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    Re: Aviation Bubble Sextant
    From: Ken Gebhart
    Date: 2010 Aug 29, 14:04 -0500

    I'm wondering if mjans is listening since he has not come back with  
    the type of sextant he has.  Or are we just talking to each other?
    
    I would like to add that ANY Pioneer (Bendix) sextant (not just the  
    A-5/ A-7) suggests collimation alongside the bubble.  This includes  
    the A-14/ A-15.
    
    Ken Gebhart
    On Aug 29, 2010, at 5:37 AM, Gary LaPook wrote:
    
    > I have attached the refraction and coriolis correction tables from  
    > H.O. 249.
    >
    > You will note that the temp correction can be ignored except in  
    > very extreme cases.. For example, only for altitudes below 10� to  
    > you apply it at all. At sea level for the temperature range of  5�  
    > to 47� C there is no temp correction even for low altitudes.
    >
    > gl
    >
    > Gary LaPook wrote:
    >> In the past I have uploaded manuals for many types of bubble  
    >> sextants and they can be found in the archives by entering the  
    >> URLs below.
    >>
    >>
    >>
    >> Most of them allow you to collimate the object in the center of  
    >> the bubble. The only one that doesn't is the Pioneer/ A-5/ A-7.
    >>
    >> You want the bubble to be about two sun diameters in size.
    >>
    >> Align the body in the center of the bubble or to the horizontal  
    >> right or left, any other alignment results in loss of accuracy. If  
    >> you were to use the top or bottom of the bubble you would have to  
    >> allow for the size of the bubble which will be different every  
    >> time you form it. Trying to use a top and a bottom of the bubble  
    >> sight and then average them presents two problems, the bubbles  
    >> often change size while in use (some bubble designs are worse than  
    >> others in the respect) and the second problem is you will have to  
    >> work fast since the altitude of the body is probably changing.
    >>
    >> Collimation needs to be achieved near he horizontal center of the  
    >> field of view, not off to the left or right. Up or down  
    >> displacement is OK but sideways displacement means that the  
    >> instrument is tilted sideways resulting in inaccuracy.
    >>
    >> If standing on earth the only corrections you make are index  
    >> correction and refraction. No dip, no, semi-diameter, no temp and  
    >> pressure ( they are too small to worry about given the inherent  
    >> accuracy of bubble sextant sights.) If shooting the moon then you  
    >> add in parallax in altitude.
    >>
    >> If flying you need to correct for coriolis.
    >>
    >> I agree with Frank and recomend "American Air Navigator" by  
    >> Mattingly.
    >>
    >> gl
    >>
    >>
    >> A-5
    >> http://www.fer3.com/arc/m2.aspx?i=109441
    >>
    >> MA-1
    >> http://www.fer3.com/arc/m2.aspx?i=109439
    >>
    >> MA-2
    >> http://www.fer3.com/arc/m2.aspx?i=109438
    >>
    >> A-6
    >> http://www.fer3.com/arc/m2.aspx?i=109442
    >>
    >> A-12
    >> http://www.fer3.com/arc/m2.aspx?i=109443
    >>
    >> Balldrop
    >> http://www.fer3.com/arc/m2.aspx?i=109444
    >>
    >> MK IX
    >> http://www.fer3.com/arc/m2.aspx?i=109447
    >>
    >> MK 5
    >> http://www.fer3.com/arc/m2.aspx?i=109448
    >>
    >> Pioneer octant
    >> http://sites.google.com/site/fredienoonan/topics/pionneer-octant
    >>
    >>
    >>
    >> Frank Reed wrote:
    >>>
    >>> mjans, you wrote:
    >>> "I have recently acquired my grandfathers WWII Bubble sextant. He  
    >>> has long passed away and left me no instruction on how to use it.  
    >>> No manuals, no spoken wisdom, nada... I've figured out how to  
    >>> make the bubble and how to collapse it, etc.. but still haven't  
    >>> quite figured out what corrections I need to apply to the  
    >>> sighting. When figuring out Ha from Hs, would I need to apply an  
    >>> Index correction? Since there doesn't seem to be an index mirror,  
    >>> I don't really see why?"
    >>>
    >>> Gary Lapook will no doubt have lots of advice for you, but I'll  
    >>> throw in my two or three cents before he gets back.
    >>>
    >>> For instructions and manuals, you can pick up old air navigation  
    >>> textbooks on ebay and at abebooks.com. One that I like is the  
    >>> "American Air Navigator" published during the Second World War.  
    >>> It's beautifully illustrated, and it is a detailed, complete  
    >>> account of the methods of airborne celestial navigation. Also,  
    >>> ask away here on NavList. There are plenty of people who would be  
    >>> happy to discuss any questions, from the most basic to the most  
    >>> esoteric.
    >>>
    >>> Think of index error as being like "tare weight" on a scale. We  
    >>> need to check the offset when the instrument should be reading  
    >>> zero. The exact geometry and construction of the device is  
    >>> immaterial. For a bubble sextant, make some observations for  
    >>> known altitudes. Gary has often suggested using the altitude of  
    >>> Polaris when it is near the meridian since it is changing very  
    >>> slowly and easily calculated for a known latitude.
    >>>
    >>> You asked:
    >>> "When taking a sighting, how big should the bubble be and where  
    >>> do I place the object? In the bubble or on top?"
    >>>
    >>> The bubble should be about a degree or so in diameter. It doesn't  
    >>> seem to be critical. You can place the object on the upper edge  
    >>> of the bubble, the side of the bubble, the center of the bubble,  
    >>> the lower edge of the bubble as you find convenient, but it needs  
    >>> to be consistent with your measurement of the index error. For  
    >>> some reason, using the center of the bubble seems not to have  
    >>> been recommended historically. I don't understand that since it  
    >>> actually seems to give better sensitivity in altitude  
    >>> observations with it centered since the bubble acts like a little  
    >>> lens. On land, you can take an altitude at the top edge of the  
    >>> bubble, then one from the bottom edge of the bubble, and average  
    >>> the two. That yields excellent results.
    >>>
    >>> You asked:
    >>> "This one seems to have a timer too, is there anyway to  
    >>> mechanically change the timing interval? It seems to only take a  
    >>> sight for about a minute!"
    >>>
    >>> The timer is not for taking extended sights, like LAN sights  
    >>> (which you mentioned later). It's for averaging. The idea is that  
    >>> your hand will be shaking, the airplane is vibrating from its  
    >>> engines and machinery, and in smaller planes and/or rougher air  
    >>> the airplane may be rolling and pitching around quite a bit. So  
    >>> you look through the instrument and do your best to keep the Sun  
    >>> aligned with the bubble for a minute (or two, depending on the  
    >>> type of averager). The average altitude observed over that  
    >>> interval will generally be much better than any single  
    >>> instantaneous altitude
    >>>
    >>> You asked:
    >>> "Assuming I was in an old DC-3 flying at 5000 ft, taking a LAN  
    >>> sight... What corrections would I apply.. I assume, temperature,  
    >>> pressure and altitude (pressure or density alt??) but would  
    >>> airspeed apply too? If so.. True Airspeed or Ground Speed?"
    >>>
    >>> Yes, temp, pressure, and altitude. You can find tables for this  
    >>> in many places. There is a fairly important correction for the  
    >>> Coriolis acceleration which depends on ground speed.
    >>>
    >>> You asked:
    >>> "Would the correction be altitude only instead of Dip?"
    >>>
    >>> Since you don't use the visible horizon (and it's essentially  
    >>> never visible from flight altitudes anyway), there is no dip  
    >>> correction. The altitude correction is a correction for air  
    >>> density which could, in theory, be made by adjusting the air  
    >>> pressure, but in practice it's a separate correction.
    >>>
    >>> -FER
    >>>
    >>>
    >>> ----------------------------------------------------------------
    >>> NavList message boards and member settings: www.fer3.com/NavList
    >>> Members may optionally receive posts by email.
    >>> To cancel email delivery, send a message to NoMail[at]fer3.com
    >>> ----------------------------------------------------------------
    >>>
    >>
    >>
    >>
    >
    > 
    
    
    
    
    

       
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