Wire cabane struts for foam board planes.

Wire Cabane and Interplane struts for foam board planes.

[UPDATE - 31/08/2016 - I've recently simplified my fixing method for holding the wire cabanes onto the box fuselage deck. The saddle type system used in my Polikarpov PO2/U2 works well for that specific situation where the cabanes mount right at the front of the box fuselage, but it is a bit fiddly. Looking back at my original AVRO 539b with cabanes mounted just to the deck, it's still flying after 3 years (and I'll admit to a few unpleasant nose plants) with no signs of coming apart. So I decided to rework that 'basic' design and included a slim version in the modified SE5 plans article. [link below]. I will update the AVRO539b plans, but you can easily adapt them to this newer, simpler design by referring to the SE5 plans or some of the new 'assembly'  images below. At the end of the day, these are just a melting pot of suggestions that can be modified to your needs. Happy building.]

My first cabane elements appeared in my Velie Monocoupe.

This was a mix of wire, MDF (Medium Density Fibreboard), BBQ skewers (bamboo) and foam. I was able to create sufficient strength and rigidity to allow that ‘magic’ of an empty cabin space beneath the wing which has always been a great feature of this plane and its derivatives.

The basic structure beneath the surface was a simple four sided box. Wire on top and to the front, MDF on part of the base and the rest from foam. A short length of BBQ skewer glued on the top of the foam back plate allowed the connection between wire and foam.

The system has stood the test of time and is still in place in the rebuilt Morphocoupe, which is still flying.

From there it was clear the concept could be expanded, and I did this with my Avro 539B. The key to this was a sandwiched 'foam and tongue depressor' platform that was stable enough to screw an assembled wire cabane structure onto using plastic saddle fittings.

Again, the magic airspace beneath the wing was created. But the frame was a 'one-off' design and I've now simplified the system.

This is the simplified method, using gift card. I've only shown one set of holes, but you'd need to drill four sets, a pair for each zip-tie. Once the skewer is in place under the gift card, you could drill down through box fuselage deck to create these holes. You could also pre-drill them, first putting the card in the right place on top of the deck and marking the drilling points.

This diagram shows another way to do it, using that original sandwich of tongue depressors or lollipop sticks above and below the box fuselage deck. This is probably the very simplest method I've developed, as there's no drilling required, you just poke the zip-ties through the foam. The only downside is you need to either hang the power pod a little lower or shape the top of the power pod around these projections.

Looking back at another biplane build, the Polikarpov PO2, it had the tricky problem of needing the front cabane element mounted right at the nose of the fuselage.

After a bit of thought I came up with this method, where two skewers are embedded in the fuselage sides. Wire saddles catch on the skewers and provide the mounting points for the cabane frame. This arrangement allows the front cabane struts to sit pretty close to the nose, and the loading is passed down into the skewer. The PO2s natural triangulation of the cabanes gives rigidty, and the assembly sits on the same popsicle stick platform, being held down by the zip ties connecting to the wire saddles. 

The system actually works fine, and is very robust. Here you can see the embedded skewers in the fuselage sides, running right up to the fold in the nose taper, and how the ends of the wire saddles are hooked over the skewer.

So, once you have a mounting system, how do you build up your cabane frame. In my plans I've worked out the wire dimensions, so once the components are bent-up into shape they can then be assembled. Here's a typical 'test assembly' of the cabane struts onto the mounts using cable ties to hold the frame down and hold the elements in roughly the right positions. From the top it will look very similar for all the mounting methods I suggest. Next stage is joining all the wires with thread and CA along the top.

Here's a typical join. I arranged the frames so that the 'gaps' in the wire do not overlap - the second frame 'bridges' the gap in the first frame and strengthens it, and the first frame bridges the gap for the second frame. To be sure of the strength of the frame I added another three groups of wrapping. Finally, I snipped off the top cable ties. They were just there to position the wires as I wound the thread on. I've probably put about forty to fifty turns on each group. The thread needs to be reasonably firm, but even a slightly loose join will stiffen up when you drip-on the thin CA. The strength comes from the multiple turns, and from each thread bonding to the next - its actually incredibly strong.

Once you've finished all the thread and CA joins, you can try a test fit of the wing.

If everything's OK, the final stage is to fit corner centering pads to the wing. This helps to align the wing with the middle rectangle of wires, so that the wing aligns correctly every time it's removed and replaced. You might notice I 'nipped-off' the right-angle corners of the triangle, so that it fits better and doesent foul the thread and CA join. At this stage it's more important to align the wing relative to the fuselage, rather than the cabane frame. Nobody will notice if the frame is slightly askew, but they will notice if the wing is skewed.

For a more rigid wire to wire join in the cabane assembly, you can use wraps of thin copper wire and solder. It achieves the same effect as the thread and CA but is unlikely to release. I've found the thread and CA can break away from the wire in a bad shock, but it will not part. Having a 'break away' join in the assembly can actually stop damage spreading through the airframe, which is a bonus. And if you find a loose join, its easily fixed with a drip or two of thin CA. A little give and flexibility is no bad thing when absorbing the shocks of poor landings etc.

The only other part of the 'biplane equation' is the wing 'interplane struts'. I developed a system for embedded mounting points for the Velie Monocoupe wing, which has again stood the test of time.

This wing and strut assembly is still flying on the Morphocoupe. I was recently told this is still called a cabane because it connects wing to fuselage.

Here's the fitting system in use on the AVRO. Because it links wing to wing this is called an 'interplane' strut.

It's just a servo arm with a short length of BBQ skewer through the mount hole. Placed through the wing at the right point and glued in place. There's other simple solutions using common modelling parts...

 ...because the interplane fitting on the back of the top wing on my Avro ended up so close to the edge of the bottom panel, I notched the panel and used a control horn that passed through the top of the wing.

Here's the pins ready to go through the top. If I remember my assembly sequence, I glued the horn in place as you see it, pressed the top panel onto the pin ends to mark the foam, and used a bit of stiff wire to make the holes. When the wing was glued-up, the pins slipped easily through the foam and projected out the top.

Once the little ratchet plate was fitted and the protruding ends snipped-off it wasn't too obvious. You can see them just at the back there on the top wing.

Here's the same system shown in the drawings for the PO2. By choosing a good long servo arm, more than one wire can be attached to that point on the wing. If you want you can dress up the interplane struts with BBQ skewers and insulating tape to bulk them out, which is what I did with the Velie Monocoupe. There's loads more photographs and detail in the build articles for the Monocoupe and AVRO.

If you look at the most recent plans for the SE5, you'll get a good idea how the newer system works. My design might look complicated, but its actually very simple and quite quick to assemble. There will be other ways to do this, and I welcome input on alternatives or obvious improvements.

Just to show you how good this system can look, here's a model SE5A built by John Evans in Canada, using my recently modified and simplified SE5 plans. He's significantly enhanced the detail, adding; tape fairings to bulk out the cabanes and the landing gear, the exhaust pipes, pilot and of course the brilliant paint job. He's also doubled-up the ailerons, putting them on the top wing as well. Built in Adams foam, even with the added detail and paint it weighs 977grams with a 2200mah LiPo - about 150 grams less than my heavy UK foam board version.

So that's a quick round-up of various methods for fixing biplane wings to foam board using wire cabanes. There will be other methods, and there are some designs just using simpler rigid glued-in skewer and dowel cabanes and struts which work well, though getting all the struts glued-in at the same time is apparently a bit tricky.

So why do I champion this wire frame system? For a start, it's been around for a long time, and plenty of balsa builders use similar assemblies. But here's a list of other reasons;

1. Resilience - In my flight videos I often show when my biplanes have bad landings, or, most of the time, the planes tumble-landing into long grass. I do this for a reason - the plane seldom suffers damage! But I know that in similar circumstances, many rigid assembly biplanes would lose a wing or suffer some kind of 'shock' damage to the cabane assembly. This resilience is a result of carefully chosen design elements, both from the ‘real’ and ‘model’ worlds.

2. Longevity - I’m not expecting anything but a real hard nose-in-the-dirt crash to stop the plane flying. The same can’t be said for many more rigid designs.

3. Flexibility - Even ‘at the field’ I can completely dismantle my planes down to separate fuselage and wings, which is great if I need to do a bit of ‘fettling’ or minor repair to skewer holes and the like.

4. Appearance - I feel it looks a little closer to the ‘real thing’, which is always nice when you are modelling a real subject.

Rigid designs have their advantages too, but from a model flying perspective, especially where conditions and flying skills are not always perfect, I feel a plane’s ability to cope with a few knocks and keep flying is a real bonus.

Cheers, alibopo.