Gyroscopic Instruments
Gyroscopic instruments may be driven either electrically or by vacuum. In most light aircraft the Turn Coordinator (TC) is electrically driven. Usually the Heading Indicator (HI) and Attitude Indicator (AI) are vacuum driven.Gyroscopic Principles
Any spinning object possesses gyroscopic characteristics. The central mechanism of the gyroscope is a wheel similar to a bicycle wheel. It's outer rim has a heavy mass. It rotates at high speed on very low friction bearings. When it is rotating normally, it resists changes in direction.
The gyroscope exhibits two predominant characteristics:
- Rigidity in Space
- Precession
Rigidity in Space
The gyroscope resists turning. When it is "gimbaled" ( free to move in a given direction) such that it is free to move either in 1, 2 or 3 dimensions, any surface such as an instrument dial attached to the gyro assembly will remain rigid in space even though the case of the gyro turns. The Attitude Indicator (AI) and the Heading Indicator (HI ) use this property of rigidity in space for their operation. The HI responds only to change of heading. The AI responds to both changes in Pitch and in Roll.
Precession
Precession is the deflection of a spinning wheel 90 ° to the plane of rotation when a deflective force is applied at the rim. If a force is applied the top of the rim (the plane of rotation), the precession (turn) will be 90° in the horizontal plane to the left. The Turn Coordinator (TC) uses this precession property. For example, then taxiing on the ground, the Turn Coordinator will move, with the small airplane in the instrument showing a bank, even though the aircraft is level. The banking of the small aircraft presentation indicates only that the aircraft is turning.The Vacuum System
The Attitude Indicator (AI) and the Heading Indicator (HI) n light aircraft are usually driven by a vacuum system. The principal components are shown below. Not shown are auxiliary devices such as valves, filters etc. A pump provides the vacuum to the AI and HI through a system of vacuum lines. A Vacuum Gauge is attached to the lines which gives the pilot an indication that adequate vacuum is being generated.
Heading Indicator (HI)
The Heading Indicator (HI) uses the principle of Rigidity In Space for it’s operation. The Gyro is mounted such that it registers changes around the vertical axis only; i.e. direction changes. The compass card attached to the gyro appears to the pilot as though it is turning. In reality, it and the attached gyro are remaining rigid in space, while the aircraft and case turn about the gyro.
The HI is not automatically synchronized with the magnetic compass. It must be set to the compass heading while level on the airport surface prior to take-off.
The HI gyroscope may precess in small amounts over time. Therefore, the HI should be checked against the compass in 15 minute intervals. The check should be done only while flying in straight, level and un-accelerated flight. If adjustment is required, the heading can be reset using the adjustment knob shown.
The compass card has letters for the cardinal headings N, E, S, and W. Each numbered interval is every 30 degrees. The graduations are further divided by the longer marks every 10 degrees, and intervening short marks at the 5 degree points.
A significant advantage of the HI over the magnetic compass is its steadiness in turbulence and various aircraft movements. As will be discussed later in the section on the magnetic compass, the compass can have several errors introduced during turns, acceleration and deceleration. The HI is unaffected by these maneuvers and by turbulence, and is a reliable instrument as long as the precession re-adjustment in made in timely fashion.
Some makes of HI’s may "tumble", losing their gyroscopic characteristics if subjected to more than 55 degrees of pitch or bank. In this condition, the heading card spins rapidly, and cannot be used for navigation until reset by the adjustment knob.
Attitude Indicator (AI)
The Attitude Indicator shows rotation about both the longitudinal axis to indicate the degree of bank, and about the lateral axis to indicate pitch (nose up, level or nose down). It utilizes the rigidity characteristic of the gyro. It is gimbaled to permit rotation about the lateral axis indicating pitch attitude, and about the longitudinal axis to indicate roll attitude.
The principal parts of interest to the pilot are:
- The miniature wings attached to the case remain parallel to the wings of the aircraft.
- The horizon bar which separates the top (light) and bottom (dark) halves of the ball
- The degree marks on the upper periphery of the dial. The first 3 on both sides of center are 10 degrees apart, then 60 degree bank marks, and 90 degree bank arks. Fifteen degrees of bank is called a standard rate turn.
The adjustment knob is used to adjust the wings up or down to align with the horizon bar. This allows adjustment to the height of the pilot. Preferably, the adjustment should be made when level on the ground.
When the wings are aligned with the horizon bar, the aircraft is in level flight. If the wings are above the horizon bar, the aircraft is in a climb. Wings below the horizon bar indicates a decent. The upper blue part of the ball represents the sky. The miniture airplane wings (fixed to the case) represent the wings of the aircraft. In the past, the instrument has been refered to as "an artificial horizon". When in a left turn, the blue portion of the ball will have rolled to the right, as tho you were looking at the horizon over the nose of the aircraft. In a right turn, the blue portion will have rolled to the left.
Turn and Slip Indicator
The instrument is comprised of two components.
- The turn needle is an electrically driven gyroscope which indicates the rate of turn. The marks (often called the doghouse) on either side of center represents a bank angle of 15 degrees. This is termed a Standard Rate Turn. The rate of turn is 3 degrees per second. It takes 2 minutes to turn 360 degrees.
- The glass level containing the black ball is called the Inclinometer. It provides the pilot with a measure of the Turn Quality. During both straight and level flight and during turns the ball should stay centered.
Turn Coordinator
The Turn Coordinator is similar to the Turn and Slip indicator. It is found in more modern aircraft. The main difference is in the presentation of the turn. A miniature airplane is used to show the bank instead of a needle.
There are 2 marks on each side. The upper ones indicate level flight when the wing align with them. The lower marks indicates a bank angle of 15 degrees, which produces a standard rate turn. When the aircraft turns right, the minature aircraft in the instrument indicated a right bank. When turning left, it indicates a left bank.
Turn Quality
The inclinometer in both the Turn and Slip Indicator and the Turn Coordinator measures the turn quality. As mentioned previously, when in a turn, part of the lift of the wing goes into "turning" the aircraft. This is called the Horizontal Component of Lift (HCL). This HCL is directed toward the center of the turn. Also, a force directed outward and away from the center of the turn exists. This is called Centrifugal Force (CF). For the turn to be coordinated these two opposing forces must be equal. When they are equal, the ball in the inclinometer will remain in the middle.
When too much rudder is applied, a skid results. The Centrifrugal Force is bigger than the Horizontal Component of Lift (HCL). This makes the ball go toward the outside of the inclinomenter (i.e. the ball in NOT in the middle).
When insufficient rudder is applied, a slip results. The Horizontal Component of Lift (HCL) is larger than the Centrifrugal Force (CF). The ball rolls toward the lower side( or inside) on the inclinometer.
The term step on the ball is often used as a memory aid in overcoming a slip or skid. In actuality, more rudder pressure (or less bank) must be applied in a slip. Less rudder pressure (or more bank) must be applied in a skid. In both cases correction must be made so that HCL = CF to keep the ball centered.
