How I understand a flat (rudder-only) turn:
- AOA between relative wind and the fuselage of the airplane causes a sideslip in the direction of yaw.
- Vertical stabilizer weathervanes into the relative wind to cause a turn.
The only difference I see between a coordinated turn and flat turn is that the horizontal lift component comes from the wing in a coordinated turn but from the fuselage "airfoil" in a flat turn.
Why is the flat turn less efficient, if both methods of turning work the same in principle?
Picture credit: Pilot Effect (YouTube)