Cut-away view of an oleo strut. Source: Rainbow Aviation ServicesOleo-pneumatic (oil/gas) shock struts, or oleo struts, are shock absorbers that cushion forces associated with aircraft landings and ground maneuvers such as taxiing. Oleo struts are critical elements of aircraft landing gear, connecting an aircraft’s wheels to the airframe to provide the main path through which load forces are transmitted from the ground to the airframe.

By converting a portion of the aircraft’s kinetic energy to heat, oleo struts absorb and dissipate forces associated with landing. They minimize the accelerations experienced by the airframe and its occupants as the aircraft makes contact with the runway surface during landing – including damping the recoil to reduce bounce – and as it travels along the ground during taxiing maneuvers.

Types of shock absorbers

Many types of shock absorbers are used on aircraft. Oleo struts are a kind of fluid-spring shock absorber filled with gas — usually dry air or nitrogen — and hydraulic fluid. Other types of fluid-spring shock absorbers include the oil-filled liquid spring and the air-filled pneumatic shock absorber.

Another category of shock absorbers are those using a solid spring, such as a steel coil spring, steel leaf spring or rubber spring consisting of a stack of rubber disks.

[Discover flat springs, compression springs, air springs and gas springs on Engineering360.]

Solid-spring shock absorbers are cheaper, highly reliable and require less maintenance than shock absorbers filled with fluids like gas or oil. Solid-spring shock absorbers, however, suffer from low efficiency,

Efficiency = A / (L x S)

where A is the energy absorbed by the strut during its stroke, L is the maximum load on the strut during the stroke, and S is the maximum stroke resulting from the maximum load.

Rubber disc shock absorbers are seen in one of the two main landing gear legs of a Mooney M20F Executive 21, a single engine piston aircraft that seats a pilot and up to 3 passengers.

As aircraft size scales up, the magnitudes of the shocks that must be absorbed rises. Solid-spring shock absorbers with sufficient capacity to absorb the shocks of larger aircraft weigh too much compared to oleo-pneumatic shock struts due to the lower efficiency of solid-spring shock absorbers. As a result, solid-spring shock absorbers are mainly limited to use on smaller aircraft.

Oleo-pneumatic shock struts are the most common type of shock absorber used on modern aircraft. Not only do oleo struts have a higher efficiency and damping-to-weight ratio than any other type of shock absorber, they also dissipate energy in a more optimal way, storing and releasing energy at controlled rates during the compression and expansion cycles of the strut to provide a relatively constant impact force.

The same principle by which oleo struts work is used in some gas-hydraulic buffers to absorb energy in industrial, rail and elevator applications by controlling equipment deceleration and dissipating impact forces. The principle is also similar to that used in hydropneumatic suspension systems in some automobiles and tractors.

How oleo struts work

Schematic showing some of the main components of the left main landing gear of an Embraer EMB 190 (left) and the uncompressed and compressed states of its oleo shock strut (right). Source: R Lernbeiss and M Plöchl. Simulation model of an aircraft landing gear considering elastic properties of the shock absorber.

Oleo struts absorb and dissipate shock loads using a combination of two fluids — a gas and a hydraulic fluid — contained in two chambers — a cylinder and a piston. The lower piston — attached to the axle upon which a wheel or wheels are mounted — travels up and down in the upper cylinder, which is fixed to the aircraft structure through the landing gear.

The lower chamber is filled with hydraulic fluid and the remaining space in the upper cylinder is filled with dry air or nitrogen. The two chambers are separated by an orifice plate that allows the hydraulic fluid to travel between the lower and upper chambers. Bearings maintain alignment and smooth motion between the piston and cylinder and seals prevent leakage of hydraulic fluid.

Diagram illustrating the interaction between the gas (light blue) and hydraulic fluid (red) in an oleo strut during compression of the strut. Source: Rainbow Aviation Services

As the aircraft touches down, the piston travels up into the cylinder, forcing hydraulic fluid through the orifice into the upper chamber. Gas in the upper chamber, functioning like a spring, is compressed by the hydraulic fluid to absorb the impact of touchdown as well as bumps during taxiing. During the recoil, the gas expands and forces hydraulic fluid back into the lower chamber causing extension of the strut as the piston travels back down out of the cylinder. The compression of gas and movement of fluid through the orifice generates heat that is transferred via convection and conduction through the strut to the airframe and atmosphere.

The rate at which hydraulic fluid enters and leaves the upper chamber, and thus the speed at which compression and expansion of the gas occurs, is controlled by the size of the orifice. The hydraulic fluid, therefore, restricted by the orifice, acts to dampen the movement of the piston during shock absorption and recoil.

Design variations

Schematic showing some of the main components of an oleo-pneumatic shock absorber. Source: Currey, N. Aircraft landing gear design: principles and practices.Many design variations of oleo struts exist. Some oleo struts contain a tapered metering rod that moves along with the piston through the center of the orifice to vary the orifice size. This continuously adjusts the rate at which hydraulic fluid enters the upper chamber, allowing a higher rate of flow at the moment of touchdown and progressively lowering flow rate as the strut reaches its point of maximum compression. This optimizes the magnitude and duration of force on the airframe due to the landing loads, spreading the force over the longest duration possible and keeping the force relatively constant over that duration.

Some oleo struts contain valves that restrict hydraulic fluid flow during recoil to prevent rapid piston motion that results in hard bounce-backs. Others have valves that open when the strut encounters an unusual loading force, such as bumps on rough fields. The pressure changes cause the valve to open and pass extra fluid to reduce maximum forces on the airframe.

Other oleo strut design variations include devices in which the piston is located above the cylinder, alternative orifice arrangements, multiple compression and recoil orifices and multiple chambers.

Diagram illustrating elements of a double-acting oleo-pneumatic shock absorber. Source: Currey, N. Aircraft landing gear design: principles and practices.

Double-acting shock struts contain multiple air/oil sections to effectively separate the load-stroke response into two operating regimes to optimize shock absorption performance for each. For example, the impact upon landing gear touchdown involves very different initial loading compared to bumps encountered during taxiing on an unpaved field. At touchdown, there is no initial load on the oleo strut, whereas during taxiing, the initial load includes the full weight of the aircraft. The load-stroke response of double-acting shock struts is designed to handle each of these situations in an optimal way.

A common design feature in oleo struts is a separator plate that creates a seal between the gas and hydraulic fluid to prevent mixing between the two fluids. The separator is allowed to freely float so as not to restrict fluid motion. The separator eliminates the potential for issues like frothing of the hydraulic fluid that would interfere with adiabatic compression of the gas by cooling it.

Most oleo struts are equipped with a torque link, or scissor link, that connects the upper cylinder and lower piston to prevent rotation of the piston within the cylinder and to maintain alignment of the wheel. The torque link contains a joint in the middle to allow piston retraction and extension.

Oleo struts are also often accompanied by shimmy dampers. These hydraulic shock absorbers contain a piston and a flow restrictor that prevent rapid oscillating motion of the nose or main landing gear without interfering with slower movements associated with steering.

Design considerations

Oleo struts are designed around a number of parameters including compression ratios (the ratio of the pressure in the strut in its static position divided by the pressure in the strut in its extended position), as well as the loads, strokes and pressures in extended, static and compressed positions.

Important considerations when designing oleo struts include the sink speed of the aircraft, landing gear load factors and the strut stroke.

The white oleo-pneumatic shock strut cylinder is clearly visible in this image of part of one of two main landing gear legs for the Boeing 787. Source: BoeingThe sink speed describes the aircraft’s vertical speed at the moment it touches down. A typical sink speed for transport aircraft at design landing weight is 10 ft/s.

The landing gear load factor is the maximum acceptable load in the shock strut and is arrived at by summing the static load plus the dynamic reaction load and then dividing by the static load. Typical landing gear load factors for large transport aircraft range from 0.75 to 1.5, up to 5.0 for fighter jets.

Stroke encompasses both the strut stroke — the distance the piston travels — and the wheel stroke — the vertical distance the wheel moves. Some designs utilize a levered approach in which the strut stroke is less than the wheel stroke to minimize stowage space required for the landing gear.

Among different aircraft types, the distance from the static strut position to the fully compressed position varies widely as a percentage of the total stroke of the piston, from percentages in the single digits to nearly 50%. A commercial passenger aircraft like the Boeing 737-200 has an extension of 15% (2.1 in from static to compressed out of a total stroke of 14 in).

Hydraulic fluids used in oleo struts are commonly required to conform to standards specifying minimum performance criteria, such as

• MIL-PRF-5606 for mineral oil-based hydraulic fluids,
• MIL-PRF-83282 for synthetic hydrocarbon-based hydraulic fluid with better fire resistance than MIL-PRF-5606, or
• MIL-PRF-87257 for synthetic hydrocarbon-based hydraulic fluid with better low-temperature viscosity performance than MIL-PRF-83282

Servicing

Maintaining the correct fluid to gas ratio is critical to the proper function of an oleo strut. Parameters that control this ratio are the level of hydraulic fluid and the pressure of gas in the oleo strut.

Servicing an oleo strut involves the following typical steps:

1. Bleed air or nitrogen gas by opening a valve near the top of the upper cylinder.
2. Fill the strut to the proper level with hydraulic fluid by attaching a tube to the filler valve and compressing and extending the strut to suck fluid into it.
3. Inflate the strut with dry air or nitrogen to the specified pressure.

Resources

Currey, N. S. (1988). Aircraft landing gear design: principles and practices. American Institute of Aeronautics and Astronautics.