The Art of the Helicopter John Watkinson - Karatunov.net

268 The Art of the Helicopter
(a)
(b)
Fig. 7.5 (a) When a helicopter accelerates at 270M in the northern hemisphere, dip will pull the north pole
of the compass down and increases the apparent heading. (b) In a sharp bank there will also be acceleration
towards the centre of the turn and a further acceleration error will occur.
Erroneous explanations will be encountered attributing some of the rotation of the
compass disc to the acceleration alone, but a moment’s thought will confirm that a
couple cannot be conveyed through a single pivot. It is almost certain that there will
be a question on compass acceleration errors in one of the pilot’s examinations, and
the correct answer will be obtained following the process below:
(a) Visualize the manoeuvre and the attitude of the rotor disc. The compass disc will
be very nearly parallel to the rotor disc.
(b) If the disc is tilted north down/south up or vice versa there is no effect. If the tilt is
east down/west up or vice versa there will be an error.
(c) Visualize the tilted compass disc in the earth’s dipping magnetic field. In the
northern hemisphere the north-seeking pole dips; in the southern hemisphere the
south-seeking pole dips.
(d) The dip will turn the tilted disc. Visualize whether this increases or reduces the
heading read off against the lubber line.
Here is an example: a machine turns to port through north in the southern hemisphere.
The rotor disc and compass disc will be tilted down on the west side, so the dip in
the southern hemisphere will pull the south-seeking pole down. The compass disc is
turned clockwise when seen from above. This causes the heading in degrees to be less
than it should be. The pilot should turn to a greater reading or ‘overshoot the turn’
to compensate. In practice the error is countered by turning to a greater or a lesser
indicated heading that will become the correct heading when the turn ceases and the
compass settles.