22 January, 2007

Designing the SGB88

That carbon arrow shaft really inspired me. It inspired me to throw out all sorts of methods and techniques and try a whole bunch of new stuff. I decided to call this project the SGB88.

The first new thing that I tried was to use AVL to design the wing. I looked around and picked a planform that I liked, and then tinkered with a few combinations of sweep, panel spacing, dihedral and airfoil until I got one that had both a good L/D and power factor. I stuck with an 88" span for no good reason, other than it was somewhere between 2 and 2.5 meters.

I picked an arbitrary design load case to work towards. I reasoned that I might have to perform a 10 foot radius pullout at 35 mph (works out to 4.2g). I used this to generate the bending and shear loads for the wing. You can see a pretty load plot from AVL above, and the even more useful plot of bending moment and shear as a function of span.

From this and some additional calculations to look at load in the spar caps and deflection during this maneuver, I decided that a built up wing simply wasn't going to cut it. I had hoped to keep the total aircraft weight under 24 oz (with 8 oz for powertrain and LiPo), but I was faced with the real possibility of a wing fold in normal flight.

Given that I ended up deciding to use Drela's AG34-36 set, I made some templates and cut a set of cores for the right wing. I then epoxied carbon fiber and balsa on each skin, and joined the three sub sections together. With the inclusion of a carbon joiner sleeve (aluminum rod joiner), two servos, wiring and flaps/ailerons, the right wing weighs just under 8 ounces, but is very solid. So much for the 24 ounce airplane, but now I will try to shoot for 32.

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