Oleo Strut
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An oleo strut is a
pneumatic Pneumatics (from Greek ‘wind, breath’) is a branch of engineering that makes use of gas or pressurized air. Pneumatic systems used in Industrial sector, industry are commonly powered by compressed air or compressed inert gases. A central ...
air–oil hydraulic
shock absorber A shock absorber or damper is a mechanical or hydraulic device designed to absorb and damp shock impulses. It does this by converting the kinetic energy of the shock into another form of energy (typically heat) which is then dissipated. Most sh ...
used on the
landing gear Landing gear is the undercarriage of an aircraft or spacecraft that is used for takeoff or landing. For aircraft it is generally needed for both. It was also formerly called ''alighting gear'' by some manufacturers, such as the Glenn L. Martin ...
of most large
aircraft An aircraft is a vehicle that is able to fly by gaining support from the air. It counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil, or in a few cases the downward thrust from jet engines ...
and many smaller ones. This design cushions the impacts of landing and damps out vertical oscillations. It is undesirable for an airplane to bounce on landing as it could lead to a loss of control, and the landing gear should not add to this tendency. A steel
coil spring A selection of conical coil springs The most common type of spring is the coil spring, which is made out of a long piece of metal that is wound around itself. Coil springs were in use in Roman times, evidence of this can be found in bronze Fib ...
stores impact energy from landing and then releases it, while an oleo strut instead absorbs this energy, reducing bounce. As the strut compresses, the
spring rate A spring is an elastic object that stores mechanical energy. In everyday use the term often refers to coil springs, but there are many different spring designs. Modern springs are typically manufactured from spring steel, although some non- ...
increases dramatically because the air is being compressed. The
viscosity The viscosity of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water. Viscosity quantifies the inte ...
of the oil dampens the rebound movement.


History and applications

The original design for the oleo-pneumatic shock-absorbing strut was
patent A patent is a type of intellectual property that gives its owner the legal right to exclude others from making, using, or selling an invention for a limited period of time in exchange for publishing an enabling disclosure of the invention."A p ...
ed by British manufacturing conglomerate
Vickers Armstrong Vickers-Armstrongs Limited was a British engineering conglomerate formed by the merger of the assets of Vickers Limited and Sir W G Armstrong Whitworth & Company in 1927. The majority of the company was nationalised in the 1960s and 1970s, wi ...
during 1915. It had been derived from the recuperative gear design of the
Vickers gun The Vickers machine gun or Vickers gun is a water-cooled .303 British (7.7 mm) machine gun produced by Vickers Limited, originally for the British Army. The gun was operated by a three-man crew but typically required more men to move and o ...
, controlling
recoil Recoil (often called knockback, kickback or simply kick) is the rearward thrust generated when a gun is being discharged. In technical terms, the recoil is a result of conservation of momentum, as according to Newton's third law the force requ ...
by forcing oil through precisely sized orifices. Vickers' oleo strut was first applied to an aeroplane by the French aircraft company
Breguet Aviation Breguet or Bréguet may refer to: * Breguet (watch), watch manufacturer **Abraham-Louis Breguet (1747–1823), Swiss watchmaker ** Louis-François-Clement Breguet (1804–1883), French physicist, watchmaker, electrical and telegraph work * Brégue ...
. The design proved to be viable and was extensively adopted across the aviation industry for fixed undercarriages, becoming simply referred to as an "Oleo unit" or undercarriage leg. However, Vickers' initial design had placed air above the oil, an arrangement that did not pose any problem until the introduction of
retractable landing gear Landing gear is the undercarriage of an aircraft or spacecraft that is used for takeoff or landing. For aircraft it is generally needed for both. It was also formerly called ''alighting gear'' by some manufacturers, such as the Glenn L. Martin ...
during the mid-1930s. The engineer Peter Thornhill devised a novel undercarriage strut that used a free-floating piston, not only being a lighter arrangement but enabling the whole strut to be inverted and to work while at an angle, eliminating the weakness of using an oil and air mixture. Oleo-pneumatic technology was subsequently reused by the manufacturer to produce several other products, including hydraulic railway buffers and industrial shock absorbers. During 1926, the Cleveland Pneumatic Tool Company designed and introduced its own oleo strut, one of the first to be purpose-designed for use on airplanes. The company subsequently marketed the product as an ''Aerol'' strut, which had entered widespread use within the
United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territorie ...
within the space of a decade. By 1931, innovations in the field were being made in the
United Kingdom The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) or Britain, is a country in Europe, off the north-western coast of the continental mainland. It comprises England, Scotland, Wales and North ...
,
France France (), officially the French Republic ( ), is a country primarily located in Western Europe. It also comprises of Overseas France, overseas regions and territories in the Americas and the Atlantic Ocean, Atlantic, Pacific Ocean, Pac ...
, and North America. During subsequent decades, the oleo strut became commonly used for aviation purposes around the world. By the twenty-first century, a wide range of different shock-absorbing struts were in use, but typically employ common principles, despite considerable variations in size, weight, and other performance criteria. Furthermore, refinements continued to be made to the technology behind the oleo strut. During 1954,
hydropneumatic suspension Hydropneumatic suspension is a type of motor vehicle suspension system, designed by Paul Magès, invented by Citroën, and fitted to Citroën cars, as well as being used under licence by other car manufacturers, notably Rolls-Royce ( Silver Sha ...
was introduced, which utilizes the same principle of a gas that compresses (
nitrogen Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...
) and a fluid that does not; in this application, an engine-driven
pump A pump is a device that moves fluids (liquids or gases), or sometimes slurries, by mechanical action, typically converted from electrical energy into hydraulic energy. Pumps can be classified into three major groups according to the method they u ...
is used to pressurize the
hydraulic fluid A hydraulic fluid or hydraulic liquid is the medium by which power is transferred in hydraulic machinery. Common hydraulic fluids are based on mineral oil or water. Examples of equipment that might use hydraulic fluids are excavators and backhoe ...
. Another such example was a US
patent A patent is a type of intellectual property that gives its owner the legal right to exclude others from making, using, or selling an invention for a limited period of time in exchange for publishing an enabling disclosure of the invention."A p ...
filed by Jarry Hydraulics during 1958. During the 1960s, the British
Ministry of Technology The Ministry of Technology was a department of the government of the United Kingdom, sometimes abbreviated as "MinTech". The Ministry of Technology was established by the incoming government of Harold Wilson in October 1964 as part of Wilson's am ...
sponsored research into theoretical studies into improved oleo-dampening technology. In 2012, it was proposed that the vibration-dampening qualities of the oleo strut could be enhanced by using semi-active control to adjust fluid viscosity. The use of oleo struts for electric-powered automatic guided vehicles has also been evaluated. According to Engineering360, by 2019, the oleo-pneumatic strut had become the most common type of shock absorber in use on modern aircraft. In particular, the oleo strut has seen heavy use on the largest cargo airplanes in the world, such as the
Antonov An-124 Ruslan The Antonov An-124 Ruslan (; russian: Антонов Ан-124 Руслан, , Ruslan; NATO reporting name: Condor) is a large, strategic airlift, four-engined aircraft that was designed in the 1980s by the Antonov design bureau in the Ukrain ...
; it reportedly provides for a rough-field landing capacity while carrying payloads of up to 150 tons. This design also cushions the
airframe The mechanical structure of an aircraft is known as the airframe. This structure is typically considered to include the fuselage, undercarriage, empennage and wings, and excludes the propulsion system. Airframe design is a field of aerospa ...
from the impacts of
taxiing Taxiing (rarely spelled taxying) is the movement of an aircraft on the ground, under its own power, in contrast to towing or pushback where the aircraft is moved by a tug. The aircraft usually moves on wheels, but the term also includes aircra ...
, resulting in greater levels of comfort for passengers and crew alike. In non-aircraft use, the Quadro range of motor scooters use the oleo strut, which is claimed to give favourable low speed lean characteristics.


Operation

An oleo strut consists of an inner metal tube or
piston A piston is a component of reciprocating engines, reciprocating pumps, gas compressors, hydraulic cylinders and pneumatic cylinders, among other similar mechanisms. It is the moving component that is contained by a cylinder and is made gas-tig ...
, which is attached to the wheel axle, and which moves up and down in an outer (or upper) metal tube, or cylinder, that is attached to the
airframe The mechanical structure of an aircraft is known as the airframe. This structure is typically considered to include the fuselage, undercarriage, empennage and wings, and excludes the propulsion system. Airframe design is a field of aerospa ...
. The cavity within the strut and piston is filled with gas (usually nitrogen, sometimes air—especially on
light aircraft A light aircraft is an aircraft that has a maximum gross takeoff weight of or less.Crane, Dale: ''Dictionary of Aeronautical Terms, third edition'', page 308. Aviation Supplies & Academics, 1997. Light aircraft are used as utility aircraft co ...
) and
oil An oil is any nonpolar chemical substance that is composed primarily of hydrocarbons and is hydrophobic (does not mix with water) & lipophilic (mixes with other oils). Oils are usually flammable and surface active. Most oils are unsaturated ...
(usually hydraulic fluid), and is divided into two chambers that are connected by a small orifice of a precise, calculated size. When the aircraft is stationary on the ground, its weight is supported by the compressed gas in the cylinder. During landing, or when the aircraft taxis over bumps, the piston slides up and down. This movement compresses the gas, which acts as a spring, and forces oil through the orifice, which acts as a damper. A tapered rod is used on some designs to change the size of the orifice as the piston moves, providing greater resistance as compression of the strut increases. Additionally, a check valve is sometimes used to uncover additional orifices so that damping during compression is less than during rebound. Oleo struts absorb and dissipate forces by converting a portion of the accumulated kinetic energy into thermal energy.
Pneumatic Pneumatics (from Greek ‘wind, breath’) is a branch of engineering that makes use of gas or pressurized air. Pneumatic systems used in Industrial sector, industry are commonly powered by compressed air or compressed inert gases. A central ...
systems like the oleo strut generally have long operating lives, and the construction is not unusually complex for maintenance purposes. Nitrogen is usually used as the gas instead of air, since it is less likely to cause
corrosion Corrosion is a natural process that converts a refined metal into a more chemically stable oxide. It is the gradual deterioration of materials (usually a metal) by chemical or electrochemical reaction with their environment. Corrosion engine ...
. The various parts of the strut are sealed with
O-ring An O-ring, also known as a packing or a toric joint, is a mechanical gasket in the shape of a torus; it is a loop of elastomer with a round cross-section, designed to be seated in a groove and compressed during assembly between two or more par ...
s or similar elastomeric seals, and a scraper ring is used to keep dust and grit adhering to the piston from entering the strut.


See also

*
FedEx Express Flight 80 FedEx Express Flight 80 was a scheduled cargo flight from Guangzhou Baiyun International Airport in the People's Republic of China, to Narita International Airport in Narita, Chiba Prefecture (near Tokyo), Japan. On March 23, 2009, the McDon ...
– bounce during landing leading to crash


References

{{Aircraft components Aircraft undercarriage Telescopic shock absorbers