NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Aviation Bubble Sextant
From: Ken Gebhart
Date: 2010 Aug 29, 14:04 -0500
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 >>> ---------------------------------------------------------------- >>> >> >> >> > >