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    Re: Bubble Sextants
    From: Gary LaPook
    Date: 2008 Dec 16, 11:23 -0800
    Ken, I hate to disagree with you but I have taken many shots in small planes being hand flown solo without an autopilot using the technique that you described. Trim the plane, put it on heading, put sextant to eye, center star in bubble, put sextant down and then turn the plane back onto heading since it invariably wandered off heading while looking thru the sextant. This would cause a "wander error" since the heading was changing at the moment of colimation and this wander error was of an unknown amount and of a scale larger than the normal wander error contemplated in the navigation manuals because it is caused by the rate of heading change at that moment. Using the averager, even with half of the A-10A disk covered with marks, produced good shots.

    gl
    --- On Tue, 12/16/08, Ken Gebhart <GEBHART@CELESTAIRE.COM> wrote:
    From: Ken Gebhart <GEBHART@CELESTAIRE.COM>
    Subject: [NavList 6764] Re: Bubble Sextants
    To: NavList@fer3.com
    Date: Tuesday, December 16, 2008, 11:00 AM

    I am of a different opinion.  I heard a story about Wiley Post and  
    Charles Lindburgh making aa oceanic flight to evaluate a bubble
    sextant in an airplane. I may have this backwards, but Wiley would
    hold the plane steady while Lindy made sextant shots. Upon return,
    they announced that some kind of averager was needed because the body
    kept moving up and down as it was chased by the bubble. However, it
    is my experience that if you do NOT have someone hold the plane
    steady, or an autopilot is NOT used, then successful instantaneous
    shots can be made. All that is required is that the plane is trimmed
    straight and level before the shot. Then after the shot (which takes
    no more than 15 seconds if you have the sextant pre-set) if the
    planes heading and speed has not changed, you have a good shot.

    Ken Gebhart
    On Dec 16, 2008, at 11:55 AM, Gary J. LaPook wrote:

    >
    > Yes you do NEED an averager.
    >
    > The most perfected bubble sextants are probably the Kollsman
    > instruments. The periscopic instruments are too large for use in a
    > small cockpit but the hand held Kollsmans, the MA-2 and the pendulous
    > mirror MA-1, work real well in a small cockpit and I have taken many
    > sights with these in cockpits as small as a Cessna 172. The bubble
    > type
    > can be used for sun sights without the need of a battery to illuminate
    > the bubble during the day but the mirror type needs electricity to
    > light
    > up the reference reticle even during the day. Most of these have
    > avergers.which work off of a wind up clockwork mechanism and do not
    > need
    > electricity to operate but the most modern have an electronic averager
    > that is better than the clockwork models but do need 28 volts to
    > operate. These would probably work on an aircraft 24 volt system which
    > are actually 28 volts, when the engine is running, but I haven't
    > tried
    > this. Even the clockwork averagers are more sophisticated than
    > those in
    > other sextants. The electronic avergers were put on the periscopic
    > models in the 1980s but not on the hand held models. It is very simple
    > to swap out these averagers and put an electronic one on the handheld
    > sextants only requiring removing 4 screws, takes about 5 minutes. I
    > did
    > this with the MA-1 mirror type since it required electricity anyway. I
    > left the mechanical averager on my MA-2 since I like the fact that it
    > will operate during the day without a power connection. The light
    > bulbs
    > that come in these require 28 volts and it is simple to make a power
    > supply out of three nine volt batteries. If you prefer to make a power
    > supply for 12 or 6 volts there are replacement light bulbs in these
    > voltages. For about $25 you can make a 28 volt supply that works
    > off of
    > a 12 volt source. For practice on the ground you can buy a lawn
    > sprinkler power transformer which puts out 28 volts.
    >
    > I also like the A-10A and I have flown across the ocean using this
    > type.
    > Its averger is not as sophisticated as the Kollsman's but is easy to
    > use. Some people have had problems with them due to lack of
    > lubrication
    > during 50 years of non use but they can be refurbished in a couple of
    > hours.
    >
    > Check with Celestaire to see if they have reconditioned sextants
    > available.
    >
    > gl
    > Peter Fogg wrote:
    >> Welome Garry
    >>
    >> As an initial step, try entering some of those terms, eg; "bubble
    >> sextant", etc, into the Search box at:
    >> http://www.fer3.com/arc/
    >>
    >> There has been much discussion of these devices on this List and you
    >> will find a wealth of information there. Then feel free to pose
    >> further questions here, as quite a few members use and have restored
    >> such machines (although I'm not one of them!).
    >>
    >> On Tue, Dec 16, 2008 at 1:40 AM, JODIE COST <gjmooseair@yahoo.com
    >> <mailto:gjmooseair@yahoo.com>> wrote:
    >>
    >> I am a commercial pilot here in Alaska and just starting my
    >> education in celestial navigation and I have two questions for
    >> the
    >> group.
    >>
    >> As I have yet to find anyone teaching celestial navigation in the
    >> Anchorage area what home study courses or text do you recommend ?
    >>
    >> Since I plan on using these skills on land and from the cockpit I
    >> an interested in a good bubble sextant, but which one? From what
    >> I've read, the R.A.F. MK IX seems to be highly recommended but
    I
    >> have yet to find a reliable source for them ( I won't use ebay

    >> for
    >> the purchase precision instruments). The U.S.Navy MK 5 and the
    >> U.S.A.A.F. A-14 are available in overhauled condition but
    don't
    >> seem to be as highly recommended as the MK IX.
    >>
    >> Since I plan to use this sextant for navigation in flight
    >> would an
    >> averaging device be useful?
    >>
    >> Thank you in advance for your help.
    >>
    >> Garry Cost
    >>
    >>
    >>
    >>
    >>>
    >
    >
    >




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