the bicycle wheels increase speed, they become more stable in  their plane of rotation. This is why a bicycle is unstable and  maneuverable at low speeds and stable and less maneuverable  at higher speeds.  By mounting this wheel, or gyroscope, on a set of gimbal  rings, the gyro is able to rotate freely in any direction. Thus,  if the gimbal rings are tilted, twisted, or otherwise moved,  the gyro remains in the plane in which it was originally  spinning.  [Figure 8-18]  Precession is the tilting or turning of a gyro in response to a  deflective force. The reaction to this force does not occur at  the point at which it was applied; rather, it occurs at a point  that is 90° later in the direction of rotation. This principle  allows the gyro to determine a rate of turn by sensing the  amount of pressure created by a change in direction. The rate  at which the gyro precesses is inversely proportional to the  speed of the rotor and proportional to the deflective force.  Using the example of the bicycle, precession acts on the  wheels in order to allow the bicycle to turn. While riding  at normal speed, it is not necessary to turn the handle bars  in the direction of the desired turn. A rider simply leans in  the direction that he or she wishes to go. Since the wheels  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  applied to the top of the wheel to the left. The force actually  acts 90° in the direction of rotation, which has the effect of  applying a force to the front of the tire, causing the bicycle  Several flight instruments utilize the properties of a gyroscope  for their operation. The most common instruments containing  gyroscopes are the turn coordinator, heading indicator, and  the attitude indicator. To understand how these instruments  operate requires knowledge of the instrument power systems,  gyroscopic principles, and the operating principles of each  instrument.  Any spinning object exhibits gyroscopic properties. A wheel  or rotor designed and mounted to utilize these properties is  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 refers to the principle that a gyroscope  remains in a fixed position in the plane in which it is spinning.  An example of rigidity in space is that of a bicycle wheel. As  Figure 8-18.  Regardless of the position of its base, a gyro tends to  remain rigid in space, with its axis of rotation pointed in a constant  direction.  Rigidity in Space  Gyroscopic Principles  Gyroscopic Flight Instruments  Figure 8-17.  Altimeter trend vector.  3600 3500 3400 3300 3200 ° ° VOR 1  270  2  1  1  2  -500 4 200  4 100  4 000  3 900  3 800  3 700  3700  40 80 000  130  120  110  90  80  70  1  100  9  TAS 100KT  270  4 100 3000 3 900 20 80 4 000 3 100 4 000 3 900 80 70 975 -125 3 100 4 000 3 900 70 30 950 -250 4 000 3000 3 800 45 05 3 925 -375 Altitude trend vector  Turn rate trend vector  Airspeed trend (increasing)  Precession  8-15 