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 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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