US2693242A

US2693242A - Variable pitch screw propeller

Info

Publication number: US2693242A
Application number: US183115A
Authority: US
Inventors: Arthur E Elmer
Original assignee: Individual
Current assignee: Individual
Priority date: 1950-09-05
Filing date: 1950-09-05
Publication date: 1954-11-02
Anticipated expiration: 1971-11-02

Description

NOV- 2, 1954 A. E. ELMER VARIABLE PITCH SCREW PROPELLER 2 SheetS-Sheet 1 Filed Sept. 5, 1950 @ma `23a Now 2, 1954 A, E, ELMER I 2,693,242

VARIABLE FITCH SCREW PROPELLER Filed Sept. 5, 19,50 2 Sheets-Sheet 2 Inventor E15-'ca /-ZLW- E. 5271er l eicient in operation.

United States Patent O 2,693,242 y VARIABLE PITCH SCREW PROPELLER Arthur E. Elmer, Gloucester, England Application September 5, 195i), Serial No. 183,115 7 Claims. (Cl. 170-16016) This inventionrelates to variable pitch screw propellers and isl concerned with propellers of this character intended more particularly, though not exclusively, for use with models, such as model aircraft or boats.

The main object of the invention is to provide a variable pitch screw propeller of extreme simplicity which may be of small size and yet be of robust construction.

Another object is to provide a variable pitch screw propeller which will be economical in production and Further objects are the provision of an improved and simplified construction of screw propeller which will have substantially constant -speed characteristics for all practical purposes in the contemplated manner of use of the propeller and which will be completely automatic in operation.

With one or more of these objects in View the invention comprises a variable pitch screw propeller wherein each blade is separately mounted in the hub and provided near its root with a bob-weight a component of the centrifugal force due to which tends to turn the blade about its axis to coarsen its pitch angle against a spring force tending to hold the blade in the fine pitch position,` the arrangement being such that as the rotational speed of the propeller increases the bob-weight moves about the blade axis to coarsen the pitch angle of the blade and thereby maintain such speed substantiallyvconstant.

According to another feature of the invention means are provided to enable each blade to 'oe set in its fine pitch position, the movement of the bob-weight to coarsen the pitch angle of the blade rendering said means ineffective whereby, after coarsening of the pitch angle has occurred, upon substantial reduction of the rotational speed of the propeller the blades may turn about their axis past the fine pitch position into a fully feathered position. This may be achieved, for example, by making the bobweight with an eccentric portion adapted to abut against a shoulder on the hub or a member associated therewith to form a fine pitch stop, movement of the bob-weight I about the blade axis to coarsen the pitch angle of the blade removing said portion from its abutment whereafter due to the cccentricity of said portion it turns about the axis of a pin on which the bob-weight is pivotally mounted to take said portion out of the path of the abutment whereby, upon reduction of the rotational speed of the propeller below the predetermined value,v the blade returns past the ne pitch stop into the fully feathered position.

In carrying the invention into elfect in connection with a model aircraft screw propeller, for example, the propeller mayvhave two, three or more blades separately mounted in the hub. One embodiment of the invention in the form of a two-bladed propeller for model aircraft will now be described, by way of example, with reference to the accompanying drawings, in which:

Figure 1 is a section on the line 1-1 of Figure 2 showing the propeller with the blades partially broken away and a portion of the engine shaft on which the propeller is mounted,

Figure 2 is a plan view of the propeller illustrating one of the bob-weights in the ne pitch position, the blade profile superimposed in full lines being that of the associated blade also in the line pitch position, the remaining blade profiles shown in broken lines being those of such blade in the coarse pitch and fully feathered positions,

Figure 3 is a front elevation with the blades removed and with the bob-weights again illustrated in their une pitch positions, l

2,693,242 Patented Nov. 2, 1954 Figure 4 is a section on the line 4-4 of Figure 2 showing the mounting of the bob-weight on one of the blade roots,

Figure 5 is a section substantially on the line 5-5 of Figure 1 illustrating a Vernier adjustment for the propeller setting, and

Figure 6 is a diagrammatic side view of the upper half of the hub shown in Figure l and illustrating an arrangement of cam surfaces on the feathering and coarse pitch stops to facilitate resetting of the propeller to fine pitch from the fully feathered position.

Referring to the drawings, the hub 10 of the propeller is split along its vertical transverse centre-line as shown at 11, 11 in Figure 2, being constructed for example as two identical die-

cast halves

12, 13, and has a substantially cylindrical bore 14 extending from end to end therethrough. The hub is also formed with a bore 1S extending therethrough from front to back to receive a reduced extension 16a, 16h of the engine shaft 16, the forward end of the portion 1Gb of the latter being threaded at 17 to enable a nut 18 to be screwed thereon to retain the hub on the shaft. In addition to its function as a hub-retaining nut the member 18 constitutes a fairing for the front of the hub, being of streamline form annularly cored for lightness around a central spigot portion 18a. The latter is axially drilled as shown at 18e, the outer end of the drilling being tapped at 18d so that when it is engaged with the thread 17 on the engine shaft the annular clamping face 15b of the nut bears on the front wall of the hub 10 and presses its rear wall against the shoulder formed between the main portion of the engine shaft 16 and the reduced extension 16a of the latter. Due to the fact that the spigot portion 18a of the nut extends beyond the annular clamping face 1819y it projects into the bore 1S and is made of the same diameter as the reduced portion 16a of the engine shaft so that it closely fits the bore and forms a radial location for the front end of the hub equivalent to that provided by the reduced portion 16a of the engine shaft at the rear end. The nut 18 is also provided with a diametral drilling 18e near its front end for the reception of a tommy bar to facilitate tightening and removal of the nut.

The

blades

19, 19 of the propeller may be of Wood or moulded plastic and their roots are formed with tapped bores 9 to receive the screwed shank portions 20a of metal adaptors 20. The end 20b of the latter which extend beyond the blades are of cylindrical form to constitute extensions of the root ends of the blades and enter the bore 14 inthe hub. A spring washer 21 is interposed between the annular end face of the blade root and the shoulder formed between the portions 29a and 20h of the adaptor for a purpose to be referred to hereinafter. Located in annular grooves 10a formed one towards each end of the bore 14 are blade-retaining rings 22 through the central sleeve-like boss portions 22a of which pins 23 pass so that the heads 23a of the pins abut the inner ends of the sleeve portions. The portions of the pin Shanks passing through the sleeve portions 22a of the blade-retaining rings are plain but their intermediate portions 23b are threaded to enable the pins to be screwed into tapped portions Zlc 'i of bores formed axially in the metal adaptors 20. Towards their inner ends such bores are plain, as shown at 26d, and they terminate in plain portions 20e separated from the portions 20d by annular shoulders 201i. Due to this construction, when the pins 23 are screwed into the tapped portions 20c of the adaptor bores, plain portions 23C of the pins pass into the bore portions 20d while their reduced ends 23d enter the portions 20e of the bore and projectslightly therefrom. The pins are screwed inwardly until just sufficient clearance is left between the end of the portions 23C thereof and the shoulders 20j to give the desired amount of end float whereafter the projecting ends of the portions 20d of the pins are riveted over as shown at 23e. By this means the blades are held in the hub with the annular end surfaces of the ends 20h of the blade adaptors lying against the blade-retaining rings 22.

Around the sleeve portion 22a of each blade-retaining ring a torsion spring 24 is arranged one end 24a of which is bent radially inwards to enter a radial bore in the head 23a ofthe respective pin 23 while the other end 2417 projects longitudinally to pass into a hole 22b in the blade-retaining ring 22. The latter, as illustrated in. Figure 5, has a pair of diametrically

opposed recesses

37, 37 formed in its peripheral. edge into which inwardly extending lugs b carried by the hub project. in order to prevent turning movement of the blade retaining ring relatively to the hub. By this. means the spring 24 may be arranged to exert a torsional preload' on the blade tending to turn the blade into the fine pitch position which is represented by the blade profile 19a drawn. in full lines in Figure 2.

A pin 25, Figures 2 and 4, extends radially outwards through the blade adaptor 20 and. on the outer end of the pin a small bob-weight 2'6 is mounted so as to be rotatable thereon. Offset from a diameter of the bobweight is a radially and inwardly projecting portion 27 of the substantially rectangular shape shown in- Figure 6 and which, with the bob-weight in the position in which it is set for take-off, abuts against a shoulder 28, Figures 2 and 3, formed by a cut-away portion 29 of the outer end of the wall of the. hub 10. This shoulder 28 locates the bob-weight 26 to one side of the plane containing the blade axis and the longitudinal centre-linevof the propeller, i. e. the plane of Figure l, preventing the spring 24 turning the blade 19 to a finer pitch angle and thus constituting a fine pitch stop. By reason of this positioning of the bob-weight 26 to one side of the aforesaid plane the centrifugal force due to the bobweight when the propeller is rotated will have a component acting normal to this plane and this centrifugal twisting moment will tend to turn the blade 19 about its axis in a direction opposite to that in whichv the torsion spring 24 acts.

At a point angularly spaced from the line pitch stop or shoulder 28 by an amount equal to the predetermined power range of the propeller (approximately 20 inthe embodiment illustrated in the drawings) the end face of the hub 10 is formed with a projecting portion30, Figures 2 and 6, which provides a shoulder 31 at the other end of the cut-away portion 29 of the wall from the shoulder 28. This shoulder forms a coarse pitch stop limiting the rotation of the pin and the bobweight 26 carried thereby about the blade.v axis in a counter-clockwise direction in Figure 2, and the consequential rotation of the blade 19 to the coarse pitch position which is represented. by the blade profile 19b drawn in dotted lines in this figure. portion 32, Figure 2, is formed on the end face of the hub which provides a shoulder 33 lying, like the shoulder 31, in the path of the pin 25 so as to limit ther rotation of the latter about the blade axis in a clockwise direction in the figure. the shoulder 33 the consequential rotation of the blade 19 brings it to the fully feathered position represented by the blade profile. 19C drawn in chain-dotted lines in Figure 2, this shoulder thus constituting a feathering stop.

The propeller illustrated in the drawings. is designed so as to be adjustable for service at any desired rotational speed within a wide range, say between 4000 r. p. m. and 12000 r. p. m. The adjustment meanswhereby the propeller may be set for a given r. p. m. of the engine shaft 16 are illustrated in Figure 5 from which it will be seen that the head 23a of the pin 23 is formed with three.

radial bores

34, 35, 36 at equiangular spacing. Moreover the blade-retaining ring 22, in addition to the pair of diametrically opposed

recesses

37, 37 to which reference has already been made, is provided with additional pairs of

recesses

38, 38; 39, 39; 40, 40 each of which pairs may be engaged with the lugs 10b on the hub instead of the

pair

37, 37. A Vernier adjustment of the torsional preload exerted by the spring 24 on the blade 19 is thereby possible by inserting the end 24a of the spring into any selected one of the

bores

34, 35, 36 and engaging the lugs 10b with any selected pair of recesses in the blade-retaining ring 22 in order to vary the angular position of the hole 22b therein, and thus the angular position of the end 24b of the spring, relatively to its other end 24a. Moreover the spring loading is chosen or adjusted in relation to the force generated by the bob-weight 26 such that the latter will not operate to move the blade 19 towards coarse pitch until this pre-selected rotational speed is reached.

In Figures 2, 3 and 4 the propeller is illustrated with Another projecting With the pin so rotated until it abuts.

the parts in the take-off position, i. e. with the internally projecting portion 27 of the bob-weight lying against the fine pitch stop 28, initial take-olf r. p. m. of the engine shaft 16 being insuicient to move the bob-weight from such stop. Upon take-olf, however, as the forward speed of the model increases towards flying speed the r. p. m. of the engine shaft increases. until the aforesaid preselected speed is reached whereupon the bob-weight 26 movesabout the blade axisthe rate of the torsion spring 24 being such that a small increase in revolutions above the predetermined rotational speed causes such movement-the blade 19 turning about its axis from the position 19a in Figure 2 towards the position 19b, as dictated by the bob-weight. This coarsening of the pitch angle of the blade reduces the rotational speed of the propeller and conversely a tendency for revolutions to fall causes the blade to move towards finer pitch.

By reason of this movement of the bob-weight 26 it leaves the shoulder or ne pitch stop 28 and the centrifugaly force' generated by the offsetv portion 27 of the bob-weight causes the latter to be rotated on its pin 25 through whereby such offset portion is turned out of the pathv of the shoulder or stop 28. Thus when the engine stops, as for example by reason of the fuel carried by the model becoming exhausted, due to the rotational speed of the propeller falling substantially, the blade 19 turns backwardly about its axis past the line pitch position and into the fully feathered position 19e in Figure 2. Such feathering occurs very rapidly due to the fact that immediately the bob-weight 26 passes from one side to the other of the plane of Figure l the component of the centrifugal force normal to suchplane is reversed in direction and its effect is thus added to the energy of the spring 24. This automatic feathering of the blade 19 results in theI trailing edge of the blade .being disposed forwardly instead of its leading edge, as clearly shown in Figure 2, but this will be of little consequence in the case of model propellers and will result in substantially increased length of flightof the model as compared with models fitted with the fixed pitch or adjustable pitch propellers at present in usev on model. aircraft.

The arrangement of torsion spring 24 and bob-weight 26 and the automatic pitch-changing and feathering operationsdescribed above for one blade is, of course, identical on the other blade, or in the case of a threeor fourbladed propeller on all the other blades, and it will be appreciated from the.` foregoing description that the propeller will operate throughout the range of engine powers at substantially constant speed, that is to say having been set by the adjustment means described with reference to Figure 5 to rotate at a given speedit will absorb the power delilered by the engine within, say r. p. m. of that Spee Means may beprovided to effect automatic re-setting of the bob-weight to the take-off position at the end of a iiight upon rotation of the blade by hand from the feathered to the coarse pitchposition. Such means comprise a cam surface 41 formed on the projecting portion 32 of the hub end face so as to extend in an inclined direction from the upper end of the shoulder 33 forming the feathering stop. This cam. surface lies in the path of the olfset portion. 27` ofthe bob-weight 26 so that immediately p rior to the pin 25. striking the shoulder 33 the portion 27 strikes the cam surface and rides up the incline causing the bob-weight, and itself, to turn about the pin 25 in the directionof the arrow to the position shown in chaindotted lines in Figure 6. A similarinclined cam surface 42 is provided on the projecting portion 30 of the hubend face and extends from the upper end of the shoulder 31, as shown in Figure 6. When the blade 19 is rotated back to the coarse pitch position with the bob-weight turned as describedv the offset portion 27 of the bob-weight strikes this `second cam surface 42 immediately in advance of the pin 25 contacting the shoulder 31 as a result of which the bob-weight is caused to rotate about the pin from the position of the portion 27 shown in dotted lines in the direction of the full line arrow so that as soon as the blade is released the spring 24 returns the blade and bob-weight assembly towards ne pitch and the portion 27 encounters the shoulder 28 and retains the bob-weight in the position shown in Figure 2. Thus re-setting of the propeller for take-off may be accomplished without touching the bob-weights which enables a spinner 43 to be attached to the propeller. as shown diagrammatically in Figure 1, without causing diculties in the re-setting operation.

The spring washers 21 interposed between the end faces of the blade roots and the adjacent shoulders of the adaptors 20 serve as a simple means for permitting the adjustment and setting of the pitch of the blades 19, constituting blade-locking washers in that their frictional bearing on the `end faces of the blade roots holds `the blades in the positions to which they are set relatively to the adaptors 20.

I claim:

1. A variable pitch screw propeller comprising in combination a hub, a plurality of blades rotatably mounted on said hub, a plurality of bob-weights one of which is carried by each blade near its root and is positioned relatively to its respective blade so as to be rotatable about an axis normal to said blade and so that, with the latter in the fine pitch position, the bob-weight is located to one side of the plane containing the blade axis and the longitudinal centre line of the propeller to exert, when the propeller is rotating, a centrifugal force a component of which tends to turn the blade about its axis to coarsen the pitch angle of the blade an eccentric abutment carried by each bob-weight and eccentrically arranged with respect to said axis about which the bob-weight is rotatably mounted, spring means connected to the hub and each separate blade respectively which tend to turn each blade about its axis in the opposite direction towards the fine pitch position, a plurality of abutment means carried by the hub and each of which cooperate, when the propeller is'stationary, with one of said eccentric abutments to limit the movement of each blade towards the tine pitch position under the influence of said spring means, each of said eccentric abutments moving to a position in which it is ineffective to engage with the respective abutment means carried by the hub during movement of the bob-weight to coarsen the pitch angle of the blade whereby, after coarsening of the pitch angle has occurred, upon substantial reduction of the rotational speed of the propeller the blade is turned about its axis by the spring means past the fine pitch position into a fully feathered position.

2. A variable pitch screw propeller according to claim l, wherein said spring means comprise a plurality of helical torsion springs each of which is arranged coaxially of a blade and is connected at its opposite ends to the hub and blade respectively and such springs, in turning each blade past the ne pitch position towards the fully feathered position, cause the respective bob-weight to pass from one side to the other of said plane whereby said component of the centrifugal force is reversed in direction and its effect added to the energy of the spring to ensure rapid feathering.

3. A variable pitch screw propeller according to claim l, further characterised by a plurality offblade-retaining rings iixed against movement axially and angularly of the hub, a plurality of members each arranged coaxially of a blade and having a head portion disposed in a bore in the hub to bear against the respective blade-retaining ring and a shank portion screwing into the root of the blade to draw the latter inwardly against said blade-retaining ring, a plurality of coil springs forming said spring means each arranged round a sleeve portion of a blade-retaining ring and having one end anchored thereto and the opposite end to the respective head in order to exert a torsional eiect on the blade, a plurality of alternative angularly spaced spring anchorages in each of said heads for said opposite end of the spring, and means for providing alternative location for each blade-retaining ring in the hub in a number of angular positions different from said plurality whereby Vernier adjustment of the torsion of each spring is provided.

4. A variable pitch screw propeller comprising in combination a hub, a plurality of blades rotatably mounted on said hub, a plurality of pins projecting substantially radially outwards of each respective blade root and which is positioned relatively to its respective blade so that, with the latter in the line pitch position, the pin being located to one side of the plane containing the blade axis and the longitudinal centre line of the propeller, a plurality of bobweights each of which is rotatably mounted on its respective pin and each having an eccentric portion carried by its bob-weight and which is eccentric relative to the axis of the respective pin whereby on rotation of the propeller each bob-weight exerts a centrifugal force a component of which tends to turn the respective blade about its axis to coarsen the pitch angle of the blade, spring means connected to the hub and each separate blade which tend to turn the latter in the opposite direction towards the tine pitch position, abutment means for each blade, said abutment means being carried bythe hub, against which said eccentric portion of each of said bob-weights abuts to limit the movement of each blade towards the ne pitch position under the iniiuence of said spring means, movement of each bob-weight about the respective blade axis to coarsen the pitch angle of the blade removing said eccentric portion from its abutment whereafter, due to the eccentricity of said portion, it turns about the axis of its pin out of the path of the abutment means whereby upon reduction of the rotational speed of the propeller below a predetermined value each blade is returned by the spring means past said abutment means, and a feathering stop for each blade arranged on the hub and disposed in the path of said eccentric portion against which said eccentric portions nally abut after passing said abutment means.

5. A variable pitch screw propeller according to claim 4, wherein each of said eccentric portions is a radially and inwardly projecting portion of the bob-weight of subtantially rectangular shape oifset from a diameter of the atter.

6. A variable pitch screw propeller according to claim 4, further characterised by cam means carried by each feathering stop which cooperate with said eccentric portion as each bob-weight moves into abutment with the respective feathering stop to effect turning movement of the bob-Weight about said pin towards its original position of abutment, a coarse pitch stop for each blade, said coarse pitch stops being carried by the hub, and additional cam means carried by each coarse pitch stop and operative, when each blade is turned about its axis until the respective bob-weight passes said abutment means to bring said eccentric portion into contact with said additional cam means, to effect further turning movement in the same direction of the bob-weight about said pin into substantially its original position in which said eccentric portion automatically engages the abutment means when the spring means turns-the bob-weight towards the abutment means as soon as the blade is released.

7. A variable pitch propeller according to claim 4, further characterised by a coarse pitch stop for each blade, said coarse pitch stops being carried by the hub, against which said pins abut to limit the turning movement of the blades towards the coarse pitch position, each blade being coaxially mounted in a bore in the hub and the latter being formed adjacent the outer end of each bore with a shoulder formed longitudinally of the hub and of suiiicient height to lie in the path of said eccentric portion in its abutment-engaging position but of insuicient height to lie in the path of said pin, such shoulder forming part of said abutment means, and two further shoulders angularly spaced one to each side of said shoulder and each of suicient height to lie in the path of said pin and formy said coarse pitch and feathering stops respectively.

References Cited in the iile of this patent UNITED STATES PATENTS Number Name Date 1,940,200 Wingler Dec. 19, 1933 1,977,031 Zipay Oct. 16, 1934 2,013,930 Squires Sept. l0, 1935 2,264,568 Hamilton Dec. 2, 1941 FOREIGN PATENTS Number Country Date 402,038 Italy Feb. 13, 1943 541,206 Great Britain Nov. 17, 1941