Examining the Turn Indicator!
The turn indicator serves to indicate when the aircraft is in a turn.
The turn indicator is a rate gyro that shows the rate at which an aircraft is moved along its vertical axis or "axis of turn". The
turn indicator contains a vertically spinning gyro which rotates around a horizontal axis. If you examine the illustration above, you see
that the gyro is contained within a fixture that is aligned horizontally with the aircraft's longitudinal axis. The gyro is free to tilt
left and right. Let's examine how the turn indicator actually depicts the rate of turn.
When the aircraft's nose is moved to the left, the force is applied to right front of the gyro. Due to gyroscopic precession that we discussed earlier, the force acts 90 degrees from the point of application in the direction of rotation. This force then tilts the top of the gyro to the right. Examine the illustration below. Since the gyro tilts in the opposite direction of turn, if the indicating needle was fixed directly to the gyro, we would receive a false direction of turn. If we turn left, we don't want the instrument telling us that we are turning right. In order to remedy this, a redirecting mechanism is needed.
The greater the turning force applied to the gyro, the greater the tilt of the gyro will be. The gyro's tilt is inhibited by the springs that are attached to the gimbal.
The springs are calibrated with the markings on the turn indicator's face. When the aircraft turns, a force is applied to the gyro. The faster the aircraft turns, the greater the
force applied to the gyro. When the indicating needle aligns with the one of the two off-set marks on the indicator face, the aircraft is established in a"Standard Rate Turn".
A standard rate turn is defined as a turning rate of 3 degrees per second. If you enter a standard rate turn to the right, it will take two minutes to arrive back at your entry heading. 360 degrees divided
by 3 degrees per second equals 120 seconds or 2 minutes. One thing to remember about the turn indicator is that it is not a bank indicator. This instrument only shows when the aircraft is turning. You can
bank the aircraft in one direction and apply opposite rudder to counteract the turning force and the turn indicator will indicate nothing but a vertical needle as long as the aircraft does not turn.