I finally got around to finishing my 48" UFO build. It took longer that I thought because I had a few setbacks, but first let me describe the project.
After building 2 successful UFOs and looking at lots of plans for Mantas, bi-planes with disc wings, and other oddly shaped aircraft that could be reduced to a flying disc at the most basic level I decided I wanted to try something large.
I knew the wing loading could be low - maybe 3oz / sq ft - but how low was to be seen. My first step was to buy the sheet of 8 ft x 4 ft 1/2" Dow bluecor insulation at Lowes for $12. The plan was to make this a KFm2 with the step on top being a semi-circle of 48" diameter.
I cut the full 48" dia circle and the semi-circle quickly. After fitting them and doing some tests I realized that the weight of the back half caused it to flex and bow and bend. Like a cattail on the end of a reed has a lot of leverage when the wind blows.
Then I realized I needed a strong spar of some sort to hold the back half of the craft straight and level with the front half. I used a 3 ft 1/4" wooden dowel from Michaels for this. Melted a trench into the center panel all the way along the center thrust line and glued the dowel in with hot glue and gorilla glue. Didn't have to be neat, just straight down the center. It would be covered later with the fuselage.
After glueing the spar down the center it was time to cover the front half with the 1/2" KFm step. Used 3M Super 77 adhesive for this (as always) and got it lined up pretty nice. Don't worry too much, will trim and sand to perfection later.
Once I had the top step in place I test it for strength. Pretty good, no flex. BUT, it's not level across. The Dow foam baord had a little bit of bow in it. The best way t straighten it out at this point is to turn it over and glue another layer on the bottom so it can't bow. So I did. The bottom layer is 6mm FFF foam, not 1/2", because it's thinner and lighter. This thing already weighs about 1000g with no electronics!
Now it has turned into a KFm4 airfoil. No problem, though. I trim the layers evenly and then use a sanding block to round the leading edge 180 degrees around the circle. Didn't do the back half.
Did the electronics next. The heaviest motor I have is about 100g. The heaviest battery is a 2200mah 3S. I put them all up front and did a balance test. It was crazy tail heavy. Looking around the house for something heavy I found a bunch of bananas. I tried several bananas and found that 3 bananas + the electronics up front balanced it nicely. The bananas weighed close to 500g.
After cutting the palce for the electronics and getting the motor mounted, I moved down to the servos and elevons. I had to custom make some control rod guides for the large control rods and the deep foam. I ended up using some servo control arms drilled out glued vertically into the foam. This did a great job preventing the control rods from bowing.
There was another problem, however. The weight of the large elevons was too much for the first pair of servos. One stripped out farily quickly becuase when I would lift the UFO off the ground the elevons would drop and pull the servo arms. One too many times I guess.
I installed some larger servos and was more careful about lifting it after that. The elevons still drop and I can hear the servos moving but they have held up so far. Also, I glued some plywood pieces under the servo tabs and screwed the servos down instead of glue. This has eliminated the servo "wobble" that I noticed when they were glued into the foam pockets. These servos are mounted firmly and elevon movement is more precise (no slop).
The easiest part of the whole build was cutting the fuselage and glueing it on the craft. No problem at all with that...
After everything was installed and the UFO was ready for a maiden flight, I still had to balance it at 25% (1 ft) from the nose. My 3 bananas were black and soft so I had to find something else heavy and compact. Aha! I had some D-Cell batteries in a drawer. 4 of them placed in the battery tray and taped securely to the top balanced it out.
I would have preferred a heavier motor and maybe a 4000mah battery but I used what I had and made it work. Anyway, I ended up using a Turnigy EDF motor rated for 300W @ 60A. I only had a 40A ESC so I propped down to about 30A using my watt meter.
With the Turnigy motor, I used a 2200mah 3S lipo, 40A ESC, 11x4.7 APC style prop, and 2 HXT900 servos.
Here are some pictures and a short video. The hat cam stops recording a lot and I don't know why it does that. Sometimes I don't get any usable video at all from this thing.