At hobbyking.com you can find lots of cheap drone parts, including several flight controllers which are open source.

They are usually based on the Arduino platform.

There are several open source projects that support the flight controllers, multiwii was the first one (works with any arduino, and a wii nunchuck).

Several arduino-based flight controllers are now sold that contain a CPU and all the sensors you need.

Baseflight is based on multiwii, supports (at least one) flightcontroller with 32 bit arm processor (normal arduino is 8 bit AVR)

Cleanflight is another project, based on baseflight with support for more hardware.

I don't know of one where your phone is the controller though, but your probably could make that work through some telemetry solution (I know of some you can at least easily connect to Android phones)

In any case, you don't want your phone to do the actual flight controller calculations, that would be way too much lag, you need that in realtime, on the drone, so you will need a flight controller on the drone. (also, strapping the phone to the drone would be too heavy, unless you build a big drone, which is expensive)

You can then control the flight controller through a serial connection. The telemetry I mentioned is serial-over-radio.

Wlan could also work, there are small cheap wlan-chips you can buy from places like Adafruit and Sparkfun.

A problem with wlan though is the latency, but should be ok for a slow drone

I doubt you will find anything that works right out of the box though, it will take some programming of your own and lots of research.

100$ should be possible, for a small one, but it will not be easy to do. (as in, just buying a complete thing with everything and an SDK), you will need to buy it in parts and construct it yourself.

... or just wait, I'm sure someone will release something like you want within a year or two :P

I have seen lots of SDKs online for mobile to drone, and also lots of 'out of the box' drones use mobile phones for controlling them, instead of traditional joysticks (?). this enables them to be light / less expensive. So I doubt there is that much lag?

Those all have flight controllers.

You can let the phone replace the radio in an RC-system, and use it to send steering information, but you will need intelligence on the receiving end too.

Since a drone is an unstable flying machine, you need a flight controller to keep it level.

The flight controller constantly polls the sensors to calculate the orientation of the drone and adjusts the motor speed controllers.

Sensors is at least a gyro and an accelerometer, but can also include barometer, magnetometer, gps, and more.

Advanced drones, like the ones you see doing impressive stuff like playing tennis on youtube also use external sensors (cameras) to track its position.

You really want to react on the sensor information as fast as possible

To send that info to the phone to be processed and then back again would mean the information is too old, which might mean under or overcompensation and make it wobble out of control. Even the control loop on the flight controller needs to written with hard realtime requirements, which might make it troublesome to get a mobile to do it even if you strapped it to the drone, because of its multitasking OS.

Good thing though is you can get a very small and light flight controller for pretty cheap.

So, you need the flight controller to handle the finer points of keeping the drone in the air, then you can use whatever to send signals to the flight controller to tell it to go up/down left/right etc.

ARDrone uses wifi for it, but it is a slow drone. (which is a good idea if the pilot is unexperienced...)

For a racer drone, you also need as little lag as possible on the control signal, but I guess that is a special case :P

Good thing though is you can get a very small and light flight controller for pretty cheap.

Where? I've been looking for a PC to radio (more convenent to program than phones). I couldn't find one, only arduino projects that require you to build the radio from scratch.

I don't think we are talking about the same thing... Not sure what you are looking for. (disclaimer if the stuff below is totally useless to you...)

With flight controller, I mean the SoC + sensors mounted on the drone and used to fly it, that does not include communication with the flight controller.

These cost anything between <10 to >200 euro depending on features and brand (http://www.hobbyking.com/hobbyking/store/uh_listCategoriesAndProducts.asp?cwhl=XX&pc=1762&idCategory=1763)

To communicate with the flight controller from your PC, you could use something like this: http://www.hobbyking.com/hobbyking/store/__55560__HKPilot_Transceiver_Telemetry_Radio_Set_V2_915Mhz_.html

That's rs232-over-radio, not very high band width, but nice range and should be enough for sending control signal. (no video)

Other alternatives would be XBee (https://www.sparkfun.com/products/8742) or Wifi (https://www.sparkfun.com/products/13678), with higher band width, but shorter range.

Usually (as in normal hobby use) they are controlled by PPM-signals from an RC receiver though. I don't know of any way to send such signals with your PC, but I'm guessing it is easier to bypass it with one of the above things, at least for drone flight controllers, since they all have serial ports too.

(second disclaimer: These are not plug-and-play solutions, but will need some getting hands dirty with code and possibly a soldering iron :) )