4 5
20p9 two-line cutline
goes here
20p9 two-line cutline
goes here
32 MODEL AVIATION January 2003 32
■ Mike Hurley
40-size sport airplanes to 40% monster International Miniature
Aerobatic Club (IMAC) airplanes are using 1⁄16 balsa to sheet
almost everything including the wings. We use 4 x 48-inch sheets
as a basis, and cut them down from there.
There’s a big difference in density of balsa; in one batch we
found a sheet that weighed 11 grams and another that was 70
grams! The heavy wood should be a tad stronger, but I doubt that
there is any sheeting application that needs lumber like that 70-
gram plank. Imagine the variations you could have if you were to
blindly start gluing sheets together at random. It really is better to
know what you have from the start by weighing each sheet on a
good digital scale. I picked mine up at an office-supply store for
$29.
Use the lighter, more bendable woods for shorter runs like the
stabilizers or for places that require a tight bend like the turtledeck.
But remember that the light wood is very soft and gouges easily,
making handling and your finishing job a delicate process. Use
harder wood if you want a more robust surface to finish.
A soft, contest-grade sheet weighs something in the
neighborhood of 14-18 grams (4- to 6-pound density). We used
Sheeting Foam
Components
THIS INSTALLMENT of Project Extra will cover sheeting and
finishing foam parts. As I stated in the introduction to the project,
this construction series is intended to span model-building in general
but still apply to the Extra 300LX. This article will give some
insight into sheeting foam and an overview of today’s most common
sheeting methods. As you will see in this article, there are many
ways to accomplish the sheeting portion of any aircraft, and you can
successfully sheet the foam for your Extra with any of these
methods.
For a step-by-step account of how we chose to do the sheeting on
our Extra, go to the AMA Web site—www.modelaircraft.org—for
this month’s PDF download of Project Extra construction details.
Choose Your Wood: The Extra has many foam parts. The wings,
stabilizers, rudder, a large hatch, and the turtledeck are foam. We
experimented with various methods of attaching the wood to the
foam and gluing the individual sheets together, and we came up with
several acceptable ways to do both.
Before we get started, it would benefit you to take a look at the
wood you are using for sheeting your foam parts. Most models from
Fully assembled in the shop, the 35% Extra 300LX takes quite a
bit of building space, but it will be envied out on the field!
Volume II:
02sig1.QXD 11.21.02 3:02 pm Page 32
February 2003 33
The trick to a perfect layout, and to remembering what’s under
the sheeting, is to first make a template with poster board.
Whether you’re using Duco, Ambroid cement, cyanoacrylate, or
wood glue, applying the glue is the trickiest part.
To begin edge-truing sheets before gluing them, use a
long straightedge and X-Acto knife. Cut off only what’s
necessary to straighten the edges.
Erik built this sanding fixture as a final step to a perfect edge for
gluing. Run sheets back and forth across the block, and edges will
be ready for gluing.
these everywhere but the main wings. We
used planks that came in at 18-22 grams (6- to
8-pound density) for the wings, and we tried
to avoid any wood exceeding 24-25 grams.
Before any foam was glued, we weighed the
individual parts and made sure that the left
and right sides were as near the same weight
as possible.
Choose Your Glue: Edge-truing the sheets
before you start gluing them together helps
the process immensely. Our builder Erik
Richards made his edge-truing guide from
particleboard and a piece of angle aluminum
with 150-grit sandpaper glued to it. Trim the
edges with a four-foot straightedge and a
sharp hobby knife or a razor blade, then get a
final edge by lightly guiding the sheets along
the edge-truing surface.
There are several ways to approach gluing
sheets together and several glues to choose
from. We’re striving for an analysis of the
pros and cons of these differing methods. I
experimented with some of the more common
methods for this article, edge-gluing with
cyanoacrylate (CyA), Duco, or Ambroid
model-airplane cements, and common
carpenter’s wood glue. All of these methods
produce acceptable results and with practice
can be quite nice.
All methods start by taping the edge-trued
sheets together down the entire length of the
sheet on one side, and flipping it over and
bending at the tape line to expose the edge.
The glue is applied to the edge, and the sheets
are folded back together.
Thick CyA applied in a very thin line with
a fine tip to the edge of the sheet worked very
well with a little practice. As you move the
bottle along the edge, the glue seems to drag
out of the bottle. Thin CyA can be a mess and
results in severe hard spots that are
unsandable if you try to avoid the edge-gluing
method and soak the seam with it. The thick
CyA works well because it gives you a little
more time to align the edges before it sets.
Once the edges are together, dab up any
excess glue with a paper towel and sand
lightly with a block and 150-grit paper.
Before the glue hardens completely, flip the
sheet over, peel off the tape, and sand again.
The trick is to keep the glue to a minimum
because any overspill will harden the wood
and become difficult to sand. After I got the
knack of using the CyA I liked it very much.
It was the quickest of the edge-gluing
methods and yielded a clean, smooth sheet of
wood. On the other hand, too much glue will
result in hard edges that cannot be sanded out,
so the glue application is very important.
I’ve read that CyA can soften with heat, so
I did a test. I heated the sheeting with my
covering heat gun and measured the
temperature with a Raytek digital surface heat
gauge. I got the wood hot enough to melt the
foam underneath it (250 degrees Fahrenheit)
with no noticeable sign of softening the CyA.
Okay, so it’s not a completely scientific test,
but surely the act of covering your airplane or
Photos by Michael Ramsey Graphic Design by Carla Kunz
02sig2.QXD 11.21.02 2:15 pm Page 33
A stress-bearing plate is inserted in wing and stabilizer to
support tube sockets. An ink-pad stamper was used to mark
light plywood for cutting the tube hole.
allowing it to sit in the sun will not affect the
joints glued with CyA.
Working with old-fashioned wood glue
takes speed and practice. I could not get the
glue to lay out in as fine a bead as the CyA
even with the use of a glue syringe, so
cleanup was a bit more involved. After the
pieces are folded together, you need to work
quickly to scrape the excess with a plastic
squeegee. A damp rag will help to remove the
remainder, but the glue starts to set fast. The
added step of having to retape both sides after
a cleanup and sanding meant that there was
quite a bit more work involved, and the need
to let it set overnight added to the time factor.
As was CyA, wood glue was very hard once
it set up completely; if you don’t get it level
and clean before the drying process, you end
up with an unsandable raised edge.
Old-timers may remember Duco and
Ambroid cements from the stick-and-tissue
models of our youth—a time before CyA was
invented. The method with these cements is
very much like with wood glue, but it does
not dry as quickly, so after the glue is applied
and the sheets are folded back together, it’s
just a matter of carefully scraping the glue off
with a squeegee and wiping down the wood
with a paper towel. Do a quick, light sanding
with 150 on a block and retape. Flip the sheet
over, peel off the tape, and clean up and sand
this side like the first, and retape to let it dry
overnight.
Erik likes this method the best because the
classic airplane cements give you more time
to sand before they start to set up. With Duco
or Ambroid it may be easier to achieve a
seam-free finish in the end. I prefer the CyA
because it is much quicker, and, being a little
less picky, I was satisfied with the result of
the seams.
Another method has popped onto the
scene in just the last few years that many
people are using, especially in the Scale
Aerobatics arena. If you are going to sheet
your wings using Probond polyurethane glue,
you may completely forgo gluing the sheets
together. The polyurethane glues expand
while drying, forcing their way into every
crack and crevice; this action will glue the
sheets together in the process. Simply tape the
sheets together on the outside as you normally
would, and move on to the sheeting process.
Most of the time only a small amount of
glue gets past the tape onto the outside of the
wood. Sanding the sheets smooth and clean
afterward was possible with this method, but
it took a light touch and some patience.
Prepare the Foam Cores: Before we discuss
the various sheeting methods, let’s prepare the
foam cores. We must first determine where a
number of points will need attention, such as
servo bays, hard points for control horns,
alignment dowels, etc. We like to make a
template from thin poster board so after the
cores are sheeted we can align the template
and know where we located all the stuff that
was buried in the foam under the sheeting.
We’ll know where the servo bays are, where
the dowels go, and so on.
An easy approach to an accurate template
is to slide poster board under the plan.
Working on a building surface that allows
pins to penetrate, place a pin at each corner of
A router attachment on a Dremel tool is perfect for cutting the
reliefs for inlaying balsa in the foam core.
After the hole is fitted and the tube is in place, cut the stressbearing
plate to fit.
Besides the servo rails and cutouts, a stress plate is also
inserted under the balsa for the control-horn dowels.
34 MODEL AVIATION
02sig2.QXD 11.21.02 2:15 pm Page 34
the part and at each corner of the objects that
you want to locate. We wanted to bury dowel
stress-bearing plates for the control-horn
dowels and 1⁄4 x 3⁄8 x 21⁄4-inch spruce servo
rails. Remove the poster board from under the
plan and connect the dots (holes that you
made with the pins). Later we’ll use the
template for marking and cutting the hinge
lines, so be sure to add that to the template.
We used a Dremel tool with a router
attachment to cut bays for the bearing stress
plates and servo rails. The servo rails were
sunk into the foam enough for the servo to fit
nearly flush with the surface of the wing or
stabilizer. Be sure to account for the skin’s
thickness when making your depth
calculation.
Since Erik will be vacuum-bagging his
wings, he added very soft balsa to the rails to
temporarily bring them flush with the foam.
This way the sheeting will stay consistent
under the pressure from the vacuum-bagging.
Later when the servo bays are cut out, he
will cut away the balsa so that the servos
can sit farther down into the bay and stay
flush with the wing’s surface.
The control-horn stress-bearing plates are
there to tie the top and bottom stressed skins
together and make for an even fulcrum for the
horn to pivot around the hinge. A 1/2-inch
dowel will pass through and adhere to both
plates. A bolt will pass through the dowel and
act as the control horn. To me this is just
about the ultimate control-horn configuration
for foam surfaces. It’s extremely strong and
rigid, uses the entire thickness of the surface
for mechanical advantage, and is serviceable
in that the bolt does not need to be epoxied in
place and can be removed and replaced at any
time.
The stress plates are installed so that they
protrude slightly into the cut area for the
leading edge (LE) of the elevator or aileron.
Later when those surfaces are cut from the
wing or stabilizer, the stress plates will be cut
too, then they are sure to glue flush against the
LE material. The corners on everything that
we install in the cores are rounded. The
rounded edges have less tendency to
produce a stressed point or a failure point.
Make the tube socket stress-bearing plate.
In the foam core is an open bay where the
stress plate holds the wing tube socket. This
stress plate is very important; without it, the
foam would start to crumble in just a few
short flights. We measured a piece of light
plywood to fit perfectly from side to side
inside the bay but had excess hang out of the
opening on both sides of the core. We used
the tube socket end dipped in an ink pad to
mark the location to make a hole in the stress
plate for the socket.
After the hole is marked and cut to fit
perfectly around the socket, place it back in its
location with the tube in place, and trace the
outline of the wing core. The idea is for this
plate to glue the tube socket, the foam, and the
wing skins together, so leave it slightly
oversize when cutting to fit so that you can
sand it perfectly flush with the core and the
sheeting will bond to it. We will also fill the
bay with scrap foam and sand it flush so that
the whole assembly is securely glued together.
It’s time to glue the dowel stress plates, the
wing tube socket, the wing tube socket stress
Finished foam core after wood pieces are glued in and sanded
flush. It’s important that sanding be smooth and there are no
gaps or openings to show after sheeting.
The assembly process entails an internal TE. Tying in the
sheeting to the TE makes for a much stronger finished product.
These are the parts for the tube socket assembly and the stressbearing
plate. Epoxy was used to glue everything.
Scrap foam was used to fill the opening. Once it’s glued in place,
it will all be sanded to fit. The end of the tube socket is capped
with 1⁄8 light plywood.
February 2003 35
02sig2.QXD 11.21.02 2:16 pm Page 35
36 MODEL AVIATION
Use 1⁄2 light contest-grade balsa for the LE. Cut it slightly
oversize, and attach it using a liberal amount of epoxy.
Once the vacuum has started to draw, all wrinkles and folds
must be smoothed away. Any imperfections left in the bag will
be permanently marked in the sheeting.
Shuck was placed on top of bagged wing and light weight was
used to hold core flat. When glue is dry, cut ends off bag just
inside seal and reuse for next wing.
When the bag is smooth and even, turn up the regulator to reach
approximately 8 inches Hg.
To ensure a straight contoured LE, mark the centerline with a
piece of masking tape. Use a razor plane to work it to shape. Do
the final finish with a sanding block.
plate, and the servo rails into place before sheeting. In our foam cores,
made by FlyingFoam.com, the channels for the servo wires are already
cut into the main wing foam. For the stabilizers we’ll just use a 1/2-inch
copper tube to cut a channel later. Cap the end of the tube socket with
1⁄8 light plywood and sand it flush. This will act as a tube stop. Without
it, the wing or stabilizer tube could migrate into the foam and cut as it
went.
We used epoxy to assemble everything. Other glues that are
compatible with foam, such as carpenter’s glue or Probond
polyurethane, are fine too. After the glue is set, sand the installed
components flush to the core’s surface. Make sure that there is at least
3⁄8 inch of excess socket material extending from the root of the wing
or stabilizer. You can see that the stress-plate bay has been filled with
scrap foam, the servo rails and stress plates are in and flush, and that
everything should glue nicely to the sheeting we are about to apply.
The servo bays and holes for the control-horn dowels will be cut after
the sheeting is in place.
On our wings and rudder we decided to bury our trailing edges
(TEs) inside the sheeting. This method is structurally much stronger
than butt-gluing the TE to the wing after it is sheeted. One of the worst
enemies of large control surfaces, especially long ailerons, is flex. A
flexible surface is more prone to failure because of flutter. We are
striving for very light yet very rigid surfaces, and although it may be a
bit more work to build the TE into the wings, the strength advantage is
well worth the effort.
The idea is simple. Glue the TE to the bare foam and shape it to
create an extension of the foam, then sheet over the whole assembly.
The wings on the Extra were not designed to have completely sharp
TEs, but the cores from FlyingFoam.com are cut oversized and go to a
point so that you can cut them to achieve your desired final result. We
02sig2.QXD 11.21.02 2:16 pm Page 36
February 2003 37
cut exactly 1 inch off the TE of the wings and
rudder and added a half inch of balsa to create
our TE, then we sanded the balsa flush with
the core. After the wings are sheeted your TEs
will already be done, and all that will be
needed is a cursory sanding to true up the TE.
Now it’s time to fit the wings and
stabilizers to the fuselage and sand them to get
a flush fit. Since you can’t cut off the phenolic
tube at this point, it may be a good idea to
make a thick dummy root rib that will fit over
the phenolic and fit tight against the fuselage
side. Adjust by sanding the foam wing or
stabilizer root until you get a nice, tight fit.
Sheeting Techniques: With the construction
of a foam wing, the strength is achieved by
the shearing action between the sheeting and
the foam. The thickness of the wing also plays
a role. The density or strength of the sheeting
itself is the least critical factor in the strength
of the wing. So the method by which the
sheeting is glued will, in large part, be the
determining factor in the wing’s strength. A
spar may help, but I have never found them
necessary when the sheeting is done properly.
I don’t claim to have all the answers, and
for most of us the ultimate way to accomplish
the task at hand may be a combination of
methods. Let’s look at three methods for
applying the sheeting to the foam: epoxy or
Probond with weights for holding the sheeting
to the skins, contact cement, and vacuumbagging.
Erik will vacuum-bag his wings.
You can get in-depth info about what we did
during that process in the more detailed online
construction manual. For the purpose of this
article we’ll pay a quick visit to each method
rather than try to give in-depth instruction.
A close friend of mine, Norm Cassella, a
longtime builder with more than 60 completed
1⁄3-scale Lasers and countless other models,
uses spray contact cement for all of his foam
sheeting. His models have proven themselves
in time with countless hours baking in the sun.
Many of them are still flying weekly and in
perfect condition after 10 years!
The completed Extra you see in this article
was built with the method I’m going to
describe. The trick is to use an industrial grade
of spray glue. Some builders have used 3M
#77 and have had less than satisfactory
results. We have had great luck with 3M
08074 spray trim adhesive, available at your
local automotive-parts store. It won’t melt the
foam when applied correctly, it doesn’t lift in
hot weather, and it holds fast for years without
drying out.
Contact cement is probably the quickest
way to sheet foam, and although it is far less
work than the other methods, it may not be the
easiest! If the two pieces to be glued touch in
the wrong spot, you could easily have a ruined
wing because they instantly adhere on contact
and will not separate without destroying one
of the parts.
As with any sheeting method, a good, flat
table or work surface is essential to attain a
straight wing panel. We were lucky to obtain
a couple of 5 x 3-foot slabs of machined
marble. It’s about as perfect as you can find
without the expense of a machinist’s table.
With contact glue, trial-fitting is the key.
Start with the TE and roll the wing on the flat
surface to the LE. Mark the corners and
double-check the marks. Spray the contact
cement on both surfaces to be glued and let
stand until tacky as per the directions on the
can. It usually takes approximately 10
minutes. Too much build-up will take longer
to dry and may melt the foam, so don’t overdo
it. You’re after a medium, even coat. Getting
the TE flat on the first contact is very
important to obtain a straight TE. Tack down
the TE, then just roll the panel flat on the table
to the LE and you’re done! I use epoxy to
adhere the end caps and edges and make sure
they are glued well to any exposed sheeting.
This ensures that the sheeting will not lift
from an edge.
Newsflash: While I was sitting here
writing, Erik weighed the two sheeted wing
panels complete with TEs, phenolic sockets
installed, servo rails, and dowel plates. They
are exactly the same weight! (We had to
weigh them in grams to verify … ) That’s
how accurate you can be when you pay
attention to the details. Amazing! (Again, he
is vacuum-bagging with epoxy.)
Most modelers use the weighting method,
and it works virtually the same with epoxy
and polyurethane. Trim your glued or taped
sheets to fit in the shucks as tightly as you
want. We leave roughly 1⁄4-inch overhang all
the way around.
Erik likes to seal the wood before
applying any epoxy so that it does not soak
into the wood too heavily. He prefers to
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02sig2.QXD 11.21.02 2:16 pm Page 37
use hairspray for its light weight and
spray-on convenience, but Balsarite will
seal the wood slightly better. Hairspray is
also ready to sand in just a few minutes,
whereas Balsarite must dry for several
hours. Sand the hairsprayed surface with
320-grit paper on a foam block, and wipe it
clean with a tack cloth or vacuum it.
Weigh the epoxy (or at least use a
measuring vessel) to get the same amount on
each wing panel. Spread it evenly with a
plastic spreader or your spouse’s departmentstore
credit cards (I find that the Neiman
Marcus and Lord & Taylor cards work the
best … ) until the whole surface has a shine.
Don’t allow any significant buildup. Push any
excess off onto some waxed paper.
On your flat building surface, place the
assembled wing core and skins into the shucks
with the top of the wing down. Align the
assembly and place a flat and true piece of 3⁄4-
inch particleboard that spans the entire core on
the top to spread the weight that you are about
to add. Most people use bricks or heavy
blocks to weight the core. It takes at least 200
pounds or more on a wing this size to get a
good, tight bond evenly across the wing
surface. Add the weight, and let the cores dry
overnight.
As I mentioned, Erik wanted to use the
vacuum-bagging method for our airplane.
Vacuum-bagging is a process in which the
wing is placed in a plastic sleeve and sealed,
allowing the air to be pulled out with a
vacuum pump, which creates uniform
pressure around the entire wing assembly. At
8 inches Hg, the equivalent would be to place
approximately 1,800 pounds on the wing
panel. The vacuum also tends to pull the glue
farther into the porosity of the wood and the
foam, creating a better bond with less glue.
This method requires special tools and
materials. On average you can get started
vacuum-bagging for roughly $300. See the
sidebar for a list of tools, materials, and
suppliers.
The wing and skins are inside the plastic
sleeve while the shucks are lightly weighted
on the outside to hold everything in place. In a
picture you can see the vacuum pump, the
sleeve, and various hardware to get everything
doing its job. To follow our procedure stepby-
step, go to the AMA Web site. After the
sheeting is in place, we will have to install and
shape the LEs, root plates, and tips. We used
1⁄8 light plywood for the wing and stabilizer
roots and 1⁄8 balsa for the tips with 1/2-inch
contest-grade balsa sheet stock for the LEs.
Shape them per plans. You can read more
about the procedure in our Web construction
manual.
Good luck with the sheeting process, and
remember that the goal is to get a straight
wing panel. Next month we’ll finish the foam
parts, fully trammel the aircraft for perfect
alignment, and cut out and hinge all the
control surfaces. MA
Mike Hurley
11542 Decatur Ct.
Westminster CO 80234
[email protected]
Equipment for Bagging Wings
The Composite Store
Box 622
Tehachapi CA 93581
(800) 338-1278
www.cstsales.com/Vacuum_Bagging/
vacuum-bagging-systems.htm
Aerospace Composite Products
14210 Doolittle Dr.
San Leandro CA 94577
Order desk: (800) 811-2009
Technical assistance: (510) 352-2022
www.acp-composites.com/acp-vbs.htm
Following is a list of the items we used:
• Pump and regulator
• Vacuum line
• 3 yards 36-inch-wide nylon tube (bag
for wings)
• 3 yards 18-inch-wide nylon tube (bag
for stabilizers)
• 1 roll of tacky seal
• 4 yards 2-inch breather strip
—Mike Hurley
