Now let’s shift to the drag force during landing — Page 280, Lesson 347

Let’s pick up right where we left off — we were comparing propeller and jet aeroplanes during the landing roll, and I want to finish that thought before we move into drag. Here’s the key point I want you to hold onto: a propeller aeroplane can use reverse thrust earlier in the landing roll than a jet aeroplane, and it can keep that reverse thrust applied all the way until the aeroplane comes to a full stop. That capability gives the propeller aeroplane a greater braking advantage over the jet during landing. So, in summary, the usable period of reverse thrust in the landing roll is shorter for a jet than for a propeller aeroplane. And because of that difference, the authorities have laid down less stringent landing performance regulations for propeller aeroplanes. We’ll get into the precise nature of those regulations later in the book — for now, just understand the cause-and-effect chain: more usable reverse thrust means a greater braking advantage, which means the regulators can be a bit more lenient on the performance rules for props. Now let’s shift to the drag force during landing. You may recall there are several forms of drag, but the two main types are parasite drag and induced drag. However — and this is important — while the aeroplane is on the ground, during both take-off and landing, wheel drag must be considered alongside the aerodynamic drag. The aim of the landing is to bring the aeroplane to a stop safely within the confines of the runway. To decelerate, sufficient rearward-directed forces need to act on the aeroplane. So, in addition to reverse thrust, aerodynamic drag plays a crucial role in landing. Let’s deal with induced drag first. Induced drag is dependent on lift and is proportional to angle of attack. During the airborne section of the landing, there is still a large amount of lift being generated, and the angle of attack is relatively high. That means induced drag is far higher than in cruising flight. But here’s the crucial change: when the aeroplane’s nose wheel touches the runway, the angle of attack drops to almost nil, and induced drag is consequently reduced to zero. So the picture you should have is this: induced drag is at its strongest while you’re still airborne in the flare, because you’re holding a high angle of attack to generate lift. The moment the nose wheel contacts the runway, that source of drag effectively disappears — it goes to zero. That’s why, on the ground, you can’t rely on induced drag to help you stop; you’re left with parasite drag, wheel drag, and reverse thrust. We’ll look at parasite drag next.