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Note for Aircraft Drawing Engineering - ADE By chetan g

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AERONAUTICAL INSTRUMENTATION ****MODULE4 MODULE 4 GYROSCOPIC FLIGHT INSTRUMENTS SYLLABUS: ➢Gyroscope and its properties ➢Types of gyros-mechanical, ring laser gyros, fiber optic gyros and their limitations ➢Gyro horizon ➢Direction indicator ➢Turn and bank indicator. Chetan Ghatage, Asst. Prof Department of EIE, RNSIT Page 1

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AERONAUTICAL INSTRUMENTATION ****MODULE4 GYROSCOPE AND ITS PROPERTIES As a mechanical device a gyroscope may be defined as a system containing a heavy metal wheel, or rotor, universally mounted so that it has three degrees of freedom: (i) Spinning freedom about an axis perpendicular through its centre (axis of spin XX1) (ii) Tilting freedom about a horizontal axis at right angles to the spin axis (axis of tilt YY1) and (iii) Veering freedom about a vertical axis perpendicular to both the spin and tilt axes (axis of veer ZZ1). The below figure shows the references established by gyroscope . Fig 4.1: Gyroscope The three degrees of freedom are obtained by mounting the rotor in two concentrically pivoted rings, called inner and outer gimbal rings. The whole assembly is known as the gimbal system of a free or space gyroscope. The gimbal system is mounted in a frame as shown in figure below, so that in its normal operating position, all the axes are mutually at right angles to one another and intersect at the centre of gravity of the rotor. The fundamental properties of a Gyroscope. The two fundamental properties of a Gyroscope are: i) Gyroscopic inertia or rigidity ii) Precession Both these properties depend on the principle of conservation of angular momentum, which means that the angular momentum of a body about a given point remains constant unless some force is applied to change it. Chetan Ghatage, Asst. Prof Department of EIE, RNSIT Page 2

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AERONAUTICAL INSTRUMENTATION ****MODULE4 Angular momentum is the product of the moment of inertia (I) and angular velocity (U) of a body referred to a given point-the centre of gravity in the case of a gyroscope. Rigidity: The property which resists any force tending to change the plane of rotation of its rotor. This property is dependent on three factors: (i) the mass of the rotor, (ii) the speed of rotation, and (iii) the distance at which the mass acts from the centre, i.e. the radius of gyration. Precession: The angular change in direction of the plane of rotation under the influence of an applied force. The change in direction takes place, not in line with the applied force, but always at a point 90' away in the direction of rotation. The rate of precession also depends on three factors: (i) the strength and direction of the applied force, (ii) the moment of inertia of the rotor, and (iii) the angular velocity of the rotor. The greater the force, the greater is the rate of precession, while the greater the moment of inertia and the angular velocity, the smaller is the rate of precession. The limitations of a free gyroscope and a displacement gyroscope. The limitations of a free gyroscope are: • Apparent drift • Real drift • Transport wander a) Apparent drift The earth rotates about its axis at the rate of 15⁰ per hour, and in association with gyro-dynamics, this is termed the earth rate (we). When a free gyroscope is positioned at any point on the earth's surface, it will sense, depending on the latitude at which it is positioned, and the orientation of its spin axis and its input axis,various components of the we as an angular input. Chetan Ghatage, Asst. Prof Department of EIE, RNSIT Page 3

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AERONAUTICAL INSTRUMENTATION ****MODULE4 Thus, to an observer on the earth having no sense of the earth's rotation, the gyroscope would appear to veer or drift, called as apparent drift. b) Real Drift Red drift results from imperfections in a gyroscope such as bearing friction and gimbal unbalance. Such imperfections cause unwanted precession which can only be minimized by applying precision engineering techniques to the design and construction. c) Transport wander Consider a horizontal-axis gyroscope which is set up initially at the North Pole, with its input axis aligned with that of the earth, and then it will exhibit an apparent drift equal to we. Assume now that the gyroscope is transported to lower latitude, and with its input axis aligned with the local vertical component of we. During the period of transport, it will appear to an observer on the earth that the spin axis of the gyroscope has tilted in a vertical plane, until at the new latitude it appears to be in the position, termed as transport wander. Apparent tilt, or transport wander would also be observed if, during transport, the input axis were aligned with either a local N-S component, or a local E-W component of we. Transport wander will appear simultaneously with drift, and so for a complete rotation of the earth, the gyroscope as a whole would appear to make a conical movement. The limitations of a displacement gyroscope are: • Gimbal lock • Gimbal error a) Gimbal Lock This occurs when the gimbal orientation is such that the spin axis becomes coincident with one or other of the axes of freedom which serve as attitude displacement references. Consider example, the case of the spin axis of the vertical-axis gyroscope becoming coincident with the ZZ1, axis of the outer gimbal ring. This means that the gyroscope would lose its spin axis, and since the rotor plane of spin would be at 90⁰ to the ZZ1 axis but in the same plane as displacements in roll, then the stable roll attitude reference would also be lost. If, in this 'locked' condition of the gimbal system the gyroscope as a whole were to be turned, then the forces acting on the gimbal system would cause the system to precess or topple. Chetan Ghatage, Asst. Prof Department of EIE, RNSIT Page 4

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