US2620888A

US2620888A - Blade tracking mechanism for lifting rotors

Info

Publication number: US2620888A
Application number: US733673A
Authority: US
Inventors: Harold T Avery
Original assignee: Individual
Current assignee: Individual
Priority date: 1947-03-10
Filing date: 1947-03-10
Publication date: 1952-12-09
Anticipated expiration: 1969-12-09

Description

Dec. 9, w52 H. T. AVERY 2,620,883

' BLADE TRACKING MECHANISM FOR LIFTING ROTORS Filed MaICh lo, 1947 2 SHEETS-SHEET l BY @at 4 fw 4free/Veys.

H. T. AVERY BLADE TRACKING MECHANISM FOR LIF-T ING KOTORS -2 sx-mETs--srmn'r 2 Filed March 10, 1947 INVENTOR. /zfwaw IAA/52V BY @7L/.4.4

Patented Dec. 9, 1952 BLADE TRACKING MECHANSD'I FOR LIFTING RO-'IORS Harold T.Avery, Oakland, Calif.

Application. March 10, 1947,'Senzial4 No. 733,673

Claims.

The present invention relates t'o sustaining rotors for aircraft, and particularly to rotors of the articulated type, that isl to rotors comprising blades each attached to a central hub by one or more hinges including a flapping hinge permitting of vertical angular displacement of the blade relative lto the hub.

In such a rotor each blade is, at. each instant, angularly positioned about its flapping hinge in accordance with the resultant of' all forces acting on the blade at the instant, including the effect of the momentum and inertia of the blade itself. It is the usual practice yin Asuch, rotors, particularly in helicopters, to provide some means for effecting cyclic control of pitch so that the blades may be Vcaused to vfollow some non-symmetrical path or to oiTSet the nen-symmetrical effect on the blades of forward flight or other translational movements of the craft.

Unless each blade is an exact duplica-te of each other blade both mechanically and in aerodynamic shape it will tend to seek a ldifferent angular position and to track higher -or lower under at least some conditions of flight. For instance, if one blade has a slight ltwist tending to give it a slightly higher pitch setting near the tip than that lof the other blades it will tend to track higher than the other blades. It is customary to provide some means whereby the pitch setting may be decreased on a blade which is thus found to be tracking too high, but since such means normally serves to reduce pitch over the entire blade the best that it can do is to permit of setting .the blade so that the pitch over the inner portion of the blade (that is the portion closer 'to the hub) will be enough less than that of .the other blades to counterbalance under certain operating conditions the remaining excess of pitch setting in the outer portion of the blade. n this manner the blade may be made to track properly on .the test stand where the adjustments are made, `but under certain *flight conditions wherein the proportion of the blade lift contributed by the outer portion of the blade differs considerably from that pertaining under the test conditions the blade will again fail to track properly with the other blades,

The immense variety in the nature and location of the `difference in aerodynamic shape that may occur between blades, together with the eX- and aerodynamic forces set up by discrepancies if' Cl. 17d-1.60.13 )4 in blade tracking. Present methodsof retrackingl blades are however veryslow, awkward and expensive, and though they are capable of providing good tracking `und-er` certain chosen sets of operating conditions., they will not in general, prevent the lblades from going out of proper tracking and causing vibration under other sets of operating conditions. Y

Itis an object of this invention `to cause the various blades, of a rotor to automatically seek the same path `or track in spite of differences in construction vand/or aerodynamic Sharpe of the respective blades.

It is a further object to provide such automatic 'tracking continuously during flight and in spite of changes in the operating conditions.

It is a further object to equalize the average effective flapping positions of all blades at each point in the circuit, taking into 'account displacement due to exing of the blades as well as those due to angular displacementv of the blades on their flapping hinges.

lit is an 'object to provide Such retracking in a manner that will not affect the average collective pitch of all blades.

It is a further object to control the rate of retracking so that on the one hand it will be sufficiently great so that tracking errors will be minimized substantially as rapidly as they 'Will occur, and yet on the other hand retraeking will not occur so rapidly as to produce adverse etfects upon the cyclic control of the rotor.

It is also an object to vary the rate of retracking in accordance with the amount of the difference in blade positions, so that large tracking errors can be corrected at rates which, if brought into play b-yslight'cyclic control displacements of the blades, would unduly disturb such slight control displacements.

It is a further object to restrict such variation in the rate of retracking to substantial-ly the maximum range of diierences in blade positions that may be related to tracking errors, so that the rateof retracking will increase vwith increase in size of the tracking error up to the maximum tracking errors that may be anticipated, but will not continue to increase throughout all of the much greater range of differences in blade positions which occur on account vof cyclic control displacements within which `greater range further increase in the .rate of retraeking would serve no useful purpose but would unnecessarily increase te@ adverse selects-er cycli@ @Qn-trol- The manner in which the foregoing, together with aslitorel @bg-legis an@ advantages Vf the Rotor construction and drive The invention is illustrated in Figures 1 and 2 as applied to a three-bladed rotor constructed for the most part, substantially as disclosed in my co-pending United States Patent Application, Serial Number 665,653, filed April 29, 1946, now Patent No. 2,539,562, dated January 30, 1951.

As particularly shown in Figure l each blade (the three such blades being identified in Figure 2 as Illa, IIlb, and IUc respectively) comprises a skin or covering integrally mounted on ribs |2 which in turn are integrally attached to the tubular blade spar I3, which spar ter- L minates inwardly in a bearing retainer |4 containing a ball thrust bearing I5 co-axial with the adjacent portion of the spar. This bearing serves to attach the blade to the connecting link I6 in a manner permitting of the blade being rotated about the spar axis relative to the link I6, to effect changes in the pitch setting of the blade. Connecting link I6 is in turn attached by means of flapping hinge |1 to lugs I8 integral with hub member I9. Hub member turn pivotally mounted by means of roller bearings and 2|, for rotation about cylindrical member 22 and about the co-axial cylindrical member 23 which is attached to member 22 by means of a plurality of bolts 24, which bolts together with their co-operating nuts 3|) serve to attach both members 22 and 23 to the cylindrical member 3| which is integrally attached to ring 32 and fuselage frame members 33.

Attached to the bottom of hub member I3 by means of a plurality of bolts 34 is a ring 35 having downwardly extending lugs 36 for receiving the roller bearing 20. The spherical roller thrust bearing 31 is interposed between ring 35 and cylindrical member 23, thus serving to transmit to the framework of the craft the upward thrust of hub member I9, which is primarily the force which sustains the craft in fiight. Attached to the top of hub member I9 by means of a plurality of bolts 38 (Fig. 2)

is a plate 39. Interposed between this plate and a flange 40 (Fig. 1) of cylindrical frame member 23 is ball thrust bearing 4|, which serves to sustain the rotor when it is not exerting an upward lift on the craft.

The drive for the rotor cornes from the engine, not shown, through transmission shaft 42, which shaft is guided in the upper surface 44 of the cylindrical frame member 22. Integral with the upper end of shaft 42 is gear 50 which meshes with idler 5|, which idler is rotatably mounted on stud 52 which is integrally mounted in member 22. Idler 5I in turn meshes with teeth 53 cut into the inner face of hub member I9. Hub member |9 is thus rotated upon the fixed cylindrical member 22, 23 by the rotation of shaft 42 by the engine.

Collective and cyclic pitch setting As previously described, blade i0 is attached to connecting link I6 in a manner permitting of |9 is in 2.

the blade being rotated about the blade axis relative to link I6, to effect changes in the pitch setting of the blade. Blade I0 is rotatably positioned about this blade axis by the vertical posi- `tioning of the blade pitch controlling means which comprises link 6| connected by a ball and socket joint to arm (Fig. 2) integral with the blade. The length of link 6| is rendered adjustable by constructing it of two sections screwed together and locked by nut 62 (Fig. l).

The vertical positioning of pitch control link 6| is effected by a pitch control spider 65 comprising arms 64 integral with ring 65a, each link 6| being connected by means of a ball and socket joint to a bell-crank 63 pivotally mounted on a corresponding one of these arms 64. Each pitch control bell-crank 63 is maintained in a fixed position relative to its respective arm 64 except to the extent that it may be displaced relative thereto by the automatic blade tracking mechanism, which is described hereinafter. Pitch control spider 65 is rotated about pitch control ring 66 on ball bearings 61 in unison with the rotor, the force for rotation being supplied by pin 68 which is integrally mounted on hub member I9, and protrudes through a slot in arm 39 integral with pitch control spider 65.

Pitch control ring 66 is integrally attached to pitch control hub 10 by means of arms 1| which arms include necked down round portions 12 which pass through cylindrical member 3| in slots 13. Pitch control hub 10 is attached to collective pitch control rod 14 by ball and socket joint 15. One cyclic pitch control assembly is illustrated in Figure l, comprising cyclic pitch control arm integrally attached to pitch control ring 66 and attached to cyclic pitch control rod 8| by ball and socket joint 82. There is also another cyclic pitch control assembly (not shown) which comprises another cyclic Ditch control arm 80 and rod 8| like those just described, but this assembly is attached to ring 66 at a location displaced ninety degrees from that of the assembly illustrated. The collective pitch control rod 14 and the cyclic pitch control rods 8| are connected by a linkage such that vertical movement of the collective pitch control rod 14 will be accompanied by the same amount of vertical movement of both of the cyclic pitch control rods 8|, but the cyclic pitch control rods 8| are each capable of independent vertical movement relative to collective pitch control rod 14. Such a type of linkage is shown on pages 122-130 of the June 1945 issue of Aviation magazine, and a different form of such a linkage is shown in my co-pending United States Patent Application, Serial 630,745, led November 26, 1945, which issued as Patent No. 2,546,881 on March 21, 1951.

The operation of the pitch control assembly is as follows: The upward movement of collective pitch control rod 14 (automatically accompanied by similar upward movement of both rods 8|) raises ring 66 without altering its angle of tilt, thereby simultaneously increasing the pitch on all of the rotor blades |0 by means of the linkages previously described. The upward or downward movement of one of the two cyclic pitch control rods 8| relative to collective pitch control rod 14 inclines ring 66 and hence pitch control spider 65 relative to the rotor hub axis at an angle dependent upon the position of the cyclic pitch control rod 8| relative to rod 14. The inclination of pitch control spider 65 causes the rotor blades to increase and decrease their pitch actricesL cyclical-ly; i. e., the pitch is increased by that 'segment lof the pitch control lspider that 'is raised and conversely the pitch is decreased by that segment of the pitch control spider that is lowered on account of the inclination of the pitch control spider. Thus spider 65 and the mechanisxn which controls it, as above described consti'tutes the primary pitch adjusting means fcr the rotor, but its action is subject to alteration by the secondary pitch adjusting means hereinafter described.

Mechanism responsive to blade flemme Because of the fact that rotor blades are ordinarily constructed of relatively slight vertical dimension and sustain relatively great aerodynamic loads of uneven and cyclically changing distribution along the length of the blade, such blades are ordinarily subjected to considerable flexing in flight, particularly in a vertical direction. If a blade which is attached to the rotor hub by a flapping hinge is appreciably exed, the effective flapping position of the blade, and therefore its tracking lof the other blades, is dependcnt upon the direction and `amount of the ilexingoi the blade as Well as upon the vertical angular displacement of the root section of the blade about the flapping hinge. In order to take both of these factors into account I provide mechanism which is quantitatively responsive to blade iiexure as well as to displacement of the blade about the napping hinge. This mechanism includes a lever 91 (Fig. l) which is angularly positioned about napping hinge Il in response to both vertical flexing of the blade and angular displacement of the blade on napping hinge This angular positioning of lever 91 is effected by the following mechanism:

Lugs 90 are integrally attached to tubular blade spar I3. Flexible cables or wires r| are attached to these lugs by means or" pins 92 and are led along paths vertically displaced from the neutral axis of the blade through guides 53, each of which guides may consist of a pair of rollers mounted on a lug integral with spar i3. Near the root of the blade each cable 9| is attached to a connecting link 911i by means of a connecting pin 95. Connecting links Qd are pivotally attached by means of pins 96 to lever 97|. Pins dii are placed considerably further apart than are the pins 92 at the outer ends of cables 9|, so that the angular displacement oi lever di will be correspondingly less than that of lugs The reason for this is so that lever 9i will be angularly displaced in proportion to the average eiTective angular displacement or" the blade relative to the hub due to blade flexing, as measured at the napping hinge. This angular displacement is substantially that of a line connecting the flapping hinge to a point on the blade somewhere in the vicinity of the average effective center of lift of the blade, and is in any case very considerably less in amount than the angle through which the outer tip portion of the blade has been ilexed. Therefore, if lugs 90 are located rather close to the blade tip, pins Sli should be about three times as far apart as pins 92 so that lever el will be rocked through about one-third the angle through which lugs 9c are deflected, while if lugs 90 are located further in on the blade the ratio of the distance between pins 95 to that between pins $22 should be correspondingly decreased, so that in any case, whether the spacing is determined by theory or by test, the rflexing of the blade in flight shall displace lever 91 'through an angle equal to that 6 through which the unilexed blade would have to be rocked .around flapping hinge in order to produce substantially the same amount of change of blade tracking eiiect on the craft.

These same connections to lever 91 will also cause that lever to rock about napping hinge in unison with the blade, .for any given condition of blade exure. Hence lever 91 is angularly positioned about hinge in accordance with the average effective blade position taking into account both blade iiexure and rocking ofthe blade upon the flapping hinge.

Blad/e tracking mechanism As previously indicated it is highly desirable to eliminate all differences in blade tracking under all conditions of operation and mechanism is therefore provided which is sensitive to any differences that may exist between the angular position of the various levers 97 associated with the respective blades, and which is automatically operable to decrease such differences by adjustment of the pitch of one 0r the other or of both of the blades with reference to which the difference exists. In the preferred form of blade tracking mechanism herein described pitch is automatically adjusted on both of the blades concerned.

This blade tracking mechanism consists of two major parts: the first part constitutes a tracking control means comprising a valve system together with the linkages which supply that system With indications of the differences in blade tracking, and the second part constitutes an operating means comprising a hydraulic system that alters blade pitch in accordance with the responses of the valve system, in a manner adapted to eliminate the differences in blade tracking.

The valve system and its linkages are as follows: In conjunction with each blade there Vis provided a slide |00 which moves in a guide I0! and is connected to the corresponding lever Si (previously described) by a slide extension H32, connecting link |03, and pins |04 and |05. Vertical pin |06 is integrally attached to slide |00, and ts in the fork of one arm of a bell-crank |01 (see also Fig. 2) which is pivotally mounted on pin |08 which in turn is integrally mounted in hub member IS. The other arm of the bellcrank is pivotally attached by means of a pin ||0 to a link |09, Which link is made adjustable in length by constructing it of two pieces clamped together by means oi bolts l2 extending through slots Each link |09, in addition to being connected at one end to a bell-crank 01, as previously described, is pivotally connected at the other end to one end of a lever ||3 by means of a pin I4. 'At its opposite end lever I3 is provided with a fork which spans the one of the previously mentioned vertical pins Ii which is positioned` by the next adjacent blade. Near its mid-point lever IIS is pivotally attached to a valve-positioning link H5 by means of a pin H5. Link I5 thus constitutes an element jointly positioned by two of the blades in accordance with the difference of the eiiective napping angle of the two blades. Each link ||5 is normally held in fixed relation to a corresponding slide valve member I1 by means which nevertheless permit of its being yieldably displaced relative thereto in either direction. This means comprises a spring l I8 which yslips over tenons Hd, which are identically shaped on both link H5 and member Il?, as parts of identical H-shaped openings in the two members. Each member H5 lies directly on top of the corresponding member ||1 and in the plan view, as illustrated in Figure 2, the portions of these two members extending outwardly from pin I6 are identical. This construction is generally similar to that of the spring link illustrated in Figure 50 of Avery Patent No. 2,271,240 dated January 12, 1942. However, each member H terminates inwardly with an arcuate surface concentric with the corresponding pin ||6, as shown in dotted lines in Figure 2, while the corresponding slide Valve member ||1 extends on inwardly and includes as an integral part thereof a slide valve |20, the three such valves shown in Figure 2 being identified as |20a, |2013, and |20c respectively. Each slide valve |20 slides in valve sleeve |2| (the respective sleeves being identified as |2la, |2|b, and |2|c) and its movement therein is limited by stop lugs |22 integral with valve |20. The purposes of the Valve override spring ||8 is to permit movement of valvepositioning link ||5, in either direction after slide valve |20 and the integral extension ||1 thereof may have reached the limit of their travel and are prevented from moving by stop lugs |22.

It is to be understood that, as illustrated in Figure 2, the above described valve mechanism is repeated three times for a three-bladed rotor, and that each valve is associated with two adjacent blades. In a manner hereinafter set forth the valve mechanism just described operates to displace one of the three slide valves |20 from neutral whenever there is a difference in effective flapping angle between the two blades with which said slide valve is associated.

Each slide valve |20 thus constitutes an adjustable element positioned by the blades for controlling the blade tracking operation, and, in the manner hereinafter described, these valves act as the controlling members for the pitch adjusting or altering portion of the blade tracking means.

As previously mentioned the second part of the blade tracking mechanism consists of the hydraulic system which constitutes the operating means for altering blade pitch to reposition the blades so as to maintain proper tracking. Hydraulic pressure and hydraulic flow for all three blades is supplied by the single hydraulic pump |30 which is attached to plate 39 by means of a plurality of bolts |3|, and plate 39, as previously described, is attached to hub member |9 by means y of a plurality of bolts 38. Hydraulic pump |30 is equipped with a conventional spring loaded bypass |32 which is used to permit all excess hydraulic fluid pumped to flow back to intake while maintaining constant hydraulic pressure on the output side of the pump. Hydraulic pump |30 is operated by shaft |33 (Fig. 1) which is turned by spur gear |34 which is integrally attached to shaft |33. Spur gear |34 is rotated by idler gear |35 which turns on stud |36 which is integrally attached to plate 39. Idler gear |35 is, in turn, rotated by idler gear |31 which is free to rotate on stud |38 which is integrally attached to plate 39. Idler gear |31 is rotated whenever the rotor is turning by virtue of the fact that it meshes with gear teeth |39 cut on the inside face of flange 40 of cylindrical frame member 23.

output of the pump to the other of'said tubes |44. As indicated in Figure 1 each of these tubes |44 includes a flexible looped portion |44a, through which it is connected to a hydraulic cylinder |45, the three such cylinders being labelled |45a, |451), and |45c, respectively, in Figure 2. These cylinders constitute operating units peculiar to each blade. Each such hydraulic cylinder |45 contains a piston |41 backed up by a spring |46 against which the hydraulic pressure must press. Integrally connected to each piston |41 is a piston rod |48 which has a right angle bend at its outer extremity, this bent portion passing through slot |49 in pitch control bell-crank E3. Each piston |41 constitutes a pitch adjusting unit of the previously mentioned secondary pitch adjusting means, for displacement of a piston |41 will rock the associated bell-crank 63 and through it raise or lower the corresponding pitch control rod 6| to adjust the pitch setting of the associated one of the blades I0. The pumping of fluid into a cylinder |45 forces its piston |41 and piston rod |48 outward, which through bellcrank 63 forces pitch control rod 6| upward which increases the pitch of the corresponding blade and causes the blade to rise due to aero- Therefore increase in the volume of fluid in a cylinder |45 raises the respective blade and conversely a decrease in such volume lowers the blade.

Slide valve |20a (Fig. 2) is positioned in response to differences in the effective flapping angle of blades |0a and |0b and in turn regulates the flow to and from the hydraulic cylinders |45a and |45b which adjust the pitch of these two blades, while slide-valve |20b is similarly positioned by differences between blades |0b and |0c and similarly adjusts the pitch of these two blades to eliminate such differences, and slide-valve |200 is similarly positioned by differences between blades |0c and |0a and adjusts the pitch of these two blades to eliminate such differences.

Operation of the blade tracking mechanism is as follows: For the purpose of illustration assume that blades |012 and |0c are tracking, but that blade |0a is travelling higher than the other two. This will cause the lever 91 which is associated with blade |0a to rotate slightly counterclockwise (as viewed in Fig. l) about flapping hinge |1. This will cause vertical pin |06 on the corresponding slide |00 to move inward, carrying inward the lower end (as viewed in Fig. 2) of the lever ||3 which is associated with slide-valve |20a. This will cause slide-valve 20a to move inward in valve-sleeve |2|ia. The inward movement of said vertical pin |06 will however also cause counterclockwise rocking of the bell-crank |01 with which it is associated, thus through link |09 causing outward movement of the right end (as viewed in Fig. 2) of the lever |3 associated with slide-valve |20c, thus causing said slide-valve to move outward in valve sleeve |2 |c. The inward movement of slide-valve |20a will cause hydraulic fluid to be pumped from the cylinder |45a, which is associated with blade |0a, and into the cylinder |45b, which is associated with blade |011. This will lower the pitch on blade Illa and raise the pitch on blade |0b, thus causing decrease in the effective flapping angle of blade |0a and increase in that of blade |0b. The outward movement of slide-valve |20c will cause hydraulic fluid to be pumped from the cylinder |45a and into the cylinder |45c, which is associated with blade |0c. This will lower still further the pitch on blade |0a and raise the pitch on blade |0c thus further 9 lowering the path of blade Illa and raising that of blade |00. Thus blade Illa will be lowered at twice the rate. that blades ||lb and ||lc are raised. This process continues until all of the 4blades are tracking again, whereupon the centering of valves ld terminates the process.

Thus if any blade stands at a flapping angle different from the mean of the napping angles of all blades it will be caused to change its flapping angle inthe direction of that mean at a rate substantially proportional to the amount of its departure from such mean Value.

In view of the fact that the above described mechanism is operated in such a mannerr as to eliminate differences in eiective flapping positions of the blades it would', if it operated rapidly enough, seriously interfere with the cyclic differences in flapping position, which constitute the accepted method of exercising control of the craft through an articulated rotor. To avoid this, either the openings |53 (Fig- 2*) in all of the valves |2il, or the'xedparts with which they cooperate, or both, are made of very slight dimension in the directionperpendicular to the plane of Figure 2 andV they therefore serve to markedly throttle the rate of fluidflow, therebyserving as rate control means for the operating means. As shown in Figure 2 these openings |50 are of considerable width in the plane of Figure 2'.v By thus providing openings |50 which are of considerable width in the direction of travel of valves |26 (which is in the plane of Figure 2)- and of very slight eiiective dimension in the direction perpendicular to the plane of Figure'Z, themarked throttlin-g effect provided by valves |253 is selectively altered by displacement of a valve |20 so that the ow through the valve opening is approximately proportional to the displacement of the valve from the position at which it completely cuts oli the flow. This fact, combined with the fact that the Valve displacement is arranged to normally be proportional to the difference in eifective flapping angle, regulates the flow to an amount which is at least roughly proportional to such dierence, thus providing for correction of the majority of each tracking error in approximately a constant period of time regardless of the amount. of such error. With such an arrangement the functioning ofv this mechanism does not appreciably affect the amplitude of cyclicchanges in blade position due to the fact that such changes normally involve equal amounts of movement oi each blade above and below its mean napping position and therefore in passing from its upper to its lower extreme,y position or vice versa it will be subject to two. substantially equal and opposite increments of correction by the blade tracking mechanism, which increments will therefore substantially balance' each other. The action of the blade tracking` mechanism will have a tendency to advance all cyclic changes in blade path by a small and` substantially constant and predictable azimuth angle, which therefore. can be allowed for in orienting the cyclic control mechanism. It is therefore, practicable to construct the valves with openings su-iiiciently large to correct, for instance, approximately one-half of any given tracking error within a single-cycle of rotor rotation.

By constructing valves |211A so-as to provide. a

proportionately regulated throttlingpeiect on the hydraulic iiow the opposing requirements for promptly eliminating tracking errors and yet knot disturbing cyclic. blade; differences: are reconciled. Considerable latitude is possibleliin the degree of 10 such throttlingV that will give satisfactory results but there are rather` sharply dened limits to such latitude. For instance if the throttling limits the flow to a rate proportional' to the dif-` ferences in effective blade positions, and ifr the vertical dimension of openings |50 is such that the iiow during one cycle of normal rotor operation will eliminate half the diiierence in blade positions, then it will require 4.3 cycles to eliminate 95% of` the error or about onek and a fraction seconds, which is fully as short a time as willv be required for introducing anyv marked change in flight conditions. Therefore, the response is rapid enough toA provide substantially instantaneous correction oftracking errors.

The amountof correction that would be introduced in one quarter cycle isindicative of' the amount of'efiect' on cyclicqcontrol, and' this should be a small fraction of the total error, not more than one-third in any case and preferably less. Any more rapid introduction of correction than this would commence to introduce secondaryv irregularities of objectionable size, due' principally to the fact that the previously mentioned' tendency of the blade tracking'corrections to balance out i-n each half cycle of rotor operation is not perfectly attained if the amount off cyclic tilt of the rotor is not constant overthev halfcyc'le. For instance if the-amount, of cyclictilt is 5% greater or less during the lasthalf' of such a half cycle period than it is during the" iirst half of it, and if the correction effective yin a quarter cycle is 0.33 of the difference existing then the'secondary irregularity introduced would' be ofthe amount of cyclic differenceV ind flapping position of two adjacent-blades. This-fact limits the rate at w-h-ich correction' should be introduced to not more than 0.801of the tracking error'in one cycle, for this rate corresponds to correction of 0.33 of the tracking error in a quarter cycle, which as above indicated is substantially the maximum that can be tolerated. At this rate 95% of the tracking error would be eliminated'in 1.86 cycles, which is extremely prompt: correction.

|The lower limi-t on rate` of correction is, im.- posed by the fact thatv retracking should take place substantially as* rapidly as flight conditions can change. On this basisv not over 4 seconds should be allowed for 95% correction, which at 200 R. P. M. represents 13.3 cycles, which corresponds to 0.20of the tracking error corrected in one cycle. At this rate only 0.054V of' the error is corrected in one-quarter cycle, which would reduce the secondary irregularitiesabove mentioned to one-sixth of the limiting values described.

Hence satisfactory operation will be secured if the rate of correction is held substantially within the range of 20% to 80% of the error corrected in one cycle of normal rotor operation, but` departure much if any beyond this range oommences toY introduce either too slow a response or too marked'irregularities upon rapid change of cyclic tilt.

.As previously indicated the rate of correction maybe established at any selected value byappropria-tely choosingtheV vertical dimension of valve openings |50. In order to secure the; co1'- rection of a substantially constant fraction of any tracking error duringl each cycleit would be necessary for the remainder of the hydraulic system to. be lof su-iiciently large 'cross-section to produce negligible friction loss. However, the permissible latitude in retracking rates is' so 11 great that very considerable friction losses can be tolerated, so long as they are not so great as to cause the resultant retracking rate to fall substantially outside the limits hereinabove indicated as acceptable for blade differences within the range of such differences that may be due to tracking errors. For diierences in blade positions greater than those which may be due to tracking errors, and such greater diiferences will ordinarily occur cyclically in connection with rotor tilt, it will not only be permissible for the tracking correction per cycle to become less than 0.20 of the difference, but highly desirable for it to do so for large cyclic differences If the valve area is so related to the characteristics of the remainder of the hydraulic system that the amount of valve opening corresponding to the maximum difference in blade position caused by tracking errors permits almost as much hydraulic flow as the amount of flow that would pass through the system with the valve entirely removed, then the maximum rate of retracking for any amount of blade difference will be only slightly greater than that required for proper correction of maximum tracking errors. However, to provide greater latitude in the design of the system, and particularly in order to permit of faster correction of maximum tracking errors combined with sharper limitation on the maximum rate of retracking I prefer to provide the limit stops |22, previously described, arranged to limit the movement of valve |20 in each direction, and hence prevent any further increase in the rate of retracking, after the valve has been displaced from neutral by at least the distance corresponding to the maximum blade difference due to tracking error. In order that tracking errors combined with large cyclic blade differences may produce selective positioning of the valves adapted to reduce the tracking errors throughout a considerable part of the cycle, and in view of the fact that such selective positioning ceases while the valve is held against its limit stop, I prefer to place the limit stops so that they do not become effective until the valve has been displaced considerably beyond the position corresponding to the maximum tracking error itself, and to have the friction losses in the system such that they gradually reduce the percentage of error corrected per cycle for valve displacements intermediate between those corresponding to maximum tracking error and those at which the stops |22 are encountered.

With the blade tracking mechanism constructed in the particular manner which has been illustrated and above described, a difference in effective flapping angle of any two blades automatically initiates a readjustment of pitch on both of said blades in a manner tending to eliminate the difference. It is not essential, however, that pitch be readjusted on both blades for the difference could be eliminated by applying the correction to either of the two blades. For instance the valve |200 is positioned in accordance wlth the difference between the napping angles of blades Illa and |c. Through two openings in the valve sleeve |2|c it is connected to tube |5| which connects it to the pitch adjusting mechanism of blade |Ua, which is the leading one of these two blades, while through two other openings in the valve sleeve |2|c it is connected to tube |52 which connects it to the pitch adjusting mechanism of blade Ic, which is the following one of these two blades. As previously described, if blade Illa is positioned at a higher flapping angle than blade Ille, hydraulic fluid 12 pumped through tube |5| operates to decrease the pitch on blade la, and that pumped through tube |52 operates to increase the pitch on blade |00, until the diierence in flapping angle is eliminated. However, if tube |5| and its openings through sleeve |2|c were removed the difference would still be eliminated, for the iluid pumped through tube |52 would still operate to increase the pitch on blade lOc until the difference disappears. Thus if the three tubes which lead forward from the three valves and correspond to tube |5| were thus eliminated, or flow through them was permanently cut off, the blade tracking system would operate by bringing each blade to automatically track the blade ahead of it. If instead of eliminating the tubes corresponding to tube |5| the three corresponding to tube |52 were eliminated or plugged, the blade tracking system would operate by bringing each blade to automatically track the blade next behind it. However, by including vboth sets of tubes appreciably prompter and more satisfactory tracking can be secured. For instance if as previously assumed for illustration blades |0b and |60 are tracking but blade |0a is travelling higher, then with tubes |5| eliminated valve |20a acts to lower blade Illav to eliminate the difference between blades lila and Ib, while valve |20c acts to raise blade Ic to eliminate the difference between blades |0a and |Dc. Since there is initially no difference between blades |0b and IOc, valve |20b does not initially alter the pitch of blade Ilb, but as blade |00 is raised a difference between the positions of blades |01) and |0c develops and serves to raise blade lb at a rate proportional to the amount it remains below blade |0c which would be at a slower rate than if it were also controlled by the difference between blades |0a and |017, as it will be in case all the tubes illustrated in Figure 2 are functioning.

What I claim is:

1. In an aircraft having a sustaining rotor including a hub and a plurality of blades each attached to said hub by means permitting of change in the angle subtended between the blade axis and the rotor axis and means permitting of change in the pitch angle', and primary pitch adjusting means connected to each of said blades; blade tracking means independent of said primary pitch adjusting means and comprising secondary pitch adjusting means, pitch controlling means for each of said blades comprising a member jointly positioned by said primary and said secondary pitch adjusting means, a member connecting each of said pitch controlling means to its respective blade so as to angularly position the blade in pitch, rate control means independent of said primary pitch adjusting means for controlling the rate of operation of said secondary pitch adjusting means, and including an adjustable element and connections from said blades to said adjustable element to position said adjustable element in accordance with differences in the angles subtended between the respective blades and the rotor axis.

2. The invention set forth in claim l in which the secondary pitch adjusting means comprises a plurality of pitch adjusting units each connected to a respective one of said blades to control the pitch angle thereof; said rate control means comprises a plurality of rate control units each including an element selectively positionable to control the rate of operation of one of said pitch adjusting units; andsaid connections include a plurality of members each connected to one' of "said elements and toja plurality of blades including the' blade' to which the respective' pitch adjusting unit is related. t

3. The* invention set forth in claim l, `in which said blades 'are' flexible', in combination with a member in' conjunction with each blade; Yeach of said members Vbeing mounted on the hub'for movement relative thereto' in response to vertical ilexing ofthe blade, a part integrally attached to theb'lade' inthe' outer half of the' length there` of, and a flexible link connected to said part and to said member and extending inward along the blade and guided'relative thereto so' as to impart Amovement to saidv member upon vertical flexing of said' blade, and in which said positioningl of said adjustable element of. said rate control means by said blades is eiiected through said members. l l

4. The invention set forth' in claim l, in which said means permitting of change in the anglesubtended between the blade axis andthe rotor` axis comprises a flapping hinge and a. flexible root portion of said blade, in combination with-a member in conjunction with each blade, each of said members being mounted on the hub for movement relative thereto in joint response to`V the flexing of the blade andthe displacement of the blade upon' its hinge relative to the hub, a part integrally attached to the bla'dein theouter-hali thereof and a flexible link connected to said part and to said member and extending inward along the blade and guided relative thereto so as to impart movement to saidmernber upon vertical'flexing and/or'displacement of 'said blade relative to said hub, a connectionr from said flexible link to said member along a line removed from the axis of said flapping hinge, and inA which said'positioning of said adjustable element of said rate control means by said bladesis eiiected' through said members.

5. In an aircraft having a sustaining rotor including a hub and a plurality of blades each attached to said hub by means permitting ol change inthe flapping angle and'l by means permitting or change in the pitch angle of the blade; pitch adjusting means for each of said blades comprising a, cyclic pitch control member, manually positionable means mechanicallj7 control'- ling the position of said member, a pitchadjusting member., means for automatically positioning said member under joint control of two of said blades in accordance with the respective napping 'anglesV of said two blades,Y a pitch setting'mem'- ber connected to said blade at a point remote from the blade axis so as to determinev the angular position of said blade about said axis, and positioning means for said pitch setting member jointly positioned by said cyclic pitch control member and said pitch 4adjusting member and consisting of mechanical linkage extending .from said cyclic pitch control member and said pitch adjusting member to said pitch setting member.

6,. The invention set forth inwclaim 5 wherein the means for automatically positioningthe vpitch radjusting member comprises 'rate control means selectively settable to corresjxondinglyV determine the rate o such positioning, an element, connections 'from said element to 'each of two' of said blades r'to position said element in accordance with the difference between the effective ilapping angles of said two blades, and means connecting said element to said rate control means to set said rate control means in accordance with the position of said element.

7. The invention set forth in claim 6 in com- 14 bination with a surface connected to saidrate control means and la `cooperating surface fixed in the path or movement of said rst mentioned surface, so as to'block said surf-ace and thereby limit the range of positions to which said rate controll means may be set by said element.

8; in anairc'raft having a sustaining rotor including a hub and a plurality offlexibleblades attached to said hub bymeans including a pitch changing hinge constituting a pivotal support having an axis longitudinal of the blade; blade tracking-means including a pitch adjusting memberrconnected to one of said blades at -a point remote fromv said Ypitch changing hinge so-asto selectively alter the pitch thereof in laccordance with the displacement of said pitch adjusting member, means for automatically displacing said pitch adjusting member under joint control of two ofA said blades in accordance with the difierence in the vertical-flexural positionl ofsaid two blades including two parts each associated with a respective one of said two blades and integrally attached Ato the blade in the outer half lof the length thereof, two flexible links each associated with a respective one of said two blades and connected to the respective' one of said two parts and extending inward along the blade and guided relative thereto along a path vertically displaced from the neutral axis or the blade, a lever connected to each of said two links and jointly positioned by them, operating means for applying power from said source tosaid pitch adjusting member to displace same, adjusting means for selectivelyadjusting said operating means tocontrolY the direction andv rate at which said power is supplied, and a connection from said lever to said adjusting means to adjust` said operating means in joint response to the ilexing of `saidtwo blades.

t 9. Inl `an aircraft having aV sustaining rotor including a hub and a plurality of ilexibleblades each attached to said hub by means compri-sing a flapping hinge and a pivotal mounting substa-ntially parallel to the axis of the blade;-blade tracking means including a pitch adjusting member connected to one of said blades at a poi-nt remote from said pivotal mounting so as to angularly displace said blade von Vits respective pivotal mounting in accorda-ncefwi-th the displacement of said pitch adjusting member, means for automatically displacing said pitch adjusting member under join-t `control oftvv'o-of said blades in accordance with the difference inf'their effective vertical positions as determined -bytheir respective angular displacements about their respective flapping hinges plus their respective verlticalflexura-l displacement, including two parts each 1associated with a respectiveonefof said two blades and integrally attached itc, ther-'blade in vtheeu'ter half 1of the length thereof, ytwo-ficxible -linksfeac'h attached toa respective 4one of said parts uand extending 'inw-ard along the blade and-guided'relative "thereto along a 'rpath verticallyY displaced from the neutral axis-of the blade, a lever, la connection from-each of said links tosaid lever, each of 'said connections being vertically displaced-'from the respective napping hinges, `a source of power, operating means for uapplying lpower'fronrsaid source 'to said pitch adjusting member tri/displace same, Vadjusting"means for selectively adjusting said operating 'means to control the direction and rate at which said power is supplied, yand a connection from said lever to said adjusting means to adjust said operating means in joint response to the flexing of said two blades.

10. In an aircraft having a sustaining rotor including a hub and a plurality of blades each attached to said hub by means permitting of change in the flapping angle of the blade and means permitting of change in the pitch angle of the blade; blade tracking means comprising a hydraulic pump, a hydraulic cylinder having a piston, a hydraulic connection from the pump to the cylinder to operate the piston, a valve interposed in said connection between said pump and said cylinder for controlling the operation of said piston by said pump, linkage connecting said valve to two of said blades to position said valve in accordance with the difference of the flapping angles of said two blades, a, manually positionable pitch controlling element, a pitch controlling member connected to said element and said piston so as to be jointly positioned by said element and by said piston, and mechanical linkage connecting said pitch controlling member to one of said two blades to establish the pitch angle thereof.

11. The invention set forth in claim 10 in in which said valve is provided with ports which are of slight dimension in the direction perpendicular to the travel of the valve and to the direction of flow of the fluid through the valve as compared to their dimension in the direction of travel of the valve.

12. In an aircraft having a sustaining rotor including a hub and a'plurality of blades each attached to said hub by means permitting of change in the flapping angle of the blade and a pivotal mounting substantially parallel to the axis of the blade; blade tracking means including a plurality of pitch adjusting members each connected to a respective one of said blades so as to angularly position said blade on its respective pivotal mounting, a plurality of controlling members mounted on the hub for individual displacement relative thereto, linkage connecting each of said blades to two adjacent ones of said controlling members and thereby connecting each controlling member to two adjacent blades to automatically displace said controlling member relative to said hub in accordance with the difference between the flapping angles of said two blades, an operating connection to each pitch adjusting member from the two controlling members which are connected to the blade to which said pitch adjusting member is connected for displacing said pitch adjusting member.

13. In an aircraft having a frame and a sustaining rotor including a hub rotatable with respect to said frame and a. plurality of blades each attached to said hub by means permitting changes in the flapping angle and pitch thereof; pitch changing mechanisms comprising a pitch control spider tiltably supported with respect to its center, a plurality of bell-crank levers, a fulcrum on said spider for each of said bell-crank levers, each fulcrum mounting its respective lever for rocking movement in a vertical plane and means connecting an arm of each of said levers with one of said blades for changing the pitch of said connected blade upon displacement of said fulcrum in a vertical plane or upon rocking of said spider upon its fulcrum; separate primary and secondary adjusting means connected with said pitch changing mechanism, said primary adjusting means comprising a train of mechanism 16 carried by said frame and connected to said spider to tilt same; and said secondary adjusting means comprising a plurality of hydraulic cylinders mounted on said spider, a, piston in each of said cylinders connected with a second arm of a respective one of said bell-crank levers; and a control system for said secondary adjusting means comprising a source of fluid pressure,

a connection between each of said cylinders and said source, valve means including an adjustable element interposed in each of said connections, a lever fulcrumed on each of said adjustable elements, means including a connection between one of said blades and a point on one of said levers spaced from the fulcrum thereof for adjusting said element in one direction upon upward flapping movement of one of said blades, and means including a connection between another of said blades and a point on one of said levers oppositely spaced from the fulcrum thereof for adjusting said element in the opposite direction upon upward flapping movement of said other of said blades.

14. The invention set forth in claim 13 in which said valve means includes a throttling passage partially opened in selective amounts by the displacement of said adjustable element whereby the amount of fluid supplied to said cylinders by said source of fluid pressure is proportioned to the difference between the flapping angles of ythe blades connected to said element.

l5. In an aircraft having a frame and a sustaining rotor including a hub rotatable with respect to said frame and a plurality of blades each attached to said hub by means permitting changes in the flapping angle and pitch thereof; pitch changing mechanisms comprising a pitch control spider, a plurality of devices mounted on said spider and movable either by said spider or relatively thereto in a substantially vertical direction, and means connecting each of said devices with one of said blades for changing the pitch of the connected blade upon either such movement of said device; separate primary and secondary adjusting means connected with said pitch changing mechanisms, said primary adjusting means comprising manually operable means for moving said spider and the said devices mounted thereon in a substantially vertical direction, and said secondary adjusting means comprising means carried by said hub and responsive to said blades upon changes in the flapping angles thereof for moving said devices relatively to said spider in a substantially vertical direction.

HAROLD T. AVERY.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,800,470 Oehmichen Apr. 14, 1931 2,356,692 Platt Aug. 22, 1944 2,397,154 Platt Mar. 26, 1946 2,397,489 Jenkins Apr. 2, 1946 2,408,489 Stalker Oct. 1, 1946 2,439,089 Hodson Apr. 6, 1948 2,444,070 Stanley June 29, 1948