Question: I read somewhere that when the aircraft banks to turn, its angle of attack increases.Since angle of attack is the angle between the relative wind and chord line, how does turning increase it? Is it because the relative wind is now from different direction or is it because the wing that goes higher meets the air at a higher angle?

Answer: 
The aircraft turns to change its speed vector. The absolute value of the vector stays the same, but its direction is changed. A force is needed to change the direction of movement of any mass, and the wing is used to provide this force in addition to the lift force. This is why lift is increased in a turn.

[![Actual and desired speed in a turn](https://i.stack.imgur.com/qV14B.png)](https://i.stack.imgur.com/qV14B.png)

For turning, the aircraft needs to add a force in the direction of the red vector in the sketch above. This it can either do by pointing its nose to the right, and then the fuselage will create a small lateral force, albeit at a heavy price in drag. Or the airplane uses its wing to provide this force, which is a much cleverer way of creating the desired force because the wing is by far the most efficient "force generator" of an aircraft.

[![Front view of forces on turning aircraft](https://i.stack.imgur.com/Vv0pr.png)](https://i.stack.imgur.com/Vv0pr.png)

Note that as the turn progresses, the red vector will point in the opposite direction of the initial (blue) speed vector, which means that the speed in the initial direction is reduced to zero when a 90° turn is completed. The horizontal component of the lift force accelerates the aircraft in the new, desired direction and decelerates it in the old direction.

Lift can be increased either by speeding up, by flying in denser air or by increasing the angle of attack. The first two options are not very practical, so it is the angle of attack change which gives the aircraft the additional lift for turning. This angle of attack increase is controlled by pulling the stick gently back, thereby reducing lift on the horizontal tail so the aircraft can pitch up.

If you need more formulas to calculate the precise angle of attack change, look at [this answer](https://aviation.stackexchange.com/questions/8012/what-does-it-take-to-turn-a-747-around-180-degrees/8014#8014) for flight mechanics in banked flight and [this answer](https://aviation.stackexchange.com/questions/14508/what-is-the-method-to-calculate-a-finite-wings-lift-from-its-sectional-airfoil/14559#14559) for how lift depends on angle of attack.
The question itself is open ended but, in the simplest terms, it depends on how you execute the turn - are you flying a constant airspeed or a constant angle of attack?

In either instance, in a level turn, the load factor will increase with the inverse cosine of the bank angle.

If flying a constant airspeed turn, then an increase in lift coefficient will command a higher angle of attack as dynamic pressure remains constant: CL-turn = CL-wings level (L-turn/L-wings level)

If flying a constant angle of attack turn (i.e., same angle of attack in the bank as that used in level flight), the dynamic pressure has to increase to support the higher wing loading at a constant lift coefficient. Since dynamic pressure is a function of the velocity squared, the velocity commanded to maintain level flight within a banking turn varies with the square root of the load factor: V-turn = V-wings level x (load factor)^1/2 ...all other factors being equal.