Right, let's get into the landing gear shock absorption — Page 94, Lesson 128

Right, let's get into the landing gear shock absorption. We've got a few different ways to absorb that landing impact, and I want to walk you through them properly. First up, we have spring steel legs. These are usually found at the main undercarriage positions. The leg itself is a tube, or a strip of tapered spring steel. The upper end is attached by bolts to the fuselage, and the lower end terminates in an axle, on which the wheel and brake are assembled. So the whole thing flexes on landing to absorb the energy. Next, we have rubber cord. When rubber cord is used as a shock absorber, the undercarriage is usually in the form of tubular struts. These are designed and installed so that the landing force is directed against a number of turns of rubber, in the form of a grommet or a loop. So the rubber twists and stretches to take the load. Now, the big one — the oleo-pneumatic strut. Some fixed main undercarriages, and most fixed nose undercarriages, are fitted with this type of shock absorber. The design varies considerably between individual struts, but I want to point out one thing worth considering: the fitting of spats. Spats are an aerodynamic fairing, which may be required to minimize the drag of the landing gear structure. The drawback is that they can pick up mud when landing or taking off from grass airfields. That mud can add considerably to the weight of the aircraft, and it may affect take-off performance. So, to avoid that eventuality, if any mud has been picked up, the spats must be removed, cleaned, and replaced before the next take-off. Let's look at the anatomy of the oleo-pneumatic strut itself, because it's a clever piece of engineering. We have an upper cylinder and a lower cylinder. Inside, we have a fluid and a gas, separated by a separator piston. There's also a piston that moves within the assembly. At the bottom, we have the axle, and at the top, the fluid filler plugs for adding fluid, and a gas inflation valve for charging the gas. Now, the key to how it works is the flutter valve. When the leg extends, the extension of the leg causes the flutter valve to rise, restricting the flow of the fluid. That restriction is what damps the movement. We also have sealing rings — one on the separator piston and others around the assembly — to keep the fluid and gas where they belong. And we have an upper torque link and a lower torque link, which keep the two cylinders aligned and prevent them from rotating relative to each other. So, in short: the gas acts as the spring, compressing to absorb the initial impact, and the fluid, forced through the restricted flutter valve, provides the damping to control the rebound. That's the oleo-pneumatic principle in a nutshell.