3 Gyroscopes
All you have to do to work out total DI error is to take the equation:
Real Wander Although we have noted that the latitude nut does introduce a real wander, this is dealt with elsewhere. Under this heading we consider only gyro imperfections that produce a known rate of wander. We have no way of calculating the value of the wander. If it is to be taken into consideration then it is given to us in degrees per hour, with a positive or negative change of gyro heading. This is the fourth and last error we consider.
Total drift = real wander + earth’s rotation. + latitude nut + transport wander Then fill in the values, as given for real wander, and according to latitude and E/W groundspeed as appropriate for the others, then apply the signs from the table. It works!
Points to Watch Watch out for aircraft flying in one hemisphere with their latitude nuts set for the correct latitude value in the other hemisphere. Remember that mean latitude must be taken for ER and TW, if the aircraft is flying with a north or south track component. Do not use the mean latitude for a latitude nut correction, it only has one set value for the flight and it might be quite different to the latitude at which you are operating. Remember also that earth rotation and latitude nut corrections are rates, in degrees per hour, whereas transport wander is a value in degrees.
Total DI Error Taking all these errors into account we can say that total drift is made up of: Real wander + earth’s rotation + latitude nut + transport wander We know how to calculate the values to put into the equation, but we have to consider the signs of the different errors. Real wander will be given, with its sign, so we need to have a quick table to work out the others. It is:
Example:
Northern Hemisphere
Southern Hemisphere
Earth’s Rotation
-
+
A perfect DI has its latitude nut set for 60°N. The aircraft starts from 50ºN 003ºW and flies for 3 hours ending up at 42ºN 0045ºE. At the end of the flight, what is the total DI error?
Latitude Nut
+
-
Solution:
Transport Wander East
-
+
Transport Wander West
+
(a) This is a perfect DI, so real wander is zero. (b) Earth rotation produces an error of
-
Figure 3.8 + = - =
anti-clockwise or left hand rotation clockwise or right hand rotation
In this table we can see that the Earth’s rotation will make DIs in the northern hemisphere under read, and that the latitude nut, if correctly set for the northern hemisphere will have the opposite sign, which it is designed to do. Likewise we can see, that as we said earlier, flying west in the northern hemisphere will introduce an error opposite in sign to the Earth’s rotation, cancelling to zero if you are flying west at exactly the same speed as the Earth is rotating. 3.6
15 x sin 46 º/hr, using the mean latitude = 15 x 0.719 = 10.8º º/hr
which, over 3 hours, is 10.8 x 3 = 32º, it is a negative error in the northern hemisphere, -32º. Drifting clockwise/right hand.
(c) The latitude nut error is:
v6.1.5
15 x sin 60 º/hr = 15 x 0.866 = 13.0 º/hr
over 3 hours the LN error is 3 x 13.0 = 39º, it is a positive error in the northern hemisphere, +39º.
Instrumentation