Summary
Highlights
This lesson introduces the design and construction of fixed and retractable landing gear, including hydraulic retraction, steering, and emergency systems. It focuses on the functions of landing gear, which include ground maneuvering, supporting the aircraft, providing clearance for components, facilitating loading, absorbing landing shocks, and enabling deceleration. The landing gear is effectively dead weight when airborne, leading to extensive research in weight and space reduction.
There are two main landing gear layouts: the tricycle type, with a nose wheel and two main gear units, and the tailwheel (or taildragger) type, with a tailwheel behind the main wheels. The tricycle layout offers advantages such as reduced risk of tipping, improved pilot visibility, and a more level platform for loading, making it the preferred choice for most modern aircraft. The main gear units support up to 90% of the aircraft's weight and absorb initial landing shocks, while the nosewheel provides steering. Fixed, non-retractable landing gear is common for slower, lighter aircraft due to its simplicity, lower cost, and reduced maintenance, despite increasing drag. Higher performance aircraft use retractable gear to minimize drag, though this adds weight, complexity, and maintenance.
Fixed landing gear commonly uses three types of shock absorption systems: spring steel legs, rubber cords, and oleo-pneumatic struts. Spring steel legs are typically used for main landing gear, consisting of a tapered steel tube or strip bolted to the fuselage. Rubber cord systems utilize tubular struts designed to direct landing forces against a loop of rubber turns. Oleo-pneumatic struts are found in some fixed main gears, most fixed nose gears, and almost all retractable landing gears, varying in design but operating on the same principle.
An oleo-pneumatic strut consists of two concentric cylinders: an upper outer cylinder fixed to the airframe and a lower inner cylinder attached to the wheels and axle. The cylinders contain hydraulic fluid and compressed gas, separated by a free-floating piston. Torque links prevent rotary movement and limit vertical movement between the cylinders, absorbing torsion loads during ground maneuvering. The gas supports the aircraft's weight, cushions taxiing bumps, and absorbs landing shocks, while the fluid dampens oscillations and controls cylinder compression and extension rates. A piston with holes restricts fluid flow, thereby dampening movement. A flutter valve, a free-floating circular plate with a central hole and peripheral holes, is fitted to the lower cylinder piston assembly, limiting extension movement more than compression movement to prevent bouncing upon landing. A gas leak is indicated by the strut not extending fully, revealing uneven amounts of shiny metal on the leg.
Various methods are used to connect wheels to the undercarriage leg, including fork, half-fork, cantilever, dual wheel, and multi-wheel bogie or truck systems. These methods are generally self-explanatory based on their names. The lesson concludes by summarizing the key takeaways: the three types of fixed landing gear, the role of gas in supporting the aircraft and absorbing shocks, the fluid's function in dampening oscillations, the purpose of torque links, and the common methods for connecting wheels.