Let me begin with the introduction — Page 84, Lesson 87

I want to walk you through the start of Chapter 5, which is all about using the Navigation Computer for distance, speed, time, and conversions. This builds directly on the ratio technique we saw in the last chapter. Let me begin with the introduction. In the previous chapter, we learned how to use a proportioning, or ratio, technique on the Navigation Computer when dealing with problems of rates — that is, quantities per minute or quantities per hour. The example we used was a rate of climb, expressed as 700 feet per minute. By setting '700' on the outer scale against '1' on the inner scale, we could read off the number of feet gained on the outer scale in 2 minutes, 3 minutes, 4 minutes, and so on, by looking at the corresponding inner scale values. Now, there's a key feature on the inner scale of the Navigation Computer: the red arrow. That red arrow is the indicator of '1', and it's used as the inner scale reference if the answer we want is in the same units. For example, if we use the red arrow to indicate '1 minute', then the answer will come out in minutes and decimals of a minute. This is useful when the rate is given in quantity per minute, which is how rate of climb or descent is usually expressed. But here's the important distinction: if we use the red arrow to indicate '1 hour', then the answer will be in hours and decimals of an hour. That may not be as useful, because few of us work in decimals of an hour. Usually, even if the rate is given 'per hour', we want the answer in minutes, or hours and minutes. That's where the '60' index comes in. The '60' index is the black triangular arrow on the inner scale. If we have a rate given per hour — for instance, fuel flow is usually given in pounds or kilograms per hour, but we usually want the answer in minutes or hours and minutes — then we put the '60' index against the hourly rate. Let me give you the example from the text: suppose our fuel flow is 2000 kilograms per hour. By putting 60 minutes on the inner scale — instead of one hour — against 2000 on the outer scale, we are effectively dividing 2000 by 60. Then, if you look at the inner scale, you can read off the fuel used in any number of minutes directly. So to summarise the two indices: the red arrow is the '1' index, used when your rate is per minute and you want answers in minutes. The black triangular '60' index is used when your rate is per hour but you want answers in minutes. This distinction is fundamental for all the speed, time, distance, and fuel problems we're about to work through.