NavList:

Greg R. wrote:

With the exception of the red numbers on the outside rings, your MB-2A
computer looks remarkably similar to the Jeppesen CR series (CR-3,
CR-6, etc.) - is it maybe a military version (or an earlier version)of
the Jepp one?

Gary LaPook responds:



The MB-2A is very similar to the MB-9 pictured here:

http://www.rekeninstrumenten.nl/pages%20and%20pictures/12071.jpg

except the true airspeed goes up to 1800 knots on the MB-9 while it only goes up to 1000 knots on the MB-2A. Since I don't fly planes that can exceed 1000 knots I prefer the MB-2A since its more limited speed range allows an expanded scale for the range it covers.

It is very similar to the Felesenthal PT computer pictured here:

http://www.rekeninstrumenten.nl/pages%20and%20pictures/12141.jpg

in that each of these computers solve the wind triangle with trig, no vector diagram is drawn.

The Jeppesen CR-3 pictured here:

http://sliderule.mraiow.com/wiki/Jeppesen_CR-3

uses a diagram on the back to determine wind factors so its method is completely different than the previous computers.  All of these are similar in that they have the standard time-speed-distance scales and they allow for compressibility in computing true airspeed.

The original E-6B pictured here:

http://www.rekeninstrumenten.nl/pages%20and%20pictures/12081.jpg

uses a wind vector diagram on the back and does not allow for compressibility in TAS computations.

gl


Greg R. wrote:

With the exception of the red numbers on the outside rings, your MB-2A
computer looks remarkably similar to the Jeppesen CR series (CR-3,
CR-6, etc.) - is it maybe a military version (or an earlier version)of
the Jepp one?

And I assume this isn't the other calculator that you alluded to
earlier (Barger, or something similar?) - it's late and I can't find
that particular post right now.

--
GregR




--- "Gary J. LaPook" <glapook@pacbell.net> wrote:

  

Gary J. LaPook wrote:

    

Being the conscientious navigator that I am, I followed my usual
practice of memorizing some data from the 2008 Nautical Almanac so
that I would have it available for emergency use.

The first thing I memorized was the GHA of Aires at 0000 Z January
      

1,
    

2008 which is 100º 01.9' and also remembering that Aires advances
59.139' each day. With this information you can calculate GHA Aires
      

for 0000 Z on June 9, 2008 which is the 161st day of the year but
      

is
    

only 160 days from January 1st. So multiplying 59.139' times 160
      

days
    

gives  157º 42.2'  to which you add the starting value of 100º
      

01.9'
    

to come up with the GHA Aires on June 9th at 0000 Z of  257º 44.1'.
      

To
    

this you add the change of GHA for the time since 0000 Z (3 hours
      

42
    

minutes 10 seconds for the Vega shot) by multiplying the time
      

interval
    

by the rate of change of 15.041º per hour making 55º 41.6' making
      

the
    

GHA Aires at the time of the Vega shot of 313 º 25.7'.

I also memorized the SHAs and the Declinations of ten of the
navigation stars ( nobody could memorize all 57) which should be
enough for emergency use as tabulated for July 1st so that the
      

values
    

will be reasonable for the whole year. Fortunately this included
      

the
    

three stars used in this exercise. So now adding the SHA of Vega,
      

80º
    

41' we end up with the GHA of Vega of 34º 06.7' and using the D.R.
      

as
    

the A.P. we get an LHA of 274º 48' and the declination of 38º 47'
      

N.
    

(rounded to the whole minute)

Using these values on my Bygrave slide rule (see attached work
      

sheet)
    

since I have no tables with me, I computed Hc of 23º 59'.

The Hs given was 24º 05.5' Computing the dip correction in my head
      

of
    

4.5' (the square root of 20 must be between 4 and 5 ) and applying
      

the
    

refraction correction of minus 2 gives an Ho of 23º 59' giving and
intercept of zero and an azimuth of  58.1 º.  I long ago memorized
      

the
    

refraction table for altitudes above 10º in The Air Almanac and in
      

H.O
    

249, the cutoff values are 63-33-21-16-12-10º , zero above 63, 1
      

above
    

33, 2 above 21, 3 above 16, 4 above 12 and 5 above 10.

I used the same procedure for Spica and Pollux getting another zero
      

intercept for Pollux, Zn of 290.2º and a  4 NM away for Spica with
      

a
    

Zn of 171.7º.

Since I am on the road I do not have any of my plotting tools with
      

me
    

so I had to make do with what I found in my briefcase. I used my
      

MB-2A
    

flight computer since it had an azimuth scale and I used a pad of
paper  with a right angle at the corner as my straight edge for
plotting the LOPs. I used a tape measure from IKEA to measure the
length of the intercept (see photo.)  I plotted the LOPs and found
      

the
    

fix by bisecting the three angles giving a fix .4 NM west of the
      

A.P.
    

(D.R.) and 2.8 NM north of it. (Plotting a fix as a distance from
      

the
    

A.P. like this is common in aerial practice and it is often done on
      

an
    

E-6B.) Adding the 2.8 NM north to the D.R. latitude gives a fix
latitude of 34º 16' North. To convert the .4 NM west  to a
      

longitude
    

you divide the .4 NM by the cosine of the latitude, .82, to find
      

the
    

difference in longitude of  .5' so the fix longitude is 119º 19.5'
West (rounded to either 119º 19' or 20'.)  ( I got the cosine of
      

34º
    

by finding the sine of 56º on the MB-2A sine scale, used for wind
correction calculations.)

My fix might not be in agreement with others but I used a
      

refraction
    

table tabulated in whole minutes, I only memorized the stars'
positions to the nearest minute and I did not have any plotting
      

tools
    

to use but my position is certainly good enough for emergency
navigation and done without an almanac, tables or electrons.

gl
1

m_burkes@msn.com wrote:

      

Captain Lecky would be proud of those dividers ha! Speaking of
interpolation I have found a neat way to get around that pesky DSD
        

and
    

interpolation tables by using the aviation E6B computer or the
equivalent nautical slide rule. Essentially the set
        

up:d-value/60=d-
    

correction/declination minutes. Yes the calculator offers the
        

proof.
    

Mike Burkes
On Jun 12, 11:44 pm, Anabasi...@aol.com wrote:


        

Thanks for the nice exercise Greg.  I literally had to dust off
          

the  ship's
    

Vol III of HO 229 and deflower a Plotting sheet 925 to work this
          

one  out.
    

Since I was bereft of electronic gadgets, I did this with a
          

plotting sheet,
    

2 triangles, a pair of dividers, 2 books, a pencil, and small
          

piece of scratch
    

paper (wouldn't have reams of paper in the Lifeboat).  I have
          

attached a
    

picture in to this message with the plot and the tools.

My Lat is a bit lower (plotting or math error?).  I used an
          

assumed  position
    

method and HO 229.  I had to assume we were drifting and no
          

current (didn't
    

advance or retard the lines).  I had not done a full HO 229  paper
          

reduction
    

of a star in many years, and I had to think a second to remember
          

how to use
    

the interpolation pages on the inside covers for the declination
          

interpolation.
    

I usually whip those off with the calculator.  Still,  I got
          

pretty close to
    

the computer solutions with Lat 34deg 11.9' N and  Longitude
          

119deg 16.0'W.
    

As to how you would get an Eastern sight on the west coast, you
          

would have  2
    

options in general.  The first would be a back sight.  This would
          

be
    

particularly difficult with a regular sextant at such a low
          

altitude.  The  other
    

option would be to use a bubble sight tube or other artificial
          

horizon.  If you
    

were across a bay, you could also use a dip short of  the horizon
          

table.
    

That's all I can think of at the moment.

Jeremy

**************Vote for your city's best dining and nightlife.
          

City's Best
    

2008.      (http://citysbest.aol.com?ncid=aolacg00050000000102)

gregExerPlot.jpg.JPG
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