So you wanna build a wing?

This page should always be under construction, with your help!

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.


Carbon spars are very desirable, for their weight and strength. Pre made carbon spars are the strongest for their size, but handmade ones are comparable if the same amount of carbon is used. The best spars are no good if the spar web is not up to the task, and this is where the end grain balsa or carbon web is extremely effective.

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.

For some more information, have a look around in

Nic Wright's Model Aircraft Supplies

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 spar caps


I was shown how to make sharp trailing edges by a friend called Klaus Mittendorff. Klaus lives just over 4 kilometers from where I was staying for a short time in Toowoomba and I often found that I turned up at the right time to see Klaus doing some precision building. (Klaus was a precision engineer as a first trade and it shows with how he goes about his modeling.) I have learned quite a bit by watching and assisting and would like to pass on some of the techniques that I have witnessed. Klauses' trailing edges were the first thing that I noticed and they are a relatively easy thing to achieve.

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.


This portion on leading edges and paint is reproduced with permission from Nic Wrights notes.(with small additions from me to make some parts a little clearer)

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 Worrall.

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.

E-mail Andy MacDonald