Gyroscopes 3
Earth’s Rotation We stated in the last chapter that the apparent drift due to the Earth’s rotation is: 15 x sin latitude, in degrees per hour. The latitude in this case is the latitude of the actual position of the aircraft. If you are dealing with a period of time with an associated latitude change - flight north or south - then you take the mean aircraft latitude for the period. This is one of the four errors we consider.
Latitude Nut
Figure 3.7
The latitude nut - and in early instruments it is a real nut, screwed in and out to produce the necessary imbalance and drift - is set to produce the opposite error to earth’s rotation. Therefore the error is also 15 x sin latitude in degrees per hour, but in this case the latitude is the figure set on the latitude nut scale. It is not always the same as the aircraft’s actual latitude, as it is set by the maintenance crew, and cannot be re-set in flight. This is the second of the errors we consider.
The air is fed through two nozzles mounted on the outer gimbal, itself tied to aircraft horizontal, which, when the rotor is erect, impinge equally on both sides of the buckets. When the rotor is out of alignment the stream of air strikes the buckets asymmetrically and produces a side force which precesses to re-erect the gyro. As the aircraft banks in a turn, the erection system will try to make the gyro erect to the new position in space of the aircraft horizontal, but this is not significant at the relatively small angles of bank at which this instrument is used. In straight and level flight, the gyro re-erects to what is both earth and aircraft horizontal.
The latitude nut induces a real wander to counter the apparent wander of earth rotation
Errors
Transport Wander
As aircraft instruments are necessarily imperfect friction and imbalances will create real wander. The better the instrument the less this value will be.
As the gyro is moved from one point on the Earth to another the gyro maintains its orientation in space. The direction of true north, however, changes, and the further you travel in an east/ west sense the greater the change. Transport wander is the apparent loss of alignment caused by east/west travel and its value is simply the convergency between two points.
The DI will be subject to apparent wander, both because of the Earth’s rotation and because of transport wander. In addition to this, we introduce an adjustable correction for earth’s rotation, the latitude nut attached to the inner gimbal, which is an out-of-balance force that produces a real wander equal to and opposite in sign to the Earth’s rotation error - if it is correctly set. We dealt with these in general in the previous section but we must now look at the mathematics of these errors.
Instrumentation
Transport wander (º) = change of longitude x sin mean latitude. Transport wander in an easterly direction will have a different sign from transport wander in a westerly direction.
v6.1.5
3.5