US3711045A - Kites - Google Patents

Abstract

A box kite employs narrow lifting bands producing improved flight performance and flight stability, and lighter structural loading, permitting efficient use of round-sectioned body members and channel fittings achieving a simple yielding structure which prevents breakage and is inexpensively manufactured and easily assembled and disassembled. The construction is suitable for modular arrangement having several individual kites attached abreast.

Description

United States Patent 1 1 Holland, Jr.

[ 1 Nov. 16, 1973 54] KITES [76] Inventor: Raymond Prunty Holland, Jr., 1702 West Third Street, Roswell, N. Mex.

[22] Filed: Nov. 27, 1970 21 Appl. No.: 93,277

[52] US. Cl. ..244/153 R [51] Int. Cl ..'..B64c 31/06 [58] Field of Search ..244/153 R, 154, 155 R [56] References Cited UNITED STATES PATENTS 607,129 Potter ..244/153 R 2,057,148 10/1936 Hopkins ct al ...244/l53 R 1328,14 l/l920 Fcrgusson ...244/l53 R 3,494,578 2/1970 Coreton ..244/153 R FOREIGN PATENTS OR APPLlCATlONS 555,542 8/1943 Great Britain ..244/153 R l/1910 France ..244/153 R 11/1905 Great Britain.....

4/1914 France ..244/153R OTHER PUBLICATIONS J. B. Millet Scientific Kite-Flying. The Century Magazine Vol. LlV, 1896. Pages 70-72, 77.

Primary Examiner-Milton Buchler Assistant Examiner-Paul E. Sauberer [57] ABSTRACT A box kite employs narrow lifting bands producing improved flight performance and flight stability, and lighter structural loading, permitting efficient 'use'of round sectioned body members and channel fittings achieving a simple yielding structure which prevents breakage and is inexpensively manufactured and easily assembled and disassembled. The construction is suitable for modular arrangement having several individual kites attached abreast.

5 Claims, 5 Drawing Figures KITES The present invention relates to improvements in box kites and has particular reference to the commercially successful Skyscraper or narrow-band box kite which, as compared to existing box kites, employs relatively reduced lifting surface area and yet is capable of flight in both lighter winds and stronger winds, by virtue of its novel aerodynamic form and structural benefits derived from that aerodynamic form.

Although the box kite as a general class of kite has been known for 75 years, box kites are still imperfect and subject to inventive improvement.

Existing box kites employ lifting surface bands which are broad in chord length, the perpendicular width or chord length of each band being typically about onefourth of the overall length of the kite. The intended purpose of the broad band in box kites has been to provide the kites with sufficient lifting surface area to enable them to support their weight in light winds. Yet these existing box kites experience difficulties due to the broad bands: The edges of the broad lifting bands cannot be kept taut unless structure is provided which is either relatively heavy or consists of multiple elements and is therefore relatively expensive; the kite tends to be unstable, preventing successful launches in gusty wind, the kite typically rushing off sideways upon being launched, often diving into the ground; the kite requires a bridle to correct its stability and trim problems, and this bridle requires repeated adjustment by the kite flier without ever fully producing the desired results; breakage occurs when the kite strikes the ground; and when the launch is successful, the rate of climb tends to be sluggish and the angle of flight tends to be low. The broad bands on existing box kites, held in tension around a frame of parallel longitudinal body members, tend to bend the body members allowing the band edges to slacken. To correct this the body members have been made thick and heavy, requiring weight of structure, consequently defeating the intended purpose of the broad band (preventing flight in light winds). Sometimes the bands have been provided with doubled or bound edges, or strings have been placed in the edges, or extra bays of spreaders have been used to support the body members in order to support the bands, all of which have added to the complexity and the manufacturing cost of this kite. The attachment of the spreaders to the body members also has been unsatisfactory. in some box kites a fitting is used, bound to the body member and having a socket to receive the end of the spreader; these are expensive. In other constructions the end of the spreader is slotted and fits over the body member; this construction, if made of wood, which is usually necessary to meet price competition, splits the spreader and destroys the kite when it strikes the ground. The present invention corrects all of these difficulties in a simple construction.

It is the primary object of this invention to produce a simple, inexpensive, high performance box kite which is crash-proof.

In more detail, the objects of this invention include the following:

To produce a box kite of efficient and stable aerodynamic form, which produces a relatively large amount of lift per unit of lifting surface area, and a relatively large amount of lift per unit of induced drag,

thereby resulting in a relatively fast rate of climb, a relatively high angle of flight and relatively light air loads applied to the kite structure, thereby producing a relatively light pull on the flying string, permitting a relatively thin" light string to be used without risk of breakaway, and by virtue of the thin light string with its relatively reduced wind drag, permitting much higher and steeper flights to be accomplished, the light applied air loads also permitting relatively light kite structure to be used, thereby saving weight and improving kite performance in all departments.

-To produce a box kite having a sturdy, economical, lightweight, simple rigid structure which is shock-resistant, yielding or collapsing instead of breaking upon striking the ground, and which avoids extra bays of spreaders and/or reinforcements in the edges of the bands such as are common in existing box kites.

To produce a box kite which requires no bridle, nor any adjustment by the kite flier, and which flies correctly with proper stability and trim every time it is launched in ordinary winds.

To produce a kite which, by virtue of these improvements, is able to fly in a wind which may be either lighter, or stronger, or gustier than the lightest, or the strongest, or the gustiest wind, in which any simple box kite could fly before this invention.

To produce a box kite which is easily assembled and disassembled for packaging, carrying, and flying.

To produce a box kite, which, by virtue of its simplicity and margins of strength, rigidity, and flight performance is suitable for modular construction, employing several kites flown together attached abreast.

Referring now to the drawing,

FIG. 1 shows at the left the typical proportions of existing box kites 1, compared to the box kite 2 of this invention, at the right, with its narrow chord lifting surfaces and reduced lifting surface area.

FIG. 2 shows a perspective view from slightly beneath and to one side of the box kite 2 of this invention, having strut-like circular-sectioned longitudinal body members 3; closed band lifting surfaces members 4, made of tough cloth-like flexible sheet material, at-

tached to body members 3; fittings 5, each having a first channel-shaped recess engaging a member 3, and a second channel-shaped recess opposite the first recess engaging a strut-like spreader member; spreader members 6, engaging in the second recess on fitting 5, just described; and flying line 7. I

FIG. 3 is a view taken at 33 of FIG. 2 showing the manner in which longitudinal member 3 and spreader member 6 fit into the recessed channels in fitting 5. The first recess in fitting 5 receives member 3 along the channel, producing bearing contact of significant length between fitting 5 and member 3, fitting 5 thereby supporting member 3 against bending moments caused by the end thrust of spreader member 6 and the tension in the material of band 4. Fitting 5, if forced, may rotate around the center of curvature of circular sectioned member 3, carrying the end of spreader 6 with it, bending spreader 6. If the bending becomes large, spreader 6 will pop out of the second recess in fitting 5 before spreader 6 breaks.

FIG. 4 is a view at 4-4 of FIG. 3 indicating how spreader 6, if forced, may move in the second recess of fitting 5, either by rotation or by sliding bodily in the channel, as indicated by the dotted lines. If abnormally severe loads occur displacing member 6 it will disengage from the channel in fitting before fracture occurs in any part. v

The assembly and disassembly of the kite may be seen from FIGS. 2, 3 and 4 to be simple and convenient. In the knocked down form the body members 3 remain permanently attached to bands4, in which form the unsupported material of bands 4 is readily rolled around the longitudinal members 3. For assembly, the kite is unrolled and fittings 5 are pressed on longitudinal members 3, and the ends of Spreaders 6 are pressed in-the second recesses on fittings 5 as shown. The movements which fittings 5 may make relative to round section member 3 consisting of rotational and sliding movements, and which spreaders 6 may make relative to fittings 5, as illustrated in FIGS. 3 and 4, are of assistance in getting tautness producing spreader 6 into place. Member 6 may be bent or sloping in being put in place, after which, by means of the deformable structure described, it may be straightened and squared up to bring lifting surfaces 4 to full tautness. Friction forces are sufficient to hold these parts in place and preserve tautness in flight, but are not sufficient to hold the parts rigidly in a crash loading; the automatic disengagement of one or more of members 6 from fitting 5 or fitting 5 from member 3 prevents breakage of any part.

The chord length of lifting surface 4 is the perpendicular distance between the leading edge 9 of this surface and the trailing edge 10 of the surface. It is this dimension which is small on the kite of this invention relative to the overall length of the kite, producing resultsfdirectly and indirectly, which have heretofore been unknown to kite builders and which would seem to be contrary to common sense in that the kite of this invention uses reduced lifting area to be able to fly better.

FIG. 5 shows multiple modular box kites made up of four units and two units of box kite 2, respectively, bound together abreast at adjoining longitunal members 3 including short portions of these members 8, which extend forward and rearward of the most forward and most rearward parts of lifting surfaces 4.

To understand the apparent anomaly that the overall flying ability is improved by reducing lifting surface area one must keep in mind the aerodynamic fact that every surface producing lift distorts the pattern of airflow behind that lifting surface. When two lifting surfaces follow each other in tandem, the more forward surface turns the flow downward at the more rearward surface and reduces the lift at that more rearward sur face. As two lifting surfaces in tandem, each at the same angle to the remote wind direction, move relatively closervto each other, this lift-reducing 'effect at the rearward surface becomes relatively larger. But if they are moved farther apart, some of the lift otherwise lost is restored. In other words, when two equally inclined surfaces in tandem are more widely separated fore and aft they produce more total lift than when they are close to each other fore and aft.

According to elementary aerodynamic theory every lifting surface generates a downwash velocity com ponent at that lifting surface, and this downwash velocity component persists unchanged behind the lifting surface, regardless of the distance behind the lifting surface. This, in itself, would say that a greater rearward separation of the rearward behind the forward surface would not restore lift at the rearward surface. This apparent contradiction and commonly held belief is explained by the existence of three aerodynamic effects in addition to simple theoretical downwash:

l. The local circulation around the forward surface produces downwardly inclined flow close behind that surface in addition to and distinct from the theorectical downwash velocity component mentioned above. This downward flow due to circulation decreases rapidly as the distance behind the forward lifting surface is increased, so that lift is regained from this effect as the rearward surface moves farther rearward.

2. There are three dimensional flow effects which reduce the net downwash at the downstream lifting surface.v Vortex trunks trail behind the outboard tips of the forward surface, and the theoretical downwash exists only between these trunks. Outboard, beyond the centers of these tip vortices, there is no downwash but an upwash instead. Of especial significance is the fact that the trailing vortex trunks move laterally toward each other as they trail rearwardly so that the rearward lifting surface, when it has the same lateral span dimension as the forward surface, as in this case, comes into the upwash flow at its extreme outboard tips, an effect which generously restores lift at the rearward surface when the fore and aft separation of the two surfaces is sufficient.

3. Vertical separation of the rearward surface below the forward surface enables the rearward surface to escape part of the theoretical downwash from the forward surface, producing a progressively increasing lift benefit at the rearward surface as the rearward surface moves rearward and downward, as is the case on a box kite.

The primary non-dimensional scale of measurement for comparing these aerodynamic effects is the chord length of the forward lifting surface; the greater the separation of the forward and rearward lifting surfaces when measured in units of such chord lengths, the greater will be the benefits of this separation.

Hence, the logic of the use of the narrow band on the box kite is this: lt separates the rearward from the forward surface, aerodynamically, and enables the rearward surface to produce lift which it otherwise could not produce. This compensates for its smaller size.

Surface area intended to produce lift but located where it cannot do so efficiently, as by broad bands located close behind one another, nevertheless produce drag, a force acting downwind. This drag makes the kite sluggish, prevents high angles of flight, increases structural loads on the kite's frame and covering, requires heavier structure, increases the load in the flying line, often breaking it, and requires a thick heavy flying line which rapidly burdens the kite as it attempts to climb, principally because of the downward and downwind wind pressure on the flatly inclined thick string, so that the height which the kite can reach is soon limited. These faults are corrected by the present invention.

The Airplane Kite Company performed carefully controlled tests on the evening of May 27, 1970 to investigate the aerodynamic effect of band width on box kites. Two kites were tested, each with longitudinal members 2 being 36 inches long and with eachspreader member 6 having a length of 14% inches. The chord length of band 4 was 6 inches on one kite and 4 inches on the other. Both of these kites were of the proportions taught by this patent, their ratios, length/chord being 9 in one case and 6 in the other, whereas this ratio is typically about 4 and often less, in existing box kites. The test was made to determine whether the benefit of the narrower chord was still present as the band chord became very narrow compared to previous practice. The weights of the kites were accurately known and they were flown on identical flying lines, at the same height, side by side in a steady wind which was observed to be gradually decreasing in velocity. The flying line was attached 6 inches behind the leading edge of the front band, in the same position on both kites.

The average lifting surface loading of the narrow band kite, total weight/total lifting area, was 6 percent greater than the loading of the wider band kite.

When the wind velocity decreased sufficiently the first kite to settle to the ground was the wider band kite. The kite with the narrower bands continued to fly.

The test was repeated with longer flying lines, allowing the kites to reach stronger winds higher up, sufficient to support them. Once again, as the wind velocity decreased the more lightly loaded kite with the wider bands settled out first, and a short time later as the wind velocity decreased further the narrow band kite came down also. These tests showed conclusively that the kite with the very narrow bands developed a lift coefficient at least 6 percent greater than that of the kite having somewhat wider bands.

The tests also demonstrated visually that the kite with the narrower bands climbed faster and flew at a steeper angle. Similar differences, but of greater magnitude, are also apparent when narrow band box kites are flown alongside conventional broad band box kites.

box kite may use members 3 which are circular in section, supported by a single bay of spreaders.

The circular section of longitudinal member 3 solves the problem of obtaining suitable sticks for kite manufacture, since hard wood dowels may now be used which are available commercially. Thecircular section of member 3 also permits the solution of the breakage problem,.in that the joint between the spreader 6 and member 3 may now be made to yield under load, by the use of fitting 5 which can rotate on member 3. This same fitting 5, conveniently available as a short length of rigid extrusion, also provides the opportunity for a yielding connection relative to spreader 6, furthering the objective of a crashproof structure. With such a construction, the parts can simply turn on each other, or shift, or come apart, instead of breaking when the kite strikes a solid object.

Then, also, fitting 5enables member 3 to be still further reduced in size and weight. The only appreciable load on member 3 occurs where it is in contact with band 4, and fitting 5 is present in this same region, in contact with member 3, supporting it, serving the same function as a wing rib. For example, in an extreme form, fitting 5 may be made as long as the chord length of band 4, reaching from the leading edge 9 to the trailing edge 10 of band 4, thereby removing substantially all of the stress from member 3.

The narrow band box kite, finally, achieves aerodynamic stability unknown to the broad band box kite. The narrow chord aerodynamic surfaces have greater values of the aerodynamic parameter, dC /da, than broad chord surfaces; the relative increase of this quantity is especially great at the rearward surfaces in the presence of the forward surfaces, in flight. As a result, aerodynamic damping in pitch and yaw are These aerodynamic advantages of the narrow lifting V band are the key that unlocks a chain of related benefits which are not available to the conventional broad band box kite.

A narrow lifting band in tension requires less weight of structure for its support than does a broad lifting band. The principal load that determines how large longitudinal member 3 must be is the tension in band 4. Leading edge 9 and trailing edge 10 of band 4 must be kept reasonably taut. When band 4 is narrow, leading edge 9 and trailing edge 10 are close together and the bending moment which they exert on longitudinal member 3 is correspondingly small. As a result, member 3 may be smaller and lighter, with substantial weight savings for the kite, or member 3 may be made longer without any net weight increase.

The total air loads on the kite become smaller as band 4 is narrower, because of the drag-producing surface area is reduced. As a result, all structural members and flying string may be made lighter, and a kite having the required structural strength and rigidity to fly in strong winds, now being lighter, can fly in lighter winds also.

Whereas broad band box kites need members 3 which are either of deep cross section or are supported by more than one bay of spreaders 6, the narrow band greatly increased for the narrow band box kite as compared to the broad band box kite. Except in the strongest and gustiest winds, the narrow band box kite can be launched without risk of diving to the ground.

The best point for attaching the flying line to the narrow chord box kite of this invention lies just rearward of the forward band, where it may be attached directly to member 3, without a bridle. On the broad band box kite, even if the stability were good enough to permit a single tie point to be used, that tie point would lie in the region of the rear portion of the forward band, an inconvenient place to tie the line, because it would have to pass through the material of the band. Of course, because of the difficulty in making the broad band box kite trim properly in pitch, a branched bridle is ordinarily used, and this requires proper adjustment of the lengths of the two branches of the bridle, a complexity which often combines with the marginal stability and marginal damping of the kite to frustrate the kite flier.

On. the kite of this invention, on the other hand, the attachment of the string is so simple and the flight behavior so reliable, that these narrow chord box kites may be lashed together abreast by twos or fours, with the flying line attached without a bridle, as shown in FIG. 5, and they exhibit good stability, impressive high angles of flight, and because of their efficient low drag aerodynamic form, only moderate string pull. The unique structural design described above gives these kites the structural margins which preserve structural integrity in these configurations.

I claim:

1. A box kite comprising a longitudinal body member, a lifting surface of sheet material in the form of a closed band attached to said body member, a fitting having a straight extrusion-like form with substantially constant cross-section, said fitting comprising two straight channel-shaped recesses opening oppositely from each other, the first of said recesses engaging said body member in the region of attachment of said lifting surface, and a spreader member engaging said fitting in the second of said recesses in said fitting.

2. In claim 1 said channel-shaped recesses in said fitting being unobstructed fore and aft, whereby said body member and said spreader member are movable forward and rearward in said recesses, permitting convenient assembly and producing tautness, as described.

3. In claim 1, said body member having an overall length at least five times as great as the mean chord length of said lifting surface.

4. In claim 1 said body member comprising a short portion extending forward of the leading edge of the most forward lifting surface on said kite and a short portion extending rearward of the trailing edge of the most rearward lifting surface on said kite, said kite comprising two modular units adjoined abreast, bound

Claims (5)

1. A box kite comprising a longitudinal body member, a lifting surface of sheet material in the form of a closed band attached to said body member, a fitting having a straight extrusion-like form with substantially constant cross-section, said fitting comprising two straight channel-shaped recesses opening oppositely from each other, the first of said recesses engaging said body member in the region of attachment of said lifting surface, and a spreader member engaging said fitting in the second of said recesses in said fitting.

2. In claim 1 said channel-shaped recesses in said fitting being unobstructed fore and aft, whereby said body member and said spreader member are movable forward and rearward in said recesses, permitting convenient assembly and producing tautness, as described.

3. In claim 1, said body member having an overall length at least five times as great as the mean chord length of said lifting surface.

4. In claim 1 said body member comprising a short portion extending forward of the leading edge of the most forward lifting surface on said kite and a short portion extending rearward of the trailing edge of the most rearward lifting surface on said kite, said kite comprising two modular units adjoined abreast, bound together at said short portions of said body members, each of said modular units being an individual kite capable of flight as a separate kite.

5. In claim 2, said body member being of circular cross section in the region of contact with said fitting member within said first channel-shaped recess, whereby said fitting is movable in rotation around the longitudinal axis of said body member, and is movable to disengage from said body member upon the removal of said spreader member, thereby relieving destructive stresses on impact, as described.

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