The remaining flight instruments are gyroscopic instruments. A gyroscope refers to a spinning weight, designed to utilize the gyroscopic properties of rigidity and precession. When spun up to high speeds, a gyroscope becomes rigid in space and refuses to move.
Precession refers to the reaction of a gyroscope when a torque is applied to its axis of rotation. The reaction to a deflective force on a gyroscope occurs at a point 90 degrees later in the direction of rotation.
The reason for this can be understood by looking at four key points on the gyroscope as it turns. If a torque is applied to the gyroscope's rotational axis, it can be seen that the gyroscopes near point is being lifted and its far point is being pushed downward. As successive pieces of the gyroscope rim move through these points, they are not deflected to a different direction up and down.
Let's declare a starting point at the near and far points. End points are named, say 10 degrees later in the direction of rotation. Look at what happens to the material in the gyroscope's rim as it progresses from the start points to the finish points. The angle of the material is not being changed. This is because both the start and finish positions are moving up and down.
Now look at the sides of the gyroscope, the points parallel to the deflective force. We declare the start positions at these two points, and two finish positions 10 degrees later in the direction of rotation. This time, the finish positions are moved up and down, but the start positions remain stationary. So, the material passing through these points is being deflected up and down, resulting in an opposing force on the gyroscope according to Newton's third law.
These reactive forces sum to result in a reaction to the deflective force at a point 90 degrees later in the direction of rotation.
Gyroscope Power Sources
Gyroscopes are normally spun up by an electric motor or by air blowing through an internal turbine wheel. This air is normally generated by an engine driven vacuum pump. On some aircraft, an external venturi tube is used to generate suction for this purpose.
A common configuration is for the attitude and heading indicators to be powered by an engine driven vacuum pump, while the turn coordinator is powered electrically. This setup gives redundancy in case of a vacuum pump or electrical system failure.
The vacuum system is monitored by the pilot via a vacuum gauge in the cockpit. Electrically driven gyroscopes contain a red flag, which disappears when the instrument is powered.
The attitude indicator provides an immediate, direct, and corresponding indication of any change of aircraft pitch and bank in relation to the horizon.
The relationship of the miniature airplane to the horizon bar is the same as the relationship of the real aircraft to the actual horizon.
Attitude Indicator Errors
Following a 180 degree turn, the attitude indicator will indicate a slight climb and bank in the opposite direction. If a 360 degree turn is made, the turn error from the two 180 segments will cancel each other out and no error will be apparent.
In addition to turn errors, the attitude also exhibits acceleration and deceleration errors. If the airplane speeds up or slow down, the horizon bar will move up or down, respectively.