DESCRIPTION

The present invention relates to boomerangs.

FIELD OF THE INVENTION

Boomerangs are well known devices which typically comprise a pair of members disposed at an angle to one another. The members are typically so shaped that if the device is thrown it will describe a path by which it returns to the vicinity of the thrower.

SUMMARY OF THE INVENTION

The present invention provides a boomerang of different construction to known boomerangs.

In accordance with the present invention there is provided a substantially flat boomerang device comprising a flat central ring member having inner and outer circular boun¬ daries and defining a central aperture, at least two wing members projecting outwardly from the outer boundary of the ring substantially in the plane thereof, each wing member being of asymmetrical aerofoil section and being arranged to act in the same direction as all of the other

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wings, such that when the device is thrown with a rotary motion the wings produce lift and also turn.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention will now be described, by way of example, with reference to the accompanying drawings, in which :-

FIGURE 1 is a plan view of one embodiment of a four winged boomerang in accordance with the present invention; FIGURE 2 is a side elevation of the boomerang of Figure 1;

FIGURE 3 is a plan view of one embodiment of a three winged boomerang device in accordance with the present invention;

FIGURE 4 is a side elevation of the boomerang of Figure 3;

FIGURE 5A is a transverse section along the lines A - A of Figures 1 and 3;

FIGURE 5B is a transverse section along the lines B - B of

Figures 1 and 3;

FIGURE 6 shows a plan view of a boomerang device in r accordance with the present invention which is to the same general design as the boomerangs of Figures 1 to 5 but has - backswept wings; ,

FIGURE 6A is a section along the line A - A of Figure 6;

FIGURE 6B is a section along the line B - B of Figure 6;.

FIGURE 6C is a section along the line C - C of Figure 6; FIGURE 7 shows a plan view of a boomerang device in accordance with the present invention which is to the same general design as the boomerangs of Figures 1 to 5 but has

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forwardly swept wings.

FIGURE 7A is a section along the line A - A of Figure 7; and

FIGURE 7B is a section along the line B - B of Figure 7.

^{'• '} DESCRIPTION OF THE INVENTION In Figures 1 and 2, there is shown a four winged boomerang device in accordance with the present invention which comprises a central ring 10 having four wings 11 projecting normally outwardly therefrom at 90° spacings. The ring 10 has an inner circular boundary 12 and an outer circular boundary 13. The ring 10 further defines a central aperture 14.

As shown in Figure 5B the ring 10 has a symmetrical aero- foil section which is flat underneath and symmetrically arched on top.

The use of a ring 10 with a central aperture 14 has the advantage that, in flight, air can pass through the middle of the ring 10 which reduces dracj. Also, the aerofoil section of the ring 10 has the advantage of imparting gliding lift to the boomerang device in flight. The circular ring shape is found to conserve spin efficiently especially compared to more angular shapes such as triangles or squares, and this is found to be an important advantage in obtaining good flight characteris¬ tics. Further, as shown in Figure 5A, each wing 11 has

asymmetrical aerofoil section whereby one side is flat and the other side is curved. The portion of the wing 11 adjacent the edge 11a which will be leading in rotational flight is thicker than the portion of the wing 11 adjacent the edge lib that will be trailing. Each wing 11 is shaped so that its thicker edge 11a faces the same way as the thicker edges of all of the other wings 11.

As shown, the wings 11 are all of the same length and width and thickness and are mounted symmetrically about the ring 10. However, it is not absolutely essential for the boomerang device to be symmetrical. It is preferred that the centre of gravity of the device be at the centre of ^" the ring 10. If the wings 12 were in some way asymmetrically disposed either in location or size, shape or weight then, if desired, the centre of gravity could be maintained at the preferred location by appropriately counterweighting the opposite side of the device.

It must be emphasised that it is preferred for the wings 11 be the same and to project outwaru y from the ring 10 in a symmetrical manner such that each pair of adjacent wings are spaced apart by the same angle such as 90°, this means that in the preferred embodiment each wing 11 makes an equal contribution to the flight of the boomerang device. Further, the wings 11 are preferably spaced so that they are non-overlapping when viewed in plan.

Still further, in the construction shown the ring 10 is of substantially the same thickness as the wings 11 at their

greatest thicknesses and the ring 10 is completely flat on one side and aerodynamically, symmetrically arched on the other. However, there is no reason why the wings 11 and ring 10 could not be of different thicknesses. It is preferred to have a flat side on the reverse to the symmetrical aerofoil side of the ring 10. It is preferred that the ring 10 be of symmetrical_ aerofoil section since in use as the device rotates, a side of the ring 10 which is leading at one moment will be trailing at the next. Further, as ^' shown the boomerang device may comprise an outer circular ring 15. This outer ring 15 is partly included for safety purposes. Preferably, the outer ring 15 has a reduced thickness such as about 1/2 thickness compared to the rest of the device and is circular or elliptical in cross-section to reduce any adverse effect on flight characteristics of the boomerang. Further, the junctions between the wings 11 and the outer ring 15 may comprise skirt portions 16 to reduce drag. The skirt portions 16 are flat portions with flat opposed surfaces and they are located in the corners between the wings 10 and the outer ring 15.

Still further, it is found that the wings 11 may be bent slightly out of the plane of the ring 10 away from the flat side thereof for increased flight stability. The angle of bend may typically be up to 5° such as about 2°.

In use, the boomerang device of Figures 1 and 2 is thrown in similar manner to a conventional boomerang. That is,

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the device may be launched at an angle of about 45° into the wind. The device is launched in relation to the ground at an angle between vertical and say 40° off vertical leaning outwardly from the thrower. Further, the device is thrown with a spinning action so that the whole device adopts a rotating motion. The interaction of the turning lift produced by the wings 11 in rotating flight and the gyroscopic progression of the whole device results in a curving flight which returns the device to the vicinity of the-thrower.

In Figures 3 and 4 ^' there is shown a device similar to that shown in Figures 1 and 2 except that it comprises three symmetrically disposed wings projected outwardly at-120° spacings. The number of wings can vary widely. However, if the number of wings increases it is thought that mutual interference between wings will also increase due to air turbulence and the overall amount of drag will increase. Preferably, the boomerang device of the present invention has from 2 to 6 wings such as 3 or 4.

For best results it is preferred that the overall propor- tions of the device be maintained within certain limits. For example, the ration g between x (ring 10 outer diameter) and d (overall diameter) is preferably between about 0.3 and about 0.6. For a four winged device it is preferably about 0.41 while for a three winged device it is preferably about 0.51.

Other preferred ratios are as follows:-

1 (wing length) about 0.2 to about 0.35 (preferably d (overall diameter) about 0.28 for four wings, about 0.24 for three wings)

(average) w (wing width) about 0.05 to about 0.1 (preferably d (overall diameter) about 0.08 for four and three wings)

_ (ring width) : about 0.05 to about 0.1 (preferably d (overall diameter) about 0.07 for four wings, about

0.076 for three wings)

_ (greatest wing thickness) : about 0.008 to about 0.015 d (overall diameter) (preferably about 0.01 for four wings and about 0.013 for three wings)

Also, when the outer safety ring is used it is preferred that the ratio between the total wing area and the total inner ring area be between about 0.5 and 1.8.

Further, the ratio between the wing length (in cm) and the total volume of the device (in cm-*- ^* ) is preferably greater than 1.

Clearly the various ratios will tend to vary with the number of wings used and also the degree of raking of the wings 11 which will be described below in relation to

Figures 6 and 7.

It is envisaged that the boomerang device of the present invention will be manufactured in one piece by being moulded from plastics material. However it can also be manufactured from other light materials such as plywood. In particular, the boomerang devices of the present invention can be moulded in a single piece from flexible thermoplastics material particularly a resilient material such as polypropylene. This allows a user to bend the wings himself at will to any desired shape either up or down. If the wings are bent down the device tends to have a wide low circular flight which is good in windy condi¬ tions. If the wings are bent up, the device tends to have a high hovering flight which is good in still conditions. With a resilient material such as polypropylene the boomerang device tends to return to its substantially flat shape in time.

In Figure 6, there is shown a boomerang device of the same general type shown in Figures 1 and 2 having four equiangularly spaced wings 11. Like reference numerals denote like parts.

However, the wings 11 do not extend diametrically, as in Figures 1 and 2, outwardly from the ring 10, but are backswept. The wings 12 may be backswept by an angle in the range from 2 to 20° preferably 3 to 10°. In Figure 7, there is shown a boomerang device also of the same general type disclosed in Figures 1 and 2 but having four equiangularly spaced wings 11 which are forwardly

swept. The wings 11 may be forwardly swept by an angle in the range from 5 to 30° preferably from 10 to 25° such as 16°.

As shown in Figures 6A and 7A, the rings 10 have a symmetrical aerofoil cross-section as with the embodiment of Figures 1 and 2. Also, as shown in Figures 6B and 7B, each wing 11 in each of these embodiments has an asym¬ metrical aerofoil cross-section having a thicker portion adjacent a leading edge 11a, and a thinner portion adjacent a trailing edge- lib.

Also, it should be' noted that in the embodiment of Figure 7, the wings 11 are tapered in plan so that the sides of the wings 11 converge together slightly away from the ring 10. Further, as shown in Figures 6 and 6C, the outer leading edges, in use, of the wings 11 of the embpdi ent of Figure 6 of the present invention may be provided with undercut portions 18 which improves the flight characteristics of the boomerang device of Figure 6 By sweeping the wings either forwardly or backwardly, drag . is reduced and, he device is able to remain spinning for longer than would otherwise be the case.

Sweeping the wings also enables a larger flight surface to be incorporated within a smaller diameter. This favourably affects the lift to spin ratio and increases the relative size of the inner ring 10. Further, the direction of the sweep affects the trajectory

of the device. Forward swept wings (as in Figure 7) produce a wide low circular flight while the backswept wings (as in Figure 6) give a high, elliptical hovering flight. The use of an inner ring 10 compared to a small hub more efficiently conserves the spin of the device since it acts like a flywheel. Further, the inner ring 10 produces lift in addition to the wings 11 since it also has an aerofoil section. The lift produced by the inner ring is more of a gliding lift than a turning lift.

The wings 11 have higher lift on the side of the advancing wing since it has a higher airspeed than the retreating wing on the other side. Thus, the boomerang tends to lift on its advancing side and to dip on its retreating side. This is what is termed the rolling moment.

The inner ring 10 also acts as a convenient and safe means of catching the device on the return.

Further, the boomerang device in all embodiments of the present invention is substantially planar and is devoid of projections deviating at a large angle from the general plane of the boomerang device such as an axial stick, stem, handle or the like as is found in some prior art devices. Further, the outer drag skirts 16 of the boomerang of the present invention help to reduce spin drag and thus increase effective flight duration. It should be noted that in Figures 6 and 7, the drag skirt 16 adjacent the

trailing edge lib of each wing 11 is larger than the drag skirt adjacent the leading edge lib. The skirts 16 also provide a convenient flange to accept the index finger of the thrower. This enables the thrower to more readily impart spin to the boomerang when throwing.

Modifications and variations such as would be apparent to a skilled addressee are deemed within the scope of the present invention. For example, the aerofoil sections shown in the drawings have flat reverse surfaces. These surfaces could if desired be made concave especially when the boomerang devices are moulded from plastics material.