 | The Standard Foam Wing Sheeting Method The
old contact cement or epoxy and stacking weight on the core and cradle
sandwich is the most common foam wing
sheeting procedure everyone remembers as being somewhat heavy. There
is only one chance when using contact adhesives to adhere the wing sheeting.
Any mistakes here and you have ruined your wing because when the sheeting
comes in contact with the foam there is no repositioning the sheeting.
The contact adhesives tend to increase the weight to unacceptable levels for
upper level competitive control line aerobatics. The brush on varieties such as Core Bond tend
to be heavy and sink in between the foam beads to add extra weight.
When using Core Bond it is important to apply the material to the sheeting
and the foam and allow to dry completely before joining the sheeting to the
foam. Products such as 3M 77 are easier to apply since they are
sprayed on. Some of the 3M products are not foam friendly so do not
use anything but 3M 77 before testing on scrap foam. When using 3M 77
spray the wood and foam |
| Wood Selection
Since the wood quantity is rather large in
a foam wing it is important to select good light wood. Five pound wood
seems to be the norm. The lighter four pound wood is very rare.
I have purchased thousands of 1/16 wing sheeting sheets and only had twenty
or so true four pound pieces. For a sheet of wood to be an average of
four pounds per cubic foot there has to be some wood in the sheet that is
less than four pounds. When wood becomes less than four pounds per
cubic foot it is almost without structure. Another way around the
weight of the sheeting is to use thinner wood. Riley Wooten of Lone
Star Balsa will custom plane sheeting to your specifications if you buy in
quantity. If using the vacuum bagging technique then carbon veil can
be placed under the sheeting with great results. I have always sheeted
my foam wings with the grain running parallel to the leading edge which
helps the wood make the bend around the leading edge easier. The A
grain wood at the leading edge and the C grain wood at the trailing
edge. A grain wood is easy to roll or bend parallel to the grain and
is the type to use when molding parts from sheet stock and a foam wing is a
molding of sorts. The C grain tends to resist the bending forces parallel
to the grain and is suited for the straight flat trailing edge. It is
also a good idea to apply the sheeting over the pre sanded leading edge and
trailing edge. The wing becomes stronger when the sheeting is applied
over the leading and trailing edges. Place masking tape on the foam
and precede to razor plane and sand the LE and TE down removing the tape
only when final sanding. Use a sanding block that is at least as long
as the part you are sanding. |
| Edge Gluing
Now here is the part that takes some finesse.
The hardest part is getting the wood to but together without any gaps.
If you have a joiner at your disposal it makes the job effortless. I
made a fixture to hold multiple sheets side by side and run them over a
joiner to true the edges up. If you are without a joiner then the old
straight edge and and sanding bar will work fine. The bar has to be as long
as the wood sheet you are sanding. I have found that even though the
wood has been trimmed with an Xacto it still needs to be sanded. I
believe that the Xacto tends to wander with the grain slightly. Hang
the wood over your workbench about 1/16 of an inch and sandwich with
another straight edge to keep it flat and lightly sand. Once the edges
come together without any gaps I use masking tape to hold the sheets
together. Hang one piece of wood over the edge of the bench letting
the masking tape to act like a hinge. Apply a bead of medium CA glue
to the edge of the sheet. Just enough glue to cover the edge no more
and no less. This takes some practice to get right but when you get it
right you will be rewarded with very little sanding. Once the glue
bead is applied I flip the sheeting on the table flat and rub the seam with
a paper towel until the CA has cured. I continue gluing the sheets
together until have a large sheet. By gluing the sheets one at a
time to the sheet that is becoming larger you are able to trim the
large sheet if it is becoming distorted. Distortion usually occurs when you
have to pinch them together to close any gaps. Enough sheets are glued
together to make one sheet and when cut diagonally makes up two pieces for
one wing half. |
| Sanding The Sheeting I
have had greet luck with building over glass. Glass is always easy to
clean and can be shimmed flat within a few thousands of an inch with
newspaper. If the glass is placed onto a metal desk it will hold
tolerances that are hard to hold when using a wood or any other surface that
will absorb moisture. This flat surface becomes mandatory when
sheeting wings or sanding the sheeting or for building an airplane for that
matter. The sanding pad that I use is 1/4 aluminum about 4 inches by
ten inches with 100 grit paper. The 100 grit sounds a little coarse
but with this much surface area on the sanding pad it is required.
Place the sheeting on the glass and sand with the grain or diagonally to
it. If the glue is applied properly you will not have to sand the
seams very much. Most of the sanding will have to be done to get the
sheet thickness mismatch evened out. Sand both sides until you are
satisfied with the results. Hold the sheet up to the light and candle
to see if the seams are sanded properly. |
| Not All Foams Are Created Equal Most
foam wings are constructed from Expanded Bead Polystyrene or Styrofoam as
most of us know it. The foam is manufactured in large billets ranging
from four foot by eight foot blocks to four foot by sixteen foot blocks.
These blocks or billets are around three feet thick. The billets are
then placed onto a conveyer and cut into sheets with multiple hot
wires. As with any other material there are many varieties by many
manufacturers. I have purchased foam that was one pound per cubic foot
but varied in texture somewhat since it was from a different manufacturer.
DU Pont an others are in this business. Most modelers are aware
that the one pound foam is the way to go but fail to realize the difference
in quality of foam. The lumber yards are a source of foam but of very
low quality. The problem with the lumber yard or building supply
variety is the amount of regrind material. Regrind material is a
recycled foam material that is mixed into the virgin bead material for a
filler. When the regrind is mixed into the new material it becomes
hard. These hard beads do not cut as smooth as the virgin bead
material and create unsightly gouges or lines in the core. Some of the
lines can be sanded out but I have had the wire come to a standstill trying
to melt through one of these hard regrind spots. Contaminants such as
wood chips can also find their way into the foam on occasion also.
Therefore it is wise to go to a foam manufacturer and specify one pound per
cubic foot virgin bead expanded bead polystyrene. Since the cost in
transporting one pound foam is high there are many manufacturers locally if
you look in the Yellow Pages. The cost is prohibitive in shipping this
material so try to locate it locally. |
| Epoxy Vacuum Bagging Wood To Foam Here
is where the process begins to show some promise for the competitive
aerobatic airplane. The vacuum pump opens up a whole new set of
possibilities. It makes the heavy contact cement system obsolete along
with setting cinder blocks and sand bags on the wing and cradle. Not
only is there a weight savings in glue but you gat 100 percent
adhesion. When you vacuum bag the sheeting to the foam you can use
less epoxy than with the stack and weight method. I believe that this
is due to the vacuum pulling the glue out of the grain and the amount of
pressure applied from the vacuum. It is important to obtain a vacuum
pump that you can regulate such as the one Aerospace Composites sells.
The vacuum can be no greater than eight inches of mercury. If the
vacuum exceeds this amount it will crush the one pound per cubic foot
foam. Also of importance is not to core the wing out until after the
sheeting has been applied. Since the wood is already bonded to the
foam at this point more foam can be removed from the wing. I will
cover a process in the future where almost all of the foam is removed from
the wing. When applying the glue on the wood use a low viscosity epoxy
such as Hobby Poxy Smooth And Easy. Scrape the glue thin enough so
that the glue is only in the grain of the wood. The surface of the
wood should have a damp look without being shiny. If using carbon veil
under the sheeting, thinner sheeting can be used. Experiment
with sample pieces to see how much epoxy it will take to soak through the
veil and bond to the foam. I ran some experiments on this and had good
luck with just putting the epoxy on the wood and letting the glue wick
through the veil and bond to the foam under vacuum. Pattern R/C wings
were sheeted with the veil located where the ailerons were cut out.
This increased the trailing edge rigidity and stiffened the ailerons.
Carbon Fiber tow could be placed under the sheeting to act as a spar
also. Other variations to this theme are sheeting the leading edge of
the wing back to the high point and placing strip ribs in the foam and
removing the foam after completion. |
| Coring The Foam Wing When
using the vacuum bagging technique coring will have to be done after the
bagging is complete. Other wise the core will crush. I tried to bag a
wing that was already cored under low vacuum and you wouldn't believe how
fast I removed the quick connect fitting after watching the wing almost collapse.
This was only a test but one didn't need to repeat. When coring
a wing it is necessary to make a separate set of templates. The
templates become too flimsy if they are designed to do both cutting and
coring operations. Here again I found it beneficial to suspend the
wing and let the wing rotate around the hot wire. |
| Foam Cutting Machines There
are a few machines that are commercially available and when the instructions
are followed produce excellent cores. There is no one cutter that will
do all of the different variations of wings possible. There are the
high aspect sailplane wings that need different methods and setups than a
short low aspect ratio combat core. High taper wings such as a jet
airplane are inherently hard to cut due to the tip melt-away when
cutting. The control line wing is probably one of the easiest wing
configurations to cut. I developed my own cutter that cuts both sides
of the wing at the same time. It speeds up the process and if you are
careful when making the templates the wings are accurate. The Feather
Cut machine is a good cutter that will cut only one side at a time.
When using this machine it is important to have a flat work surface to lay
the foam block on when cutting since a warped work surface will transfer
into the airfoil when |
| The Hybrid Foam/Balsa/Carbon/Built-up
Wing The idea behind this wing is to vacuum
bag the leading edge sheeting over the pre-placed full span spar capped with
carbon and place ribs behind the leading edge sheeting. The leading
edge sheeting extends to the high point of the wing. The ribs are of the I
beam variety and are placed into grooves cut into the foam and sanded flush
with the surface of the foam. The trailing edge is then glued to the
ribs and the trailing edge sheeting and cap-strips are then
added. After the LE sheeting has been bagged down to the foam
then the leading edge is completely cored down to the wood. The
remaining foam left between the ribs is removed. Basically the foam is
used as a fixture to hold all of the ribs and sheeting in place then
removed. It is a hard process to present without pictures so there
will be more pictures and info on this process in the future. |
| 
This picture shows a wing with a spar installed ready for the leading edge
sheeting to be vacuum bagged down over the leading edge to the spar.
Balsa strip ribs will be laid into grooves behind the sheeted leading
edge. Notice that the leading edge is already tack glued to the
foam. It is not necessary to glue the leading edge completely to the
foam since the foam will be removed after the bagging process. The
spar is a built up balsa warren truss configuration. |
| Template Material The
best template material for cutting foam is a product referred to as
Formica. It is used in counter top applications and can be purchased
in sheets at your local building supply store. Another good source of
this material is a kitchen cabinet makers shop. They usually throw the
scrap away and will gladly give you their scraps. I try to get the
thicker material which seems to be flatter. There is a thin variety
but it seems to curl too easily. Solid light colors seem to work the best
since you will be able to see the centerline on the material. The type
of cutter you are using will dictate the design of the template. Templates
that cut only one side of a wing at a time will vary from one that is
designed to cut the top and bottom simultaneously . |
|  |
| |
|  This
picture shows the leading edge of a template. The center lines have
been scribed into the template for reference. The wood strip on the
front is a stop for the foam cutter I designed. The hoes are drilled into
the template for large modeling pins to secure the template to the foam
block. |
| 
Template that has been attached to the center line running
the circumference of the foam block. |
| 
The simple adjustable height gauge that is used to scribe
center lines around the foam block. |
| 
The trailing edge of a template. The hole is where the
cable hooks to when pulling the template and block through the cutting
wires. The wood cross piece is a stop. |
