Document
3,879,153 4/1975 Breuner Calif.; David A. Peters, Fairview FOREIGN PATENTS OR APPLICATIONS 1,044,232 11/1953 France .....
The United States of America as 1938 United Kin represented by the Administrator of Grafel; John R. Manning ABSTRACT 416/ 104; 4 1 6/ 14 1 ; scribes various cross section distributions and the in- 13 Claims, 23 Drawing Figures \
U.S. Patent Dec. 28, 1976 Sheet 2 of 5 3,999,886
PRINCIPAL ELASTIC
A X E S
Ell6 4
PR I NC I PAL
ELASTIC AXES
5 8"
52"
54"
HIGH STIFFNESS
AXIS OF (E) MATERIAL
ROTATION
0 LOW STIFFNESS (
,E) MATERlAL
PLANE OF
- ROTAT IO hl
x
aten& Dec. 28, 1976 Sheet 3 of 5 3,999,886
70-
' 6
U.S. Patent Dec. 28, 1976 Sheet 4 of 5 3,899,886
-108
BLADE
LAG MOTION
U.S. Patent Dec. 28, 1976 Sheet 5 of 5 3,999,886
IO
Q"
-. 2 - .4 -. 6
-.8 - 1.0
PITCH -LAG COUPLING
FIG I3
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1 2
inclination provided lies between 30" and 45" with respect to the plane of rotation of the blade. HINGELESS HELICOPTER ROTOR WITH A specific arrangement providing pitch-lag coupling IMPROVED STABPLITV includes a hollow rigid torque tube which surrounds a FIELD OF THE INVENTION 5 flexible blade shank. The torque tube is connected to This invention relates to helicopter rotors and, more the blade portion of the rotor blade at one end and is Particularly 9 to a hingeless helicopter rotor having im- connected to the blade shank at the other end by means proved stability characteristics.
of a shear pin and socket assembly. The geometry of the shear pin assembly and the rigid construction of the BACKGROUND OF THE INVENTION l o torque tube forces the blade shank to twist responsive rotors can be grouped genera1 to bending thereof which takes place in a direction categories, hinged and hingeless. Hinged, Or articu- parallel to the plane of rotation, thus providing cou- lated, rotors permit movement both perpendicular and pling bemeen the blade pitch and bending movements parallel to the plane Of rotation Of the to reduce plane of blade rotation. This arrange- parallel to the blade stresses. Hingeless rotors eliminate the need for 15 ment is suitable for hingeless rcPtors that include a pitch the hinges and utilize elastic deflections of the blade to change bearing in the rotor hub.
relieve stress. Hingeless rotors provide reduced rotor complexity and cost as well as improved flying qualities Another specific arrangement involving the torque tube and blade shank provides pitch-lag coupling by and maneuverability. ' Previously known hingeless rotor arrangements &a- 20 proper arrangement Of the pitch link*The pitch link is used by the operator to adjust blade pitch for control- racteristicly suffer from a number of substantial draw- ling the rotor. The Proper arrangement of the pitch-link backs. For example, nearly all hingeless configurations are prone to aeroelastic instabilities involving rotor constrains the blade shank to twist when lead-lag bend- blade motions parallel to the plane of rotation. These ing ofthe blade shank Occurs thus Providing the desired instabilities can be characterized as isolated rotor insta- 25 Pitch% Coupling- This arrangement is suitable for bilities not involving participation of the helicopter hingeless rotors that do not include a Pitch change fuselage, or as coupled rotor-fuselage instabilities in- bearing in the rotor hub.
volving coupling between rotor blade motion and fuse- Other features and advantages of the invention will lage motion. In both cases, the elimination of blade be set forth in, or will be apparent from, a detailed hinges introduces strong structural coupling between 30 description of the preferred embodiments found here- individual blade motions and the combined rotor-fusel- inbelow.
age motion. In many cases these coupling phenomena BRIEF DESCRIPTION OF THE DRAWINGS are undesirable and Droduce rotor instabilitv. A Dartic- . .
ularly important class of instabilities involves a coupled FIG. 1 is a simplified perspective view of a conven- rotor-fuselage instability called ground resonance. This 35 tional helicopter rotor illustrating the location of the instability may occur whenever the fundamental lead- principal elastic flexural axes; lag bending frequency of the blade is less than the rotor FIG. 2 is a perspective view similar to that of FIG. 1 rotational frequency. Instability problems such as those illustrating the location of the principal elastic flexural referred to are usually handled with trial and error axes in a blade constructed in accordance with the design techniques and, in many instances, auxilliary 40 invention; lead-lag dampers must be installed to eliminate the FIG. 3 is a perspective view similar to that of FIG. 1 instability. However, this solution, because of the Cost illustrating rotor blade that results from the basic and complexity thereof, tends to coupling the pitching of the blade to the lead-lag bend- advantages provided by the use of a hingeless rotor.
ing of the blade; FIG. 4 is a perspective view of a rotor blade sDar SUMMARY OF THE INVENTION constructed of uniform stiffness material which ilius- In accordance with the invention, a hingeless heli- trates the locations of the principal elastic flexural axes copter rotor is provided wherein the inherent damping associated with the blade; characteristics of the rotor, and hence the stability of FIGS. 5A, §B and §C are transverse cross sectional the rotor blade motion, are improved. In addition to the 50 views of three different embodiments of a blade con- basic advantage regarding the prevention of castas- structed of non-uniform stiffness materials, illustrating trophic instability, the helicopter rotor can be made the locations of the principal elastic flexural axes pro- simpler, lighter and less costly, thereby providing ad- vided by these constructions; vantages regarding overall performance, efficiency and FIG. 6 is a transverse cross sectional view of a blade cost. Generally speaking, the invention comprises a 5 5 constructed of non-uniform stifFness materials used to hingeless helicopter rotor having a blade construction illustrate a method of calculation of the location of the wherein the principal elastic flexural axes of the blade principal flexural axes; are inclined relative to the plane of rotation of the FIG. 7 is a perspective view of a preferred embodi- blade, and including an arrangement for providing 60 ment of an arrangement for coupling pitching with pitch-lag coupling, i.e., for varying the pitch of the lead-lag bending for a configuration including a pitch blade in relation to the degree of bending of the blade change bearing in the rotor hub; in a plane parallel to the plane of rotation of the blade, FIGS. $A, 8B and 8C are diagrammatic, highly sim- this bending being referred to as "lead-lag'' bending.
plified views of the assembly in FIG. 7 used in illustrat- In one embodiment, the blade is constructed of mate- 65 ing the correspondence between lead-lag bending and rials having different stiffnesses to provide the desired pitching for three different conditions; FIGS. 9A, 9B inclining of the flex axes, different cross sectional con- and 9C are transverse cross sectional views of the figurations of stiff and less stiff materials producing different angles of inclination. Preferably the angle of blades of FIGS. 8A, 8B and 8C;
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FIG. 10 is a perspective view of an arrangement for blade 20, which is indicated at 24, points in the direc- coupling pitching with lead-lag bending for a configura- tion of rotation of blade 2 0 . It should be pointed out tion not having a pitch change bearing in the rotor hub; that when reference is made to zero pitch or to positive FIGS. 11A, 11B and 11C are diagrammatic simpli- or negative pitch, this does not include the pitch of the fied views of the assembly in FIG. 10 used in illustrating 5 blade that the helicopter operator has direct control the correspondence between lead-lag bending and over and that is used for maneuvering and control.
pitching for three different conditions; Referring again to FIG. 3, as blade 20 “lags” back from FIGS. 124 12B and 12C are transverse cross sec- rest position 28 to a lag position 30 during blade rota- tional views of the blades of FIGS. 11A, 11B and 11C; tion, pitch-lag coupling provides for the pitch of blade and 10 20 to increase 3 . 8 8 so that its major axis 24 is inclined FIG. 13 is a diagram or graph which plots aerody- from the plane of rotation 22 by this angle. When blade namic lead-lag damping as a function of pitch-lag cou- 20 “leads“ rest position 30 so as to be located in a pling for various inclinations of the principal flexural position such as indicated at 32, the major axis 24 is axes. declined from the plane of rotation 22 so as to provide 15 a negative pitch of -A€). The rotation in question can DESCRIPTION OF THE PREFERRED be produced in several ways including appropriate EMBODIMENTS rigging of the blade pitch controls, Le., the aforemen- As mentioned above, in accordance with the inven- tioned operator controls, or by using special blade tion a rotor blade is provided which combines ( 1 ) in- designs. Both of these techniques are conventional and clined principal elastic flexural axes and (2) pitch-lag 20 are employed in some helicopters now in use. Two coupling, that is, coupling of the pitching of the rotor approaches for achieving pitch-lag coupling will be blade, Le., the tilt of the blade with respect to its plane discussed below.
of rotation, with lead-lag bending, i.e., bending which Before discussing the embodiment for providing occurs in a plane parallel to the plane of rotation of the pitch-lag coupling, an exemplary embodiment which blade. 25 provides inclination of the principal flexural axes will To provide a better understanding of the invention, be considered wit!^ reference to FIGS. 4, SA to 5C and certain background considerations will be briefly inves- 6 . As discussed below, the concept illustrated by the tigated. Refemng to FIG. 1 , arrows 10,12 illustrate the embodiments of FIGS. SA to 5C and FIG. 6 are incor- locations and the directions of the principal flexural porated in the embodiment providing pitch-lag cou- axes of a rotor blade 14a of a conventionally designed 30 pling. It should be emphasized that inclination of the helicopter rotor assembly generally denoted 14. As principal flexural axes can be achieved in a number of illustrated, the first principal flexural axis 1 0 coincides ways and the embodiments discussed below are given with the major axis of the cross section of the blade 14a as examples only.
i.e., the longitudinal cross sectional dimension of the FIG. 4 shows a blade spar 40 which is of conventional blade, and points in the direction of rotation which is 35 design and which includes principal elastic axes 42, 44 indicated by arrow 1 6 . The second principal flexural that generally coincide with the geometric axes of sym- axis 12 is perpendicular to axis 1 0 . metry of the blade or blade spar cross section. If the It should be noted that FIG. 1 is an idealization of a blade spar is rectangular in cross section and of uni- conventional rotor blade. In reality, the principal flex- form stiffness material, then the principal elastic flex- ural axes may be inclined at a slight angle to the plane 4 0 ural axes will coincide with the axes of symmetry of the of rotation as an indirect result of slight non-uniformi- rectangle. In order to rotate the principal axes, a blade ties in stiffness of the blade materials and because of o r spar in accordance with a preferred embodiment of twist of the rotor blade. These small inclinationsare not the invention is constructed of materials of dissimilar significant however. In this manner, the flexural axes are oriented stiffness.
As discussed hereinabove, in accordance with the 45 at the desired angle with respect to the plane of rota- present invention, the principal flexural axes are tion by selective distribution of stiff and less stiff mate- steeply inclined. Thus, referring to FIG. 2 , the primary rials. Refemng to FIG. SA, there is shown a cross sec- flex axes indicated by arrows lo’, 12’ are inclined an tion of blade 50 which is basically constructed of a angle 0, from the plane of rotation of blade 14a‘. The relatively low stiffness material 54, but which includes pitch of the blade 14a’, which can be defined as the 50 first and second portions 52 that are constructed of a inclination of major axis 18’ relative to the plane of high stiffness material and are arranged at opposite rotation of blade 14u’ is not changed from its orienta- corners of blade 50 as illustrated. This cross-sectional tion shown in FIG. 1 . An arrangement for inclining the configuration provides rotation of elastic flexural axes flexural axes without changing the pitch of the blade to the positions indicated at 56 and 58. For non- will be discussed below. At this point it will suffice to 55 uniform stiffness cross sections with lumped concentra- note that in conventionally designed blades, the two tions of stiffness, such as illustrated, the orthogonal primary bending modes take place along the principal principal axes will pass approximately through and flexural axes and remain relatively uncoupled; blades between the areas of high stiffness. Two further config- with steeply inclined principal axes produce significant urations are illustrated in FIGS. 5B and 5C, with ele- coupling of the primary bending modes. 60 ments similar to those of FIG. SA being given the same in FIG. 5B and with As was also mentioned previously, the invention con- numbers with primes attached cerns providing pitch-lag coupling in addition to incli- double primes attached in FIG. 5C.
nation of the principal flexural axes. Referring to FIG. The orientation of the actual flexural axes can be 3, a rotor blade assembly is illustrated which includes a calculated from the stiffness distribution of material rotor blade 20. Rotor blade 20, in the rest position 65 within the blade cross section and the geometry of the illustrated in solid lines, (i.e., when pointing in the cross section. Referring to FIG. 6, an exemplary blade direction indicated by arrow 28) has a pitch of zero 60 is shown which is of arbitrary cross section and degrees. Under these conditions, the major axis of which is constructed of high stiffness material 62 and
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5 6 stiffness materials. Socket 88 is formed in a second low stiffness material 64 arranged in an arbitrary con- flange 80b of shank root 80. The shear pin 86 is secured
figuration producing inclined axes 5 and v. The x and y
axes shown in FIG. 4 are parallel and perpendicular, to and extends outwardly from one end of the torque respectively, to the plane of rotation of the blade and tube 84 and is inserted into socket 88, thus pinning the torque tube 84 to the shank root 80 and hub 72. The pass through the neutral axis of the blade cross section. 5 The bending stiffness parameters for this nonuniform geometry of the shear pin 86 and socket 88 prevents stiffness cross section are defined by the following for- linear transverse deflections at this point, but permits angular deflections and rotations. Since the shear pin mulas: 86 and socket 88 are positioned vertically above the (E&, = .fJ E(x,y)y2dxdy 10 blade shank 78 and the torque tube 84 can not bend or twist, the blade shank 78 must twist during leading or ( E O u u = JJ E(x,y)x*dxdy lagging movements so as to keep the pin 86 in socket 88. The arrangement described above produces a posi- = JJ E ( ~ , y ) x y d x d y tive pitch for blade lag and a negative pitch for blade where E is the stiffness of the blade material, i.e., 15 lead- Young's Modulus and ( E l ) is the bending stiffness. In FIGS. 81?+ to 8c and 9A to 9 c illustrate the operation of the system in FIG. 7 . Referring t~ FIGS. SA and 9A, the case of a uniform stiffness cross section these for- mulas reduce to the cqmmon case where ( E I ) , = EI,,, when blade 90 is at rest, the torque tube, indicated at etc., where I==, I,, are the usual cross section moments 94 and the blade shank 92 are aligned vertically with of inertia and I,, is the cross section product of inertia. 20 each other so that the inclination of blade axis indi- The principal flexural axes of the blade are the axes TJ cated at 98 With respect to the plane of rotation 96 is and 4, inclined at an angle 8, where 8 is given by the zero degrees. Again, as mentioned above, the pitch referred to does not include th'e pitch that results from following formula: the motion of pitch link 82 in FIG. 7 , which is operator 25 controlled and not coupled to blade lead or lag. When the blade 90 lags back from the rest position thereof as 2 ( E I ) , , e = - tan-' indicated in FIG. SB, the blade shank 92 bends and c ( E O z z - ( E O v u .
torque tube 94, being rigid, forces the blade shank 92 to &st thereby producing a positive inclination of the The bending stiffnesses about the 6 and rl axes are given 30 blade axis 98 with respect to the plane of rotation 96.
by the following formulas: Similarlv. as illustrated in FIGS. 8C and 9C. when the blade leads the rest position thereof so as to cause the
(E')(( = (EI),&oszO + (EI),pinZO - (EI),+in 20
blade shank 92 to bend forward, the torque tube 94 forces the blade shank to twist thereby producing a
(El)?? = (EI),&inZO + (EI),,,cosZO + (El),& 20
35 negative angle between the blalde axis 98 and the plane sin of rotation 96.
(E')?( = [ ( E I ) ~ , - ( E I ) J 7 + (EI)," (cos28 - sin%) Refemng to FIG. 1 0 , a further embodiment accord- ing to the invention is illustrated. A helicopter rotor In order for the inclination angle 8 to be Properly 100 comprising a blade 101, a blade shank 102, a defined, and for the inclination of the principal flexural 40 torque tube 184, a pitch link 106 and a rotor hub 108, axes to produce the desired beneficial effects, it is nec- is shown. Rotor shank 1Q2 inco(rporatesinclined princi- essay that the bending stiffness parameters ( E V p n d pal flexural axes as described hereinabove. This em- (E')?? be unequal or that ( E L not be equal to bodiment has as an advantage the elimination of the zero.
rotor hub bearing, thereby simplifying rotor hub 108.
FIG. 7 illustrates a preferred embodiment incorpo- 45 Except for the combination of inclined principal flex- rating both inclined flexural axes and pitch-lag COW ural axes and pitch-lag coupling, the configuration pling. A helicopter rotor assembly 70 includes a rotor shown in FIG. 10 is known in the art. The desired pitch- hub 72 with four pitch change bearings in the hub lag coupling is obtained from the kinematics of pitch bearings 74. A rotor blade 76 has a blade shank 78 link 106 and is described herein below.
attached thereto, blade shank 78 terminating in a shank 50 Refemng to FIGS. PIA and I2A, rotor 100 is shown root 86) that is received in a hub bearing 74. Shank root in the rest position, i.e., with no lead-lag bending. Point 80 includes a pitch link connection flange 80a which is 115 in FIG. 12A is fixed, and its vertical motion in the coupled to a pitch link control member 82 controlled direction of the arrow 110 is controlled by the helicop- by the helicopter operator. Vertical motion of pitch ter operator. Pitch link 106 is tilted ,away from the link control member 82 produces rotation of blade 76, 5 5 vertical plane by an angle 8. Vertical motions of point 78 and shank root 80 on the bearing 74 SO 115 in the direction of arrow 110 are for conventional blade shank as to provide the pitch changes that the helicopter blade pitch control for maneuvering.
operator requires for control purposes. Blade shank 78 Referring now to FIG. 11B, rotor blade 101 is shown is constructed of materials of different stiffnesses SO as lagging during rotation due to bending of rotor shank to provide the inclined principal flexural axes described 60 1Q2. As seen in FIG. I2B, lag bending forces torque above and is connected to the blade 76 at a point which tube 104 and blade 1 8 1 to increase its pitch by an angle is about 15 to 20% of the distance between the center + A 8 by the kinematics of torque tube 104, pitch link of the rotor and the blade tip. 106 and fured point 115. Refemng to FIGS. 11C and An arrangement including a torque tube 84, a shear 12@, similar action forces torque tube 104 and blade 86 and a shear pin socket 88 to provide the desired 65 101 to decrease its pitch -A@ when blade 101 leads pin pitch-lag coupling. The torque tube 84, which is hollow during rotation.
so as to permit receipt of blade shank 78 therein, is Referring to FIG. 13 there is shown a diagram which rigidly fixed to blade 76 and is constructed of high illustrates the benefits of the invention. In the diagram,
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4. A rotor as claimed in claim 1, wherein said blade the vertical axis represents aerodynamic damping of the free vibrations of the blade at the lead-lag natural is constructed of materials which are of different stiff- frequency and is given in terms of percent of critical nesses and are geometrically arranged to provide incli- nation of said principal flexural axes.
damping. The horizontal axis represents pitch-lag cou- pling which is defined as the ratio of the increase in the 5 5. An improved hingeless rotor comprising blade pitch angle to the increase in the blade lead an- a rotor hub; gle. Aerodynamic lead-lag damping is plotted as a func- a blade connected to said rotor hub for rotation tion of pitch-lag coupling for six different values of e,, therewith, said blade having the principal flexural the angle of inclination of the principal flexural axes. axes thereof inclined from the plane of rotation o f The values of aerodynamic lead-lag damping pertain to 10 said blade; a helicopter rotor blade with a Lock number of 8, and means for varying the pitch of said blade in relation- having bending stiffness with respect to the principal ship to the degree of bending of said blade in a flexural axes such that the two bending natural fre- plane parallel to the plane of rotation of said blade; quencies of the blade (when not rotating) are equal to said blade including a blade portion and an elongated 0.7 and 0.458 times the normal operating rotational l 5 shank portion connected to the rotor hub, said frequency of the rotor. Furthermore, the pitch angle of pitch means comprising a hollow rigid torque tube the rotor blade is zero; that is, the rotor is operating at which surrounds said shank portion and is affixed the zero lift condition, which is the most critical with to said blade portion at one end thereof; and regard to aerodynamic lead-lag damping that is asso- means for coupling said torque tube to said rotor hub ciated with ground resonance instability for a helicop- 2o such that transverse linear deflections of said ter resting in ground contact. It is noted that hingeless torque tube are prevented while angular and rota- rotor blades normally exhibit approximately M to 1 tional reflections are permitted thereby forcing the percent damping due to hysteresis of the blade material blade shank to twist in response to bending thereof (this damping being referred to as structural damping) which takes place in a direction parallel to the and that this value is not included in FIG. 13. Damping 25 plane of rotation.
on the order of 4-5% may be required to prevent rotor 6. A rotor as claimed in claim 5, wherein said shank instabilities such as ground resonance. FIG. 13 demon- portion is connected to said hub through a shank root strates that percentage values for aerodynamic damp- portion and said torque member is coupled t o said ing up to 11% result from a combination of pitch-lag blade root portion.
coupling with inclined principal flexural axes for angles 30 7. A rotor as claimed in claim 5, wherein said cou- of inclination from about 30" to 45". The curves in FIG. pling means comprises 13 also demonstrate that decidedly superior results are a socket formed in said shank root portion, and achieved by combining the techniques of inclining the a shear pin which extends outwardly from the top of principal flexural axes and providing pitch-lag coupling 35 said torque tube at the other end of said torque since when either one of the other technique is not tube and is received in said socket.
used, the resulting aerodyamic damping falls off to near 8. A rotor as claimed in claim 1, wherein a plurality zero. It is not necessary that the pitch-lag coupling be of said blades connected to said hub for rotation there- provided as a result of kinematics of specially arranged with, each of said blades having the principal flexural mechanical linkages, pins, sockets, etc.; but the inven- 4o axes thereof inclined from the plane of rotation of said tion includes embodiments wherein the pitch-lag cou- blades and including means for varying the pitch of that pling may be provided by the inherent elastic or struc- blade in relation to the degree of bending of that blade in a plane parallel to the plane of rotation of the blades.
turd properties of the blade spar.
Although the invention has been described with re- 9. An improved hingeless helicopter rotor comprising spect to exemplary embodiments thereof, it will be 45 a rotor hub; understood that variations and modifications can be a blade connected to said rotor hub for rotation effected in these embodiments without departing from therewith, said blade having the principal flexural the scope or spirit of the invention. axes thereof inclined from the plane of rotation of We claim: said blade; means for varying the pitch of said blade in relation- 1. An improved hingeless helicopter rotor comprising 50 a rotor hub; ship to the degree of bending of said blade in a plane parallel to the plane of rotation of said blade; a blade connected to said rotor hub for rotation said blade including a blade portion and an elongated therewith, said blade having the principal flexural shank portion connected to said rotor hub, said axes thereof inclined from the plane of rotation of pitch varying means comprising a hollow rigid said blade when said blade has zero pitch; and torque tube which surrounds said shank portion means for varying the pitch of said blade in relation- and is affixed to said blade portion at one end ship to the degree of bending of such blade in a plane parallel to the plane o f rotation of said blade thereof; and a pitch link member coupled to the other end of said to damp blade oscillations and increase blade sta- torque tube so as to force said blade shank to twist bility. 60 in response to bending thereof which takes place in 2. A rotor as claimed in claim 1, wherein the angle of inclination of the principal flexural axes lies between a direction parallel to the plane of rotation.
approximately 30" and 45" as measured from the plane 10. An improved hingeless helicopter rotor adapted of rotation of said blade. to damp lead-lag oscillations and reduce the likelihood 3 . A rotor as claimed in claim 1, wherein said blade 65 of ground resonance comprising is constructed of materials of non-uniform stiffness, a rotor hub; which provide inclination of said principal flexural a rotor, the root portion of said rotor being a blade axes. shank;