Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND ART
1. Field of the Invention
The present invention relates generally to conversion assemblies for use on
tilt rotor
aircraft for converting from a helicopter mode to an airplane mode, and vice
versa. In particular,
the present invention relates to a method and apparatus for stabilizing the
articulating rotor
portion relative to the stationary structure of the aircraft while in the
airplane mode.
2. Description of Related Art
Tilt rotor aircraft are hybrids between traditional helicopters and
traditional propeller
driven aircraft. Typical tilt rotor aircraft have rotor systems that are
capable of articulating
relative to the aircraft fuselage. This articulating portion is referred to as
a nacelle. Tilt rotor
aircraft are capable of converting from a helicopter mode, in which the
aircraft can take-off,
hover, and land like a helicopter; to an airplane mode, in which the aircraft
can fly forward like
a fixed-wing airplane.
The design of tilt rotor aircraft poses unique problems not associated with
either
helicopters or propeller driven aircraft. In particular, certain static and
dynamic loads are
generated by the tilt rotor assemblies that are not present in either
conventional helicopters or
fixed wing aircraft. While in the aircraft mode, aircraft stability is
maintained by a support
assembly referred to as a "downstop" assembly. The downstop assembly has two
main purposes.
First, the downstap assembly must provide vertical stiffness in order to react
against the
downward forces required to keep the nacelle from rising throughout the flight
envelope. Second,
the downstop assembly must provide enough lateral stiffness to ensure flight
stability. The exact
amount of lateral stiffness is based upon aircraft geometry, flight envelope
requirements, adjacent
part stiffness, and several other factors that are unknown until flight
testing is underway.
Therefore, it is desirable that the downstop assembly be tunable in such a way
that redesign of
adjacent parts is not required as a result of the need to increase or decrease
the lateral stiffness.
If the lateral stiffness is matched or tuned to a particular aircraft's
minimum lateral stiffness
requirement, then the aircraft's wing structure can be isolated from damaging
lateral static and
oscillatory loads.
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Certain attempts have been made to isolate the static and dynamic loads
created between
the wing structure and the nacelle while the tilt rotor aircraft is in the
airplane mode. In some
tilt rotor aircraft, the lateral loads have been isolated by a downstop
assembly having long vertical
blade. In this application, the height of the vertical blade requires a large
fairing to be used, thus
increasing the frontal drag of the aircraft. Other tilt rotor aircraft have
minimized the height of
the downstop assembly, but at the cost of introducing lateral loads into the
wing structure. Thus,
although great strides have been made in the design of tilt rotor aircraft,
the problem of isolating
lateral nacelle loads from the wing structure by using a package that is
small, adjustable, and
vertically stiff has not been adequately resolved.
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BRIEF SUMMARY OF THE INVENTION
There is a need for a tilt rotor aircraft having a low-height tunable tilt
rotor downstop
assembly.
It is an object of the present invention to provide a low-height tilt rotor
downstop
assembly for isolating lateral loads while providing high vertical stiffness.
It is another object of the present invention to provide a tunable tilt rotor
downstop
assembly for isolating lateral loads.
It is yet another object of the invention to provide a tilt rotor downstop
assembly for
isolating lateral loads that does not intrude into the wing structure.
It is yet another object of the present invention to provide a low-height
tunable tilt rotor
IO downstop assembly for isolating both static and dynamic lateral loads.
It is yet another object of the present invention to provide a low-height
tunable tilt rotor
assembly downstop having an L-shaped striker arm, the downstop being tunable
by adjusting the
physical dimensions of the longer leg of the L-shaped striker arm.
It is yet another object of the present invention to provide a tilt rotor
aircraft in which
lateral nacelle loads are isolated by a low-height tunable tilt rotor downstop
assembly.
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BRIEF DESCRIPTION OF THE DRAWINGS
Figure lA is a perspective view of a tilt rotor aircraft in an airplane mode.
Figure 1B is a perspective view of a tilt rotor aircraft in a helicopter mode.
Figure 2A is an exploded perspective view of a striker assembly of a low-
height tunable
tilt rotor downstop according to the present invention.
Figure 2B is an assembled perspective view of the striker assembly of Figure
2A.
Figure 2C is a cut-away view of Joint A of the striker assembly of Figure 2A.
Figure 3 is a front view of the striker arm of the striker assembly of Figures
2A and 2B.
Figure 4 is an exploded perspective view illustrating the attachment of the
striker assembly
of Figures ZA and 2B to a prop-rotor gear box assembly.
Figure 5 is an exploded perspective view of a cradle assembly of the low-
height tunable
tilt rotor downstop according to the present invention.
Figure 6 is a perspective view illustrating the attachment of the cradle
assembly of Figure
5 to an outboard wing rib and a forward wing spar.
Figure 7 is a perspective view of the assembled low-height tunable tilt rotor
downstop
according to the present invention, including the striker assembly of Figures
2A and 2B and the
cradle assembly of Figures 5 and 6.
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DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to Figures 1 A and 1 B in the drawings, a typical tilt rotor
aircraft 11 is
illustrated. Tilt rotor aircraft 11 has an airframe 13 and wings 15a and 15b
coupled to airframe
13. As is conventional, wings 15a and 15b terminate with tilt rotor assemblies
17a and 17b,
respectively. Fairings 18a and 18b for reducing drag are disposed between tilt
rotor assemblies
17a and 17b and wings 1 Sa and 1 Sb. Tilt rotor assemblies 17a and 17b each
may include an
engine, a transmission, and a gear box (see Figure 5) for driving prop-rotors
19a and 19b.
Conversion actuators (see Figure 8) control the position of tilt rotor
assemblies 17a and 17b
between an airplane mode, as illustrated in Figure lA, and a helicopter mode,
as illustrated in
Figure 1B. In the airplane mode, tilt rotor aircraft 11 can be flown and
operated like a
conventional fixed-wing propeller driven aircraft. In the helicopter mode,
tilt rotor aircraft 11
can take-off, hover, land, and be operated like a conventional rotary wing
aircraft or helicopter.
Refernng now to Figures 2A-2C in the drawings, the preferred embodiment of a
low-
height tunable tilt rotor downstop according to the present invention is
illustrated. A striker
assembly 31 includes a base member 33 configured to receive an angled, tunable
striker arm 35.
Base member 33 is preferably made of aluminum, but may be made of any other
sufficiently
rigid material. Base member 33 includes a plurality of mounting apertures 36.
Striker arm 35
is generally L-shaped having a post portion 37 and an leg portion 39. Striker
arm 35 is
preferably made of titanium, but may be made of other materials for which the
mechanical
properties, in particular bending stiffness, may be adjusted, or "tuned," by
altering the geometrical
dimensions of striker arm 35. This tuning feature of striker arm 35 plays a
central role in the
present invention, and will be discussed in more detail below.
Post portion 37 and leg portion 39 of striker arm 35 intersect at a generally
cylindrical
corner portion 41. Corner portion 41 includes a cylindrical bore 43 that
passes transversely
through corner portion 41 along an axis 45. Bushings 47 are coupled to the
interior of bore 43
on each end of bore 43. Bushings 47 are preferably anti-friction bushings,
such as bushings
having a teflon lining. Bushings 47 have an interference fit with bore 43, but
may be coupled
to bore 43 by other well known means. Leg portion 39 has a transverse width w
that is generally
constant over the length of leg portion 39. Post portion 37 preferably tapers
inwardly from
comer portion 41 to a tip portion 49. Tip portion 49 is generally cylindrical
along an axis 51.
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Leg portion 39 extends away from corner portion 41 and terminates at a forked
end 53 having
an upper flat 53a and a generally parallel lower flat 53b. Striker arm 35 will
be explained in
more detail below with respect to Figure 3.
Base member 33 includes a plurality of lugs 55a and 55b. Lugs 55a and 55b are
generally
parallel. Lugs 55a and 55b include bores 57a and 57b, respectively, passing
therethrough.
Bushings 61a and 61b are coupled to the interior of bores 57a and 57b,
respectively, along an
axis 59. Bushings 61a and 61b are similar in construction to bushings 47.
Bushings 61a and 61b
are preferably anti-friction bushings, such as bushings having a teflon
lining. Bushings 61 a and
61b are preferably interference fit into bores 57a and 57b, but may be coupled
to lugs 55a and
55b by other well known means.
A slip bushing 63 is received by bushing 61 a. Slip bushing 63 is held in
place between
a bushing flange 61c of bushing 61b and a washer 65a. A bolt 67 passes along
axis 59 through
washer 65b, bushing 61b, bushing 63, and washer 65a; and is releasably
received by a nut 69
having a pin 71. In this manner, an anti-friction pivot joint A (see Figure
2C) is created, about
which post portion 37 and leg portion 39 pivot.
Continuing with reference to Figures 2A-2C in the drawings, base member 33
includes
a second plurality of lugs 73a and 73b. Lugs 73a and 73b are generally
parallel to each other
and parallel to axis 59. Lugs 73a and 73b include bores 75a and 75b,
respectively, passing
therethrough. Bushings 77a and 77b are coupled to the interior of bores 75a
and 75b,
respectively, along an axis 77. Bushings 79a and 79b are similar in
construction to bushings 47.
Bushings 79a and 79b are preferably anti-friction bushings, such as bushings
having a teflon
lining. Bushings 79a and 79b are preferably interference fit into bores 75a
and 75b, but may be
coupled to lugs 73a and 73b by other well known means.
A retainer pin 81 is received through bushings 79a and 79b. Retainer pin 81
has a pair
of recessed flats 83a and 83b. Flats 83a and 83b are generally parallel to
each other and parallel
to axis 59. It is preferred that at least recessed portions 83a and 83b of
retainer pin 81 are coated
with an anti-friction material, such as KARON, which is commercially available
from the
Kamatics Corporation of Bloomfield, Connecticut. Retainer pin 81 is free to
rotate within tabs
73a and 73b about axis 77. Flat recessed portions 83a and 83b are configured
to slidingly receive
fork 53, thereby forming a sliding and pivoting joint B (see Figure 2B).
Because fork 53 is
CA 02314835 2000-08-02
allowed to slide relative to retainer pin 81, and rotate relative to axis 77,
leg portion 39 will flex
by bending as a lateral load is applied to post portion 37. However, leg
portion 39 has sufficient
stiffness to prevent flats 53a and 53b from translating enough relative to
tabs 73a and 73b such
that fork 53 releases from retainer pin 81. In other words, the sliding
connection of fork 53 with
retainer pin 81 allows post portion 37 to pivot about axis 59, i.e., joint A.
As shown in Figure 2B, striker arm 35 passes from joint A to joint B along a
slot 90 in
base member 33. Slot 90 allows leg portion 39 of striker arm 35 to remain in a
generally
horizontal position and flex or bend in a vertical plane without restriction.
Slot 90 is configured
to accommodate variations in the vertical thickness of leg portion 39, as will
be explained in
more detail below. In addition, slot 90 allows striker assembly 31 to maintain
an overall low
vertical height or profile. Although the terms "vertical" and "horizontal" are
used herein, it
should be understood that these terms are used only for ease of explanation
and are not intended
to be limiting as to the directions in which the present invention functions.
With striker assembly 31 configured and assembled in this manner, the lateral
loads
1 S indicated by the arrows in Figure 2B generated by tilt rotor assemblies
17a and 17b while in the
airplane mode, are transferred from tip portion 49 of post portion 37 to leg
portion 39 and fork
53. Because post portion 37 is short, providing the low-height feature of the
present invention,
post portion 37 does not bend significantly. The lateral loads are transferred
to leg portion 39
by post portion 37 rotating about axis 59. As leg portion 39 bends, the
lateral loads generated
by tilt rotor assemblies 17a and 17b are isolated and absorbed, thereby
preventing the lateral loads
from being transferred to wings 15a and 15b. Therefore, wings 15a and 15b do
not require
additional structural support to react against the oscillatory vibration
loads. This provides
tremendous savings in terms of weight and cost.
Referring now to Figure 3 in the drawings, striker arm 35 is illustrated in a
front view.
As is shown, post member 37 and leg member 39 form an angle a about axis 45.
Angle a is
not restricted; however, angles greater than 115° may adversely effect
the low-height feature of
the present invention. Post portion 37 has a vertical height h, as measured
from the lowest point
of tip portion 49 to axis 45; and leg portion 39 has a length 1, as measured
from the end of fork
53 to axis 45. Due to the low-height feature of the present invention, height
h is smaller than
length I. It should be noted that axis 45, about which corner portion 41 is
concentric, and axis
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51, about which tip portion 49 is concentric, do not have to be parallel. In
general it is preferred
that axis 51 be made parallel to the mast centerline of tilt rotor assemblies
17a and 17b. It
should be understood that for certain tilt rotor aircraft, axis 45 and axis 51
may be parallel
without significantly affecting the functionality of striker arm 35.
Leg portion 39 has a selected vertical height, or thickness t, as measured
from a lower
surface 91 to an upper surface 93. Based upon thickness t, the leg portion 39
has a selected
vertical cross-section, or thickness profile. It is preferred that striker arm
35 be made of a rigid
material, for which the bending stiffness of leg portion 39 may be selectively
varied according
to thickness t, and the corresponding thickness profile. It is preferred that
width w and length
1 of leg portion 39 remain constant so as not to require changes to retainer
pin 81 or slot 90 (see
Figure 2B). For example, if striker arm 35 is made of titanium, has length 1
of about 7.0 inches,
height h of about 2.5 inches, and thickness t varying from about 0.66 inches
near corner portion
41 to about 0.38 inches near fork 53, then leg portion 39 of has a bending
stiffness range of
about 50,000 pounds per inch to about 150,000 pounds per inch.
Because it is preferred that width w and length 1 of leg portion 39 be
constant, the
bending stiffness of leg portion 39 may be selectively determined by altering
thickness t of leg
portion 39. In other words, striker arm 35 may be tuned to a selective bending
stiffness by
altering the thickness profile of leg portion 39. It will be apparent that the
bending stiffness of
leg portion 39 will increase as thickness t increases. Thus, for similar
materials, the bending
stiffness of leg portion 39 is greater for a thickness profile having a
variable thickness t,, than
for a thickness profile having a variable thickness t; and the bending
stiffness of leg portion 39
is less for a thickness pmfile having a variable thickness tz, than for a
thickness profile having
a variable thickness t. It is preferred that tip portion 49 of post portion 37
be coated with a very
hard material, such as tungsten carbide, to resist fretting against the
surface of a V-block 115 (see
Figure 5). The interface between tip portion 49 and V-block 115 will be
explained in more detail
below.
Referring now to Figure 4 in the drawings, assembled striker assembly 31 of
Figure 2B
is shown being coupled to a prop-rotor gear box assembly 101. A prop-rotor
gear box assembly
101 is disposed within each tilt rotor assembly 17a and 17b (see Figures lA
and 1B). Prop-rotor
gear box assemblies 101 drive rotor hubs 19a and 19b. Each prop-rotor gear box
assembly 101
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is adapted to be coupled to striker assembly 31, preferably by the inclusion
of studs 103 disposed
on a coupling portion 104. Studs 103 are aligned with and releasably received
by mounting
means 36 of base member 33. A shear boss 105 is coupled to base member 33 to
provide
additional support against shear forces acting between striker assembly 31 and
prop-rotor gear
assembly 101. A scrim 107, preferably an epoxy scrim, is bonded to base member
33 to provide
fretting protection. A solid shim 109, preferably made of a metallic material,
is disposed between
scrim 107 of base member 33 and coupling portion 104 of prop-rotor gear
assembly 101 to
provide adjustment capability. Although striker assembly 31 has been shown and
described as
being coupled to prop-rotor gear box assembly 101, it should be understood
that striker assembly
31 may be coupled to other components of tilt rotor assembly 17a or 17b.
Referring now to Figure 5 in the drawings, an exploded perspective view of a
cradle
assembly 111 of the low-height tunable downstop according to the present
invention is illustrated.
Cradle assembly 111 includes an attachment portion 113 and yaw restraint
portion, or V-block
115. Attachment portion 113 is preferably made of a rigid metallic material,
such as aluminum.
V-block 115 is carried in a trough portion 117 of attachment portion 113. V-
block 11 S is
adjustably coupled to attachment portion 113 by fasteners, preferably bolts
119. Trough portion
117 is preferably lined with shims 121a and 121b. Shims 121a and 121b are
preferably
aluminum peel shims which allow vertical and lateral adjustment, respectively,
of V-block 115.
A spacer plate 123 is disposed on a forward internal face 125 of trough
portion 117. Spacer plate
123 is necessary on forward internal face 125 because tilt rotor assemblies
17a and 17b exert
rotor thrust forces upon V-block 115 in the forward direction. Spacer plate
123 preferably
includes an epoxy coating to prevent fretting. Spacer plate 123 is coupled to
trough portion 117
by conventional fastening means 127, such as bolts or rivets.
V-block 115 is made of a rigid metallic material, such as titanium. V-block
115 has a
rounded V-shaped groove interface portion 129 configured to releasably receive
tip portion 49
of post portion 37 as tip portion 49 rotates downward with each tilt rotor
assembly 17a and 17b
during conversion into airplane mode. Striker interface portion 129 includes
inclined surfaces
130a and 130b that converge to form a generally longitudinal trough 130c.
Trough 130c is
generally transverse to the lateral loads, or yaw loads, shown in Figure 2B.
Because striker
interface portion 129 is subjected to oscillatory loads from tip portion 49,
it is desirable that
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striker interface portion 129 have a very hard surface to resist fretting.
Thus, it is preferable that
V-block 115 be made of a hard metallic material, and that at least striker
interface portion 129
be coated with a very hard material, such as tungsten carbide. To ensure that
V-block 115 does
not fret relative to attachment portion, it is preferred that V-block 115 be
coated with an adhesive
material, such as epoxy, on all surfaces that are in contact with shims 121a
and 121b.
Attachment portion 113 includes mounting apertures 131.
Referring now to Figure 6 in the drawings, cradle assembly 111 is illustrated
coupled to
wing 15b. Attachment portion 113 of cradle assembly 111 is adapted to be
coupled to at least
one wing spar and at least one wing rib. Cradle assembly 111 does not intrude
into the interior
of wing 15b. Preferably, attachment portion 113 is coupled to a forward wing
spar 135 and an
outboard wing rib 137 by conventional fastening means 133, such as bolts or
rivets, through
mounting apertures 131. As is shown, trough portion 117 may extend outboard in
a cantilevered
fashion beyond outboard wing rib 137 to ensure that the low-height feature of
the present
invention is maintained. Attachment portion 113 is configured to allow
attachment of cradle
assembly 111 to wing 1 Sb, while not interfering with other components of wing
15b, such as
aperture 139 through which a conversion actuator spindle 143 (see Figure 7)
passes. Although
cradle assembly 111 has been shown and described as being coupled to forward
wing spar 135,
it should be understood that cradle assembly 111 may be coupled to other
components of wing
- 15a or lSb.
Referring now to Figure 7 in the drawings, the components of Figures 2A-6 are
illustrated
in an assembled fashion. Conventional hydraulic conversion actuators 141 are
used to convert
tilt rotor assemblies 17a and 17b between the airplane mode and the helicopter
mode. Conversion
actuators 141 pivot about spindles 143 as conversion actuators 141 actuate
tilt rotor assemblies
17a and 17b by exerting forces on pylons 145. Tilt rotor assemblies 17a and
17b pivot about
spindles 147 that pass through rear portions 149 of wings 15a and 15b. It
should be apparent that
cradle assembly 111 may be coupled to coupling portion 104 of prop-rotor gear
assembly 101,
and striker assembly 31 may be coupled to wings 15a and 15b without affecting
the functionality,
tunability, or low-height feature of the present invention.
In operation, tilt rotor assemblies 17a and 17b are rotated downward from the
helicopter
mode (see Figure 1B) to the airplane mode (see Figure lA). It is preferred
that tip portion 49
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be forced against V-block 115 at a selected preload while tilt rotor aircraft
11 is in the airplane
mode (see Figure lA). Because striker assembly 31 is coupled to prop-rotor
gear assembly 101
via coupling portion 104, as tilt rotor assemblies 17a and 17b reach the
airplane mode, tip
portions 49 of post portion 37 of striker arm 35 are forced into contact with
V-blocks 115. In
this manner, the selected preload is transferred from cradle assembly 111 to
wing 15a. As long
as the selected vertical preload is maintained, tilt rotor aircraft 11 will
remain stable in the aircraft
mode. If the selected preload is not maintained, tilt rotor aircraft will
become unstable due to
the oscillatory loads. The present invention provides a means of reacting the
vertical preload
between wings 15a, 15b and tilt rotor assemblies 17a, 17b; and a means of
isolating and
absorbing both static and dynamic lateral flight loads between wings 15a, 15b
and tilt rotor
assemblies 17a, 17b. It is desirable that tilt rotor assemblies 17a and 17b
receive a selected
downward preload from conversion actuator 141 (see Figure 7) such that tip
portions 49 remain
in contact with V-block 11 S throughout the flight envelope of the aircraft.
As long as the
selected preload is maintained, tip portion 49 will not move relative to V-
block 115, and the yaw
loads, or lateral loads, will be effectively restrained. In the preferred
embodiment of the present
invention, V-block 115 does not latch onto or lock onto tip portion 49. It
should be understood
that latching or locking mechanisms may be desirable in certain situations or
installations. As
is shown, cradle assembly 111 wraps around forward wing spar 135 and outboard
wing rib 137.
This allows cradle assembly 111 to maintain a low-height.
It should be apparent from the foregoing that an invention having significant
advantages
has been provided. Providing an L-shaped striker assembly with a short
generally vertical post
portion and a longer generally horizontal leg portion that can be selectively
tuned by merely
alteriag the thickness, allows the present invention to absorb or dampen
oscillatory vibration loads
without intrusion into the wings, while maintaining a low-height. While the
invention is shown
in a limited number of forms, it is not limited to just these forms, but is
susceptible to various
changes and modifications without departing from the spirit thereof.
