In aircraft instruments, gyros are used in attitude, compass and turn coordinators. These instruments contain a wheel or rotor rotating at a high RPM which gives it two important properties: rigidity and precession. The rotor or gyro can be electrically or vacuum / pressure driven by a pump on the engine.
Construction wise the gyro is fixed in the instrument by rings or gimbals and these give the gyro certain motions of freedom. It is these motions or movement in each planes which allow for certain characteristics used in these instruments.
Pilots flying under VMC will normally only rely on these instrument when getting out of IMC situations. Keep in mind that to be proficient in flying on instruments you will need regular training with a safety pilot, aka flying under the hood.
Without being current on instruments, most if not all VFR pilots, will probably crash when attempting to do this.
Rigidity & Precession
These two properties are unique to a rotating mass. Below an explanation of how they work and their application in aircraft instruments.
Whilst small, the rotor of a gyroscopic instrument must rotate at a very high RPM. Giving them inertia, also called rigidity and they maintain this alignment to a fixed point in space. This basically happens to every rotating object: wheel, propeller etc.. For example: this rigidity gives the moving bicycle its stability preventing it from falling over while riding it.
A number of factors have their influence on rigidity: the mass of the rotor, its RPM or angular velocity and finally the distance of the mass to the axis of rotation. The larger the distance the greater the rigidity with equal rotational speed. Again, a bike has large wheels and can rotate slowly to obtain enough stability.
When you apply a force to a point around the spinning rim of the gyro, the rotor will tilt as if the force was 90° further in the direction of motion as shown in the image. This apparent displacement of the force is called precession.
The amount of precession depends on the following factors: strength and direction of the force applied, the amount of inertia of the gyro (mass concentration on the rim), diameter and the RPM or rotational velocity of the gyro.
To conclude: the rate of precession in a free gyro is directly proportional to strength of the force and inversely proportional to the RPM and the moment of inertia. Thus the more mass and RPM a gyro has the more stable it is and maintain its position to a fixed point in space.
The gyro rotor is held in place by rings or better known as gimbal rings. These allow for freedom of motion in three dimensional planes as required by the instruments of the aircraft. Not all instruments will need all the planes of movement at the same time, this depends on their function, see the next pages.
Planes of movement
There are three possible motions for a gyroscope: the plane of rotation of the gyro; the plane of applied force and as a result: the plane of precession. Please refer to the image above for more detail and three dimensional view.
Depending on how you setup or mount the gyro in the gimbal rings it will have number of planes the gyro can move in, each useful to the pilot in different instruments. Below you will find a list of possible installations:
- Rate gyro can move in one plane (not the plane of rotation) and the movement in the third plane is used to measure the precession. You will see this type of gyro in a Turn Coordinator or Turn and Bank indicator (old models).
- Tied gyro moves in all three planes but kept in one plane by an outside force, usually air jets in case of the Direction Indicator (Gyro compass)
- Earth gyro has freedom of movement in all three planes but is held in one plane by Earths gravity. You will find this gyro in an Attitude Indicator.
- Space Gyro moves in all three planes and is stabilized to a fixed point in space. You will apparently see this gyro move due to the Earths rotation while in fact its not moving at all, space wise.
These gyro instruments are delicate pieces of equipment and will need proper care and handling by maintaining the vacuum system to very high standards. Only then can they be relied upon in actual instrument conditions by the pilot where basic seat of the pants flying often fails.