US2615520A - Reinforced propeller blade - Google Patents

Description

Oct. 28, 1952 c. A. LIEDHOLM 2,615,520

' REINFORCED PROPELLER' BLADE Filed Nov. 26, 1947 INVENTOR.

ATTORNEY.

Patented Oct. 28, 1952 UNITED! STATES VTENYTV'V REINFoRcsn 'PROBELLVERJBLADE Carl A. Liedholm, Mountain Lake, N. J assignor to Curtiss-Wright Corporation, a corporation of Delaware I Application November 26, 1947, Serial No. 788,211

part to high localized stresses induced by vibration, andin part to the inevitable microscopic or macroscopic checks and cracks which may appear anywhere in the mass of the steel of which the blade is formed. Such failures are further contributed to by imperfections in the welding of blade components to one another, .which allow, scattered zones where localized stresses may be high enough to cause localfailures and to thereby initiate cracks which may grow to serious proportions as the blade continues to operate at high stress levels. These faults and cracks may occur despite precision techniques in the fabrication of blades and despite zealous and careful inspection of allsurfacesof. the propeller blade. It will berealized that the problem is of rather critical nature since blades operate .at high stress levels, and the {mass of material of which they are composed is held to the minimum toreduce the weight of the blade. I

I.'.;In spectionof the exterior of a fabricated propellerblade, prior to final surface finishing,v is comparatively simple magnetic, macroscopic and visual inspection can be made with little -difficulty. Few blade failures result from flaws having their inception at the blade outersurface. :The interior surfaces of hollow steel propeller blades, however, are much more difficult to' inspect since remote inspection techniques must be practiced with the aid of optical and other instruments which maybe inserted into the hollow of the blade from the butt end. I have found that most failures of propeller blades occur from interior inceptions,'which are contributed to by inspection difficulties mentioned above and to difficulties attendant to fine surface finishing of the interior surfaces of propeller blades.

;raising tolerable operating stress level and in =reducing the likelihood of failure in propeller blades.

,-.- -Objects of this invention are to increasethe -1 Claimd; (o1. fin-r159) resistanceof hollow steel propeller blades to failures due to vibratory stresses'and to other stresses which may be imposed thereon, andto effect such resistance tofailure by internally precompressing either all or part of the interior surface of a hollow steel propeller blade and increasing its strength and fatigue resistance-by a suitable metallurgical technique such as by nitriding.

Further objects of the invention will become apparent from the following description and claim,- and from the drawings. The drawings and the description: associated therewith, however, are not to be construed as limiting the scope of the invention since various other means for accomplishing objectives of the invention will become apparent to those skilled in the art;- a

. In the drawings in which similar reference characters represent similar parts, Fig. l is a a v Fig. f is a section similar to Fig. 3 at the blad trailing edge; and v I Fig. 5 is an enlarged fragmentary section of a blade leadingedgeportion showing an alterna- .tive mode of practicing the invention.

The propeller blade to which the invention. is applicable is represented in its entirety in Fig. 1 and comprises a tubular shank portion l0 provided if desired with a flange l2 utilized in connection with retention of the propeller blade in a propeller hub. The blade also may be provided if desired with an annular cuifring l4 which provides an abutment against which a. blade embracing cuff may be secured against displacement under the influence of centrifugal force. .These elements [2 and 14 do not form a partbf the invention and are illustrated merely as examples of a particular blade organization. The eifective portion of the blade aerodynamically, comprises a large paddle shaped member 16 constituted by a camber plate l8 and a thrust plate 20, thetwo plates preferably being tapered in-thickness longitudinally-of the blade-and hav- I and adjacent the trailing edge 24 are ordinarily thickened and are integrally united with one another through the medium of welds. Further, and particularly where blades are large] and wide, the blade may be centrally reinforced by a rib 26 extending longitudinally and centrally of the blade and secured as by welding to thickened portions 28 of the camber and thrust plates. Frequently, the rib 26 is formed in part on each blade plate and the two parts are united along the blade neutral axis to comprise the entire rib. As inferred, a rib such as 26 may or may not be necessary in the blade, depending upon its size and design requirements.

In that type of blade wherein separate camber and thrust plates are preformed and are subsequently united at their leading and trailing edges, the weld at said edges is applied from the outside of the blade and a fillet 30 is formed within the leading edge, a similar fillet 32 being formed internally of the blade adjacentthe trailing edge.- The interior surface of the welded fillet is ordinarily more or less non-uniform in its surface finish and unless it be finished smoothly as by filing or grinding, high and low points and other faults may exist on the fillet as shown at 34 and 36 in Fig. 3, which under blade operating conditions form points of high stress concentration which may lead to fatigue failure. The proce dures for securing a fine finish in blade fillets are rather costly and time consuming but up to this time are fully justifiable in view of the improved fatigue resistance of the blade which is produced. The fillets however, are not the only points at which. failures may initiate, and in certain types of blades, due to the pattern of stress distribution occasioned by the blade design andv by the vibratory stresses and steady state stresses imposed upon the blade in operation, failures can occur at other-parts of the blade surface. An analogous situation to failures may also exist in blades where welds are made in' other blade zones than the leading and trailing edges.

I have found that by superficially increasing the strength of and precompressing the interior surface of a propeller blade to the proper depth, the effect of localized faults in the blade surface is minimized and that the fatigue resistance of a propeller blade can be raised to levels heretofore not obtainable. A preferred method of securing the precompression indicated at 38 is to nitride the interior surfaces of the propeller blade. This may be easily accomplishedby' following'theusual nitriding procedure; namely and in general, the soaking of thebla-de for anappropriate length of time at al:u )'ut-9l5' F. while-nitrogen carryin gas such as ammonia is passed over the surface to lie-nitrided. This forms iron nitride in the steel, increasing its strength and hardness and-causing a. slight growth of the material. This growth causes precompression of the material since it is constrained by the enveloping unn'itrided blade 'material'. In the case of the hollowblade this technique is extremely simple-- in. its accomplishmentsince the blade proper may be soaked at the proper temperature in a. non-oxidizing atmosphere and ammonia gasmaybe passedinto the blade hollow through anappropriategas' connection. associated with the. butt" of the propeller blade. A. nitridedl case having reat fatigue strength and hardness, develops over the inner surface of the propeller blade toa depth offrom .005 to 030 inch depending upon the time of soaking',.the inner margins of the caserblending' gradually into the parent metal.

In some types of propeller blades, nitr'iding of the entire interior surfaceof the blade maybe unnecessary, in which case those portions of the blade surface where nitriding is notzdesired may 4 be coated with an appropriate plating which pre vents nitriding. The nitrlding may be localized at such zones, for instance, as the fillets at the inside of the leading edge and the trailing edge of the blades as noted in Fig. 5 at 4|).

The best depth of the precompressed or nitrided layer varies with blades of difierent design. It should be a depth to allow substantially the same high operating stress levels to be maintained both in the outer and inner surfaces of the blade material, such stress levels being the maximum to allow freedom from blade failure. Expressed in another way, the depth of the nitrided layer should be such that failures of the blade due to stress overload, normally applied to test samples, should start from the inner surface and from the outer surface with substantially equal frequency. When such a condition prevails, the safe operating stress level of the blade is at a maximum and is materially greater than that attainable with untreated blades, or with overor under-treated blades. Even so, moderately overor undertreated blades should be better than untreated blades.

A considerable amount of testing has been accomplis'hed on propeller blades, otherwise identical, some having nitrided inner surfaces and others being unnitrided. Actual nitrided blade samples under enforced test conditions have failed in a stress range of the order of a minimum of 40,000 pounds per square inch whereas the un-nitrid'ed samples have failed at stresses of the order of a minimum of 23,000 pounds per square inch, the latter failures having started from the blade interior surfaces.-

While nltriding of the steel of the propeller blade is referred as a means for internal surface precompressing and, incidentally, hardening and strengthening, other means may be employed for increasing the prestress, hardness and strength of the material, such as case carburiz'ing and cold working. It is appreciated that the prior art has taught the treatment of propeller blades to ditfei'entiate strength and hardness in the wall of the bladebut in all instances of which applicant has knowledge, emphasis the art has been placed upon increasing the strength and hardness of the exterior surfaces of the blades either by electro-deposition of hard metal, cold working, surface carburization or nitriding. The primary purpose of external strengthening and hardening was to increase propeller blade resistance to abrasion and external erosioh and naturally to increase the strength Of the blade in general. However, suchprior art-- expedients' while constru tivefcrimproving abrasion resistance, do not provide a solution for the underlying robl ms or reinforcing blades against vibration induced stress or of equalizing failure incepticns between the outside and inside blade surfaces. Nor do the prior teachings provide means to overcome imperfections inthe interior of the blades where surface finishing. and inspection is extremely difiicult.

The phenomenon of increasin'gfatigue strength of propeller blades by nitridihg or other modes of internal hardening is believed to consist in the proposition that the treated surfaces are placed in a stateof controlled compressive stress which delays fault inception where the surface finish may be imperfect- It is known that the technique of nitriding causes a growth of the material of the order of a few per cent whereby the nitrided layers are inevitably compressedand the metal layers adjacent thereto are correspondingly placed in tension. The controlled compressive stress at the interior blade surface, by actual test materially increases the fatigue strength of the propeller blades despite minor defects which may occur on the interior surfaces of the blade or on the interior surfaces of the relatively unfinished fillets or Welds which occur at various parts of the blade. When techniques such as cold work or other hardening processes are used on the interior surfaces of the propeller blade, it is considered that such techniques should properly include the creation of a state of precompression on the inner blade surfaces.

Though two embodiments illustrating the invention have been shown and described, it is to be understood that the invention may be applied in other and various forms. Changes" may be made in the arrangements, without departing from the spirit of the invention. Reference should be had to the appended claim for definitions of the limits of the invention.

What is claimed is:

A hollow steel propeller blade comprising an integral, substantially homogeneous shell of airfoil profile having thickened leading and trailing edge portions and relatively thinner shell portions therebetween, said thinner shell portions extending across the blade chord from a short distance rearward of the leading edge to a short distance forward of the trailing edge, the interior surface layers of said leading and trailing edge portions 6 deviating from the homogeneity of the rest of the blade and being nitrided to a depth of from .005" to .030, to precompress said surface layers, and to increase the strength and hardness thereof better to resist the action of operating stresses thereat, said nitrided surface layers terminating where the thickened leading and trailing edge blade portions blend in thickness to said I thin shell portions.

' CARL A. LIEDHOLM.

REFERENCES CITED The following references are of record in the Metals Handbook, 1948 Ed., pub. by A. S. M., 7301 Euclid Ave., Cleveland, Ohio, p. 700.

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