Sesiones de aprendizaje

The turn indicator comprises of a horizontal spin axis gyro, supported in a gimbal ring, and is mounted with its plane of rotation acting along the fore and aft or roll axis (X – X1) of the aeroplane.

Chapter 17 Turn and Balance Indicator

Instrumentation

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It uses a rate gyro, and has freedom of movement in the rolling plane only. The rotor is either electrically driven, and includes a power failure warning flag, or is air driven. Both types of drive are structured to produce a low rotor speed of approximately 9000 rpm, because in level flight, the gyro axis is maintained in its horizontal position by an adjustable spiral spring.

The spring attaches between the gimbal and the instrument case. A pointer is also attached to the gimbal, and moves over a scale showing the aeroplane’s rate of turn, which is positioned adjacent to the zero datum mark, when the gyro is in its horizontal position (i.e. when the aeroplane is in level flight). A damping device, usually a piston cushioned by air in a cylinder, is fitted to the gimbal to ensure that the instrument reacts smoothly to changes in the rate of turn, and at the same time reacts to a definite turn rate without pointer oscillation.

When the aeroplane turns, the gyro precesses, thus tilting the rotor and gimbal ring until the precessing force matches the tension of the spring. At this point, the precession ceases, and the gyro remains inclined for the duration of the turn, giving an indication of the actual rate of turn, shown by the pointer’s position on the scale. When the aeroplane stops turning the gyro returns to its original horizontal position under the action of the spring.

Turn and Balance Indicator Chapter 17

OPERATION

For example, when an aeroplane enters a left turn, the gyro axis, which is rigid, opposes the turn, and a force is experienced about the vertical input axis.

The gimbal ring also turns with the aeroplane, but the resultant turning moment resists due to the rigidity of the gyroscope, which precesses about the longitudinal (X – X1) axis. During a left turn, a force applies at the front pivot of the gimbal ring, which is the same as applying a force at point F on the rotor rim. Due to primary precession, a subsequent force acts 90° later in the plane of rotation (i.e. at point P), causing the gimbal ring to tilt about the fore and aft axis. The pointer, connected to the gimbal ring, also moves, and in doing so indicates the direction of turn via reverse gearing. The rate of turn can also be established, since the force exerted by the spring is directly proportional to the amount of gimbal deflection.

During a left turn, the gyroscope, in precessing, stretches the spring until the force it exerts prevents further deflection of the gyro. As the gimbal ring deflects under the influence of force P, the stretched spring exerts a downward force where attached to the gimbal. This equates to a force pressing on the left-hand lower part of the gyro rotor (i.e. opposite to force P), and when precessed through 90°, produces a rotational force about the input axis, acting at point K on the rim. Force K acts in the same direction as the original turning force, F, known as Secondary Precession. When the rate of turn is established, force F reaches a constant value, and when force K reaches the same value, (i.e. the forces applied are equal and opposite), the gyro is unable to tilt any further). Force F is due to the rigidity of the gyro, and force K is a precessing force. The angle of tilt is therefore entirely dependent on the magnitude of force F, whilst the rate of turn is a function of gyro tilt.

The scale showing the rate of turn is calibrated in what are termed standard rates and, although seldom marked on the instrument, are classified by the numbers 1 to 4, corresponding to turn rates of 180°, 360°, 540°, and 720° per minute respectively. On commercial aeroplanes, the scale is normally only graduated to indicate rate one turns, since turns in excess of this rate, are not

Chapter 17 Turn and Balance Indicator

Instrumentation

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ERRORS

The turn indicator does not suffer from apparent wander because the spring prevents topple in the vertical plane, and drift in the horizontal plane is impossible due to the instruments construction. Mechanical or real wander is also normally negligible, providing that the spring tension is correctly adjusted.

Erroneous indications may however be caused if the rotor speed fluctuates too far from its normal operating rpm. If the instrument case of an air-driven gyro is not airtight, air draws into the case via the leaks, resulting in a loss of efficiency. This results in a reduction in the rotor speed and the pointer indicates a lesser rate of turn. Similarly, if the speed is too high, the pointer indicates a higher rate of turn than that flown. The most likely fault is if rotor speed falls below the design rpm, which results in both the gyro rigidity and the precessional forces reducing. Of these, the reduction in the precessional forces is the most important as they are no longer able to overcome the spring tension to the same degree. The turn indicator will, therefore, under read. In effect, the following rule is easy to remember and summarises this:

Under speed of the rotor under indicates the rate of turn.