Crashworthy landing gear

A shock strut assembly for an aircraft landing gear includes a trunnion fitting, a compressible oleo strut subassembly, a wheel subassembly, an energy dissipating subassembly, and a mechanical load control subassembly. The oleo strut subassembly includes a cylinder member having an upper end mounted in combination with the trunnion fitting by means of the mechanical load control subassembly and a piston member having a lower end affixed in combination with the wheel subassembly and an upper end slidably inserted in the cylinder member. The energy dissipating subassembly includes a cylindrical cutter member secured in combination with the cylinder member and the trunnion fitting and a frangible tube member mounted in concentric combination with the cylinder member so that the lip of the upper end thereof is disposed in abutting engagement with the cutter member. The mechanical load control subassembly includes a shear ring, a retainer nut, complementary flange cutouts formed in the endface of the cylinder member, and complementary torsion key slots formed in the trunnion fitting. The shear ring has a configuration that segregates the landing forces induced in the shock strut assembly during landings and includes a cylindrical body having opposed shear flanges extending outwardly therefore. Each shear flange includes a torsion key. The shear ring is mounted in locked combination with the cylinder member and the trunnion fitting by means of the threaded engagement of the retainer nut with the trunnion fitting wherein the retainer nut engages the shear flanges. In the locked configuration, the cylindrical body abuts the cylinder member, the shear flanges engage the complementary flange cutouts of the cylinder member and the torsion keys are disposed in the complementary torsion key slots. During normal landing, the shear ring prevents movement of the cylinder member with respect to the trunnion fitting. In a crash landing, the shear flanges shear at a predetermined axial load failure limit, which allows the cylinder member to be displaced relative to the trunnion fitting. Displacement of the cylinder member causes the frangible tube member to interact with the cutter member, causing fracturing of the frangible tube member for crash landing energy dissipation.

TECHNICAL FIELD 
The present invention relates to aircraft landing gear, and more 
particularly, to a crashworthy aircraft landing that incorporates a 
mechanical load control and a frangible tube member for predictably 
controlling energy dissipation by means of the landing gear in the event 
of a crash landing. 
BACKGROUND OF THE INVENTION 
A significant percentage of the operating time of a helicopter involves 
low-speed, low-altitude flight regimes and/or hovering operations. 
Accidents occurring during these modes of helicopter operations involve 
high vertical descent rates with the helicopter in a near normal flight 
attitude. While there is some degree of uncertainty vis-a-vis the flight 
attitude at ground impact of helicopters involved in high-altitude and/or 
high speed accidents, to the extent that the helicopter pilot is able to 
exercise the autorotation technique, such helicopters will impact the 
ground in a near normal flight attitude. In these type of accidents, the 
landing gear system, whether of the skid-type or the wheel-type, is the 
first element of the helicopter to impact the ground. As such, landing 
gear systems are typically designed with the constraint that such systems 
must be capable of attenuating or dissipating a large degree of the impact 
energy experienced in a crash landing situation. For example, the FAA 
requirement for civil aircraft is that such aircraft must exhibit 
structural integrity after a free fall ground impact from a height of 8.0 
inches (equivalent to a sink rate of 6.55 ft/sec). Military aircraft 
requirements are typically more stringent, requiring structural integrity 
after a free fall ground impact from a height of 26.8 inches (equivalent 
to a sink rate of 12 ft/sec). In addition, landing gear systems should be 
designed so that once the energy-absorbing capability of the landing gear 
system is exceeded, the landing gear system reaction to the crash landing 
does not increase the risk of danger to any occupants of the helicopter, 
e.g., controlled penetration the cockpit and/or cabin areas of the 
helicopter and/or avoiding rupturing the fuel cells of the helicopter. 
The survivability constraint is typically accommodated by design of the 
landing gear system and/or the undercarriage of the helicopter so that a 
large percentage of the impact energy arising from a crash landing is 
attenuated or dissipated by the undercarriage and/or landing gear system. 
For example, some helicopters are designed with crushable tub structures, 
i.e., the portion of the fuselage below the passenger compartment, which 
are designed to crush during a crash landing to attenuate or dissipate the 
impact energy. This type of design is similar to that used in the 
automotive industry for attenuating or dissipating the impact energy 
generated in head-on crashes. 
With a skid-type landing gear system, the skids are designed to attenuate 
the energy generated by normal landings by elastic deformation of the 
skids. The skids are operative to crush in response to the impact energy 
of a crash landing. The crushing of metal skids absorbs a significant 
percentage of the crash landing energy. While skid-type landing gear 
systems are generally effective, one drawback to such systems is that the 
degree of degradation of the skids over time due to normal landings may 
not be readily observable by visual inspection. In addition, replacement 
of the skids due to degradation arising from normal landings is a labor 
intensive and expensive process. 
Wheel-type landing gear systems typically incorporate a compressible oleo 
strut subassembly that is operative to attenuate the energy generated by 
normal landings. Energy attenuation is achieved by stroking which causes 
compression of a compressible gas in the oleo strut subassembly. This type 
of energy attenuation is generally effective in decoupling landing loads 
from the helicopter, and in addition, does not result in any significant 
degradation of the landing gear system over time due to multiple normal 
landings. To react the impact energy of a crash landing, wheel-type 
landing gear systems may employ shear pins which are operative to transfer 
the impact energy of the crash landing from the oleo strut subassembly to 
the landing gear trunnion. 
The shear pins are inserted in aligned apertures in the oleo strut 
subassembly and landing gear trunnion and are designed to fail at a 
predetermined load level (as a result of a crash landing) to effectuate 
the transfer of the impact energy of the crash landing from the oleo strut 
subassembly to the landing gear trunnion. There are several disadvantages 
arising from the use of shear pins. First, shear pins do not have a high 
degree of durability. Load transfer between the oleo strut subassembly and 
the landing gear trunnion is subject to a high stress gradient due to the 
geometry of the shear pins and the corresponding apertures. This can 
result in local yielding and degradation over time due to multiple normal 
landings. Secondly, the mechanical degradation of the shear pins and/or 
aligned apertures is not readily apparent during a visual inspection. In 
addition, the replacement of worn and/or damaged shear pins and/or the 
oleo strut subassembly and/or the trunnion (due to aperture wear and/or 
damage) is a labor intensive, time consuming, and expensive proposition. 
Finally, shears pins react all of the loads, i.e., vertical, drag, side, 
and torsional loads, arising from normal landings. It is difficult to 
analytically predict the degree of damage to the shear pins and 
corresponding apertures from all loading conditions, and as such, it is 
difficult to predict with a high degree of certainty at what axial crash 
load, i.e., ultimate shear loading, the shear pins will shear at. In 
addition, normal wear and/or degradation of the shear pins and/or 
corresponding apertures directly affects shear pin tolerances and 
interfits, which has a significant impact on the ultimate shear loading at 
which the shear pins fail. 
A need exists to develop a durable, predictable, reliable, and maintainable 
mechanical means to control the functioning of a wheel-type landing gear 
system in response to a crash landing. 
DISCLOSURE OF THE INVENTION 
One object of the present invention is to provide a mechanical load control 
subassembly for a crashworthy landing gear shock strut assembly that 
segregates the landing forces exerted on the mechanical load control 
subassembly into vertical, drag, side, and torsion loads to minimize local 
yielding and degradation of the mechanical load control subassembly. 
Another object of the present invention is to provide a mechanical load 
control subassembly for a crashworthy landing gear shock strut assembly 
that segregates the landing forces exerted on the mechanical load control 
subassembly into vertical, drag, side, and torsion loads such that the 
mechanical load control subassembly is reliable and predictable in failing 
at a predetermined axial load level. 
A further object of the present invention is to provide a mechanical load 
control subassembly for a crashworthy landing gear shock strut assembly 
that is readily inspectable and replaceable. 
One more object of the present invention is to provide an energy 
dissipating subassembly for a crashworthy landing gear shock assembly that 
dissipates crash landing energy by a mechanical fracturing mechanism. 
These and other objects of the present invention are achieved by a shock 
strut assembly for a crashworthy aircraft landing gear that comprises a 
trunnion fitting mounted in combination with the aircraft, a wheel 
subassembly, a compressible oleo strut subassembly operative to attenuate 
energy coupled into the aircraft by the wheel subassembly during normal 
landings, the compressible oleo strut subassembly including a cylinder 
member having a lower end and an upper end mounted in combination with the 
trunnion fitting, a piston member having a lower end affixed in 
combination with the wheel subassembly and an upper end slidably inserted 
in the lower end of the cylinder member, and a floating piston mounted for 
sliding movement in the piston member, a mechanical load control 
subassembly locked in combination with the upper end of the cylinder 
member and the trunnion fitting, the mechanical load control subassembly 
being operative during normal landings to restrain relative movement 
between the cylinder member and the trunnion fitting and being operative 
during a crash landing in response to a predetermined axial load to allow 
upward displacement of the cylinder member with respect to the trunnion 
fitting, and an energy dissipating subassembly disposed in combination 
with the cylinder member and operative in response to the crash landing to 
mechanically dissipate crash landing energy induced in the aircraft by the 
wheel subassembly during the upward displacement of the cylinder member. 
The described embodiment of the mechanical load control subassembly 
comprises a shear ring having a cylindrical body and opposed shear flanges 
extending outwardly from cylindrical body, each said shear flange having a 
torsion key, a retainer nut, the upper endface of the cylinder member 
having complementary flange cutouts, and the trunnion fitting having an 
upper internal shoulder having complementary torsion key slots. The shear 
ring is locked in combination with the upper end of the cylinder member 
and the trunnion fitting by threaded engagement of the retainer nut with 
the trunnion fitting wherein the retainer nut engages the shear flanges of 
the shear ring, and wherein in the locked combination the cylindrical body 
abuts the cylinder member, the shear flanges engage the complementary 
flange cutouts of the cylinder member, and the torsion keys are disposed 
in complementary torsion key slots. Each shear flange comprises an inner 
restraint segment and an outer shearable segment defined by a shear line 
such that in the locked combination the retainer nut engages the outer 
shearable segments of the opposed shear flanges and the inner restraint 
segments of the opposed shear flanges engage the complementary flange 
cutouts of the trunnion fitting. The opposed shear flanges have a 
predetermined thickness such that, in response to the predetermined axial 
load from the crash landing, the opposed flanges are sheared along the 
shear lines thereof wherein the upward displacement of the cylinder member 
with respect to the trunnion fitting can occur. 
The described embodiment of the energy dissipating subassembly comprises a 
cylindrical cutter member secured in combination with the cylinder member 
and the trunnion fitting, and a frangible tube member mounted in 
concentric combination with the cylinder member. The frangible tube has a 
lower end secured in combination with the lower end of the cylinder member 
and an upper end disposed in abutting engagement with the cylindrical 
cutter member wherein the upward displacement of the cylinder member 
during the crash landing causes the frangible tube member to interact with 
the cylindrical cutter member such that the frangible tube member is 
fractured during the upward displacement of the cylinder member with 
respect to the trunnion member. The cylindrical cutter member includes an 
arcuate fracture surface, and the upper end of said frangible member is 
disposed in abutting engagement with the arcuate fracture surface.

BEST MODE FOR CARRYING OUT THE INVENTION 
Referring now to the drawings wherein like reference characters identify 
corresponding or similar elements throughout the several views, FIGS. 1A, 
1B, 1C depict a shock strut assembly 10 for a crashworthy wheeled landing 
gear system for a helicopter. The shock strut assembly 10 according to the 
present invention includes means for mechanically dissipating energy 
generated during a crash landing and mechanical means for predictably 
controlling the operation of the energy dissipating means during a crash 
landing. The mechanical control means is readily integrated in combination 
with the shock strut assembly 10, is durable, easily replaceable, and 
highly predictable in failing at a predetermined load level during a crash 
landing. 
The described embodiment of the shock strut assembly 10 comprises a 
trunnion fitting 12, a compressible oleo strut subassembly 14, a wheel 
subassembly 16 that includes an axle 18 and wheels 20, an energy 
dissipating subassembly 50, and a mechanical load control subassembly 60. 
The trunnion fitting 12 includes pins 22 for mounting the shock strut 
assembly 10 in rotatable combination with the helicopter fuselage (by 
means of trunnion bearings TB as indicated in FIG. 1B) such that the shock 
strut assembly 10 is alternately retractable for extended flight 
operations and extendible for near ground flight operations, e.g., aerial 
taxing, low level hovering, and/or landing. 
The oleo strut subassembly 14 includes a cylinder member 24 having an upper 
end mounted in combination with the trunnion fitting 12 by means of the 
mechanical load control subassembly 60 as described hereinbelow in further 
detail and a piston member 26 sized for sliding movement within the 
cylinder member 24. The lower end of the piston member 26 is secured in 
combination with the axle 18 (for the described embodiment, the piston 
member 26 is integrally fabricated in combination with the axle 18; 
alternatively, the piston member 26 and the axle 18 can be fabricated as 
separate elements and secured in combination by a conventional technique 
such as welding or bolts) and the upper end of the piston member 26 is 
inserted within the cylinder member 24 as illustrated in FIG. 1B for 
sliding displacement with respect thereto. 
The inserted end of the piston member 26 includes a centering cam 28 having 
an orifice 28O, a split ring "karon-type" bearing 30 (karon-type is used 
herein in the sense of exhibiting a low coefficient of friction similar to 
Teflon and having the additional characteristic of being suitable for 
post-fabrication machining to final form), and a rebound valve 32. A 
bearing--centering cam 34 is provided on the inner surface of the lower 
end of the cylinder member 24. A floating piston 36 is slidably mounted 
within the piston member 26 and functions as a separator between the oil 
and compressible gas volumes of the oleo strut subassembly 14. The volume 
within the piston member 26 between the floating piston 36 and the axle 18 
is filled with a compressible gas. For the described embodiment, the 
compressible gas is nitrogen. A torque subassembly 38 comprising first and 
second torque arms 40, 42 is rotatably coupled between the axle 18 and the 
cylinder member 24 as illustrated in FIG. 1C (see also FIG. 1A). The 
centering cam 28, the split ring bearing 30, the rebound valve 32, the 
bearing-centering cam 34, and the torque subassembly 38 control the 
sliding interaction of the piston member 26 with respect to the cylinder 
member 24 during normal and crash landings. 
A fill port 44 is mounted in combination with the upper end of the cylinder 
member 24 by means of a cylinder cap 46 as illustrated in FIG. 1B (see 
also FIG. 4). The volume within the cylinder member 24 and within the 
piston member 26 between the floating piston 36 and the centering cam 28 
is filled with oil, e.g., an oil as defined by Mil-H-5606, utilizing the 
fill port 44. For the described embodiment of the oleo strut subassembly 
14, approximately 236 in.sup.3 (7.2 lbs) of oil is required to fill such 
volume. 
During normal landings, the landing gear is extended by rotating the 
trunnion fitting 12 about the trunnion bearings TB. With the shock strut 
assembly 10 in the extended position, the floating piston 36 assumes an 
equilibrium position as a result of opposing pressure forces exerted 
thereon by the oil and the compressible gas in the oleo strut subassembly 
14. Landing loads are coupled through the wheels 20 and the axle 18 to 
cause the piston member 26 to be displaced upwardly into the cylinder 
member 24, i.e., the oleo strut subassembly 14 is compressed. Since the 
oil in the oleo strut subassembly 14 is essentially an incompressible 
liquid, the upward displacement of the piston member 26 decreases the 
overall internal volume of the cylinder member 24, forcing oil in the 
cylinder member 24 to be displaced through the orifice 28O into upper 
portion of the piston member 26. The displaced oil exerts a biasing force 
against the floating piston 36 to cause displacement thereof from the 
equilibrium position towards the axle 18. Such displacement of the 
floating piston 36 causes compression of the compressible gas in the lower 
portion of the piston member 26. The compression of the compressible gas 
in the oleo strut assembly 14 attenuates the landing loads induced in the 
landing gear, thereby effectively decoupling the landing loads from the 
helicopter. Concomitantly, the displacement of the piston member 26 causes 
volume changes in the zone between the inner surface of the cylinder 
member 24 and the outer surface of the piston member 26. The rebound valve 
32 controls the transfer of oil into and out of this zone to provide a 
damping action vis-a-vis oil displacement occurring in the oleo strut 
subassembly 14 as a result of the landing loads. The mechanical loss 
control subassembly 60 is operative to maintain the cylinder member 24 in 
static relation to the trunnion fitting 12 during normal landings, i.e., 
no displacement of the cylinder member 24 relative to the trunnion fitting 
12 during normal landings. 
The energy dissipating subassembly 50, which functions as the means for 
mechanically dissipating energy generated during a crash landing, 
comprises a cylindrical cutter member 52 having an arcuate fracture 
surface 54, a restraining shoulder 56, and an interactive surface 57 as 
illustrated in FIG. 2 (see also FIG. 1B) and a frangible tube member 58. 
The cylindrical cutter member 52 is mounted to the bottom of the trunnion 
fitting 12 (see FIG. 1B, 2) with the restraining shoulder 56 in abutting 
engagement therewith such that a clearance gap 59 is defined by the 
cylindrical cutter member 52, i.e., between the surface of the cylinder 
member 24 and the frangible tube member 58. Preferably, the interactive 
surface 57 is coated or lined with a "karon-type" material which allows 
the clearance gap 59 to be defined to a tight tolerance. The interactive 
surface 57 acts as a bearing surface and a centering member during 
operation of the shock strut assembly 10 during a crash landing. 
The frangible tube member 58 is disposed in concentric combination with the 
cylinder member 24 as illustrated in FIG. 1C (see also FIG. 1B). The lower 
end of the frangible tube member 58 is secured in combination with the 
lower end of the cylinder member 24 and the upper lip of the frangible 
tube member 58 is disposed in abutting engagement with the arcuate 
fracture surface 54 of the cylindrical cutter member 52. The upper end of 
the frangible tube member 58 may be machined, e.g., beveled or chamfered, 
to facilitate the initial fracturing thereof due to interaction with the 
cylindrical cutter member 52 during a crash landing as described in 
further detail hereinbelow. The frangible tube member 58 is fabricated 
from a material having moderate ductility and high fracture toughness 
since mechanical fracturing of such a material in a crash landing results 
in the dissipation of a large amount of energy (see example hereinbelow). 
For the described embodiment of the shock strut assembly 10, the frangible 
tube member 58 is fabricated from 2024 aluminum and has a thickness of 
about 0.125 inches. 
The mechanical load control subassembly 60 of the described embodiment is 
exemplarily illustrated in FIGS. 3-4 and includes a shear ring 62, a 
retainer nut 80, complementary flange cut-outs 82 formed in the endface of 
the cylinder member 24, and complementary torsion key slots 84 (only one 
slot is visible in FIG. 3) formed in an internal shoulder 12IS of the 
trunnion fitting 12. The shear ring 62 comprises a cylindrical body 64 
having opposed shear flanges 66 depending outwardly from the upper end 
thereof. Each shear flange 66 includes an inner restraint segment 68 and 
an outer shearable segment 70 as defined by a shear line 72. Each outer 
shearable segment 70 includes a torsion key 74 depending downwardly 
therefrom. 
With the cylinder member 24 of the oleo strut subassembly 14 inserted 
within the trunnion fitting 12, the shear ring 62 is mounted in 
combination with the cylinder member 24 and the trunnion fitting 12 as 
illustrated in FIG. 4 and secured in combination therewith by means of the 
threaded engagement of the retainer nut 80 in combination with the 
trunnion fitting 12. The outer surface of the cylindrical body 64 of the 
mounted shear ring 62 abuttingly engages the inner surface of the upper 
end of the cylindrical member 24, the inner restraint segments 68 of the 
shear flanges 66 abuttingly engage the complementary flange cutouts 82 of 
the cylindrical member 24, and the torsion keys 74 are engaged in the 
complementary torsion key slots 84 of the trunnion fitting 12. The lower 
endface of the engaged retainer nut 80 abuttingly engages the outer 
shearable segments 70 of the shear flanges 66 of the mounted shear ring 
62. A cover 48, for the described embodiment the cover 48 is fabricated 
from 7075-T73 aluminum, is affixed to the upper end of the trunnion 
fitting 12 primarily to provide environmental protection for the trunnion 
fitting 12. 
The inherent simplicity of the mechanical load control subassembly 60 
according to the present invention enhances the maintainability of the 
shock strut assembly 10. The mechanical condition of the shear ring 62 may 
be readily inspected for wear and/or cracking by removing the trunnion 
cover 48, disengaging the retainer nut 80, and removing and visually 
inspecting the shear ring 62. A degraded shear ring 62 is readily 
replaceable utilizing the foregoing procedure. 
As noted hereinabove, the shear ring 62 is operative to prevent upward 
displacement of the cylinder member 24 with respect to the trunnion 
fitting 12 during normal landings. This function of the shear ring 62 is 
achieved by the abutting engagement of the inner restraint segments 68 of 
the shear flanges 66 with the complementary flange cutouts 82 of the 
cylinder member 24. 
The configuration of the mechanical load control subassembly 60, and in 
particular the shear ring 62, provides for the segregation of the landing 
loads induced in the landing gear during normal landings. During a normal 
landing, the landing gear is subjected to axial, torsional, drag, and side 
loads (collectively the landing loads). These landing loads are 
transmitted from the wheel subassembly 16 through the oleo strut 
subassembly 14 to the trunnion fitting 12 by means of the mechanical load 
control subassembly 60. More specifically, the axial landing loads are 
transmitted as a shear load acting on the shear flanges 66 and the 
retainer nut 80, the drag and side landing loads are transmitted as 
bearing loads acting on the abutting surfaces of the cylindrical body 64, 
the cylinder member 24, and the trunnion fitting 12, and the torsion 
landing loads are transmitted as torsion loads acting on the torsion keys 
74 and the complementary torsion key slots 84. 
The segregation of the landing loads facilitates the sizing of the shear 
ring 62 to react the drag, torsion, and side landing loads with a margin 
of safety that precludes premature failure of the shear ring 62 due to 
these landing loads. While some torsion stress is transmitted through the 
shear flanges 66, such torsion stress levels are significantly below the 
endurance limit of the shear flanges 66. Pragmatically, therefore, only 
axial landing loads are reacted by the shear flanges 66 (as shear stress). 
These characteristics of the mechanical load control subassembly 60, and 
in particular, the shear ring 62, means that local yielding and 
degradation thereof due to normal landing loads are minimized, thereby 
providing enhanced durability. 
Since the shear ring 62 is subjected primarily to axial landing loads, and 
in light of the mounted configuration of the shear ring 62 with respect to 
the cylindrical member 24 and the retainer nut 80, the shear ring 62 fails 
in shear along the shear lines 72 of the shear flanges 66 when subjected 
to crash landing loading. These characteristics of the mechanical load 
control subassembly 60 makes design of the shear flanges 66 to fail at a 
predetermined axial load limit that is representative of a crash landing 
condition extremely practical. Calculating the ultimate shear load 
capability for a fiat plate configuration fabricated from a specific 
material can be accomplished by one skilled in the art using known 
techniques. For the shear ring 62 described herein, the shear ring 62 is 
fabricated from 6061-T6 aluminum using conventional techniques, and the 
shear flanges 66 have a thickness of about 0.19 inches based upon the 
predetermined axial load failure limit described hereinbelow. The relative 
simplicity of the configuration of the shear flanges 66 allows the 
thickness dimension thereof to be held within a tolerance of .+-.0.001 
inches (about 1.3%). 
In a crash landing, the piston member 26 is displaced upwardly into the 
cylinder member 24. The axial crash landing loads acting on the oleo strut 
subassembly 14 are transmitted through the oil therein to the cylinder cap 
46, from the cylinder cap 46 to the cylinder member 24, and from the 
cylinder member 24 to the inner restraint segments 68 (as shear stress) of 
the shear flanges 66 of the shear ring 62. When the axial crash landing 
loads reach the predetermined axial load failure limit of the shear 
flanges 66, the shear flanges 66 fail in shear along the shear line 72 
such that the cylinder member 24 is no longer restrained in static 
combination with the trunnion fitting 12. Continued upward displacement of 
the piston member 26 forces the cylinder member 24 to be displaced 
upwardly with respect to the trunnion fitting 12. The upward displacement 
of the cylinder member 24, which is controlled in part and facilitated by 
the interactive surface 57 of the cylindrical cutter member 52, causes the 
frangible tube member 58 to interact with the cylindrical cutter member 
52, resulting in fracturing of the frangible tube member 58. The 
fracturing of the frangible tube member 58 that occurs due to the 
continued upward displacement of the cylinder member 24 results in a 
significant dissipation of the impact energy generated as a result of the 
crash landing (see example hereinbelow). As the cylinder member 24 strokes 
during the crash landing sequence, the energy dissipated by the 
progressive fracturing of the frangible tube member 58 is such that the 
axial load acting on the landing gear remains relatively constant at the 
predetermined axial load failure limit. Thus, the energy dissipating 
efficiency of the shock strut assembly 10 according to the present 
invention is relatively high. The configuration of the arcuate fracture 
surface 54 of the cylindrical cutter member 52 facilitates removal of the 
fractured pieces of the frangible tube member 58 to preclude interference 
with the continuation of the fracturing mechanism. 
The embodiment of the shock strut assembly 10 described hereinabove is 
designed for use in the S-92.TM. HELIBUS.TM. helicopter being developed by 
Sikorsky Aircraft Corporation, a subsidiary of United Technologies 
Corporation (S-92 and HELIBUS are trademarks of Sikorsky Aircraft 
Corporation). The sizing and material composition of the shear ring 62 as 
described hereinabove are based upon the design criteria that: (1) the 
mechanical load control subassembly 60 have the capability to withstand 
approximately 120,000 landings (approximately 30,000 flight hours) without 
failure; and (2) the shear flanges 66 repeatably fail in shear at a 
predetermined axial load failure limit of about 32,000 lbs. This 
predetermined axial load failure limit allows the landing gear system, and 
in particular, the shock strut subassembly 10 described herein, of the 
S-92.TM. HELIBUS.TM. helicopter to accommodate a significant percentage of 
the crash landing energy that results from a 26 ft/sec sink rate (which is 
equivalent to a free fall crash landing from a height of about 10.5 feet). 
The ability to accommodate a 26 ft/sec sink rate is one of the safety 
design criteria of the S-92.TM. HELIBUS.TM. helicopter. 
A 26 ft/sec sink rate is equivalent to dropping the S-92.TM. HELIBUS.TM. 
helicopter from a height of approximately 10.5 feet. Based upon a gross 
weight of approximately 24,000 lbs for the S-92.TM. HELIBUS.TM. 
helicopter, this results in the generation of about 3,023,106 in-lbs of 
energy. The S-92.TM. HELIBUS.TM. helicopter has a tricycle-type landing 
gear system consisting of two main landing gears and a nose landing gear. 
Full fracturing of the frangible tube member 58 of the shock strut 
assembly 10 of each of the main landing gears and the nose landing gear 
dissipates about 1,511,553 in-lbs of the energy generated in a crash 
landing under these circumstances (about 503,851 in-lbs of energy in the 
nose landing gear and about 503,851 in-lbs of energy in each of the main 
landing gears). Combining the energy dissipated by the frangible tube 
members 58 with the energy dissipated by the compression of the 
corresponding oleo strut subassemblies 14 and the energy dissipated by the 
destruction of the wheels 20 results in a total energy dissipation of 
about 2,267,330 in-lbs by the tricycle-type landing gear system. The total 
energy dissipated by the tricycle-type landing gear system represents 
approximately 75% of the energy generated during a crash landing of an 
S-92.TM. HELIBUS.TM. helicopter at a sink rate of 26 ft/sec (the energy 
dissipated by fracturing of the frangible tube members 58 accounts for 
about 50% of the energy dissipated). 
A variety of modifications and variations of the present invention are 
possible in light of the above teachings. It is therefore to be understood 
that, within the scope of the appended claims, the present invention may 
be practiced otherwise than as specifically described hereinabove.