CGS Ground School Technical The turn and slip indicator © Crown Copyright 2012 No Part of this presentation may be reproduced without the permission of the issuing authority. The views expressed in this presentation do not necessarily reflect the views or policy of the MOD.

The turn and slip indicator The turn and slip indicator is actually two instruments in one. The turn indicator is the top instrument which indicates the aircrafts rate of turn. The slip ball is the bottom instrument that resembles a curved spirit level tube and tells the pilot if the aircraft is in balance or not. L R

The turn indicator The turn indicator uses a rate gyro. It is made up of: A rotor that is spun at approximately 9000rpm by an internal motor. It is aligned with the aircrafts longitudinal axis and its rotational axis is the lateral axis. A cradle (gimbal) that is allowed to rotate about the longitudinal axis. A base that is fixed to the inside of the instrument case.

Gyroscopic principles All rotating objects behave in the same way due to gyroscopic principles. A basic gyroscope consists of a rotor that is suspended in two gimbals (an inner gimbal and an outer gimbal). If the rotor is spinning quickly enough, the whole assembly can be moved around, turned upside down or even thrown into the air and the rotor will remain upright. This is due to something called the conservation of angular momentum. It is this tendency to remain upright that is used in a turn indicator. As briefly mentioned before, the turn indicator is fitted with a rate gyro. This differs from a basic gyro because movement is only allowed along two of the three axes. The axis that is fixed is the one about which the turn indicator measures movement – the normal (vertical) axis.

Gyroscopic precession When a force is applied to a gyroscope, the resultant force will occur 90° removed in the direction of rotation. This is known as a precessing force. If the gyroscope is restricted in one of its axes, the rotor will tilt. SENDING A FIRST SOLO DISCUSS THE FOLLOWING

The turn indicator If the aircraft yaws or turns, the turn and slip indicator will move with the aircraft. The spinning rotor inside however will try to remain upright. This is not possible because the gyro’s movement around the normal axis is restricted. Due to the precession, the force trying to rotate the rotor about the normal axis acts at 90° removed in the direction of rotation. This resultant force, tilts the rotor, which subsequently turns the needle. The needle then indicates the direction and rate of turn. L R L R

Rate of turn The turn indicator in the Viking is calibrated to measure a rate 1 glider turn. This means that when the needle is deflected and aligned with the white marker, the aircraft will be turning at a rate of 540° per minute at 50kt (9° per second). In the Vigilant, it is calibrated to measure a standard rate 1 turn. (360° per minute). L R 1 MIN TURN

Calibration Calibration is achieved by controlling the movement of the gimbal around the longitudinal axis. This is done by attaching the gimbal to the base with calibrated springs and ensuring the rotor is spinning at the correct speed. If the springs are too tight or the rotor speed too slow, the turn indicator will indicate a rate of turn slower than the actual turn. If the springs are too slack or the rotor speed too fast, the turn indicator will indicate a turn much faster than the actual. L R 1 MIN TURN

The slip ball The slip ball is a heavy metal ball that is free to move around in a glass tube. Its movement is restricted by a thick fluid. When the aircraft is in straight balanced flight, the ball will sit between the ‘goalposts’ due to gravity. If the aircraft is put out of balance, the instrument will move with the aircraft, leaving the ball to follow the original flight path. In this example, the aircraft is yawed to the right. The instrument (being attached to the aircraft also moves right), inertia then sends the ball to the left. This indicates to the pilot to press the left rudder pedal to achieve balanced flight. L R 1 MIN TURN

Balanced turns If the aircraft is in a turn, the ball will tell the pilot whether the aircraft is skidding or slipping. This time, when in balance, the ball is kept in the middle of the goalposts due to gravity pulling it down and centripetal force pulling it out of the turn. When these two forces are balanced, the aircraft is in a balanced turn. L R 1 MIN TURN Centripetal force Gravity

Balanced turns An aircraft is said to be skidding when the rear of the aircraft is sliding out of the turn. This would be indicated by the needle and ball being on opposite sides. In this example, left rudder is required in a right turn to return to balanced flight. An aircraft is said to be slipping, when the rear of the aircraft is sliding in towards the centre of the turn. This would be indicated by the needle and ball on the same side. In this example right rudder would be required to achieve balanced flight. L R 1 MIN TURN

THE END Any Questions?