An airplane, like any moving object, requires a sideward force to make it turn. In a normal turn, this force is supplied by banking the airplane so that lift is exerted inward as well as upward. The force of lift is thus separated into two components at right angles to each other (Fig. 3-5). The lift acting upward and opposing weight is called the vertical lift component. The lift acting horizontally and opposing inertia or centrifugal force is called the horizontal lift component. Thus the horizontal lift component is the sideward force that forces the airplane from straight flight and causes it to turn. The equal and opposite reaction to this sideward force is centrifugal force. If an airplane is not banked, no force is provided to make it turn unless the turn is skidded by rudder application. Likewise, if an airplane is banked, it will turn unless held on a constant heading with opposite rudder. Proper control technique assumes that an airplane is turned by banking, and that in a banking attitude it should be turning.
Banking an airplane in a level turn does not by itself produce a change in the amount of lift. However, the division of lift into horizontal and vertical components reduces the amount of lift supporting the weight of the airplane. Consequently, the reduced vertical component results in the loss of altitude unless the total lift is increased by (1) increasing the angle of attack of the wing, (2) increasing the airspeed, or (3) increasing the angle of attack and airspeed in combination. Assuming a level turn with no change in thrust, the angle of attack is increased by raising the nose until the vertical component of lift is equal to the weight. The greater the angle of bank, the weaker is the vertical lift component, and the greater is the angle of attack for the lift/weight balance necessary to maintain a level turn.