Hi Bob,

I've been reading Compass Errors from page 5.24 to 5.26 of the book and I need your help understanding compass dip.

The paragraph below has me confused.

The displacement of the compass needle means that there is more weight on one side of the pivot than on the other. In the Southern Hemisphere, the northern end of the compass needle sits higher than the southern end. The northern end can be thought of as the 'heavy end'.

My interpretation of this statement is this - If the northern end sits higher (the side with the S painted), the southern side would be heavier (the side with the N painted). Is this correct?

Thank you.

The compass needle attempts to align itself with the local direction of the earth's field. Because it is suspended from above [like a picture frame], there is a greater mass on the northern side of the pivot than on the southern side. This extra mass is what we are referring to when we talk about the 'heavy' end.



Bob

Thanks Bob. Still need more clarification.

In your diagram, the S and N, are these the poles of a magnet or what is painted on the compass card?
Further in your text, for acceleration and deceleration errors, you mention the heavy side as the side with the S painded on it?

Still quite confused.

The 'hevy' end is the end with more mass under the pivot. Coloured orange in the figure. It is the north pole of the magnet and has 'S' painted on it. If you are facing east the compass needle will lie across your line of sight and so the centre of gravity of the needle will be to the left of the pivot. That means that if you suddenly accelerate, inertia will cause the 'heavy' end to lag behind and there will be a swing towards the pilot. i.e. towards the 'S'; So the compass gives a false indication that you are turning towards the south.

Likewise, if you suddenly decelerate, the 'heavy' inertia will cause the 'heavy' end to continue away from the pilot, causing the compass needle to swing the 'N' towards the pilot. This causes a false indication of a turn towards the north.

If you are facing west, the same effect occurs. If you are facing north or south you are looking along the compass needle so the centre of gravity is aligned with the pivot and there is no effect.

Bob

Maybe this will help also. The compass needle actually stays fixed with the north pole pointing to the north. As the aircraft turns onto various headings, it is actually the aircraft turning around the fixed card that makes the compass indications change in the viewing window. Note that 'N' is on the south pole of the needle and 'S' is on the north pole while 'E' is on the western side and 'W' is on the eastern side. This is necessary to allow the pilot to see the correct indication of his/her direction as he/she looks through the viewing window. See 1, 2 and 3 in the figure below.


The compass needle is not horizontal because of the 'dip' in the earth's magnetic field and so the centre of gravity of the needle is not directly under the pivot. If an acceleration occurs on a heading of east or west, the inertia acts through the centre of gravity pulling the north pole end of the needle towards the viewing window. Because that is the end with 'S' painted on it, the pilot observes an apparent turn to the south [1 and 4 or 2 and 5].

Acceleration on South or North has no turning effect because the centre of gravity and the pivot are in line with direction of travel.

Thank you for the explanation and the diagrams Bob. This is now crystal clear to me.

I also understand now that the 'heavy bit' is on the opposite side of which the magnetic dip naturally occurs in each hemisphere.

Since a compass manufactured in the northern and southern hemisphere would work differently, I assume this is taken into consideration for aircraft being manufactured in the USA but will be flown exclusively in Australia?

Actually the different hemispheres are not a problem because the compass needle is suspended from above as in the first diagram. That means the the needle is under the influence of gravity as well as the dip in the earth's field. Because it is suspended from above, gravity tends to bring it to a horizontal position and the earth's field tries to tilt it up. The resulting angle of the needle is called 'residual dip' and it is what we are interested in here.

In the northern hemisphere, the needle simply dips the other way.

Thanks Bob.

With regard to overshoot and unershoot, do we use the degree of angle of bank as the reference or would 15 degrees always be okay?

I was shown something along the lines of - if you're turning from E to N, for every 30 degrees turn, increase overshoot by 10 degrees.
For example turing from 090 to 030 - overshoot by 20; turning from 090 to 000 overshoot by 30.

There isn't a definite number of degrees because the angle of dip of the earth's field varies greatly with latitude. For example you would need to allow more in Hobart than in Cairns. Also you would need to allow a lot more if you were doing a turn with more bank. However for Australian latitudes in general, providing you keep the turn to about rate 1, the rule you were shown would work pretty well. You would have to experiment with your aircraft and location to be very accurate.

Luckily, it doesn't matter much in practice, if you set the DG to the compass in level flight, all turns you make on the DG will only need the normal lead to allow for the rate of turn as turning and acceleration error don't apply to the DG.

Great. Thank you Bob.