Landing Gear — Page 100, Lesson 140

I want to walk you through the nose wheel steering system and then the undercarriage configuration. We're in the landing gear chapter, and I've already covered the basic steering operation, so let's pick up with what happens when you centre the control valve. When the control valve is in its neutral position, fluid is free to flow between the steering jacks. That's the castoring condition. What that means in practice is the nose wheel is free to swivel, so the aircraft can be towed on the ground without fighting the steering hydraulics. And after you've made a turn with the steering wheel, when you release it, the nose wheel can return to the central position because the fluid just flows between the jacks. Now here's a nice detail — if the aircraft is being towed and the nose wheel is turned, that angular movement is transmitted back through the follow-up linkage to the steering wheel. So the steering wheel in the cockpit actually moves as the nose wheel is castored during towing. There's also a quick-release pin, often provided, so the steering jacks can be disconnected. That lets ground crew turn the nose wheel through large angles during servicing, angles the steering system wouldn't normally allow. Now, damping. Restrictors are fitted in the pipelines between the control valve and the steering jacks. Those restrictors provide damping for the nose wheel steering operation — they smooth out the motion so the steering doesn't oscillate. Next, nose wheel shimmy. This is an important one. Because the tyre side walls are flexible, an unstable, rapid sinusoidal oscillation can be induced into the nose undercarriage. That oscillation is called shimmy. It's a rapid side-to-side vibration. Excessive shimmy, especially at high speeds, can set up vibrations throughout the whole aircraft, and it can be dangerous. What increases the tendency to shimmy? Worn or broken torque links, wear in the wheel bearings, and uneven tyre pressures. All of those make shimmy more likely. And there are several ways to reduce it. A hydraulic lock across the steering jack piston. Fitting a hydraulic damper. Fitting heavy self-centring springs. Double nose wheels. Or twin contact wheels. Each of those is a design measure to suppress that oscillation. Now let's move to undercarriage configuration. As modern aircraft grew in size and all up weight — that's AUW — wheel loading increased. Wheel loading is defined as the static load on each wheel of the landing gear at aircraft take-off weight. Since the main undercarriage carries a large proportion of the aircraft weight, the main wheels are the greatest problem. Wheel loading, measured in pounds per unit area, has a direct bearing on the type of surface the aircraft can operate from. So the role of the aircraft directly affects the undercarriage configuration. An aircraft with high wheel loading would damage the surface of a low-strength runway. Now, it's very expensive to strengthen the very long runways required for modern transport aircraft. So undercarriages that confer low wheel loadings are in considerable use. The approach is to replace large single wheels using high-pressure tyres with a number of small wheels using low-pressure tyres. The larger aircraft — the B747, B777, and A340 — may have 10 to 18 wheels in their landing gear. And more than two main legs may be provided to spread the load. The 747, 777, and A340, for example, have both wing gear and body gear. Finally, the actual configuration chosen for the aircraft is determined by two things: the problem of stowage when the gear is retracted, and the load-spreading consideration. So it's not just about the runway — you also have to fit all those wheels and legs into the airframe when they retract. That's the castoring and damping behaviour, shimmy and its cures, and how wheel loading drives the whole undercarriage layout.