DG Flugzeugbau DG 202, Flight Manual

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Manual Compass 46--MFK--1 BOHLI MAGNETTECHNIK AG
Edition 01.2007 Page 3 of 16
The inclination varies with geographic latitude. Over the magnetic poles the needle would
point down vertically; on the magnetic equator it would lie horizontally. One can notice this
effect on long flights going North--South or vice--versa.
The effect shows on the Bohli compass in this manner: the imaginary circle, which the pointer
mark describes on the dial during a circle on the plane, becomes wider in lower latitudes and
narrower in higher latitudes. At a given inclination (latitude), the diameter of the imaginary
circle (described by the red mark) can easily be visualized. Circles for three typical inclinations
are painted on the dial. Directional indication is not influenced by a change of latitude. The
local variation must, of course be accounted for, as with many other compasses.
Since the magnetic needle maintains its position like a platform, a damping device is superflu-
ous. Directional indication is therefore immediate and without any noticeable oscillation. The
chosen magnet exerts a high directional force; at European latitudes it is about 2.5 times
stronger than for conventional “declination compasses”.
There is very little deviation caused by interfering magnetic fields (inside the cockpit) which
are moving around the compass needle during circling. If the source of such interference is
more than 20 cm (8 in) away, it may be ignored. The off--center effect caused by such magne-
tic disturbances is automatically compensated when levelling the compass. The four main
magnetic directions can therefore be established on the instrument with precision.
The use of this compass is limited to areas with a magnetic inclination between 40 ° and 75 ° .
This includes Europe, Iceland, the USA (including part of Alaska), Mexico, southern part of
Canada and all of Asia located between 30
° and 65 ° northern latitude. A special model is avai-
lable for the southern hemisphere, suitable in Australia, New Zealand and South Africa.
The first units of this compass were used during the 1972 World Gliding Championships in
Vrsac (Yugoslavia) and have proved successful.
It is not surprising that the glider pilots, in particular, are showing great interest in this instru-
ment. Navigation is more complex in gliding than it is in power flying. Gliding requires frequent
transition between straight flight and narrow circling (for gaining altitude). In competitive soa-
ring accuracy is imperative, for instance in maintaining an exactly determined wind correction
angle, or when rolling out of a turn onto a precise heading (this problem will be treated in de-
tails in a separate article).
For this we need quick an accurate directional information. Furthermore it allows the pilot du-
ring circling to determine track radials towards special land marks, clouds or other circling gli-
ders.
In addition, the compass can be utilized for centering thermals. After maximum climb, the pilot
makes a 270
° turn (3/4 circle), then files straight for a short moment and continues circling.
This will usually be a faster method for getting into the center than by guessing time, using
land marks on the ground or other methods.
The instrument can serve the same purpose as a gyrocompass but it needs no energy, is al-
ways ready for use and does not drift off. It is equally suitable for visual and instrument flying.