Sectioning the wing

From a point of view of structural design and weight, it would be best to have as few wing pieces as possible. Though a one-piece airplane would be great regarding weight, it would be great mess in transportation, and not to forget, in needed workshop space. Originally, I was thinking of a three piece airplane: two wings and one center section. This is pretty much how the Horten borthers sectioned many of their allwings. My center section was originally going to be 800 mm wide and the wings roughtly 6900 mm long. Pretty long wings, if you ask me…

Well, reality is always different than theory, and „The Long“ workshop is actually not long enough for such an endevour. Raimer and Walter Horten had quiet other possibilities including workshop space, which I simply do not have. However, I’m actually looking for a design, which could fit into a regular garage if necessary. So, I had to rethink the sectioning today.

The obvious solution is to split the wings into two pieces each. This reduces the length of the largest parts to below 5000 mm, which fits into a regular garage.

„The Long“ workshop with largest wing section inside:

SW-I_Garage

Another good aspect of splitting the wing is transportation. Having five sections makes your live much easier when you have to move the airplane from one place to another. A relative small trailer (2000 mm x 4000 mm) should be enought to transport „Miss Snow White“:SW-I_Transport

Dividing the wing into two sectioins has also a downside, which is added structural weight. Anyway, I do not have another choice and the benefits outweight. By the way, I decided to split the wing where the elevon start. This seems to be a good position…

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First sit-down testers in the mockup

I managed yesterday to get the mockup ready for a low weight sit-down test. The difference between a model and a mockup is huge: A full sized person should be able to enter, sit down and exit the mockup without breaking it. You have to make the structure strong enough to take-up the load of an adult (100 kg or 220 lb). I started today in the morning with lower weights. I hope to be able to sit down myself soon—without rupture of the mockup and myself.

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Three new workshops

After moving into our new home in Stuttgart, Schneewittchen has finally gotten a proper workshop! Well, there are actually three of them:

„The Small“ for diverse stuff:

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„The Long“ for wings:

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„The Large“ for the center section:

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Rib designs

Schneewittchen will be made of a mixture of carbon composites and wood. In more detail: laminated layers of uni-directional carbon profiles for the main and aft spars, a carbon sandwitch for the leading edge (torsion box) and wood/composite ribs for the rest (aft of main spar). Due to the mixture between wood and composites, I analyzed the desgin of the ribs in detail.

In principle, the ribs are similar to usual wooden truss ribs found in vintage gliders. This type of ribs is elaborated, however, very light and strong at the same time. The idea is to modify this traditional method and use carbon composites for the truss elements, in particular, the curved upper and lower profiles. These elements define the shape, which should be as precise as possible, also at loaded conditions.

Usually wood strips are bent into form and glued together with the other elements to form a truss. This works fine, however, the strips are stressed and the shape always differs slightly. Instead of using wooden strips, a composite sandwitch profile can be cured in a thread-milled form resulting in an almost stress free profile of perfect shape. At least in theory…

So much on the theory. Some figures on strength, stiffnes, etc. are needed before ribs can be build ribs this way. I started by making different sandwiches for the upper and lower profiles and testing their ultimate strengths. After having found the way to build the main profiles, I moved on to testing the joints. Having all needed figures, I built two rib prototypes, which are still waiting to be loaded and tested for their ultimate strength.

A picture is worth a thousand words:

Shear loading:

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Bending:

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Joints:

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Prototype of rib number 10:

rib
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Always start with a mockup

The aerodynamic design ir practically done and it’s time to start with the full-sized airplane. But before I put my hands on it, a mockup of the center section is needed to decide on how to design the cabin. A cabin is not needed in an all-wing, at least from the point of view of aerodynamics. Nevertheless, you need to sit somewhere, unless you decide to use a prone position, like in the Horten IV. To be honest, I prefere a more conventional sitting position.

After getting some ideas and letting a friend cut wood for me, I started to build a full-sized mockup to get an idea of the sitting position, instrument and control positions, and not to underestimate, how the view out of the plane is going to be. Essentially, I decided to build the mockup of two spars and wooden ribs. It will be covered with Ceconite Superflite 104A. Surely not he cleanest build, but it should do enough to get a feeling of how it’s going to be piloting the Schneewittchen.

There is still a lot to do. Anyway here are some pictures:

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