An aircraft requires a sideward force to make it turn. In a normal turn, this force is supplied by banking the aircraft so that lift is exerted inward, as well as upward, as shown in the figure below. The force of lift during a turn is separated into two components at right angles to each other. One component, which acts vertically and opposite to the weight, is called the “vertical component of lift.” The other, which acts horizontally toward the center of the turn, is called the “horizontal component of lift” or centripetal force.
The horizontal component of lift is the force that pulls the aircraft from a straight flight path to make it turn. Centrifugal force is the “equal and opposite reaction” of the aircraft to the change in direction and acts equal and opposite to the horizontal component of lift. This explains why, in a correctly executed turn, the force that turns the aircraft is not supplied by the rudder. The rudder is used to correct any deviation between the straight track of the nose and tail of the aircraft into the relative wind. A good turn is one in which the nose and tail of the aircraft track along the same path. If no rudder is used in a turn, the nose of the aircraft yaws to the outside of the turn. The rudder is used rolling into the turn to bring the nose back in line with the relative wind. Once in the turn, the rudder should not be needed.
Banking the aircraft into a turn produces no change in the total amount of lift developed. Since the lift during the bank is divided into vertical and horizontal components, the amount of lift opposing gravity and supporting the aircraft’s weight is reduced. Consequently, the aircraft loses altitude unless additional lift is created. This is done by increasing the angle of attack until the vertical component of lift is again equal to the weight. An important fact for pilots to remember when making constant altitude turns is that the vertical component of lift must be equal to the weight to maintain altitude.
The rate of turn is a measure of how fast an aircraft makes a heading change. It is measured in degrees per second. At a given airspeed, the rate at which an aircraft turns depends upon the magnitude of the horizontal component of lift. As the angle of bank is increased, the horizontal component of lift increases, thereby increasing the rate of turn. Consequently, at any given airspeed, the rate of turn can be controlled by adjusting the angle of bank. However, to provide a vertical component of lift sufficient to hold altitude in a level turn, an increase in the angle of attack is required. Since the drag of the airfoil is directly proportional to its angle of attack, induced drag increases as the lift is increased. This, in turn, causes a loss of airspeed in proportion to the angle of bank. A small angle of bank results in a small reduction in airspeed while a large angle of bank results in a large reduction in airspeed. Additional thrust must be applied to prevent a reduction in airspeed in level turns. The required amount of additional thrust is proportional to the angle of bank.
If the bank angle is held constant and the airspeed is increased, the radius of the turn increases. A higher airspeed causes the aircraft to travel through a longer arc due to a greater speed. An aircraft travelling at 120 knots is able to turn a 360° circle in a tighter radius than an aircraft travelling at 240 knots. In order to compensate for the increase in airspeed, the bank angle would need to be increased. Think about a bike, the faster it goes, the more difficult it is to make a sharp turn.
In a slipping turn, the aircraft is not turning at the rate appropriate to the bank being used, since the aircraft is yawed toward the outside of the turning flight path. The aircraft is banked too much for the rate of turn, so the horizontal lift component is greater than the centrifugal force. Equilibrium between the horizontal lift component and centrifugal force is reestablished by either decreasing the bank, increasing the rate of turn, or a combination of the two changes.
A skidding turn results when the centrifugal force is greater than the horizontal lift component, pulling the aircraft toward the outside of the turn. Correction of a skidding turn thus involves a reduction in the rate of turn, an increase in bank angle, or a combination of the two.
From a practical point of view, it is often easiest to remember to “step on the ball”. For example, if during a right turn, the ball in the inclinometer is displaced to the right, the pilot must apply more right rudder. If during a right turn, the ball in the inclinometer is displaced to the left, the pilot is applying too much right rudder, therefore the pilot must reduce the total amount of right rudder being applied.