Illustration of the face of a turn-and-slip indicator
In aviation, the turn and bank indicator (or turn and slip 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.
Use
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 (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). In clouds, 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 detects whether the aircraft is side-slipping during a turn. In a properly co-ordinated turn, no sideslip should be present because having a non-zero sideslip angle causes extra aerodynamic drag.
Operation
Precession causes a force applied to a spinning wheel to be felt 90° from the point of application in the direction of rotation.
Rate instruments such as the turn-and-slip indicator operate on gyroscopic precession. Precession is the characteristic of a gyroscope that causes an applied force to produce a movement, not at the point of application, but at a point 90° from the point of application in the direction of rotation.
The turn indicator is a small gyroscope spun either by air or by an electric motor. The gyro is mounted in a single gimbal with its spin axis parallel to the lateral axis of the aircraft and the axis of the gimbal parallel with the longitudinal axis. When the aircraft yaws, or rotates about its vertical axis (aircraft), it produces a force in the horizontal plane that, due to precession, causes the gyro and its gimbal to rotate about the gimbal axis. It is restrained in this rotation plane by a calibration spring; it rolls over just enough to cause the pointer to deflect until it aligns with one of the doghouse shaped marks on the dial, when the aircraft is making a standard-rate turn.
The ball or pendulum is a "balance indicator" and is called an inclinometer.It is used to display if a turn is in balance and it does this by measuring the relative strength of the force of gravity and the force of inertia caused by a turn. The ball is placed inside a curved glass tube that is partially filled with a liquid, much like the fluid used in a compass. When the aircraft is flying straight-and-level, there is no inertia acting on the ball, and it remains in the center of the tube between two wires.
Variations
A Taylorcraft turn and bank Indicator
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.
The major limitation of the turn-and-slip indicator is that it senses rotation only about the vertical axis of the aircraft. It tells nothing of the rotation around the longitudinal axis, which in normal flight occurs before the aircraft begins to turn. A turn coordinator overcomes this problem by having its gimbal frame angled upward about 30° from the longitudinal axis of the aircraft.
See also
References
