In the last chapter, we set up ratios for rates like climb — Page 84, Lesson 87

We're starting Chapter 5 of your General Navigation book, and this is where the Navigation Computer really earns its keep. In the last chapter, we set up ratios for rates like climb. Now we're formalizing that into the core of distance, speed, time, and conversions. Let me walk you through the setup. The computer has two circular scales, an outer and an inner. When we set a rate, we align a quantity on the outer scale against a time on the inner scale. For a rate of climb of 700 feet per minute, we put 700 on the outer scale over 1 on the inner scale. Then any value on the inner scale—2, 3, 4 minutes—reads out the feet gained on the outer scale. That's the ratio technique. Now, the key refinement. On the inner scale there's a red arrow. That's the indicator of '1'. It's your inner scale reference when the answer you want is in the same units as the rate. If you set the red arrow to mean 1 minute, then your answers come out in minutes and decimals of a minute. That's handy when your rate is per minute, like climb or descent rates. But here's the catch. If you set the red arrow to mean 1 hour, your answers come out in hours and decimals of an hour. And few of us think in decimals of an hour. Usually, even when the rate is given per hour, we want the answer in minutes, or hours and minutes. So for that case—and fuel flow is the classic example, given in pounds or kilograms per hour, but we want minutes or hours and minutes—we use the '60' index. That's the black triangular arrow. We put the 60 index against the hourly rate. Let's say fuel flow is 2000 kilograms per hour. By putting 60 minutes on the inner scale, instead of 1 hour, against 2000 on the outer scale, we're effectively dividing 2000 by 60. That gives us the flow per minute directly. So you have two reference points on the inner scale: the red arrow for '1' and the black triangular arrow for '60'. The red arrow is your base '1' index, and the black triangle is your base '60' index. Choosing which one to use depends entirely on the units you want in your answer. If you want per minute, use the red arrow as 1 minute. If you want per hour, use the red arrow as 1 hour. If you want minutes from an hourly rate, use the 60 index. That's the foundation. The rest of the chapter builds on this—the yellow scale, the blue scale, fuel consumption, distance and volume conversions. But this ratio setup, and knowing which index to use, is the heart of it.