Propeller Or Fan Shrouds

Abstract

An improved propeller shroud comprising a toroidal cavity formed on its inside surface along its entire periphery level with the blades of said propeller and along the meridian of the shroud and movable flaps mounted on downstream of same cooperating with a cavity bottom comprising a retractable wall with or without streamlined partition walls adapted within said toroidal cavity and along substantially meridian planes of said shroud.

Description

The present invention concerns improvements to propeller or fan shrouds. The advantages conferred by providing a shroud round a propeller or a fan have long been known.

However, the prior art shrouds involve some difficulty in fabrication because of the smallness of the permissible clearance between the tip of the propeller (or fan) blades and the inner wall surface of the shroud, for the smaller this clearance the higher the efficiency of the propeller (or fan). This in turn requires great rigidity and small concentricity tolerances on the stationary and moving parts.

Further, prior art shrouds are often equipped with diffusers, of which the design and associated devices must allow for the flow separation phenomena which occur on the trailing edges of the shroud and which it is of advantage to reduce to a minimum in order to ensure as favourable as possible an aerodynamic "balance" for the overall system.

It is the object of the present invention to mitigate these drawbacks and constraints and provide a method and means for resolving the problems of fitting a propeller inside its shroud without the need to observe the requirements for minimum clearance between the tips of the blades and the inner wall surface of the shroud, for great rigidity, and for the small tolerances mentioned precedingly, and/or the problems of causing the fluid driven by the propeller to flow along the downstream walls of the shroud without any separation phenomena.

In accordance with the invention, the inner wall of the shroud is formed with a toroidal peripheral cavity level with the propeller blades, within which the marginal eddies produced by the blade lift are trapped. In this way the flow continuity between the blades and the shroud is assured without the need to observe the close tolerances referred to precedingly.

In accordance with a second teaching of this invention, the cavity has disposed therein, along the substantially meridian plane of the shroud, partitions which are so profiled as to foster rotation of the air in a substantially meridian direction counter to the normal direction of rotation of the air driven by the blades.

It is a third teaching of the invention that the toroidal cavity has a large diameter of the same order of magnitude as the thickness of the shroud, whereby it is possible if desired to eliminate the entire upstream portion of the shroud, which becomes redundant. The bottom of said cavity may either be fixed or be formed by a retractable wall.

In accordance with a fourth teaching of this invention, the shroud for carrying the subject method of this invention into practice has movable flaps associated thereto, whereby a convergent or divergent stream of fluid is obtained on exit from the shroud.

The present invention includes in its scope such shrouds as embody the subject method of this invention.

Further particularities and advantages of the invention will become more clearly apparent from the description which follows with reference to the accompanying nonlimitative exemplary drawings, in which:

FIG. 1 is a highly diagrammatic perspective view of a shroud section according to the invention;

FIG. 2 is a sectional view in elevation of an improved shroud according to an alternative embodiment of the invention;

FIG. 3 illustrates an improved shroud according to a third possible embodiment of the invention;

FIGS. 4 and 5 are schematic sectional views in elevation of an improved shroud according to a fourth possible embodiment of the invention, with flaps and the devices associated thereto depicted in their "divergent efflux" and "convergent efflux" positions respectively;

FIG. 6 is a perspective view of an example of a movable flap associated to the shroud of FIG. 2;

FIG. 7 is a schematic top view of a succession of movable flaps in their "divergent efflux" and "convergent efflux" positions;

FIG. 8 is a perspective view of a detail of certain component parts of FIGS. 4 and 5, in the position corresponding to that of FIG. 5;

FIGS. 9 and 10 are diagrammatic sectional views in elevation on an enlarged scale of the mechanism for actuating the flaps;

FIG. 11 is a sectional plan view on a smaller scale of an improved shroud according to the invention;

FIGS. 12 and 13 are schematic sectional views in elevation of an improved shroud according to yet another alternative embodiment, with flaps and their associated devices shown respectively in their "divergent efflux" and "convergent efflux" positions; and

FIGS. 14 and 15 are schematic sectional views in elevation on an enlarged scale, respectively portraying the fitting method and the flap actuating mechanism for the embodiment illustrated in FIGS. 12 and 13.

Referring to the accompanying drawings and more particularly to FIGS. 1 and 2, it will be seen that the inner wall surface of shroud 1 is connected to the body 1a of the engine by braces 1b and that, in accordance with a first embodiment of the subject method of this invention, said inner wall surface is formed with a toroidal cavity 2 level with the propeller blades 3. It will immediately be appreciated that when the blades 3 rotate in the direction of arrow F, air will be caused to eddy in a continuous flow along the arrows F.sub.1. The marginal vortices generated by blade lift will thus be "trapped," whereby this simple form of embodiment ensures flow continuity between the blades and the shroud.

In the embodiment illustrated in FIG. 2, the toroidal cavity 2 is likewise formed on the inner wall surface of shroud 1, but in this case profiled partitions 5 are disposed therein along substantially meridian planes in order to form sectors 4, clearly shown in FIG. 11.

As the propeller blades 3 rotate, the presence of the partitions constrains the air to rotate in the direction of arrows f.sub.1 instead of being entrained in the direction of rotation of the propeller. Further, this rotation in the direction of arrows f.sub.1 causes the layer of air on exit from the shroud to flow along the downstream side of the shroud walls without any flow separation phenomena, as depicted by the arrow f.sub.2.

With the problem of close separation resolved thus, it will be appreciated that diffusers or movable flaps (such as 7) can be associated to a shroud devised in accordance with this invention, so as to obtain a variably converging or diverging efflux, thereby making it possible to ensure optimal matching of the internal throughput to the fan blade pitch, irrespective of the velocity of the external fluid. In FIG. 2 the solid lines illustrate a fixed flap or a movable flap in its divergent position, and the dashlines show the same movable flap in its convergent position.

In one possible form of embodiment, the flaps assume the form shown in FIG. 6 and are hingedly mounted about a hinge-pin 8 fast with the trailing edge of the shroud, and these flaps cover one another partially after the fashion of scales and, upon being actuated by any convenient means well known per se, are caused to occupy the divergent (A) or convergent (B) positions (see FIG. 7).

The advantages as to nonobservation of the tolerances, continuity of flow, and flow along the trailing edges of the shroud walls without flow separation phenomena are to be found once more in the case of the embodiment shown in FIG. 3, but with an additional advantage. In accordance with this embodiment, the toroidal cavity 2 is given a large diameter of the same order of magnitude as the thickness of the shroud, thereby enabling all the upflow portion of the shroud (shown in dashlines in the drawings) to be dispensed with since it becomes redundant. Clearly, the bottom 6 of such a cavity will be fixed in that event. In accordance with the present invention, partitions 5 (shown in dot-dash lines) may or may not be provided. The advantages and possibility of associating flaps on the downstream end, referred to precedingly, is likewise applicable to this particular embodiment which is illustrated in FIG. 3.

FIGS. 4, 5 and 8 to 11 illustrate another possible embodiment of the invention which is especially advantageous in the case of vertical takeoff craft. It is well known, indeed, that the exhaust jet must be divergent in still air and that the ratio of the outer diameter to the inner diameter of the shroud must be greater than 1:20. In high speed forward flight, on the other hand, the shroud must be of small thickness in order not to create parasite drag, while the intake must be noncambered or cambered inwardly slightly, and the exit must be convergent. The form of embodiment shown in these figures provides a compromise solution on the configurations imposed by limit operating conditions.

As in the case of the embodiment illustrated in FIG. 3, this alternative embodiment includes profiled partitions 5 positioned substantially along the meridians of the shroud and a bottom consisting of a retractable wall 9. These partitions are hollow and preferably sufficiently thick to house the various control means to be described hereinafter. The retractable wall is preferably made of an elastic material and is rigidly united with the shroud at 1c; its trailing edge carries a rod 10 actuated by a link 11 housed within 5 and hinged at 12. Rod 10 is furthermore guided in its motion by a circular arc-shaped groove 13. Thus it will be comprehended that when the link 11 is actuated (by any convenient means well known per se), the elastic wall or membrane 9 can assume either of two limit positions depicted in solid lines and dashlines respectively in FIG. 9 and represented in FIGS. 4 and 5. In the configuration of FIG. 4 this membrane forms the bottom of the toroidal cavity, whereas in the position shown in FIG. 5 the bottom of the cavity is retracted. The advantages of this particular embodiment will emerge hereinbelow.

In combination with this retractable bottom, the invention provides for an annular flap 7a which may either be devised as shown in FIG. 6 or be made, in accordance with an alternative embodiment, of some resilient material which can be fitted and controlled as follows (see FIG. 10).

The flap 7a has its leading edge 14 made fast with the partitions 5 by any convenient means (such as a keeper ring). A metal reinforcement 15 is buried in its midst and carries a set-square-shaped part 16 which is hinged at 17 and connected at 18 to a rod 19 which is controlled by a link 20 pivotally connected to the shroud 1 at 21. When the link 20 is in the position shown in solid lines in FIG. 10, the annular flap 7a will be in the position shown in solid lines on the same figure. Conversely, when the link 20 is caused to move into the position shown in dashlines in FIG. 10, the part 16 pivots about the hinge point 17 and moves the flap 7a into the position shown in dashlines, whereupon the interconnection point 18 moves to the position 18a.

Actuation of retractable wall 9 (by means of link 11) and of flap 7a (by means of link 20) can be synchronized by any convenient means which it would be unnecessary to describe here since they are familiar to the specialist in the art. As a result of such synchronism, retractable wall 9 and flap 7a may concurrently occupy variable positions, the limit positions being shown diagrammatically in FIGS. 4 and 5.

The advantages of this embodiment and its satisfactory operating efficiency in the case of application to vertical takeoff craft in particular will readily be appreciated. Indeed, an examination of FIGS. 4 and 5 shows that in case of a divergent exhaust for takeoff (FIG. 4), the stream escaping from the periphery of the propeller is impelled forwardly (arrows f3) and thereafter sucked in once more to form a "trapped" vortex in the partitioned cavity sections of the shroud (arrows f4). An appropriate degree of twist to the blade tips will impart to the air recycled in said vortex and to the air immediately adjacent to the neighboring layer the high total head needed to stabilize the diffusion process on the streamlines flap 7a.

Conversely, in the case of the convergent exit during forward flight (FIG. 5), it will be noted that the shroud is highly "transparent" to the air flow (arrows f5) and creates little parasite drag because of the thinness of the aerofoil sections and the small wetted area. The retractable wall 9 accordingly provides a compromise between the ideal shapes for operation in still air (FIG. 4) and in high-speed flight (FIG. 5), respectively.

Referring now to the alternative embodiment shown in FIGS. 12 through 15, the inner wall surface of the shroud 1 connected to the engine pod 1a, by means of braces 1b is formed with a toroidal cavity 2 level with the propeller blades 3, which cavity has a diameter of the same order of magnitude as the thickness of the shroud, thereby making the entire upstream portion of the shroud redundant and enabling it to be dispensed with. In accordance with the invention, streamlined partitions (in dot-dash lines) may or may not be positioned substantially along the meridians of the shroud. The bottom of the shroud is formed by a retractable wall 9a which may be either rigid or elastic.

The upstream edge of said wall supports a rod 10a guided along a slideway 13a formed in the shroud 1. This rod is connected to a nut or threaded sleeve 21 cooperating with a screw 11b which is rotated by a suitable gear train represented schematically on FIG. 15 by reference numeral 22. Rod 10a also forms a hinge pin for the link 19a, the other end of which is pivotally connected at 8b to the upstream end of a flap 7c capable of pivoting about a downstream hinge pin 8a. A well 23 is provided in the downstream part of the shroud for receiving the flap 7c when the same is in its "convergent efflux" position (see FIG. 13 and the position shown in dot-dash lines in FIG. 15). Clearly, if the bottom 9a is rigid enough, the links 19a can be dispensed with.

From the foregoing description (taken in conjunction with FIG. 15) it will be seen that when the gear train 22 is activated by any convenient means well known per se, the nut 21 will move along the screw 11b, entraining with it the rod 10a which slides along the slideway 13a, and that at the same time the upstream end of link 19a will follow the same motion and its end 8b will drive the upstream end of flap 7c and cause the downstream end thereof to pivot about 8a, whereupon said flap will assume the position shown in dot-dash lines in FIG. 15 as it penetrates into the shroud well 23.

The two limit positions of the flaps 7c are shown in FIGS. 12 and 13. FIG. 12 corresponds to the divergent configuration of the streamlines F.sub.6 subsequent to trapping (F.sub.7) in the shroud with its toroidal cavity (provided or not with partitions 5), while FIG. 13 corresponds to the convergent configuration of said streamlines.

In the form of embodiment shown in FIGS. 4, 5 and 8 through 11, actuation of the retractable wall and of the flap 7c is synchronized, as a result of which synchronism the retractable wall and the flap 7c can simultaneously occupy variable positions, the limit positions being shown diagrammatically in FIGS. 12 and 13. The advantages offered by this embodiment will again be appreciated from the explanations given precedingly, and it is to be noted in addition that, in its convergent exit configuration, the shroud no longer has only one thin aerofoil section of minimum drag.

It is to be understood that the present invention is by no means limited to the description given with reference to a preferred exemplary embodiment, and that many changes and substitutions of parts may be made without departing from the scope of the invention.

Claims

I claim:

1. An improved blade propeller or fan shroud comprising a toroidal cavity along its entire periphery level with the blades of said propeller or said fan and along the meridian of the shroud wherein said toroidal cavity has a width of the same order of magnitude as the thickness of the shroud, comprises a bottom so that the upstream portion of said shroud is made redundant and, along substantially meridian planes of said shroud, streamlined partition walls, whereby rotation of the air is assisted in a substantially meridian direction counter to the normal direction of rotation of the air driven by the blades, and movable flaps mounted on the downstream end of said shroud.

2. An improved propeller or fan shroud, comprising on its inner surface and along its entire periphery a toroidal cavity having a width of the same order of magnitude as the thickness of the shroud, a cavity bottom comprising a retractable wall, movable flaps mounted on the downstream end of said shroud, and means operatively connected between said flaps and said bottom whereby to cause motion of the latter to impart motion to said flaps between two limit positions, to wit a divergent exhaust position and a convergent exhaust position.

3. A shroud according to claim 2, wherein said retractable wall is united therewith by its upstream edge, its downstream edge comprising said means for imparting motion to said movable flaps.

4. A shroud according to claim 2, wherein said motion-imparting means include a linkage system operatively connected to said movable flaps.

5. A shroud according to claim 4, wherein said movable flaps pivot about their upstream edge.

6. A shroud according to claim 4, comprising a well on its downstream side, wherein said movable flaps pivot about their downstream edge in order to retract, in their limit convergent exhaust position, into said well and form a single thin aerofoil section with said shroud.

7. A shroud according to claim 2, comprising a slideway-forming groove, one edge of said retractable wall sliding therein.

8. A shroud according to claim 3, further comprising streamlined partitions within said toroidal cavity along substantially meridian planes of said shroud.