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Tetrahedral Kite Plan - Construction


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Requirements


Construction

Naming of the Parts

Here is a diagram of the various parts of the 4 cell tetrahedral kite this plan will refer to...

[Diagram]
The four faces of the kite are the front, rear, left and right faces.

When flying the leading edge is at an angle pointing into the wind. The trailing edge, which does not have of the covers over it, is perpendicular to the wind, and parallel to the ground. The other four edges are side edges -- two on each of the side faces of the kite.

Step 1 -- Spars

Before you can start you must chose the kites one dimension - the individual cell length, which I will refer to as L. The final kite itself will be twice this length in total for the 4 cell version, and a bit over 3 L by 2 L when folded flat, in a diamond shape. Of course for a 10 cell version the kite will be 3 L along on each edge.

In my example kite, I chose a length (L) of 33 cm (1/3 meter) more for conveniance than design (it was a prototype). I recommend however that L be 40, 50 or even 60 cm in length.

Fiberglass is very flexiable, and 6mm dowel not ultra strong, however the ridged structure of a tetrahedron makes all the spars support each other so the rods flex or lack of strenght has little overall effect. In fact another plan in this workshop makes a Straw Tetrahedral Kite from plastic drinking straws!.

I recommend for cell sizes (L) of 50cm or more that spars of 6mm wooden dowel be used instead. Carbon Fibre could also be used though it is probably too expensive, and for a kite of this type, its strenght is not really required, though its light weight would be a benifit.

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For our 4 cell tetrahedral kite, cut the spar rods (whatever you are using) into...
6, 2×L length spars for the external edges.
and 10 spars of L length for the internal braces of the cells.

Mark the center of all the longer spars, where the various edge joints will be positioned.

All the spars should also fit tightly into the plastic tubing (same inner diameter as the spars being used. If it is loose wrap one or two turns of sticky tape around the ends of all the spars.

DO NOT however do this for the ends of the trailing edge spar unless the tubing is a very very loose fit. The reason for this is that the ends of the trailing edge will be pushed in and pulled out of the plastic tubing repeatedly during its lifetime, allowing the kite to be flattened. See the assembly photos of step 4.

Step 2 --- Joints

Due to the number of spars being joined together, it is the joint themselves which is the most important aspect of construction. This 4 cell kite uses 10 joints of 3 different types to connect together the 16 spars you cut above.

Unless otherwise stated DO NOT insert the ends of the spars into the plastic tubing. If you do, you may not be able to pull them out so easily, so you can slide the rods into the cover spar pockets.

Corner Joints
[photo] [photo]
4 Required.

This is the simplest of the joints and is commonally used for delta shaped stunt kites, such as a ``Peter Powell Stunter''. It is used to hold the external spars in the larger tetrahedral shape. Do not insert the spars, yet.

A plastic tube of at least 6 cm in length is (forcibly) inserted into a slit cut near one end of another plastic tube, about 5 cm in length. The Leading and Trailing Edge Spars will (in step 4) be inserted into the shorter tube, while the Side Spars is inserted into either end of the longer tube. This creates a solid, but flexible joint.

Leading & Trailing Edge Joints
[photo] [photo]
2 Sets Required.

At center of the leading and trailing edge, 4 internal spars come together at a point in the middle of the spars. Thus the joint has to connect together 4 spar ends and let an external spar continue though it. That is quite a large number of spars at the same point.

To do this, a hole is made in the middle of two lengths of plastic tubing (about 6 cm long). For 3mm Fibreglas it is enough just to piece the tubes with a nail or small philips head screw driver. For 6mm dowel (or larger) a hole (smaller than the spar diameter) will have to be punched out of the plastic tubing. A leather hole punch is ideal, but a drill bit of a 4mm brass tube with the edge sharpened has also been suggested. Remember the hole must be smaller than the spar diameter so that it will be a good very tight fit, while not weakening the plastic tubing too much.

Push the leading and trailing edge spars through the two plastic tubes and then tie the tubes very tightly together with either: a bit of wire, twist tie, or small length of string (kite line); so that they stay together forming a single joint.

Center the joint in the middle of the spar for a 4 cell version, or every L apart for larger multi-cell versions of this kite. Wrapping a bit of tape around the spars on either side of the tube will ensure that the joint will not slide around.

Side Edge Joints
[photo] [photo]
4 Sets Required.

The Side joints is very tricky to design if you want the kite to fold flat for transport. Also as I do not use a spar horizontally across the middle of the front and rear faces of the kite the joint only has 3 internal spars joining in the middle of the side spar.

The cross piece is not needed, the lower cells being supported by the upper cells and the trailing edge spar, thus cutting down the weight of the kite to a degree.

A plastic tube, about 7 cm long, is folded double, and the end of another smaller tube ( about 4 cm long ) is placed inside the fold. As before a hole is then punched though ALL the tubes (the one folded over so is pierced twice) a bit away from the fold, so that the holes lines up.

The tubes are then pushed onto the side spars - first the longer tube, then the end of the smaller tube and finally the longer tube (folded over) again. and the join pushed to the middle of the spar. Some more tape around the spar on either side of the joint will again hold it in place.

NOTE: When the frame work is finally assembled, the smaller (middle) tube of these joints will hold the internal spars which goes to the middle of the trailing edge. This is important as it is the smaller tube which will then do all the bending and twisting when the kite is folded flat.

As mentioned in the ``Side Joints'', I leave out one of the internal bracing spars to save weight. If you want to keep that spar OR if you are creating a 10 cell ( 3 L per edge ) version you may like to retain those internal spars. In that case you may like to substitue the ``Leading & Trailing Edge Joints'' for the side joints.

However doing this will make the kite a lot more difficult to fold flat. In the 10 cell version you will also have have to slide the middle cell covers over the spars, before, you can can place the second join along the spar. making a real puzzle to put the kite together on the flying field.

Step 3 --- Covers

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Photo'ed above is one of the covers I used. It is made of poplin (polyester cotton), though ripstop would probably be better. Thay consist of 2 equilateral triangles of length L with a generous hem allowance to allow the spars to slip into the hems, and still be able to hem the spar tubes. I suggest 2 cm hems if you use 3mm fibreglass and 3cm for 6mm dowel.

Actually the hem should be generous enough for the plastic tubing as well, as the ends of the joints will usually will also slide into the ends the spar pockets as well.

Before creating the spar pockets however cut out the four corners of the cover, hemming these if the material requires it. I also attached a small loop at the end points of the cover to loop over the spar joints and help pull the cover tight. I have found though, that if the cover is properly made these loops are not really required due to the triangular shape.

NOTE: If the spars you are using are thicker than the 6mm in diameter, you may like to add a gap of about the diameter of the spar, between the equilateral triangles. Remember that as with all box kites a tight cover is a good cover, so becareful with measurments.

All the sewing is reasonably straight, (except maybe the cut out corners) so you should have no real problems in this respect. Also as the cell is completely braced you should not need to worry about the bias of the fabric (not that ripstop has much give to it). Though having the bias set across the longest width is probably the best for uniformity.

Step 4 --- Assembly

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Shown above is a photo of the bare frame so you can see the spar construction, and another with the kite frame properly covered.

The frame must be assembled while at the same time carefully slipping on each cover, first on the external spars, and then inserting the inner spars into the hem. If you placed loops on the ends of the covers you must do the lower, leading cell, covers first, so that you can insert spars (and joint tubing) through the loops while covering the upper cells.

On the lower cells, do not insert the external spars into the corner joints until both spars have been inserted into the hems of the cell covering. After this insert the spine of the cell and finally the internal bracing spars from the leading edge to the lower folded tubing of the side joints.

When covering the trailing (upper) cells, it is much easier to insert the spine of the cells into the framework first. This spar is inserted into the upper folded tube of two side joints. The middle tube should not yet be in use. Thread the two external spars though both hems together. After this pull the cover under the just already inserted cell spine.

If loops are attached to the covers, thread the trailing spar though them so that each loop side on the opposite side of the center trailing edge joint. All that is left is to very carefully insert the last internal spars into the hem between the trailing edge joint and the middle tube of the side joints.

Remember the middle tubes of the side joints MUST hold the internal spars which are connected to the middle trailing edge joint. If it does not, the plastic tubing at the sides will twist and bend awkwardly when the kite is folded.

[photo] [photo]

The above photo's show pictures of both just the bare frame and the covered kite as it would appear when folded flat. To fold the kite, both ends of the trailing spar is pulled out of both corner joints. Only the middle of the spar is left attached to the kite, via the trailing edge joint.

The kite would only have been able to fold flat like this, as I did not add spars across the front and rear faces of the kite, and the flexible side joints.

Step 5 --- Bridling

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You have actually quite a number of choices in the bridling of the kite. The various books I have read showing the tetrahedral kite (about, not its construction!) recommend that the kite line is attached directly (or indirectly via a loop of line) to the middle of the wind-most cell, without any bridle line.

To bridle it here I attached a small loop of line to the leading spar in the middle of the cell, 1/4 of the spars length or 1/2 L of the way down the spar. The loop is threaded through the though a small hole in the middle of the leading cell (See photo above). I also wound some tape on either side on the spar to stop it slipping up and down the spar and pulling on the cover stretched across it.

ASIDE: To stop the poplin cover, I used in my prototype, from fraying I placed a drop of super (crazy) glue here to be absorbed by the material and then let it dry, before making the small hole, for the loop.

Another method is to just properly bridle the kite from either: both corner joints of the leading spar, OR from the middle joint and the wind-most corner. If you do bridle the kite however you should provide a good length of line for the bridle (5 to 6 L is good).

In very high winds the kite does not seem to be very stable. I found that just attaching the flying line to the wind-most corner directly seems to work very well. Trial and error in this case seems to be the best teacher.

Aside...

Two other ways of bracing are available, other than the full, or near full (as above). See the tetrahedral hints and tips page. The purpose of these methods is to reduce the need for so many spars, thus making each tetra cell lighter, though posibily not as strong.

For dowel, the mast-bracing method works well. For stronger spars like carbon fiber tubing, the corner-bracing method can be used. For lighter spars such as straw or BBQ skewers, only the fully braced method is recommended.

STOP THE PRESS #2

A new tetrahedral kite design simular to the above is now available from TetraLite Kites. While I can't tell you the construction technique (you have to buy their manual for US$20, and it is worth it), their kites are based on 1/8 inch dowel or BBQ Skewers, and plastic tubing. The joining method is based on a Rec.Kite newsgroup discussion.

If you look on thier pages you will see series of pictures of a 34 cell tetrahedral kite being folded flat into something a mere 3 inches thick. This kite is also so very very much lighter in weight, due to the materials used.

--- Anthony Thyssen.


Created: 16 June 1996
Updated: 8 August 1997
Author: Anthony Thyssen, <anthony@cit.gu.edu.au>
WWW URL: http://www.cit.gu.edu.au/~anthony/kites/tetra/plan/