Aircraft landing gear shock absorbing strut

Abstract

An aircraft landing gear shock absorbing strut 10 having a first end provided with a mounting bearing 14 for coupling to an aircraft, and a second end provided with a wheel assembly, bogie beam or sim

Description

5 Background An aircraft landing gear shock absorbing strut is a structural assembly arranged to carry the weight of the aircraft when on the ground, and also absorb landing loads. A major component of an aircraft landing gear shock absorbing strut is therefore a shock 10 absorber. The shock absorber can be pressurised to achieve a particular shock absorber extension length, specific to a type of aircraft. This pressurisation level is calculated using the weight of the aircraft, the ambient temperature around the aircraft and static design 15 spring curves. A known type of shock absorber contains a generally incompressible liquid, such as oil, for damping purposes, as well as an elastic gas, such as nitrogen or helium, for spring purposes. Such shock absorbers can be referred to as "oleo-pneumatic" shock 20 absorbers. EP3069994 provides an example of an oleo-pneumatic shock absorber strut. The present inventors have devised a new type of oleo-pneumatic shock absorber strut which has one or more of the following advantages relative to known oleo-pneumatic shock absorber strut: 25 - reduced likelihood of bottoming out less frequent service intervals Summary 30 According to a first aspect of the invention, there is provided an aircraft landing gear shock absorbing strut according to claim 1. Thus, the aircraft landing gear shock absorbing strut according to the first aspect of the invention is provided with an electrical heating element which is operable to heat gas 35 within the oleo-pneumatic shock absorber. As such, the spring curve of the oleo-pneumatic shock absorber can be modified in use to tailor to the landing weight and external temperature conditions, thereby reducing the risk of the shock absorber bottoming (reaching a fully compressed state) upon landing. Additional advantages are 26 08 25 that while the aircraft is on the ground, the shock absorber closure can be maintained at a constant degree during refuelling, which can avoid a requirement for ground support equipment. 5 The electrical heating element is mounted externally on the oleo-pneumatic shock absorber to heat the gas through the oleo-pneumatic shock absorber. The electrical heating element is in the form of a cylindrical sleeve. This can provide effective heat transfer between the heating element and a component of the oleo-10 pneumatic shock absorber, which in turn radiates the heat to the gas. The electrical heating element can be disposed around the exterior of the outer cylinder to heat the gas through the outer cylinder. The heating element can have one end region located at or near the end of the outer cylinder closest to the mounting bearing, 15 which in use is where gas will rest for an unseparated oleo-pneumatic shock absorber when in the deployed condition. The heating element can extend along at least one tenth, one quarter or half of the outer cylinder towards the second end of the outer cylinder, from which the inner cylinder projects, and in some embodiments the heating element can extend along the majority of the outer cylinder. 20 The electrical heating element can be disposed around the exterior of a portion of the inner cylinder which projects from the outer cylinder when the oleo-pneumatic shock absorber is fully compressed, to heat the gas through the inner cylinder. 25 The aircraft landing gear shock absorbing strut can further comprise an insulating layer provided around the circumference of the oleo-pneumatic shock absorber to thermally insulate the shock absorber. The insulating layer can have one end region located at or near the end of the outer cylinder closest to the mounting bearing, which in use is where gas will rest for an unseparated oleo-pneumatic shock absorber when in the deployed 30 condition. The insulating layer can extend along at least one tenth, one quarter or half of the outer cylinder towards the second end of the outer cylinder, from which the inner cylinder projects, and in some embodiments the insulating layer can extend along the majority of the outer cylinder. 35 In embodiments where the electrical heating element is disposed around the exterior of the outer cylinder, the insulating layer can have the same size as the electrical heating element. 26 08 25 The aircraft landing gear shock absorbing strut can further comprise an air channel provided between the insulating layer and the oleo-pneumatic shock absorber, the air channel being open at first and second ends to atmosphere via first and second openings to enable the passage of air through the air channel to cool the gas, the first opening 5 being provided with a ventilation control member movable between an open condition in which the first opening is free to permit the passage of air through the air channel and a closed condition in which the ventilation control member restricts the passage of air through the air channel. 10 The air channel can extend around the circumference of the oleo-pneumatic shock absorber. The temperature sensor can be arranged in contact with a region of the oleo-pneumatic shock absorber at which the gas is proximate, in use, when the aircraft landing gear 15 shock absorbing strut is in the deployed condition, for take-off and landing. The temperature sensor can be located within the oleo-pneumatic shock absorber to directly sense the temperature of the gas. 20 The controller can be communicatively coupled to a shock absorber closure sensor to provide a closure level signal indicative of the level of shock absorber closure. The controller can be programmed to estimate what the level of closure should be for an estimated level of fluid within the oleo-pneumatic shock absorber and the controller can operate the heating element or ventilation control member to increase or decrease the 25 temperature of the gas if the closure level is less than or greater than the estimated level of closure. This can allow the health of the shock absorber to be monitored and additional compensations made, extending the time between services. The heating element can comprise a plurality of discrete heating bands disposed along 30 the axis of the outer cylinder. In accordance with a second aspect of the invention, there is provided an aircraft comprising one or more aircraft landing gear shock absorbing struts according to the first aspect. 35 In accordance with a third aspect of the invention, there is provided a method of changing the internal pressure or closure of the oleo-pneumatic shock absorber of the 26 08 25 aircraft landing gear shock absorbing strut according to any preceding claim, the method comprising: tuning the electrical heating element on to heat the gas within the oleo-pneumatic shock absorber; and 5 turning the electrical heating element off when the temperature sensor senses a first temperature. The method can comprise: moving the ventilation control member to the open condition when the 10 temperature sensor senses a second temperature which is greater than the first temperature. The method can comprise: progressively switching off bands of the heating element from the bottom towards 15 the top as the shock absorber closes. Brief Description of the Drawings Embodiments of the invention will now be described with reference to the accompanying 20 drawings, in which: Figure 1 is a schematic diagram of an aircraft landing gear shock absorbing strut according to an embodiment of the invention; 25 Figure 2 is a flow chart of a method according to an embodiment of the invention; and Figure 3 is a schematic diagram of an aircraft landing gear shock absorbing strut according to an embodiment of the invention. 30 Detailed Description By way of a non-limiting overview, embodiments of the invention relate to the active control of gas spring temperature within an aircraft landing gear shock absorber to compensate for shock absorber closure under differing load and environmental 35 conditions. Gas compressibility is used in many shock absorber applications as the energy storing component. Existing shock absorbers are passive devices requiring compromise to the load stroke spring curves. The risk of bottoming (i.e. full compression of the shock absorber) at low environmental temperature conditions has to 26 08 25 be balanced against acceptable breakout load, maximum closure and out stop load at ambient conditions. Careful design of the compression ratio allows closure and spring rate to be engineered for successful on-ground operation. Gas compressibility is affected by ambient temperature, which can in use be very cold, leading to a reduction in internal 5 pressure. Embodiments of the invention include an electrical heating element operable to raise the temperature of the shock absorber and in particular the gas spring component. Due to the universal gas law, the internal pressure of the shock absorber is directly proportional to the gas temperature. By providing the heating element, not only can the shock absorber be made to function in more uniform manner at various external 10 temperatures, but also the heating element can actively be used to tune the shock absorber for optimal operation between aircraft heavy and light ground manoeuvring conditions, as well as for varied landing loads. This can be advantageous in all aircraft operation where extreme environmental temperature differences are experienced; for example, landing gear which experience extreme low temperature cold soaks over night 15 before being fuelled and taxied at maximum ramp weight (MRW). Figure 1 shows an aircraft landing gear shock absorbing strut according to an embodiment of the invention generally at 10. The shock absorbing strut 10 defines a main structural load bearing assembly of the landing gear assembly. 20 The shock absorbing strut 10 includes an oleo-pneumatic shock absorber having an outer cylinder 12 defining the main fitting for direct coupling to an aircraft 100 via a main bearing 14 or hinge. An inner cylinder 16, which can be referred to a "sliding tube", is slidably mounted within the outer cylinder 12 for linear movement between 25 extended and compressed conditions. The outer cylinder 12 and inner cylinder 16 are hollow so as to collectively define a variable volume chamber containing a liquid L such as oil and a gas G such as nitrogen. The chamber is sealed by dynamic seals (not shown). A lower end of the inner cylinder 16 is provided with a ground contacting assembly coupling 18 for coupling to a wheel assembly, bogie beam or the like (not 30 shown). The spring function of the shock absorber is provided by the contained gas G. The liquid L serves as a damping fluid. In the illustrated embodiment, the shock absorber is "unseparated" in that the liquid L and gas G are free to mix within the shock absorber. 35 As such, when the landing gear is deployed and the shock absorber is generally vertically orientated, as shown in Figure 1, gravity causes the gas G to settle above the liquid L, which serves as a damping fluid. 26 08 25 The shock absorbing strut 10 is provided with a temperature sensor 20 arranged to sense the temperature of the gas G within the oleo-pneumatic shock absorber. The temperature sensor 20 can comprise one or more sensors directly monitoring the gas temperature or indirectly inferring the gas temperature from the damping fluid or shock 5 absorber mechanical components. In the illustrated embodiment, the temperature sensor 20 is a metallic probe mounted in direct contact with a surface of the outer cylinder 12 at the upper end, where gas G will settle when the strut is in the deployed condition for take-off and landing. Thus, the 10 upper region can be used to indirectly measure the temperature of the gas G. In other embodiments, the temperature sensor 20 can be provided at a suitable location within the shock absorber to directly measure the gas temperature, for example any free space at the top of the gas chamber that the piston of the inner cylinder 16 does not 15 pass through in use. The shock absorbing strut 10 is also provided with an electrical heating element 22 operable to heat the gas within the oleo-pneumatic shock absorber. The electrical heating element 22 is coupled to a power source (not shown) which can be dedicated to 20 the heating element 22 or can form part of the aircraft. In this embodiment the electrical heating element 22 is mounted externally on the oleo-pneumatic shock absorber to heat the gas G through the oleo-pneumatic shock absorber. The electrical heating element 22 is arranged in the form of a cylindrical 25 sleeve, which can for example comprise a plurality of elongate heating element rods arranged with equiangular spacing along and around the circumference of the outer cylinder 12. The heating element 22 extends along the majority of the outer cylinder 12, but in other embodiments the heating element 22 can have one end region located at the end of the outer cylinder 12 closest to the mounting bearing 14 and can extend 30 along just some of the outer cylinder 12 so as to be localised around the gas G. In other embodiments the heating element 22 can extend from that location by at least one tenth, one quarter or half of the outer cylinder 12 towards the second, lower end of the outer cylinder, from which the inner cylinder 16 projects. 35 The heating element can comprise a plurality of discrete bands arranged in series along the outer cylinder 12 which can be independently switched on or off. Moreover, in the case of a two-stage shock absorber, the heating element can be provided around an internal cylinder containing gas, in the oil part, to indirectly heat the gas. Alternatively, 26 08 25 the heating element can heat gas indirectly via heating the oil, although this is less efficient. The shock absorbing strut 10 is also provided with a controller 30 electrically coupled to 5 a power supply (not shown) and the heating element 22 by a wired connection for example and communicatively coupled to the temperature sensor 20. The controller 30 can be configured to turn the heating element 22 on to heat the gas G until the temperature sensor 20 senses a predefined temperature. The controller 30 can be a single computing device or a distributed system with individual computing devices for 10 each element for example, communicatively coupled together by wired or wireless connections. The controller 30 can be mounted on a component of the shock absorbing strut 10 itself or alternatively can be mounted on another part of the aircraft. The controller 30 can for example be mounted on a bracket attached to the main fitting in a convenient place. The controller 30 can include or be coupled to an ambient temperature 15 sensor for measuring the air temperature adjacent to the shock absorber. As shock absorber static internal pressure is a function of the shock absorber seal diameter and the aircraft mass, control of the gas temperature allows the gas volume / pressure to be adjusted. Direct or indirect monitoring of the gas temperature can therefore be used to control the shock absorber static closure. 20 Pressure in a loaded gas spring oil damped shock absorber spring is generated by the axial load over the seal area. Embodiments of the invention make use of the gas laws for a closed system P1V1 / T1 = P2V2 / T2. In static conditions the temperatures T1 and T2 are the same, so the shock absorber closes as the pressure increases as the seal area is 25 nominally constant. Actively controlling the shock absorber fluids and gas temperature allows a modification to the above equality as energy can be added or released from the system, thereby allowing modification of the shock absorber load stroke curve. Shock absorber spring curves are affected by ambient temperature. To provide a spring 30 pressure that gives acceptable performance over the majority of the external temperatures encountered in use, there is a risk that the shock absorber may run out of travel (stroke) leading to impact between components, known in the art as bottoming. Taking control of the shock absorber temperature by application of heat to the system can therefore result in a simpler shock absorber design. 35 The gas law PV=nRT for a closed system, where P is the pressure, V is the volume n is the number of moles of gas, R is the universal gas constant and T is the temperature applies. As the system is closed, equilibrium is maintained at all conditions and the term 26 08 25 n and R are not required with the equality for a different condition becoming P1V1 / T1 = P2V2 / T2. For a shock absorber, at isothermal closure the temperatures are constant simplifying to 5 P1V1 = P2V2. Embodiments of the invention can therefore adjust the amount of energy in the system by control of the gas temperature. 10 For example, the likelihood of bottoming can be reduced by increasing the spring temperature. In this case V1=V2, so the equality becomes T2=P2 / P1*T1. As the pressure is directly proportional to the aircraft weight, the equation shows the increase in temperature is also a linear function of aircraft weight, making it straightforward to control. 15 Using an operating temperature at above the maximum ambient for minimum aircraft weight condition requires energy input to heat the system. Higher operating weight require higher energy inputs to compensate for spring volume changes under pressure increases. Use of higher than ambient temperatures allow the system to release energy. 20 In the illustrated embodiment, the aircraft landing gear shock absorbing strut 10 also includes an optional thermal insulating layer 26 provided around the circumference of the oleo-pneumatic shock absorber to thermally insulate shock absorber and the gas G within it from the external environment. The insulating layer 26 can for example be 25 formed from any suitable aerospace insulation material such as a polymer foam or fibre wool. In the illustrated embodiment, the aircraft landing gear shock absorbing strut 10 also includes an optional air channel A provided between the insulating layer 26 and the oleo-30 pneumatic shock absorber, the air channel being open at first and second ends El, E2 to atmosphere via first and second openings to enable the passage of air through the air channel to cool the gas, the first opening at end El being provided with a ventilation control member 24 movable between an open condition in which the first opening is free to permit the passage of air through the air channel and a closed condition in which the 35 ventilation control member restricts the passage of air through the air channel. The ventilation control member 24 can take any suitable form and can for example comprise actuatable valves provided at one or both ends of each portion of the air channel A, which can be controlled by the controller 30 to open and close the air channel A. The air 26 08 25 channel A preferably extends around the circumference of the oleo-pneumatic shock absorber. The air channel A can for example comprise a plurality of elongate passages at equiangular spacings around the shock absorber. 5 The controller 30 can be communicatively coupled to a conventional shock absorber closure sensor 28 to provide a closure level signal indicative of the level of shock absorber closure / compression. The closure sensor 28 can for example comprise a Rotary Variable Differential Transformer sensor mounted on a torque link or dressings slave link to give a signal related to shock absorber closure. The controller 30 can be 10 programmed to estimate what the level of closure should be for an estimated level of fluid within the oleo-pneumatic shock absorber and the controller 30 can operate the heating element 20 or ventilation control member 24 to increase or decrease the temperature of the gas G if the closure level is less than or greater than the estimated level of closure. This can allow the health of the shock absorber to be monitored and 15 additional compensations made, extending the time between services. In some embodiments, heating can be regulated solely according to measured closure to maintain constant height and in other embodiments the controller can process both sensor signals to provide functionality such as only maintaining ride height beyond a certain tolerance. 20 In embodiments where an air channel and ventilation control device are provided, the controller can be configured to apply heat if below a target temperature and vent heat if above the target temperature, resulting in a common shock absorber spring behaviour regardless of ambient temperature. Where an ambient temperature sensor is provided, 25 the controller can advantageously be configured such that the system does not try to vent when ambient temperature is near or above the shock absorber temperature. In some embodiments the controller can be manually switched on by flight deck command or the heating and / or venting functions can be triggered by a temperature 30 deviation greater than a certain amount. Referring to Figure 2, embodiments of the invention extend to a method 30 ofchanging the internal pressure of oleo-pneumatic shock absorber of the aircraft landing gear shock absorbing strut according to an embodiment of the invention. 35 At step 22, the method comprises turning the electrical heating element on to heat the gas within the oleo-pneumatic shock absorber. 26 08 25 At step 24, turning the electrical heating element off when the temperature sensor senses a first temperature. The method can comprise moving the ventilation control member to the open condition 5 when the temperature sensor senses a second temperature which is greater than the first temperature. The method can comprise progressively switching off bands of the heating element from the bottom towards the top as the shock absorber closes. When the shock absorber is 10 fully extended, the gas will occupy a large proportion of its length. As the shock absorber is progressively loaded, the gas will be compressed to a progressively smaller volume and a smaller proportion of the length of the shock absorber. The top of the gas will stay in the same place, but its lower surface will rise. This can provide a more energy efficient way of heating the gas. 15 Referring to Figure 3, an aircraft landing gear shock absorbing strut according to a further embodiment of the invention is shown generally at 40. The shock absorbing strut 40 of this embodiment is similar to the shock absorbing strut 10 of Figure 1 and for brevity the following description will focus on the differences. 20 In this embodiment, the shock absorbing strut 40 includes a "separated" oleo-pneumatic shock absorber, where the gas G is separated the liquid L by a separator piston 42 slidably housed within the bore of the inner cylinder 44. The electrical heating element 46 in this embodiment is disposed around the exterior of 25 a portion of the inner cylinder 44 which projects from the outer cylinder 48 when the oleo-pneumatic shock absorber is fully compressed, to heat the gas G through the inner cylinder 44. Insulation and an air channel are optionally provided. It is however preferred to place the heating element inside the gas chamber G in this embodiment. 30 Moreover, the shock absorber is inverted as a "capsule type" shock absorber, where the inner cylinder 44 is generally upright when the landing gear is deployed and is indirectly coupled to the aircraft by a mechanical main fitting structure (not shown), which does not form part of the oleo-pneumatic shock absorber chamber. However, embodiments of the invention extend to separated shock absorbers with similar configurations to that 35 shown in Figure 1, where the outer cylinder is provided with the mounting bearing and defines the main fitting. Although the invention has been described above with reference to one or more preferred embodiments, it will be appreciated that various changes or modifications may be made without departing from the scope of the invention as defined in the appended claims. The word "comprising" can mean "including" or "consisting of" and therefore 5 does not exclude the presence of elements or steps other than those listed in any claim or the specification as a whole. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. 26 08 25 26 08 25

Claims

1. An aircraft landing gear shock absorbing strut having a first end provided with a mounting bearing for coupling the shock absorbing strut to an aircraft, the shock absorbing strut having a second end provided with a ground contacting assembly, the shock absorbing strut comprising:an oleo-pneumatic shock absorber comprising an inner cylinder slidably coupled within an outer cylinder to define a variable volume chamber containing a liquid and a gas;a temperature sensor arranged to sense the temperature of the gas within the oleo-pneumatic shock absorber;an electrical heating element operable to heat the gas within the oleo-pneumatic shock absorber; anda controller communicatively coupled to the temperature sensor and electrical heating element and configured to turn the electrical heating element on to heat the gas within the oleo-pneumatic shock absorber until the temperature sensor senses a first temperature, wherein the electrical heating element is mounted externally on the oleo-pneumatic shock absorber to heat the gas through the oleo-pneumatic shock absorber, wherein the electrical heating element is in the form of a cylindrical sleeve.

2. The aircraft landing gear shock absorbing strut according to claim 1, wherein the electrical heating element is disposed around the exterior of the outer cylinder to heat the gas through the outer cylinder.

3. The aircraft landing gear shock absorbing strut according to claim 1, wherein the electrical heating element is disposed around the exterior of a portion of the inner cylinder which projects from the outer cylinder when the oleo-pneumatic shock absorber is fully compressed, to heat the gas through the inner cylinder.

4. The aircraft landing gear shock absorbing strut according to any preceding claim, further comprising an insulating layer provided around the circumference of the oleo-pneumatic shock absorber to thermally insulate the shock absorber.

5. The aircraft landing gear shock absorbing strut according to claim 4, further comprising an air channel provided between the insulating layer and the oleo-pneumatic shock absorber, the air channel being open at first and second ends to atmosphere via first and second openings to enable the passage of air through the air26 08 25channel to cool the gas, the first opening being provided with a ventilation control member movable between an open condition in which the first opening is free to permit the passage of air through the air channel and a closed condition in which the ventilation control member restricts the passage of air through the air channel.

6. The aircraft landing gear shock absorbing strut according to claim 5, wherein the air channel extends around the circumference of the oleo-pneumatic shock absorber.

7. The aircraft landing gear shock absorbing strut according to any preceding claim, wherein the temperature sensor is arranged in contact with a region of the oleo-pneumatic shock absorber at which the gas is proximate, in use, when the aircraft landing gear shock absorbing strut is in the deployed condition, for take-off and landing.

8. The aircraft landing gear shock absorbing strut according to any of claims 1 to 6, wherein the temperature sensor is located within the oleo-pneumatic shock absorber to directly sense the temperature of the gas.

9. The aircraft landing gear shock absorbing strut according to any preceding claim, wherein the heating element comprises a plurality of discrete heating bands disposed along the outer cylinder.

10. An aircraft comprising one or more aircraft landing gear shock absorbing struts according to any preceding claim.

11. A method of changing the internal pressure of oleo-pneumatic shock absorber of the aircraft landing gear shock absorbing strut according to any preceding claim, the method comprising:turning the electrical heating element on to heat the gas within the oleo-pneumatic shock absorber; andturning the electrical heating element off when the temperature sensor senses a first temperature.

12. The method according to claim 11, when dependent on claim 5, further comprising:moving the ventilation control member to the open condition when the temperature sensor senses a second temperature which is greater than the first temperature.26 08 25

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