Turn Indicator and Balance Ball
A complete description of this instrument can be found in NATOPS. In a practical
aerodynamic sense, the balance ball indicator (Figure 17) shows directly and
immediately the longitudinal alignment of the aircraft, nose to tail. This information
enables the pilot to detect and eliminate aircraft yaw by trimming the ball to the
center of the inclinometer (while holding the aircraft attitude constant on the attitude
gyro), thus maintaining balanced flight.
Use and Limitations. The turn needle (Figure 17) is a wing attitude crosscheck
instrument in constantrate turns. The needle is calibrated for a fourminute turn;
that is, if the aircraft is in a turn with one needlewidth deflection, it will take four
minutes for the aircraft to turn 360º. This means that a oneneedlewidth deflection
of the needle yields a turn of approximately 1½º per second. If a twoneedlewidth
deflection is maintained, the aircraft will turn twice as fast, or at 3º per second. At any
given airspeed, a specific angle of bank (AOB) may be set on the gyro, which will
give an exact needlewidth deflection if in balanced flight (ball centered). If in
unbalanced flight, we would have difficulty coordinating the gyro attitude and turn
indicator for a specific turning rate.
Similarly, if the wings were level and the ball displaced, a yaw (turn) develops in a
direction opposite from the displaced ball, adversely affecting heading control.
If the ball is displaced to the right or left, which will occur when a slip or skid takes
place, it indicates unbalanced flight. This condition may be corrected by application
of rudder pedal pressure on the side to which the ball is displaced. When the ball is
again centered, use rudder trim to relieve control pressure. During the trimming
operation, the pilot must ensure the attitude is controlled by observing the miniature
airplane on the gyro. If the attitude (nose and wing) is not held constant during the
rudder trimming operation, the aircraft nose will yaw as rudder pressure and/or trim is
applied and the wings will attempt to roll in the direction of the yawing moment.
Therefore, the pilot should trim the balance ball only when stabilizing the nose and
wing attitude on the gyro. Finally, check the ball position and repeat the trimming
steps if necessary.
1-10 INTRODUCTION TO BASIC INSTRUMENTS
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