The Navigation Computer - Triangle of Velocities — Page 136, Lesson 127

I want to walk you through how the navigation computer solves the triangle of velocities. We've already seen how to plot the wind vector on the computer's face. Now let's interpret what we see. Look at where the wind mark is positioned on the computer. In our example, it sits about 12 degrees to the right of the centre line. That tells you something critical: you have 12 degrees of starboard drift. Remember, starboard means right. Your heading is 000° — that's due north, straight up the centre line. Because the wind is pushing you 12 degrees to the right, your actual path over the ground — your track — becomes 012°. Now, how do we get ground speed? You read it off against the radial arcs on the computer. In this example, the ground speed comes out to 118 knots. These are exactly the same answers we got earlier by doing a scale drawing on paper. All the navigation computer has done is create an analogue model of that scale drawing — it's a physical, mechanical way of doing the same geometry. Let me describe what's happening on the computer's face using the proper vector names. The centre line represents the Air Vector. It points toward 000° — due north — and its length represents 100 knots true airspeed. That's your aircraft's motion through the air mass. Connected to that is the Wind Vector, which you have plotted downwind yourself — you physically moved the wind dot or slid the grid to represent the wind direction and speed. The resultant vector — the one that connects the start of the Air Vector to the end of the Wind Vector — is the Ground Vector. This is your actual path over the earth. It tracks up the 12° starboard drift line, and from that line you read both your track direction and your ground speed from the vector's length. So on the computer, the centre line is your heading and airspeed through the air. The wind mark shows drift. The line from the centre dot to the wind mark gives you track and ground speed. That's the triangle of velocities, modelled mechanically on the computer's face. Now, the book gives you a table of practice examples with headings, wind velocities, true airspeeds, and the resulting tracks and ground speeds. These are for you to work through on your own computer. I'll let you try those one at a time.