Hi , i want to build a quadcopter but i can't figure out some issues. First , i know that Ardupilot or Multiwii based on Arduino. And they both have good ground station softwares. I have an Arduino Mega , so i need to know , can i use my arduino instead of ardupilot hardware? (of course with necessary sensor attachments)
I plug my arduino , and apm software flashed it's firmware like it is an Ardupilot. And it didn't send me any errors. So i felt like i can do it with arduino 
Thanks for your help.
You can use any arduino you like. Just use an accelerometer or gyroscope for the equilibrium. You just need to write a good program. I am doing the same at the moment. I will make my own PCB with an atmega328P.
I use a 2.4GHz receiver you find in model planes, and with the PulseIn() I can read out the servo signals and use them for the control.
I am building my program with a kind of fly-by-wire system. With my transmitter I can set the equilibrium state for the X and Y axis of the gyroscope. And the arduino steers the engines to meet these states. If I move the joystick forward, the Equilibrium state will shift and the Qcopter will tilt forward and move forward. The drawback of this is that I cant make saltos or other stunts, but I think it is a reasable safe way to do this.