US2041849A - Propeller - Google Patents

Description

. Patented May26, 1936 UNITED STATES PATENT oFF- ca 2,041,849 j mornnnnn 7 Ernest a. McCauley, mm. Ohio Application July 1, 1932, Serial No. 620,462

' 48 Claims. (01. 170 159) This invention relates'to propellers, and more particularly to aeronautical propellers of the type in which a plurality of blades are connected to a hub portion. A q

It is well known that propellers have been previously constructed of conventional airfoil sections and of solid metal construction; likewise attempts have been made to build propellers of stampings of thin sheet.metal. Heretofore in the construction of propellers of this character the blade sections were of such thickness that they did not have a very high efiiciency above a tip velocity of approximately 950 feet per second and at speeds beyond this {range their efficiency fell off considerably due to the lack of sufficient rigidity in the crosssectional area of the blade sections, whichpermitted the outer end or .tip portion of the blade to violently flutter and eventually'crystallize and break off, due to fatigue of the thin material.

Furthermore, previous attempts toprovlde a metal propeller with substantially solid blades have in general resulted in merely decreasing the cross-sectional area of the blade section a small proportion below those used on the conventional wood propellers, and propellers constructed thusly, while they have shown advantages of from two to five percent in efliciency over the wood propellers, have nevertheless had their lim- 0 itations in that they were not highly emcient at the prevailing aeronautical tip speeds and were extremely ineflicient in the ranges of tip velocities, at and beyond the speed of sound which is approximately 1186 feetp'er second.

0 Further disadvantages have been the limita tions on the engine speeds, the increasing of'the engine speeds would causethe tip velocities of the propellers to be increased beyond their best 'eflicient operating range, and flutter violently,

v0 thereby producing objectionable noises. I

Itis therefore a primary object of this invention' to provide a propeller, of such construction that will'permit an increase in the present engine speeds with a consequent increase in the .5 'efflciency and horsepower of the engines, and that will operate eiiiciently at speeds in the range at and beyond the speed of sound. Accordingly, I provide a propeller of metal so constructed and arranged as to afford very thin aerodynamically i0 eiiicient blade sections of relatively small crosssectional area'in similar proportion to the width and thickness thereohwhich sections vary in width and thickness thruout the length of the blade and are so designed as to prevent a fluti5 tering of the blade, and especially at the outer end or tip portion thereotwhen the same is rotating at high velocity and to possess an extremely high rigidity to withstand torsional and thrust stresses that are applied to said sections.

Furthermore, I provide a propeller of this 5 character with an airfoil of such shape or contour that will improve the aerodynamic proper ties of the propeller by diminishing the prejudicial deformation due to thrust and torque at high tip velocities that will approach or exceed 10 950 feet per second, which would be substantially equivalent in a small propeller, to approximately 5 feet diameter running at 3600 R. P. M., and in a large propeller approximately feet diameter running at 1250 R. P. M., and that will 15 resist during such velocities bending or flexion suiiiciently to prevent failure of the. section against the ensuing thrust and torque loadings and avoid excessive noises.

A still further object of this invention is ti) provide in a blade of this character a distribution of the material in such manner as to provide a propeller of utmost strength and rigidity to withstand the high loadings due to centrifugal force, and the combined bending stresses of thrust and torque. Another object of my invention is the provision of a. propeller of this character having a hub portion in abutting engagement. with the blades to provide a solid base on the hub for clamping the blades, and means detachably connected to said blades and hub for retaining the blades against the entire action of the centrifugal force and a major portion of the rotational torque loads, said means permitting of an angular setting of the blades.

'A'still further'object of my invention is to provide a very accurate and simple means 01' balancing the blades. j e

Various other-objects and advantages will ap- 40 pear from an inspection of the following specifl cation in connection with the drawing which is hereto annexed and forms a part thereof and in i which Fig. 1 is a plan view of a propeller constructed in accordance with my invention;

Fig. 2 is a view similar to Fig. 1 showing an enlarged view of the propeller blade; I

Fig. 3, Fig. 3a, Fig. 312, Fig. 30, Fig. 3d;Fig. 3e,

Fig. 3 .Fig. 39, Fig. 3h, Fig.3i, Fig. 3f, are trans- 0 the blade showing the general contour or proflle of the blade airfoil sections;

Fig. 5, Fig. 5a, Fig. 5b, and Fig. 5c, are a fractional outline of a blade tip portion together with corresponding various cross section showing a variation having the various cross sections with the median line located on line N-N;

Fig. 6, Fig. 6a, Fig. 6b, and Fig. 6c are a view similar to Fig. 5, Fig. 5a, Fig. 5b, and Fig. 50 showing a further variation;

Fig. 7 is an enlarged fractional view in longitudinal cross-section of the blade showing the root end anda portion of the shank;

Fig. 8 is a front elevational view showing the hub in quarter section and fragmentary portions of the blades;

Fig. 9 is a plan view corresponding to Fig. 5 in the position, and

Fig. 10 is an enlarged view of the right of Fig. 8.

Referring to the drawing and with particular reference to Figs. 8 and 9 the hub portion iii of the propeller comprises the axle portion i2 and the integral radially extending socket portions I4 which extend a relatively short distance laterally away from the axle portion.

The hub portion is preferably in the form of a steel drop-forging, the axle portion having an internal cylindrical spline bore i6 provided with splines H by means of which the hub is securely mounted on a propeller shaft (not shown). The socket portions H are preferably formed with shallow cylindrical bores I that extend inwardly to a surface corresponding to the outer wall surface of the axle portion. The ends of the socket portions I4 are each formed with outwardly turned flange 20, the outwardly presenting faces 22 of which are disposed in planes at right angles to the longitudinal axis a:-a: of the hub or propeller and the inwardly presenting faces 24 of which are made frustro-conical for a purpose hereinafter described.

As herein disclosed a novel propeller blade 26 preferably made of metal in the form of a steel forging and in a manner hereinafter described is attached to each of the hub socket portions by providing the root of the blade with a projection 28 that is received within the bore i 8 of the socket portion and with an outwardly turned annular flange 32 positioned at a relatively short distance from the outer end of the projection 28 so as to be capable of having its face 34 about the face 22 of flange 20 of the hub, this projection serving, when the blade is connected to the hub, for a purpose hereinafter described.

Each blade is attached as shown in Figs. 8 and 9, to the hub and held against centrifugal forces by means of a split clamp 36 having split-clamp parts 38 and 40, each of which is of substantially semi-cylindrical construction having inturned flanges 42 and 44, the former flange being formed with a straight inner face 46 contacting with the face 48 on the propeller blade and the latter flange having a frustro-conical face 50 contacting with the face 24 on the hub flange.

' The clamp parts 36 and 40 of each pair of clamps 36, that are disposed on opposite sides of the axle portion of the hub are provided with lugs 52 so that the clamp parts may be secured together by means of bolts 54 which, when tightened, cause the clamp parts ,to move closer together in the direction of the longitudinal axis of the propeller and due to the tapered faces or surfaces 24 and 50 the blades are caused to move slightly inwardly towards the propeller axis of rotation z-z with the flanges of the blade held in tight abutting engagement with the flanges of the hub so that the blades are held rigidly against a solid base on the hub. The clamp parts 36 and 40 of each clamp form practically the sole restraining means for holding the blades against centrifugal force and serve to take substantially the entire load in the transmission of rotational torque force to the blades.

Each of the blades, as shown consists of a cylindrical root portion 56, as shown in Fig. 2 and Fig. 3, a shank portion 56, comprising Fig. 3a and Fig. 31) having the leading and trailing edges 4 and 5 respectively, and an upper surface 9 and a lower surface 6 as illustrated, and an airfoil portion 30, with cross sections as shown in Fig. 30, Fig. 3d, Fig. 3e, Fig. 31, Fig. 3a, Fig. 3h, Fig. 3i, Fig. 37', having the leading and trailing edges 2 and 3 respectively, and upper surface 60 and lower surface 62, as also shown in Fig. 4, which planfonn profile in Fig. 2 between the sections of Fig. 30 to Fig. 37' provide a contour of the leading and trailing edges of opposite convex curved profiles, that has the maximum width at a point substantially midway of the blade length, from the axis of rotation, and is preferably in the form of a steel forging which may be heat-treated to a high tensile strength, and is formed with relatively thin sections in which the material is so distributed as to present the ut-' most strength 'and rigidity to the forces of centrifugal, thrust and torque. With the use of special alloy steel, such as nickel-chrome molybdenum or its equivalent, heat treated to a tensile strength of approximately from 150,000 to 175,000 lbs. per square inch, it will be apparent that thinner blade sections may be employed than is possible to obtain with other materials of less tensile strength provided, however, a proper distribution of the material is made without sacrifice in the efficiency of the blade section ranging substantially in diameters from 5 feet to 15 feet or greater.

In accordance with my invention, therefore, in order to provide an airfoil section of such contour and thinness that greater efllciency may be obtained without substantially increasing the weight of the blade, as a whole, when compared with other blades of the same diameter now generally in use and made in the form of solid aluminum alloy or steel forgings, I have designed an airfoil section, as shown in Fig. 4, having a profile such that the upper surface 60 thereof is convexly arcuate and the lower surface 62 is in the form of an ogee or reverse curve, both surfaces merging into somewhat sharply rounded edges 2 and 3 and so arranged that the swelling or thicker portion is disposed adjacent the leading edge 2 and the arched or thin portion adjacent the trailing edge 3. There is thus provided a blade in which the leading edge portion is formed with a double-convex surface and the trailing edge portion with a concavo-convex surface, the point of 'tangency or merging of the convex and concave portions 68 and 10 of the lower surface 62 occurring approxi mately one third the chord length of the airfoil section from its leading edge 2 as represented by the line Y-Y on Fig. 2.

The double convex or leading edge portion 12 has each cross section relatively gradually increasing in thickness from the tip end of the blade to the inner end of the shank portion to into the cylindrical section it at the root 'end' relatively gradually increasing in" thickness throughout the length of the airfoil-portion '30 of the blade, and in a corresponding manner, so that the trailing edge portion becauseof its peculiar arched construction which maybe formed by. forging in suitable dies, that will not only have a high strength weight ratiobut will also possess suflicient rigidity and stiffness to withstand bending and deflection from the thrust loadings and will furthermore supplement the leading edge portion 12 to provide sufiicie'nt strength to withstand the forces of torque load-g ings.

According to; these improvements thegreater mass of the material will be located near the leading edge and the center of gravities 16 both in plan and elevation of the blade sections from tip, to root are disposed in a straight line that is coincident with the longitudinal axes x- -xand E-E respectively, of the blade or propeller. This may be changed so that the median line of grave will follow closelyjalong the path of the line 25 i -Y on the leading edge portion i2, and may even crossover the axis X-X near the outer' extremity of the blade to the trailing edge portion it. The centrifugal force will thus assist in-keeping the blade as a whole from deflecting rearwardin the path of rotation and add further strength to'theblade in this direction.

The shank portion '58 of the blade has its cross sections gradually varying from a circular sec- .tion it at its inner end to an airfoil section W at its inner end that consists of Fig. 3, Fig. 3a,

and Fig. 3b, which planform in Fig; 2 provides a contour of the leading edge i and trailing edge 5 of opposite concave curved profiles that merge portion of the blade, and. is formed with a longitudinally disposed conical bore tithe base of which is disposed at the root end of the blade. The dimensions of this cone are such that the walls of the shank portion are of sumcient thickness as to provide the required strength'to sustain the operative stresses of the propeller and each cross section thereof has its material increasing in area in continuation of'the gradual increase in the cross sections of the airfoil portion it to provide ablade whose cross-sectional areas gradually increase from tip to root. With that a highlyrigid and emcient blade is obtained. V

The cylindrical projection or extention M of the blade root is made to have a close turning fit with its complementary socket or bore it so as to center the blade and readily permit an adjustment thereof to vary the pitch angle and, also, provides a bearing area which together with the bearing area afforded by the abutting faces 22 and 3d of the fiangestil and 32 respectively resists deflection in any direction of the blade at the point of engagement with thehub.

By reason of its construction mypropeller is the proper balance.

capable of being horizontally'balanced without adding weight as has been the practice heretofore. It is, of course, well known that after assemblingthe blades complete with the hub a balance must be obtained and in accordance with my invention this may be. obtained by facing off the necessary amount of material of the abutting face 34 of the flange 32 of that blade which is heavy in balance thus allowing the weight of the entire blade to be moved inwardly towards the axis ZZ of rotation of the hub when assembly is again made to thereby obtain In Fig. 5, Fig. 5a, Fig. 5b, and Fig. 50, I have shown a propeller blade constructed in a man-' ner as hereinabove described except that the tip end portionis provided with a substantially flat bottom face or under surface 86 which gradually merges longitudinally into the ogee or reverse curve of the lower surface 62 of the adjoining airfoil portion, and having the leading and trailing edges 6 and 1 respectively, merging with 2 and 3 at a. point substantially as shown in Fig. 5a. the equivalent to Fig. 3%. r

A further variation of propeller construction embraced in my invention is shown in Fig. '6, Fig. 6a, Fig. '61), and Fig. 6c. The propeller blade of this propeller is similar to thatdescribed in connection with Fig. 3 to Fig. 37', except that the center of gravities of the blade'sections which lie on the neutral line N--N, are inclined downwardly and outwardly with respect to the longitudinal axisE-E as viewed in Fig. 6 or in the direction of thrust or forward movement of the propeller, starting from a point about one fourth the length of the blade measured from the root end-and terminating at the tip end thereof.

' It will thus be seen that with this construction the centrifugal force will provide a restoring force tending to'position the center of gravity line N-N in coincidence with the longitudinal axis E-E of the propeller and will thereby prevent the bending of the blade, especially at the tip end, by the thrust loads exerted thereon.

. While the foregoing description defines a blade made of solid steel construction, it is intended that the scope of this invention pertaining to the characteristics of the airfoil section may be applied to blades of other suitable materials, and other variations from the airfoil described and the method to provide rigidity in a thin section may be made by those skilled in the art, without departing from the spirit of the invention. Reference will therefore be had to the appended claims for a definition of the limits of the invention.

What I claim and desire to secure by Letters Patent is: Y

1. A propeller blade'having an outer portion of airfoil cross-section of the character described, said outer portion gradually increasing in cross- ,sectional area from the tip inward, a cylindrical root portion adapted for anchorage to a propeller hub and a. shank portion exterior of said hub connection of cross-section gradually varying from a cylindrical to theouter portion airfoil section, saidblade having a longitudinally disposed conical bore extending thruout the length of the root portion and into the shank portion,

said conical bore being of such diameter and depth as to provide a gradually increasing crosssectional area of the blade from the tip towards the root andsubstantially thruout the entire length of the blade.

, ly the entire length of the root and shank portions.

3. A propeller blade having an outer portion of solid airfoil cross-section of the character described, said outer portion gradually increasing in cross-sectional area from the tip inward, a cylindrical root portion adapted for anchorage to a propeller hub and a shank portion exterior of said hub connection of cross-section gradually varying from a cylindrical to the outer portion airfoil section, said blade having a central longitudinal tapering bore of circular crosssection formed within the root and shank portions, said bore tapering inwardly from the root of the blade and in such manner as to provide a gradually increasing cross-sectional area from the tip towards the root and substantially thruout the entire length of the blade.

4. A propeller blade having an outer. portion'of airfoil cross-section of the character described, said outer portion gradually varying incross-sectional area, a cylindrical root portion adapted for anchorage to a propeller hub and a shank portion exterior of said hub connection of cross-sec-' tion gradually varying from a cylindrical to said airfoil section and said blade having the leading edge portion that extends forward approximately from the longitudinal axisof the blade of such varying cross-sectional area throughout the length as to provide a tapering beam portion along the leading edge of sufficient strength to sustain a major portion of the torque stress-in service operation of the blade.

5. A propeller blade in which the airfoil crosssections are formed with concavo-convex lower surfaces and convex upper surfaces having similar proportions in'width and thickness and'in which the material is so distributed as to locate the median line of gravity in substantial coincidence with the blade axes in both longitudinal planes thereof.

6. A propeller blade in which the airfoil crosssections are formed with concavo-convex trailing edge portions disposed rearward of the longitudinal axis of the blade and reduced in mass sufficiently to locate the median line of gravity in substantial coincidence with the blade axes in both longitudinal planes thereof, so that the trailing edge portion of arched construction forms a high strength weight ratio to provide sufllcient strength to withstand deflection from thrust loadings.

7. A propeller blade in which the airfoil crosssections are formed with concavo-convex trailing edge portions disposed rearward of the longitudinal axis of the blade and reduced in mass sufliciently to locate the median line of gravity in substantial coincidence with the blade axis in the planform plane, and forward with respect to the thrust side of the blade, in the plane of rotation, said trailing edge portion of arched construction in connection with the restoring action of centrifugal force will provide utmost rigidity to withstand deflection from thrust loadings.

8. A propeller blade havingits median line of gravity for substantially the blade length disposed at substantially one-third the length of the blade sections measured from the leading edge and having that portion between the median line and leading edge'conflned with in convexly curved upper and lower surfaces and constituting a tapering beam, extending along the leading edge throughout the length of the blade from root to tip, to sustain a major portion of the torque stress' in the blade.

. 9. A propeller blade having the median line of gravity for substantially the blade length disposed at substantially one-third the length of the blade sections measured from the leading edge and having that portion between the median line and leading edge confined within convexly curved upper and lower surfaces throughout the length of the blade, and constituting a virtual tapering beam, the relative thickness of the cross-sections of said beam increasing gradually from the tip to the root end of the blade.

10. A propeller blade having a'hollow shank inner portion and anouter portion having the and lower surfaces and the trailing portion formed with concavo convex surfaces that mergerespectively with said upper and lower leading edge surfaces, the sections of said. blade diminishing in area from the root to the tip at a progressively decreasing rate, whereby the ratio of thickness vto thevchord of the cross-sections will provideutmost rigidity andstrengt'h to the blade.

12, A propeller comprising a hub having blade mounting members, blades having their inner ends mounted against and within said members and means for clamping the inner ends of said mounting members, blades having their inner ends mounted against and within said members and means for clamping the inner ends of said blades to the members of said hub in such a man'- ner that the driving force of said hub is transmitted thru said means to said blades, said blades and members having telescoping parts for locating said blades within said hub, said means for clamping the ends of said blades with the hub members for attaching the bladesto the hubs in such manner that the driving force of said hub is transmitted thru said means to said blades.

14. A propeller comprising a hub having sockets provided with flanges, bladeshaving their inner ends formed with flanges and projections for abutting with the flanges of said hub and said projections are received within said sockets for centralizing said blades, and means for attaching the blades to the hub in such manner that substantially all the driving stresses of hub operation are transmitted thru said flanges to the blades.

15. A propeller comprising a hub having a plurality of flanges and sockets to abut against similar flanges on the root end of the blades provided with projections to be mounted in said sockets,

said sockets to provide-a driving means from said hub to said blades and means for retaining said blades with said hub.

16. An airplane propeller comprising a pluralityoi' blades having cylindrical flanges and pro- Jections at their inner ends, a hubhaving sockets and cylindrical base flanges inter-fitting and in abutting engagement with said projections and flanges of said blades, and means for attaching said blades to said hub for rigidly securing the blades to the hub in such manner that the centrifugal force of said blades is transmitted through said means to said hub, and the, rotational torque force'is transmitted through the abutting of said flanges on the hub'and blades.

17. A propeller blade of solid steel having an airfoil section of the character described, with theleading edge portion having suflicient mass to improve the aerodynamic properties of the propeller by diminishing the prejudicial deformation due to thrust and torque stresses exerted on the blade, said airfoil so constructed and arranged to resist bending or fiexion of the blade from the ensuing thrust and torque loadings.

18. A propeller blade of solid steel having an airfoil section of the character described with the leading edge portion having suflicient mass to provide rigidity from the root portionof the blade to-the tip portion to improve the aerodynamic properties of the propeller by diminishing the" edge portion is of such proportion for utmost a strength and rigidity to withstand the combined loading of centrifugal force and thrust force to prevent excessive flutter of the blade and provide a silent propeller.

2d. A propeller blade of solid steel having an airfoil section of the character described, so constructed and arranged that a distribution of the material throughout the blade length along the leading edge portion, and .by arching the trailing edge portion is-of such proportion for utmost strength and rigidity from the root portion of throughout'the length of the blade substantially from the root portion to the tip, so constructed- 5 and arrangedthat the leadingedge portionis,

formed with a double convex surfaces, and thetrailing edge portion is formed with a concavoconvex surfaces, the point of tangency of the con- "vex and concave portions on the lowerv surface of the airfoil occurring approximately one-third of the chord length of the airfoil section from the leading edge, said portion of. the blade sections confined within'the double convexsurfaceswhose width and thickness graduallydecreases'from root to tip comprises a tapering beam; along the entire leading edge portion of the. blade to add rigidity against rearward deflection of the blade in the path of rotation against the forces of torque.

T tions confined withinthe concavo-convex surfaces and approximately two-thirds of the chord length of the sections comprises an arched construction of varying width and thickness along the trailing edge sections of the blade,.to provide rigidity against forward deflection of the blade in the path of rotation against the forces of thrust.

24. A propeller blade having an airfoil section I throughout the length of the blade substantially from the root portionto the tip, so constructed and arranged that the leading edge portion is formed with a double convex surface, and the trailing edge portion is formed with a concavoconvex surface, the point of 'tangency of the com --vex and concave portions on the-lower surface of the airfoil occurring approximately one-third of the chord length of the airfoil section from the leading edge, said sections gradually decreasingin area from the root portion to the tip of the blade for the purpose described.

25. In a propeller blade, a planformprofile, of such a character that the contour of the leading edge and the trailing edge of the outer portion of the blade from the tip inward are formed of convex curved profiles, and merge into concave ,curved profiles at the inner shank portion, and

- of such'a character that the contour of the leading edge and the trailing edge of the outer portion of the blade from the tip inward are formed by convex curved profiles, and merge into concave curved profiles at the inner shank portion, and terminate into a cylindrical profile at theroot end portion of the blade, said planform profile having the maximum width of the blade at a point substantially midway from the axis of ro tation.

27. In a propeller blade,'-an airfoil of the character described comprising .concavo-convex surfaces, a planform. profile, of such a. character thatthe contour of the leading edge, and-the trailing edge of the outer portion of the blade'from the tip inward are formed by convex curved profiles, and merge into concave curved profiles at the inner shank portion, and terminate into a cylindrical profile at the root end portion of the blade.

28. In a propeller blade of solid steel, having a shank portion at the inner end, and an airfoil portion extending therefrom, said airfoil portion having a concave-convex lower surface of relative similar proportions throughout the length thereof.

29. In a propeller blade of solid steel, having an end projection, a cylindrical flanged root end adjoining said projection, a shank portion adjoining said root end, and an airfofl portion adjoining said shank portion, said-airfoil portion having a concavo-convex lower surface of relativesimilar proportions throughout the length thereof.

30. In a propeller blade, a planform profile, of such a character that the contour of the leading edge and the trailing'edge of the outer portion of the blade from the tip inward are formed of convex curved profiles, and merge into concave curved profiles at the inner shank portion, and

terminate into a cylindrical flanged profile at the root end portion of the blade provided with a cylindrical projection at the innermost, end of the blade.

31. In a propeller blade, a planform profile, of such a character that the contour of the leading edge and the trailing edge of the outer portion of the blade from the tip inward are formed of convex curved profiles, and merge into concave curved profiles at the inner shank portion, and terminate into a cylindrical fianged profile at the root end portion, and a cylindrical projection adjoining the flanged profile at the innermost. end of the blade.

32. In a propeller blade, a planform profile, of such a character that the contour of the leading edge and the trailing edge of the outer portion of the blade from the 'tip inward are formed by convex curved profiles, and merge intoconcave curved profiles at the shank portion, and terminate into,

a cylindrical profile at the root end portion of the blade, said planform profile having the maximum width of the blade at a point intermediate the tip and root end portion.

33. In a propeller blade, having a shank portion at the inner end, and an airfoil portion extending therefrom, said airfoil portion having a concave-convex lower surface for a greater portion of the blade length, and merging into a relatively fiat lower surface substantially throughout the outer extremity of the blade.

34. In a propeller blade, having a hollow shank I portion at the inner end, and an airfoil portion extending therefrom,'said airfoil portion having a concavo-convex lower surface for a greater portion of the blade length, and merging into a relatively fiat lower surface substantially throughout the outer extremity of the blade.

35. In a propeller blade, having a cylindrical 'lower surface throughout the outer extremity of the blade.

3'7. In a propeller blade, having a cylindrical flanged root end and projection, a shank portion adjoining said root end, and an airfoil portion extending therefrom, said airfoil portion having. a

concave-convex lower surface fora greater length of the blade, and merging into a relatively fiat lower surface throughout the outer extremity of the blade.

- 38. A propeller blade having two opposite faces, one face having a convex surface for the entire length of the blade, the other face having a concavo-convex surface for a substantial portion of the length of the blade and having a relatively fiat surface for. the portion adjacent the extremity of said blade.

39. A propeller blade having two opposite faces, one face having a convex surface for the entire length of the blade, the other face having a concave-convex surface for a substantial portion of the length of the blade and having a relatively fiatsurface for that portion of its length where the air velocity traversing said surface will approach or exceed, in normal operation, the velocity of 950 feet per second.

40. In a propeller blade having a hollow root and shank formation and an airfoil portion-with surfaces of the character described, said outer portion gradually increasing in area from the tip inward to the root end, the thrust surface having both concave and convex portions merging into each other.

41. In an aircraft propeller bladev detachable from a driving hub, having a hollow root portion and a solid airfoil portion extending therefrom,

said airfoil portion increasing in area from the tip inward to the root portion with the thrust surface part concave and part convex each merging into the other in varying proportions.

42. In a solid metal propeller blade having an airfoil portion with thin airfoil sections gradually increasing in thickness throughout the length of said airfoil portion of the blade, which are formed by arching to provide a high strength-weight ratio in the airfoil portion of the blade. 7

43. In a solid metal propeller blade having an airfoil portion with thin airfoil sections gradually increasing in thickness throughout the length of said airfoil portion of the blade, which are formed byarching of said airfoil sections to provide a high strength-weight ratio of suificient rigidity and stiffness to withstand deflection against the stresses imposed in service operation of the blade.

44. In a solid metal propeller blade having an airfoil portion for the greater length of the blade, 'said airfoil portion so constructed and arranged that rigidity is built into a portion of the airfoil portion by arching the section to withstand deflection against the thrust forces in service operation of the blade;

45. In a solid metal propeller blade having an airfoil portion for the greater length of the blade, said airfoil portion so constructed and arranged that rigidity is built into one of the surfaces of the airfoil portion by arching the section to. withstand defiection against the thrust and torque forces in service operation of the blade.

46. A propeller having a hub provided with a plurality of solid bases and a cylindrical socket within said bases, blades having flanges and projections inter-fitting and in abutting engagement with said bases and sockets, and means for attaching said blades to said hub to rigidly hold said blades.

4'7. A propeller blade having a rounded leading edge with a radius whose curvature gradually increases from the tip to the'root portion of the blade, and a leading edge portion confined within oppositely curved convexed upper and lower surfaces and merging with said rounded leading 76 10 flanged root portion, a shank portion,'and an outer portion of airfoil section, said outer portion having rounded leading and trailing edges, with a leading edge portion confined within oppositely curved convexed upper and lower surfaces, and a trailing edge portion having-convexed upper and concaved lower surfaces that merge with the upper and lower convex surfaces of the leading edge portion respectively at a point substantially onethird the chord of the blade.

ERNEST G. mum. 1

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