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Патент USA US3078977

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Feb. 26, 1963
Filevd Aug. 15, 1960
"6 )6 4:!” 4
JA/Vt‘j 14/. 590W”
Patented Feb. 2%, 1%53
tubular member 2.8 to provide sliding bearing support
with the inner surface of casing 12. Ports 2% are pro
vided in the side wall of tubular member 28 near bushing
3% to provide intercommunication between compartments
24 and 26. The lower end of casing 12 includes a pair
of annular bushings 32 inserted in a stepped diameter por
Filed Aug. 15, 1966, Ser. No. 4%,566
tion and held in position by threaded cap 34-. Bushings
4 Claims. (Cl. 183-96)
32 include annular slots for receiving fluid seals 36. The
The present invention relates to a shock stru-t particu~
nut 43
is tubular
onto the16threaded
carries a end
portion of
larly adapted for use in a helicopter landing gear.
One of the problems facing the helicopter landing gear
tubular member 16 to clamp the diaphragm 38 into place.
designer is that of ground resonance. Ground resonance
A metering ori?ce 42 is formed in the diaphragm 33 and
is a phenomenon which can occur during either landing
a metering pin 44, which is operatively carried by mov
or take-off operation. It occurs when a slightly out-of
able tubular member 23 through the removable end plate
balance condition of the spinning rotor blade induces a 15 46, is movable through the metering ori?ce 42; to control
pitching and rolling motion to the airframe on its wheels.
the size of the ori?ce which is available for metering fluid
Rotor unbalance can be aggravated by the blade drag
therethrough. Nut as contains a fluid seal 48 mounted
hinges used on most rotor assemblies. Drag hinges are
thereon which provides a {?uid tight compartment 5%‘
the couplings between the blades and the rotor shaft. At
formed between diaphragm 38 and end plate 45. All
low speeds, the drag hinges can permit unequal spacing
?uid entering or leaving compartment 56 must pass
of the blades thus producing the out-of-balance condi—
through ori?ce 42. Fluid forced from chamber 5t} passes
through ori?ce 42 and through the additional openings 5'2
The kinetic energy of the pitching or rolling mass of
provided in the sidewall of tubular member 16 into cham
the airframe is transferred to the landing gear. When
ber 22.
this energy is transferred at a rate greater than the energy 25
When the strut is fully compressed such as illustrated
dissipating rate of the landing gear, a self-excited reso
in PEGURE 1, the liquid level of the hydraulic fluid rises
nance can occur. There have been instances of catas
to the level indicated by the legend, approximately up
trophic failure of major components of helicopters re
to annular bushing 3d. The remaining volume contains a
sulting from loadings imposed under these ground reso
compressible ?uid medium such as air or the like.
nance conditions. Accordingly, it is an object of the 30
The parts and structure described up to this point com
present invention to provide a landing ‘gear shock strut
prise a conventional aircraft shock strut of the variable
James W. Brown and Raymond 3. Black, South Bend,
ind, assignors to The Bendix Corporation, South
Bend, Ind, a corporation of Delaware
for helicopters having linear damper means for reducing
ground resonance oscillation.
metering ori?ce type primarily adapted for absorbing
impact energy during aircraft landings. As the aircraft
Another object of the present invention is to provide
containing such a strut approaches a landing, the movable
in combination with a shocl: strut of the variable ori?ce 35 tubular member 28 is fully extended providing a maxi~
type and a metering pin for controlling the ori?ce area,
mum volume in compartment 53%}. As the aircraft wheels,
a damping piston secured to the metering pin for damp
which are secured in any well-known manner to the lower
ing low amplitude vibrations.
end of movable member 28, touch down and the aircraft
Other objects and features of the invention will become
load is transferred from the wings, rotor or other load
apparent from a consideration of the following descrip
supporting airfoil to the landing gear, tubular member 23
tion which proceeds with reference to‘ the accompanying
drawings, wherein:
elescopes within members 12 and id. The telescoping
action forces ?uid from compartment St} to chamber 22
FlGURE 1 is a section view of a shock strut in accord
through ori?ce 42 at a rate permitted by metering pin
ance with the present invention, and;
44 which may have a variable area along the portion of
FIGURE 2 is an enlarged section view of a portion
its length remaining below Wall member 74% to achieve the
of PEGURE 1 providing a clearer illustration of the linear
most c?icient load absorbing characteristic. As ?uid is
damper portion of our shock strut.
forced into and occupies the volume in chamber 22 the
Referring to FIGURE 1, a shock strut, incorporating
compressible fluid or air contained in said chamber is
the novel principles of the present invention, is generally
compressed. As the aircraft comes to rest and all its
indicated by reference numeral ill, and includes an outer 50 weight is supported by the landing gear, the full aircraft
tubular casing 12 which normally is secured to an air
wcicht is in effect riding on a compressed column of air.
frame by means of a bracket partially shown at 14. A
Such action provides a highly e?icient aircraft landing
second tubular member 16 contained concentrically within
energy absorbing means, however, small cyclic vibrations
casing 12 is threadedly secured to a retainer 18 at its upper
or variations are not effectively countered since such a
end which in turn is secured to the upper end of casing
strut is primarily designed to meet large stroke high en
12, by means of nut
such that the tubular members 12
ergy absorption conditions. The previously described
and 15 are relatively ?xed. The outer surface of tubular
phenomenon of ground resonance may induce cyclic vi
member 16 and the inner surface of tubular casing 12
brations which build up to a high magnitude in a shock
de?ne a hydraulic fluid containing annular cham er 22 60 strut designed to meet the landing conditions pointed out
which is further separated into two compartments 2% and
above. Accordingly, our invention comprises the utili
26 by a movable tubular member 2:} which is telescoped
zation of linear damper means in combination with the
between said ?xed tubular members. An annular bush
above described variable metering ori?ce shock absorber
ing as is threadedly secured to the upper end of movable
to provide a shock strut that is not only capable of ef?
ciently absorbing landing energy, but in addition effective
ly and e?iciently dampens out small amplitude vibrations
induced by conditions such as ground resonance. The
structure comprising the resonance damper will now be
A hollow cylinder 53 comprised of two threadedly
joined tubular sections 64} and 62 is positioned within the
78 becomes operative as a damping agent, as only the
relatively small restrictive passage 84} is operative to pass
?uid to opposite sides of piston 78. During extension
of the shock strut, for example after take off, ?uid enters
the chamber between piston '1" 3 and wall 66 through pas
sages 68 and 80. It has been found unnecessary for air~
craft installations to provide means for permitting rapid
shock absorber movement in the extending direction, al
though quite clearly the provision of such means is mere
second ?xed tubular member 16 at a position determined
by spacing sleeve 64; inserted between the lower end of
cylinder 58 and diaphragm 38. Cylinder 5t} abuts at its 10 ly carrying forward the ideas disclosed herein and would
be within the ambit of the ordinary mechanic.
upper end against retainer 18. At the threaded juncture
Although only one embodiment of our invention has
of the tubular sections 69 and 6272. Wall member 65 is
been disclosed, it should be understood that this is merely
retained and includes a small calibrated ori?ce 6S and
the preferred embodiment, and that’ other arrangements
two spring loaded check valves 75) and 72 which permit
flow in opposite directions when the check valve spring 15 employing the novel aspects de?ned in the claims are
within the scope of this invention.
loading force is overcome. A second wall member ‘74
We claim:
is secured to the lower end of cylinder 62 and with the
1. A shock strut comprising first and second relatively
side Walls of said cylinder and wall member 66 forms a
movable telescoping elements de?ning a variable volume
chamber 76 which contains a column of hydraulic ?uid.
Wall member '74 has an opening 75 therein for receiving 20 ?uid containing chamber, an ori?ce plate having a ?uid
control ori?ce ‘formed therein movable with said ?rst
metering pin 44in a fluid tight manner. A piston 73 is
telescoping element and forming one side wall of said
attached to the upper end of metering pin 44 and includes
‘variable volume chamber, a metering pin contoured over
a ?'cd small bore calibrated passage 8i} and a second
a portion of its length secured to said second telescoping
'ar_,e bore relief passage 32. The piston 78 operates in
the column of ?uid in chamber 76 and the damping force, 25 element and movable therewith, said metering pin ar
ranged to extend through said ‘fluid control ori?ce of said
required under short stroke high frequency cycling of
ori?ce plate to de?ne a variable effective area control
the shock strut is provided by passing ?uid from one
ori?ce, a cylinder member movable with said ?rst tele
side of piston 72'; to the other through restrictive passage
scoping element containing a con?ned column of hydrau
The di?ference in fluid volume in chamber 76 due
to the varying displacement of metering pin 44 is com 30 lic ?uid 'coaxially arranged with said metering pin, said
cylinder member having a ?uid tight opening at one end
pensated for by allowing excess fluid from chamber 76,
for receiving said meteringpin while preventing liuid
above piston 8%, to pass through ori?ce 68 in wall as to
transfer during the full operative range of shock strut
a compensating chamber 84 formed in the upper tubular
movement, and a piston member secured to said metering
section it}. A movable piston or wall member 36 is pro
pin within said cylinder member for movement there
vided in tubular section st} to provide a separation barrier
with, said piston member having a relatively small diam
between the hydraulic .?uid in chambers 34 and '76 and
eter restrictive passage formed therethrough to permit
the compressible lluid or air in the uppermost section of
?uid transfer from opposite sides of said piston with said
the shock absorber. Hydraulic ?uid entering chamber
cylinder member at a controlled rate.
54 thus force-s piston 36 upwardly to compensate for the
2. A shock strut as claimed in claim 1 wherein said
variation in fluid displacement in chamber 76 due to the 40
piston member includes a second relatively large diameter
entrance therein oi metering pin 44. During the exten
passage formed there-through in parallel ?ow relationship
sion portion of the shock absorber cycle, the reverse ac
with said relatively small diameter restricted passage to
tion takes place and the excess fluid required is returned
permit relatively unrestricted ?uid transfer from opposite
from chamber 3d to chamber 76 through ori?ce 63 by
sides of said piston, pressure responsive spring loaded
action of the air pressure on the top of ?oating piston 35.
valve means for normally closing said large diameter pas
if desired, a spring member may also be employed to urge
sage, said valve means further operative to open said rela
piston as downwardly to insure more positive positioning
large diameter passage under the influence of a
and reduce the probability of cavitation in chambers 76
Check valve ‘it! is provided in Wall member 66 50 hydraulic fluid pressure dilierential of a predetermined
minimum value during the compression stroke of said
to further reduce the probability of cavitation and to per
mit rapid transfer of hydraulic ?uid during long strolte
rapid movements of metering pin 44.
telescoping elements.
3. A shock strut of the hydraulic-pneumatic type com
A movable disc member dd is slidably mounted on the
prising ?rst and second relatively movable telescoping
upper end of metering pin 44 immediately below piston
elements de?ning a variable volume iluid containing
66 and includes a valve member 92 secured thereto ar
chamber, ?uid control means de?ning an ori?ce commu
ranged to close large diameter passage
Disc 3%} and
valve 92 are urged upwardly in a direction to close pas
nicating with said variable volume chamber movable with
said ?rst telescoping element, an elongated metering pin
sage 82 by force producing spring 94 concentrically
extending through said ori?ce for controlling the slice
mounted around metering pin 4d held in position by a 60 tive ?ow controlling area thereof movable with said sec
tired disc member 95 also secured to metering pin 44.
ond telescoping element, a hollow cylindrical member
During normal long stroking of the shock absorber such
containing a column of hydraulic fluid arranged within
as occurred in the previously described aircraft ‘landing,
movable with said ?rst telescoping element in co
sufficient pressure is built up in chamber 76, between pis
alignment with said metering pin, said cylindrical
ton 78 and Wal 6:3, to unseat valve 92 against the action 65 member including a ?rst ?xed wall member and a second
or" spring 94, thus permitting rapid transfer of fluid in
hamber 76 from above to below piston 73. The pres
sure built up between piston '78 and wall 66 is also high
enough to unseat check valve 72 to allow for rapid com
?xed wall member disposed in said cylinder to de?ne
a ?red volume ?uid containing compartment in coopera
tion therebctween said ?rst ?xed Wall member having an
pensation for displacement of metering pin 44. Shock 70 opening formed therein for receiving said metering pin
therethrough so that one end of said metering pin is re—
absorbing characteristics under these conditions are con
ciprocable in said ?xed volume compartment, and a piston
trolled by metering pin 44 and ori?ce 42 con?gurations
member secured to said metering pin within said ?xed
and are not materially attested by the resonance damper.
volume compartment, said piston member including a ?rst
After the aircraft has come to rest and piston 78 is not
restrictive ?uid permitting passage formed therethrough
traversing chamber ‘76 rapidly, valve 532 closes, and piston
for damping the movement of said piston and metering
References Cited in the ?le of: this patent
pin members.
4. A shock strut as claimed in claim 3 wherein said
piston member includes a relatively large relief passage
formed therein, valve means operative with said relief 5
spring means
to control
the hydraulic
to bias said
in a pas-
sage closing position, said valve means being operative
to open against the action of said spring means when the
pressure di?erential across said piston exceeds a prede- 10
termined value.
Elliott _______________ __ Sept 5, 1939
’ A
run 16 """"""""
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Carbon —————————————— —- D?c- 18: 1956
Muller et a1 ------------ -- Dec- 3, 1957
OrShansky ___________ __ May 12, 1959
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