Here’s the scenario — Page 221, Lesson 258

Let’s pick this up right where the idea of excess drag left off, because the whole descent story hinges on it. We just said that increasing excess drag — by deploying speed brakes, putting the undercarriage down, or reducing thrust — steepens the descent angle. Now I want to flip that completely on its head, because the opposite situation is what you need after an engine failure. Here’s the scenario. One engine has failed. Your priority is no longer to get down quickly. Your priority is to cover the greatest possible horizontal distance, so that you have a large area in which to select a suitable landing field. To fly the greatest distance, you need the smallest possible angle of descent — and that is sometimes called the minimum glide angle. Now, think about what we just established. Steeper descent means more excess drag. So the logical inverse is: to descend at the smallest possible angle, excess drag must be minimum. That’s the key relationship — minimum glide angle requires minimum excess drag. So where does excess drag hit its minimum? Look at the drag curve. The speed at the very bottom of that curve — the speed where drag itself is at its minimum — is called VMD, which stands for velocity for minimum drag. At VMD, drag is at its lowest value. And since, with no power, excess drag is simply drag minus thrust, and thrust is zero in a glide, the excess drag is minimum exactly at VMD. So the rule is: to descend at the minimum possible angle, you fly the aeroplane at VMD. Now, there’s another way to look at glide performance, and that’s through the lift over drag ratio — commonly called the lift drag ratio. Before we use it, I need to set up a little geometry for you. In a glide descent, the resultant of drag and lift balances the force of weight. From your Principles of Flight work, you’ll remember that the resultant of lift and drag is called the total reaction. If you take the drag force line and move it upwards, you form a triangle of forces. In that triangle, the angle between the lift vector and the total reaction is called angle gamma. And here’s the beautiful part: that angle gamma is the same as the angle of descent. So if lift and/or drag values change, both angle gamma and the glide angle change together — they’re locked to each other. Now, the payoff. A typical modern jet has a maximum lift drag ratio of about 19. And where that ratio reaches its maximum, the value of angle gamma will be at its minimum — which, of course, means the shallowest glide. That’s the connection: maximum lift drag ratio gives you the minimum glide angle, and that happens at VMD. So to tie it all together: after an engine failure, you want maximum range in the glide. That means minimum glide angle. Minimum glide angle means minimum excess drag. Minimum excess drag happens at VMD. And VMD is where the lift drag ratio is at its maximum — about 19 for a typical modern jet.