Bohli Compass, how to use?

[janus]

Someone wrote: It can still be fun though. And where can I get a
Cook compass?

This tripped me. I recently bought into an SZD55, like all
polish gliders it has a Turn and Slip indicator. but unlike most
it has a Bohli compass.
It really tells me nothing! There is a tiny needle which points
into a glass globe with headings marked on it and the needle
makes a circular path over the glass globe as I make a 360° turn.
I understand the needle is supposed to be inclined with the
earths mag. field, but how does this relate to the markings on
the glass globe?
Can anyone explain?

On top of it I have to take it out when I compete, because it is
a cloud flying aid... As if the GPS isnt.

Krister Engvoll
Norway

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[JohnPegase]

The Bohli should be on a rotatable mount. Turn the mount until the compass is
level with the horizon, and the needle goes round as you circle and indicates
the correct direction you are flying in. So if you quickly roll back to level,
you come out on the heading you just saw. Then turn the compass back to being
level with the horizon again. So before you go into cloud, get a stable turn
established, level up the compass, and if the turn doesn't alter, you can come
out of cloud pointing in the correct direction. If you turn accurately, the
needle traces out the same circle on the compass markings. If the circle
becomes off centre, your angle of bank has changed. Adjust the compass again
to get a good circle, and you can see your angle of bank from the compass mount
against the instrument panel.

That's the theory, I've only tried some of it and only a few times, so i can't
say how easy it is to the adjustments in cloud. hope this helps a bit.

John Wright, 742

[Peter Nyffeler]

It is very important that the Bohli compass is properly adjusted.
For E-W adjustment the glider-tail must be hold up as it would fly.

Ask them for a manual. There e-mail address is bo...@bohli-magnete.ch

----------------------------------------------------------------
PETER NYFFELER, ETH-Zuerich, Physical-Chemistry
mailto:nyff...@phys.chem.ethz.ch, <http://www.chem.ethz.ch/~peny >
To my hobbies: Soaring <http://www.chem.ethz.ch/~peny/sf>
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