US1828258A - Propeller - Google Patents

Description

Oct. 20, 1931.

H. F. scHMxDT 1,828,258

PROPELLER Filed Jan.' 11, 1930 2 sheets-sheer 1 ,my/n. v+m-5.73.55?! l' Hna in 39.1.

s m n 13 415mm xs wiens 1 a 1n Inch. 2

ATTORNEY OSL 20, 1931. H. F. SCHMIDT 1,828,258

PROPELLER Filed Jan. ll, 1930 2 Sheets-Sheet 2 Fm. lo.

` LWITN Es: 'INVENTOR l@ v y, H.Fcm1n)m rBY Patented Oct.` 20, 1931 UNITED STATES PATENT OFFICE HENRY E. SCHMIDT, OE LANSDOWNE, PENNSYLVANIA, ssIGNoR To WESTINGROUSE ELECTRIC a MANUFACTURING COMPANY, A CORPORATION OE PENNSYLVANIA PROPELIER Application mea January 11, 1930. serial No. 420,213.

My invention relates to propellers and particularly to propellers of the high speed type 1n which the projected area` of the propeller blades is a substantial percentage of the disc area, as used, for example, in blowers and pumps, and it has for an object to provide a propeller of the foregoing character which shall be so formed and proportioned as to have improved operating characteristics and the blades of which can be readily formed from suitable material or blanks.

It has for a further object to provide a propeller of the foregoing character which shall be relatively lighter in weight and have greater structural strength than the propellers heretofore provided, and it has for still another object to reduce the working stresses prevailing in the root portions of blades of propellers of this character.l

These and other objects are effected by my invention as will be apparent from the following description and claims, taken in connection with the accompanyingr drawings, forming a part of this application, in which:

Fig. 1 is a plan view of one form of propeller constructed in accordance with my invention;

Fig. 2 is a cross-section of a propeller blade taken normal to its working faces;

Fig. 3 is a developed view of the peripheral or tip edges of the propellerl blades shown 1n Fig. 1 and of a cross-section through the root portions of the saine propeller blades;

Fig. 4 is a stress diagram of a propeller blade of the character shown in Figs. 1 to 3 except that it has uniform thickness from root to tip;

Fig. 5 is a stress diagram of the same type of propeller blade having a straight taper from root to tip, the blade tapering to zero thickness at the tip;

Fig. 6 is a stress diagram of a propeller of the same type arranged in accordance with my invention and provided with a straight taper from root to tip, the blade having some finite thickness at the tip;

Fig. 7 is a stress diagram of the propeller blades of Figs. 1 to 3, which blades are arranged in accordance with the preferred form of my invention, the tip having some finite thickness and the cross-section of the blade being a variable or curved taper, as distinguished from a straight taper, from root to tip;

Fig. 8 is an enlarged radial cross-section of the blade embodied in the propeller shown in Figs. 1 to 3 and upon which the stress diagram shown in Fig. 7 is predicated;

Fig. 9 is a cross-section through the hub of the propeller and is taken on the line IX-IX of Fig. 1; and, v

Fig. 10 is a cross-section through a pro- 'separately formed propeller blades 11 attached thereto, Each of the blades 11 has radially-diverging leading and trailing edges 12 and 13, respectively, the peripheral or tip edge 14 of each blade being of substantially greater length than the root portion 15. As will be apparent from Fig. 1, the total projected area of the blades 11 is a very substantial percentage of the propeller disc area. It will also be noted that, as shown in Fig. 3, the blades have a uniform thickness from their leading edges 12 to their trailing edges 13.

It is the general practice to form propeller blades of the foregoing character out of plate material, whereby the blade not only has a uniform thickness fromtheleading edge to the trailing edge, but also has a uniform thickness in a direction from its root portion to its tip portion. However, I have found that blades so formed have excessive tensile`- stresses set up in their root portions due to the effect of centrifugal force and that, to

under Stress must be progressively reduced from the base 15 to the tip 14. This can beaccomplished by properly tapering the thickness of the blades from their roots to their tips; and, in order to reduce the stresses at the blade root to a minimum, a variable or curved taper should be employed. This will be apparent from Fig. 2, wherein it will be noted that the blade decreases in thickness from the root portion to the tip portion 14 and that a variable or curved taper is employed in order that the centrifugal stresses prevailing in the root portion 15 may be reduced to a minimum.

The foregoing will be app arent by reference to Figs. 4 to 7, wherein, there is illustrated, by way ofexainple, a comparative study of the stresses in propeller blades of various cross sections. figures pertains to stress calculations made in connectlon with a propeller of the type shown in Figs. 1 to 3 and used in a blower casing for circulating 100,000 ou. ft. of air per minute against a static pressure of 161/2 inches of water. When operating at full capacity, the ropellers are driven at a speed of 3520 P. M. However, the study of propeller blade stresses set forth in the forgoing figures is predicated upon approximately overspeed, or a speed of 4320 R. P. M.

Referring to Fig. 4, this shows a section of a propeller blade made of plate material and having uniform thickness from the leading to the trailing edge and from the root to the tip. This is the type of blade that has commonly been employed heretofore in propeller type blowers of the character referred to.

' Fig. 4 shows the areas and stresses prevailing in a blade of this character. The radial dimensions of the blade are indicated as abscise areas as ordinates on the right hand s t ,side of the curve and the stresses as ordinates on the left hand side of the curve. It will be noted that the blade has a radius at its root of 7 inches and at its tip of 21 inches. It will also be noted that the blade has a stress area at its root portion of only 19 sq. inches and a stress area at its tip portion of approximately 47 sq. inches. The stress area of the blade at any radius may be said to be the developed length of the blade from the lead- .ing edge to the trailing edge at that radius multiplied by the thickness of the blade at that radius. In other words, from inspection of Fig.' 3, it will be apparent that the greater thickness of the-blade.

vveloped lengths of stress area at the tip is the developed length measured from 12 to18 multiplied by the thickness of the blade while the stress area at the root portion is the relatively shorter length 12' to 13 multiplied by the relatively It will be -noted from inspection of Fig. 1 that the depropellers` of the. .type upon which the present invention is predicated are materially greater at the tip than at the root. Hence, if the thickness of the blade is uniform from root to tip. the stress area is greatest at the tip and smallest at the root. From the curve in Fig.'4 it will be apparent that, because of the small area near the root and the large amount of metal near the tip, the tensile stress at the root is exceedingly pers to zero thickness The data given in these,

high. It will be seen from the curve that the tensile stress at the root of a blade of this character is 54,000 lbs. per Sq. inch.

This stresscan be reduced by tapering the blade from root to tip. The limiting case for such tapering is that in which the blade taat the tip, as in Fig. 5. From -t-hiscurve, it will be noted that the area under stress increases from the tip to near the root, and consequently a stress offonly 19,400 lbs. per sq. inch is set up in the root. However, in actual practice, the blade must have some finite thickness at the tip depending upon the size of the blade. A thickness of 1/8 lof an inch is not unreasonable for the propeller given in the present example. A blade with a straight taper from root to` tip and a tip thickness of 1/8 inch is shown in Fig. 6, the area under stress increasing from the tip to near the root and the stress at the root being 23,400 lbs. per sq. inch.

I have found that a blade having a variable or curved cross-section and decreasing in thickness'from the root to the tip will have a lower stress than one whose cross section is a straight taper, such as shown in Fig. 6. Referring` now to Fig. 7, I show a form of blade which has a variable or curved ta er and wherein the stress area of the blade ecreases progressively from the root to the tip and which involves a maximum tensile stress at l the blade root of only 17 ,500 lbs. er sq. inch.

Referring now to Fig. 8, I s ow an enlarged view of a cross-section through the blade upon which the curve shown in Fig. 7 is predicated. From inspection of this figure, it will be apparent that, in order to facilitate manufacture, I form the blade of three straight-sided tapered sections 17, 18 and 19 joined by curves of large radii at 21 and 22, the whole approaching or simulating a continuous curve. The surface 17 18 and 19 is referred to herein as a curved surface inasmuch as a curve may be said to be com osed of an infinite number of straight lines Joined together. As stated heretofore, such a blade section involves a maximum tensile stress at the blade root of onl 17,500 lbs. per sq. inch.

It will therefore lie apparent that, in accordance with my invention, the preferred form of blade embodied in the propeller shown in Figs. 1 to 3 has a uniform hickness from the leading to the trailing edge and has a taper, preferably a variable or curved taper, from the root to the tip so as to provide a progressively decreasing stress area from the root to the tip. One of the important objects of my invention is to provide an efficient propeller blade which can be readily formed from plate or sheet material and for a description of the method of manufacture employed, reference may be had to my copending application entitled. Method of manufacturing propellers, Serial No. 420,- 212, filed January 11, 1930 and assigned to the Westinghouse Electric & Manufacturing Company.

Referring now to the other proportions of my propeller blades, I prefer, as -disclosed and claimed in my U. S. patent, No. 1,596,459, issued August 17, 1926 for propeller pump and assigned to the Westinghouse Electric & Manufacturng Co. to so form the propeller that the projected area of the blades is a substantial portion of the disc or annulus area. Preferably, as shown in Fig. 1, the projected area is approximately of the disc area. In addition, as shown in the radial pitch of the blades constant but to have the axial pitch increasing from the inlet or leading edge 12 of the blade to the outlet or trailing edge 13, the pitch ratio of the trailing edge being greater than the pitch ratio of the leading edge. By the term pitch ratio is meant theratio of the pitch of a blade at any point to the diameter of the propeller. For example, in the present embodiment, the propeller has a diameter of 41.5 inches, a pitch of 34 inches at its leading edge and a pitch of 56.5 inches at its trailing edge. The pitch ratio of the leading edge is therefore .82, and the pitch ratio of the trailing edge is 1.37. The ratio of pitch ratios of the trailing edge to the leading edge is therefore 1.37 divided by .82 Which is equal to 1.67.

In order to facilitate the manufacture of the propeller, I form the blades 11 separately from the hub 10 and attach the blades to the hub, by some means such as Welding. Referring to Fig. 2, it will be noted that the root portion 15 of the blade is secured in a suitable recess or groove 23 cut in the hub, the blgde being Welded to the hub as at 24 and In Fig. 10, I show a blade 25 similar to the blade 11 shown in Fig. 2, but in Fig. 10 the hub of the propeller is provided with helicoidal ribs or projections 26 which are grooved to provide a suitable slot 27 to receive the root portion of the blade. The blade may then be held in the slot by rivets 28 assing through the projections 26 and the lade root. It will be obvious that other means of attaching the blades to the hub may be employed.

Having arrived at a form of propeller blade which is of minimum weight and maximum strength, I also prefer to provide. a form of propeller hub which has these same characteristics. Accordingly, the hub is made cylindrical or hollow and is provided With a centrally-disposed, interior web or bracing member`31 Which is bored at 32 to receive a propeller shaft (not shown). This web 31 is preferably formed integrally with the hub cylinder 33 and is provided with laterally-extending fianges 34` which serve to form an extended bearing on the shaft and thus tend to increase the rigidity of the entire structure.

Fig. 3, I prefer to have A hollow hub requires interior bracing .to prevent distortion duel to the centrifugal force of the blades. In order to provide ad-v ditional bracing means for the hub, I fit the discs 35 and 36 into the opposite ends of the hub cylinder 33 and secure these discs in place lin any approved manner, preferably by fusion o f metal, as by welding at 37. The discs 35 and 36 are bored to receive the propeller shaft at 38 and $39respectively, and While these discs are not shown as being provided With flanges, such as the ianges 34 on the Webs 31, it will be understood that it is Within the province of my invention to provide either, or both of these discs with such flanges, should conditions render this desirable.

I may also provide abutment means, such as the circumferential ribs 41, which are disposed about the inner wall of the hub cylinder 33 near each end of the latter and serve as stops for the respective disc members. This simple arrangement considerably expedites the assembly of lt-hese end discs, slnce it provides for ready adjustment of the respective discs to their proper locations, it merely being necessary to push each disc into the cylinder until it rests against its respective abutment means, after which it may be welded to the cylinder as indicated at 37.

From the foregoing, it will be apparent that I=have devised a propeller embodying a combinationv of many novel features, all of which `cooperate to produce a propeller of minimum weight, maximum 'strength and 100 high efficiency. By proportioning the propeller so that the projected area of the blades bears a. relation to the disc area of the order given herein and by having the pitch of each blade increasing from the leading to the 105 traling edge and by also having the rat1o of pitch ratios of the trailing edge with respect to the leading edge of the order given herein, increased capacity is obtalned and consequently a smaller propeller may be ut1- 110 lized to translate a given quantity of Huid. The foregoing, in cooperation with the uniform thickness of blade from its leading to lts trailing edges and the tapered thickness from the root to the tip so as to provide progres- 115 sively decreasing stress areas from the root to the tip, provides a propeller of mlnimum weight and size and maximum strength for a given capacity as Well as a propeller the blades of Which can be readily formed from 120 plate material.

While, in the present embodiments, I show a form of propeller blade wherein both Working faces of the blade curve from their root to their tip portions, it will be obvious 125 that the propellers of the prior art may be materially improved by curving only one of their Working surfaces so as to provide aprogressively decreasing stress area. While 1t is to be understood that the latter form of 13o y propeller is Within the province of my invention, I prefer, as shown in the accompanying drawings, to curve both .Working faces in order that the blade may be more symmetrical and in order to avoid eccentric loading of the same. Furthermore, While I have shown a propeller having a large projected area in proportion to the disc area and a uniform thickness of blade from the leading to the trailing edge, my invention also contemplates providing progressively decreasing stress areas from the root to the tip portions .1n propellers wherein the thickness of the blades is not uniform from the leading to the trailingr edges.

`While I have shown my invention in two forms, it will the art that it is not so limited, but is susceptible of various other changes and modifications without departing from the spirit thereof, and I desire, therefore, that only such limitations shall be placed thereupon as are imposed by the prior art or as are specifically set forth in the appended claims.

What I claim is:

l. A propeller embodyinga hub and blades, said blades having substantially straight leading and trailing edges Which diverge from their root to near their tip portions so as to provide a greater developed length of blade from the leading tothe trailing edges near the tip portionthanneartheroot portion, said bladeshaving a substantial uniform radial cross-section from their leadin to their trailing edges and said blades aving a Working surface radially `concaved' toward the other lWorking surface, the concavity of said working surface being such that the stress areas so progressively increase from the tip of the blade to the root of the blade as to effect minimum stress at the root, the stress area at any radius bein the developed length of the blade from the leading edge to the trailing edge at that radius multipled by the thickness of the blade at that radius.

2. A propeller embodying a hub and blades, said blades having substantially straight leading and trailing edges which diverge from their root to near their tip portions so as to provide stress areas which' are, circumferentially, relatively long at the tip portion and relatively short at the'root ortion, said blades having a substantiall uniform radial cross section from their leading to their trailing edges and said blades having a Working surface radially concaved toward the other working surface, the concavity of said workthe str ess areas mum stress at the root, the `stress area at any radlus being the developed length of the be obvious to those skilled in' 3. A propeller embodying a hub and blades, said `blades having substantially straight leading and trailing edges which di verge from their root to near their tip portions so as to provide a greater developed length of blade from the leading to the trailing edges near the tip portion than near the root portion, said blades having a substantial uniform radial cross-section from their leading to their trailing edges and said blades having their oppositely-disposed working surfaces radially concaved toward each other, the concavity of said working surfaces being such that the stress areas so progressively increase from the tip of the blade to the root of the blade as to effect minimum stress at the root, the stress area at any radiusl being the developed length of the blade from the leading edge to the trailing edge at that radius multiplied by the thickness of the blade at that radius.

4. A propeller as claimed in claim 3 wherein the blades have a total projected area which is equal to or greater than 1,/2 of the propeller disc area.

5. A propeller as claimed in claim 3 Wherein the total projected area of the propeller blades is approximately of the propeller disc area.

6. A propeller as claimed in claim 3 wherein the propeller blades have an axial pitch which increases from the leading to the trail- ,ing edge.

A propeller as claimed in claim 3 wherein the blades have a constant radial itch and an axial pitch which increases fiom the leading to the trailing edge.

8. A propeller as claimed in claim 3 wherein the pitch ratio of the trailing edge of each blade is greater than the pitch ratio of the leading edge and the ratio of pitch ratios of the trailing edge to the leadingV edge is ap proximately 1.67.

9. A propeller as claimed in claim 3 wherein the total rojected area of the blades is approximatefy 2/3 of the discarea, the radial pitch of the blades is constant and the axial pitch ofthe blades increases from their leading to their trailing edges.

In testimony whereof I have hereunto subscribed my name this 21th day of January,

HENRY F. SCIIMIDT.

root of the blade as to effect minil

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