Landing Gear Types - Fixed and Retractable Landing Gear
Among the various types of aircraft landing gear, all are currently sorted into two main categories: fixed and retractable. Fixed landing gear simply refers to gear that is permanently attached to the airframe. Retractable landing gear refers to a system that allows landing gear to be extended and retracted from the aircraft as needed. The use of both arrangements primarily relies on aircraft speed and internal volume.
If you regularly fly on commercial airlines, you are likely familiar with retractable landing gear to some degree. This configuration is the most commonly used setup on standard commercial aircraft. Large aircraft retraction systems customarily rely on hydraulics for operation. A landing gear parts supplier, and/or a landing gear parts distributor, can expect to encounter conventional hydraulic systems components. These include, but are not limited to priority valves, actuating cylinders, and selector valves. Another vital part in the retractable gear layout is the gear door. Gear door operations and arrangements vary by aircraft. Most arrangements retract the landing gear into the fuselage, if there is space. This is typical in aircraft such as the Boeing 747 and Airbus A340. If space is not available, extra space is built extending from the fuselage.
Though the retractable configuration is the most common among commercial aircraft, fixed landing gear is still in production. It is most frequently seen on relatively small, light-weight, single engine aircraft. On occasion, it is also seen in light-weight twin engine planes. In a fixed arrangement, the landing gear parts are attached to the airframe, which exposes it to the slipstream in flight. This creates what is referred to as parasite drag. This occurs due to the extension of the landing mechanics outside of the airframe. As long as the aircraft is relatively slow, it will benefit more from having a fixed landing gear system. This is due to the fact that retractable landing parts are quite heavy. Induced drag is drag created simply due to the added weight of retractable landing gear parts. Overall, in order for fixed landing gear to be more beneficial to an aircraft, the potential parasite drag must amount to less than the potential induced drag of retractable mechanisms.
Landing gear arrangements have changed drastically over time. Leonardo Da Vinci is believed to be one of the first conceptualize a formal landing gear system for his “flying machines”. Fast forward to the 1920’s, fixed landing gear was positioned, statically, under the fuselage. It was not until 1927, that research found a considerable amount of fuselage drag was contributed by fixed landing gear. Aircraft enthusiasts might also be familiar with the 1930’s Boeing Monomail, which is one of the first planes credited with the initiation of retractable landing gear. Nearing a century after the birth of commercialized retractable landing gear, both fixed and retractable forms are still in use.