What you're looking for is a flight controller with open source firmware. You have several options for this, but Arducopter and MultiWii are the most common. I prefer arducopters, but it's more of a 'PC versus Mac' debate: there are upsides and downsides to both, and in the end, both do a pretty good job. They're basically Arduino sketches loaded on an ATMEGA, so customizing it is pretty straightforward.
I would highly recommend building it yourself, not buying a ready-made quad. It's cheaper in the long run, (search 'DJI phantom replacement parts' for instance), more fun and you'll learn more. I don't think you can write a decent stability algorithm without knowing exactly what your hardware does. One word of 'warning' though: the people behind Arducopter/MultiWii really know what they're doing. If you're expecting to write a better stability algorithm than the current ones, well, you're in for quite a challenge. Having said that, if you choose to contribute to the open source projects: power to you!
For about USD250 you can assemble a decent quad that's: - Fun to fly - Has replaceable parts (a necessity, you WILL crash) - Is upgradeable (GPS for instance, or better ESC's, better motors, etc.) - Has an open source flight controller
You should be able to make one yourself for this amount. Take for instance a nice 250 size FPV quad: - ARF-kit: USD 100,- (Frame, motors, escs, wires, etc.) - HKPilot (APM based) flight controller: USD 71,- - Transmitter/Receiver: USD 54,- - Keychain camera - Ass. propellers (buy spares. You WILL crash!) - Batteries (3-cell lipo) and charger
Kit: http://www.hobbyking.com/hobbyking/store/__52863__HobbyKing_...
Flight controller: http://www.hobbyking.com/hobbyking/store/__51498__HobbyKing_...
Transmitter/receiver: http://www.hobbyking.com/hobbyking/store/__8992__Turnigy_9X_...
Regarding flight times: I have a slightly bigger quad (350 size). With gopro attached it weighs in about 1150 grams, and I get about 6-7 minutes of flying time with a 3-cell 2200mah lipo. I'm always experimenting on improving flight times, and I think I can squeeze a couple of minutes more out of it with a little tweaking. OTOH, 3-cell lipos are less than 15 bucks so I always bring 3 or 4 of them with me.
A couple of extra words if you're new to flying quads: - Start off with a light, cheap microquad. For instance the Cheerson CX-10. At 20 bucks it'll cost less than a wrecked frame and 2 props, and it will teach you how to fly these things. Plus, they're light so there's no damage when you crash them.
- Please, please be careful with the larger quads. Don't fly over people, don't fly over buildings, roads, etc. Not only is this dangerous, (+2 lbs travelling at 40mph with spinning blades...) you're ruining the hobby for everybody else when things go wrong. Same goes for cameras strapped to your quad: respect privacy.
- I've said it before but here it goes again: You. Will. Crash. Don't think 'I'll be extra careful' or (in my case) 'I'm a seasoned RC pilot, I think I'm able to fly this thing without crashing': you're wrong. I promise. You will crash, and you will break stuff. Not the end of the world, but get some spare props and a frame if you can.
- I've had nothing but excellent experiences with Chinese webshops like dx.com, banggood.com or miniinthebox.com. Sure, quality is less than European brands like Graupner, but it'll work. Your first car probably wasn't a brand new Audi, but a banged up '89 civic, right? It'll get the job done just fine.
One last thing: I don't know if you have a wife/husband, but be prepared to spend a lot of time tinkering. It's an addictive hobby, I can tell you...