Turn_and_bank_indicator

Turn and bank indicator

[turn-uhn-bangk]

In aviation, the turn and bank indicator shows the rate of turn and the coordination (slip or skid) of the turn. The rate of turn is indicated from a rate gyroscopically and the coordination of the turn is shown by either a pendulum or a heavy ball mounted in a curved sealed glass tube. No pitch information is provided.

A more correct term for this instrument is Turn and Slip because it is not the aircraft angle of bank that is sensed. For this an Artificial Horizon (AH) instrument or equivalent is required.

The rate of turn is the rate at which the airplane is changing its heading, sensed by a rate gyro held in gymbals and restrained by springs. Tick marks, sometimes called "dog houses" on some makes of instruments because of their shape, 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. In cloud, 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 ball or pendulum indicator senses side-force at the instrument. This detects whether the aircraft is side-slipping during a turn. In a properly co-ordinated turn, no side-slip should be present because it causes extra aerodynamic drag. This is because side-slip is when the airflow is at a lateral (sideways) angle to the centre-line of the fuelage. Such a lateral airflow angle is conventionally known as "beta", compared to the angle of the wing chord to the airflow which is known as "alpha", the "angle of attack".

In slow speed aircraft such as gliders and helicopters, a yaw string can be fitted on the outside of the cockpit canopy. This senses the sideslip (beta) angle directly and the pilot can make corrections to "keep the string in the middle". It often consists of a piece of wool 3 or 4 inches (8 or 10 centimetres) long. The yaw string serves the same purpose as the ball in the turn and slip indicator except that it is more sensitive and removes the need to look at the instrument panel.

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