US2340133A - Propeller blade - Google Patents

Description

E. MARTIN PROPELLER BLADE Filed Dec. 14, 1940 ATTORNEY suppressing vibration of. the

Patented Jan. 25, 1944 PROPELLER BLADE Erle Martin, West Hartford, Conn., assignor to United Aircraft Conn.,

Corporation, East Hartford, a corporation of Delaware Application December 14, 1940, serial No. 370,171

(ol. 17o-159) 14 Claims.

This invention relates to improvements in propeller blades and has kparticular reference to an improved metal blade foran aeronautical propeller.

An object of the invention resides in the provision of an improved propeller blade having f energy dissipating means incorporated therein for blade during propeller operation.

A further object resides` in the provision, in a metal propeller blade of the character indicated, of vibration suppressing means carried by the outer portion of the blade without materially weakening the blade or changing its weight.

Other objects and advantageswill be more partlcularly pointed out hereinafter or will become apparent as the description proceeds.

In the accompanying drawing, in which like reference numerals are used to designate similar parts throughout, there is illustrated a suitable structural embodiment for the purpose of disclosing the invention. The drawing, however, is for the purpose of illustration only and is not to be taken as limiting or restricting the invention since it will be apparent to those skilled in the art that various changes in the illustrated construction may be resorted to without in any way exceeding the scope of the invention.

In the accompanying drawing,

Fig. 1 is an elevational view of a propeller blade showing the incorporation therein of vibration suppressing means constructed according to the invention.

Fig. 2 is an elevational view on an enlarged scale of the outer or tip .portion of the blade lllustrated in Fig. 1.

Fig. 3 is a cross-sectional view on an enlarged scale taken on the line 3-3 of Fig. 2.

Fig. 4 is a cross-sectional view of a fragmentary portion of the propeller blade tip showing a rivet aperture .extending therethrough. f

Fig. 5 is a longitudinal sectional view of a tubular rivet.

Fig. 6 is an elevational view of the tubular rivet, and

Fig. 7 is a sectional view of a fragmentary portion of the propeller blade tip similar to' Fig. 4 showing the rivet of Fig. 5 and the plug of Fig. 6 assembled in operative position therein.

Referring to the drawing in detail, the numeral I0 generally indicates the metal propeller blade secured at its root or base end in a -propeller hub generally indicated at l2. The blade maybe se- A cured in the hub for turning movements about the longitudinal axis of the blade to change the a plug for :filling` propeller pitch in a manner well known to the art.

It is a well known fact that propellers are subject to severevibration conditions arising from various sources such as the vibratory components of engine torque, eccentric rotation of the propeller shaft and dynamic and aerodynamic unbalance of the propeller itself. These vibrational effects are particularly severe in the case of anaeronautical propeller because of the great length of the )propeller blades in proportion to their weight and Width and the high speed at which such propellers are customarily driven and such vibrational conditions may become particularly Vsevere 'in the case of all metal propeller blades since the metal does not have as great internal damping characteristics as does the material of a blade formed of wood or other non-metallic material. It has been found that the outer or tip portion of metal blades usually have a greater amplitude of vibration induced movement than Y any other portion of the blade and that frequently a node exists between the extreme outer or tip portion of the blade and the inner portion. Such a nde usually occurs in the outer-half of the blade where the section of the blade is relatively thin and the concentration of stresses about the location of the node has frequently led to fatigue failure of the propeller blade.

In carrying out the invention, the outer part of theblade is first divided or split by means of a saw cut disposed substantially in a plane including the neutral surface of the blade, that is, the surface including vthose points Within the blade which remain free of strain when the blade is bent. The plane Vof the saw cut does not exactly follow the neutral surface since it is more convenient from a manufacturing standpoint to have the saw cut straight and it has been found entirely satisfactory to-provide a straight saw cut through the width of the blade with substantially equal amounts "of metal on opposite sides of the cut, as is particularly shown in Fig. 3. While the extent of the saw cut relative to the length of the blade will depend upon the vibrationalcharacteristics of the particular blade I somewhat greater tharrone-,third of the total blade length. y

After the saw cut has beenf made, a sheet of some suitable material having relatively high externa! friction characteristics and a comparatively high internal hysteresis is inserted in the cut to fill the same. 'I'he thickness of the saw cut and of the inserted material will also depend upon the vibration characteristics of the particular blade shape and the material of which the blade is made and upon the damping characteristics of the inserted material. It has been found that a relatively thin sheet of material such as Micarta or Formica is 'adequate because of the relatively high external friction when in contact with the surfaces of the metal blade parts and its high internal hysteresis effect.

After the sheet of material I4 has been inserted in the saw cut provided in the outer portion of the blade, a plurality of tubular rivets I 6 are inserted through apertures provided through the blade and the damping material. These apertures i8 are preferably provided with-flared ends as indicated at 20 and 22 and the tubular rivets I6 may have one end pre-flared, as indicated at 24; to nt the flare of one end, of an aperture. After the tubular rivet has been inserted through the aperture its straight end is flared outwardly, as indicated by broken lines `in Fig. 5, to llt the ilare at the opposite end of the: aperture and clamp the two parts of the propeller blade on 4opposite sides of the 'material Minto firm contact with the sheet of inserted material. The rivets I8 are preferably made of some relatively strong and consequently comparatively heavy material, such as mild steel, and are made tubular in order to avoid an unnecessary increase in the Hweight of the outer blade portion. .After the tubular rivets are in place they may be illled with suitable plugs 26 of light weight material such as aluminum which plugs may be provided with flared or expanded end portions to fl1l"the contact with the inserted material when the blade tip bends under the stress of aerodynamic or vibration induced forces.

'I'he above described construction provides an arrangement in which the outer portion of the propeller blade constitutes a composite cantilever beam. Since the friction material I4 is disposed substantially along the neutral plane or surface of the original blade form this material does not tend to move as the blade is bent. The two portions of the blade on opposite sides of the saw cut will bend, each about its own neutral plane or axis with the result that one of the surfaces in*v contact with the friction material lwill'be'in a stateof compression and the alternative surface will be in a state of tension.' Since both parts separated by the saw Vcut are held together at the -bottom of the saw cut there will be a slight relative motion between the opposite faces of the saw cut whenever the blade is bent. This relative motion will create friction between the inserted friction material and the/contacting surfaces of the outer blade portion which friction will damp out vibrational forces tending to bend 'the blade by dissipating the energy of these forces. The internal hysteresis of the friction material adds to the frictlonal damping effect by dissipating a portionof the energy of the blade bending forces in internal friction of the material. While a suitable constructional embodiment has been illustrated in the accompanying drawing and hereinabove described for the purpose of disclosing the invention, it is to be understood that the invention is not limited to the particular embodiment so illustrated and described, but that such changes in the size, shape and arrangement of the various parts may be resorted to as come within the scope of the sub-joined claims. Having now described the invention so that others skilled in the art may clearly understand the same, what it is desired to secure by Letters Patent is as follows: v`

' 1. A flexible metal propeller blade having a laminated outer portion including the blade tip and a non laminated inner portion including the blade shank and having major dimensions of length from the tip end to the shank end thereof, breadth from the leading edge to the trailing edge thereof, and thickness between the face sur.

lfaces thereof, the laminated outer portion comprising separate closely adjacent blade parts in- .25 tegral with the blade shank and whose major surfaces lextendvlengthwise of said blade, solid sheet material substantially coextensive with the made parts of sindA iaminated'pornon and hem under compression between and in frictional con- 30 tact with said parts and movable, upon blade vibration, relative to the blade part surfaces with which it is in contact and when so moved dissipatins energy through slidingsurface friction.

2. In a flexible metal propeller blade having a laminatedportion and a nonlaminated portion and having major dimensions of length from the shank end to the tip end thereof, breadth from the leading edge to the trailing edge thereof and thickness between the-face surfaces thereof, said 4'() laminated portion including blade laminations each having a surface'extending lengthwise of said blade from a location adjacent the tip end ofthe blade-to the nonlaminated portion, alternate laminations integral with the metal of said blade, a lamina of a material different from the metal of said blade secured between each two adjacent blade metal laminae, and means mainv taining said material under compression between and in frictional contact with said blade metal laminae, to dissipate energy through sliding surface friction upon relative movement of adjacent laminae during blade vibration. y

3. In a flexible solid metal propeller blade of airfoil contour having a neutral plane within said liliv contour extendingalong the major axis of the blade and chordwise of the blade from adjacent the leading edge to adjacent the trailing edge thereof, the tip of said blade having a slot formed therein and defined by maior surfaces substantially'parallel with saidv neutral plane and-ex tending inwardly from the blade tip end and terlminating less than two-thirds ofthe blade length from said tip, an insert of sheet material held under compression in said slot and in frictional 05..engagement with the tip end portion of said blade, said sheet material, uponl blade vibration,

being movable relative to the tip= end portion of the blade with which it is in contact and when so moved dissipating energy through sliding surface friction. f

4. In a flexible metal propeller blade having a thickness between the rface surfaces thereof said laminated portion including integral extensions of said nonlaminated portion, vibration suppressing means in said laminated portion comprising sheet material extending less than twothirds of the blade length toward said blade shank f from adjacent the tip endof the blade and having its major surfaces extending in the direction of .blade length and breadth said material being substantially coextensive with said integral extensions and located intermediate the blade thickness and assembled in frictional contact with said integral extensions of said nonlaminated portion, to dissipate energy through slidtact and when so moved dissipating energy through sliding surface friction.

8. A metal propeller blade having a nonlaminated shank portion and a tip portion comprising two parts integrally joined at one end to each other andA to said shank portion and separated -by a narrow slot disposed in a plane extending along a chord of the blade and along the major 1 axis thereof and extending from adjacent the between the face surfaces thereof, vibration suppressing means comprising, blade laminations each having a major surface thereof extending lengthwise of said blade each lamination extend` ing only from a location greater than one-third the blade'length from the shank end to adjacent the tip end thereof, said blade tip being slotted to form said blade laminations constituted bythe metal of said blade, and a lamination of solid sheet material -secured between and in frictional contact with each two adjacent blade metal laminations and movable, upon blade vibration, relative to the laminations with which it is in contact and when so moved dissipating energy through sliding surface friction.

6. In a exibie metal propeller blade having major dimensions of length from the shank end to thel tip end thereof, breadth from the leading edge to the trailing edge thereof and thickness between the face surfacesthereof. vibration suppressing means comprising, blade laminations each having a surface extending lengthwise of said blade from adjacent the tip end and terminating less than two-thirds of the blade length from said tip and extending across the breadth of said blade between the leading edge and the leading edge/of the blade tip'portion to adjacent the trailing edge thereof and inwardly from the blade tip to the nonlaminated shank portion, solid sheet material in said cut substantially filling the same and constituting with saidy two parts blade tip laminations, and means for holding said laminations ink frictional engagement, so that relative movement of said laminations, upon blade vibration, will dissipate energy through sliding surface friction. f 1

9. A flexible metal propeller blade having `a laminated outer portion including the blade tip and a nonlaminated inner portion including the blade shank and having major dimensions of length from the tip end to the shank end thereof, breadth from the leading edge to the trailing edge thereof, and thickness between the face surfaces thereof, the laminated outer portion comprising separate closely adjacent blade parts integral with the bladeshank and whose major trailing edge thereof, alternate laminations belng metal and having a portion integral with each other and said blade, and a lamina of a material different from the metal of said blades secured between each two adjacent blade metal laminae and movable, upon blade vibration, relative to the blade metal laminae with which it is in contact and when so moved dissipating energy through sliding surface friction.

'7. In a flexible solid metal propeller blade hav- -ing major dimensions of length fromthe shank end to the tip end thereof, breadth from the leading edge to the trailing edge thereof and thickness between the face surfaces thereof, a split in the tip portion of the blade extending from the blade tip end through said tip portion in the direction of the blade length to a location less than two-thirds of the blade length from the blade tip and through said tip portion in the direction of blade breadth intermediate the thicknessof the blade tip portion separating said tipportion into two parts integrally united at their inner ends with each other and with the remainder of said blade, solid sheet material held under compression between and in frictional contact with said parts and movable, upon blade vibration, relative to the blade part surfaces with which it is in consurfaces yextend lengthwise of said blade, solid sheet material having different internal friction characteristics than that of the material of said blade held under compression between and in frictional contact with said parts and movable,

-upon blade vibration, relative to the blade part surfaces with which it is in contact and when so moved dissipating energy through sliding surface friction. v

10. In a flexible metal propeller blade having a laminated portion and a nonlaminated portion and having major dimensions of length from the shank end to the tip end thereof, breadth from the leading edge to the trailing edge thereof and thickness between the face surfaces thereof, said laminated portion including blade laminations each having a surface said blade from a location adjacent the tip end of the blade to the nonlaminated portion, alternate laminations being constitutedby the metal of said blade and integral therewith, a lamina of a materialdifferent from the metal of said blade and having different internal friction characteristics than that of thematerial of said blade secured between each two adjacentbiade metal laminae, and means maintaining said material under compression between said blade metal laminae, to dissipate energy by sliding surface friction upon relative movement of adjacent laminate upon blade vibration.

11. A metal propeller blade having a tip portion comprising two parts integrally joined at one end and separated by a narrow slot disposed in a plane extending along a chord of the blade and along the major axis thereof and extendingfrom adjacent the leading edge of the" blade tip portion to adjacent the trailing edge thereof and inwardly from the blade tip to a locationless than two-thirds of the blade length from the blade tip, an .insert of a material, having different internal friction characteristics than that of the material of said blade in said cut substantially filling the same and constituting with said two parts blade tip laminations, and means for holding said laminations in vfrictional engagement so that relative movement of said blade and said insert inciextending lengthwise of dent to blade vibration will dissipate energy through sliding surface friction.

12. A flexible metal propeller blade having a laminated outer portion including the blade tip and a nonlaminated inner portion including the blade shank and having major dimensions of length from the tip end to the shank end thereof,

breadth from the leading edge to the trailing edge thereof, and thickness between the face surfaces thereof, the laminated outer portion comprising separate closely adjacent blade parts integral with the blade shank and whose major surfaces extend lengthwise of said blade, solid sheet material having energy absorbing internal friction characteristics held under compressions between and in frictional contact with said parts and movable by said blade parts and relative to the blade part surfaces with which it is in contact and when so moved by blade vibration, dissipating energy through sliding surface friction and through internal friction of said solid sheet material.

of said blade and integral therewith, a lamina of a material different from the metal of said blade and having energy absorbing internal frictional characteristics secured between each two adjacent blade metal laminae to absorb energy through internal friction upon movement of said material by said blade under blade vibration, and means maintaining said material under compression between said blade metal laminae, to

dissipate energy by sliding vsurface friction upon relative movement of adjacent laminae.

14. A metal propeller blade having a nonlaminated shank portion and a tip portion comprising two parts integrally joined at one end to each other and to said shank portion and separated by a narrow'slot disposed in a plane extending along a chord of the blade and along the major axis thereof and extending from adjacent the leading edge of the blade .tip portion to adjacent the trailing edge thereof and inwardly from the blade tip to the nonlaminated shank portion, an insert of solid sheet material having energy absorbing internal friction characteristics, in said cut substantially filling the same and constituting with said two parts blade tip laminations, and means for holding said laminations in frictional engagement, so that relative movement of said blade and said insert incident to blade vibration will dissipate energy through sliding surface friction and movement of said insert by said blade will absorb energy through internal friction of said insert.

ERLE MARTIN.

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