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_ March 22, 1938. A. E. LARSEN ET AL ‘ 2,111,975 ‘ ROTATIVB WINGED AIRCRAFT Filed Dec. v19, 1935 :5 Sheéts-Sheet. 1‘ ATroR'NEYJ " ‘ March 22, 1938‘. _ ’ I I . ‘ A_ E_ LARSEN ET AL ' ROTATIVE WINGED ' 2,111,975 AIRCRAFT Filed Dec‘. 19, 1955 3 sheets-sheet '2 mcsua _ 10 go’ 30 40' 5o 60 % BLADE’ LENGTH 70 - INVENTORS >60 90 511100 a“? 51W Wv?ééw ATTORNEYS - March 22, '1938. A. E.- LARSEN ET. AL ‘ ROTATIVE WINGED AIRCRAFT Filed ‘Dec. 19, 1935 P 2,111,975 I s Sheets-Sheet z INVENTORS ’ W44’ - # WT‘MM ATTORNEYS Patented Mar. 22, 1938 , ‘ 2,111,915 ’ UNITED STATES ‘PATENT-v orrice . 2,111,915 , ROTATIVEf-WINGED AIRCRAFT Agnew E. Larsen, Huntingdon Valley, aiulPaul H. Stanley, Glenside, Pa., assignors to ‘Auto giro Company of America, Willow Grove, Pa", ' . i a corporation of Delaware * ~_ _ Application December 19, 1935', Serial No. 55,142 20 Claims. , (01. 244-18) This invention‘ relates to aircraft having ro tative sustaining blades or wings, and is especially concerned with that type of. craft in which a sustaining rotor is employed incorporating a plurality of blades which are pivotally connected with a common generally upright axis mecha nism. ' . characterized by a novel incidence relation be tween inboard and outboard portions thereof. Special arrangements in the structure, such as features of the spar, nose, ribs, ballast weights, etc., are also of importance in the working out 5 of the primary objects, as will appear more‘ fully Y hereinafter. I One of the major objects of the invention is to increase the e?lciency of craft of this type, more ' - - , How the foregoing, together with. other objects and advantages are" attained, vwillappear more fully in the following description’ referring to the 10 The invention also overcomes a diving tendency accompanying drawings, in‘w'hich Figure 1‘ is a plan view of the structural mem sometimes experienced at high forward speeds. I 10 particularly to increase the rotor e?iciency. ' Still further, it is an object of the inventio'n‘to provide a rotor the blades of which are of reduced :15 .weight as compared with prior constructions and which are further of simpli?ed construction. The foregoing important and more or less gen eral objects are served by various features of con-‘ bers of a blade according tov the invention, the outer covering being removed; Figure 2 is an enlarged sectional view of the 15 blade taken substantially as indicated by the sec tion line 2—-2 on Figure 1; - \ Figure 3 is a top plan view of one of the ribs ' employed in building up the blade; - struction such as the following: ' The invention contemplates a blade of novel 20 form, more speci?cally, the improved blade has a thickness ratio varying by steps which is con veniently obtained with maintenance of uniform chord, thus without necessarily reducing the‘ '25 solidity of the rotor. According to the invention, Y ‘ Figure 4 is a partial sectional view takenas in- ' dicated bythe section line 4—4 ‘on Figure l and illustrating a ballast weight and the mounting therefor; v Figure 5 is a fragmentary top view of structural members of the blade, this view also showing the 25 ballast weight; ‘ ' the blade is composed of several portions, pref Figure 6 is a view containing a graph and a erably three, each one of which is of substantially uniform thickness but the several portions, re-_ blade'in outline and illustrating a number of fea spectively, being of di?erent thicknesses, an out: ‘ . tures discussed fully hereinafteryand Figure 7 is‘ a view of the blade section (in out 30 board portion,‘ for example, being of reduced thickness ascompared with the adjacent inboard portion. v Similarly, the invention contemplates the em ployment of a spar of step—down construction, 35 blade, particularly illustrating the relation of incidence settings of di?erent portions of _ the blades. _ - As mentioned at the outset, the invention is 35 the several portions of the spar preferably being substantially coextensive with the portions'of the concerned with a blade for an aircraft sustaining blade mentioned above as being of di?erent thick rotor of the type incorporating an axis mecha nesses. ' l' > ' In addition to the above, the construction of 40 this invention involves a blade of substantially ?xed center of pressure (preferably well forward or at about 25% of the chord from the leading edge) for all normal angles of attack above a 30 line) at various points along the length of the nism, ‘thisbeing shown at ‘i in Figure l, to which a plurality of blades are pivoted. The blade ap pearing in Figure 1 is secured to the axis or hub 40 ‘I by means of a “?apping” articulation 8, exten sion link 9 and “drag" articulation Ill. The blade is vthus free to "?ap” in a direction generally - predetermined minimum, for example, 2° positive. . transverse the mean rotative path of travel and The blade section employed in accordance with this invention further has a rearward travel of the center of pressure below said predetermined minimum; This type of blade we have foundto provide very material improvement in smooth; '50 ness of operation, especially when maintaining the sectional center of gravity substantially co incident with or slightly ahead of the sectional center of pressure, this latter ‘also constituting an important feature of ' the invention. 55 The improved blade herein disclosed is‘; also also to move fore and aft substantially within 45. the path of travel. The articulations, of course, serve to interconnect the hub member and the main structural element of the-blade which takes the form of a spar ll. At the inner end of the spar a fitting l2 may be provided, this ?tting 60 having fork or equivalent parts adapted to co operate with the pivot Hi. The fitting I 2 and also an additional sleeve B at the inner end of the spar serve, to increase rigidity toward the root end of the blade, this being of importance in the 55 2 ‘2,111,975 , ' . ‘ cantilever’ support of the blade and for other , weight itself, and a web ll, lying in the plane reasons. In general,‘ moreover, it might be said . of the blade, is welded to the sleeves 30. that because of bending and tension loads at the root, greater strength and related characteristics are required toward the root end, whereas aero dynamic efficiency, because of the high speed, This construction provides a rigid mounting for the weights 28 which ishighly effective as against the action of centrifugal force, in spite of the fact that the weight is. spaced a substantial distance is of greatest importance toward the tip of the ‘ from the spar. blade. Numerous features of the present inven From Figures 1 and 6 it will be seen that the -tion carry out the relationships just mentioned weights 28 are all located in the outer half of the blade. It is the-outboard portion of the blade 10 10 more completely than has been possible hereto fore. ' Turning again to the spar, attention is called to the fact that the inboard portion of the spar designated by the arrows lid, is of substantially 15 uniform cross-section or diameter but ‘larger reduced size. and I I0. Before considering the effect of these weights and also of various other factors already men tioned, reference is now made to the incidence setting of the blade. As will best be understood 20' from examination of Figures 6 and 7, the blade Sections lib and lie are also respec is~of smaller diameter than portion Ilb, so that 20 the spar is of stepped construction. The spar, moreover, is preferably composed of a single drawn tube and has short tapered sections l4 and I5 joining sections lia-—Ilb, and Ilb-llc, respectively.' At the outer end of the section Ilc 25 an extension i6 is provided, and ‘this extension carries a tip part I‘! shaped to ‘conform with the point about 85% of the blade length from the root. In consequence, the tip has 1%° lower in erably, although not necessarily, made of wood, 55 60 ' 65 76 out from the root end thereof outwardly toward the tip. This primary washout, if carried to the group being apertured to engage the spar in the The attachment of the ribs to the spar may be accomplished (see ?ange 2| with extended lips 22 which may de 35 sirably be welded to the spar. The ribs are pref 50 has a progressive decrease in incidence or wash desired tip-taper. The spar carries ribs I811, I82) and l8c, each ‘ Figures 2 and 3) by means of a ?tting l9 riveted to the rib as at 20, this ?tting having a sleeve or 45 - extreme tip of the blade, would give the blade 25 about 74° lower incidence at the tip than at the root. An additional washout of 1°, however, is superimposed on the primary washout, this ad 30 respective sections thereof, ' outer portion of the blade, does not impose the total centrifugal load thereof on the outermost section of the spar which, as already noted, is of than the intermediate and outboard sections I lb tively of uniform diameter, although portion “0 '40 that requires ballasting, but the distribution dis closed, while concentrating the weight in the ditional decrease in incidence commencing at a Rfl cidence than the root. As indicated in the graph of Figure 6, the blade is set on the hub with its ' root end at an incidence of approximately 41/2’ above zero lift, and in view of this, the tip set ting approximates 2%° above zero lift. The in and the same applies to the nose strip 23 which . cidence settings just mentioned (see also Figure in accordance with the present invention is of '7) are the positions of the blade section (at fairly large section, with two purposes in view: different points along the blade) measured from ?rst, an increase in rigidity of the blade in the that position which, with air?ow perpendicular plane or path of rotation and, second, location to the axis of rotation, would give zero lift. As of the center of gravity well forward. This seen in Figure 7, with a blade section of the type latter object is also served by location of the spar shown, the chord line of the blade in zero lift position (represented by line z—z) is slightly well forward in the blade section. More spe ci?cally, it will be noted that the center of the negative with respect to the plane a:—.1: perpen spar is located less than 25% of the chord from dicular to the axis of rotation r-r, the negative the leading edge. These features of construction, angle being about 1.2" as shown. together with fairly robust ribs, particularly inv Attention is now directed to the fact that in the trailing edge portions thereof, may afford Figure 7 the full line blade outline A is taken at sufiicient rigidity in the path of rotation even a point toward the inner end of the blade and without the necessity of employng a supple-' that the chord line H of this section is posi mentary spar or stringer which heretofore has tioned at ‘it/2° positive angle with reference to been located intermediate the main spar and the the line 22-22 representing the zero lift position. trailing edge. The rear ends of the ribs are em Figure 7, it will be noted, also shows two addi braced by a trailing edge stringer 24 which may tional blade sections B and C in dot-and-dash be applied in'the form'of a trough or V and then outline. Sections B and C both have substan pinched together at its front edges as at 25. The tially the same chord as section A, although sec nose of‘ the wing is preferably also surrounded tion B is of reduced thickness as compared with by a plywood covering 26 which, as clearly seen section A and section C, in turn, is of reduced in Figure 2, is recessed into the edges of the ribs. thickness as compared with section B. The This nose covering also adds considerable rigidity points at which these three sections are taken in the path of rotation. is also indicated by corresponding letters on the The blade, of course, is covered with a fabric blade appearing in Figure 6, and from this lat 21 which may be suitably doped. ter view it will be seen that section A is taken As seen in Figure 1, the blade further in in the region where the spar has largest diame corporates a plurality of ballast weights 28, one ter, section B in the region where the spar has of which appears to best advantage in Figures 4 intermediate diameter, and section C in the outer and 5. This weight, it will be observed, is dis section of the blade where the spar is of smallest posed well forward in the nose of the blade and diameter. _ is preferably supported on the spar by means of Turning again to Figure 7, it will be noted that a pair of bolts 29 which pierce the spar hori the chord line b-b of the section B is positioned zontally at the neutral axis and thus result in at a 4° positive angle with reference to line z-z, minimum weakening thereof. A pair of spacer and from comparison with Figure 6 it will be sleeves 30 are disposed between the spar and the seen that section line b-b is positioned at the 40 60 55 60 65 a. 75 2,111,975 r 3 \ point where the graph shows 9. 4° incidence. above zero lift position. The expression “angle Similarly, the chord line 0-0 of section C is of attac " is'herein usedto de?ne the angle be tween the chord line and the line of air?ow at any given instant. For various reasons (notably positioned at about a 314° angle with reference to line e--z, as also appears from the graph of Figure 6. 1 ' the effect of .blade ?apping and attitude and di From the foregoing it will be seen that al though the same chord dimension is maintained rection of ?ight of the craft), the angle of attack is almost continuously ?uctuating and may as substantially throughout the entire length of the sume a value which is eithergreater or less than blade, the thickness ratio of the blade is varied, 10 with the section. of least thickness at the tip and the section of greatest thickness at the root. Ac-' cording to the invention, the thickness of section A is maintained substantially uniform through out the portion of the blade which is coextensive with the inner spar section Ila. The section B, of intermediate thickness, is maintained through out the portion of the blade corresponding to section llb of the spar, and the section C, of minimum thickness, is maintained throughout 20 the portion of the blade corresponding to the out the incidence setting. ‘ ‘ Reverting now to the characteristics of the blade section employed, below approximately 2° positive, the center of pressure of the section adopted moves rearwardly from the 25% position quite rapidly. . ' V 1 ‘ The reduction in the incidence setting- of the 16 blade toward thetip, as herein contemplated, has been found to materially reduce vertical ?uctuation in forces or "bouncing" transmitted 1 from the rotor to the‘ machine. ‘By the differ ence in incidence setting, a more‘ uniform lift distribution is applied to the blade throughout is of stepped thickness, the portion ‘of greatest its cycle, and, in consequence, bending moments thickness being located inboard and the portion in the blade are’reduced. Reduction in bending of least thickness toward the tip, as is desirable moments in-the blade is of importance since this reduces ?uctuation i'n lift delivered to the'hub. 25 from the standpoint of structural and vaerody namic emciency of the rotor. The present Furthermore, under certain conditions of ?ight, the advancing blade of a rotor having ?apping stepped construction, in contrast with a gradu ally tapering shape, is of substantial advantage blades, will, through a restricted range of posi-_ tions, be operating at a low angle of attack.- A ‘since, with the varying oblique flow of air en '30 countered during rotation of the rotor, the effec negative torsional moment would normally occur 20- ‘ boa'rd portion of the spar llc. Thus, the blade 30' tive blade section is distorted to a much smaller as a result of this in a blade having an aerofoil degree than with prior forms of blades. The only points of material distortion, in fact, are those at section of the general character used in accord- - the junction points between adjacent portions of‘ 35 the blade, at which points the blade tapers rather sharply from the. larger thickness‘ to the next smaller thickness. This tapering at the junction points is substantially coextensive with the tapered portions l4 and iii of the spar. ' The step-down blade construction, while main taining uniform chord dimension, is of further ‘ advantage since it provides a material and highly desired reduction in drag in the outboard portion of the blade without a noticeable reduction in lift 45 and further without reducing the solidity of the ance with the present invention. However, the reduction in blade incidence toward the tip has an advantageous eiiect in minimizing this tor 35 sional moment and thus reducing any diving tendency produced by the moment. The location of the sectional center of gravity ' coincident with or slightly ahead of the center of pressure of the blade, as herein contemplated, 40' is also of importance, and with a blade of the type herein shown, the ballast weights aid in pro viding the relation of center of gravity to center of pressure just referred to. The relation of the C. G. and C. P. overcomes detrimental cumula 45 tive action producing torsional de?ection of the I rotor, i. e., the ratio of the total blade area to the disk area swept by the blades. Maintenance of . blade and causing undesirable loads on the ma the‘ solidity is of advantage chie?y .in retaining the desired vlow ?ight speed characteristics of 50 the rotor. chine under the effect of vertical accelerations. due to maneuvers or “bumps”. ‘ ' 50 Another advantage of- the blade structure here dimension from the leading edge. This condi in disclosed, particularly the ‘employment of a ' rather robust leading edgestrip, a plywood cov ering for‘ the nose part of the wing, and other features increasing the stiffness of the'blade in the plane of rotation, is that the increase in 55 ‘ stiffness in the plane mentioned increases the v‘natural vibration frequency in this plane and , tion is substantially maintained throughout the this, in tum,- reduces‘ tendency toward'grou'nd In considering some further features of the in-, vention, it should‘be noted that the section em- ployed in all portions of the blade is preferably ‘one of several known forms having a substan as tially .?xed center of pressure located well for wardly, for'example, at about 25% of the chord ‘several steps of the blade, although the thickness 60 ratio is slightly modi?ed. ' By way oi’ example, the thickness ratio in the innermost portion of the blade may be about Hill/2%, in the interme diate portion about 111_/2%,jand in the outboard portion about 10%. . ~ - With- further reference to the blade section, resonance in the rotor. ' V . ‘ The step-down construction of the blade and 60 other features thereof which reduce theneces sity for employing a, secondary spar as herein before referred to, is of substantial advantage in simplifying construction. Note particularly that with the step-down construction, the number of while the center of pressure as above indicated . different types of ribs is reduced, since all ribs in - is substantially ?xed, this condition is maintained the region of each of the spar sections Ila, llb in a highly e?icient section only above a prede and llc may be of one type, both as to external termined minimum aerodynamic angle of~attack. This minimum, according, to the present‘ inven tion, preferably approximates 2° positive. It is here to be noted that the expression “angle of attac " is not to be confused withthe .“inci ‘ dence setting" of the blade which in the present 75 description refers- to'the angle of the chord line I contour as-well as to diameter of spar receiving aperture. ' ‘ ‘ _ ' ’ 1n summarizing the advantages, it should be noted‘ that various features of the structure, in cluding the step-down spar, the reduction ‘in thickness ratio of the blade, the C. G. and C. P. relation, and incidence settings are of ‘importance 75 4 2,111,975 V in reducing weight while providing adequate inboard end of the blade. ployed, improving the smoothness of rotor op 8. For an aircraft sustaining rotor, a generally upright axis mechanism and a sustaining blade secured to the axis mechanism with freedom for movement in a direction generally transverse the mean rotative path of travel of the blade, said eration, reducing the tendency to set up torsional or other vibrations due to periodic ?uctuation in the position of the center of pressure, and sub stantially eliminating “bouncing" of the rotor. We claim: 1. A blade for an aircraft sustaining rotor, an 10 inboard portion thereof being of substantially blade having an airfoil section with a substan tially ?xed center of pressure throughout a range of aerodynamic angles of attack above a prede 10 termined angle, the blade setcion further having form thickness, the two portions being joined by a angles of attack below said predetermined angle, ' 2. A blade for an aircraft sustaining rotor, said blade having a spar, an inboard portion of which is of substantially uniform cross-section and an outboard portion of which has a smaller but sub stantially uniform cross-section, and means de -20 ?ning the blade surface, an inboard portion thereof being of substantially uniform thickness, and an outboard portion thereof being of smaller but substantially uniform thickness, the two por tions being joined by a relatively short tapered 25 _ uniform thickness, and an outboard portion thereof being of smaller but substantially uni relatively short tapered portion. 15 providing relatively positive incidence toward the strength, even where ballast weights are em portion. 3. A blade for an aircraft sustaining rotor, an inboard portion thereof being of substantially uniform thickness, and an outboard portion thereof being of smaller but substantially uni '30 form thickness, the two portions being joined by a relatively short tapered portion, and the two portions further having substantially the same chord. 4. A blade for an aircraft sustaining rotor, an v35 inboard portion thereof being of substantially uniform thickness, and an outboard portion thereof being of smaller but substantially uni form thickness, the two portions being joined by a relatively short tapered portion, and said two 40 portions further having a gradual or progressive incidence washout providing relatively positive ‘incidence toward the inboard end of the blade. 5. A blade for an aircraft sustaining rotor, an inboard portion thereof being of substantially 45 uniform thickness, and an outboard portion thereof being of smaller but- substantially uni form thickness, the inboard and outboard por tions both having a substantially ?xed sectional center of pressure at about 25% of the chord from the leading edge throughout at least a major part of the normal range of angles of attack. 6. A blade for an aircraft sustaining rotor, an inboard portion thereof being of substantiallyuni form thickness, and an outboard portion thereof 55 being of smaller but substantially uniform thick ness, the inboard and outboard portions both hav ing a substantially ?xed sectional center of pres sure at about 25% of the chord from the leading edge throughout at least a major part of the nor 80 mal range’of angles of attack, and each portion further having its sectional center of gravity sub-' stantially coincident with the center of pressure. '7. A blade for an aircraft sustaining rotor, an inboard portion thereof being of substantially 66 uniform thickness, and an outboard portion thereof being of smaller but substantially uni a rearward travel of'the center of pressure at an outboard portion of the blade being set at a positive incidence with respect to its zero lift 15 position, which incidence, however, is such that when the blade is on the advancing side of the rotor during high speed forward ?ight, the aero dynamic angle of attack of the outboard portion is below said predetermined angle, and an in 20 board portion of the blade being set at a positive incidence with respect to its zero liftposition, which incidence is sufficiently above said prede termined angle to maintain an angle of attack above the predetermined angle when the blade is 25 on the advancing side of the rotor during high ' speed forward ?ight. 9. For an aircraft sustaining rotor, a generally upright axis mechanism and a sustaining blade secured tolthe axis mechanism with freedom for 30 movement in a direction generally transverse the mean rotative path of travel of the blade,'said blade progressively decreasing in incidence from a point toward the root end thereof outwardly substantially to the tip, the decrease in incidence in about the outer 15% of the blade length be ing substantially uniform per unit of length and being greater ‘per unit of lengthv than the de crease in the inner 85% of the blade length, and the decrease of incidence in the inner 85% of the 40 blade length also being substantially uniform per unit of length of the blade. 10. For an aircraft sustaining rotor, a gener ally upright axis mechanism and a sustaining blade secured to the axis mechanism with free 45 dom for movement in a direction generally trans verse the mean rotative path of travel of the bIadeLsaid blade progressively decreasing in in cidence from a point toward the root end thereof outwardly substantially to the tip, 'the decrease in 50 incidence in about the outer 15% of the blade length being substantially uniform per unit of length and being greater per unit of length than the decrease in the inner 85% of the blade length, . and the decrease of incidence in the inner 85% 55 of the blade length also being substantially uni form per unit of length of the blade, the total decrease in incidence from the root to the tip of the blade being approximately 1%°, the blade being mounted on the axis mechanism with its 60 root end portion set at approximately ‘ll/2° posi tive lift incidence with respect to its zero lift position and the section of the blade substan tially throughout the ‘length thereof having a substantially ?xed center ‘of pressure at angles of attack above approximately 2‘? and further 05' having a rearward travel of the center of pres form thickness, the inboard and outboardpor tions both having a substantially ?xed sectional sure at angles of attack below approximately 2°. 11. A blade for an aircraft sustaining rotor, center of pressure at about 25% of the chord from 70 the leading edge throughout at least a major part said blade incorporating a spar, an inboard por 70 of the normal range of angles of attack, each tion of which is of substantially uniform section portion further having its sectional center of and an outboard portion of which is of smaller gravity substantially coincident with the center of _ but substantially uniform section, means carried pressure, and said two portions still further hav on said sparv and de?ning the surface or form 75 ing a gradual or progressive incidence washout of the blade. said means being of substantially 75 2,111,976 uniform thickness throughout a portion of the blade substantially coextensive with said in board portion of the spar and further being of reduced but substantially uniform thickness w . , 5 said portions further being- of substantially uni form thickness throughout its length and an out board portion being of reduced thickness as com pared with an inboard portion, and a ballast throughout’ a portion of the blade substantially . weight in the blade toward its leading edge in coextensive with said outboard portion of the spar. . I the- outboard portion thereof. 16. Ablade for an aircraft sustaining rotor, 12. A blade for an aircraft sustaining rotor, said blade incorporating ‘a spar and means de ?ning the blade shape, substantially coextensive said blade incorporating a spar, an inboard por 10 tion of which is of substantially uniform section . portions of said means and of the spar being of 10 and an outboard portion of which is of smaller different thickness and cross-section, respective but substantially uniform section, means carried ly, an outboard portion having substantially uni- ‘ on said spar and defining the surface or form- of ‘form blade thickness and spar cross-section but the blade, said means being of substantially uni form thicknessthroughout a portion of the blade substantially coextensive with said inboard por tion of the spar and further being of reduced but substantially uniform thickness throughout a being smaller than the blade thickness and spar _ section in an inboard portion. 15 17.‘ For an aircraft sustaining rotor, a blade of spar and‘rib construction, an. inboard portion of the spar being of substantially uniform cross portion of the blade substantially coextensive . section, an outboard portion thereof being of re 20 with said outboard portion of the spar, and a duced but substantially uniform cross-section, 20 ballast weight positioned forwardly of the spar and an intermediate portion of the spar being tapered to join said inboard and outboardpor in said outboard portion. . 1 13. A blade for an ‘aircraft sustaining rotor, tions, and a plurality of ribs mounted on said spar in spaced relation, the length of said inter said blade incorporating _a spar, an inboard por tion of which is of substantially uniform section mediate section and the spacing of said ribs be 25 and an outboard portion of which is of smaller ing approximately the same. 18. For an aircraft sustaining rotor, 'aigener but substantially uniform section, means carried ' on said spar and de?ning the surface or form of the blade, said means being of substantially uni form thickness throughout a portion of the blade substantially coextensive with said inboard por tion of the spar and further being of reduced but substantially ‘uniform thickness throughout a portion of the blade substantially coextensive 35 with said outboard portion of the spar, and a so ballast weight secured to the spar adjacent the ally upright axis mechanism and a sustaining ' blade secured to the axis mechanism with free dom for movement in a direction generally trans 80 verse the mean rotative path of travel of the blade, said blade progressively decreasing in in cidence from a point toward the root end thereof outwardly substantially to the tip, the decrease in incidence in an. outer portion of the blade 85 length being substantially uniform per unit- of outer end of said inboard portion of the spar. length and being greater per unit of length than 14. A blade for’ an aircraft sustaining rotor, the decrease in the inner portion of the- blade said blade incorporating a spar, an inboard por— ‘length, and the decrease in incidence in the inner portion of the blade lengthralso being substan- L 40 tion of which is of substantially uniform sec tion and an outboard portion of which is of tially uniform per unit of length. 19. A blade for an aircraft sustaining rotor, smaller but substantially uniform section, means carried on said spar and de?ning-the surface or. the blade being composed of at least three por- ' tions' of different thickness each of which is of form of the blade, said means being of substan ' tially uniform thickness throughout a portion of ' substantially uniform thickness throughout its ' length. ' the blade substantially coextensive with said in board portion of the spar and further being of reduced but substantially uniform thickness throughout a portion of the blade substantially coextensive with said outboard portion of the spar. and a plurality of ballast weights carried 20. A .blade for an aircraft sustaining rotor, said blade including a spar, means carried by the spar and de?ning the blade contour, a ballast weight at least the majpr mass of which lies in board portion. and the the plane of the blade chord, the ballast weight. by the spar, one in the outboard portion thereof , further being horizontally offset from the spar, and another adjacent the outer en'd of the in; and means interconnecting the ballast weight ' 15. A blade for an aircraft sustaining rotor, portions of said'biade being of diiferent thick ness but substantially the same chord, each‘ of spar. ' ' ' AGNEW LARSEN. PAUL H. STANLEY.