| Gauss |
Yes, it is a free action and Flyby Attack does not have a straight line requirement like Ride-By Attack (via Charge) does.
Basically, all Flyby Attack does is gives you a standard action at any point during a move.
So, if you want to fly in, attack, and then do a 180degree turn (burning 10' movement) and fly out you may do this using just Flyby Attack and the normal rules for flying.
What Wingover does is it allows you to spend a free action to do a 180degree turn without making a Fly check or burning 10 feet of your movement to do it.
- Gauss