CoD: Use of P8 Compass

(trying to learn how this thing works)
Check if this is modelled correctly in CoD!!!
Edit: first tests made. On ground I find a small variation on actual heading, but once in air it works well, it seems. More test to be done. Fascinating stuff :slight_smile:

A few posts below, and in the wiki, we talked about how to set the directional gyro, using the compass. Why we need to do that? why the 2 instruments?

Quote from Robert J. Goebel in “Mustang Ace”. He was flying Spifires at this time:

I was almost due south of the field (the course given to me was 345º , north Northwest). I rotated the cover glass on the compass and set its two parallel lines to the desired heading. But when I turned the aircraft to align the pointer on the card between the lines, I put the little index arrow at the wrong end.

Unbeknownst to me, I was flying the reciprocal, away from the airfield instead of toward it.

This was a common mistake when using only the compass. To alleviate the problem, the gyro was added. Once it was setup, it gave you the heading you were flying.

Now, how to set a course we want to fly in the compass?
A P8 compass has a rotating bezel (which has the 360º degrees marked on it) and a needle that always points North. In the late P-type compass we have modelled in CoD, the North point of the needle is marked with a small crossbar on it (it was a red arrow before, hence the RAF expression “fly red on red”, now changed for “tee on tee”)

So, to fly heading 135º, you would rotate the bezel until 135º is in the 12 o’clock position of the compass casing:

Now turn the aircraft until the north point of the needle is pointing to the big “N” in the bezel:

That’s it. As long as the needle is between the 2 parallel yellow lines, you are in the correct course, better if you keep them parallel.

Finally, I will quote Peter Dodds about how to turn the aircraft into the desired heading.
Note: check if this is modelled.
Edit: first test indicates that yes, it is modelled. After stopping your turn, the needle oscillates for a few seconds.

Turning Onto Your Heading
The earth’s lines of magnetic force do not lie parallel to the surface but
are inclined towards the poles (magnetic inclination). For reasons to do
with the way the compass compensates for this with balance weights, the
needle does not always point north when the aircraft is turning.
When turning north, the needle actually moves ahead of the aircraft’s
rotation, and when turning south, the needle lags behind the aircraft’s
turn. This leads to one of those numerous mnemonics with which aviation
abounds – UNOS. “Understeer north – oversteer south”.
When turning north, turn until the needle points beyond the north point
on the bezel. When turning clockwise (from west to north) let the needle
swing to a heading in the region of 030º and when turning anticlockwise,
let it swing into the 330º region. As you roll wings level, the needle will
continue to move, and if you have got it right, settle down pointing north.
Similarly when turning south, roll out wings level when the needle points
some 30º before north. So when turning clockwise (from east to south)
roll out of the turn when the needle points to around 030º and when
turning anticlockwise, turn as the needle approaches the 330º region. As
you roll wings level, the needle will fall back, and if you have got it right,
settle down pointing north.
The good news is that when turning east or west the effect is cancelled
out and you can roll wings level as the “10º before heading” comes up, the same as for a D.I.
The precise amount of understeer and oversteer necessary is a function of
several factors – turn rate, heading required, climbing and descending,
accelerating or decelerating for example, but it can easily be 30º or more.
You can only get it right reliably with experience.

Thanks Nepe, this is fascinating stuff and very rewarding when the process is understood.

Yours pics aren’t showing btw.

Another nice touch I discovered is that you can adjust your altimeter. When starting a mission it shows your height above sea level but you can wind on/off height so that it reads 0ft or aerodrome level - the QFE. Neat!



post updated with pics