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All my Kollsman periscopic sextants with the pendulous mirror (made in Germany
and UK, under Kollsman licence) have the two minutes averager.

Werner


Am Mittwoch, 25. November 2009 20:24:22 schrieb Gary LaPook:
>  Even if the individual sights within the average are more accurate with
> the MA-1 than with the bubble instruments the mirror is still subject to
> the same accelerations as the bubble. These are of two types, random,
> caused by turbulence, and periodic, caused by the normal oscillations of
> the aircraft about its three axes such as phugoid and  dutch roll, with
> are sinusoidal in nature.  To eliminate or minimize the random errors
> "more is better." Taking a greater number of sights will result in a
> better average so taking more sights during the two minute period works
> for both bubble and mirror artificial horizons. The natural oscillation
> period of large aircraft is about 40 seconds and to minimize the errors
> caused by them it is best if the averaging period matches the period of
> the aircraft or exact multiples of it.  Two minutes matches three such
> periods while one minute is only one and a half periods and so will
> leave an inherent error in the data and the bias will depend upon where
> in the cycle the observation starts.
>
> gl
>
> douglas.denny@btopenworld.com wrote:
> > May I offer a possible explanation?  It is only a guess however.
> > The pendulous reference is easier to use than a bubble, being an horizon
> > indicator rather than a circle in which the star is placed.  This I
> > believe gives simpler and greater accuracy of observation than a bubble
> > where the observer has to estimate the centre placement of the star in
> > the bubble, and hence would require less time to complete an assessment
> > of the star alignment with the reference.
> >
> > The oscillation period of a large aircraft I would have expected to be
> > within a minute anyway,  so anything above one minute is not necessarily
> > going to increase accuracy on this count. The only increase in accuracy
> > would be if there are longer term accelerations present (such as an
> > unwanted inadvertent side-slip) corrected within the time period of
> > observation.
> >
> > The only answer must be ultimately that tests would have been carried out
> > by Kollsman and they probably found little difference in results for the
> > one minute as opposed to two minute observation periods.
> >
> > Douglas Denny.
> > Chichester.  England.

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