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6
Now look at the number 15 on the disk. Between 15
and 16 each calibration mark is equal to .2 and
would be read as 15.2, 15.4, etc. If you were solving
a problem with an airspeed of 150 knots, the first
calibration past 15 (150 in this case) would be 152.
The spacing changes again at the number 30,
where each calibration becomes .5, and at 60,
where each calibration equals 1. Before you read a
value from the disks, be sure you understand what
each line of calibration is equal to.
You will use the slide rule side to establish
ratios. With the numbers matched, the ratio is 1 to
1. Now set the number 60 (the rate arrow) directly
opposite to (or, “lined-up” with) 12 on the outer disk
(
see
Figure 2). Notice that all of the numbers on the
outer disk are exactly twice the value of the num-
bers on the inner disk: 90 is opposite of 180, 15 is
opposite of 30, 3.5 is opposite of 7.0. You will use
these ratios in solving time-speed-distance and fuel
consumption problems.
Look at the inside scale on the disk, where the
lines of calibration look like clock times: 9:00, 6:00,
etc.—these express hours. The inner scale is in
hours and the middle scale is in minutes. 1:10 is
directly below 70; one hour and ten minutes is the
same as seventy minutes. 5:00 is printed below 300
minutes, and five hours is the same as 300 minutes.
To convert minutes to seconds, place the rate arrow
opposite to the minutes on the outer scale and read
seconds opposite to the “seconds” arrow, just to the
right of 35 on the inner scale.