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called the miles scale and the minutes or time scale, respectively (Figure 4.12).
Figure 4.12. Dead Reckoning Computer Slide Rule Face.
4.7.3. The numbers on each scale represent the printed figure with the decimal point moved any number
of places to the right or left. For example, the numbers on either scale can represent 1.2, l2, 120, 1200,
etc.
4.7.4. Since speed (or fuel consumption) is expressed in miles (or gallons or pounds) per hour (60
minutes), a large black arrow marked speed index is placed at the 60-minute mark.
4.7.5. Graduations of both scales are identical. The graduations are numbered from l0 to 100 and the unit
intervals decrease in size as the numbers increase in size. Not all unit intervals are numbered. The first
element of skill in using the computer is a sure knowledge of how to read the numbers.
4.8. Reading the Slide Rule Face. The unit intervals that are numbered present no difficulty. The
problem lies in giving the correct values to the many small lines that come between the numbered
intervals. There are no numbers given between 25 and 30 as shown in Figure 4.14, for example, but it is
obvious that the larger intermediate divisions are 26, 27, 28, and 29. Between 25 and (unnumbered) 26,
there are five smaller divisions, each of which would, therefore, be .2 of the larger unit. A mental
estimate will aid in placing the decimal point.
122 AFPAM11-216 1 MARCH 2001
Figure 4.13. Dead Reckoning Computer Wind Face.
AFPAM11-216 1 MARCH 2001 123
Figure 4.14. Reading the Slide Rule Face.
4.9. Problems on the Slide Rule Face:
4.9.1. Simple Proportion. The slide rule face of the computer is so constructed that any relationship
between two numbers, one on the miles scale and one on the minutes scale, will hold true for all other
numbers on the two scales. Thus, if the two 10s are placed opposite each other, all other numbers will be
identical around the circle. If 20 on the minutes scale is placed opposite 10 on the miles scale, all
numbers on the minutes scale will be double of those on the miles scale. This feature allows one to
supply the fourth term of any mathematical proportion. Thus, the unknown in the equation could be
solved on the computer by setting 18 on the miles scale over 45 on the minutes scale and reading the
answer (32) above the 80 on the minutes scale. It is this relationship that makes possible the solution of
time-speed-distance problems. This can also be solved algebraically: (45X=18x80 therefore
X=[18x80]/45).
4.9.2. Time, Speed, and Distance:
4.9.2.1. An aircraft has traveled 24 miles in 8 minutes. How many minutes will be required to travel 150
miles? This is a simple proportion which can be written as:
Setting the 24 over the 8 on the computer as illustrated in Figure 4.15 and reading under the 150, we find
the answer to be 50 minutes.
4.9.2.2. A problem that often occurs is to find the GS of the aircraft when a given distance is traveled in
a given time. This is solved in the same manner, except the computer is marked with a speed index to
aid in finding the correct proportion. In the problem just stated, if 24 is set over 8 as in the original
problem, the GS of the aircraft, 180 knots, is read above the speed index as shown.
Given: GS 204 knots
Find: Distance traveled in 1 hour 15 minutes (75 minutes)
124 AFPAM11-216 1 MARCH 2001
Solution: Set the speed index on the minutes scale to 204 on the miles scale. Opposite 75 on the minutes
scale, read 255 NM on the miles scale. The computer solution is shown in Figure 4.16. The solutions for
time and speed when the other variables are known follow the same basic format (Figures 4.17 and
4.18).
Figure 4.15. Solve for X.
Figure 4.16. Solving for Distance When Speed and Time Are Known.
AFPAM11-216 1 MARCH 2001 125
Figure 4.17. Solving for Time When Speed and Distance Are Known.
Figure 4.18. Solving for Speed When Time and Distance Are Known.
4.9.3. Seconds Index. Since 1 hour is equivalent to 3,600 seconds, a subsidiary index mark, called
seconds index, is marked at 36 on the minutes scale of some computers. When placed opposite a speed
on the miles scale, the index relates the scales for converting distance to time in seconds. Thus, if 36 is
placed opposite a GS of 144 knots, 50 seconds is required to go 2 NM; and in 150 seconds (2 minutes 30
seconds), 6.0 NM are covered. Similarly, if 4 NM are covered in 100 seconds, GS is 144 knots (Figure
4.19).
4.9.4. Conversion of Distance. Subsidiary indexes are placed on some computers to aid in the
conversion of distances from one unit of measure to another. The most common interconversions are
those involving statute miles, NM, and kms.
126 AFPAM11-216 1 MARCH 2001
Figure 4.19. Seconds Index.
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