The duct elbow consists of two parts.
Angle between oblique face and longitudinal axis of each part is
Angle A between axes of two parts is adjustable.
Turn the right part 180 deg. to get max A = 2B.
At the connection place, pipe section is an ellipse that reduces flow area. The less angle B is, the more flow area is.
So the elbow with more parts (3, 4, …) is recommended.