Note: Descriptions are shown in the official language in which they were submitted.
li FI~L~ OF TflE Il~lV~NTION
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This invention relates to the control of flapping in
',ihelicQpter blades, and ~ore particularly to a compound hub spring
llstructure having nonlinear spring characteristics.
¦ PRIOR ART
It is known to provide a helicopter with hub springs in
order to oppose and mini~ize flapping tendencies in the blades
during flight rnaneuvers. In U.S. Patent No. 3,807,896 a pair of
~Iconcentric tubes extend along the teetering axis, and are coupled
j,respectively to the blade and to the ~ast to oppose, by torsion,
the fl~pping of the blades. In U.S. Patent ~o. 3,804,552 an
~elastomeric bearing is mounted between a rotor yoke flange which
,rotates with the blades about the flapping axis, and a main mast
'flange that is fixed to the ~ast.
' The presence of a hub spring induces forces which oppose
flap~ing. Depending upon the spring rate, greater or less
vibration in the air frame may occ-~r by way of the r~ast. ~ligh
sprin~ rates are desired during flight ~aneuvers in order to
prevent excess flapping. Under level flight conditions, low
~0 j~spring rates are desirable in order to m,inimize vibration and
¦~imProve stability.
In accordance with th* present invention, a compound hub
¦,sprin~ system is provided wherein at low flap angles the spring
~'rate is low. Beginning at a predetermined flap angle, the spring
~5 I rate is increased. The increase in spring rate can be tailored to
a desired transition characteristic to avoid or minimize hard
contact between the rotor and the mast during high flap angles.
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UMMARY OF THE INVENTI~N
A compound hub spring structure for controlling
flapping in main helicopter rotor blades is provided. The
structure applies a restraining force having a low spring
rate at low flap angles, and a high spring rate at flap
angles greater than a predetermined flap angle.
~ n accordance with an aspect of the invention there is
provided a rotor blade mounting for a flapping main rotor
of a helicopter with a rotatable drive mast, comprising:
a trunnion attached to the mast, said trunnion including
structure extending from opposite sides thereof to define
a teetering axis; a yoke including bearing means secured
to said trunnion for pivotal movement ab~ut said teetering
axis, said yoke having rotor blades attached thereto;
first elastomeric spring means connected between said yoke
and said trunnion for continuously opposing flapping of
the rotor blades with a first restraining shear bias of
predetermined spring rate, said first spring means being
disposed in spaced relationship with the teetering axis;
and second elastomeric spring means mounted on said yoke
for opposing flapping of the rotor blades beyond a
preselected flapping angle with a second restraining bias
of non-linear spring rate so that overall the
¦ flap-opposing bias is non-linear, said second spring means
being positioned on opposite sides of the mast in spaced
relationship with the teetering axis.
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~ ore particularly, the main helicopter rotor blades
are coupled to the main rotor by means exerting a linear
restraining force at a low spring rate. Further, means
such as a resilient block mounted for rotation with the
rotor about the flapping axis applies a restraining force
exhibiting a high spring rate characteristic prior to hard
contact between the rotor and the mast.
In one aspect of the invention, the spring rate of the
resilient block increases as the blade flapping angle
increases for angles exceeding a predetermined flap angle.
In a further aspect, the spring rate of the resilient
block is controlled by varying the area of the contact
zone between the resilient block and the mast, to provide
a non-linear increase in spring rate as the flapping angle
increases.
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i DESCRIP~ION OF THE DRA~1INGS
The novel features believed characteristic of the
invention are set forth in the appended claims. 1Ihe invention
' itself, however, as well as further objects and advantages thereo[
will best be understood by reference to the following detailed
description of an illustràtive embodiment taken in conjunction
with the accompanying draw;ngs, in which:
Il FIGURE 1 is a side view partially in section of a
j~mast-yoke coupling embodying the present invention;
1~ li FIGURE 2 is a top view partially in section of the
coupling of Figure 1 taken along lines 2-2;
FIGURE 3 is a graph illustrating the operation oE the
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present invention;
li FIG~RE 4 is a sectional view of a rectangular hub spring
L5 li in combination with tubular resilient pads in accordance with the
!invention;
FIGURE 5 is a side sectional view of a mast-yoke
coupling having a tapered resilient block for nonlinear spr.ing
`action; and
IGURE 6 is a top view of the coupling of Figure 5 taker
par~ially in section to further illustrate the resilient block.
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;! DESCRIPTION OF PREFERRED ~MBODIME~TS
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', FIGUR~S 1 an _ 2
` Figures 1 and 2 illustrate a mast-yoke coupling 10
le~bodyin~ a compound hub spring structure in accordance with the
'invention~
~, Coupling 10 includes a pillow block 11 with an outer
¦bearing plate ~2 secured to the pillow block by means of bolts 13.
¦ A hub sprinq plate 14 is secured to a hub trunnion 15 by means of
;~bolts 16. The pillow block 11 is coupled to the hub trunnion 15
~by means o~ an elastomeric bearing 17 formed on a cylinder 18 in a
bore in pillow block 11. Cylinder 18 is secured to the trunnion
¦l15 by the bolts 16.
The bearing 17 is co~.prised o laminated elasto~eric
j,cylinders interleaved. with thin metallic cylinders located betwee~
,the pillow block 11 and the inner cylinder 18.
,¦ A yoke 19 is formed integral with pillow block 11 and
carries a ring 20 at the lower surface of the yoke 19. A
~resilient elastomeric block 21 is adhered to the inner walls of
'the ring 20 concentric to main rotor mast 22. ~he inner surface
~0 of ring 21 is conical with the apex located at the ~xis of `trunnion 15.
. ~langes 14a of plate 1~ extend between plate 12 and an
! inner plate 23 contiguous to pillow block 11. Plates 12 and 23
l~are secured together by spacers 24, and are secured to pillow
liblock 11 by bolts 13. ~lastomeric shear pads 25 are adhered to
,`confronting surfaces of plates 12 and 23 and to the surfaces of
flange 14a.
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i~ In accordance with the invention, the elastomeric
,bearing 17 perrnits flapping of helicopter blades about the axis o!
¦~trunnion 15. Plates 12 and 23, secured to pillcw bloclc 11, rotat~
iwith the pillow block upon blade flapping. As the plates 12 and
123 rotate, the pads 25 and 26 are subjected to a shearing force
~since flanqe 14a is fixed to trunnion lS.
During high flap angles ring 21 will approach mast 22.
~When ring 21 comes in contact with the rotor mast hard contact
ibet~een the rotor mast and the assembly 10 is ~voided or minirnize(l
IThe pad 21 serves 2s a hub spring~ applying a force opposing
~1apping that can be readily tailored to fit design criteria.
¦ FIGURE 3
Figure 3 illustrates in graph form the force generated
Iliby the preferred embodiments of the present invention during
l~helicopter blade flapping.
The restraining force generated by the elastomeric
bearing 17 with increasing flap angle is illustrated by the line
30. The restraining force monotonically increases with increasirlc!
!flap angle. The combined effect of the elastomeric bearing 17 and
?.0 l~the shear ~ads 25 and 26 is illustrated by line 31. The combined
l~restraining force is essentially linear but higher.
~ h*n the flap angle of the roto~ assembly 10 is so grea~
¦`that the resilient block 21 contacts mast 22, the combined
~restrainin~ force may be illustrated by the ~raph line 32 for one
¦,configuration of pad 21. The cornpound hub spring structure thus
provides a linear restraining force illustrated by the graph line
131 until the resilient block 21 engages the rotor mast 22 as
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indicated by a point 33. Ther~after, the restraining force
,'increases at a greater rate as illustrated by the graph line 32.
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Il' Figure 4 illustrates a mo~ified form of hub sprinq 12,
,jl4a, 26 of Figure 2.
il Referring to Figure 4, a short cylinder 40 is secured to
the outboard end of a hub trunnion 41 along with a plate 42 by
means of bolts 43. Pillow block 44 houses a bearing for the hub . .
¦!trunnion 41, an outer cylinder 46 is secured to block 44 by means
Iliof bolts 48. An elastomeric cylindrlcal shear pad 5~ is adhered
,to the outer surface of cylinder 40 and the inner surface o~ :
¦¦cylinder 46.
¦ In operation a shearing force is ap~lied to the shear
l`pad 50 as pillow block a4 rotates about trunn~on 41. :
¦~ FIGURES 5-and 6
¦I Figures 5 and 6 illustrate a modif lcation to the
;compound hub spring structure of Figures 1 and 2.
'; More particularly, a rotor assembly 60 is mounted for
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rotation with a main rotor mast 61. The rotor asse~bly is
~0 comprised of a pillow block 62, a yoke 63 integral with the pillo~.
block, and a grip 64 coupling a helicoper blade (not shown) to the
,yoke 63,
i ~ block 65 encircles the mast 61. A 5haped elastomeric
`.ring 66 is adhered to the inner surfaces of ring 65D The upper
~5 ~and lower surfaces of ring 65 are conical and converge to a nose
which engages ~ast 61. As best seen in Fi~ure 6 the rin9 66 is
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~shaped to contact mast 61 at.points which are perpendicular to tne
flapping axis. ~ing 66 preferably will be formed in two halves o[.
.like shape eac~l adhered to half rings 65, 65a which are connected
.;by bolts 67 and secured to yoke 64 by bolts 68. The split ring
,structure 6~, 65a permits the nonlinear hub spring to be ad~ed to
'existing aircraft without undue modification~
., Unlike the resilient block 21 o Fi~ure 1 r the resilient'
Iblocks 66 provide a nonlinear restraining force as illustrated by
l'curve 34 of Figure 3. The nonlinearity may be selected by adiust-
¦'ment of the shape of the block 66. With a fine nose contact,
jinitial forces will be small, increasing as the area of contact
ith the mast 61 increases~ It will be understood that the linear
hub svring may be of the form disclosed in said U~S. Patent
! 3,807,896. Thus, the restraining force generated by a compound
Il.hub spring structure comprising the el~stomeric bearing 17, the
shear pads 25 and 26 oF Figure 2, and the resilient block 66 may
be illustrated by the line 31 for low flap angles as indicated to
;the left of point 33. When the rotor assembly 60 teeters with
increasing flap angle, the resilient blocks 66 are deformed
~0 ~against the mast 61. Such a condition is illustrated by curve 3~
jto the right of point 33, wherein a nonlinear restraining Eorce is
`generated to oppose the helicopter blade flappiTlg.
Thus a compound hub spring system is provided wherein
the spring rate increases linearly from a low rate under flight
~5 ~conditions s~ch as hover where the blade flap angle is low, and
~rapidly increases nonlinearly from a predeter~ined flap angle
during flight conditions where hi~h fl.ap angles may occue.
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llaving described the invention in connection with
icertain specific embodiments thereof, it is to ~e understood that
rther modiEications may now su~gest themselves to those skilled
in the art, and it is intended to cover such modifications as fall
S within the scope o~ the appended clai~s.
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