Many pilots do not reach a complete understanding of what makes an airplane turn. Such an understanding is certainly worthwhile, since many accidents occur as a direct result of losing control of the airplane while in turning flight.
 In review, the airplane is capable of movement around the three axes. It can be pitched around the lateral axis, rolled around the longitudinal axis, and yawed around the vertical axis. Yawing around the vertical axis causes most misunderstanding about how and why an airplane turns. First, it should be kept in mind that the rudder does not turn the airplane in flight.

Although most pilots know that an airplane is banked to make a turn, few know the reason why. The answer is quite simple. The airplane must be banked because the same force (lift) that sustains the airplane in flight is used to make the airplane turn. The airplane is banked and back elevator pressure is applied. This changes the direction of lift and increases the angle of attack on the wings, which increases the lift. The increased lift pulls the airplane around the turn. The amount of back elevator pressure applied, and therefore the amount of lift, varies directly with the angle of bank used. As the angle of bank is steepened, the amount of back elevator pressure must be increased to hold altitude.
 

In level flight, the force of lift acts opposite to and exactly equal in magnitude to the force of gravity. Gravity tends to pull all bodies to the center of the Earth; therefore, this force always acts in a vertical plane with respect to the Earth. On the other hand, total lift always acts perpendicular to the relative wind, which for the purposes of this discussion is considered to be the same as acting perpendicular to the lateral axis of the wind. With the wings level, lift acts directly opposite to gravity. However, as the airplane is banked, gravity still acts in a vertical plane, but lift will now act in an inclined plane.  As illustrated in figure 1-40, the force of lift can be resolved into two components, vertical and horizontal. During the turn entry, the vertical component of lift still opposes gravity, and the horizontal component of lift must overcome apparent centrifugal force. Consequently, the total lift must be sufficient to counteract both of these forces.  The total resultant lift acts opposite to the total resultant load. So long as these opposing forces are equal to each other in magnitude, the airplane will maintain a constant rate of turn. If the pilot moves the controls in such a manner as to change the magnitude of any of the forces, the airplane will accelerate or decelerate in the direction of the applied force. This will result in changing the rate at which the airplane turns.

Figure 1-40.—Forces acting on an airplane in a turn.