I've seen way too many people during the closed and open beta as well as post release talking about the chopper controls being impossible and clunky.
Without dragging anything out about this, just follow the below instructions:
When at a reasonable altitude as to not run into buildings or trees, hold down the (W) and (Shift) key. This will cause the chopper to start going forward nose down. As you gain more speed, the nose will continue to dip until you are propelling into the earth.
As soon as your chopper starts heading towards the ground, just let go of (W). Continue to hold shift. Your chopper will even out and fly perfectly forward, as if on autopilot.
As you continue to hold down shift, if you want to raise your chopper, just hold (S). If you want to go lower, hold (W). When you have made your corrections on altitude, just let go of the (S) or (W) key while continuing to hold (Shift), and you will continue on like normal.
If you want to be a badass and swerve left or right around buildings or trees, you can do so easily without losing speed or control of forward motion. Just hold shift and turn left of right. Once you decrease your speed enough, you will notice you can't hold shift and go forward anymore, the chopper only goes straight up. Repeat step 1.
After learning HOW they set up the controls it makes perfect sense. Flying is easier than ever while maintaining a lot of control over the chopper. It's not Arma 3 controls but that doesn't mean it's bad. I hope this guide helped you.