In aviation, the turn and balance indicator, aka Turn and Slip indicator, and the turn coordinator variant are essentially two aircraft flight instrument in the one device, A rate of turn indicator that displays the rate that the aircraft heading is changing and the balance indicator or slip indicator that displays the (slip or skid) of the turn.
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The turn and balance indicator is often called under various names interchangeably such as the turn and slip indicator, or turn and bank indicator While the turn co-ordinator has a different working mechanism.
Although sometimes called the turn and bank indicator this instrument does not give bank information.
The turn indicator is a gyroscopic instrument and works off the principle of precession. The gyro is mounted in a gimbal with the gyros rotational axis in-line with the the lateral (pitch) axis of the aircraft and the gimbal with limited freedom around the longitudinal(roll) axis of the aircraft.
As the aircraft yaws this applies a torque force to the gyro around the vertical axis and due to precession the torque force moves 90degrees in the direction the gyro is spinning, Thus the gyro and gimbal will rotate with limited freedom against a calibrated spring. This torque force against the spring will reach equilibrium and the angle that the gimbal and gyro rest at is connected to the display needle displaying the rate of turn.
The hash marks, sometimes called "dog houses" on some makes of instruments because of their shape, With the needle lined up against them generally show a 'standard rate turn'. The standard rate (Rate One) for most airplanes is three degrees per second, or two minutes per 360 degrees (marked "2 min. turn"; some turn-and-slip indicators used in faster aircraft like the Concorde are marked "4 min. turn", while gliders are usually set to one minute turns). Under instrument flight rules, using these figures "timed turns" can be made in order to conform with the required Air Traffic patterns. For a change of heading of 90 degrees, a rate one turn for 30 seconds is required.
The balance indicator, which in earlier aircraft was a pendulum with a dashpot for damping, is nowadays usually a ball sealed inside a liquid filled curved glass tube, With the liquid acting as a damping medium. This ball due to its inertia and the force of gravity gives an indication of whether the aircraft is slipping, skidding or in balanced flight. With the ball centred in the middle of the tube. The aircraft is said to be in balanced flight, if in the inside(wing down side) of the turn the aircraft is slipping and if on the outside(wing up side) of the turn is said to be skidding.
A simple alternative to the balance indicator is a yaw string.
The turn coordinator is a development of the turn and balance indicator with the major difference being the display and and the axis the gimbal is mounted in. The display is that of a miniature airplane as seen from behind as this looks similar to that of an attitude indicator "NO PITCH INFORMATION" is usually written on the instrument To avoid confusion.
The gimbal pitched up 30 degrees from from the lateral axis, This causes the instrument to respond to roll as well as yaw. Thus the instrument will display a change sooner as it will react to the change in roll before the aircraft has started to to yaw. Although this instrument reacts to changes in roll it does not display the roll attitude.
The turn coordinator should be used as a performance instrument when the attitude indicator has failed. Called "partial panel" operations, this can be unnecessarily difficult or even impossible if either the pilot does not understand that the instrument is showing roll rates at as well as turn rates, or the internal dashpot is worn out. In the latter case the instrument is said to be underdamped; in turbulence it will indicate large full-scale deflections to the left and right, all of which are roll rate responses. In this condition it may not be possible for the pilot to maintain control of the aircraft in partial-panel operations in instrument meteorological conditions. For this and other reasons many highly experienced pilots prefer the "older" turn and bank indicator design.
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