If you have any idea of how a flying wing stays in the air, you will know that conventional swept flying wings use washout, or wing twist that decreases the angle of attack towards the tips, to obtain stability. There are some wings on these pages that don't use this principle. The Spin Off is one of these. It uses the positive pitching S5010 as its section and therefore needs no washout. These wings will not display the favourable stall characteristics of a washed out wing however and may need to be flown with caution.
The maintaining of the washout in the conventional wings is critical to their performance and it is important to build them with a hefty consideration towards TORSIONAL rigidity. This can be achieved by composite construction using the fabric oriented so the fibres lay at 45 degrees to the lengthwise direction of the wing. It is VERY important for the skins to be able to transfer loads to each other. If the skins are not joined by either a glass or balsa cap then you are wasting all the effort of your carefull design.
The smaller flying wings may be quite suitable for planking with balsa, but for the time and effort involved you may as well have a go at vacuum bagging.
One of the things that really suprised me with the Co7 was the dual
tow hooks. Some of you may find this as a surprise, but my first wings
were Klingbergs with one towhook in the center.
Why bother, you ask? Well, think of the loads imposed with a conventional single towhook. During launch, the wings are trying to lift up (naturally) and the towhook is pulling down. All of this contributes to a MASSIVE bending stress in the center of the wing. Needing to have a load bearing structure to absorb the launching forces on the towhook also restricts design. If the center of gravity is behind the main wing structure, how are you going to place the towhook without massive forces on the fuselage? Sure you could do it, but with a fuselage that strong you won't need ballast. If you don't want to mount your single towhook on a fuselage, then it will have to be on the main wing and that will limit the amount of sweep you can use before you are mounting a towhook on the trailing edge.
The answer then is to use two towhooks.
This simple design feature relieves the tow loads trying to bend the wing at the center, which is still trying to lift up against the tow loads. The loads are of course halved at each attachment point, by being shared between the two, but also the wing is not getting the bending load in the center.
The other advantage is the fact that a three part wing is smaller
when disassembled and doesn't have an awkward transfer of forces
throught the center joiner, which sometimes can cause structural
problems in a swept wing configuration. The joiners in a three part
wing can be encapsulated between the spar caps resulting in a much
stronger construction, the same as conventional tailed aircraft with
two and three piece wings. The tow hooks can be installed in the ply
end plates of the wing centre section and also serve to tie the wing
skin, spar and joiner tube forces together.
The swept wings that are joined at the center can utilise a thick
reinforced spar web to hold the joiner tube. A tapered ply web being
well suited for the job.
A premade carbon web of two layers of 180 gm carbon at 45 degrees is
the easiest to install.
Once the spaces for the spar caps have been sanded/hotwired to shape, the cores are cut down the center of the spar cap groove. A little bit of experimentation will show what sort of wire will cut the approximate gap to equal the width of the carbon web material. Some carefull measuring and cutting will yield the same gap for the balsa, although with another cut and maybe some careful sanding
Keep in mind you can add a second web of carbon if the spar is
curved in the center section as it is with the CO7.
With the carbon or balsa web material cut to match the depth of the web required, it is now a matter of slightly abrading the corner of the web slot to allow a greater bonding area for the spar caps.
With the spar web accurately glued to the cores and the cores
accurately glued together, the spars are installed, either prefab or
carbon tows style. With the prefabricated sort it is important to get
an excellent bond, Cabosil and resin can be used here as used by Klaus
in the trailing edge section below. With the carbon tow style of
construction, it is sometimes recommended to bag and shape the spars
first and then bag the skin, if you are not familiar with the level you
can fill the spar slot, before it will leave a lump in the skin.
I will be doing it always, as I prefer not to risk all that work, not to mention the cost of the carbon skins!
If you wrap the joiner tubes in kevlar tow and superglue or resin,
and then fuzz them up with a file, they will key nicely into the carbon
I must say at this point that Klaus forgoes the vacuum bag for a
large number of big pavers and a very strong bench. He cuts his cores
out of the densest white foam that he can get his hands on and he uses
the prefabricated carbon spar caps available from Composite Structures
Technology as they work out to cost as much as the tow constructed ones
and are stronger and easier to work with.
Klaus cuts his cores so that the beds end at the trailing edge of
the wing and then he cuts the last 4 millimeters off the trailing edge
of the core. (Trailing edge trick number 1) Then he cuts the first 5
millimeters off the leading edge. I thought that this was a bit odd at
first but then I saw why. Klaus uses 14 thou Mylar as skins to transfer
the fibreglass skins to the foam core. The skins are cut to start at
the trailing edge and almost meet at the leading edge. They also
overlap the sides of the core by about 5 mm to cover the ply end plates
that are placed on the core before pressing.
The trailing edges of the Mylar are taped together with a gap that
ensures that when they are folded together, the inside surfaces touch.
This is the second part of the trick for the perfect trailing edge.
After being polished with release agent, the skins are painted with
a two pack paint and let dry for a few days. ( It doesn't pay to let
the paint dry for too long) Then the glass cloth which alternates in
bias by 45 degrees is put on. The last wing center panel had four
layers of a specially sized 1.4 ounce glass top and bottom. The glass
is staggered back from the trailing edge of the Mylar by a few
millimeters each layer and then the core is placed in the skins exactly
4 mm forward of the Mylars taped trailing edge. I almost forgot, the
cores are squeegeed with a fairly viscous mix of cabosil and resin.
This has proved to add minimal weight (about 4 grams per wing) but add
an amazing amount of bonding strength. This is crucial to stop
delamination which is the major failure factor with any skinned wing,
particularly glassed fibre skins.
Just before the skins are folded over the core which is lying in
the lower bed, a wet out 8k carbon roving is placed where the four mil
of core trailing edge was cut away. Then 3 to 5 strands of glass tow
(depending on the size of the piece being built) are laid dry on top of
the carbon roving on the side towards the core. Then a firm mix of
micro balloons and resin is squirted along the leading edge of the core
where the other piece was removed. This is done with freezer bag with
the corner cut off and it makes the job so easy it is unbelievable.
Then the top skin is folded over the core and the trailing edge of
the Mylar is aligned with the top and bottom beds and then a board is
placed on top and the brickies labourers come in and pile a sizable
weight of pavers on top until we wonder just how much more the bench
can take. The leading edge mix squeezes out between the gap in the
Mylar and is caught by the oven bake that I neglected to mention
separates the Mylar skins from the core beds. I also forgot to mention
that there is a carbon tow applied to the front of the core before the
leading edge mixture is squirted in. This sandwich is then left for the
mandatory time and when it is peeled apart, the trailing edge is
finished with a few swipes of sandpaper and the leading edge can be
finished to profile with the minimum of sanding. With a bit of painting
it looks perfect. This technique gives a strong trailing edge with an
average thickness of 1/4 of a millimeter. Yes..... Damn fine trailing
edges. And we all know how important trailing edges are for maintaining
airflow over the aft portion of the wing.
Achieving very accurate leading edge profiles is easy when you know
how. The procededure I use is based on an idea passed on to me by Dave
Control stations are established at 200mm intervals along the span.
The profile is cut to shape exactly at these control stations using a
.5 inch flat file and a template of the leading edge. The surface of
the leading edge at the control stations is then marked with a ball
point pen which leaves ink on the surface only. The leading edge is
then abraded between the control stations with a long sanding bar until
the ink starts to disappear.
The checking templates are used to confirm the accuracy of the
section between the control stations.
The more you practice with this technique, the easier it gets, but
be carefull at first as it is very difficult to replace material if you
take too much off. There really is no need to change to a finer paper
at the end.
The 80 grit finish will provide a good key for the primer/filler.
(two pack acrylic is recommended, applied with a a piece of foam
plastic). If the primer/filler contains agents which melt foam and part
of the foam core is exposed during the shaping operation then the foam
should be dug out and filled with micro balloons / epoxy before the
application of paint.
The finishing coat should be a soft paint with a high colour
density. It will be necessary to abrade and polish the paint when it
has fully cured to provide a smooth surface for a minimum disturbance
of the laminar flow boundary layer. A soft paint can be polished with
little effort which will keep the heat generated by polishing to a
minimum. There is a good chance that that skins will be locally thin at
the the leading edge and heat will soften thin parts of skin and the
underlaying foam causing depressions. This is most likely to occur on
light weight carbon or glass skins.