by Mark Atwood
THIS ARTICLE WILL cover the process of
bonding the wing skin to the foam. It assumes
that you have already equipped the wing (with
servo trays, spars, retract plates, etc.) and made
and prepared the balsa wing skins.
The main goals to achieve when sheeting a
wing (or stabilizer or surface) are to make it
straight, light, and strong. Strength in most
cases comes from the shape and structure of
the materials you use.
With the possible exception of Pylon
racers, which endure very high loads, most
wings need little more than a small single spar
or, for a two-piece wing, a well-secured wing
tube. The phenolic type is sufficient to meet the
strength requirements of the aircraft, which in
this article is a 2-meter F3A RC Aerobatics
(Pattern) model. Given that, the main focus
needs to be on light and straight.
Some builders labor intensively to core out
foam material. They do this not to save the
weight of the foam (which weighs practically
nothing), but to remove material where glue
would have to be located. The process is timeconsuming,
and if done improperly it can
critically reduce the structure’s stress-handling
ability.
If the builder has properly selected the
wood materials and the foam cores are made
from the lightest material possible (.75-1
pound in density), this method can be used to
obtain the lightest structure possible.
The weight of the glue is the enemy when
building this light. The following is a simple,
low-tech, and low-skill method for producing
an extremely light, straight wing, with a
minimal amount of glue and fuss. In addition,
this construction method is fast because you
won’t have to labor over hollowing out delicate
wing cores.
To start, identify all the places on the wing
core that will eventually be cut out or cut
through during the building process. This
typically includes the ailerons, flaps, servo
openings, landing-gear openings, etc. The wing
shown will have an aileron cut out, as well as a
servo opening on the bottom.
Use a permanent marker (they write best on
the foam) to outline every place that will be
cut. When marking the aileron cutout, keep in
mind that you’re likely cutting an inner and
outer boundary around the aileron so that there
is room to cap the wing surface with balsa and
to cap the edges of the aileron.
Draw accurate centerlines on the foam
wing’s root and tip. Measure the center point
of the wingtip (or root) at various points from
the LE to the TE, and connect the points to
form the centerline. Take your time; your wing
LIGHTWEIGHT WING SKINNING
July 2008 49
will be only as straight as your centerline.
Once that is complete, test-fit the wing
skins and foam cores into the foam shucks
from which the wing was cut. Trim the balsa
skins so that there is approximately 1/4 inch of
overhang on all sides. This will make it easier
to line up the wing in the shucks after the glue
is applied.
Lay the entire assembly on a flat surface. I
like to use a piece of old pool-table slate; it’s
flat and sturdy. You can often get a nice piece
for free from a local billiards repair shop. It
will probably be slightly chipped and unusable
for a pool table but perfect for a level building
board.
Use a height gauge (a simple homemade
version is shown) to measure the distance from
the tabletop to the centerline. The centerline of
the wingtip should be parallel to the board, as
should the centerline of the root. They will
probably not be at the same height because
most wings have some dihedral in them, but
both centerlines must be parallel or you’ll build
a twist into the wing.
Do not assume that the wing shucks are
straight! Stresses that are internal to the foam
often cause it to twist slightly upon cutting.
Check to make sure you can get the position
close. You’ll spend the time to get it perfect
after you apply the glue.
Pros use this simple
secret to make wings
as light as possible
50 MODEL AVIATION
Photos by the author
To make sure the wing layup stays straight,
a simple, homemade height gauge is used to
accurately measure how even the centerline
is along the building surface.
Any syringe will work, but the Monoject’s
(from Sig) curved tip allows the user to drag
it smoothly across the surface. Rinsing the
syringe with acetone after use will allow it to
be used three to four times.
The author applied his assembly method to four Mike Hester-designed Black Magic
airframes. This lightweight technique is a time-saver.
Wing-core preparation begins with the location and cutout
procedure for the wood and composite reinforcement and the servo
box. Hot-wire cutting tools are preferred.
The author used a marker to draw the perimeter of the aileron
cutout. The dotted line represents the hinge line and won’t be cut.
Also marked for glue are the half rib, which secures the wing tube,
and a small basswood spar.
For illustration purposes, the author drew the entire checkerboard
glue pattern on the wing. The black outline around the perimeter
also represents a glue-application area.
Notice that the epoxy bead is small and that no lines have actually
been drawn on the wing other than the critical areas such as aileron
cutouts. The rest of the checkerboard is fairly arbitrary.
• Foam core and shucks.
• Balsa skins (made to fit).
• Height gauge (homemade or professional).
• Glue syringe (from Sig Manufacturing).
• Two- or three-hour slow-set epoxy (adhesive—
not resin).
Materials
Needed:
• 1/2- to 1-inch plywood board (slightly larger than the
panel being sheeted).
• Sharpie marker. (Various colors are recommended.)
• 100-plus pounds of weight (cinder blocks).
• Hard, sturdy, flat surface. (Pool-table slate is
ideal.) MA
—Mark Atwood
July 2008 51
Cutting Wing-Tube Holes
in Foam Cores
Bevel the inside of the wing-tube sleeve and harden the cutting
edge with thin cyanoacrylate. Using a sharp knife, fine sandpaper,
and a rasp, hone the hardened edge to a sharp point; the bevel edge
should point inside.
Rotating the sharpened sleeve while gently pushing it into the
foam cuts a correct-size hole. The foam will break loose roughly
every 2 inches and needs to be ejected.
Properly aligning the wing tube in the foam cores can be
challenging. The photo shows a tube guide constructed from 3/4
plywood with changeable 1/8-inch Masonite tube-guide plates to
accommodate different tube sizes. The base is clamped to the
workbench and has jackscrews (nylon wing bolts) at the front
corners for adjusting the guide fixture to align the tube parallel to
the bench top.
The wing core is supported on a baseplate, which is shimmed to
locate the vertical center of the hole and set the desired dihedral.
The ends of my core blanks are parallel to the aircraft centerline
and butt against the edge of the guide fixture to establish the wing’s
sweep angle. The hole is located slightly above center at the root so
the outer end of the tube is near center.
Visit the National Society of Radio Controlled Aerobatics
District 8 Web site for more information and great building tips. MA
—Jim Hiller
[email protected]
Sources:
National Society of Radio Controlled Aerobatics District 8
www.nsrca.org/d8/
The dihedral built into the foam cores is easy to see; the tip is higher
than the root. The wing will be true as long as the root and tip are
parallel to the table. Particleboard evenly distributes the cinder
blocks’ weight across the wing.
After 24 hours the wing can be removed and finished with LE and TE
caps, wingtip, servo cutout, aileron cutout, and capped. It’s ready for
covering and weighs just 10.1 ounces.
Once you’re satisfied that the wing can be positioned correctly, it’s
time to mix some epoxy. I use roughly 1 ounce to sheet both sides of a
2-meter Pattern model’s wing panel (approximately half of a 900-
square-inch wing).
I like to use a two-hour or three-hour slow-set epoxy—preferably
one that’s not too thin. This is contrary to many sheeting methods,
which spread a thin epoxy across all the foam. A thin epoxy spread too
thin will be quickly absorbed into the foam, and you risk getting no
adherence.
If you lay a bead of thicker epoxy on the foam, you’re guaranteed to
get a great bond everyplace glue is located. The trick is knowing where
to put it.
Any of the various brands of glue syringes works well to distribute a
consistent bead of adhesive onto the foam. Fill the syringe with glue (it
will hold roughly 1/2 ounce of epoxy) and place a small bead
(approximately 1/16 inch in diameter) on all the areas marked on the
foam.
When placing the glue on the perimeter, a line within 1/2 inch of the
edge is sufficient. The glue will spread when the skins are placed on and
weighted.
Begin making a checkerboard of glue lines, leaving just about 2-inch
squares. I draw lines of glue back and forth and then up and down until
there are no open areas larger than 2 x 2 inches.
Once you have distributed the glue on the foam, place the wing skin
carefully over the foam, flip the wing over (I place it in the foam shuck),
and repeat on the other side.
With glue on both sides and the wing carefully positioned in the
shucks, place a 1/2 or 3/4 piece of plywood or particleboard over the wing
to weight it down. Using the height gauge, check to make sure the
centerlines on the root and tip are parallel to the building surface.
The line at the root should be the same height from the board all the
way from the LE to the TE. The same goes for the centerline on the tip,
although the tip height may vary from the root height. If both lines are
parallel, you are sure to have a straight wing.
If the lines are not parallel, make small adjustments to move the
wing in the shucks to straighten it out. Do not be afraid to have the wing
slightly out of alignment with the shuck if that’s what it takes to get the
measurement straight.
(Editor’s note: Those who build foam wings designed with washin
should follow the plans and measure to make sure the correct incidence
is maintained, as the author described where the plans were followed to
ensure that the wing was built without incidence in the wingtips.)
Once you have the measurements straight, add weight on top of the
plywood board to apply pressure while the glue cures. Somewhere
between 100 and 150 pounds is necessary to ensure a good bond. Cinder
blocks weigh nearly 50 pounds each, and they are inexpensive and easy
to find. Allow the glue to cure for a full 24 hours before removing the
weight.
The result will be a solid, straight wing. And by using less than an
ounce or so of epoxy, it will be a light one. MA
Mark Atwood
[email protected]
Edition: Model Aviation - 2008/07
Page Numbers: 49,50,51
Edition: Model Aviation - 2008/07
Page Numbers: 49,50,51
by Mark Atwood
THIS ARTICLE WILL cover the process of
bonding the wing skin to the foam. It assumes
that you have already equipped the wing (with
servo trays, spars, retract plates, etc.) and made
and prepared the balsa wing skins.
The main goals to achieve when sheeting a
wing (or stabilizer or surface) are to make it
straight, light, and strong. Strength in most
cases comes from the shape and structure of
the materials you use.
With the possible exception of Pylon
racers, which endure very high loads, most
wings need little more than a small single spar
or, for a two-piece wing, a well-secured wing
tube. The phenolic type is sufficient to meet the
strength requirements of the aircraft, which in
this article is a 2-meter F3A RC Aerobatics
(Pattern) model. Given that, the main focus
needs to be on light and straight.
Some builders labor intensively to core out
foam material. They do this not to save the
weight of the foam (which weighs practically
nothing), but to remove material where glue
would have to be located. The process is timeconsuming,
and if done improperly it can
critically reduce the structure’s stress-handling
ability.
If the builder has properly selected the
wood materials and the foam cores are made
from the lightest material possible (.75-1
pound in density), this method can be used to
obtain the lightest structure possible.
The weight of the glue is the enemy when
building this light. The following is a simple,
low-tech, and low-skill method for producing
an extremely light, straight wing, with a
minimal amount of glue and fuss. In addition,
this construction method is fast because you
won’t have to labor over hollowing out delicate
wing cores.
To start, identify all the places on the wing
core that will eventually be cut out or cut
through during the building process. This
typically includes the ailerons, flaps, servo
openings, landing-gear openings, etc. The wing
shown will have an aileron cut out, as well as a
servo opening on the bottom.
Use a permanent marker (they write best on
the foam) to outline every place that will be
cut. When marking the aileron cutout, keep in
mind that you’re likely cutting an inner and
outer boundary around the aileron so that there
is room to cap the wing surface with balsa and
to cap the edges of the aileron.
Draw accurate centerlines on the foam
wing’s root and tip. Measure the center point
of the wingtip (or root) at various points from
the LE to the TE, and connect the points to
form the centerline. Take your time; your wing
LIGHTWEIGHT WING SKINNING
July 2008 49
will be only as straight as your centerline.
Once that is complete, test-fit the wing
skins and foam cores into the foam shucks
from which the wing was cut. Trim the balsa
skins so that there is approximately 1/4 inch of
overhang on all sides. This will make it easier
to line up the wing in the shucks after the glue
is applied.
Lay the entire assembly on a flat surface. I
like to use a piece of old pool-table slate; it’s
flat and sturdy. You can often get a nice piece
for free from a local billiards repair shop. It
will probably be slightly chipped and unusable
for a pool table but perfect for a level building
board.
Use a height gauge (a simple homemade
version is shown) to measure the distance from
the tabletop to the centerline. The centerline of
the wingtip should be parallel to the board, as
should the centerline of the root. They will
probably not be at the same height because
most wings have some dihedral in them, but
both centerlines must be parallel or you’ll build
a twist into the wing.
Do not assume that the wing shucks are
straight! Stresses that are internal to the foam
often cause it to twist slightly upon cutting.
Check to make sure you can get the position
close. You’ll spend the time to get it perfect
after you apply the glue.
Pros use this simple
secret to make wings
as light as possible
50 MODEL AVIATION
Photos by the author
To make sure the wing layup stays straight,
a simple, homemade height gauge is used to
accurately measure how even the centerline
is along the building surface.
Any syringe will work, but the Monoject’s
(from Sig) curved tip allows the user to drag
it smoothly across the surface. Rinsing the
syringe with acetone after use will allow it to
be used three to four times.
The author applied his assembly method to four Mike Hester-designed Black Magic
airframes. This lightweight technique is a time-saver.
Wing-core preparation begins with the location and cutout
procedure for the wood and composite reinforcement and the servo
box. Hot-wire cutting tools are preferred.
The author used a marker to draw the perimeter of the aileron
cutout. The dotted line represents the hinge line and won’t be cut.
Also marked for glue are the half rib, which secures the wing tube,
and a small basswood spar.
For illustration purposes, the author drew the entire checkerboard
glue pattern on the wing. The black outline around the perimeter
also represents a glue-application area.
Notice that the epoxy bead is small and that no lines have actually
been drawn on the wing other than the critical areas such as aileron
cutouts. The rest of the checkerboard is fairly arbitrary.
• Foam core and shucks.
• Balsa skins (made to fit).
• Height gauge (homemade or professional).
• Glue syringe (from Sig Manufacturing).
• Two- or three-hour slow-set epoxy (adhesive—
not resin).
Materials
Needed:
• 1/2- to 1-inch plywood board (slightly larger than the
panel being sheeted).
• Sharpie marker. (Various colors are recommended.)
• 100-plus pounds of weight (cinder blocks).
• Hard, sturdy, flat surface. (Pool-table slate is
ideal.) MA
—Mark Atwood
July 2008 51
Cutting Wing-Tube Holes
in Foam Cores
Bevel the inside of the wing-tube sleeve and harden the cutting
edge with thin cyanoacrylate. Using a sharp knife, fine sandpaper,
and a rasp, hone the hardened edge to a sharp point; the bevel edge
should point inside.
Rotating the sharpened sleeve while gently pushing it into the
foam cuts a correct-size hole. The foam will break loose roughly
every 2 inches and needs to be ejected.
Properly aligning the wing tube in the foam cores can be
challenging. The photo shows a tube guide constructed from 3/4
plywood with changeable 1/8-inch Masonite tube-guide plates to
accommodate different tube sizes. The base is clamped to the
workbench and has jackscrews (nylon wing bolts) at the front
corners for adjusting the guide fixture to align the tube parallel to
the bench top.
The wing core is supported on a baseplate, which is shimmed to
locate the vertical center of the hole and set the desired dihedral.
The ends of my core blanks are parallel to the aircraft centerline
and butt against the edge of the guide fixture to establish the wing’s
sweep angle. The hole is located slightly above center at the root so
the outer end of the tube is near center.
Visit the National Society of Radio Controlled Aerobatics
District 8 Web site for more information and great building tips. MA
—Jim Hiller
[email protected]
Sources:
National Society of Radio Controlled Aerobatics District 8
www.nsrca.org/d8/
The dihedral built into the foam cores is easy to see; the tip is higher
than the root. The wing will be true as long as the root and tip are
parallel to the table. Particleboard evenly distributes the cinder
blocks’ weight across the wing.
After 24 hours the wing can be removed and finished with LE and TE
caps, wingtip, servo cutout, aileron cutout, and capped. It’s ready for
covering and weighs just 10.1 ounces.
Once you’re satisfied that the wing can be positioned correctly, it’s
time to mix some epoxy. I use roughly 1 ounce to sheet both sides of a
2-meter Pattern model’s wing panel (approximately half of a 900-
square-inch wing).
I like to use a two-hour or three-hour slow-set epoxy—preferably
one that’s not too thin. This is contrary to many sheeting methods,
which spread a thin epoxy across all the foam. A thin epoxy spread too
thin will be quickly absorbed into the foam, and you risk getting no
adherence.
If you lay a bead of thicker epoxy on the foam, you’re guaranteed to
get a great bond everyplace glue is located. The trick is knowing where
to put it.
Any of the various brands of glue syringes works well to distribute a
consistent bead of adhesive onto the foam. Fill the syringe with glue (it
will hold roughly 1/2 ounce of epoxy) and place a small bead
(approximately 1/16 inch in diameter) on all the areas marked on the
foam.
When placing the glue on the perimeter, a line within 1/2 inch of the
edge is sufficient. The glue will spread when the skins are placed on and
weighted.
Begin making a checkerboard of glue lines, leaving just about 2-inch
squares. I draw lines of glue back and forth and then up and down until
there are no open areas larger than 2 x 2 inches.
Once you have distributed the glue on the foam, place the wing skin
carefully over the foam, flip the wing over (I place it in the foam shuck),
and repeat on the other side.
With glue on both sides and the wing carefully positioned in the
shucks, place a 1/2 or 3/4 piece of plywood or particleboard over the wing
to weight it down. Using the height gauge, check to make sure the
centerlines on the root and tip are parallel to the building surface.
The line at the root should be the same height from the board all the
way from the LE to the TE. The same goes for the centerline on the tip,
although the tip height may vary from the root height. If both lines are
parallel, you are sure to have a straight wing.
If the lines are not parallel, make small adjustments to move the
wing in the shucks to straighten it out. Do not be afraid to have the wing
slightly out of alignment with the shuck if that’s what it takes to get the
measurement straight.
(Editor’s note: Those who build foam wings designed with washin
should follow the plans and measure to make sure the correct incidence
is maintained, as the author described where the plans were followed to
ensure that the wing was built without incidence in the wingtips.)
Once you have the measurements straight, add weight on top of the
plywood board to apply pressure while the glue cures. Somewhere
between 100 and 150 pounds is necessary to ensure a good bond. Cinder
blocks weigh nearly 50 pounds each, and they are inexpensive and easy
to find. Allow the glue to cure for a full 24 hours before removing the
weight.
The result will be a solid, straight wing. And by using less than an
ounce or so of epoxy, it will be a light one. MA
Mark Atwood
[email protected]
Edition: Model Aviation - 2008/07
Page Numbers: 49,50,51
by Mark Atwood
THIS ARTICLE WILL cover the process of
bonding the wing skin to the foam. It assumes
that you have already equipped the wing (with
servo trays, spars, retract plates, etc.) and made
and prepared the balsa wing skins.
The main goals to achieve when sheeting a
wing (or stabilizer or surface) are to make it
straight, light, and strong. Strength in most
cases comes from the shape and structure of
the materials you use.
With the possible exception of Pylon
racers, which endure very high loads, most
wings need little more than a small single spar
or, for a two-piece wing, a well-secured wing
tube. The phenolic type is sufficient to meet the
strength requirements of the aircraft, which in
this article is a 2-meter F3A RC Aerobatics
(Pattern) model. Given that, the main focus
needs to be on light and straight.
Some builders labor intensively to core out
foam material. They do this not to save the
weight of the foam (which weighs practically
nothing), but to remove material where glue
would have to be located. The process is timeconsuming,
and if done improperly it can
critically reduce the structure’s stress-handling
ability.
If the builder has properly selected the
wood materials and the foam cores are made
from the lightest material possible (.75-1
pound in density), this method can be used to
obtain the lightest structure possible.
The weight of the glue is the enemy when
building this light. The following is a simple,
low-tech, and low-skill method for producing
an extremely light, straight wing, with a
minimal amount of glue and fuss. In addition,
this construction method is fast because you
won’t have to labor over hollowing out delicate
wing cores.
To start, identify all the places on the wing
core that will eventually be cut out or cut
through during the building process. This
typically includes the ailerons, flaps, servo
openings, landing-gear openings, etc. The wing
shown will have an aileron cut out, as well as a
servo opening on the bottom.
Use a permanent marker (they write best on
the foam) to outline every place that will be
cut. When marking the aileron cutout, keep in
mind that you’re likely cutting an inner and
outer boundary around the aileron so that there
is room to cap the wing surface with balsa and
to cap the edges of the aileron.
Draw accurate centerlines on the foam
wing’s root and tip. Measure the center point
of the wingtip (or root) at various points from
the LE to the TE, and connect the points to
form the centerline. Take your time; your wing
LIGHTWEIGHT WING SKINNING
July 2008 49
will be only as straight as your centerline.
Once that is complete, test-fit the wing
skins and foam cores into the foam shucks
from which the wing was cut. Trim the balsa
skins so that there is approximately 1/4 inch of
overhang on all sides. This will make it easier
to line up the wing in the shucks after the glue
is applied.
Lay the entire assembly on a flat surface. I
like to use a piece of old pool-table slate; it’s
flat and sturdy. You can often get a nice piece
for free from a local billiards repair shop. It
will probably be slightly chipped and unusable
for a pool table but perfect for a level building
board.
Use a height gauge (a simple homemade
version is shown) to measure the distance from
the tabletop to the centerline. The centerline of
the wingtip should be parallel to the board, as
should the centerline of the root. They will
probably not be at the same height because
most wings have some dihedral in them, but
both centerlines must be parallel or you’ll build
a twist into the wing.
Do not assume that the wing shucks are
straight! Stresses that are internal to the foam
often cause it to twist slightly upon cutting.
Check to make sure you can get the position
close. You’ll spend the time to get it perfect
after you apply the glue.
Pros use this simple
secret to make wings
as light as possible
50 MODEL AVIATION
Photos by the author
To make sure the wing layup stays straight,
a simple, homemade height gauge is used to
accurately measure how even the centerline
is along the building surface.
Any syringe will work, but the Monoject’s
(from Sig) curved tip allows the user to drag
it smoothly across the surface. Rinsing the
syringe with acetone after use will allow it to
be used three to four times.
The author applied his assembly method to four Mike Hester-designed Black Magic
airframes. This lightweight technique is a time-saver.
Wing-core preparation begins with the location and cutout
procedure for the wood and composite reinforcement and the servo
box. Hot-wire cutting tools are preferred.
The author used a marker to draw the perimeter of the aileron
cutout. The dotted line represents the hinge line and won’t be cut.
Also marked for glue are the half rib, which secures the wing tube,
and a small basswood spar.
For illustration purposes, the author drew the entire checkerboard
glue pattern on the wing. The black outline around the perimeter
also represents a glue-application area.
Notice that the epoxy bead is small and that no lines have actually
been drawn on the wing other than the critical areas such as aileron
cutouts. The rest of the checkerboard is fairly arbitrary.
• Foam core and shucks.
• Balsa skins (made to fit).
• Height gauge (homemade or professional).
• Glue syringe (from Sig Manufacturing).
• Two- or three-hour slow-set epoxy (adhesive—
not resin).
Materials
Needed:
• 1/2- to 1-inch plywood board (slightly larger than the
panel being sheeted).
• Sharpie marker. (Various colors are recommended.)
• 100-plus pounds of weight (cinder blocks).
• Hard, sturdy, flat surface. (Pool-table slate is
ideal.) MA
—Mark Atwood
July 2008 51
Cutting Wing-Tube Holes
in Foam Cores
Bevel the inside of the wing-tube sleeve and harden the cutting
edge with thin cyanoacrylate. Using a sharp knife, fine sandpaper,
and a rasp, hone the hardened edge to a sharp point; the bevel edge
should point inside.
Rotating the sharpened sleeve while gently pushing it into the
foam cuts a correct-size hole. The foam will break loose roughly
every 2 inches and needs to be ejected.
Properly aligning the wing tube in the foam cores can be
challenging. The photo shows a tube guide constructed from 3/4
plywood with changeable 1/8-inch Masonite tube-guide plates to
accommodate different tube sizes. The base is clamped to the
workbench and has jackscrews (nylon wing bolts) at the front
corners for adjusting the guide fixture to align the tube parallel to
the bench top.
The wing core is supported on a baseplate, which is shimmed to
locate the vertical center of the hole and set the desired dihedral.
The ends of my core blanks are parallel to the aircraft centerline
and butt against the edge of the guide fixture to establish the wing’s
sweep angle. The hole is located slightly above center at the root so
the outer end of the tube is near center.
Visit the National Society of Radio Controlled Aerobatics
District 8 Web site for more information and great building tips. MA
—Jim Hiller
[email protected]
Sources:
National Society of Radio Controlled Aerobatics District 8
www.nsrca.org/d8/
The dihedral built into the foam cores is easy to see; the tip is higher
than the root. The wing will be true as long as the root and tip are
parallel to the table. Particleboard evenly distributes the cinder
blocks’ weight across the wing.
After 24 hours the wing can be removed and finished with LE and TE
caps, wingtip, servo cutout, aileron cutout, and capped. It’s ready for
covering and weighs just 10.1 ounces.
Once you’re satisfied that the wing can be positioned correctly, it’s
time to mix some epoxy. I use roughly 1 ounce to sheet both sides of a
2-meter Pattern model’s wing panel (approximately half of a 900-
square-inch wing).
I like to use a two-hour or three-hour slow-set epoxy—preferably
one that’s not too thin. This is contrary to many sheeting methods,
which spread a thin epoxy across all the foam. A thin epoxy spread too
thin will be quickly absorbed into the foam, and you risk getting no
adherence.
If you lay a bead of thicker epoxy on the foam, you’re guaranteed to
get a great bond everyplace glue is located. The trick is knowing where
to put it.
Any of the various brands of glue syringes works well to distribute a
consistent bead of adhesive onto the foam. Fill the syringe with glue (it
will hold roughly 1/2 ounce of epoxy) and place a small bead
(approximately 1/16 inch in diameter) on all the areas marked on the
foam.
When placing the glue on the perimeter, a line within 1/2 inch of the
edge is sufficient. The glue will spread when the skins are placed on and
weighted.
Begin making a checkerboard of glue lines, leaving just about 2-inch
squares. I draw lines of glue back and forth and then up and down until
there are no open areas larger than 2 x 2 inches.
Once you have distributed the glue on the foam, place the wing skin
carefully over the foam, flip the wing over (I place it in the foam shuck),
and repeat on the other side.
With glue on both sides and the wing carefully positioned in the
shucks, place a 1/2 or 3/4 piece of plywood or particleboard over the wing
to weight it down. Using the height gauge, check to make sure the
centerlines on the root and tip are parallel to the building surface.
The line at the root should be the same height from the board all the
way from the LE to the TE. The same goes for the centerline on the tip,
although the tip height may vary from the root height. If both lines are
parallel, you are sure to have a straight wing.
If the lines are not parallel, make small adjustments to move the
wing in the shucks to straighten it out. Do not be afraid to have the wing
slightly out of alignment with the shuck if that’s what it takes to get the
measurement straight.
(Editor’s note: Those who build foam wings designed with washin
should follow the plans and measure to make sure the correct incidence
is maintained, as the author described where the plans were followed to
ensure that the wing was built without incidence in the wingtips.)
Once you have the measurements straight, add weight on top of the
plywood board to apply pressure while the glue cures. Somewhere
between 100 and 150 pounds is necessary to ensure a good bond. Cinder
blocks weigh nearly 50 pounds each, and they are inexpensive and easy
to find. Allow the glue to cure for a full 24 hours before removing the
weight.
The result will be a solid, straight wing. And by using less than an
ounce or so of epoxy, it will be a light one. MA
Mark Atwood
[email protected]