Flight Instruments

the bicycle wheels increase speed, they become more stable in Airspeed trend (increasing) their plane of rotation. This is why a bicycle is unstable and 4000 130 4200 maneuverable at low speeds and stable and less maneuverable 2 120 4100 at higher speeds. Altitude trend vector 110 4000 1 1 40 400045 By mounting this wheel, or gyroscope, on a set of gimbal 100 33909000 3925 9 80 -500 3000 05 -375 rings, the gyro is able to rotate freely in any direction. Thus, 90 3800 1 38 3 0 1 0 00 if the gimbal rings are tilted, twisted, or otherwise moved, 80 270° 3700 2 4 3 00 9 0 5 7 0 0 the gyro remains in the plane in which it was originally 70 3700 390030 -250 spinning. [Figure 8-18] TAS 100KT 3100 270° 3600 400080 VOR 1 3500 3975 -125 Precession 390070 Turn ra3t4e0 0trend vector 4100 Precession is the tilting or turning of a gyro in response to a 3300 3400000 2 0 0 deflective force. The reaction to this force does not occur at 80 the point at which it was applied; rather, it occurs at a point 3200 3900 that is 90° later in the direction of rotation. This principle Figure 4-27. Supporting Instruments 3100 allows the gyro to determine a rate of turn by sensing the Figure 8-17. Altimeter trend vector. amount of pressure created by a change in direction. The rate at which the gyro precesses is inversely proportional to the Gyroscopic Flight Instruments speed of the rotor and proportional to the deflective force. Several flight instruments utilize the properties of a gyroscope for their operation. The most common instruments containing Using the example of the bicycle, precession acts on the gyroscopes are the turn coordinator, heading indicator, and wheels in order to allow the bicycle to turn. While riding the attitude indicator. To understand how these instruments at normal speed, it is not necessary to turn the handle bars operate requires knowledge of the instrument power systems, in the direction of the desired turn. A rider simply leans in gyroscopic principles, and the operating principles of each the direction that he or she wishes to go. Since the wheels instrument. are rotating in a clockwise direction when viewed from the right side of the bicycle, if a rider leans to the left, a force is Gyroscopic Principles applied to the top of the wheel to the left. The force actually Any spinning object exhibits gyroscopic properties. A wheel acts 90° in the direction of rotation, which has the effect of or rotor designed and mounted to utilize these properties is applying a force to the front of the tire, causing the bicycle called a gyroscope. Two important design characteristics of an instrument gyro are great weight for its size, or high density, and rotation at high speed with low friction bearings. There are two general types of mountings; the type used depends upon which property of the gyro is utilized. A freely or universally mounted gyroscope is free to rotate in any direction about its center of gravity. Such a wheel is said to have three planes of freedom. The wheel or rotor is free to rotate in any plane in relation to the base and is balanced so that, with the gyro wheel at rest, it remains in the position in which it is placed. Restricted or semi-rigidly mounted gyroscopes are those mounted so that one of the planes of freedom is held fixed in relation to the base. There are two fundamental properties of gyroscopic action: rigidity in space and precession. Rigidity in Space Figure 8-18. Regardless of the position of its base, a gyro tends to Rigidity in space refers to the principle that a gyroscope remain rigid in space, with its axis of rotation pointed in a constant remains in a fixed position in the plane in which it is spinning. direction. An example of rigidity in space is that of a bicycle wheel. As 8-15