But here's another UAV approach, that's quite interesting,
and has high potential for DIY UAV hobby experiments:

The AirBurr Project: Robust Flight in Cluttered Indoor Environments
... http://lis.epfl.ch/airburr/
Check out this video:
... http://www.youtube.com/watch?v=GgdI0oiPY0s

Instead of a quadcopter's 4 motors, it's a "flying egg", with only 2
facing concentric prop motors, and a pair of servos running an X-vane.

This gives you a number of advantages over quadcopters:
- 2 vs 4 prop motors - Cheaper, and if the other hardware
is light enough it MAY have more flying time.
- Simpler control algorithms (but, code is "weightless" AND a one-time cost...)
- Geometry & CG gives higher inherent stability (it's "pendulum-like")
- Uses identical standard props, vs having to find matched LH/RH pairs.
- Doesn't require as many sensors on board, JUST to fly.
- Resilient to collisions, with a self-righting ability.
(If it is grounded, it just rights itself and takes off again! :-)

Disadvantage:
- Thrust is on centerline, so IF you intend to lift something
you have to think hard about airflow. Quadrotors may be a better
choice for dangling things below it, like cameras or object
pick-n-place contests.  But as a simple flying camera-droid UAV
(vs a "lifter"), IMHO this "flying egg" is a great choice.

Question - Do you guys think this would be cheaper & easier to
make, and/or longer flying than a quadcopter?  (esp with something
like the cheap $5/pair flyweight micro-servos Paul H once found.)  

*** PAUL - Do you still have a link for those $5/pair micro-servos?

I'm not sure, but control is so much simpler and stability is inherent,
you may even be able to get away without an onboard computer
and sensor suite, and just use a basic R/C rig with servo mixing to fly it.

Bumper cage - Try my recycled windshield wiper stainless
steel strips. (FREE, in every auto supply's outdoor trash bins,
the day AFTER a rain. :-) Yea, they're not as light as carbon fiber,
but they're a whole lot cheaper!

QUESTION: How do you easily weld stainless steel strips together?
What's the proper welding tech? (A DIY cap discharge spot welder?)

Hmmm.. If you make the bumper egg or teardrop-shaped, and
get the CG right, you MAY be able to get away without the
legs, by making it act like a "Weeble" toy and be self-righting:
... http://en.wikipedia.org/wiki/Weeble

So... Thoughts?  IF you ever wanted to attempt a UAV copter,
WOULD this be a better or cheaper choice than a quadcopter?

How would YOU make a UAV?

- Keith Mc.