US3796399A

US3796399A - Kite

Info

Publication number: US3796399A
Application number: US00253843A
Authority: US
Inventors: H Wechsler
Original assignee: Individual
Current assignee: Individual
Priority date: 1972-05-16
Filing date: 1972-05-16
Publication date: 1974-03-12
Anticipated expiration: 1991-03-12

Abstract

A kite of generally triangular configuration having leading and trailing edges between which are disposed a pair of open cells to which a bridle and tethering lines are attached. The frontal portion adjacent the apex of the kite body is partially flexible and coacts with the frontal cell to define an open mouth which partially closes in response to changes in air current for stabilizing the kite in flight.

Description

United States Patent [191 Wechsler [111 3,796,399 l Mar. 12, 1974 KITE [76] Inventor: Harold R. Wechsler, 2 Buena Vista Rd., Suffer-n, NY. 10901 [22] Filed: May 16, 1972 21 J Appl. No.: 253,843

[52] U.S. Cl. 244/153 R, 244/D1G. l [51] Int. Cl. B646 31/06 [58] Field of Search... 244/153 R, 154; D34/l5 AF;

244/153 R, 154, 155 R, DIG. 1

[56] References Cited UNITED STATES PATENTS 131961157 8/1963 Levine 244/153 R UX 984,295 2/1911 Peuvot.... 698,634 4/1902 Conyne 244/153 R Perkins 244/153 R Hartig 244/153 R X Primary Examiner-Duane A. Reger Assistant Examiner-Paul E. Sauberer Attorney, Agent, or Firm-Fricdman & Goodman [57] ABSTRACT A kite of generally triangular configuration having leading and trailing edges between which are disposed a pair of open cells to which a bridle and tethering lines are attached. The frontal portion adjacent the apex of the kite body is partially flexible and coacts with the frontal cell to define an open mouth which partially closes in response to changes in air current for stabilizing the kite in flight.

8 Claims, 13 Drawing Figures PAIENIEUm 12 I914 SHEU 2 BF 3 FlGd.6

KITE

BACKGROUND OF THE INVENTION The present invention relates generally to kites, and more particularly to kites of tailless variety.

As is well known, kites have a long history of development and have countless structural aspects and configurations, all of which conform generally to the principle of aerodynamics as best understood by kite developers and persons skilled generally in the field of aero dynamics. I

The conventional kite generally employs a tail for purposes of stabilizing the kite in flight so as to diminish, to some extent, the tendency for the kite to nosedive and plunge downwardly in response to changes inwind velocity, wind direction and air current changes in general. The result, therefore, is that kite ethusiasts must employ a tethering line of greater weight and tensile strength than is most effective for flying the kite to its maximum or zenith capacity, this because of the still rather ineffective stability that the tail of the kite can impart and the requirement for the kite enthusiast to periodically pull upon the tethering line drastically to alter the nose-dive.

Recent kite enthusiasts have generally resorted to kites of the tailless variety which, because of various structure and congigurations associated therewith, have obviated the need in general for the cumbersome tail and which do have a more effective flying stability associated therewith than the typical tail kites;

However, the tailless kites, as conventionally understood, still require a rather heavy tethering line for purposes of resisting tearing thereof while the kite is in flight, as when subjected to a nose-dive or downward plunge, the kite enthusiast, at that point, generally pulling rather heavily or drastically upon the tethering line so as to stabilize the kite. Moreover, tailless kites, because of their configuration, may collide with the ground when in a nose-dive at low elevations, thereby, subjecting the kite to damage or repair.

Still other disadvantages associated with the conventional tailless kites are that they do not have the most effective and rapid ascent properties, do not have a maximum vertical altitude component relative to a minimal horizontal altitude component, cannot be effectively lofted in a cross-wind or in a wind load of minimal degree, and usually have high drag factors associated therewith.

SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide a tailless kite which is provided with structure which stabilizes the kite as the kite is retrieved and permits the kite to settle gently to the ground in a generally horizontal attitude in the absence of violent colli- SlOn.

It is a further object of the present invention to provide a kite which has what may be characterized as a brake for obviating nose-dives and which permits the wings thereof to assume a negative dihedral angle when tension is released through the tethering line at low elevations.

It is still a further object of the present invention to provide a tailless kite having a rapid ascent capacity, high lift capacity, a low drag factor, requires only a lightweight tethering line, can be rapidly retrieved, has

a minimal vertical altitude component relative to the horizontal altitude component thereof, may be lofted in a cross-wind or in a wind load of minimal degree, can be dismantled for storage and transportation and readily assembled thereafter, and which generally compensates for changes in wind velocity and wind direction while in flight without the kite enthusiast having to exert substantial effort, through the intermediary of the tethering line, for stabilizing the kite. To this end, the present invention relates generally t a kite comprising aflexible body of generally triangular configuration. The body includes a diverging pair of leading marginal edges, anapex from which the leading marginal edges rearwardly diverge, and a trailing edge generally opposite the apex. A pair of reenforcing struts are spaced from the apex for stiffening a major part of each of the leading marginal edges while leaving a minor frontal part of each of the leading marginal edges proximate the apex generally flexible. A crossbar is secured to the reenforcing struts to maintain the leading marginal edges in diverging relation while leav' ing a substantial portion of said body inwardly of the leading and trailing edges generally flexible. At least one open cell extends from the body through which air is drawn while the kite is in flight; the open cell and a flexible apex portion of the body inwardly of the flexible minor frontalpart of each of the leading marginal edges cooperatively defining a kite-stabilization mouth portion which opens and at least partially closes in response to air-currents in flight.

BRIEF DESCRIPTION OF THE DRAWINGS With the above and additional objects and advantages in view as will hereinafter appear, this invention comprises the devices, combinations and arrangements of parts hereinafter described and illustrated in the accompanying drawings of a preferred embodiment in which:

FIG. 1 is a partially fragmented generally top plan view of the preferred embodiment of the kite pursuant to the present invention; I

FIG. 2 is a front elevational view of the preferred embodiment;

FIG. 3 is a bottom plan view of the preferred embodiment;

FIG. 4 is a generally perspective view of the preferred embodiment;

FIG. 5 is an enlarged fragmentary perspective view of the reenforcing cross-strut as associated with one of the leading edge reenforcing struts;

FIG. 6 is a schematic plan view of the relationship of each of the reenforcing struts as associated with one another;

FIG. 7 is a generally perspective view of an alternate embodiment of the kite pursuant to the present invention;

FIG. 8 is a schematic side elevational view of kite cells and bridle therefor pursuant to the preferred embodiment;

FIG. 9 is a schematic cross-sectional view taken along the line 9-9 in FIG. 8;

FIG. 10 is a schematic cross-sectional view taken along the line 10-10 in FIG. 8;

FIG. 11 is a schematic side elevational view of the cells and bridal therefor of the alternate embodiment illustrated in FIG. 7;

FIG. 12 is a schematic cross-sectional view taken along the line l2-l2 in FIG. 11; and

FIG. 13 is a schematic cross-sectional view taken along the line 13-13 in FIG. 11.

DETAILED DESCRIPTION OF THE INVENTION Referring now to the drawings, and more particularly to FIGS. 1-6 and 8-10, the preferred embodiment of the present invention relates generally to a tailless kite denoted by the reference character 20, the kite having a body portion preferably constituted of a porous flexible cloth of natural or synthetic nature. The kite body 20 is generally defined by an upper surface 22, denoted in FIG. 1, and a lower surface 24, denoted in FIG. 3. The kite body 20 is generally triangular in contour and may be characterized therefore as being of delta configuration.

In this respect, the kite body 20 terminates in a pair of rearwardly diverging leading marginal edges 26 respectively which merge with one another at a generally frontal apex 28 and diverge away from one another rearwardly toward a trailing edge 30 of the kite body 20.

Each of the leading marginal edges 26 is provided with respective sleeves or pockets 32 such as by sewing or the like, for receiving respectively a pair of reenforcing structs 34 of light-weight wood, plastic or metallic nature. The longitudinal extent of each of the reenforcing struts 34 is substantially less than the elongate extent of the leading marginal edges 26, the reenforcing struts 34 extending from the trailing edge 30 of the kite 20 to a point spaced substantially from the apex 28 and denoted by the reference character 36, the latter which defines the forward end of each of the reenforcing struts 34 respectively. Thus, the leading marginal edges 26 are each defined in part by a major reenforced portion 26A corresponding to the elongate extent of the reenforcing struts 34, and a minor frontal portion 26B from which there is absent reenforcement and which is disposed between the apex 28 and the forward end 36 of each of the reenforcing struts 34.

A medial portion of the kite-body 20 is generally provided with what may be characterized as an open relief area 38 of generally and preferably rectangular configuration across which and in spaced relation with extends a cross-bar or reenforcing strut 40 having opposite apertured end portions 42 in which are provided a plurality of eyelets 43, as denoted in FIG. 5, for being tightly lashed and secured to the marginal edge reenforcing struts 34 such as by means of tie wires or twine 44.

Disposed below or extending from the lower surface 24 of the kite-body 20 area frontal cell 46 and a rear cell 48 which are spaced from one another through the intermediary of the open relief area 38 and which in transverse cross-section presents a generally and preferably triangular appearance as illustrated generally in FIG. 2. In this respect, the frontal cell 46 is defined in part by a concealed portion of the lower surface 24, that portion disposed directly above a pair of joined and substantially similarly shaped leaves 50 of preferably rectangular appearance, as denoted in FIG. 8. Likewise, the rear cell 48 is defined by a pair of joined similarly shaped leaves 52 and a rear portion of the lower surface 24 of the kite-body 20 disposed directly above the leaves 52, the leaves 52 as illustrated in FIG. 8 also being substantially rectangular in appearance.

As discussed above, the kite-body 20 is constituted preferably of a natural or synthetic cloth of flexible nature. However, pursuant to the principles of the present invention, a pair of longitudinal body-reenforcing struts 54 are disposed in appropriate sleeves or pockets provided in the kite-body 20, the body-reenforcing struts 54 being spaced from one another substantially by the lateral open dimension of the open relief area 38 and extend from the leading marginal edges 26 respectively to the trailing edge 30. Moreover, a pair of frontal body reenforcing struts 56 are likewise disposed within appropriate sleeves or pockets therefore, and extend in the kite-body 20 from the leading marginal edges 26 respectively to the forwardmost lateral edge of the open relief area 38. The body 20 is further reenforced at the frontal portion thereof by a central frontal bodyreenforcing strut 58 which extends from the apex 28 to the forwardmost lateral edge of the open relief area 38.

The body-reenforcing struts 54 provide longitudinal reenforcement to the kite-body 20 and define a pair of wings 59 respectively which extend between the leading marginal edges 26 respectively and each of the body-reenforcing struts 54 respectively, the wings 59 being generally movable relative to one another yet preferably retained in spaced relation and presenting a positive dihedral angle in flight, as illustrated in FIG. 2, by means of the cross-bar on reenforcing strut 40.

On the other hand, the spaced body-reenforcing

struts

56 and 58 respectively stiffen, only longitudinally, the forwardmost portion adjacent the apex 28 of the body 20 for purposes as will be further clarified below. The frontal cell 46 and rear cell 48 are interconnected to one another through the intermediary of a cell reenforcing strut 60 which is spaced from the body 20 and joined to each of the

cells

46 and 48 along the common line of joinder of each of the

leaves

50 and 52 of the

cells

46 and 48 respectively. The relationship and elongate extent of all of the reenforcing struts relative to one another is best illustrated in FIG. 6. The only clarification herein required for defining the relative elongate extend of each of the struts as illustrated in FIG. 6 is that the cell reenforcing strut 60 extends between a forward edge 62 of the frontal cell 46 to the trailing edge 30 of the kite-body 20, whereas the central body-reenforcing strut 58 extends from the apex 28 of the kite-body 20 to the rear edge 64 of the frontal cell 46.

As best illustrated in FIGS. 8-10, the frontal and

rear cells

46 and 48 are provided with

leaves

50 and 52 respectively which are of generally rectangular appearance, yet are spaced from one another in triangular relation, the foward and

rear edges

62 and 64 respectively of the frontal cell 46 and the forward and

rear edges

66 and 68 respectively of the rear cell 48, in schematic projection as illustrated of the rear cell 48, in schematic projection as illustrated in FIG. 8, appearing at right angles with the lower surface 24 of the kitebody 20.

It is significant to note that each of the reenforcing struts aforementioned, except for that of the cross-bar or cross-strut 40, are disposed within appropriate sleeves or pockets therefor, sewn or otherwise conventionally formed on the body 20, and are tightly constrained against axial or lateral movement relative to the interior of the pockets wherein the reenforcing struts are disposed, this to enhance both dimensional and flight stability of the kite when in flight. On the other hand, the cross-bar or cross-strut 40 is generally entirely exposed and secured to the marginal edge reenforcing struts 34 respectively at the opposite apertured end portions 42 of the cross-bar or reenforcing strut 40. In order to reduce the extent of wood upon wood, the wood of which each and every one of the reenforcing struts are preferably constituted of, and also for purposes of diminishing the extent of wear of the cloth of which the leading marginal edges 26 of the kite-body is constituted preferably, there is interposed between each end portion 42 of the cross-bar or reenforcing strut and the leading marginal edges 26 respectively of the kite-body 20, a cloth or other suitable padding denoted by the reference character and best illustrated in FIG. 5. 7

Referring now to FIGS. 7 and ll-l3, there is illustrated still a further or alternate embodiment of the kite pursuant to the present invention. In this respect, the alternate embodiment is generally identical with that of the embodiment as illustrated in FIGS. 1-6 and, therefore, a description of the alternate embodiment in detail is omitted herefrom for purposes of clarity and brevity and only the distinctive features differing from that of the preferred embodiment illustrated in FIGS. 1-6 will be discussed herein below. In this respect, reference characters in FIGS. 7 and 11-13 which have been duplicated from those of the FIGS. 1-6 represent identical structure of the preferred and alternate embodiments, whereas reference characters in FIGS. 7

and 11-13 which have not been duplicated from FIGS. 1-6 represent departures or distinguishing features differing from the preferred embodiment of FIGS. 1-6.

In this regard, the difference between the preferred and alternate embodiments lies principally in the configuration of the cells. The

cells

46 and 48 of the preferred embodiment, as illustrated in FIG. 8, are each provided with

leaves

50 and 52 respectively which presents a generally rectangular appearance. On the other hand, the alternate embodiment is provided with a frontal cell and a rear cell 102, the frontal cell being defined by a pair of joined leaves 104 of trapezoidal appearance, the rear cell 102 being defined by a pair of leaves 106 also of identical trapezoidal appearance.

In this respect, the frontal cell leaves 104 are each provided with a forwardly extending edge 108 which does not project further than the body apex 28 and which defines an acute angle X with that of an exposed portion of the lower surface 24 of the kite-body 28 directly thereabove, as illustrated in FIG. 11, whereas the rear edge 110 of each of the frontal cell leaves 104 appear in a schematic projection as illustrated in FIG. 11 generally at right angles with the lower surface 24 of the kite-body 20. Likewise, the rear cell leaves 106 are each provided with a rearwardly extending edge 114 which defines an obtuse angle Y with a generally concealed portion of the lower surface 24 of the kite-body 20 directly above and between the rear cell leaves 106 respectively. On the other hand, each of the rear cell leaves 106 is provided with a forward edge 112 which in schematic projection forms a right angle with the lower surface 24 of the kite-body 20, as best illustrated in FIG. 11.

Each of the embodiments, the preferred illustrated in FIGS. l-6, and the alternate illustrated in FIG. 7, is provided in a conventional manner with a bridle, the bridle being denoted by bridle segments or legs and 152 respectively, to which a tethering line 154 is connected and operated at the other end by the kite enthusiast. The bridle segment 150 is preferably shorter than that of the bridle segment 152, the bridle segment 150 being secured to the frontal cell in each of the embodiments between the forward and rear edges thereof respectively, and preferably in greater proximity to the rear edge thereof, whereas the bridle segment 152 is secured in each instance to the rear cell preferably at the forwardmost edge thereof as best illustated in FIGS. 8 and l l. The preferred length ratio of the shorter segment 150 to the longer segment 152 is within a range generally of 8:ll-19:22.5, this being the preferred length ratio range, yet as it is well understood the specific length ratio may be altered by the kite enthusiast to accommodate a specific wind load.

In operation, whether it be the operation of the preferred embodiment illustrated in FIGS. 1-6 or the alternate embodiment illustrated in FIG. 7, the kite enthusiast may secure the cross-bar or reenforcing strut 40 to the marginal edge reenforcing struts 34 respectively and mayadjust the spacing between the leading marginal edges 26 of the kite-body 20 by choosing or selecting a particular eyelet 43 through which the tie wire or twine 44 is to penetrate for lashing the cross-bar 40 to the marginal edge reenforcing struts 26 respectively. With a proper bridle arrangement and a light-weight tethering line, the kite may be lofted in generally any direction, and will immediately assume a rapid rate of ascent. The light-weight tethering line may be utilized because of the reduced drag associated with the kite body 20, this principally because of the provision of the open relief area 38 in the medial portion of the kite body 20. Moreover, the rapid rate of ascent is effected because of the provision of the frontal and rear cells through which air ingresses and egresses, the open relief area 38 also enhancing the extent of suction through the frontal cell 46 of the embodiment illustrated in FIGS. 1-6 and the frontal cell 100 of the embodiment illustrated in FIG. 7.

The relationshipof the frontal and rear cells of each of the embodiments as associated with the open relief area and which diminishes to a great extent the overall drag subjected to the kite when in flight, permits the kite to reach its zenith level or optimum height wherein the vertical altitude components is maximized and the horizontal altitude component is minimized, thereby, permitting the kite enthusiast to fly the kite almost directly overhead in a controlled manner.

Of equal importance, and possibly of even greater importance, is the fact that the relationship of structure of the kite pursuant to either of the embodiments of the present invention, will permit a rapid retrieval of the kite and a general obviation of potential collision of the kite with the ground. In this respect, the frontal cell 46 of the embodiment illustrated in FIGS. 1-6, and the frontal cell 100 of the embodiment illustrated in FIG. 7, each defines with a flexible body apex portion thereabove, that portion between the forward lateral edge of the open relief area 38 and the non-reenforced frontal portion 268 of each of the leading marginal edges 26 adjacent to the apex 28, and denoted generally in FIGS. 8 and 11 respectively by the reference character 200, an open mouth which undergoes various degrees of closing in response to changes in wind load, wind direction and air currents in general. In this respect, the flexible apex portion 200 may be characterized as a diving 7 brake which prevents the kite 20 from plunging downwardly into a violent collision with the ground. As illustrated best in FIGS. 8-13, the flexible apex portion 200 or diving brake may be moved into an upper position 200, forwardly of the

forwardmost edge

62 and 108 of the

frontal cells

50 and 104 for each of the embodiments of the present invention, presenting greater surface area or drag at the apex 28 of the kite embodiments of the present invention, thereby, stabilizing the kite in flight and imparting to the kite a horizontal attitude. The particular range of positions of the flexible apex portion 200 is best illustrated in FIGS. 10 and 13 respectively for each of the embodiments of the present invention, the range denoted generally by the

reference characters

200 and 200". Generally speaking, the flexible rear portion 202 directly above each of the

rear cells

48 and 102 of the embodiments of the present invention respectively, present a generally upwardly bowed or billowed appearance, when in flight, as illustrated in FIGS. 9 and 12 respectively.

Expanding somewhat on the operability of the flexible apex portion 200 or diving brake, the kite when retrieved to a level of approximately 100 feet or so permits the kite enthusiast to reduce the tension upon the tethering line, this resulting in an assumption of a negative dihedral angle of each of the wings 59 relative to the-cross-bar 40 and an uplift of the apex 28 to its maximum capacity, thereby, permitting the kite to settle gently in a horizontal stabilized attitude upon the ground and the obviation of a violent collision therewith, the flexible apex portion 200 or diving brake resisting nose-dive during the retrieval.

The principle difference between the preferred embodiment illustrated in FlGS. 16 and that of the alternate embodiment illustrated in HO 7 is that the preferred embodiment is less active and more stable in position at its zenith level, whereas the alternate embodiment is more active and reponsive to current changes and will seek optimum current flow in a highly stabilized condition, each of the embodiments generally altering nose-dives for stabilizing the kite.

Numerous alterations of the structure herein disclosed will suggest themselves to those skilled in the art. However, it is to be understood that the present disclosure relates to a preferred embodiment of the invention which is for purposes of illustration only and is not to be construed as a limitation of the invention.

What is claimed is:

l. A kite comprising a flexible body of generally triangular configuration;

said body including a diverging pair of leading marginal edges, a frontal apex from which said leading marginal edges rearwardly diverge, and a traling edge generally opposite said apex;

first reenforcing means spaced from said apex for stiffening a major part of each of said leading marginal edges, while leaving a minor frontal part of each of said leading marginal edges proximate said apex generally flexible;

an open relief area provided in said body through which air flows while said kite is in flight;

said open relief area being centrally disposed between said leading marginal edges;

a pair of longitudinal body-reenforcing means spaced from one another by said open relief area and extending from said leading marginal edges respectively to said trailing edge for defining a pair of wings;

said wings extending between said leading marginal edges respectively and each of said bodyreenforcing means respectively;

said wings being movable relative to one another while in flight;

second reenforcing means for being secured to said first reenforcing means to maintain said leading marginal edges in diverging relation while leaving a substantial portion of said body inwardly of said leading and trailing edges generally flexible;

said second reenforcing means retain said wings in spaced relation and permit said body to provide a positive dihedral angle while in flight;

and means for defining at least one open cell which extends from said body and through which air is drawn while said kite is in flight; said one open cell including a front portion of said body and a pair of joined members extending downwardly from said body front'portion to define a kite-stabilization mouth portion;

said body front portion being bound by said apex,

said minor frontal part of each of said leading marginal edges, said pair of longitudinal bodyreenforcing means and said open relief area;

said body front portion being movable in flight relative to said pair of joined members for permitting said mouth portion to open and at least partially close in response to air-currents while in flight.

2. A kite as claimed in claim 1, including a second open cell spaced longitudinally opposite said one open cell and extending proximate said trailing edge, said second open cell including a rear portion of said body and a pair of joined members extending downwardly from said body rear portion.

3. A kite as claimed in claim 2, wherein each of said joined members of said one open cell and said second open cell is of generally rectangular configuration.

4. A kite as claimed in claim 2, wherein each of said joined members of said one open cell and said second open cell is of generally trapezoidal configuration.

5. A kite as claimed in claim 1, wherein said first reenforcing means includes a pair of struts flxed to said pair of leading marginal edges respectively, the elongate extend of each of said struts being substantially identical with one another and less than the elongate extent of each of said leading marginal edges.

6. A kite as claimed in claim 5, wherein said second reenforcing means includes a cross-strut having a pair of opposite end portions, each of said opposite end portions of said cross-strut having a plurality of apertures spaced from one another, and means for adjustably securing the opposite end portions of said cross-strut to said pair of struts respectively through ones of said apertures.

7. A kite as claimed in claim 1, wherein said front portion of said body includes a plurality of spaced struts extending generally parallel to one another from said leading marginal edges to said open relief area.

8. A kite as claimed in claim 2, including a longitudinal strut interconnecting each of said cells to one another and secured to said cells along a line of joinder of the respective joined members of said cells.

It s m w

Claims

1. eA kite comprising a flexible body of generally triangular configuration; said body including a diverging pair of leading marginal edges, a frontal apex from which said leading marginal edges rearwardly diverge, and a traling edge generally opposite said apex; first reenforcing means spaced from said apex for stiffening a major part of each of said leading marginal edges, while leaving a minor frontal part of each of said leading marginal edges proximate said apex generally flexible; an open relief area provided in said body through which air flows while said kite is in flight; said open relief area being centrally disposed between said leading marginal edges; a pair of longitudinal body-reenforcing means spaced from one another by said open relief area and extending from said leading marginal edges respectively to said trailing edge for defining a pair of wings; said wings extending between said leading marginal edges respectively and each of said body-reenforcing means respectively; said wings being movable relative to one another while in flight; second reenforcing means for being secured to said first reenforcing means to maintain said leading marginal edges in diverging relation while leaving a substantial portion of said body inwardly of said leading and trailing edges generally flexible; said second reenforcing means retain said wings in spaced relation and permit said body to provide a positive dihedral angle while in flight; and means for defining at least one open cell which extends from said body and through which air is drawn while said kite is in flight; said one open cell including a front portion of said body and a pair of joined members extending downwardly from said body front portion to define a kite-stabilization mouth portion; said body front portion being bound by said apex, said minor frontal part of each of said leading marginal edges, said pair of longitudinal body-reenforcing means and said open relief area; said body front portion being movable in flight relative to said pair of joined members for permitting said mouth portion to open and at least partially close in response to air-currents while in flight.

5. A kite as claimed in claim 1, wherein said first reenforcing means includes a pair of struts fixed to said pair of leading marginal edges respectively, the elongate extend of each of said struts being substantially identical with one another and less than the elongate extent of each of said leading marginal edges.