 71 is offered
in 3.0mm, ¼ inch, ½ inch, and 1 inch. As with
the Spyder Foam, there are extra charges for
sheets wider than 12 inches. Rohacell also
offers IG-F, which has a finer cell structure
for reduced resin absorption. CST is starting
to carry IG-F in some weights and
thicknesses.
As a core material, Rohacell offers a
higher ratio of compressive strength-to-weight
than EPF. Rohacell IG 51 has double the
compressive strength of Highload 40 (128 psi
compared to 60 psi), but only weighs roughly
1.4 times as much. Rohacell also resists
temperatures up to 428º F (220º C), an
important consideration for models that often
sit in the sun. The maximum use temperature
recommendation for one major brand of EPF
is only165º F (74º C).
“Rohacell is really hard to beat when you
look at all the properties,” said Gail. But the
good qualities do come at a price. A 12- x 48-
inch sheet of ¼-inch Rohacell is almost $60,
or roughly $20 per tip-launch glider wing.
Shaping the Core: For EPF foam cores, the
easiest solution is to use the hot-wire
technique. The basic idea is to cut the wing
panel to outline shape, attach airfoil-shaped
templates to both ends, then use a heated wire
to cut through the foam, following the
templates.
The main limitation is that the wire only
cuts in a straight line, so panels need to be
rectangular or trapezoidal in shape. However,
by using a six-panel wing and tapering each
panel, you can approximate an elliptical
planform. The tips can be sanded after hotwiring.
Since Rohacell foam can’t be hot-wired, it
Below: Carbon-fiber cloth is oriented on the bias for
maximum torsional stiffness. A carbon I-beam spar
in the core provides bending strength.
Mark Benns’ Spin-Up 1000 was featured in Free
Flight Forum 2011, which was reviewed in the August
2011 “Free Flight Duration” column and available
through the National Free Flight Society.
Designed for windy weather, Mark Benns’
Spin-Up 1000 glider features a wing skinned
with carbon-fiber cloth over a Rohacell foam
core.
must be sanded or machined to shape.
Sanding actually goes much faster than with
balsa. As with balsa, a vacuum and face mask
should be used to control the dust. Rohacell
also cuts easily with a saw or router. For faster
and more accurate production, a jig could be
devised to hold a router at the correct angle to
shape the sloping rear of the wing airfoil.
The Skin: Possible choices for the skin
include fiberglass cloth, carbon-fiber cloth,
and carbon tissue, also known as carbon veil.
Lightweight fiberglass cloth is familiar to
most modelers. The silklike material comes in
weights as light as ½ ounce per square yard.
When using fiberglass or carbon cloth, it is
best to use a cloth with a balanced weave, one
with the same number of threads per inch in
both directions such as 0.7 ounce fiberglass
cloth with a 56 x 56 thread count. This will
ensure equal strength in both directions and
minimize the chance of warping.
Handling light fiberglass cloth isn’t easy,
but carefully unfolding the cloth onto a
smooth surface such as a large piece of
illustration board or brown wrapping paper
will help reduce snags. Smooth it out with
gentle strokes of a drafting board brush, then
mist with clear nitrate dope. This will lock the
threads of the cloth together, stiffen it slightly,
and make it easier to handle.
Carbon-fiber cloth comes in a variety of
weights; the most usable for FF are
approximately 2.25 to 2.9 ounces per square
yard. Carbon cloth is considerably stronger
and a good deal more expensive then
fiberglass cloth.
As with fiberglass cloth, carbon fabric is
almost always used on the bias, that is, with
the threads running at 45° to the wing chord.
This provides a significant increase in
torsional strength as opposed to a
chordwise/spanwise fiber orientation.
Many of the same handling techniques
used for fiberglass cloth are needed with
carbon fabric. It helps to use drafting tape to
outline an area of the cloth and then cut along
the center of the tape. This will minimize the
chance of the fabric unraveling or pulling out
of shape.
Another alternative to fiberglass or carbon
cloth is carbon tissue. Instead of being a
woven cloth, carbon tissue is a thin mat of
random fibers. Just imagine black silkspan.
Available weights range from 0.2 to 0.7
ounces per square yard. No special handling
is required; simply cut to shape with scissors.
The costs of fiberglass cloth and carbon
tissue are comparable, in the range of $16
to $18 dollars a square yard for carbon
tissue, perhaps slightly less for light glass
cloth. Carbon-fiber cloth is expensive—
roughly $150 per square yard and more for
lighter cloth. For certain applications, the
added strength, rigidity, and dimensional
stability are worth the cost.
Bagging It: Attaching the skin to the top and
bottom of the core is the easiest part of the
operation. The cloth or tissue pieces are placed
on a piece of thick Mylar and wetted out with
epoxy resin, and then the excess resin is
blotted off. Next the core is positioned over
the bottom skin and then the top skin is placed
on top of the core.
Another piece of Mylar is draped over the
assembly, and everything is placed inside a
vacuum bag. The bag is sealed and a vacuum
pump is used to remove the air from the bag.
Atmospheric pressure does the rest, evenly
pushing the skins tight against the core. The
vacuum is held until the epoxy hardens.
The result is a stiff wing that won’t flutter
on a hard launch and won’t change between
contests. More importantly, you can make
another one just like it without spending
weeks searching for a perfect piece of balsa.
Matt Gewain has created an excellent
tutorial about vacuum bagging, which can be
seen on the CST website. Other websites offer
tips, instructions, and even how-to videos.
Is it worth it? Composite glider wings require
an investment in both time and money, as does
any new technology. For some of us, the feel
of a razor-sharp plane on a piece of C-grain
balsa offers more rewards then messing with
epoxy and strange plastics. For others the
challenges of trying something new may be its
own reward.
Neither the thermal nor the stopwatch care
how the wing was made or what materials
were used.
I had the opportunity to examine two
composite-wing gliders at the Nats last
summer. The wing on Jim Buxton’s model
used cores hot-wired from blue foam and had
skinned top and bottom with two layers of 0.7-
ounce fiberglass cloth. For added bending
strength, Jim inset 0.050-inch carbon fiber
spars into the top and bottom of the core
before adding the skins.
A simple tool, made from a broken Dremel
cut-off disc with scrap-balsa depth stops glued
on either side, was used to cut the grooves for
the carbon spars. “It’s quick and easy,” said
Jim. Dihedral joints were reinforced with
carbon-fiber tow on the top only to reduce
bending on launch. “It’s scary how flat these
must be sanded or machined to shape.
Sanding actually goes much faster than with
balsa. As with balsa, a vacuum and face mask
should be used to control the dust. Rohacell
also cuts easily with a saw or router. For faster
and more accurate production, a jig could be
devised to hold a router at the correct angle to
shape the sloping rear of the wing airfoil.
The Skin: Possible choices for the skin
include fiberglass cloth, carbon-fiber cloth,
and carbon tissue, also known as carbon veil.
Lightweight fiberglass cloth is familiar to
most modelers. The silklike material comes in
weights as light as ½ ounce per square yard.
When using fiberglass or carbon cloth, it is
best to use a cloth with a balanced weave, one
with the same number of threads per inch in
both directions such as 0.7 ounce fiberglass
cloth with a 56 x 56 thread count. This will
ensure equal strength in both directions and
minimize the chance of warping.
Handling light fiberglass cloth isn’t easy,
but carefully unfolding the cloth onto a
smooth surface such as a large piece of
illustration board or brown wrapping paper
will help reduce snags. Smooth it out with
gentle strokes of a drafting board brush, then
mist with clear nitrate dope. This will lock the
threads of the cloth together, stiffen it slightly,
and make it easier to handle.
Carbon-fiber cloth comes in a variety of
weights; the most usable for FF are
approximately 2.25 to 2.9 ounces per square
yard. Carbon cloth is considerably stronger
and a good deal more expensive then
fiberglass cloth.
As with fiberglass cloth, carbon fabric is
almost always used on the bias, that is, with
the threads running at 45° to the wing chord.
This provides a significant increase in
torsional strength as opposed to a
chordwise/spanwise fiber orientation.
Many of the same handling techniques
used for fiberglass cloth are needed with
carbon fabric. It helps to use drafting tape to
outline an area of the cloth and then cut along
the center of the tape. This will minimize the
chance of the fabric unraveling or pulling out
of shape.
Another alternative to fiberglass or carbon
cloth is carbon tissue. Instead of being a
woven cloth, carbon tissue is a thin mat of
random fibers. Just imagine black silkspan.
Available weights range from 0.2 to 0.7
ounces per square yard. No special handling
is required; simply cut to shape with scissors.
The costs of fiberglass cloth and carbon
tissue are comparable, in the range of $16
to $18 dollars a square yard for carbon
tissue, perhaps slightly less for light glass
cloth. Carbon-fiber cloth is expensive—
roughly $150 per square yard and more for
lighter cloth. For certain applications, the
added strength, rigidity, and dimensional
stability are worth the cost.
Bagging It: Attaching the skin to the top and
bottom of the core is the easiest part of the
operation. The cloth or tissue pieces are placed
on a piece of thick Mylar and wetted out with
epoxy resin, and then the excess resin is
blotted off. Next the core is positioned over
the bottom skin and then the top skin is placed
on top of the core.
Another piece of Mylar is draped over the
assembly, and everything is placed inside a
vacuum bag. The bag is sealed and a vacuum
pump is used to remove the air from the bag.
Atmospheric pressure does the rest, evenly
pushing the skins tight against the core. The
vacuum is held until the epoxy hardens.
The result is a stiff wing that won’t flutter
on a hard launch and won’t change between
contests. More importantly, you can make
another one just like it without spending
weeks searching for a perfect piece of balsa.
Matt Gewain has created an excellent
tutorial about vacuum bagging, which can be
seen on the CST website. Other websites offer
tips, instructions, and even how-to videos.
Is it worth it? Composite glider wings require
an investment in both time and money, as does
any new technology. For some of us, the feel
of a razor-sharp plane on a piece of C-grain
balsa offers more rewards then messing with
epoxy and strange plastics. For others the
challenges of trying something new may be its
own reward.
Neither the thermal nor the stopwatch care
how the wing was made or what materials
were used.
I had the opportunity to examine two
composite-wing gliders at the Nats last
summer. The wing on Jim Buxton’s model
used cores hot-wired from blue foam and had
skinned top and bottom with two layers of 0.7-
ounce fiberglass cloth. For added bending
strength, Jim inset 0.050-inch carbon fiber
spars into the top and bottom of the core
before adding the skins.
A simple tool, made from a broken Dremel
cut-off disc with scrap-balsa depth stops glued
on either side, was used to cut the grooves for
the carbon spars. “It’s quick and easy,” said
Jim. Dihedral joints were reinforced with
carbon-fiber tow on the top only to reduce
bending on launch. “It’s scary how flat these
My calculations put the wing area for
the Spin-Up 1000 at approximately 152
square inches. I estimated the core weight
at between 20 and 25 grams, the carbon
fiber cloth weight at slightly more than 18
grams, the epoxy weight at 18 to 20
grams, and the carbon-fiber spar at
roughly 6 grams. That’s a total of 62 to 69
grams for the wing—surprisingly close to
Jim Buxton’s wing weight of 66 grams.
If you are accustomed to the old
javelin-launch balsa gliders, a 2.25-ounce
wing may seem heavy, but Mark’s model
is designed for windy weather with an allup
weight of 90 to 110 grams.
On the plans, Mark suggests an
alternate wing construction employing 10
pounds per cubic foot balsa. A 4- x 36-
inch sheet of 5/16-inch-thick balsa weighs
approximately 120 grams and has an area
of 144 square inches—slightly smaller
than the Spin-Up’s area. Even if half the
sheet of balsa is planed and sanded away
to make the wing, the weight would be 60
grams without a finish and the necessary
reinforcements at dihedral breaks and
throwing peg. MA
Sources:
CST-The Composites Store
(800) 338-1278
www.cstsales.com
National Free Flight Society
www.freeflight.org
Dow Building Solutions
(866) 583-2583
www.building.dow.com/na/en
Owens Corning
(800) 438-7465
www.foamular.com/foam
Evonik Industries
rohacell@globaloffice
www.rohacell.com
A2Z Corp.
(877) 754-7465
www.a2zcorp.us

New construction materials for gliders
[[email protected]]
Free Flight Duration Louis Joyner
Also included in this column:
• Composite wing gliders at the Nats
MATERIAL CHANGES: For at least 80
years, balsa has been the material of choice
for hand-launch gliders. But, just as the event
has dramatically changed from the traditional
javelin launch to tip launch, construction
materials and techniques are also starting to
change.
Javelin-launch gliders typically measured
16 to 22 inches in wingspan and weighed
roughly an ounce. Models were also
considered somewhat expendable. Serious
fliers produced large batches of gliders
knowing that some would be broken, some
wouldn’t fly well, and others (always the good
ones) would be lost in thermals. Because it is
readily available and easily worked using
simple tools, balsa was the perfect material
choice for these disposable models.
However, the new tip-launch gliders are
much larger, typically with a wingspan from
30 inches up to the maximum of 1 meter
(39.37 inches). The weight is triple that of a
javelin-launch glider. With the nearly
universal use of a DT, flyaways are greatly
reduced. Consequently the tip-launch gliders
last longer, allowing more time and money
to be invested in construction. This also
means that models can be accurately
constructed.
Although balsa is still a viable choice for
larger tip-launch models, some modelers are
exploring other options for wing construction.
Although the choices vary in the materials
and building techniques used, they all utilize a
stressed-skin structure.
Unlike a solid balsa wing, a stressed-skin
structure uses two different materials, each to
its best advantage. The top and bottom surface
“skins” provide the bending and torsional
strength; the lightweight core provides the
airfoil shape and keeps the top and bottom
skins apart under bending loads.
Stressed-skin structures are nothing new in
modeling. Many RC models utilize a foam
core covered with thin-sheet balsa or even
heavy plastic film. Others use fiberglass,
aramid, or carbon fabric to handle thinner
airfoils and higher flight loads. In FF, wings
sheeted top and bottom with thin balsa have
been used for years. Closely spaced balsa ribs
provide the core function.
More recently, thin, hard aluminum foil,
fiberglass cloth, or carbon fabric have
provided the stiff outer skin, either over a
fully sheeted wing or a foam core. In
principle, even a traditional stick-and-tissue
wing is a stressed-skin structure, with the taut
tissue adding both bending and torsional
strength to a lightweight balsa framework
core.
The Core: For tip-launch gliders, the most
popular core choices are insulation foam,
Spyder Foam, and Rohacell foam. Pink and
blue insulation foam is available at most
home stores in 4 x 8-foot sheets in a variety of
thickness.
Although inexpensive and readily
available, these are not the best choices
among insulation foams, because the
compressive strength is quite low—typically
15 to 25 pounds per square inch (psi). For a
core material, compressive strength is one of
the most important attributes.
Specialized insulating foam, designed for
certain commercial applications, has at least
twice the compressive strength. Dow
Styrofoam Highload 60 insulation, has a
compressive strength of 60 psi and a weight
of 2.3 pounds per cubic foot (pcf). The sheet
size for the pale blue material is 2 x 8 feet in
both 2-inch and 3-inch thickness.
Dow also offers slightly lighter Highload
40 with a 40 psi compressive strength in both
2 x 8 and 4 x 8 sheets and 2- and 3-inch
thickness. Cost for a 2 x 8 sheet of 2-inch
material should be less than $25, provided
you can find a local dealer that stocks it. (In
my town, the Highload foam is a special order
item with a three-pallet minimum; I didn’t
bother to ask the price.)
Owens Corning offers a similar product.
Foamular is pink, high-compressive-strength
Narrow carbon-fiber spars on the top and bottom stiffen
Jim Buxton’s glider wing. Each dihedral joint is
reinforced with carbon tow.
Jim Buxton’s tip-launch
glider sports an insulation
foam wing, skinned with
two layers of lightweight
glass cloth. Its projected
wingspan is slightly less than
the 1-meter maximum.
126 MODEL AVIATION
rigid foam. Foamular 600 has a 60 psi
compressive strength and a weight of 2.2 pcf;
Foamular 400 has a strength rating of 40 psi
and weighs 1.8 pcf.
Spyder Foam, also known as surfboard
foam, is extruded so that its maximum
compressive strength is in thickness, just
where it is needed for a wing core. Spyder
Foam has similar strength and weight
characteristics to Highload 60, with a weight
of 2.3 pcf and a compressive strength of 60
psi. Formerly available only in white, Spyder
Foam now comes in blue.
The Composites Store (CST) stocks
Spyder Foam in 1.75-inch thickness; a 12 x
36-inch sheet is $15.95 plus shipping; a 24 x
36-inch sheet is also available for $27.95, but
there is a $20 packing charge and an oversize
shipping charge for the wider sheet.
According to Gail Gewain of CST, the
manufacturer discontinued production of
Spyder Foam in late July. When the current
stock is gone, it will no longer be available.
The foams I’ve mentioned are extruded
polystyrene foam (EPF). The next foam
choice is Rohacell, a white, closed-cell acrylic
foam. Unlike polystyrene, Rohacell cannot be
hot-wired. It must be machined or sanded to
shape.
Rohacell industrial grade (IG) foam comes
in three weights: IG 31 (2 pcf) with a
compressive strength of 57 psi; IG 51 (3.2 pcf
and 128 psi); and IG 71 (4.7 pcf and 213 psi).
Rohacell is widely used in the industry as a
core material for such things as wind turbine
blades, helicopter rotors, and even hockey
sticks.
Rohacell IG 31 is available from CST in
3.0mm, ¼ inch, and ½ inch thickness, and in a
range of sheet sizes. IG 51 comes in the
widest thickness range: 1.0mm, 1.5mm,
2.0mm, 3.0mm, and ¼ inch. IG 71 is offered
in 3.0mm, ¼ inch, ½ inch, and 1 inch. As with
the Spyder Foam, there are extra charges for
sheets wider than 12 inches. Rohacell also
offers IG-F, which has a finer cell structure
for reduced resin absorption. CST is starting
to carry IG-F in some weights and
thicknesses.
As a core material, Rohacell offers a
higher ratio of compressive strength-to-weight
than EPF. Rohacell IG 51 has double the
compressive strength of Highload 40 (128 psi
compared to 60 psi), but only weighs roughly
1.4 times as much. Rohacell also resists
temperatures up to 428º F (220º C), an
important consideration for models that often
sit in the sun. The maximum use temperature
recommendation for one major brand of EPF
is only165º F (74º C).
“Rohacell is really hard to beat when you
look at all the properties,” said Gail. But the
good qualities do come at a price. A 12- x 48-
inch sheet of ¼-inch Rohacell is almost $60,
or roughly $20 per tip-launch glider wing.
Shaping the Core: For EPF foam cores, the
easiest solution is to use the hot-wire
technique. The basic idea is to cut the wing
panel to outline shape, attach airfoil-shaped
templates to both ends, then use a heated wire
to cut through the foam, following the
templates.
The main limitation is that the wire only
cuts in a straight line, so panels need to be
rectangular or trapezoidal in shape. However,
by using a six-panel wing and tapering each
panel, you can approximate an elliptical
planform. The tips can be sanded after hotwiring.
Since Rohacell foam can’t be hot-wired, it
Below: Carbon-fiber cloth is oriented on the bias for
maximum torsional stiffness. A carbon I-beam spar
in the core provides bending strength.
Mark Benns’ Spin-Up 1000 was featured in Free
Flight Forum 2011, which was reviewed in the August
2011 “Free Flight Duration” column and available
through the National Free Flight Society.
Designed for windy weather, Mark Benns’
Spin-Up 1000 glider features a wing skinned
with carbon-fiber cloth over a Rohacell foam
core.
must be sanded or machined to shape.
Sanding actually goes much faster than with
balsa. As with balsa, a vacuum and face mask
should be used to control the dust. Rohacell
also cuts easily with a saw or router. For faster
and more accurate production, a jig could be
devised to hold a router at the correct angle to
shape the sloping rear of the wing airfoil.
The Skin: Possible choices for the skin
include fiberglass cloth, carbon-fiber cloth,
and carbon tissue, also known as carbon veil.
Lightweight fiberglass cloth is familiar to
most modelers. The silklike material comes in
weights as light as ½ ounce per square yard.
When using fiberglass or carbon cloth, it is
best to use a cloth with a balanced weave, one
with the same number of threads per inch in
both directions such as 0.7 ounce fiberglass
cloth with a 56 x 56 thread count. This will
ensure equal strength in both directions and
minimize the chance of warping.
Handling light fiberglass cloth isn’t easy,
but carefully unfolding the cloth onto a
smooth surface such as a large piece of
illustration board or brown wrapping paper
will help reduce snags. Smooth it out with
gentle strokes of a drafting board brush, then
mist with clear nitrate dope. This will lock the
threads of the cloth together, stiffen it slightly,
and make it easier to handle.
Carbon-fiber cloth comes in a variety of
weights; the most usable for FF are
approximately 2.25 to 2.9 ounces per square
yard. Carbon cloth is considerably stronger
and a good deal more expensive then
fiberglass cloth.
As with fiberglass cloth, carbon fabric is
almost always used on the bias, that is, with
the threads running at 45° to the wing chord.
This provides a significant increase in
torsional strength as opposed to a
chordwise/spanwise fiber orientation.
Many of the same handling techniques
used for fiberglass cloth are needed with
carbon fabric. It helps to use drafting tape to
outline an area of the cloth and then cut along
the center of the tape. This will minimize the
chance of the fabric unraveling or pulling out
of shape.
Another alternative to fiberglass or carbon
cloth is carbon tissue. Instead of being a
woven cloth, carbon tissue is a thin mat of
random fibers. Just imagine black silkspan.
Available weights range from 0.2 to 0.7
ounces per square yard. No special handling
is required; simply cut to shape with scissors.
The costs of fiberglass cloth and carbon
tissue are comparable, in the range of $16
to $18 dollars a square yard for carbon
tissue, perhaps slightly less for light glass
cloth. Carbon-fiber cloth is expensive—
roughly $150 per square yard and more for
lighter cloth. For certain applications, the
added strength, rigidity, and dimensional
stability are worth the cost.
Bagging It: Attaching the skin to the top and
bottom of the core is the easiest part of the
operation. The cloth or tissue pieces are placed
on a piece of thick Mylar and wetted out with
epoxy resin, and then the excess resin is
blotted off. Next the core is positioned over
the bottom skin and then the top skin is placed
on top of the core.
Another piece of Mylar is draped over the
assembly, and everything is placed inside a
vacuum bag. The bag is sealed and a vacuum
pump is used to remove the air from the bag.
Atmospheric pressure does the rest, evenly
pushing the skins tight against the core. The
vacuum is held until the epoxy hardens.
The result is a stiff wing that won’t flutter
on a hard launch and won’t change between
contests. More importantly, you can make
another one just like it without spending
weeks searching for a perfect piece of balsa.
Matt Gewain has created an excellent
tutorial about vacuum bagging, which can be
seen on the CST website. Other websites offer
tips, instructions, and even how-to videos.
Is it worth it? Composite glider wings require
an investment in both time and money, as does
any new technology. For some of us, the feel
of a razor-sharp plane on a piece of C-grain
balsa offers more rewards then messing with
epoxy and strange plastics. For others the
challenges of trying something new may be its
own reward.
Neither the thermal nor the stopwatch care
how the wing was made or what materials
were used.
I had the opportunity to examine two
composite-wing gliders at the Nats last
summer. The wing on Jim Buxton’s model
used cores hot-wired from blue foam and had
skinned top and bottom with two layers of 0.7-
ounce fiberglass cloth. For added bending
strength, Jim inset 0.050-inch carbon fiber
spars into the top and bottom of the core
before adding the skins.
A simple tool, made from a broken Dremel
cut-off disc with scrap-balsa depth stops glued
on either side, was used to cut the grooves for
the carbon spars. “It’s quick and easy,” said
Jim. Dihedral joints were reinforced with
carbon-fiber tow on the top only to reduce
bending on launch. “It’s scary how flat these
must be sanded or machined to shape.
Sanding actually goes much faster than with
balsa. As with balsa, a vacuum and face mask
should be used to control the dust. Rohacell
also cuts easily with a saw or router. For faster
and more accurate production, a jig could be
devised to hold a router at the correct angle to
shape the sloping rear of the wing airfoil.
The Skin: Possible choices for the skin
include fiberglass cloth, carbon-fiber cloth,
and carbon tissue, also known as carbon veil.
Lightweight fiberglass cloth is familiar to
most modelers. The silklike material comes in
weights as light as ½ ounce per square yard.
When using fiberglass or carbon cloth, it is
best to use a cloth with a balanced weave, one
with the same number of threads per inch in
both directions such as 0.7 ounce fiberglass
cloth with a 56 x 56 thread count. This will
ensure equal strength in both directions and
minimize the chance of warping.
Handling light fiberglass cloth isn’t easy,
but carefully unfolding the cloth onto a
smooth surface such as a large piece of
illustration board or brown wrapping paper
will help reduce snags. Smooth it out with
gentle strokes of a drafting board brush, then
mist with clear nitrate dope. This will lock the
threads of the cloth together, stiffen it slightly,
and make it easier to handle.
Carbon-fiber cloth comes in a variety of
weights; the most usable for FF are
approximately 2.25 to 2.9 ounces per square
yard. Carbon cloth is considerably stronger
and a good deal more expensive then
fiberglass cloth.
As with fiberglass cloth, carbon fabric is
almost always used on the bias, that is, with
the threads running at 45° to the wing chord.
This provides a significant increase in
torsional strength as opposed to a
chordwise/spanwise fiber orientation.
Many of the same handling techniques
used for fiberglass cloth are needed with
carbon fabric. It helps to use drafting tape to
outline an area of the cloth and then cut along
the center of the tape. This will minimize the
chance of the fabric unraveling or pulling out
of shape.
Another alternative to fiberglass or carbon
cloth is carbon tissue. Instead of being a
woven cloth, carbon tissue is a thin mat of
random fibers. Just imagine black silkspan.
Available weights range from 0.2 to 0.7
ounces per square yard. No special handling
is required; simply cut to shape with scissors.
The costs of fiberglass cloth and carbon
tissue are comparable, in the range of $16
to $18 dollars a square yard for carbon
tissue, perhaps slightly less for light glass
cloth. Carbon-fiber cloth is expensive—
roughly $150 per square yard and more for
lighter cloth. For certain applications, the
added strength, rigidity, and dimensional
stability are worth the cost.
Bagging It: Attaching the skin to the top and
bottom of the core is the easiest part of the
operation. The cloth or tissue pieces are placed
on a piece of thick Mylar and wetted out with
epoxy resin, and then the excess resin is
blotted off. Next the core is positioned over
the bottom skin and then the top skin is placed
on top of the core.
Another piece of Mylar is draped over the
assembly, and everything is placed inside a
vacuum bag. The bag is sealed and a vacuum
pump is used to remove the air from the bag.
Atmospheric pressure does the rest, evenly
pushing the skins tight against the core. The
vacuum is held until the epoxy hardens.
The result is a stiff wing that won’t flutter
on a hard launch and won’t change between
contests. More importantly, you can make
another one just like it without spending
weeks searching for a perfect piece of balsa.
Matt Gewain has created an excellent
tutorial about vacuum bagging, which can be
seen on the CST website. Other websites offer
tips, instructions, and even how-to videos.
Is it worth it? Composite glider wings require
an investment in both time and money, as does
any new technology. For some of us, the feel
of a razor-sharp plane on a piece of C-grain
balsa offers more rewards then messing with
epoxy and strange plastics. For others the
challenges of trying something new may be its
own reward.
Neither the thermal nor the stopwatch care
how the wing was made or what materials
were used.
I had the opportunity to examine two
composite-wing gliders at the Nats last
summer. The wing on Jim Buxton’s model
used cores hot-wired from blue foam and had
skinned top and bottom with two layers of 0.7-
ounce fiberglass cloth. For added bending
strength, Jim inset 0.050-inch carbon fiber
spars into the top and bottom of the core
before adding the skins.
A simple tool, made from a broken Dremel
cut-off disc with scrap-balsa depth stops glued
on either side, was used to cut the grooves for
the carbon spars. “It’s quick and easy,” said
Jim. Dihedral joints were reinforced with
carbon-fiber tow on the top only to reduce
bending on launch. “It’s scary how flat these
My calculations put the wing area for
the Spin-Up 1000 at approximately 152
square inches. I estimated the core weight
at between 20 and 25 grams, the carbon
fiber cloth weight at slightly more than 18
grams, the epoxy weight at 18 to 20
grams, and the carbon-fiber spar at
roughly 6 grams. That’s a total of 62 to 69
grams for the wing—surprisingly close to
Jim Buxton’s wing weight of 66 grams.
If you are accustomed to the old
javelin-launch balsa gliders, a 2.25-ounce
wing may seem heavy, but Mark’s model
is designed for windy weather with an allup
weight of 90 to 110 grams.
On the plans, Mark suggests an
alternate wing construction employing 10
pounds per cubic foot balsa. A 4- x 36-
inch sheet of 5/16-inch-thick balsa weighs
approximately 120 grams and has an area
of 144 square inches—slightly smaller
than the Spin-Up’s area. Even if half the
sheet of balsa is planed and sanded away
to make the wing, the weight would be 60
grams without a finish and the necessary
reinforcements at dihedral breaks and
throwing peg. MA
Sources:
CST-The Composites Store
(800) 338-1278
www.cstsales.com
National Free Flight Society
www.freeflight.org
Dow Building Solutions
(866) 583-2583
www.building.dow.com/na/en
Owens Corning
(800) 438-7465
www.foamular.com/foam
Evonik Industries
rohacell@globaloffice
www.rohacell.com
A2Z Corp.
(877) 754-7465
www.a2zcorp.us