General Principles - Descent — Page 228, Lesson 268

Let’s pick up with the next factor that shapes the angle and rate of descent: the aeroplane’s configuration. And I want to start by anchoring you in the same assumption we used for weight — we’re looking at this with idle thrust, so the engine is producing no useful power. That keeps the picture clean. Here’s the core idea. If you deploy the flaps or the undercarriage, the drag increases. Look at Figure 4.11 and you’ll see that excess drag increases. Now, to balance that increase in excess drag, the nose is lowered. Lowering the nose increases what we call weight apparent thrust — that’s the component of the aeroplane’s weight acting along the flight path, pulling it forward and down. So the balance of forces is restored, but here’s the important part: the balance is achieved at a higher angle of descent, and therefore a higher rate of descent. Let me say that again slowly, because it’s the whole point. Configuration change — flaps or gear out — means more drag. To keep the forces balanced, you trade that extra drag for a steeper glide path. The aeroplane descends more steeply and more quickly. Now let’s see the same effect on the graphs. Figure 4.12 shows the drag curve for both the jet and the propeller aeroplane, with the excess drag shown as the purple area. When you deploy flaps and undercarriage, the curves move up and to the left. Moving up means more drag at any given speed. Moving left means the whole curve shifts to lower speeds. The result is that excess drag increases, and therefore the angle of descent increases for any given speed. And notice this — the speed for the minimum angle of descent, which we call VMD, is lower. VMD stands for velocity for minimum drag, and that’s the speed that gives you the shallowest glide angle. With gear and flaps out, that speed drops. We see the identical story in Figure 4.13, but now we’re looking at the rate of descent using the power required graph. Again, the purple area represents excess power required. With flaps and gear deployed, the power curves move up and left. That increases the excess power required, and therefore increases the rate of descent. And again, the speed for the minimum rate of descent, which we call VMP — velocity for minimum power — is lower. So let me summarise this configuration section, because it’s a clean, examinable result. With gear and flaps deployed, the angle of descent increases, the rate of descent increases, but the speeds for minimum angle and minimum rate of descent both decrease. More drag, steeper and faster descent, but at a slower airspeed for the best glide performance. Now let’s move to the next factor: wind. Figure 4.14 shows the effect of headwinds and tailwinds on the angle of descent. Here’s the relationship. A headwind steepens the glide angle and decreases the descent range. A tailwind does the opposite — it decreases the glide angle but increases the descent range. So if you’re flying into a headwind, you come down more steeply and you don’t travel as far horizontally. With a tailwind, you glide more shallowly and cover more ground. But here’s the subtle and often-tested point. Notice that the aeroplane in a headwind or a tailwind reaches the same descent altitude in the same time as the aeroplane flying in zero wind conditions. That’s the key demonstration. A headwind or tailwind has no effect on the rate of descent. The rate of descent is purely a function of the aeroplane’s performance in the air mass — the wind just moves the whole air mass, and the aeroplane with it. So the vertical speed stays the same; only the horizontal distance covered changes. Let me make sure you’ve got the full picture. Configuration changes the drag, which changes both the angle and the rate of descent, and it shifts the best speeds. Wind changes the angle and the range, but leaves the rate of descent untouched. Those are two distinct effects, and you want to keep them separate in your mind.