The duct elbow consists of four parts.
Angle between oblique face and longitudinal axis of each part is
B = 15 deg.
Angle A between axes of two end parts is adjustable.
Turn the brown part 180 deg. to get A of 30 deg.
Then turn orange part 180 deg. to get A of 60 deg.
Then turn green part 180 deg. to get max A of 90 deg.
In general: max A = k.2.B
Where k is number of movable parts.