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.