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What Is Undercarriage in Aircraft?

Whether you fly small jets or heavier-than-air aircraft, all will have an undercarriage that attaches the tires, skids, float, or skis to the airframe. It can be fixed, retractable, or a combination of both, where the main gear retracts while the tailwheel does not.  Knowing how the undercarriage supports your aircraft’s weight, while it is not on the ground or water, is critical to understanding how to take off, land, taxi, and maintain control of your vessel. Such knowledge is also helpful when carrying out maintenance or assessing different parts of your aircraft.

The undercarriage, in its most basic form, was found on an original Wright Flyer. It consisted of a simple pair of skids that were mounted centrally under the plane and provided sufficient balance to prevent the aircraft from tipping forward or aft. Years later, this configuration would become popular among helicopter manufacturers, but eventually fell out of favor with airplane designers just before WWI when wheels were added to the design. However, one exception to this is the tailskid which helped slow the aircraft upon landing as airplanes during this era lacked brakes and did not land on pavement.

Shortly after, retractable undercarriages were developed, offering higher cruising speeds, with the drawback of additional weight maintenance requirements. If the pilot forgot to extend the undercarriage for landing, maintenance could become extensive. By 1947, however, there were two primary types of land-based aircraft: the retractable and the fixed type. Over time, advanced materials and aerodynamics drove the development of undercarriage in the “down and welded position,” especially for new personal airplanes. This configuration’s simple design, reduced empty weight, and minimal training requirements made the fixed gear design more favorable.

Some aircraft did not have any landing gear at all. For instance, the Hughes H-4 Hercules, popularly known as the Spruce Goose, was designed for landings and takeoff on water, ridding the need for anything considered an undercarriage. Furthermore, prior to WWII, flying boats like the Boeing 314 also lacked wheels, mostly due to the fact that contemporary landing fields were not equipped to handle large aircraft. It is important to note that water as a landing surface posed fewer challenges. With so many factors and advancements to consider, we will outline various undercarriage systems.

What Is a Conventional Undercarriage?

With a conventional design, the main wheels are situated in front and lateral to the aircraft’s center of gravity. Meanwhile, at the tail, there is a smaller wheel that is steerable, using the rudder pedals. The tailwheel can be retractable which is preferred for aircraft that approach unprepared landing areas. Moreover, using a small tail wheel reduces the risk of damaging the larger nosewheel of an airplane which is present with a tricycle-type undercarriage. This type of arrangement is lighter, more cost effective, and requires less maintenance than other types.

What Is a Tricycle Undercarriage?

Today, no manufacturer offers anything other than a tricycle design because it eliminates the ground loop and high-pitched nose problem. More than that, the tricycle design offers increased stability because the airplane’s center of gravity is ahead of the main wheels. Despite its benefits, there are a few drawbacks to this arrangement. The first major issue is that weight as the nosewheel is significantly heavier than the tailwheel. If possible, add more weight if the undercarriage has steerable and/or retractable design. Unfortunately, the additional weight makes slow-speed takeoff more difficult and necessitates a higher speed than the conventional type. Another issue is the nosewheel’s tendency to shimmy, but getting the weight off the nosewheel, and checking that all the bushings and strut components are within tolerances can minimize the problem.

Fixed v. Retractable

Non-retractable undercarriages, also called fixed-gear systems, can be of the tricycle configuration, such as the Cessna 172, or conventional, like the Piper Cub. Retractable tricycle gear aircraft typically fold all three gear legs, with the exception of a few models by Rutan. In addition, a retractable undercarriage offers an increase in speed and reduces drag. Additionally, all retractables are equipped with some kind of emergency gear-extension system. From time to time, manufacturers have dabbled around the idea of automatic extension and retraction systems. Other common undercarriage designs include castering main gear, outriggers, single-wheel, tandem-wheel, and four-wheel systems.


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