WO1998015755A1

WO1998015755A1 - Dry media suspension system for aircraft

Info

Publication number: WO1998015755A1
Application number: PCT/US1997/018189
Authority: WO
Inventors: Brian Bobby Burkett; Louis Charles Hrusch; Kevin Lee Leffel; Biing-Lin Lee
Original assignee: The B.F. Goodrich Co.
Priority date: 1996-10-09
Filing date: 1997-10-08
Publication date: 1998-04-16
Also published as: AU4979497A; EP0929760A1

Abstract

The invention relates to the field of aircraft suspension systems. More particularly, the invention relates to a damped spring for an aircraft suspension comprising a dry spring and damping medium. The damped spring according to the invention is easy to seal and has little tendency to leak. According to a preferred embodiment, compression of the dry medium emulates the compression of a gas.

Description

DRY MEDIA SUSPENSION SYSTEM FOR AIRCRAFT

BACKGROUND

The invention relates to the field of aircraft suspension systems. More particularly, the invention relates to a damped spring for an aircraft suspension comprising a dry spring and damping medium.

A cross-sectional view of a prior art shock strut for an aircraft suspension is presented in Figure 1. Shock strut 100 comprises a cylinder 102, a piston 104 received within the cylinder 102, and a damping head 106. Shock strut 100 is generally representative of certain shock struts known in the art generally referred to as "air-over-oil" shock struts. Shock strut 100 is attached to an aircraft airframe 10 (shown in phantom), and a wheel truck 12 (shown in phantom). A detailed depiction of the aircraft airframe and wheel truck is not necessary here, since such structures are very well known in the art. One or more extension, retraction and/or locking mechanisms 14 (shown in phantom) are attached to the shock strut 100, along with a torque linkage 16, that holds the orientation of piston 104 throughout the strut stroke since a gear axle is often attached to the piston 104. Various other linkages and attachments may be provided as required for the specific application.

The cylinder 102 and piston 104 define a cavity 108 partially filled with an oil 110, and a pressurized gas 112, which is generally dry air or nitrogen. A piston seal 114 is provided between the cylinder 102 and piston 104, and a damping head seal 122 is provided between the damping head 106 and the piston 104. The damping head 106 comprises an aperture with a tapered metering rod 118 passing through the aperture, thereby defining a damping orifice 116 relative to stroke position. The metering rod 118 is fixed to the piston 104. A plurality of rebound dampers 120 are provided between the cylinder 102 and the piston 104. Upon landing, the cylinder 102 and piston 104 telescope and stroke toward each other, thereby compressing the gas 112 relative to its initial pressure while forcing the oil 110 to pass through the orifice 116. Forcing the oil 110 to pass through the orifice 116 develops a pressure differential across the hydraulic area defined by the damping head 122. The pressure of the gas 110 acts on the hydraulic area defined by the piston seal 114. These forces combine to resist stroking of the piston 104 toward the cylinder 102, whereby the vertical velocity of the aircraft airframe 10 toward the ground or runway is slowed and stopped during landing, and the aircraft is subsequently suspended during taxi. On take-off, the compressed gas 110 forces the piston 104 to telescope away from the cylinder 102, and the rebound dampers 120 prevent the piston 104 from moving with an uncontrolled velocity. The rebound dampers 120 comprise valves that permit unrestricted flow of oil when the piston 104 strokes toward the cylinder 102 (during landing), and that restrict flow of oil when the piston 104 strokes away from the cylinder 102 (on take-off).

The various components of shock strut 100 are configured to achieve specific performance criteria. Referring now to Figure 2, a graphical representation of vertical "Load" versus "Stroke" of the shock strut 100 is presented. As used herein, "load" is the vertical force the airframe imposes on the shock strut, and "stroke" is the distance a particular load forces the piston and cylinder to move toward each other from their initial relative position at no load. Different design criteria are employed to a certain degree depending on the specific application.

However, the following principles generally apply to the design of the shock strut 100. In Figure 2, the maximum stroke of shock strut 100 is indicated as S, and maximum aircraft weight for design is indicated as L1. For a large commercial jetliner and most aircraft, the static stroke position is the position where the aircraft weight L1 is supported during taxi. In

Figure 2, the static stroke at L1 is about 80% of the maximum stroke, indicated as 0.8S. The load versus stroke compression curve for the gas 112 is indicated as 18, which passes through the 0.8S, L1 point in Figure 2. The load at the maximum stroke S, indicated as L3, is generally on the order of three (3) times the static load L1 to assure no bottoming when a standard condition of twice the acceleration due to gravity is imposed on the aircraft airframe 10 toward the ground or runway. The maximum allowable load during landing, indicated as L2, is generally on the order of one and one-half (1 Y₂) to two (2) times the static load L1. During landing, the total vertical resistance generated is the sum of the gas compression resistance (curve 18), and the dynamic damping resistance developed by forcing the oil 110 through the orifice 116, together indicated as curve 20. The maximum stroke S is determined by the maximum landing energy (principally vertical kinetic energy) and maximum vertical load requirements. The various components comprising shock strut 100 are dimensioned such the total vertical resistance 20 quickly approaches a desired load L2, generally holds steady during most of the travel to 0.8S, and then quickly approaches the static load L1 , at which point the landing energy is absorbed and the vertical velocity of the airframe is zero (0). The size of orifice 116 changes during the stroke by means of the tapered metering rod 118, which generally increases toward its base. The orifice 116 and tapered metering rod 118 are dimensioned to provide a desired magnitude of dynamic damping resistance versus stroke. After reaching the static stroke 0.8S, the aircraft is suspended mostly by the compression resistance of the gas 112, such condition essentially corresponding to the aircraft weight being supported during taxi.

The air-over-oil shock strut is widely used in the aircraft industry because the compression of gas in an aircraft shock strut is highly nonlinear. Initially, the slope of the load versus stroke curve is relatively low which permits a greater maximum stroke S. A greater maximum stroke is desirable because the airframe slows over a greater distance, resulting in lesser maximum vertical accelerations. Subsequently, the slope more sharply increases which permits a shorter overall length of the shock strut during taxi at 0.8S when the aircraft is suspended by compression of the gas. Having a relatively short shock strut length during taxi is desirable because it decreases the moment arm subjected to drag forces, as induced by braking, side drift, runway roughness, etc. Finally, the slope increases even more sharply as the stroke approaches the maximum stroke S, which helps to prevent bottoming of the piston against the cylinder. In spite of these advantages, the oil and gasses used in an air- over-oil shock strut are difficult to seal, and tend to present a leakage problem over time. Therefore, a damped spring is desired for use in an aircraft shock strut that is easy to seal and has little tendency to leak.

SUMMARY OF THE INVENTION According to an aspect of the invention, a damped spring for an aircraft suspension is provided, comprising: a hollow media housing having a media housing aperture; a displacement rod received within the media housing aperture, the displacement rod and the media housing defining a sealed cavity, the displacement rod terminating within the sealed cavity; and, a dry medium filling the cavity, the displacement rod telescoping into the media housing and compressing the dry medium during landing of the aircraft thereby providing a vertical stopping force on the aircraft, wherein compression of the dry medium is mechanically altered as the displacement rod telescopes into the media housing to emulate compression of a gas.

According to a further aspect of the invention, a suspension method for an aircraft is provided, comprising the steps of: stopping the vertical descent of an aircraft by landing on a damped spring filled with a dry medium that is compressed during landing; and, mechanically altering compression of the dry medium during landing to emulate compression of a gas.

According to further aspect of the invention, a damping device is provided, comprising: a hollow media housing having a media housing aperture; a damping rod received within the media housing aperture, the damping rod and the media housing defining a sealed cavity; and, a dry medium filling the cavity having a storage modulus on the order of 10⁴ to 10⁶ dynes/cm² (at one radian/second). According to a further aspect of the invention, a damping device is provided, comprising: a hollow media housing having a media housing aperture; a damping rod received within the media housing aperture, the damping rod and the media housing defining a sealed cavity; and, a dry medium filling the cavity having negligible crystallization in the temperature range of -70 °C to 55 °C.

According to a further aspect of the invention, a damped spring is provided, comprising: a hollow media housing having a media housing aperture; a displacement rod received within the media housing aperture, the damping rod and the media housing defining a sealed cavity, the displacement rod terminating within the sealed cavity; and, a dry medium filling the cavity having a storage modulus on the order of 10⁴ to 10⁶ dynes/cm² (at one radian/second). According to a further aspect of the invention, a damped spring is provided, comprising: a hollow media housing having a media housing aperture; a displacement rod received within the media housing aperture, the damping rod and the media housing defining a sealed cavity, the displacement rod terminating within the sealed cavity; and, a dry medium filling the cavity having negligible crystallization in the temperature range of -70 °C to 55 °C.

According to a further aspect of the invention, a method is provided for providing stabilized performance of a dry media damped spring over repeated aircraft landings, comprising the steps of: filling a cavity inside a dry media damped spring with a dry medium for use as a damping and spring medium, the dry media damped spring having a damping head through which the dry medium is repeatedly forced thereby generating a change in pressure across the damping head during repeated aircraft landings, wherein the change in pressure developed by the dry medium is stable over the repeated aircraft landings.

In any of these aspects, the dry medium may comprise an amorphous silicon elastomer, and/or the dry medium may comprise an absorbed fluid. In any of these aspects, the dry medium may comprise an organopolysiloxane comprising 77 to 97 mole % of R(CH₃)SiO units, 2 to 23 mole % RRSiO units and 0.1 to 4.0 mole % of CH₃R(CH₂=CH)Si vinyl terminated units, or 0.1 to 6 mole % of OHRSi(CH₃)₂-O wherein R represents a group selected from the group consisting of a methyl group or phenyl group; wherein, the total of the phenyl groups out of the R groups amount from about 2 to 23 mole percent of all the R groups; from 2 to 10 weight % of a cross-linker; and from 0.1 to 2 weight % of a catalyst.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a cross-sectional view of a prior art air-over-oil shock strut.

Figure 2 is a representation of load versus stroke for the Figure 1 shock strut.

Figure 3 depicts a landing aircraft having a suspension system according to an aspect of the invention.

Figure 4 depicts the Figure 3 aircraft after landing during taxi. Figure 5 is a cross-sectional view of a shock strut according to an aspect of the invention.

Figure 6 is a cross-sectional view of a shock strut according to another aspect of the invention.

Figure 7 is a cross-sectional view of a shock strut according to another aspect of the invention.

Figure 8 is a cross-sectional view of a shock strut according to another aspect of the invention.

Figure 9 is a cross-sectional view of a shock strut having thermal compensation according to an aspect of the invention.

Figure 10 is a cross-sectional view of a shock strut having thermal compensation according to another aspect of the invention.

Figure 11 is a cross-sectional view of a shock strut having thermal compensation according to another aspect of the invention. Figure 12A is a cross-sectional view of a thermal compensator according to an aspect of the invention.

Figure 12B is a cross-sectional view of the Figure 12A thermal compensator at lesser temperature than in Figure 12A.

Figure 13A is a cross-sectional view of a thermal compensator according to another aspect of the invention.

Figure 13B is a cross-sectional view of the Figure 12A thermal compensator at lesser temperature than in Figure 12A.

Figure 14 is a cross-sectional view of a shock strut having thermal compensation according to another aspect of the invention. Figure 15 is a cross-sectional view of a shock strut having thermal compensation according to another aspect of the invention.

Figure 16 is a representation of load versus stroke for the Figure 5 shock strut.

Figure 17 is a representation of load versus stroke for the Figure 6 shock strut.

Figure 18 is a representation of load versus stroke for the Figure 7 shock strut.

Figure 19 is a representation of load versus stroke for the Figure 8 shock strut. Figure 20 is a representation of stroke versus temperature for a shock strut having thermal compensation according to an aspect of the invention.

Figure 21 is a representation of stroke versus temperature for a shock strut having thermal compensation according to an aspect of the invention. Figure 22 is a representation of stroke versus temperature for a shock strut having thermal compensation according to an aspect of the invention.

Figure 23 is a representation of stroke versus temperature for a shock strut having thermal compensation according to an aspect of the invention.

Figure 24 is an compressibility curve for a dry medium according to an aspect of the invention.

Figure 25 is front view of an aircraft having another suspension system according to an aspect of the invention. Figure 26 is cross-sectional view of damped spring according to an aspect of the invention.

Figure 27 is a cross-sectional view of damped spring according to another aspect of the invention.

Figure 28 is representation of processing piston speed versus cycles for a media processing method according to an aspect of the invention.

Figure 29 is a cross-sectional view of a damper according to an aspect of the invention.

Figure 30 is a cross-sectional view of a damper according to a further aspect of the invention.

Figure 31 is a cross-sectional view of a damped spring according to an aspect of the invention.

Figure 32 is a cross-sectional view of a damped spring according to a further aspect of the invention. DETAILED DESCRIPTION

Various aspects of the invention are presented in Figures 3-28, wherein like components are numbered alike. Referring now specifically to Figure 3, an aircraft 30 is presented comprising an airframe 32, propulsion means 34 attached to the airframe 32 for propelling the airframe 32 with sufficient force to induce and maintain flight, and a suspension system 36 attached to the airframe according to an aspect of the invention. According to an aspect of the invention, a method for stopping the vertical descent of the airframe 32 is provided by landing on a damped spring filled with a dry medium that is compressed during landing, and mechanically altering compression of the dry medium during landing in a prescribed manner, preferably to emulate compression of a gas. In the example presented, the suspension system 36 comprises main landing gear 38 and nose landing gear 40. In Figure 3, the propulsion means 34 comprise a plurality of jet engines. The main landing gear 38 comprises a shock strut 50 and a plurality of wheel and tire assemblies 52. The nose landing gear 40 comprises a shock strut 54 attached to a wheel and tire assembly 56. As will be discussed in more detail, the shock struts 50 and 54 employ a dry spring and damping medium, referred to herein as a "dry medium." The term "dry medium" means plastic powders and/or viscoelastic materials such as, for example but not limited to, elastomers or gels with or without additives.

Viscoelastic dry media materials are viscoelastic throughout the temperature range in which the medium functions as a spring and damping medium. The term "gel" refers to a material characterized by having a storage modulus equal to or slightly greater than the loss modulus. Storage modulus and loss modulus are defined by John D.

Ferry in "Viscoelastic Properties of Polymers" (John Wiley & Sons, Inc., Third Addition, TA455.P58F4 1980). The dry medium should have the capability to flow through an orifice under pressure throughout the temperature range in which the medium functions as a spring and damping medium. Although not necessary in all applications, in some applications the dry medium (i) should have negligible crystallization and no glass transition point in the temperature range of about -70° C to 55 ° C, (ii) the dry medium should have a storage modulus from 10⁴ to 10⁶ dyne/cm² (at one radian per second) throughout the temperature range in which it functions, (iii) should be flowable at low temperatures under pressure, (iv) should have a low tear strength and/or (v) high yield stress at low shear rates. Additives may be included in the dry medium for lubricity, flow control and/or to reduce hysterises losses. Any known additive can be used in the dry medium provided that the additive does not adversely affect the properties and qualities of the dry medium needed in the particular application. Examples of such additives which may affect one or more of the enumerated properties (i-v), include without limitation, molybdenum disulfide powder, paraffins and/or liquids which can be absorbed into the dry medium or mixtures thereof. Examples of liquids that can be absorbed into the dry medium include oils such as for example reactive silicon oils and nonreactive silicon fluids . Preferably if such an oil is used, up to 30 phr (based upon the dry medium) is added. Examples of silicon fluids that can be used are Masil 750 fluid and Masil SFR 70 fluid, both available from PPG Industries. Most preferably, 20 phr of Masil SFR 70 fluid, available from PPG Industries is added to the dry medium.

The dry medium can be used in applications, including, but not limited to, suspension systems subjected to temperatures generally on the order of about -70° C to 55° C depending upon the environment in which the application is used. The preferred temperature range is in the order of about -40° C to 40 ° C. A suspension system according to the invention comprises at least one shock strut that employs a dry spring and damping medium, and preferably every shock strut employs a dry spring and damping medium.

In alternative embodiments, the main landing gear 38 and/or the nose landing gear 40 may comprise a single wheel and tire assembly or a plurality of wheel and tire assemblies. In the example presented in Figure 3, the airframe 32 is a commercial airliner airframe comprising a wing 42, a vertical stabilizer 44, and a horizontal stabilizer 46. During flight, the main landing gear assemblies 38 and nose landing gear 40 are often retracted into the airframe. As presented in Figure 3, the aircraft 30 is shown in final descent about to land on a runway 48 with the main landing gear 38 and nose landing gear 40 fully extended and locked in place. During landing, the aircraft 30 descends toward the runway 48 with a vertical velocity V and a horizontal velocity H.

Referring now to Figure 4, the aircraft 30 is shown after landing during taxi on the runway 48. At this point, the shock struts have absorbed and dissipated the vertical velocity of the airplane, and V=0. Note that the length of the shock struts in Figure 3 before landing is greater than the length of the shock struts in Figure 4 after landing. As will be described in more detail, the shock struts telescope during landing while stopping the vertical descent of the aircraft 30 over the distance the shock struts shorten (the stroke). In essence, the shock struts absorb and dissipate the vertical component of the aircraft kinetic energy and potential energy. The horizontal component of the aircraft kinetic energy, is dissipated by reversing thrust on the propulsion means and/or by applying brakes after landing. During taxi, the shock struts in the main landing gear 38 and nose landing gear 40 suspend the aircraft, and absorb and dissipate any vertical accelerations induced by runway roughness. Though described with reference to a commercial aircraft airframe, the invention may also be employed in the suspensions of other types of aircraft, including commuter and general aviation aircraft, helicopters, tilt-rotors, military transports and fighters, and space vehicles. Likewise, the propulsion means may take any form, including turbojet engines, turbine and piston engine driven propellers, engine driven propfans, turbine and piston engine driven helicopter rotors, turbine and piston engine driven tilt-rotors, ramjets, and liquid and solid fuel rockets. Any such variations in the airframe and the propulsion means are considered to fall within the purview of the invention.

Referring now to Figure 5, a cross-sectional view of a shock strut 500 at zero stroke is presented according to an aspect of the invention that may be employed in the suspension system 36 of Figures 3 and 4, as well as in the suspension systems of other types of aircraft, as previously noted. In Figure 5, shock strut 500 is fully extended, and has a fully extended length indicated as D. The shock strut 500 may be attached to the airframe 32 (shown in phantom) by a knuckle 501 and, by another knuckle 503, may be attached to a wheel and brake assembly or a wheel truck 58 that carries a plurality of wheel and brake assemblies.

The shock strut 500 comprises a hollow outer housing 502 having an outer housing aperture 504, and a hollow media housing 506 received within the outer housing 502 and protruding from the outer housing 502 through the outer housing aperture 504. The outer housing 502 and media housing 506 carry shear loads induced by side-loading of the shock strut 500 during taxi. According to a preferred embodiment, the outer housing 502 and media housing 506 are both cylindrical. The media housing 506 has a media housing aperture 508. A first displacement rod 510 is fixed to the outer housing 502 and received within the media housing 506 through the media housing aperture 508.

The media housing 506 and the first displacement rod 510 define a sealed cavity 512 (boundaries indicated by heavier weight lines). A media housing seal 520 may be provided in the media housing aperture 508 between the media housing 506 and the first displacement rod 510. A damping head 514 is disposed within the cavity 512 fixed to the first displacement rod 510, and defines an orifice 516. According to a preferred embodiment, the first displacement rod 510 is hollow, and the damping head 514 defines the orifice in combination with a tapered metering rod 522 that passes through the damping head 514 inside the first displacement rod 510. The metering rod 522 is fixed to the media housing 506. Other orifice configurations are possible and contemplated within the practice of the invention, for example a fluted damping rod. A dry medium 518 fills the cavity 512. As used herein, the term "fills" means that the dry medium 518 fully occupies cavity 512, and excludes the intentional provision of a space occupied by gas or liquid within the cavity 512. The cavity 512 is filled by forcing the dry medium 518 into the cavity with the shock strut 500 fully extended (as shown) and charging the dry medium to an initial pressure Pi. During landing, the media housing 506 and the outer housing 502/displacement rod 510 telescope toward each and further compress the dry medium 516 to pressures greater than the initial pressure Pi. This movement also forces some of the dry medium 518 to pass through the orifice 516. One or more extension, retraction and/or locking mechanisms 60 may be attached to the shock strut 500, along with a torque linkage 62 that prevents rotation of the media housing 506 relative to the outer housing 502. Various other linkages and attachments may be provided as required for the specific application. A vent hole 548 may be provided to keep the pressure in an outer cavity 550 between the outer housing 502 and media housing 506 at essentially atmospheric pressure.

The dry medium 516 is at the initial pressure Pi when the stroke is zero, before any vertical load is placed on the shock strut 500. The pressurized dry medium 516 forces the media housing 506 away from the outer housing 502 when a vertical load is removed from the shock strut 500, after take-off for example, and the initial pressure ensures that the shock strut 500 will return to its zero stroke position. A ledge 524 may be fixed to the outer housing 504, and a lower bearing 552 may be disposed adjacent to the ledge 524. A spacer sleeve 526 may be inserted inside the media housing 506 resting against the lower bearing 552. An upper bearing/stop 554 is fixed to the media housing 506. The top of the spacer sleeve 526 engages the upper bearing/stop 554 at zero stroke and prevents the outer housing 504 and media housing 506 from separating any further. A maximum stroke stop 556 may be formed in the media housing 506 that engages the outer housing 502 at the maximum stroke. The two bearings 552 and 554 resist beam shear loads on the media housing 506 and outer housing 502 induced by side loads, and enhance the shear load carrying characteristics of the shock strut 500. The outer housing 502 and media housing 506 are each shown as single pieces for the sake of clarity. In practice, the various components comprising shock struts according to the invention are preferably formed from high strength steel, and assembled from multiple pieces, according to methods well known in the aircraft landing gear art. The first displacement rod 510 compresses the dry medium 518 to a pressure greater than the initial pressure Pi by decreasing the volume of the dry medium 518, from its initial volume at zero stroke, as the outer housing 504 and media housing 506 are stroked toward each other. Stroking these two components toward each other forces the displacement rod 510 into the cavity 512, and the volume of the dry medium 518 is decreased from its initial volume by the distance (the stroke) the first displacement rod 510 is forced into the cavity multiplied by the cross-sectional area 528 of the first displacement rod 510. The dry medium 518 resists this motion with a force corresponding to the pressure of the dry medium multiplied by the cross-sectional area 528.

Thus, the dry medium 518 acts as a spring.

In addition to generating a spring force, the dry medium 518 also generates a damping force when the shock strut 500 is stroked in either direction. In the example presented in Figure 5, a damping head seal 530 is provided between the damping head 514 and the media housing

506. The damping head 514 translates through the dry medium 518 as the outer housing 504 and media housing 506 are stroked toward each other, which forces the dry medium 518 to pass through the orifice 516 and develop a pressure differential across the damping head 514. One or more passages 534 may be provided in fluid communication with the orifice 516 in order to permit the dry medium 518 to pass from one side of the damping head 514 to the other through the orifice 516. Other orifice configurations are contemplated in the practice of the invention, for example an orifice without a metering rod 522. The damping force corresponds to the pressure differential multiplied by the cross-sectional area 532 of the damping head 514. The damping force and the spring force combine in summation to provide a predetermined vertical stopping force on the aircraft 30 during landing, and a predetermined suspension force L1 after landing for suspending the aircraft 30 during taxi.

The predetermined vertical stopping force and predetermined suspension force are determined by certain design criteria. Referring now to Figure 16, a graphical representation of load versus stroke is presented for the shock strut 500 used in a suspension system for the aircraft 30. In Figure 16, the maximum stroke of shock strut 500 is indicated as Sm, and the maximum static load for design (the maximum weight of the aircraft shared by the number of shock struts in the suspension system) is indicated as L1. The maximum stroke Sm is determined by the maximum landing energy (vertical kinetic energy) and maximum vertical load requirements. The static stroke at the static load L1 is indicated as Ss. For a large commercial jetliner, the static stroke position during taxi at L1 may be about 80% of the maximum stroke, but this number may vary depending on the specific application. The load versus stroke compression curve (the spring force) for the dry medium 518 is indicated as 536, and passes through the Ss, L1 point in Figure 16. The maximum load during landing, indicated as L2, is generally on the order of one and one-half (1 V) to two (2) times the static load L1.

The predetermined vertical stopping force during landing is indicated as curve 538 which, as previously noted, is the sum of the spring force (curve 536) and the damping force. The predetermined suspension force after landing for suspending the aircraft 30 during taxi is the static load L1 , and corresponds to the static stroke Ss. The various components comprising shock strut 500 are dimensioned such the predetermined vertical stopping force 538 quickly approaches the maximum load during landing L2, generally holds steady during most of the travel to Ss, and then quickly approaches the static load L1 , at which point the vertical velocity V of the airframe is zero (0). The load versus stroke compression curve 536 for the dry medium 518 is essentially fixed, so the damping force is manipulated by varying the size of orifice 516 as a function of the stroke. This preferably is accomplished by use of a tapered metering rod 522, as previously noted. The pressure differential across the damping head 514 may be proportional to the velocity of the damping head as follows: Pd * F(s) • Vdⁿ, wherein Pd is the pressure differential, F(s) is an orifice size factor as a function of stroke, Vd is the velocity of the damping head, and n is an exponent dependent upon the particular material. F(s) and n are determined experimentally. For a dry medium, n may be typically 0.4 to 2.0, but may vary depending on the composition and properties of the dry medium, and is preferably as close to 2.0 as possible. However, damping characteristics and the correlation for those characteristics varies depending on the material, and actual testing is the best way to determine those characteristics. The diameter of the metering rod 522 increases from its initial position 540 at zero stroke to its static position 542 at the static stroke Ss (see Figure 5). This causes the area of orifice 516 to decrease in a prescribed manner as the shock strut 500 strokes in order to provide the desired damping force.

A plot of pressure versus compression ratio is presented in Figure 24, and will be referred to herein as "apparent compressiblity." An apparent compressiblity 558 is determined for a particular material by starting with a defined volume of dry medium at zero pressure, incrementally reducing the volume of the dry medium, and measuring the pressure. The material exhibits some hysteresis depicted by curves 557 and 559 (shown in dashed lines). The compressibility curve is the average of these two curves. The compression ratio is the change in volume divided by the initial volume at zero pressure, and is a dimensionless quantity. In practicing the invention, the operating pressure range of the dry medium 518 is 5000 to 40,000 psi, inclusive, and for practical considerations generally does not exceed 30,000. However, pressures outside that range may be suitable for some applications.

For shock strut 500, the pressure of the dry medium 518 is governed by the apparent compressibility curve 558. A pressure at a particular stroke may be determined by calculating a corresponding compression ratio, and referring to the compressibility curve 558. The compression ratio for a particular stroke may be determined based on the initial pressure Pi (which determines an initial compression ratio CRi), the initial volume of the cavity 512 at zero stroke, and the change in the volume of cavity 512 at the particular stroke. For shock strut 500, the change in volume for all strokes between 0 and Sm is the stroke S multiplied by the cross-sectional area 528 of the first displacement rod

510. The initial pressure Pi and initial volume of the cavity 512 at zero stroke, and the cross-sectional area 528 of the first displacement rod 510 are determined by the previously mentioned design criteria and the apparent compressibility curve 558, in an iterative process. For example, assume a total stroke (Sm) of 20 inches is desired, a static stroke position (Ss) of 16 inches, an initial load of 20,000 pounds at zero stroke, a static load (L1 ) of 150,000 pounds at Ss, and a static pressure of 15,000 psi at Ss. For a cross-sectional area 528 of 10 square inches for the displacement rod 510, the total change in volume (ΔV) at Ss is 16 inches times 10 square inches, equaling 160 cubic inches. The initial pressure Pi of the dry media 518 with the shock strut 500 fully extended (at zero stroke) is the initial load of 20,000 psi divided by the area 528 of 10 square inches, equaling 2000 psi. Referring now to Figure 24, an initial compression ratio at the initial pressure Pi of 2000 psi is about 0.015, and a static compression ratio at the static pressure of

15,000 psi is about 0.065. The difference between these two compression ratios is 0.05 and corresponds to ΔVΛΛ Therefore, V equals ΔV/.06, which is 160 divided by 0.05 equaling 3200 cubic inches. The volume of dry media to achieve the prescribed performance criterion with a 10 square inch displacement rod 510 is 3200 cubic inches. Referring now to Figure 16, the load at zero stroke is determined, and the load at the static stroke is determined, and the volume V of dry media 518 is known. The rest of the curve 536 for any stroke S is determined by calculating the ΔV and compression ratio (ΔV/V) for that stroke, and determining the pressure from the compressibility curve 558. These calculations are representative of standard calculations performed in the design of liquid springs.

The load versus stroke compression curve 18 for nitrogen is presented in Figure 16 for comparison, and passes through the Ss, L1 coordinate. Note that the slope of curve 18 is much greater than the slope of curve 536 to the right of the static stroke Ss, which indicates that the shock strut 500 has more of a tendency to bottom. Therefore, according to a further aspect of the invention, the size of orifice 516 may be rapidly decreased as the stroke exceeds the static stroke Ss, which causes a sharp increase in the damping force as the static stroke Ss is exceeded. The static stroke Ss will be exceeded as part of taxi stroking, and the corresponding increase in damping force prevents bottoming. For example, the aircraft may be taxiing with the shock strut 500 at the static stroke Ss, and then pass over a bump in the runway which sharply forces the media housing 506 into the outer housing 514. The reaction of the shock strut 500 having an orifice restriction at or below the static position 542 of the metering rod is indicated as curve 546. The size of the orifice 512 is preferably restricted by providing an additional, preferably rapid, increase in the diameter of the metering rod 522 just below the static position 542. The compression curve 18 for the gas is much more non-linear than the compression curve 536 for the shock strut 500. With an air- over-oil shock strut, the slope of curve 18 is relatively flat to the left of the static stroke Ss, and sharply increases as the stroke approaches the maximum stroke Sm. The relatively flat portion of curve 18 to left of the static stroke Ss provides a long stroke over which the aircraft is decelerated during landing, and a relatively short length D during taxi.

The sharp increase in the slope of curve 18 as it approaches Sm helps to prevent bottoming. Therefore, according to another aspect of the invention, the compression of the dry medium 518 typically provides a non-linear curve of compressibility, wherein compressibility decreases as stroke increases, resulting in decreased slope as compression decreases and increased slope as compression increases. The compression of the dry medium 518 preferably emulates isothermal compression of a gas, such as nitrogen. As will be described in more detail, compression of said dry medium 518 may be mechanically altered as the displacement rod 510 telescopes into the media housing 506, preferably to emulate compression of a gas and/or to prevent the displacement rod from exceeding a maximum allowable stroke. The maximum allowable stroke is the point at which further telescoping movement of the dry medium spring is mechanically prevented, a condition commonly referred to as "bottoming", for example when the outer housing 502 contacts the media housing 506. Bottoming may cause passenger discomfort, as well as permanent damage to the damped spring.

Referring now to Figure 6, a shock strut 600 for an aircraft is presented wherein compression of the dry medium provides a load stroke of increasing curvature with a static position at the static stroke Ss, which may be 80% of the total stroke needed for landing (Sm). Shock strut 600 is similar to shock strut 500, except shock strut 600 comprises a second displacement rod 610 in addition to the first displacement rod 510. The second displacement rod 610 is preferably tubular and encircles the first displacement rod 510. A second displacement rod seal 620 is disposed between the first displacement rod 510 and the second displacement rod 610. In this example, the media housing seal 520 is disposed in the aperture 504 between the media housing 506 and the second displacement rod 620. Only the first displacement rod 510 is forced into the cavity 512 until the outer housing 502 contacts the top of the second displacement rod 610 when the stroke equals a transition stroke St. At strokes less than St, the change in the volume of cavity 512 is the cross- sectional area 528 multiplied by the stroke S. When the stroke exceeds the transition stroke St, the outer housing 502 forces the second displacement rod 610 into the cavity 512 with the first displacement rod 510, thereby providing an increased cross-sectional area 628 that includes both the first and second displacement rods 510 and 610. Thus, for strokes greater than the transition stroke St, the change in the volume of cavity 512 is the cross-sectional area 628 multiplied by the stroke S. The first displacement rod 510 alone compresses the dry medium 518 when the stroke is less than the transition stroke St, and the first displacement rod 510 and the second displacement rod 610 together compress the dry medium 518 when the stroke is greater than the transition stroke St. Providing three or more displacement rods is also contemplated as may be provided to obtain a desired compression characteristic. The cross-sectional areas 528 and 628 of the first and second displacement rods 510 and 610 are calculated as previously described in relation to Figure 24.

Referring now to Figure 17, a graphical representation of load versus stroke is presented for an embodiment of the shock strut 600. The gas compression curve 18 is presented for comparison. A stroke compression curve 636 for the dry medium 518 in shock strut 600 has a first portion to the left of St corresponding to compression of the dry medium 518 by the first displacement rod 510 alone, and a second portion to the right of St corresponding to compression by the first and second displacement rods 510 and 610 together. The slope of curve 636 to the right of St in figure 17 is greater because the volume of the cavity 512 decreases more per unit of stroke (due to the second displacement rod 610) The vertical step in the compression curve 536 at St is caused when the outer housing 502 contacts the second displacement rod 610 The magnitude of the step equals the annular area of the second displacement rod 610 (cross-sectional area 628 minus cross-sectional area 528) multiplied by the pressure of the dry medium 518 when the outer housing 502 first contacts the second displacement rod 610 A predetermined vertical stopping force during landing is indicated as curve 638, and is very similar to curve 538 of Figure 16 As previously discussed in relation to shock strut 500, the size of orifice 516 in shock strut 600 is changed as a function of stroke in order to provide a desired damping force, the compression force and damping force combining in summation to provide a predetermined vertical stopping force 638 For example, the orifice size may be locally increased in the region of the stroke at which the outer housing 502 contacts the second displacement rod 610 in order to transiently reduce the damping force to compensate for the momentary increase in spring force as the first and second displacement rods 510 and 610 dynamically telescope (stroke) into the media housing 506 under aircraft landing conditions Likewise, an additional decrease in the size of orifice 516 may be provided as the stroke exceeds the static stroke Ss in order to help prevent bottoming, such feature being optional The reaction of the shock strut 600 having an orifice restriction at or below the static position 542 of the metering rod is indicated as curve 646 Referring now to Figure 7, another embodiment of a shock strut

700 for an aircraft is presented, according to an aspect of the invention, wherein compression of the dry medium provides a load stroke of increasing curvature Shock strut 700 is similar to shock strut 500 The hollow media housing 706 and the first displacement rod 510 together define a sealed cavity 712 filled by the dry medium 518 The cavity 712 comprises a first cavity 752 and a second cavity 754 The first and second cavities 752 and 754 are separated by a transition piston 756. A transition piston seal 758 is provided between the transition piston 756 and the media housing 706 which seals the first cavity 752 from the second cavity 754. The first cavity 752 is charged to the initial pressure Pi at zero stroke, and the second cavity 754 is charged to a predetermined pressure that is greater than the initial pressure Pi. The media housing defines a first stop 760, and the dry medium 518 in the second cavity 754 urges the transition piston 756 against the first stop 760. As the shock strut 700 is stroked, the dry medium 518 in the first cavity 752 is compressed to pressures greater than the initial pressure Pi.

The dry medium 518 in the second cavity 754 remains static until pressure in the first cavity 752 adjacent the transition piston 756 exceeds the predetermined pressure. At that point in the stroke, corresponding to the transition stroke St, the transition piston 756 begins to move away from the first stop 760 and compresses the second cavity 754. The volume of dry medium 518 being compressed increases when the transition piston 756 begins to move at the transition stroke St. Before the stroke exceeds St, only the first cavity 752 is compressed and, after the stroke exceeds St, both the first and second cavities 752 and 754 are simultaneously compressed. In contrast with shock strut 600, the area of the displacement rod 510 remains constant and, hence, the change in volume of the cavity 712 is always the area of the displacement rod 510 multiplied by the stroke. The compression of the dry medium 518 is manipulated by increasing the total volume of dry medium 518 being compressed by the displacement rod 510 at the transition stroke St. The dry medium 518 in the second cavity 754 may have a composition that is the same as or different from the composition of the dry medium 518 in the first cavity 752.

Shock strut 700 also employs the tapered metering rod 522 which is fixed to the media housing 706 by an imperforate plate 762. According to a preferred embodiment, the metering rod 522 comprises a longitudinal passage 523 which permits fluid communication between the upper portion of the first cavity 752, on the low pressure side of the damping head 514, and the lower portion of the first cavity 752 adjacent to the transition piston 756. Alternatively, a perforated plate may be substituted for imperforate plate 762 that permits movement of the dry medium 518 from one side to the other of the perforated plate. If a perforated plate is employed, the perforations are preferably restricted in size to reduce sensitivity to the velocity of the damping head 514, since pressure beneath the damping head 514 is a function of the damping head velocity. Finally, the cross-sectional area of the transition piston 756 may be the same as the cross-sectional area 532 of the damping head 514, as presented in Figure 7, or the cross-sectional area of the transition piston 756 may be greater than or less than the cross-sectional area 532 of the damping head 514. Referring now to Figure 18, a graphical representation of load versus stroke is presented for an embodiment of the shock strut 700. The gas compression curve 18 is presented for comparison. A stroke compression curve 736 for the dry medium 518 in shock strut 700 has a first portion to the left of St corresponding to the compression of the first cavity 752 alone, and a second portion to the right of St corresponding to the compression of the first and second cavities 752 and 754 simultaneously. As previously discussed in relation to shock strut 500, the size of orifice 516 in shock strut 700 is changed as a function of stroke in order to provide a desired damping force, the compression force and damping force combining in summation to provide a predetermined vertical stopping force 738. Likewise, an additional decrease in the size of orifice 516 may be provided as the stroke exceeds the static stroke Ss in order to help prevent bottoming, such feature being optional. The reaction of the shock strut 700 having an orifice restriction at or below the static position 542 of the metering rod is indicated as curve 746.

Referring again to Figure 7, an alternative embodiment may be provided by placing the first stop 760 a predetermined distance below the transition piston 756, and charging both the first and second cavities 752 and 754 to an initial pressure Pi at zero stroke. In such embodiment the media housing 706 and the outer housing 502/displacement rod 510 telescope toward each other a stroke distance S during landing thereby forcing the first displacement rod 510 into the first cavity 752 and compressing the first and second cavities 752 and 754 together thereby causing the transition piston 756 to translate toward the first stop 760 until the transition piston engages the first stop 760, after which the first cavity 752 alone is compressed. The compression curve for such embodiment comprises an initial reduced slop portion that transitions to a later increased slope portion (a knee in the curve). All other features are the same as previously described in relation shock strut 700.

Referring now to Figure 8, another embodiment of a shock strut 800 for an aircraft, according to an aspect of the invention, wherein compression of the dry medium provides a load/stroke of increasing curvature. Shock strut 800 is similar to shock strut 700. Shock strut 800 comprises a media housing 806, and a transition piston 856. The media housing 806 and first displacement rod 510 define a sealed cavity 812 (indicated by heavier weight lines). The sealed cavity 812 comprises a first cavity 852 and a second cavity 854, and the transition piston 856 separates the first cavity 852 from the second cavity 854. A transition piston seal 860 is provided to seal the first cavity 852 from the second cavity 854. The media housing 806 defines a first stop 860 and a second stop 864. The first cavity 852 is charged to the initial pressure Pi at zero stroke, and the second cavity 854 is charged to a predetermined pressure which is greater than the initial pressure Pi. The pressure in the second cavity 854 urges the transition piston 856 against the first stop 860, and the transition piston 856 functions essentially the same as the transition piston 756 of shock strut 700. As the shock strut 800 is stroked, pressure in the first cavity 852 adjacent the transition piston rises. The dry medium 518 in the second cavity remains static until pressure in the first cavity 852 adjacent the transition piston 856 exceeds the predetermined pressure. At that point in the stroke, referred to herein as the first transition stroke St1 , the transition piston 856 begins to move away from the first stop 860 and compresses the dry medium 518 in the second cavity 854. As the stroke continues to increase, the first and second cavities 852 and 854 are simultaneously compressed. The transition piston 856 simultaneously compresses the dry medium in the first and second cavities 852 and 854 until the transition piston 856 engages the second stop 864. At that point in the stroke, referred to herein as the second transition stroke St2, the transition piston 856 ceases to move away from the first stop 862. At St2, the dry medium 518 in the second cavity becomes static again, and only the dry medium 518 in the first cavity 852 is compressed by further stroking of the shock strut 800. As with shock strut 700, the metering rod 522 is preferably attached to an imperforate plate 762, and the metering rod 522 preferably has a longitudinal passage 523. Providing two or more cavities and transition pistons is also contemplated as may be provided to obtain a desired compression characteristic. Referring now to Figure 19, a graphical representation of load versus stroke is presented for the shock strut 800. The gas compression curve 18 is presented for comparison. A stroke compression curve 836 for the dry medium 518 in shock strut 800 has a first portion to the left of St1 corresponding to compression of the first cavity 852 alone, a second portion between St1 and St2 corresponding to the simultaneous compression of the first cavity 852 and second cavity 854, and a third portion to the right of St2 corresponding to the compression of the first cavity 852 alone. As previously discussed in relation to shock strut 500, the size of orifice 516 in shock strut 800 is changed as a function of stroke in order to provide a desired damping force, the compression force and damping force combining in summation to provide a predetermined vertical stopping force 838. Likewise, an additional decrease in the size of orifice 516 may be provided as the stroke exceeds the static stroke Ss in order to help prevent bottoming, such feature being optional. The reaction of the shock strut 800 having an orifice restriction at or below the static position 542 of the metering rod is indicated as curve 846.

Referring now to Figure 9, a cross-sectional view of a shock strut 900 is presented having a thermal compensator, according to a further aspect of the invention. An aircraft suspension system is subjected to temperature excursions during use. Such temperature excursions depend on the environment in which the suspension system is employed.

For most systems, the temperature excursion will not likely exceed -70°C to 55 °C (-94 °F to 131 °F), and may be on the order of -23 °C to 38 °C (- 10 °F to 100°F). It is not intended to limit the invention to a particular temperature range, although functionality of the suspension system is necessary throughout the temperature range under which operation is required. When the aircraft is parked, thermal expansion and contraction of the dry medium 518 during temperature excursions increases and decreases the length D, which causes the aircraft to rise and fall. More importantly, thermal contraction of the dry medium reduces the maximum available stroke Sm, which may render the shock strut susceptible to bottoming at colder temperatures. The thermal compensator according to the invention at least partially mitigates these effects, and may fully mitigate these effects.

Still referring to Figure 9, shock strut 900 comprises the outer housing 502 and a media housing 906 received within the outer housing

502, the dry medium 518 filling a sealed cavity 912 defined within the media housing 906 and the outer housing 502. A damping orifice 516 is defined within the cavity 912. The dry medium 518 fills the cavity 912. As previously described in relation to other embodiments, the media housing 906 and the outer housing 502/displacement rod 510 telescope toward each other a stroke distance during landing and compress the dry medium 518 and force the dry medium 518 to pass through the orifice 516, thereby providing a predetermined vertical stopping force on the airframe 32 during landing and a predetermined suspension force during taxi. A temperature compensator 966 is incorporated into the shock strut 900 and subjected to the suspension force while at least partially counteracting thermal expansion and contraction of the dry medium 518 over a predetermined temperature range. According to the invention, the temperature compensator may take various forms that have sufficient strength to resist mechanical failure, and that develop sufficient force to act against the dry medium 518 in order to counteract thermal contraction, while being subjected to the suspension force.

In shock strut 900, the temperature compensator 966 is disposed within the cavity 912, and comprises an expandable and contractible bag 968 containing a fluid 970 that changes phase over the predetermined temperature range. The bag 968 is preferably made out of an elastomeric material that may be fiber reinforced. The change in phase causes the volume of the fluid 970 to increase and at least partially compensate for the decrease in volume in the dry medium 518 due to thermal contraction. According to a preferred embodiment, the fluid 970 is water if the predetermined temperature range includes the freezing point of water. Alternatively, a mixture of water and a freezing point suppressant, such as glycol or alcohol, may be employed provided that the objectives of temperature compensation are met. More than one bag 968 may be provided. The ratio of freezing point suppressant to water may be varied in each bag in order to vary the temperature at which each bag freezes, thus creating a range of temperatures over which the compensation occurs. According to a further preferred embodiment, the cavity 912 comprises a sub-cavity 972 in fluid communication with the rest of the cavity 912, and one or more temperature compensators 966 are disposed within the sub-cavity 972. The sub-cavity 972 may be defined by a perforated plate 962 to which the metering rod 522 is fixed. Referring now to Figure 20, a graphical representation of stroke versus temperature is presented for the shock strut 900. The stroke at a given static load decreases from S1 to S2 as the temperature decreases over the predetermined temperature range from T1 to T2, corresponding to temperature range over which the shock strut 900 is designed to operate. Curve 974 stroke versus temperature with the thermal compensator 966 comprising a single bag 968, and curve 976 represents stroke versus temperature without the thermal compensator 966. Note that the stroke increases at the phase temperature Tp of the fluid 970, which at least partially compensates for the thermal contraction of the dry medium 518.

Referring now to Figure 10, a cross-sectional view of a shock strut 1000 is presented having another embodiment of a thermal compensator 1066 according to an aspect of invention. In this embodiment, a media housing 1006 is received within the outer housing 502, the dry medium

518 filling a sealed cavity 1012 defined within the media housing 1006 and the outer housing 502. The media housing 506 defines a sealed compensator cavity 1068 immediately adjacent the cavity 1012. The temperature compensator 1066 comprises a compensator piston 1070 separating the compensator cavity 1068 and the 1012 cavity. A compensator piston seal 1072 seal is disposed between the compensator piston 1070 and the outer housing 502. A bimetallic actuator 1074 disposed within the compensator cavity 1068 that urges the compensator piston 1070 against the dry medium 518. In this embodiment, the bimetallic actuator 1074 comprises at least one bimetallic frustoconical ring 1076, and preferably comprises a plurality of such rings in a stack. Each bimetallic frustoconical rings 1076 comprises two frustoconical sub-rings formed from metals having different coefficients of thermal expansion, and welded together along a common face. For example, each frustoconical ring 1076 may comprise a copper sub-ring welded to an aluminum sub-ring. As temperature decreases, the aluminum sub-ring thermally contracts more than the copper sub-ring which causes the frustoconical ring 1076 to distort and force the compensator piston 1070 to move up and decrease the volume of the cavity 1012. Decreasing the volume of the cavity 1012 compensates for thermal contraction of the dry medium 518. The metering rod 522 may be fixed to the media housing 1006 by a perforated plate 1062 disposed above the compensator piston 1070.

Referring now to Figure 21 , graphical representation of load versus stroke for shock strut 1000 is presented. In this embodiment, the thermal compensator 1066 continuously compensates for the change in temperature of the predetermined temperature range delimited by T1 and T2. Curve 1078 represents the load versus stroke of shock strut 1000 with the thermal compensator 1066, and curve 1080 represents the load versus stroke without a thermal compensator. Referring now to Figure 11 , a cross-sectional view of a shock strut

1100 is presented having another embodiment of a thermal compensator 1166 according to an aspect of invention. In this embodiment, a media housing 1106 is received within the outer housing 502, the dry medium 518 filling a sealed cavity 1112 (indicated by heavier weight lines) defined within the media housing 1106 and the outer housing 502. The cavity 1112 comprises a sub-cavity 1168 in fluid communication with the rest of the cavity 1112. The temperature compensator 1166 comprises a multitude of volume-changing compensators 1170 dispersed within the sub-cavity 1168. A detailed cross-sectional view of a volume-changing compensator 1170 are presented in Figures 12A and 12B. The volume- changing compensator 1170 in Figure 12A is at a greater temperature than the volume-changing compensator in Figure 12B. Each volume- changing compensator 1170 defines a compensator volume 1172 and comprises a pair of bimetallic disks 1174 that change shape over the predetermined temperature range resulting in a change in the compensator volume 1172. In the example presented, the compensator volume 1172 is greater in Figure 12B since the temperature of the volume-changing compensator 1170 is less than in Figure 12A. Each bimetallic disk 1174 comprises two sub-disks 1176 and 1178 made from different metals having different coefficients of thermal expansion, such as aluminum and copper for example, that are welded together. The coefficient of thermal expansion of one metal is greater than the coefficient of thermal expansion for the other metal. As temperature decreases, sub-disk 1176 thermally contracts more than sub-disk 1178, which causes the bimetallic disk 1174 to distort and become more convex as shown in Figure 12B. The bimetallic disks 1174 are attached to each other around their periphery, preferably by brazing, soldering, or welding. The bimetallic disks 1174 may also be attached to each other by a rolled lip formed on one of the disks 1174, with the other disk 1174 being captive inside the rolled lip. Another embodiment of a thermal compensators 1180 is presented in Figures 13A and 13B. The volume- changing compensator 1180 in Figure 13A is at a greater temperature than the volume-changing compensator 1180 in Figure 13B. The volume-changing compensator 1180 comprises the same bimetallic disks 1174 fixed to an intermediate cylinder 1184. Each volume-changing compensator 1180 defines a compensator volume 1182. The intermediate cylinder 1184 is formed from a metal, such as copper, and the bimetallic disks 1174 may be fixed to the intermediate cylinder 1184 by soldering, brazing or welding. The bimetallic disks 1174 may also be attached to the intermediate cylinder 1184 by a rolled lip formed on the intermediate cylinder 1184, with the disk 1174 being captive inside the rolled lip. The bimetallic disks 1174 function as previously described in relation to Figures 12A and 12B, whereby the compensator volume 1182 is greater in Figure 13B since the temperature of the volume-changing compensator 1180 is less than in Figure 13A. In either embodiment, the bimetallic disks 1174 may be replaced by shaped-memory alloy disks.

Referring now to Figure 22, a graphical representation of load versus stroke for shock strut 1000 is presented wherein the volume- changing compensators 1 170 or 1180 that continuously compensate for the change in temperature over a specific range of temperatures within the predetermined temperature range delimited by T1 and T2 Curve 1186 represents the load versus stroke of shock strut 1100 with the thermal compensators 1070 or 1180, and curve 1188 represents the load versus stroke without a thermal compensator

According to a further aspect of the invention, the temperature compensator 1166 may comprise a multitude of volume-changing compensators 1170 or 1180 dispersed within the sub-cavity 1168 that rapidly change shape at a predetermined compensator temperature included within the predetermined temperature range T1 -T2 resulting in a rapid change in the compensator volume 1172 or 1182, and wherein groups of volume-changing compensators 1170 or 1180 are provided having different predetermined compensator temperatures at which the rapid change in the compensator volume occurs Bimetallic disks 1174 or shaped-memory alloy disks may be employed in this embodiment Referring now to Figure 23, a graphical representation of load versus stroke for shock strut 1100 is presented wherein the volume-changing compensators 1170 or 1180 expand in such manner over the predetermined temperature range T1 -T2 Curve 1190 represents the load versus stroke of shock strut 1100 with the thermal compensators 1070 or 1180 with groups configured to rapidly change shape at different predetermined compensator temperatures, and curve 1192 represents the load versus stroke without a thermal compensator Note the stepwise increases in stroke induced by the rapid change in compensator volume at the various predetermined compensator temperatures

Referring now to Figure 14, a cross-sectional view of a shock strut 1200 is presented having a thermal compensator 1266 according to still another aspect of invention A media housing 1206 is received within the outer housing 502, the dry medium 518 filling a sealed cavity 1212 (indicated by heavier weight lines) defined within the media housing 1206 and the outer housing 502. A compensator wall 1268 defines a sealed compensator cavity 1270 within the media housing 1206. A first compensator piston 1272 disposed within said compensator cavity 1270. A compensator medium 1274 having a greater coefficient of thermal expansion than said dry medium 518 fills the compensator cavity 1270 between said first compensator piston 1272 and said compensator wall 1268. A second compensator piston 1276 is received within a second compensator piston outer housing 1278 defined within the compensator wall 1268. A compensator linkage 1280 passes through the compensator wall 1268 and connects said first compensator piston 1272 to the second compensator piston 1276. Urging means 1282 are provided for urging the first compensator piston 1272 against the compensator medium 1274 and the second compensator piston 1276 against the dry medium 518 via the compensator linkage 1280. According to a preferred embodiment, the urging means 1282 comprises at least one spring. Various urging means may be employed, including coil springs and belleville springs. A first compensator piston seal 1284 and second compensator piston seal 1286 are provided between the respective compensator pistons and the media housing 1206. Thermal contraction of the compensator medium

1274 forces the second compensator piston 1276 against the dry medium 518 via the compensator linkage 1280 and at least partially counteracts thermal contraction of the dry medium 518 because the compensator medium 1274 contracts faster than the dry medium 518, and the urging means 1282 drives the second compensator piston 1276 into the cavity

1212 via the first compensator piston 1272. Thermal expansion of the compensator medium 1274 forces the second compensator piston 1276 away from the dry medium 518 and at least partially counteracts thermal expansion of the dry medium 518 because the compensator medium 1274 expands faster than the dry medium 518 which drives the second compensator piston 1276 away from the cavity 1212 via the first compensator piston 1272. The load versus stroke curve for shock strut 1200 is similar to the load versus stroke curve for shock strut 1000 presented in Figure 21.

According to a further embodiment, a shock strut according to an aspect of the invention comprises temperature compensation and modified compression of the dry medium 518. Referring now to Figure 15, a cross-sectional view of a shock strut 1300 is presented according to an aspect of the invention combining the modified compression features of shock strut 600 (Figure 6) with the thermal compensation features of shock strut 900 (Figure 9). Any of the modified compression embodiments presented in Figures 5-8 may be combined with any of the thermal compensation embodiments presented in Figures 9-11.

According to a further embodiment of the invention, the media housing may be reinforced on the inside and/or on the outside by a fiber reinforced plastic. In Figure 15, the media housing 906 in shock strut

1300 comprises a fiber reinforced plastic tube 1302 that lines the inside of the media housing 906. A pair of seals 1304 are provided at either end of the fiber reinforced plastic tube 1302. In the embodiment presented, the damping head seal 530 rides against the fiber reinforced plastic tube 1302. The fiber reinforced plastic tube 1302 may be formed from known materials, including fiberglass and/or polyamide and/or carbon and/or aramid fibers reinforcing an epoxy plastic, and is preferably filament wound or fabricated from sheet material and cured in an appropriate tool. As stated above, the dry medium can be any elastomer or gel, with or without additives. Preferably, the dry medium is a cross-linked amorphous silicon elastomer. The physical properties of the amorphous silicon elastomer can be varied over a wide range by the choice of the type of polymer used, the molecular weight of the resulting polymer, reinforcing fillers and other additives and the type and concentration of the catalyst so long as the material meets the desired requirements of the final application of the dry medium. The identity and amount of these are well within the purview of one of ordinary skill in the art. Some semi- crystalline silicon materials may be incorporated into the amorphous silicon elastomer provided that it does not change or destroy the characteristics needed for the particular application of the dry medium.

An example of a suitable dry medium is the silicon elastomer composition having the following components:

an organopolysiloxane comprising 77 to 97 mole % of R(CH₃)SiO units, 2 to 23 mole % RRSiO units and 0.1 to 4.0 mole % of CH₃R(CH₂=CH)Si vinyl terminated units, or 0.1 to 6 mole % of OHRSi (CH ₃)₂- 0 wherein R represents a group selected from the group consisting of a methyl group or phenyl group; wherein, the total of the phenyl groups out of the R groups amount from about 2 to about 23 mole percent of all the R groups; a cross linker ; and an optional catalyst.

Approximately 2 to about 10 weight % of the cross linker is included, whereas approximately 0.1 to about 2 weight % of the catalyst is included. Useful organopolysiloxanes include PS- 735 Copolymers, PS- 782 Copolymers and PS- 793 Copolymers; all of which are available from United Chemical. The preferred dry medium has a molecular weight in the range of approximately 5,000 to 120,000 Mw. Preferably, the storage modulus of the cured and processed dry medium is approximately in the range of 10⁴ to 10⁶ dyne/cm² (at one radian/second) as measured using a Rheometric Dynamic Spectrometer.

The amount and type of cross linker and catalyst used in the composition is dependent upon the exact organopolysiloxane and the cure system used. The identity and amount of these are well within the purview of one of ordinary skill in the art.

For example, for hydroxyl terminated polyorganosiloxanes, a condensation cure system is used. The cross linker is usually an alkoxy silane or an oligomer thereof. An example of such a component is ethyl- o-silicate or methyl octyl silane. The catalyst used in a condensation cure system is generally a organic tin compound. Examples of such organic tin compounds include dibutlytindiacetate, dioctyltinmaleate, or dibutly tin dilaurate. The use of dioctyltinmaleate as a catalysts is described in J.C. Weiss, Progress of Rubber Technology, Vol. 46, "Silicon Rubber", Elsevier, United Kingdom, 1984, p. 91. The use of the dibutly tin dilaurate as a catalyst is described in U.S. Patent Nos. 2,843,555 and 3,127, 363.

Alternatively, for vinyl terminated polyorganosiloxane, the addition cure system is used. A polyfunctional silicon hydride cross linker is added to the vinyl terminated polyorganosiloxane. An example of such a commercial crosslinker is PS-122.5 Compound, available from United Chemicals. A platinum complex catalyst is used preferably, although palladium, rhodium or ruthenium complexes may also be used. An example of a commercially available platinum complex is CPC 075 Catalyst, available from General Electric. Examples of a commercially available dry medium which can be used in the instant invention when prepared according to the manufacturer's instructions include RTV 567 Compounds, or RTV 511 Compounds or RTV 6156A and RTV 6156B Compound; all available from GE Silicones and RTVS 51 Copolymer, available from Insulcast. The most preferred dry medium is formed using the RTV 567 Compound.

Table A below lists several dry medium compositions useful for this instant invention as well as other applications requiring the desired properties of the dry medium.

TABLE A Composition Example 1 Example 2 Example 3

PS-735 Copolymer 10 (United Chemical); 3 mole% diphenyl groups

PS-782 Copolymer 10 (United Chemical); 15 mole % diphenyl groups

PS-793 Copolymer 10 (United Chemical); 22 mole% diphenyl groups

Crosslinker, PS-122.5 0.2 0.2 0.2 Compound (United Chemical)

Catalyst, CPC075 1 drop 1 drop 1 drop Catalyst (General Electric)

Tm (°C) none none weak at - 50°C

Tg (°C) -91 -68 -114

The data in Table A illustrates that these dry media gels remain generally amorphous in the temperature range of about -70° C to 55° C. The dry media preferably has no glass transition within that temperature range.

Table B illustrates some additional vinyl terminated polyorganosiloxanes as defined above which are described in the instant application.

TABLE B

Composition 4 5 6 7 8 9 10 11

RTV 6156A 10 10 5 5 3 10 Compound (GE Silicones)

RTV 6156B 10 5 10 15 10 10 9 10 (GE Silicones) CPC 095 1 2 1 2 Catalyst drop drops drop drops (General Electric)

Tg (°C) -125: -125: -125: -125: -125: -125: -125: -124: -94 -93 -88 -91 -92 -92 -92 -92

Tm (°C) none none none none none none none none

Again, this Table B illustrates other examples of dry medium falling within the parameters of the instant invention. In these examples, the commercial compounds RTV 6156A and 6156B include the cross linker as well as the catalyst.

Table C illustrates several additional commercially available dry medium compositions prepared according to the manufacturer's instructions and then processed according to the instant invention.

TABLE C

Composition Tg (°C) Tm (°C)

RTV 511 Compound -123 none (GE Silicones, diphenyl content analyzed to be about 5.5 mole %)

RTV 567 Compound -123 none (GE Silicones, diphenyl content analyzed to be about 5.3 mole %)

RTVS 51 (Insulcast) -123 none

The preferred dry medium material can be mixed with other materials that do not adversely affect its properties for the intended application. For example, it is possible to mix a low viscosity polydimethylsiloxane into the preferred dry medium silicon prior to curing. Preferably, a reactive polydimethylsiloxane (PDMS) having a low viscosity such as 75 centistokes is used. Up to 30 phr of such material can be added to the dry medium. The ability to use the PDMS is particularly surprising because the cured blend does not have a melting point or crystallization in the temperature range of -60° C to -40 ° C which PDMS itself has. Furthermore, the addition of PDMS is economically significant due to the decrease in the cost of the dry medium. Moreover, the addition of the PDMS increases the compressibility of the dry medium. These cured compositions are then processed by shearing the dry media as described below, which may then be used in various applications.

An example of another dry medium that may be suitable for some applications in the temperature range of -23 °C to 38 °C (-10 °F to 100 °F) is a silicone elastomer made from a two-component room temperature vulcanizable (RTV) composition that is commercially available from the Dow Corning Corporation under the SILASTIC^® T RTV trademark. The silicone is described in the Dow Corning product brochure entitled Information About High Technology Products © 1994, and Dow Coming's Material Safety and Data Sheet (MSDS) No. 2208458 dated July 14, 1993, both of which are herein incorporated by reference. These compositions are supplied as a two part system, one containing uncured organosiloxane polymers having relatively low initial viscosities, and the other containing a curing agent. Mixing of the two components effects a cross-linking reaction giving a cured polymer with the following physical properties: CTM 0000 Durometer hardness, Shore A 38

(points)

CTM 0137A Tensile Strength (psi) 800

CTM 0137A Elongation (percent) 385

CTM 0159A Tear Strength (die B, ppi) 95

CTM 0022 Specific Gravity (@ 25° C) 1.13 Other commercially available elastomers that may be suitable in this temperature range include, without limitation: RTV 11 , RTV 162 and RTV 133, available from General Electric; Silastic E and HS ll-RTVG, available from Dow Corning; RBC-7010, RBC-7153 and RBC-7200, available from R.B.C. Industries, Inc.; and GI-1110 and P-90, available from Silicone Inc.

Operating pressures presently expected for the dry medium 518 are in the range of 5000 to 40,000 psi, inclusive, and preferably do not exceed 30,000 psi. Standard hydraulic seals for 3000/5000 psi hydraulic systems, such as an elastomer energized cap seal, appear to sufficiently seal the dry medium throughout the expected range of operating pressures, and may be employed for the various seals disclosed herein. Wear on the seal may be reduced by setting the seal back a distance from the edge being sealed, or by providing a scraper between the dry medium and the seal. An example of a scraper is a continuous aluminum ring 0.04 to 0.05 inch thick dimensioned to provide a 0.001 inch clearance between aluminum ring and the surface being sealed. The aluminum ring may be placed in an annular channel and held in place with a snap ring. The various surfaces that contact the dry media may be plated or coated to reduce adhesion to the dry media. Tungsten carbide is an example of a coating that may reduce adhesion. Although specific examples are provided, it is not intended to limit the invention to a particular arrangement for reducing contact between the dry media and the seal, or for reducing adhesion of the dry media to the various components. Leakage is essentially eliminated, which is a great advantage over the prior art air-over-oil shock struts and springs.

Experiments with forcing the dry medium through an orifice have demonstrated that the properties tend to change as the material is sheared. This is particularly the case if an elastomeric dry medium is employed formed by curing in bulk a quantity of resin, and subsequently chopping the material into a relatively coarse particle size. The particle size is reduced with repeated cycles, and the dry medium eventually reduces to a caulk-like material. Referring now to Figure 28, for example, a plot of processing piston velocity versus a number of cycles is presented. Figure 28 is a representation of certain results obtained by repeatedly forcing the previously described SILASTIC^® T RTV brand silicone elastomer through four 2.705 mm (0.1065 inch) diameter orifices in a 8.29 cm (3.265 inch) diameter processing piston translated in a cylinder at a constant pressure of 5000 psi across the piston head. The material was prepared by curing it according to manufacturer's instructions and subsequently chopping it into roughly one-quarter inch cubes. The processing piston velocity asymptotically approaches a limit (indicated as a horizontal dashed line) as the number of cycles increases, demonstrating that the material is becoming stable. Alternatively, the material may be repeatedly cycled through one or more orifices at a constant processing piston speed until the change in pressure across the processing piston stabilizes. The material may also be stabilized by subjecting the material to high shear rates by other mechanical processes, for example a rubber mill. Whether the dry media is stable depends upon the particular application for which the dry media is intended. The dry media is preferably stabilized under more stringent conditions than that experienced in the actual application. For an aircraft shock strut application, for example, a mechanical shearing stabilization process may be employed to stabilize the dry media at a higher shear rate than the maximum shear rate experienced during aircraft landing and taxi. Finally, the shearing process affects the physical properties of the dry medium. Therefore, a compressibility curve for the dry medium should be developed for a stabilized medium. For example, Figure 24 is an example of a compressiblity curve for the SILASTIC^® T RTV material processed as described in relation to Figure 28. A stable dry medium is desired in a shock strut or dry media damped spring according to the invention employed in an aircraft landing gear or shock strut, since the damping force will remain essentially constant as the number of landings increases. The point at which the properties stabilize is a function of the change in pressure across the damping head, temperature, and the number of cycles. For example, SILASTIC^® T RTV media processed with the apparatus described above at a temperature of 94 °F while maintaining a change in pressure of approximately 4000 psi across the processing piston becomes stable at about 500-600 cycles, RTV 567 processed with the apparatus described above at a temperature of 70 °F while maintaining a change in pressure of approximately 5000 psi across the processing piston becomes stable at about 600-700 cycles, and RTV

511 processed with the apparatus described above with four 3.797 mm (0.1495 inch) orifices, rather than the 2.705 mm (0.1065 inch) orifices mentioned above, at a temperature of 130 °F while maintaining a constant change in pressure of approximately 4000 psi across the processing piston became stable at about 200-300 cycles.

A method according to the invention for providing stabilized performance of a dry media damped spring over repeated aircraft landings, comprises the steps of filling a cavity inside a dry media damped spring with a dry medium for use as a damping and spring medium, the dry media damped spring having a damping head through which the dry medium is repeatedly forced thereby generating a change in pressure across the damping head during repeated aircraft landings, wherein the change in pressure developed by the dry medium is stable over the repeated aircraft landings. The method may also comprise the step of stabilizing the dry medium by repeatedly shearing the dry medium. The dry medium may be stabilized by repeatedly forcing the dry medium to pass through at lest one orifice. The dry medium may also be stabilized by repeatedly translating a processing piston through dry medium disposed in a cylinder, the processing piston having at least one orifice. The processing piston may be translated through dry medium disposed in a cylinder while maintaining a constant change in pressure across the processing piston, the processing piston having at least one orifice. Alternatively, the processing piston may be translated through the dry medium disposed in a cylinder while maintaining a constant processing piston velocity, the processing piston having at least one orifice. Finally, the method may comprise the further steps of chopping a quantity of cured resin, placing chopped resin in a cylinder, and repeatedly translating the processing piston through dry medium disposed in the cylinder, thereby providing a stabilized dry medium.

According to the method just described, a cured elastomer may be processed to reduce its modulus from its as-cured state, as may be desired for a final application. For example, a cured elastomer, such as polyorganosiloxane, having a storage modulus on the order of 10⁷ dynes/cm² (at one radian/second) may be processed as described above until its storage modulus is on the order of 10⁴ to 10⁶ dynes/cm² (at one radian/second).

Alternatively, a cured elastomer, such as a polyorganosiloxane, having a lower modulus than desired in the final application can be used. The modulus of this material can then be increased to the preferred storage modulus on the order of 10⁴ to 10⁶ dyne/cm² (at one radian/second) by the addition of additives, preferably before cure, that form chemical bonds and/or additives that form physical bonds. An example, without limitation, of an additive that forms physical bonds is boron oxide. Examples of additives that form chemical bonds include but are not limited to silica, and calcium carbonate. Preferably, in this embodiment, five (5) weight percent of silica as well as ten (10) weight percent of boron oxide are added to the dry medium. The material is then processed by mechanical shearing until the material becomes stable, as previously described with respect to the Silastic® T elastomer. For example, the processing may take place using a processing piston having at least one orifice while keeping either the pressure across the processing piston constant or by maintaining a constant processing piston velocity. It is believed that a dry medium manufactured according to this method requires less mechanical shear processing than the method previously described herein with respect to the Silastic® T material. Referring now to Figure 25, a front view of an embodiment of a dry media damped spring 1400 for an aircraft landing gear 64 is presented, according to a further aspect of the invention. The landing gear 64 is attached to an airframe 66 (shown in phantom), and is shown after landing on the runway 48 with the landing gear 64 suspending the weight of the airframe 66. The landing gear 64 comprises a main linkage 68, and a tire 70 rotatably mounted to the main linkage 68 in known manner. The main linkage 68 comprises a first attachment 72 to which one end of the dry media damped spring 1400 is pivotally attached by a pin 74. The airframe 66 comprises a second attachment 76 and a third attachment 78. The other end of the dry media damped spring 1400 is pivotally attached to the second attachment 76 by a pin 80. Likewise, one end of the main linkage 68 is pivotally attached to the third attachment 78 by a pin 82. In Figure 26, the dry media damped spring 1400 is compressed by the weight of the airframe 66. Upon take-off, the dry media damped spring 1400 acts against the main linkage 68 and forces it to pivot downward a predetermined distance. Upon landing, the tire 70 forces the main linkage 68 to pivot up as the tire 70 contacts the runway, such action compressing the dry media damped spring 1400. Thus, the dry media damped spring 1400 absorbs the vertical component V of the aircraft kinetic energy during landing, and subsequently suspends the aircraft during taxi.

Referring now to Figure 26, a cross-sectional view of the dry media damped spring 1400 is presented. The dry media damped spring 1400 comprises a hollow dry media housing 1406 having a dry media housing aperture 508. The dry media damped spring 1400 terminates in a pair of knuckles 1401 and 1403 that attach to the first and second attachments 72 and 76. The dry media housing 1406 defines a cavity 512 filled with the dry medium 518. The first and second displacement rods 510 and 610 are received within the dry media housing aperture 508. Note that the dry media damped spring 1400 is very similar to the shock strut 600, and has all the same features except the main housing

502 is removed, and the first displacement rod 510 attaches directly to the knuckle 1403. The dry media damped spring 1400 functions in the same manner as shock strut 1400, except the dry media damped spring 1400 carries only an axial load. Therefore, the description of shock strut 600 previously provided in relation to Figures 6 and 16 also applies to shock strut 1400. A dry media damped spring according to the invention may incorporate any of the features previously described in relation to Figures 6-8, and Figures 17-19, and a dry media damped spring according to the invention is provided merely by removing the main housing 502 in each of those embodiments. The compression of the dry media 518 in a dry media damped spring according to the invention may be manipulated to emulate the compression of a gas, preferably nitrogen.

Referring now to Figure 27, a cross-sectional view of the dry media damped spring 1500 is presented, which may be substituted for dry media damped spring 1400. The dry media damped spring 1500 comprises a hollow dry media housing 1506 having a dry media housing aperture 508. The dry media damped spring 1500 terminates in a pair of knuckles 1501 and 1503 that attach to the first and second attachments 72 and 76. The dry media housing 1506 defines a cavity 512 filled with the dry medium 518. The first and second displacement rods 510 and

610 are received within the dry media housing aperture 508. Note that the dry media damped spring 1500 is very similar to the shock strut 900, and has all the same features except the main housing 502 is removed, and the first displacement rod 510 attaches directly to the knuckle 1503. The dry media damped spring 1500 functions in the same manner as shock strut 1500, except the dry media damped spring 1500 carries only an axial load Therefore, the description of shock strut 900 previously provided in relation to Figures 9 and 20 also applies to shock strut 1500 A dry media damped spring according to the invention may incorporate any of the features previously described in relation to Figures 9-14 and Figures 20-23, and a dry media damped spring according to the invention is provided merely by removing the main housing 502 in each of those embodiments

According to a further embodiment, a dry media damped spring according to an aspect of the invention comprises temperature compensation and a modified compression of the dry media 518 As previously discussed in relation to Figure 15, the dry media damped spring according to the invention may employ first and second displacement rods 510 and 610 with the thermal compensator 966 A dry media damped spring according to the invention may incorporate any of the modified compression features previously described in relation to

Figures 6-8, and Figures 17-19, with any of the thermal compensator features previously described in relation to Figures 9-14 and Figures 20- 23, and a dry media damped spring according to the invention is provided merely by removing the main housing 502 in each of those embodiments Referring now to Figures 29-32, various embodiments of damping devices and damped springs according to various further aspects of the invention are presented Referring specifically to Figure 29, a damping device 1600 is presented, comprising a hollow media housing 1606 having a media housing aperture 1608, a damping rod 1610 received within the media housing aperture 1608, the damping rod 1610 and the media housing 1606 defining a sealed cavity 1612 The dry medium 518 fills the cavity 1612 According to preferred embodiment, the dry medium 518 has a storage modulus on the order of 10⁴ to 10⁶ dynes/cm² (at one radian/second) The dry medium 518 may have negligible crystallization in the temperature range of -70 °C to 55 °C, which is preferred for use in very low temperature applications In damping device 1600, damping is generated due to friction and viscous flow of the dry medium 518. Seals 1620 are provided to seal the damping rod 1610 relative to the media housing 1606 in a manner that permits the damping rod 1610 to telescope toward the media housing 1606, thereby forcing the damping rod 1610 to translate relative to the media housing 1606, which generates a damping force on the damping rod 1610. Referring now to Figure 30, a damping device 1700 is presented which is very similar to damping device 1600. Damping device 1700 comprises a damping rod 1710 having a damping head 1714 fixed to the damping rod 1710, and the media housing 1606 and damping rod 1710 define a damped cavity

1712. The damping head 1714 defines an orifice 1716 through which the dry media 518 is forced to flow when the damping rod 1710 is translated. The damping head 1714 modifies the damping force generated by the damping device 1700 relative to damping device 1600. Referring now specifically to Figure 31 , a damped spring 1800 is presented comprising a hollow media housing 1806 having a media housing aperture 1808, a displacement rod 1810 received within the media housing aperture 1808, the damping rod 1810 and media housing 1806 defining a sealed cavity 1812, and the displacement rod 1810 terminates within the sealed cavity 1812. The dry medium 518 fills the cavity 1812. According to preferred embodiment, the dry medium 518 has a storage modulus on the order of 10⁴ to 10⁶ dynes/cm² (at one radian/second). The dry medium 518 may have negligible crystallization in the temperature range of -70 °C to 55 °C, which is preferred for use in very low temperature applications. Seals 1820 are provided to seal the displacement rod 1810 relative to the media housing 1806 in a manner that permits the displacement rod 1810 to telescope toward the media housing 1806, thereby forcing the displacement rod 1810 into the media housing 1806, which generates a combined spring and damping force on the damping rod 1810. Referring now to Figure 32, a damped spring

1900 is presented which is very similar to damped spring 1800. Damped spring 1900 comprises a displacement rod 1910 having a damping head 1914 fixed to the displacement rod 1910, and the media housing 1806 displacement rod 1910 define a damped cavity 1912. The damping head 1914 defines an orifice 1916 through which the dry media 518 is forced to flow when the displacement rod 1910 is translated. The damping head

1914 modifies the damping force generated by the damped spring 1900 relative to damped spring 1800. Reference is made to United States Patent 3,053,526 to Kendall for further information relating to such damping and damped spring devices. Comparing Figures 29-32 to Figures 5-11 and 14-15 shows that the shock struts presented in Figures 5-11 and 14-15 are actually damped springs. Therefore, the teachings provided herein with respect to damping may be applied to the embodiments of Figures 29-32, and the embodiments of Figures 31-32 may incorporate any of the modified compression features previously described in relation to Figures 6-8, and

Figures 17-19, and any of the thermal compensator features previously described in relation to Figures 9-14 and Figures 20-23, alone or in combination.

Though shown and described in a particular orientation, the shock struts, dry media damping devices, and damped springs according to the invention may be oriented as necessary for a particular application. According to an aspect of the invention, a dry media damped spring for an aircraft suspension system is provided, comprising: a hollow media housing 506 having a media housing aperture 508; a first displacement rod 510 received within the media housing through the media housing aperture 508; a second displacement rod 610 received within the media housing 506 through the media housing aperture 508, the first and second displacement rods 510 and 610 defining a sealed cavity 512 inside the media housing 506; a damping head disposed 514 within the cavity 512 and attached to the first displacement rod 510, the damping head 514 defining an orifice 516, and, a dry medium 518 filling the cavity 512, the media housing 506 and the first displacement rod 510 telescoping toward each other a stroke distance during landing and compressing the dry medium by initially forcing the first displacement rod 510 alone into the cavity 512, and subsequently forcing the first and second displacement rods 510 and 610 together into the cavity 512, the damping head being moved through the cavity 512 thereby forcing the dry medium 518 to pass through the orifice 516

According to a further aspect of the invention, a dry media damped spring for an aircraft suspension system is provided, comprising a hollow media housing 706 having a media housing aperture 508, a first displacement rod 510 received within the media housing 506 through the media housing aperture 508, the first displacement rod 510 defining a sealed cavity 712 inside the media housing 706, a transition piston 756 disposed within the media housing 706 that divides the cavity 512 into a first sealed cavity 752 and a second sealed cavity 754, the first cavity 752 having an initial pressure at zero stroke and the second cavity 754 having a predetermined pressure greater than the initial pressure at zero stroke, the media housing 706 defining a first stop 760 wherein the predetermined pressure urges the transition piston 756 against the first stop 760, a damping head 514 disposed within the first cavity 712 and attached to the first displacement rod 510, the damping head defining an orifice 516, and, a dry medium 518 filling the cavity 712, the media housing 706 and the first displacement rod 510 telescoping toward each other a stroke distance during landing thereby forcing the first displacement rod 510 into the first cavity 752 and compressing only the first cavity 752 until pressure in the first cavity 752 adjacent the transition piston 756 exceeds the predetermined pressure which forces the transition piston 756 away from the first stop 760, after which the first and second cavities 752 and 754 together are compressed, the damping head 514 being moved through the first cavity 752 thereby forcing the dry medium 518 to pass through the orifice 516.

According to a further aspect of the invention, a dry media damped spring for an aircraft suspension system is provided, comprising: a hollow media housing 806 having a media housing aperture 508, the media housing 806 defining a first stop 860 and a second stop 856 spaced from the first stop 860; a first displacement rod 510 received within the media housing 806 through the media housing aperture 508, the first displacement rod 510 defining a sealed cavity 812 inside the media housing 510; a transition piston 856 disposed within the media housing 806 that divides the cavity 812 into a first sealed cavity 852 and a second sealed cavity 854, the first cavity 852 having an initial pressure at zero stroke and the second cavity 854 having a predetermined pressure greater than the initial pressure at zero stroke, wherein the predetermined pressure urges the transition piston 856 against the first stop 860; a damping head 514 disposed within the first cavity 812 and attached to the first displacement rod 510, the damping head defining an orifice 516; and, a dry medium 518 filling the cavity 812, the media housing 806 and the first displacement rod 510 telescoping toward each other a stroke distance during landing thereby forcing the first displacement rod 510 into the first cavity 852 and compressing the first cavity 852 alone until pressure in the first cavity 852 adjacent the transition piston 856 exceeds the predetermined pressure which forces the transition piston 856 away from the first stop 860, upon which the first and second cavities 852 and 854 together are compressed until the transition piston engages the second stop 864 which ceases further movement of the transition piston 856 and returns compression to the first cavity 852 alone, the damping head being 514 moved through the first cavity 852 thereby forcing the dry medium 514 to pass through the orifice 516.

According to a further aspect of the invention, a dry media damped spring for an aircraft suspension system is provided, comprising: a hollow media housing 706 having a media housing aperture 508, the media housing 706 defining a first stop 760; a first displacement rod 510 received within the media housing 706 through the media housing aperture 508, the first displacement rod 510 defining a sealed cavity 712 inside the media housing 706; a transition piston 756 disposed within the media housing 706 that divides the cavity 712 into a first sealed cavity 752 and a second sealed cavity 754, the first and second cavities 752 and 754 having an initial pressure at zero stroke, the transition piston 756 being spaced from the first stop 760; a damping head 514 disposed within the first cavity 752 and attached to the first displacement rod 510, the damping head 514 defining an orifice 516; and, a dry medium 518 filling the cavity, the media housing 506 and the first displacement rod 510 telescoping toward each other a stroke distance during landing thereby forcing the first displacement rod 510 into the first cavity and compressing the first and second cavities 752 and 754 together thereby causing the transition piston 756 to translate toward the first stop 760 until the transition piston 756 engages the first stop 760, after which the first cavity 752 alone is compressed, the damping head

514 being moved through the first cavity 752 thereby forcing the dry medium 518 to pass through the orifice 516.

According to an aspect of the invention, a suspension for an aircraft is provided, comprising: (a) a dry media damped spring 900,1500 having a hollow media housing 906, 1506, having a hollow media housing aperture 508, a first displacement rod 510 received within the media housing 900, 1506 through the media housing aperture 508, the first displacement rod 510 defining a sealed cavity 512, 912 inside the media housing 906, 1506, a damping head 514 disposed within the first cavity 512, 912 and attached to the first displacement rod 510, the damping head 514 defining an orifice 516, and a dry medium 518 filling the cavity 512, 912 the media housing 906, 1506 and the first displacement rod 510 telescoping toward each other a stroke distance during landing thereby forcing the first displacement rod 510 into the first cavity 512, 912 and compressing the dry medium 518 and forcing the dry medium 518 to pass through the orifice 516, thereby providing a predetermined vertical stopping force on the airframe during landing and a predetermined suspension force during taxi; and, (b) a temperature compensator 966 incorporated into the damped spring 900, 1500 and subjected to the suspension force while at least partially counteracting thermal expansion and contraction of the dry medium 518 over a predetermined temperature range.

According to an aspect of the invention, a method is provided for providing stabilized performance of a dry media damped spring over repeated aircraft landings, comprising the steps of: filling a cavity inside a dry media damped spring 500, 600, 700, 800, 900, 1000, 1100, 1200, 1300, 1400, 1500 with a dry medium 518 for use as a damping and spring medium, the dry media damped spring having a damping head 514 through which the dry medium 518 is repeatedly forced thereby generating a change in pressure across the damping head 514 during repeated aircraft landings, wherein the change in pressure developed by the dry medium 518 is stable over the repeated aircraft landings. Any of the aspects described herein may further comprise compressing the dry medium 518 and forcing the dry medium to pass through the orifice 516 provides a predetermined vertical stopping force on the aircraft and a predetermined suspension force for suspending the aircraft after landing.

In any of the aspects described herein, compression of the dry medium 518 may emulate the compression of a gas.

Any of the aspects described herein may further comprise a hollow outer housing 502 having an outer housing aperture 504, the hollow media housing 506, 706, 806, 906, 1006, 1106, 1206, 1306, 1406, 1506 being received within the outer housing 502 and protruding from the outer housing 502 through the outer housing aperture 504, the outer housing 502 and media housing 506 together providing resistance to side load induced shear forces, and the first displacement rod 510 may be fixed to the outer housing 502.

It is evident that many variations are possible without departing from the true scope and spirit of the invention as defined by the claims that follow.

Claims

We claim:

1. A damped spring for an aircraft suspension, comprising: a hollow media housing having a media housing aperture; a displacement rod received within said media housing aperture, said displacement rod and said media housing defining a sealed cavity, said displacement rod terminating within said sealed cavity; and, a dry medium filling said cavity, said displacement rod telescoping into said media housing and compressing said dry medium during landing of the aircraft thereby providing a vertical stopping force on the aircraft, wherein compression of said dry medium is mechanically altered as said displacement rod telescopes into said media housing to emulate compression of a gas.

2. A damped spring for an aircraft suspension system, comprising: a hollow media housing having a media housing aperture, a first displacement rod received within said media housing through said media housing aperture, said media housing and said first displacement rod defining a sealed cavity, a damping head disposed within said cavity and attached to said first displacement rod, said damping head defining an orifice, characterized by a dry medium filling said cavity, said media housing and said first displacement rod telescoping toward each other during landing of the aircraft thereby forcing said first displacement rod into said cavity and compressing said dry medium and forcing said dry medium to pass through said orifice.

3. The damped spring of claim 1 -2, further characterized by being configured to provide a predetermined vertical stopping force on the aircraft during landing and a predetermined suspension force for suspending the aircraft after landing.

4. The damped spring of claim 3, further characterizing a temperature compensator subjected to said suspension force while at least partially counteracting thermal expansion and contraction of said dry medium over a predetermined temperature range.

5. The suspension system of claim 4, wherein said temperature compensator comprises an expandable and contractible bag disposed within said cavity and containing a fluid that changes phase over said predetermined temperature range.

6. The suspension system of claim 4, wherein said temperature compensator comprises a bimetallic actuator.

7. The damped spring of claims 1 -6, further characterizing a second displacement rod received within said media housing through said media housing aperture, said first and second displacement rods defining a sealed cavity inside said media housing, said first displacement rod alone initially being forced into said cavity, said first and second displacement rods together subsequently being forced into said cavity.

8. The damped spring of claims 1 -6, further characterizing a transition piston disposed within said media housing that divides said cavity into a first sealed cavity and a second sealed cavity, said first cavity having an initial pressure at zero stroke and said second cavity having a predetermined pressure greater than said initial pressure at zero stroke, said media housing defining a first stop wherein said predetermined pressure urges said transition piston against said first stop, wherein forcing said first displacement rod into said first cavity compresses only said first cavity until pressure in said first cavity adjacent said transition piston exceeds said predetermined pressure and forces said transition piston away from said first stop, after which said first and second cavities together are compressed.

9. The damped spring of claims 1-6, wherein said media housing defines a first stop and a second stop spaced from said first stop, and further characterized by a transition piston disposed within said media housing that divides said cavity into a first sealed cavity and a second sealed cavity, said first cavity having an initial pressure at zero stroke and said second cavity having a predetermined pressure greater than said initial pressure at zero stroke that urges said transition piston against said first stop, wherein forcing said first displacement rod into said first cavity compresses said first cavity alone until pressure in said first cavity adjacent said transition piston exceeds said predetermined pressure and forces said transition piston away from said first stop, upon which said first and second cavities together are compressed until said transition piston engages said second stop which ceases further movement of said transition piston and returns compression to said first cavity alone.

10. The damped spring of claims 1 -6, wherein said media housing defines a first stop, and further characterized by a transition piston disposed within said media housing that divides said cavity into a first sealed cavity and a second sealed cavity, said first and second cavities having an initial pressure at zero stroke, said transition piston being spaced from said first stop, wherein both said first and second cavities are compressed thereby causing said transition piston to translate toward said first stop until said transition piston engages said first stop, after which said first cavity alone is compressed.

11. The damped spring of claims 1 -10, wherein compression of said dry medium emulates compression of a gas.

12. The damped spring of claims 1 -11 , wherein said dry medium is stabilized to provide a stable change in pressure across said damping head over repeated aircraft landings.

13. The damped spring of claims 1 -12, further characterized by an outer housing having an outer housing aperture, said media housing being received with said outer housing and protruding from said outer housing through said outer housing aperture, said outer housing and media housing together providing resistance to side load induced shear forces.

14. A damping device, comprising: a hollow media housing having a media housing aperture; a damping rod received within said media housing aperture, said damping rod and said media housing defining a sealed cavity; and, a dry medium filling said cavity having a storage modulus on the order of 10⁴ to 10⁶ dynes/cm² (at one radian/second).

15. A damped spring, comprising: a hollow media housing having a media housing aperture; a displacement rod received within said media housing aperture, said damping rod and said media housing defining a sealed cavity, said displacement rod terminating within said sealed cavity; and, a dry medium filling said cavity having a storage modulus on the order of 10⁴ to 10⁶ dynes/cm² (at one radian/second).

16. The apparatus of claims 1 -15, wherein said dry medium comprises an amorphous silicon elastomer.

17. The apparatus of claims 1-16, wherein said dry medium comprises an organopolysiloxane comprising 77 to 97 mole % of R(CH₃)SιO units, 2 to 23 mole % RRSiO units and 0 1 to 4 0 mole % of CH₃R(CH₂=CH)Sι vinyl terminated units, or 0 1 to 6 mole % of OHRSι(CH₃)₂-O wherein R represents a group selected from the group consisting of a methyl group or phenyl group, wherein, the total of the phenyl groups out of the R groups amount from about 2 to 23 mole percent of all the R groups, from 2 to 10 weight % of a cross-linker, and from 0 1 to 2 weight % of a catalyst

18 The apparatus of claims 1 -17, wherein said dry medium comprises an absorbed fluid

19 The apparatus of claims 1 -18, wherein said dry medium comprises a silicon elastomer and on the order of 1 to 30 phr of silicon fluid based upon the dry medium is incorporated into said silicon elastomer

20 A suspension for an aircraft comprising the device of claims 1 - 19

21 A method for providing stabilized performance of a dry media damped spring over repeated aircraft landings, comprising the steps of filling a cavity inside a dry media damped spring with a dry medium for use as a damping and spring medium, said dry media damped spring having a damping head through which said dry medium is repeatedly forced thereby generating a change in pressure across said damping head during repeated aircraft landings, wherein said change in pressure developed by said dry medium is stable over said repeated aircraft landings

22 The method of claim 21 , further comprising the step of stabilizing said dry medium by repeatedly shearing said dry medium

23 The method of claim 21 , further comprising the step of stabilizing said dry medium by repeatedly forcing said dry medium to pass through at least one orifice

24 The method of claim 21 , further comprising the step of stabilizing said dry medium by repeatedly translating a processing piston through dry medium disposed in a cylinder while maintaining a constant change in pressure across said processing piston, said processing piston having at least one orifice

25 The method of claim 21 , further comprising the step of stabilizing said dry medium by repeatedly translating a processing piston through dry medium disposed in a cylinder while maintaining a constant processing piston velocity, said processing piston having at least one orifice

26 The method of claims 21-25, further comprising the steps of curing in bulk a quantity of resin, subsequently chopping said resin, and, stabilizing said dry medium by repeatedly shearing said dry medium

27 The method of claim 21 -25, further comprising the steps of chopping a quantity of cured resin, placing chopped resin in a cylinder, and, repeatedly translating a processing piston through said chopped resin, said processing piston having at least one orifice, thereby providing a stabilized dry medium

28 The method of claim 21-27, wherein said dry medium is an amorphous silicon elastomer and has a storage modulus less than a desired modulus in the range 10⁴ to 10⁶ dynes/cm² (at one radian/second), and said modulus is increased to said desired modulus by the addition of additives that form physical and/or chemical bonds.

29. The apparatus of claims 21 -28, wherein said dry medium comprises an organopolysiloxane comprising 77 to 97 mole % of R(CH₃)SiO units, 2 to 23 mole % RRSiO units and 0.1 to 4.0 mole % of CH₃R(CH₂=CH)Si vinyl terminated units, or 0.1 to 6 mole % of OHRSi(CH₃)₂-O wherein R represents a group selected from the group consisting of a methyl group or phenyl group; wherein, the total of the phenyl groups out of the R groups amount from about 2 to 23 mole percent of all the R groups; from 2 to 10 weight % of a cross-linker; and from 0.1 to 2 weight % of a catalyst.

30. The apparatus of claims 21 -29, wherein said dry medium comprises an absorbed fluid.