Landing Gear — Page 94, Lesson 126

I want to walk you through the landing gear chapter now. This is the start of a big topic, so let's build it properly from the ground up. First, the introduction. The landing gear has three core functions, and I want you to hold onto all three. One: it provides a means of manoeuvring the aircraft on the ground. Two: it supports the aircraft at a convenient height to give clearance for propellers and flaps, and to facilitate loading. Three: it absorbs the kinetic energy of landing and provides a means of controlling deceleration. That third one is the tricky one, and we'll come back to it. Now, the design philosophy. Once the aircraft is airborne, the landing gear serves no useful purpose. It is dead weight. So the ideal would be to replace it with some ground-based equipment. For the first two functions — manoeuvring and supporting at a convenient height — that might be possible. But for the third, absorbing the kinetic energy of landing, no satisfactory ground-based alternative exists. That's why so much research has gone into undercarriage design: to reduce weight and stowed volume when retracted. Next, the big classification: fixed or retractable. With slow, light aircraft, and some larger aircraft where simplicity is prime, a fixed, non-retractable gear is often fitted. The drag penalty in flight is offset by simplicity, reduced maintenance, and low initial cost. But with higher-performance aircraft, drag becomes progressively more important, so the gear is retracted into the wings or fuselage. The penalties there are increased weight, greater complication, and additional maintenance. Now, fixed landing gear. There are three main types. Those with a spring steel leg. Those that employ rubber cord to absorb shocks. And those with an oleo-pneumatic strut to absorb shocks. There are exceptions — rubber in compression, spring coil, and liquid spring struts — but the three main ones are spring steel, rubber cord, and oleo-pneumatic. Let's start with spring steel legs. These are usually employed at the main undercarriage. The idea is that the leg itself flexes to absorb the landing shock. It's simple, but it's a rigid structure that takes the energy through deflection of the steel. Now, the oleo-pneumatic strut is the one you'll see most in transport-category aircraft, and I want to introduce it properly because it's central to this chapter. Let me show you the construction. Look at Figure 3.1, an oleo-pneumatic strut. The name tells you the working fluids: oleo means oil, pneumatic means air or nitrogen. So this is an oil-and-gas shock absorber. The strut has an outer cylinder and an inner sliding tube — the piston. Inside, there's oil and compressed gas, typically nitrogen. When the aircraft lands, the piston slides up into the cylinder, compressing the gas, which absorbs the kinetic energy. The oil is forced through orifices — small holes — which dissipates energy as heat. That's the oleo-pneumatic principle: gas acts as the spring, oil acts as the damper. Now, the excerpt cuts off mid-sentence on spring steel legs, so let me pause there. We've covered the three functions of the gear, the fixed-versus-retractable trade-off, and the three main types of fixed gear. The oleo-pneumatic strut is the one we'll build on throughout the chapter, so make sure you're comfortable with the oil-and-gas, spring-and-damper idea before we move on.