Then by — Page 84, Lesson 89

Right, let's pick this up exactly where we left off. We've just finished the division, and I want to show you the clever bit that makes this instrument so powerful for navigation. We were dividing 2000 by 60. Now, here's the key insight: instead of putting one hour against the 2000 on the outer scale, we put 60 minutes — the inner scale — against that 2000. Because 60 minutes is one hour, we're still dividing 2000 by 60, but now the answer is expressed as a rate per minute rather than per hour. If you look at the red arrow on the inner scale, you'll see it points to 33.33. So 2000 kilograms per hour equals 33.3 kilograms per minute. That's the rate-per-minute trick, and it's the foundation for everything that follows. Now, let's move into the core of this chapter: Distance, Speed and Time. This is the bread and butter of navigation. And I want you to remember one golden rule that governs the whole instrument: the outer scale is always distance or speed, and the inner scale is always time. Always. Distance and speed live on the outside; time lives on the inside. Let's work through the three classic problems. First: How far do you go in 48 minutes at 120 knots ground speed? We set what we know. We know the ground speed, 120 knots. So we bring the black '60' triangle — that's the one-hour index — against 120 on the outer scale. That sets up our speed. Now we find 48 minutes on the inner scale, and read off against it on the outer scale. The answer is 96. And the units? We're working in nautical miles, so it's 96 NM. That's the distance problem solved. Second problem, the reverse: In 30 minutes you've covered 90 NM. What's your ground speed? Again, set what you know. Put 30 minutes on the inner scale against 90 NM on the outer scale. Now read off the ground speed against the black '60' triangle. The instrument gives you 18.0. But wait — is that 18 knots? At 18 knots you'd stall. Is it 1800 knots? At 1800 you'd strip the wings off. Common sense tells you it must be 180 knots. This is the order-of-magnitude check, and it's a habit you must build into every single calculation you do on this computer. The instrument gives you the digits; you supply the decimal point. Third problem: Ground speed 220 knots, 57 NM to go. How long will it take? Set what you know — move the black '60' triangle against 220 knots. Now bring the cursor round to 57 NM on the outer scale, because it's a distance. Read off the time on the inner scale. The answer is 15.5. Now, are we expecting 15 minutes or 150 minutes? Think about it: 57 NM is about a quarter of 220, so it should take about a quarter of an hour. The answer is 15.5 minutes. So you see the pattern in all three: set what you know, read off what you don't, and always sanity-check the order of magnitude. That's the whole of distance, speed and time on this computer.