US1663512A

US1663512A - Propeller

Info

Publication number: US1663512A
Application number: US240853A
Authority: US
Inventors: Breguet Louis
Original assignee: Ateliers dAviation Louis Breguet SA
Current assignee: Ateliers dAviation Louis Breguet SA
Priority date: 1926-09-01
Filing date: 1927-12-17
Publication date: 1928-03-20
Anticipated expiration: 1945-03-20

Description

March 20, 1928,

L. BREGUET PROPBLLER I Filed Dec. 17. 1927 44 Sheets-Sheet 1 mwN N y m March 2o, 192,8. A 1,663,512

l..l BRGUET PROPELLER Filed Dec. 1'7. 1927 4 Sheets-Sheet 2 March 20, 1928. 1,663,512

L. BREGUET PR OPELLER Filed Deo. 17, 1927 4 Sheets-Sheet 5 a'A Zg March 20, 1928.

K 1,663,512 l.. BREGUET PROPELLER Filed' Dec. 1v; 1927 4 Sheets-Sheet 4 Patented Mar. 2o, 192s.

l UNITED STATES PATENT OFFICE.

LOUIS BREGUET, OF PARIS, FRANCE, ASSIGNOR T SOCIETE ANONYME DES ATELIERS l DAVIATION LOUIS BREGUET, OF PARIS, FRANCE,l .A COMPANY OF PROPELLER.

i Application led December 17, 1927Seria1 No. 240,853, and in `lFrance September 1, 1926.

The distortions to which propellers are subjected will usually modify their properties, and chiefly in the case of propellers with thin blades. This effect -is-more strongly 5 marked in the case of metallic propellers, in which the blades consistof metallic plates which are suitably bent into shape.

The present invention has chiefly for its object a metallic propeller in which all such defects are obviated. The cross sections of the propeller blades are herein gradually increased in area from the end of the blade to the hub, and due to this form I am en-i abled to improve the aerodynamic properties of the ,propeller by diminishing the prejudicial deformation due to torsion, and also to increase the strength, the'dura2 tion andv the reliability of the propeller.

In one form of construction, the shape given to the propeller blade is such that the surface at its middle. part will be a' surface formed by rectilinear generatrices, and the surface at the end of the blade will consist of a cone whose axis is displaced to the rear of the axis of reference, and is then twisted and is optionally bent.

The propeller may be constructed without a hub, and may be secured by bolts inserted through suitable lugs provided at a point near the centre' of the propeller.

The following description with reference to the appended drawings which are` given by way of example, shows the manner in which the said invention lis carried into effect. e j l Figs. l to 7 are diagrammatic views of an explanatory nature. l

Fig 8 .shows an outline of the propeller blade, together with various cross sections, and

Fig. 9 is a corresppnding'view in the 90 position. j' Fig. 10 is a View on a larger scale, corre- 'sponding to Fig. .9, showin surface of the central part. j.

Fig. 11 is an outlineview of the middle part of the propeller, Y r

In propellers -having an approved outline according to current practice, the displacements of the centre of thrust take. place,

=ing which is about 4 per cent on the g the regulated in a direction 'such that the pitch will increase with an increase ofthrustwhen the apparatus rises, thus reducing the efficiency as well .as the number of revolutions, and hence reducing the ascensional speed.

For instance, in the case of a solid propeller in which the section of 'a blade o is shown in Fig. 1, this section representing an' will be situatedmuch to the rear when inhorizontal Hight near the ground with full admission of vgas (operation` at low thrust), so that the propeller will tend to increase its pitch upon increase of thrust when rising and to lose4 pitch upon decrease of thrust when horizontal. Due to the distorsion of the blades, the difference between the aerodynamic effects in theser two conditions will exceed the diii'erence offered by a rigid propeller, and the device will show greater variations in eiiiciency.

For a metallic propeller whose cross sections have a uniform relative thickness of 10 per cent (the ratio between the maximum thickness e of the section and the length off the section p Fig. 1) the difference betweeni the operating` angle when in level Hight' with full gas admission, and thev angle when riskaverage for the section situated at 2/3 of the radius, will be increased by per cent, by virtue of the elastic deformation due to torsion.

Ifv the propeller is so constructed as in normal practice that this diierence between these angles includesthe attacking angle correspondingto the aerodynamic factor or co-v eicient, of the aforesaid cross-section (the said coeiiieientbeing gg in which Cy -is the fhrustand ce the -timii or drift) a is pbserved that the said coeicient will be reduced by 4.5 per cent when in rising flight and by 8% for level flight with open throttle, so that the eiliciency will not be the same for the two cases. f

If due account is taken of this elastic deformation in the optimum adaptation of the' propeller for level flight, this latter will bye all 'the more unfavourable when in ascending flight. The efficiency will thus be reduced, and further, ther increase in the pitch will cause a great reduction in the number of revolutions, and hence a reductionin the ascendingspeed and in the aptitude of the aeroplane to leave the ground.

If the solid propeller is given a cross section in which the centre ot thrust is not displaced, such as a symmetrical section, the pitch ofthe propeller will usually be increased in the two aforesaid conditions of flight, i. e. level flight with open throttle and ascending flight. butl the increase will be greater when rising than on a. level, so that the prejudicial difference between the angles of incidence will increase to a less degree than in the preceding case. In fact, for a symmetrical propeller section, as shown in Fig. 2, whose centre of gravity G is situated at about 40 per cent from the leading edge A, and the centre of thrust `R1 at 22 per cent. the propeller traction when in ascending flight being about 1.4 times the traction in level Hight, the said propeller will increase in itch to a greater degree when `rising than 1n level flight, and the increase in the difference betweenthe angles of incidence will be about 35 per cent for 'a metallic propeller whose relative thickness is 10 per cent, and the variation in the aerodynamic factor will be 6.5 per cent.

To obviate such defects I provide various improvements in the construction of wood or metal propellers, whereby the neutral line or median line of gravity will be broughtv forwardly of the centre of thrust.

According to these improvements, the .rear part of the propeller is made hollow, as

'shown at 101 Fig. 3. so that the greater mass of the material will be located near the leading edge or I may make a suitable choice of heteregeneous substances, the substances having the mass or density being placed at or near the leading edge, or .like means may be employed. Referring to the case of Fig. 1, andif this is applied to a hollow' 'blade (Fig. 3) having a cross section in which the centre of thrust is displaced between R, and It1 from 40 per cent to 50 per cent of the length of the section (Fig. 3), it is simply necessary to place the neutral line G snflieiently near the leading edge in order that the moment of 'torsion which tends to reduce the pitch of the propeller will be greater at increased thrust, as when in ascending flight.

If the traction or thrust when rising is 1.4 times the traction on the level, I may place the neutral line G at a distance (l from the leading edge which is less than 22 per cent of the length of the cross section.

However, this principlevis not generally applicable, due to-practical considerations.

Like results may be obtained by curving the neutral line towards the rear of the propeller. l v i Fig. 4 is a front v/icw of a propeller blade thus arranged; it rotates about an axis O perpendicular to the figure in the direction of the arrow .F. The neutra-l line (ir of jhis blade is curved in the rear of `the plane In certain cases, this arrangement is subjected to Vcertain additional flexionstrcsses due to centrifugal force, which is a disadvantage.

T o recapit-ulate, it is observed that. a propeller blade is distorted by torsion about its yneutral line, or axis of tlie blade, by virtue of changes in the couple due to aerodynamic eflorts whose lines of action are usually displaced and whose values change.

This distortion takes place in uncompensated propeller blades in a direction such that the eilici'ency is changed, and it becomes worse according as the blade can be more readily distorted.

When in ascending Hight, the action of the aerodynamic resultante will tend to increase the pitch; the resulting increase in the leading angle causes firstlya reduction of the eflicieney of the propeller and secondly a reduction of the number of revolutions, and hence a diminution of the maximum power which the engine may be called upon to furnish when rising.

Due to these two causes, the speed in ascending flight will be reduced. y

The known methods :for obviating such defects have had but little success by reason of practical difficulties.

The metallic propeller, which is comprised in the present invention, is chiefly characterized by the following features whereby the said defects are obviated.

The relative thickness of the cross sections, i. e. the ratio of the maximum thickness to the length ot the section. increases gradually from the outer edge of the. blades to the hub. Due to this gradual increase. I can greatly increase the polar moment of inertia, i. e. the moment of inertia with reference to the centre ot gravity, relatively to cross sections which have the same area but whose relative thickness is smaller. and this affords a solid which will withstand the deformations due to torsion which act as a rule, in the ease of propellers, to change the lll) elastic distortions the efficiency.

For each cross section, the relative thlckwhich are prejudicial to l'ness of the outlines (or cross sections) is carefully selected in such manner that the variations of the angle of the aerodynamic action between the extreme conditions of functioning, which variations increase from the end of the blade to'the propeller shaft,

will correspond to the greatest possible values ot' the minimum aerodynamic coetticient of the sections, as above defined. I amthus justified in the use of outlines which are vthin at the ends and of outlines which are thicker at the hub, in which case the variations in the incidence are greater.

.The propeller according to the invention y comprises compact sections adjacent the hub which have Va stout and substantially rectangular shape. The shape of the said sections is such that their moments of inertia about any axis will allow them to withstand accidental stresses other than the stresses comprised in the usual calculations. On the contrary, the propellers whose shapein the' part adjacent the hub is that of a flat metallic member, will be subjected to an abnormal stress under accidental streses when the axis of the flexion couple ofthe forces resulting from these stresses is parallel with the main aXis'of the section.

The shape of the crosssections or outlines of the propeller according to the invention ,is shown in Fig. 8, which shows various whose Hexion couples will thus be more orl cross-sections suitably spaced apart.

By reasonl of its construction, my said propeller is adapted to'withst-and distortion caused by flexion, so that I- consider it advantageous to em loy a construction in which the neutral line Will assume `a determined form when in the inoperative position,l whereby the variations in the whole of the metal will be a minimum between the diferent rates of functioning, in spite of the relative variations of the value of the aerodynamic stresses and the centrlfug'al stresse f,

less compensated, and this will occa-'tion stresses which will be properly added to the other elastic stresses.

In Fig. 5, OG' represents 'thel neutral line of a propeller blade, which is rotatable on the axis OX and Whose equilibration is considered` in certain conditions of functioning, for instance when operating with open throttle and in level flight.

L The moment which is the resultant of the centrifugal stresses R1 and the aerodynamic stresses R2, relatively to the section S of the propeller, for example, is n1l;

In other operating conditions inif'which the centrifugal stresses are reduced 1n the ratio of 0.7 and the aerodynamic stresses are 1ncreased in the ratio of 1.3, the moment of iexlon due to the aerodynamic stresses 1s the moment ot flexion due, to the aerodynamic stresses is 1 .3 2R2d2; the resultingr moment is y 0.7E1i1d1, whence The stress. due to this moment is added to the stresses due to centrifugal force.

I may, in accordance with the invention, so dispose the apparatus that the said resulting moment will be distributed between the two operating conditions specified, by properly curving the neutral line or fibre.

rIhis arrangement is shown diagrammatically in Figs. 6 and 7. Let us consider a cross-section of a bla-de perpendicular to the neutral line; S is the surface of this'section which is shown in Fig. 7. It is required to deteromine ythe stresses in this section for given operating .conditions R of the' aeroplane.

The diagrams show firstly, the moment of the aerodynamic effects Ma acting upon the portion of the blade comprised between the i section S and the end of the blade; secondly. the moment of the centrifugal effects M acting upon the portion of the blade comprised between the section S andthe end of the said blade; this moment depends upon Vthe form which is given to the neutral line or fibre. Y

The combination of these moments affords a resultant MR, which is projected upon the main axes ot' inertia Ox 1 and Oyl at Mwl and My, 7).

If N represents the resultants of the centrifugal stresses on the portion between the section S and the end ot the blade, the stress at a` given point on the co-ordinates m, y

will be 'zNiMe..

S Ia'l MII/1m I .l

Iii/1 I j served that in any cross-section of the propeller, the points of greatest stress onv the outlineof the cross-section will be subjected to variable rates of work, which pass through maximum for determined conditions R R R" etc.

iis

If these maximum stresses are designated by am", am", amc etc. there will lbe two values which are-greater than the others, for instance wm and nm, which may be termed the extreme maximum stresses.

'lhe moment M.L which is due to the aerodynamic stresses, and the stress N, are const ant; but the moment Mc (moment of centrifugal stresses) can be changed by modi- Vf'ynig the neutral line OG (Fig. 6) so that l may regulate the rate of work at the points of maximum stress and so dispose the apparatus that. the cxtreme stresses am and am will be adjacent one another, and irrespectively of the functioning of the apparatus, the propeller will in all cases operate under the same coefficient of stress.

All propellers in which -according to the preceding calculations t-he. extreme working conditions nm and 11m differ by at least 30 per cent, are propellers whose neutral line has a form whichl is cbmprised in theupresent invention. Q f

This form which I give to the neutral line, as a result ofthe comparative observations of the stresses in different conditions vof flight, presents a new character as compared with the known arrangements in which the equilibration is observed for a given condition of functioning, or as compared with the construction in which the blades may assume a certain position, or form due to elasticity, or due to a certain degree of freedom of motion, according to the Renard device,

by which position or yform the flexion due to air reaction will be equilibrated by the centrifugal forces.

In the deformable propeller, I add to the said effects, the elastic stresses due to the variations in the curvature .Ae of the neutral line or fibre. l 'n R A p E 2J 1peller will be more reliable and more durable than a propeller having a thin cross section.

.facturedl by shaping it from a forged, or

c c n D a n prlnclple, my said propeller .is manuother, blank, starting with a straight piece of metal Whose axis is preferably parallel with the axis of the propeller which is gradually twisted or 'flattened as concerns the I-have represented by way of example in Figs. 8 to 11 and chiefly in Fig. 10 a. propeller whose central part is formed by two generatrices parallel with the plane of rotation which is the plane of the Figure 10.

In Fig. 9, the regulated surface extends from the centre of the propeller to the vertical line shown a little beyond half the radial length. From this linemto the end of the blade, the form is similar to that of a flattened cone whose axis OA is brought to the rear of the line OU which is the main axis of the propeller; the latter rotates according to the arrow F.

The said cone is twisted, and in order to give to the cross-section the proper aerodynamic' effects they may bel bent after trimming, so as to obtainthe exact neutral linedesired.

One o'f the main features of my said propeller consists in the fact that it is not provided with a hub, and the device is secured by the lugs a t) c d (Fig. 10) provided at the centre of the propeller, these being traversed by the bolts e f g L which are herein four in number, said bolts engaging and fastening the propeller'to a plate] Jmounted on the crankshaft lo of the engine (Fig. 11).

What I claim is: y

1. A propeller blade in which thecubical content of the leading edge portion is of greater specific gravity than that of the remaining portion.

2. A propeller blade having a hollow ina7 terior portion in the section remote from the leading edge to dispose. the median line of gravity forwardly of the centerl of thrust.

3. A propeller blade having a portion within its surface rearwardly of the leading edge portion reduced in mass sufficiently to locate the median line of gravity between the leading edge and the line of thrust.

4. A propeller bla-de in which the median line of gravity is situated between the' leading edge and the line of thrust, and closer to the leading edge.

5. A propeller blade having a body whose cubical content is .composed of two substances ofwidely different specific gravity, the leading edge portion being composed mostly of the heavier substance and the remaining portion composed mostly of the lighter substance tol loc-ate the median line of gravity between the leading edge portion and the line of thrust.

In testimony whereof I have signed this specification.

L" LOUIS BREGUET.