So wheel drag is low — Page 280, Lesson 349

Right, so we’ve just touched down, and I want to walk you through what happens to the drag forces during the landing roll — because this is where the physics of stopping an aeroplane really comes together. Let’s start with wheel drag. As the landing roll progresses, wheel drag increases throughout, and it reaches a maximum value just before the aeroplane comes to rest. Why? Because wheel drag is essentially the friction between the tyres and the runway — and that friction depends on how much load is on the wheels. Early in the landing run, there isn’t much weight on the wheels yet, because the wings are still generating lift. So wheel drag is low. But as the aeroplane slows, lift reduces, more weight transfers onto the wheels, and that increases the wheel drag — right up to the point just before stop. Now, the most important drag force during landing is brake drag. It’s by far the most effective retarding force because it provides the greatest stopping force. But here’s the critical relationship: brake drag is only effective if there is sufficient wheel drag — that is, sufficient friction between the tyres and the runway. If wheel drag is low, brake drag will also be low. So the brakes are only effective when there’s enough tyre-to-runway friction. That explains a key operational point. During the early part of the landing run, there’s not much load on the wheels, so wheel friction is low — and brake drag is consequently ineffective in slowing the aeroplane. But as lift reduces and more weight is placed on the wheels, brake drag becomes more effective. So brake drag increases as the landing roll progresses. And that’s exactly why pilots need to destroy lift as soon as possible after touchdown — so that braking action can be at its peak effectiveness early in the landing. The sooner you get the weight onto the wheels, the sooner the brakes can do their job. Now, in most large commercial aeroplanes, the pilots may not actually carry out the braking themselves. Instead, a highly effective automatic anti-skid braking system does it. This system can be set to low, medium, or high braking levels — that is, levels 1 through 3 — and it’s especially important to use it when landing on contaminated runways, where the friction conditions are degraded. So the chain is: destroy lift → weight on wheels → wheel drag increases → brake drag becomes effective → the anti-skid system manages the braking at the selected level. That’s the whole stopping story in one sequence.