Aircraft Landing Gear Classification on Layout
The classification of aircraft landing gear is a fundamental yet essential task, as it establishes the foundation for systematic design processes like trade-off analyses and comparative evaluations. Clear categorisation promotes effective communication amongst engineers, ensuring consistent terminology across designs and applications. For example, describing an aircraft to be composed of a ‘tricycle landing gear’ provides a sufficiently concise description of the fundamental arrangement. This article classifies the various landing gear installations, recommends a standardised naming convention that focuses on wheeled arrangements such as tricycles and bicycles, but touches upon other non-wheeled setups such as pontoons, skis, and skids.
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Landing Gear Classification – Poster
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The approach preferred within this article is to attempt to apply a consistent naming convention which is unambiguous. For the purpose of convenience, we presently assume a discussion centered on wheeled systems; however, skis, pontoons and skids are also possible and addressed within the article.
Perhaps the most common landing gear arrangement is the tricycle arrangement consisting of a single front or nose structural assembly and two main structural assemblies located close to the lengthwise mid-section of a typical aircraft. A second well-known but less common arrangement is the bicycle arrangement consisting of two structural assemblies; one at the front and a second rear-mounted assembly with both assemblies in line with each other. Legendary aircraft such as the U2 and Harrier are examples of this arrangement.
Therefore, keeping the naming convention of bicycles (and a respectful nod to early aircraft pioneers who came from a bicycle design background), the convention is extended to the single-wheeled unicycle and four wheeled quadricycle. Note: the definition of a monocycle implies the ‘rider’ or ‘power source’ is within the wheel diameter, rather than above it as in a unicycle. Because the fuselage of the aircraft is above the wheel, rather than inside it, unicycle is the correct word usage.
From this, there are three important sub-categories: this includes the inverted tricycle, the extended tricycle, and the dicycle arrangements.
This is consists of two front assemblies with a single tail-wheel assembly. This is commonly referred to as the tail-dragger arrangement. There are many examples of this arrangement from present-day helicopters to historical propeller aircraft.
These are aircraft which have the principle of a tricycle arrangement (one landing gear at the nose) but, due to aircraft size, and load capacity, may have more than two central main landing gears. Aircraft such as the A380, A340, and B747 are examples of the extended tri-cycle arrangement.
Lastly, and for completeness, is a bicycle-type configuration (two wheels), but with wheels mounted in a side-by-side configuration. This is named the dicycle. While there are a few wheeled aircraft that could fall into this category, it is required when extending the classification to non-wheeled aircraft such as those fitted with pontoons, skis, or helicopter skids. However, in all these cases identifying ‘pontoons’ would naturally imply side-by-side arrangement leaving category title perhaps to raise a few skeptically eyebrows. Nevertheless, there are several historical wheeled examples of this arrangement.
Related to the two-wheeled systems, vocabulary such as ‘dual’ or ‘tandem’ is specifically avoided because these have specific definitions at the landing gear subsystem level that are commonly used describing those system. However, it needs to be recognised that in discussions regarding skis, pontoons, etc., it’s common referred to as dual skis, rather than two skis.
Remaining there are two more discussion topics required to complete the classification. This includes a naming conventions to identify the specific structural element, and a discussion regarding auxiliary Landing Gears
There are a number of naming conventions; however, it’s recommended to use an appropriate combination of the following, which is unambiguous for the specific aircraft. The word ‘main’ in general is reserved for the structures carrying the largest loads. For a tricycle aircraft these are the structures about mid-length, but for a tail-dragger arrangements, this refers to the front two structures. A bicycle configuration may have a gear which is more appropriately called the main and tail, rather than using a front and rear definition. Aircraft with more than two mid-length structures may benefit from a convention using ‘main’ and ‘center’ landing gear (such as the DC-10), or ‘wing’ and ‘body’ landing gear (such as A380) referring to the location of the structural attachments.
Auxiliary landing gears are an additional set of structures on an aircraft designed to assist ground operation. On some aircraft, such as the B-52 and the Harrier, auxiliary landing gear play a critical role due to each aircraft’s unique design features. The B-52 uses these ‘outriggers’ to prevent wingtips from striking the ground, and the Harrier, to stabilise ground operation. Note: the U2’s wingtip structures are referred to as pogos because they detach on takeoff, and therefore are absence for landing.
Listed in the table below are a number of examples for the different landing gear layout for aircraft. There is more emphasis on wheeled assemblies with many historical and some iconic aircraft listed.
Unicycle
| Europa XS | A Single retractable wheeled assembly, with accompanying wing appendages and small tail wheel assembly |
Bicycle
| Harrier & AV-8B | Composed of dual wheeled rear LG, a steerable front or nose LG, and two wing mounted auxiliary LG. All LG’s are retractable. |
| U2 Aircraft | Composed of a larger front dual wheeled landing gear and a small but substantial dual wheeled rear landing gear. Also the aircraft two wing mounted wheel assembles which detach (fall-off) on take-off. These are referred to as pogos. |
Dicycle
|
(First Heavier-than-Air Powered Flight) |
The Wright Brothers Historic Experimental Aircraft. First Powered flight of a Heavier-than-Air Vehicle. First Flight 1903. Fitted with dual skis. |
|
(First) |
First Experiment Aircraft with Wheeled assembly in dicycle arrangement. First Flight 1906 |
| Bleriot V | Experimental Monoplane with wheeled dicycle landing gear. A detachable tail wheel seems to be used for ground transport. First Flight 1907 |
| Fokker DR-1 | WW1 German Tri-Plane (1917) with two front side-by-side mounted landing gear wheels and a tail striker |
| Sopwith Camel | WW1 British Bi-Plane (1917) with two front side-by-side mounted landing gear wheels and a tail striker |
Tricycle
| Silver Dart or June Bug (First tricycle) |
First Aircraft with Tricycle landing gear arrangement. Experimental, First Flight 1908. |
| Weick W-1A | Early tricycle wheeled aircraft (1934). Home built |
|
(First Fighter) |
First production Fighter tricycle wheeled aircraft (FF 1938). |
|
(First Bomber) |
First Large Experimental Aircraft Bomber using tricycle configuration. (1 unit)(FF 1941). |
| Heinkel He 219 | Early tricycle wheeled aircraft, Larger fighter Aircraft from WW2 characterised by long NLG structure (1942). |
|
(First Commercial) |
First Commercial Tricycle Aircraft. 1942 |
| Lockheed Constellation | Developed for troop transport, but civilian registered. Converted to commercial use after WW2. 1943 |
| Cessna 172 (First Civilian) |
First Civilian tricycle configuration aircraft (1952). |
|
(First Civilian) |
First with more than two Main Landing Gear in the tricycle arrangement. (1969) |
Inverted Tricycle – Tail-dragger
|
(First) |
First Inverted tricycled landing gear. First flight 1907. (Confirmation required, was flight performed with tail wheel attached?) |
| Supermarine Spitfire | Historic WWII fighter (FF 1936) |
| Apache Helicopter | Attack Helicopter (FF 1975) |
Quadricycle
| Curnu Helicopter No.2 | First helicopter to achieve flight, Frist with quadricycle wheeled landing gear. First flight 1907 |
| Boeing CH-47 Chinook | Front landing gear assemblies are dual wheels, while rear are single wheeled (FF 1961). |
| White Knight Two | Dual fuselage cargo aircraft to transport/launch SpaceShipTwo space plane. (FF 2008). |
