28 January, 2007

Wing Skinned

I finished skinning the SGB88 wing today. I put the joiner rod in for a test fit and snapped this picture. Keep in mind that the left wing doesn't have a LE, TE or a wingtip on yet, so this is close, but not all the way to 88 inches. the left and right hand wings were built a little differently, which is why one has a joint and the other doesn't.

I used Dow Blue foam for the wing. It has a carbon fiber ply under the balsa skin, and a carbon joiner sleeve at the root, along with ply bolt blocks and a glass reinforcement near the root. Here you can see the carbon going on the wing core prior to the balsa skin. You can also see the blue painted stab in the background.

Here is a picture of my first cut at the fuselage outline and the stab/fin joiner (25% scale). The fuse is simply 1/8 ply bulkheads with 1/16 balsa skins that I will glass with 4 oz cloth.

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.

21 January, 2007

Omei and Ascent

Over the last few months, I have added an Omei 2000 and an E-Flite Ascent to my collection. While both are ARFs, they are certainly fun to fly. I am using the Omei to check out my Futaba R149, and have had no problems thus far. It is powered by an RC Sport Evo 1480 with 40A ESC. I use a separate 1400 mAh pack for the Rx and servos, with the BEC feature disabled.

Since I have done little to the kit - apart from glassing the wing joint - I fly it like a TD ship. It floats reasonably well, and penetrates very well. Using a 2100 mAh motor pack, I was able to record a 60:31 flight (12 minutes of motor time) last December.

The Ascent has been modified from stock with an E-Flite 400 outrunner and uses a 3S 1320 Thunder Power pack for both the motor and the two SM15 servos (with an RS6UL Rx). Turning a Graupner 10x8 CAM folder, it climbs vertically, and has achieved glide:power ratios of 17:1. With some spoilers for spot landing, it could do well in an F5J competition. Given that it went together in less than 2 hours and cost $70, I'm pretty happy with that.

Reflection on my Reflection

Last summer, I had a good time flying a Great Planes Reflection Flat Out around the front yard. It was a cheap way to tinker around with 3D, and a nice way to spend some time outside on a summer day.

While my 3D flying skills never progressed much past the goof off stage, it was fun to try some of the cool moves.

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In fact, I just got off the treadmill...