General Principles - Descent — Page 221, Lesson 256

Let's pick up right where the forces in a descent left off. We've established that in a normal powered descent, the weight apparent thrust — that's the component of weight acting along the flight path — balances the excess drag. And we calculated that weight apparent thrust by multiplying weight by the sine of the angle gamma. Now, here's the key extension. If you reduce thrust even more, as shown in Figure 4.2, you create a greater amount of excess drag. That excess drag now needs more weight apparent thrust to balance it. But to get more weight apparent thrust, you have to lower the nose even more, which increases the descent angle. And for exam purposes, remember: lowering the nose is a decrease in pitch. So the central idea is this: it is the excess drag that determines the angle of descent. And notice something important — the angle gamma in our force diagram is the same angle as the angle of descent itself. Now, let's formalize this. From the force balance equation, which we wrote as DA = T + W sin γ, we can rearrange to solve for the angle gamma. That rearrangement gives us the formula for the angle or gradient of descent: Gradient of Descent (%) = (D − T) / W × 100 Let me unpack that. D is drag, T is thrust, and W is weight. So D minus T gives you the excess drag. You divide that by weight, and multiply by 100 to express it as a percentage. That's your gradient of descent in percent. So in summary: the angle or gradient of descent is controlled by the excess drag. To visualize this excess drag, we return to the thrust and drag graphs from the climbing lesson. Look at Figure 4.3 — it shows the thrust and drag curves for both a jet and a propeller aeroplane. To descend, there must be an excess of drag. On these graphs, you find excess drag by taking the area beneath the drag curve and subtracting the area beneath the thrust curve. The solid purple highlighted areas represent that excess drag. And here's the relationship: if you reduce thrust at any given speed, excess drag increases, and therefore the descent angle increases. Now, what if maximizing the angle of descent becomes a performance priority? From the theory, you'd have to maximize excess drag. Figure 4.4 shows what happens if you reduce thrust to zero. The excess drag area becomes a maximum — but to achieve that maximum excess drag, the aeroplane must be accelerated to a very high speed. You do this by closing the throttles and continuously lowering the nose, so the increasing weight apparent thrust accelerates the aeroplane. As speed rises, both excess drag and the angle of descent increase. So the angle of descent is a function of excess drag: the greater the excess drag, the steeper the angle of descent. And you could increase that angle even further if you could increase the excess drag more. That's done by deploying drag devices — the speed brakes and the undercarriage. But you must pay attention to their maximum deployment speeds. That's the complete picture of how excess drag governs your descent angle.