New toy means new furniture

We made a sturdy bench for our new toy:


It took us a couple of days to get it done. We need now a computer and some patience until we ge „The Kit“.

Composites are great, but wood is still my favority material. It spreads warmth and an organic feeling:



Hard time

The last couple of weeks have been hard: I had to revise the calculation of the loads and adapt the structure. I started with integration of the distributed loads to get the primary stresses in the spar and the forces at the joint of wing and center section. It sound easy but it isn’t!

As mentioned in my last blog entry, a huge problem is the sweep back of the wing. It makes integration of the loads more difficult to conduct and has some potentially problematic side effects on the structure. I have in particular changes in direction of the spar caps in mind. More on that later…

Sweep back has as a consequence that a portion of the torque is transformed into a bending moment. Thus, knowledge on the position of the elastic axis—also known as shear center—is a must.  Below we see a graph with the position of the main spar and the shear center (yellow) assuming a usual D-box structure:


When a shear force is applied at the shear center, the wing does not twist. It is also the center of torsion, which means that the wing tends to turn around that point when a pure torque is applied to it.

From the above figure, the shear center is slightly in front of the spar in a D-box structure. The reason is that the nose is a shear web which stiffens the structure shifting the shear center slightly off the main spar.

That’s alright for some wings, but the structure of Schneewittchen is slightly more complicated:Struktur

It is a multispar structure with several shear webs. This has as a consequence that the shear center shifts slightly aft:


The reason is simple: The other spars have also shear webs which add some extra shear stiffness. It sounds strange, but torsional stiffness increases merely by having more than one spar. Anyway, having the shear center moving aft is good, because the main spar is more or less on the ¼ chord line— also known as aerodynamic center—for which torque is mainly produced by the pitching moment of the airfoil. The above figure shows also the position of the neutral axis (in orange), under the assumption that shear webs do not resist bending. The neutral axis is free of bending stress, and hence, is used as a reference for calculation of bending stresses.

The whole process is iterative, as the size of the spar caps affects both the position of the shear center and of the neutral axis, which in turn affect the amount of torque and of bending moment. Doing it a couple of times delivers following distribution of shear, bending moment and torque:


The bending moment creates bending stresses in the spar caps, which produce axial loads in them:


So, finally I have some values to work with. The axial loads in the spars are shown up to the joint between center section and wing, because this is the place where the main spar makes a kink and loads have to change direction:


Spar caps resist only axial loads, which means trouble at the joint. Consider the extreme case of a 90° change of direction: The full bending moment is transformed in a torque. This is how a wrench works! Not to forget, the main spar is subjected to a bending moment of much more than a couple of Nm: It is rather 18’000 Nm (13’300 ft lbf). Such a high torque would be very difficult to resist. The good news are that the kink is only about 25° and it produces a torque of „only“ 2’800 Nm (2’000 ft lbf). This is trouble enough, as it results in transversal loads of ±7’600 N (1’700 lbf) at the main joint…

This loads are equilibrated by the mass of the pilot and structure. For this to properly take place, the structure needs to be suitably designed. First things first, one needs to have a good load plan:


I’ll probably be busy with this scheme and with the layout of the structure the next weeks…

Back to school

Not much visible changes have taken place since we joined the shell: We made a couple of reinforcements, which is not really worth mentioning in detail… No visible progress does not imply that nothing happend. Many things around Schneewittchen took place. We’ve been preparing and planing the pattern of the cabin. As for the center section, we will use the pattern to make a mold. The material should be delivered soon and we are finding out how and where to produce the necessary templates for hot wire cutting. Three days ago I got a special high temperature infusionable epoxy resin: EPIKOTE RIMR 935 (Hexion). We will need it to make a tool for production of the clear canopy.

Anyway, the last week felt like beeing back in engineering school: I had to revise my calculation of the structure. Sweep back makes everything much more complicated… This book on aeroelasticity by Bisplinghoff et al. has been my best friend—and my worst enemy at the same time—the last couple of days:


The good news is that my calculations on bending and torsional moments turned out to be alright!

Worth the effort

Finally! The shell of the center section is joined. It was a long and tedious way, but worth it:

We had to solve many problems to get there! It is astonishing how well it worked out considering that a couple of years ago we had not much more besides an idea.

Joining the shell was more or less straightforward. We mixed an adhesive based on expoy-resin (130 g), cotton flakes (15 g) and thixotropic agent (3 g). The adhesive was spread on the contact surfaces and the shells were pressed against each other with the mold. While the aft joint is wide, and hence, strong enough, the contact surface in the front is small. Notice also, that the aerodynamic pressure is highest here, as the stagnation point moves around this region. A bursting shell would we simply catastophal. Reinforcement was a must. We laminated and vacuum bagged a layer of biaxial non-crimp carbon fabrics (200 g) from the inside and outside. This should be enough to keep it together in all circumstances.

Here’s a video of how we joined the shells:


The next steps? Well, we  have enough to do in the center section: reinforcements of the ribs, canopy, fitting of spars, seat, controls, etc. We hope to start soon with the pattern of the cockpit and canopy. If everything works out as expectect, we should have at the end of the year a cockpit and a clear canopy (produced by Plexiweiss GmbH).

Reinforcement of ribs

Yesterday and today we laminated the reinforcement of the ribs. We used 200 g/m² non-crimpt biaxial carbon fabrics and a covering layer of 100 g/m² flax fabrics. Biaxial carbon fabrics are great when you have corners. The stiff carbon fibers have then an orientation of ±45°, so that the fibers are bent less and wrapping them around the corner is much easier. Also, this orientation is optimal for shear loads.


You might ask why we used flax fabrics. Well, they protect the load-bearing carbon fibers from external abrasion! Flax behaves much like aramid (Nomex® and co.): It takes some energy until it is damaged. Such a covering protects not only the load-bearing fibers, but also the pilot from sharp edges.

The reinforcements are curing now in vacuum at roughly 200 mbar residual pressure. The surplus resin is taken up by breather felt. We used also a covering layer of peel ply to obtain again a rough surface.



Two are better than one

The main ribs and those beside them have to take-up the weight of the pilot while entering the airplane: Climb from behind and walk on top of the wing. These ribs have to be doubled near the joint to the shell to distribute the pressure on a larger surface:



After bonding, one layer of 200 g/m² biaxial non-crimp carbon and 100 g/m² flax fabrics will further reinforce the joint.

We also made a couple of small ribs to stabilize the tail:

Diese Diashow benötigt JavaScript.

Wedding preparation

The upper and lower shells are almost ready to marry. We are making the final adjustments:

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It’s a long time since both molds were together. The next couple of weeks we’ll install some more ribs…

We also tested if the aft spar bridge fits well:

This bridge is only a piece for testing and fitting. The test piece for the main brige is beeing build by Siggi as of right now. The final bridges will be built by Eichelsdörfer GmbH using Siggi’s jigs. The material will be 1.7734.5 steel (better than SAE 4130).

CNC milling of ribs

Last time we wrote about how verly light AIREX® panels can be produced. Now it’s time to use that panels. Obviously one can use a simple saw and cut ribs off the panels. However, the ribs are large and need to be somehow transferred. A piece can be drawn in a CAD program and printed on several pieces of paper or tranferred by mere measuring and painting on the panel. Both approaches are tedious and precision is an issue. We are lucky to know somebody nearby with a self built CNC machine. With a good drawing and some experience it works like a charme. Thanks again for the help, Marcel!

Everything starts with making a good drawing a the piece:


I use a simple but good 2D-CAD program (QCad for Linux). If you look carefully at the drawing, you’ll see a chamfer at the left edge, which makes the piece more complicated (3D instead of only 2D). This drawing was imported in SolidWorks—a great but expensive 3D-CAD program—and converted via a STL file into G-Code by another tool. G-Code is what the milling machine understands. So much for the theoretical part…

Theory ends and practice starts after having a milling program. The panels have face sheets of carbon and flax fabrics. Carbon creates very stiff but brittle laminates. Flax keeps the carbon together and substantially increases robustness—similar to aramid. The AIREX® core is light but also soft. This mixture is difficult to mill properly, when the wrong router is used. Probably milling aluminum is easier than these panels. Anyway, the first time we used a router for metal. The result was pretty good, but the edges were quite fluffy. Nothing that cannot be corrected with some 400 grit sand paper. For the following pieces we bought a composite router (Karnasch 29.1783). It was not cheap, but the result was great.

Here’s a video of the machining:

I think the video speaks for itself!

We started to fix the ribs on the upper sandwich of the center section. Soon we’ll be able to „marry“ the to halves.