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    Re: Fairchild A-10A in practice
    From: Ken Gebhart
    Date: 2017 Jul 4, 12:42 -0500
    I can’t remember who did it but the name Wiley Post comes to mind. A test was made of using the bubble sextant probably around 1920. The navigator took readings while the pilot “held the plane steady”. The result was that the readings were scattered and the solution was to average them. The reason the readings were scattered was that even though the plane maintained heading and altitude, the control surfaces were disturbed by the pilot holding her steady. These motions translated into accelerations to the bubble.
    I have found that if the plane is trimmed straight and level in smooth air, and no control inputs are made either by the pilot or autopilot, then a single “spot” shot can be more accurate than averaging. Of course you have to be quick about it (5 seconds, say) and the heading and altitude cannot have changed.
    On Jul 3, 2017, at 5:38 PM, Bill Morris <NoReply_Morris@fer3.com> wrote:

    Noell and David,

    The median is preferred over the mean because it is a more robust estimate of central tendency in that it is less sensitive to skewed results and outliers. It also requires no calculation.

    Early dedicated aeronautical sextants required the observer to decide when an observation was "good" and to take a reading or press a button. However, a constant acceleration of the aircraft could give a stationary bubble even though vertical was not being sensed by it, so automatic devices that read at regular intervals were devised, that averaged or gave a median, as in the Mk IX A and A10-A , or integrated as in the German SOLD. All that was then required of the observer was to try to keep the bubble centred on the body and to keep the time. The latter could be done with a split seconds stop watch, starting it off the navigational watch, stopping the first hand at the beginning of the observation and stopping the other hand at the end, or with the help of an assistant.

    Random errors decrease in proportion to the square root of the number of observations and by about 60 observations, diminishing returns become very prominent. Additionally, the period of slow oscillation of many aircraft, called "phugoid oscillations" by Frederick Lanchester, is about 2 minutes, so 60 observations in 2 minutes tended to become the norm. No doubt this felt a very long time to be flying straight and level over hostile territory and some instruments like the MK IX BM and SOLD allowed for shorter periods.

    Bill Morris


    New Zealand

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