The Navigation Computer - Triangle of Velocities — Page 136, Lesson 125

I want to walk you through how we solve the triangle of velocities on the navigation computer — the wind face side, not the circular slide rule side. This is the practical tool that replaces drawing everything out on graph paper every time you plan a flight. Let me set the scene. On the previous page, you worked through a problem on graph paper. You had a true airspeed of 100 knots, a heading of 000° true, and a wind velocity of 240/30 — that's wind from 240° true at 30 knots. When you drew that triangle of velocities to scale and measured the result, you found your track over the ground was 012° true, and your ground speed was 118 knots. That works, but it is slow. Every flight would mean a fresh scale drawing with a protractor and ruler. There is a faster way: the navigation computer, using its wind face. Now we solve the exact same problem on the computer. Same situation: Warrior PA 28, TAS 100 knots, heading 000° true, forecast wind velocity 240/30. We want to find track and ground speed. First, we plot the wind vector. Rotate the wind face — the transparent disc — to bring 240° up to the index mark at the 12 o'clock position. That index is your reference for the direction the wind is coming from. Next, set a convenient datum to measure from. You can put 100 knots under the blue circle in the centre of the wind face. Alternatively, you can use the square grid printed at the bottom of the slide — that graph-paper-style grid works just as well. The blue circle is your reference point for the start of the vector. Now plot downwards — that is, in the 6 o'clock direction from the centre. Mark off 30 knots on the wind face. I recommend you use a soft pencil. You can use a felt-tip pen provided it is water-based — non-permanent ink — which usually has a grey barrel. Do not use spirit-based permanent pens with black barrels. The reason is simple: you will erase and re-plot these marks many times during training and in real flight planning, and permanent ink will ruin the disc. That plotted mark is your wind vector. It represents the wind speed and direction relative to your reference point. Now for the air vector. Rotate the wind disc again — this time to put your heading, 000° true, against the true heading index at the 12 o'clock position. Then move the slide to put your TAS of 100 knots under the blue circle in the centre. At this point, the computer should look like Figure 7.9. The centre line of the disc represents your air vector — the direction you are pointing and the speed you are moving through the air. The wind vector you plotted earlier now shows how the wind is pushing you off that heading. The computer has effectively drawn the triangle of velocities for you. What you read off next is your drift and ground speed. The drift is the angle between your heading and your actual path over the ground — that is your track. The ground speed is the length of the ground vector, which the computer shows you directly. In this case, you will find a track of 012° true and a ground speed of 118 knots — exactly the same result you got from the graph paper, but much faster. So the navigation computer's wind face is simply a mechanical way of drawing and solving the triangle of velocities. You plot the wind vector first, then set your heading and TAS, and the computer gives you track and ground speed without any protractor or ruler.