# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

**Re: 3-Star Fix - "Canned Survival Problem"**

**From:**Gary LaPook

**Date:**2008 Jun 15, 01:04 -0400

Gary J. LaPook had written::

"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."

Gary adds:

In case you were wondering how you can solve the wind triangle with trig on the MB-2A without a vector diagram the answer is simple, the law of sines. TAS/sin RWA = WS/sin WCA = GS/sin (RWA +/- WCA). To do this on a digital calculator first figure the Relative Wind Angle (the angle the wind is coming from compared the true course, WD - TC. Next divide the True AirSpeed by the sine of the RWA and save that value in a memory as you will use this constant twice. Next divide the Wind Speed by this constant, take the inverse sine and you have the Wind Correction Angle. Finally add or subtract the WCA from the RWA, subtract if the wind is a head wind and add if a tail wind, take the sine of this angle and multiply by the constant to give you Ground Speed.

The MB-2A does this computation for you. First place the TC arrow on the true course on the outermost RED scale and then read the RWA on the next inner RED scale lined up with the WD. This first image shows this with the TC of 50, the WD of 100 giving a RWA of 50.

Next line up the TAS on the outermost BLACK "miles"scale with the RWA on the "wind scale" which is a sine scale. The next image shows 120 knots lined up with 50 on the "wind scale."

The "constant" mentioned above is found uner the "1" index but you make no use of this and you don't even have to notice, the computer takes care of it for you.

Next look on the "miles" scale for the Wind Speed and take out the WCA on the "wind scale." The next image shows a 20 knot WS on the "miles" scale lined up with WCA of 7.3º on the "wind scale."

Subtracting this 7.3 WCA from the RWA of 50 we then set the cursor on 42.7 on the "wind scale" and read out the GS on the Miles scale, 106 knots.

gl

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