Last foam pieces on the plug

Two complex shaped foam pieces were missing on the plug. We used our new CNC machine to mill these:

Though the machine makes most of the work, a good sketch is always needed. And this isn’t easy. This is even more evident when  three dimensional pieces are milled. We used a mixture between QCad and FreeCAD to design the parts (both open source programs).

The piece at the front of the cabin was straightforwad to draw:

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The piece at the aft has a very complex shape: It has on the top a cone shaped section, which is merged to something that looks more like a trailing edge of a wing. Drawing a good transition between these two quite different shapes is challenging. We, thus, decided to keep the piece simple but still providing the basic shape. We will sand it to its final shape:

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Time consuming details

„Kleinvieh macht auch Mist!“

This little nice german proverb—which means translated: „Flock makes also muck!“— describes very well the last weeks. Though the CNC machine technically runs since a couple of weeks, not until today most important details were solved. We had to take care of having a proper workholding. This included a tooling table with many drillings and face milling the surface. It is ready up to some changes in the cooling system and vacuum cleaner:

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We had to learn a lot about milling. Without experience it is difficult to have a feeling for a good feed rate and turning speed of the spindle. And in case you are wondering: Yes, we broke a couple of tools while testing. At least nobody got hurt. I guess we found now a good combination for wood. Look at those nice chips:

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It is ready to work for Schneewittchen! There are a couple of styrofoam pieces and templates for the cabin’s plug waiting to be machined…

CNC milling machine runs

It took us roughyl three to four weeks to build the kit of our new CNC machine. Not only the mechanics took effort to get together and aligned, but also assembling the control unit and wiring the drives was quite some work… Anyway, though we moved the machine a couple of times before to set it up, today it had its first „mission“:

From now on, we can focus again on Schneewittchen. The cabin plug is waiting…

Meccano for big boys

Our new toy is finally here. A couple of hours before we picked „The Kit“ up, we realized that we should paint the room:

We’ve built a couple of large LEGO® technic sets with our kids, but this is by far our largest kit ever. This is the result of one and a half day of work:

We’ll need a couple of days more to finish it…

Foam here, foam there, foam everywhere

The tamples we got a couples of days ago have been very valuable. We started to cut the core for the plug of the cabin:

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We had some problems at the beginning, because the approach we decided to use didn’t work out as expected. Originally, we though to cut the foam perpendicularly around the template. Heavy waves was the result:

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hence, we decided to use the tamples on both sides. This has as a consequence that a tiny bit of extra material is cut when the shape is spherical. This tiny error is acceptable, as the plug core will have to be slightly adapted to our taste anyway.

The plug looks everyday more and more like a cabin:

We started a small video series on how the mold is made:

 

 

 

 

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:

schubmittelpunkt_d-box

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:

schubmittelpunkt

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:

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

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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:

struktur_zeichnung_geschnitten

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:

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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:

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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).