Turn and Balance Indicators

The turn and balance indicator, previously known as the turn and slip indicator, is essentially two instruments in one casing, which provide separate indications on a common dial. A turn indicator displays the rate of, and direction of turn, using gyroscopic principles; and a balance indicator to show whether the aeroplane is performing a balanced or unbalanced turn (skidding or side-slipping). The dial presentation of a typical turn and balance indicator is shown below.

Principle of a Turn Indicator

The turn indicator comprises of a horizontal spin axis gyro, which is 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.

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 9,000 rpm, because in level flight, the gyro axis is maintained in its horizontal position by an adjustable spiral spring.

The spring is attached 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 additionally 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 will precess, thus tilting the rotor and gimbal ring until the precessing force is matched by the tension of the spring. At this point the precession will cease, and the gyro will remain inclined for the duration of the turn, giving an indication of the actual rate of turn, which is shown by the pointers position on the scale. When the aeroplane stops turning the gyro will return to its original horizontal position under the action of the spring.

Operation of the Turn Indicator

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

The gimbal ring will also turn with the aeroplane, but the resultant turning moment will be resisted due to the rigidity of the gyroscope, and will precess about the longitudinal (X - X1) axis. During a left turn a force will be applied 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 will act 90° later in the plane of rotation, i.e. at point P, and will cause the gimbal ring to tilt about the fore and aft axis.

The pointer, which is connected to the gimbal ring will also move, and in doing so will indicate 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, will stretch the spring until the force it exerts prevents further deflection of the gyro. As the gimbal ring is deflected under the influence of force P, the stretched spring will exert a downward force where it is attached to the gimbal.

This equates to a force pressing on the left-hand lower part of the gyro rotor, ie. opposite to force P, and when precessed through 90°, will produce 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.

This is known as Secondary Precession. When the rate of turn is established, force F will reach a constant value, and when force K reaches the same value, ie. the forces applied are equal and opposite, the gyro will be 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 normally performed in these types of aeroplanes. This is because the majority of passengers do not like to experience the acceleration forces imposed during tighter turns, and it would also subject the airframe to unnecessary high load factors.

A standard rate turn is defined to be three degrees per second. This is what ATC expects when you're on an instrument clearance. It is also called a two-minute turn, because at that rate it takes two minutes to make a complete 360° turn or Rate One Turn takes one minute to complete 180°.

Errors Associated with the Turn Indicator

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 has been 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 will be drawn into the case via the leaks, resulting in a loss of efficiency.

This will result in a reduction in the rotor speed and the pointer will indicate a lesser rate of turn; similarly, if the speed is too high, the pointer will indicate a higher rate of turn than that being flown.

The most likely fault is a rotor speed falls below the design RPM, which will result in both the gyro rigidity and the precessional forces being reduced. Of these, the reduction in the precessional forces is the most important as they will no longer be 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.

Pre-flight Check

If the indicator is air driven approximately five minutes should be allowed for the rotor to reach its operating rpm prior to taxiing. With the aeroplane still stationary on the ground the turn pointer should be aligned with the zero datum, but during taxiing for take-off the pointer should respond accordingly to left and right turns.

In most light aeroplanes applying hand pressure to one corner of the flight instrument panel will also enable the turn indicator to be checked. This is because the panel is normally fitted on shockproof mountings, and any movement results in the turn pointer indicating a momentary rate of turn.

Operation of the Balance Indicator

This part of the instrument uses a mechanical method to indicate that an aeroplane is correctly banked for a given rate of turn. It uses the force of gravity, which acts upon a black ball in a liquid filled glass tube, and maintains it in its true vertical position whilst the aeroplane is in straight and level flight, as shown below.

The liquid acts as a damping medium for the ball, and two expansion chambers are concealed behind the dial, to cater for temperature changes. The back of the tube is painted on the outside with fluorescent paint to provide a contrasting background for the black ball, and the whole assembly is firmly secured to the back of the dial by a bracket. The ball itself has weight, and is thus affected by aeroplane manoeuvres.

If the ball remains in the centre the turn is balanced, and no slip or skid is present, as shown in diagram (A) below.

Diagram (B) shows the aeroplane making a left turn at a certain angle of bank. During this manoeuvre the indicator case and scale will both move with the aeroplane. The ball is additionally subject to a centrifugal reaction, since the aeroplane is in a turn, which will force the ball away from the centre of the turn.

If the turn is however carried out with the correct angle of bank the two forces will be in balance, and the ball will remain in the zero position. Any increase in airspeed during the turn will increase both the bank angle and centrifugal force.

The ball will continue to remain in line with the resultant of the two forces, as long as the bank angle is correctly maintained.

If the angle of bank for a particular rate of turn is incorrect, for example the aeroplane is under banking, as shown in diagram (C); the aeroplane will tend to skid out of the turn. This will occur because the centrifugal force predominates, and the ball is displaced away from the zero towards the outside of the turn.

By comparison if the aeroplane is alternatively over-banked, ie. the angle of bank is excessive for the rate of turn, as shown in diagram (D), the aeroplane will tend to slip into the turn, since the force of gravity will now predominate, and the ball will move away from its zero position towards the inside of the turn.

If the aeroplane skids or sideslips, the turn is said to be unbalanced, and if the ball remains in the centre, the turn is said to be balanced.

Limitations and Errors Associated with the Balance Indicator

The balance indicator has no operational limitations, and is also not subject to any errors.

Pre-Flight Check

With the aeroplane on level ground the ball should be in its central (zero) position, but during any turns when taxiing, the ball will register a skid.

Electrically Driven Turn and Balance Indicators

The internal mechanism of a typical electrical driven variant is similar to that of an air driven variant, as shown on the next page.

In this type it is important prior to flight to ensure that the ‘OFF' flag has disappeared from view, and during taxiing, the needle should indicate a turn in the correct direction, and the ball should indicate a skid. The flag will come into view if the rotor is not at its operating RPM, ie. due to a power failure, and that the instrument is unreliable.

Indications of a Turn and Balance Indicator

The diagrams show the indications that a typical turn and balance indicator would be likely to show during different types of turn.

Turn Co-ordinator

A turn co-ordinator is a development of the turn and balance indicator and is used in place of such instruments in a number of small, general aviation aeroplanes. The primary difference is in the position of the precession axis of the rate gyroscope. In this instrument the gimbal is spring-restrained and mounted with its axis is at approximately 30° with respect to the aeroplane's fore-and aft axis.

This has the effect of making the gyroscope sensitive to movements about the aeroplanes roll and yaw axes, i.e. banking, as well as turning.

Principle of Operation

The turn co-ordinator integrates both the rate of roll and the rate of turn together. It shows the pilot what the aeroplane is actually doing, and not what it has done by indicating the two rates on a display like the one shown below.

The aeroplane symbol on the turn co-ordinator moves in the direction of turn or roll, which is unlike the artificial horizon, where the symbol is fixed to the instrument case and the horizon bar moves.

If the wing of the aeroplane is lowered, even very slightly, the turn co-ordinator will immediately show a deviation from straight and level flight, whereas the turn and balance indicator will show nothing until yaw is present.

The turn co-ordinator will therefore anticipate a change from straight and level flight, whilst a turn and balance indicator will measure a deviation, thus enabling the pilot to anticipate the resulting turn. The pilot can then simply control the turn at the required rate, which is indicated by the alignment of the aeroplane with the outer scale.

The gyroscope is usually powered by a DC motor, and rotates at 6,000 rpm, although some turn co-ordinators are powered by an AC motor, which is supplied from a solid state inverter that is housed within the instrument. The inclusion of Silicon fluid or a graphite plunger in a glass tube is also often used in the instrument design, to assist in the damping out of any gyroscopic movements.