The USAF UFO: A Circle, A Rudder, A Blast

It's a Nutball without the polyhedral! It's a round Pizza Box Flyer! It's the USAF UFO!

I'm a big fan of the Pizza Box Flyer, but I'm not a fan of the pitch sensitivity. I wondered what it would be like to fly a round one, and I recalled that I'd seen something called the USAF UFO, by the always excellent GPW. Pardshaw, who runs the indespensible All Things Round thread at RC Groups, built one and loved it. I built mine along his recommendations:

• 24 inch diameter
• 1300 kv Blue Wonder (mine is 1500 kv)
• 500 Mah battery

Measure And Cut 

Making one of these is just about as easy as drawing and cutting a circle. I use Goldguy's method for building Nutballs: stick a nail in the middle of the foam, tie a string to it, run it out to a pen at your desired radius, wrap it a few times around said pen, and draw your circle. It's never perfect but it's always good enough.

Build Bigger Elevons!

Pardshaw recommended huge elevons, but what does he know? I scaled mine up from the 21 inch plan provided by GPW. I'd show Pardshaw!

It flew really sweet. It climbed at half throttle. It was very floaty, but with the throttle up it got fun. Which was the problem. The roll rate with those little elevons was slow, too slow to get out of hairy situations before smacking grass, and the whole reason you build something like this is to get into and out of hairy situations.

I cut the old elevons out and installed bigger ones. I measured 6 inches up from the rear, drew a line straight across (leaving a 2 inch strip for the tail) and cut new surfaces to fit. The throws are maybe 35 degrees and I've dialed in 40% expo. And the result? Amazing.

That said, when I was rolling this thing ten feet off the deck for the video I wished that I had even more elevon authority. Rolling at thirty feet isn't the same as rolling at ten. Pardshaw recommends that you cut them at an angle for more surface area, like so:

Bracing

I happen to have a heap of 6mm carbon fiber slats that I stole from my Dad's garage (we have that kind of relationship), so I used those for bracing. If you can't get these I imagine that bamboo skewers might be effective in conjunction with a balsa spar. Pardshaw uses pallet strapping. Any way you look at it, though, if you're using Model Plane Foam, EPP or Depron you'll want to brace your UFO. This thing pulls some G's and you don't want it going floppy. I haven't built one out of Dollar Tree Rediboard, and I suspect that it MIGHT be stiff enough withoug bracing on a 20 incher, but I'd brace it anyway.

Here is my bracing arrangement. It seems to work pretty well:

The Tail And Its History

GPW's plans call for a strange looking fuselage and tail. This is because the airplane is modeled after an image of a 1960's era flying saucer that the Air Force flew out of Area 51. Called the "Lenticular Reentry Vehicle" it was designed to bomb the USSR from space. This was all declassified in the late 1990's. Personally I was hoping for the "Little Green Men" theory to be true, but that's the way it goes. Oh well. Here's a link to Wikipedia about the LRV program, and here's a link to a picture of one of these things in flight.

My bird's tail is 12 inches long and 10 inches tall. The rudder is 4 inches wide and the fuse is 1.75 inches tall. When you glue the tail down make sure it extends BEYOND the airfoil about 1 inch. Pardshaw says that these flat discs and squares fly best when the vertical stabalizer extends beyond the wing. I braced mine with barbecue skewers glued to the leading and trailing edges of the vertical stabalizer.

Keep in mind that these are my own custom measurements for a 24 inch bird.

Thrust Angle?

Nope. I just mounted my Blue Wonder straight. Seems to be fine. 

The Guts

The following produces a 10 ounce plane all up weight, including battery:

Model Plane Foam, A-grade

1500 kv Blue Wonder Motor

Orange Rx

18 Amp ESC

2 x 9 gram servos

500 mah 3 cell battery

My Dad's "Pessimistic Motor Mount" (patent pending... you could order an FT Elements Simple Firewall)

If you don't want to work this hard you could always get the FT Power Pack from Lazertoyz.

MAKE SURE YOU BALANCE IT AT 25% OF CHORD TO START. These things do not like to be tail heavy. On a 24 inch bird the balance point is 6 inches from the leading edge.

Parting Shots

It has all the benefits of the PBF without so much pitch sensitivity. It's smooth, in a dive or a climb, but it still loops in almost it's own length. It's just as wild as a PBF but it's easier to fly. I love it. Don't think I'm going back.

Pardshaw raves about it, and I think I recall Goldguy saying that "next to a UFO, the PBF" provides the most fun-per-buck. I think I agree.

That huge tail is odd looking but MAN does it keep this thing straight, even in wind. I bet the Piyak is a hoot (it's this thing with a full length fuselage and a functioning rudder). 

The only problem is that when it turns to profile the wing dissapears. All you see is tail. It's a little disconcerting if you're far away.

Very pleased. It's my new favorite. Slap it together before lunch and fly all afternoon.

Update

Here is a 28 inch UFO powered by a Turnigy 2822/14 1450 KV motor and a 1300 mah battery. It's the second twenty-eight incher I made. The first had a standard vertical stabilizer made from 1 layer of Model Plane Foam. It flew great but the tail ripped off in a high speed dive and that was that. This version's tail is 6 inches tall and 15 inches long and made from a double layer. It flies fantastic, but it isn't as "floaty," possibly because there is less drag on the airframe. Anyway this is the maiden flight:

Original Build Thread by GPW