Progress on the center section

Though we haven’t been reporting much here, there has been substantial progress on the center section: The lower side of the shell is ready and the upper side will be soon too. It won’t take much time until we’ll join these. You probably can imagine that we are really looking forward to get these pieces together!

The production of the halfs differed slightly: While the outer face sheet of the lower half was geled when glued to the honeycomb core, we decided to let the other outer face sheet to fully cure. The reason is simple, it is quite stressful to keep gelation times when working on such large laminates. Both outer face sheets were produced using vacuum resin infusion, which yields light and great laminates. The inner face sheet needs to be glued in a geled state so that it can admit the right shape.

We made a small series of videos showing how we produced the first half (lower side). Part I, II and III, corresponding to dry layup, vacuum infusion and glueing the core, are ready and online. Enjoy!

 

Here’s a photo of the finished lower half:

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It weights roughly 6.4 kg (14 lbm), which is very good for a piece that takes up much of the the pilot’s weight.

The other half is wating for the inner face sheet. Fully cured outer face sheet:

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… and a photo while we distributed the glue for the honeycomb core (3M Scotch-Weld® 9323):

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It’s all about loads

I’ve been thinking about the layout of the spar in the center section and needed, thus, to consider the flight loads, again. About two years ago I started with an initial design of the spars, but for some reason I stopped and left the data and software in a lost and forgotten directory on my computer. Now it’s time get that programs up and running and look on the data again.

I started by sorting out the 24 cases that have to be considered to fulfill LTF-L (sorry for the German):

lastfaelle

A short translation of the table header: Number, V-n-Point, Speed, C.G. nose(k)/ tail heavy(s), Load factor, Elevon deflection left/right, Wind gust. Here’s a V-n diagram to fill that up with some graphical information:

Vn-final

A Horten is quiet different to a normal tailed airplane, and herewith I mean not only the appearance. The lift distribution is obviously another, as stabilty about the pitch and yaw axes is obtained by reducing lift at the tips (bell distribution). Moreover, in a tailed airplane the main wing can be considered to be fixed unless ailerons are deployed. In a Horten, any elevator deployment results in a change of lift distribution, and thus inevitably, in a change of loads.This has as a consequence that the maximum lift coefficient CL depends much stronger on the speed (trim position) and that in the above V-n diagram the curve between VS1 and VA is not simply a parabola. And not forget the wings have sweepback and taper…

This all makes life harder when it comes to calculating aerodynamic loads. Anyway, this is why we have computers. I fed it with the cases and let it calculate lift, drag, side-force, pitching- and all other moments. Here are the most interesing distributions, lift and pitching-moment (for the 24 cases listed above):

load torsion

The pitching moment uses the C.G. as a reference and needs to be transformed to the shear center to get the shear stresses in the shear-web and D-tube. This center depends on the spar layout, needed also to calculate the axial loads in the spar caps. I have the software ready, but I have to re-check the layout before…