Hehe, I love flying in vanilla
bf2 for this very reason. The choppers are soo easy to fly and responsive too.
The key to doing this little trick is to have either a highly sensitive mouse and/or your sensitivity set really high, for your chopper controls, in the options. That and a bit of practice and understanding of how the chopper handles.
Im not quite sure how the game engine handles this little trick, but in order to do it you have to have quite a bit of speed to begin with, then you bank sharply to either side (I prefer right since its easier for me to constantly move my mouse in that direction).
As you roll right drop your nose at the same time till you reach vertical (ideally your nose is pointed at the ground), and this is where the high sensitivity comes in, all it takes is a constant roll, or moving your mouse quickly and repetitively in the direction you want to rotate (the same direction that you dropped in from).
Its really just a barrel roll turned vertical (if you know how to do a barrel roll already). You just gotta make sure you keep the throttle up otherwise youll lose your speed and either drop straight into the ground, or end up upside down and at a low altitude.
At a less steep incline with this trick (really anwhere between 45 and about 80 degrees) you can pretty much keep it up circling forever over a fixed position. Ideal for and pesky ground troops and armor.