General Principles - Landing — Page 280, Lesson 348

We’re starting the landing phase of the performance chapter. Let’s look at the forces that actually stop the aeroplane once it’s on the runway. First, parasite drag. This is the second form of aerodynamic drag, after induced drag. Parasite drag depends on two things: the aeroplane’s forward-facing cross-sectional area — which is more accurately called form drag — and the aeroplane’s forward speed. So the bigger the frontal area and the faster you go, the more parasite drag you get. Now, in the landing configuration, the flaps and slats are fully extended. That significantly increases the form drag, and therefore significantly increases parasite drag. Then, once the aeroplane touches down and the spoilers and speed brakes are deployed, parasite drag increases even further. But here’s the key: as the speed rapidly decays after touchdown, the parasite drag decays with it, eventually decreasing to zero once the aeroplane reaches a full stop. So, in summary, the total aerodynamic drag — induced drag plus parasite drag — is very high during the early part of the landing, but very soon after touchdown it decays rapidly. Now let’s move to wheel drag and brake drag. Wheel drag is the friction force between the wheel and the runway, and also with the wheel bearings. Brake drag, on the other hand, is the friction force between the brake discs and the brake pads. Wheel drag comes into play as soon as the aeroplane touches down. But here’s the subtle part: friction is a function of the force pushing two surfaces together. Because there is still a lot of lift being generated right after touchdown, the wheel load is small during the initial part of the landing run, and therefore wheel drag is also small. As speed reduces, and as lift is destroyed by the spoilers, the wheel load increases, which in turn increases the wheel drag. So wheel drag increases throughout the landing roll and will reach a maximum value just before the aeroplane stops. Let me show you the landing distance picture. That’s the landing distance available — the length of runway from one threshold to another. And here’s the jet engine in reverse thrust mode. So the picture you should hold: aerodynamic drag is huge at the start of the landing but dies away quickly, while wheel drag builds up as lift is destroyed and wheel load increases. Both work together to bring you to a stop.