[photo] [photo]
Tetrahedral Hints and Tips

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Index

ASIDE: The following is never complete, progress usually achieved by people mailing me, which gets me back into thinking about tetras, and thus adding even more information to these pages. So the more people mail me with questions, information, or their experiences, the better the page gets.

General Notes and Problems

Tetrahedral kites are very versatile. There is not just one tetrahedral kite shape, but an infinite number of shapes and style! Sort of like lego blocks but using pyramid shapes. These are explored on my Tetrahedral Variations page.

Tetrahedrals being an unusual kite and has as a result unusual problems. 2 of the biggest is weight, and transport. Another is stability in the air.

Bridling...
The bridle point of a tetrahedral is relativally simple. The final bridal point should be a little forward of the the perpendicular from the spine and the leading point of the kite...

See this diagram I made for a old tetra plan...
[figure]

The number and actual points to which the bridle attaches to the kite varies, on the actual kite variation used, but is basically the same position regardless of model.

For tetrahedrals with multiple spines, you add bridle lines to each spine as you would for a simpler tetrahedral. Actually I find every second spine enough for 'wide' tetras. You then collect all the front bridle points together into a single point, along the center line of the kites width. The bridle lines should be long enogh so this point is 2 or so times the total width of the kite, which prevents too much side on forces from the bridle likes on the kite itself.

You would do the same with all the rear bridles. Then using a extra set of line tie the collected front bridles to the rear bridles. The bridle ring is then added to this connecting line, with its position being the only adjustment needed is needed to be made to change the angle of the bridle point.

Look closely at these two photo (click for enlargment) for examples of this type of bridle line setup...
[photo] [photo]

These extra side-by-side bridle lines also seems to improve the stability of wide tetrahedrals greatly. So much so that often extra side bridles are added to single spine tetrahedrals. For example, this tetra has some extra side bridles, rather than a rear bridle line attached to the spine.
[photo]

Weight...
Because a tetrahedral requires a lot to bracing to generate its pyramid cell shapes, and that the sails are set at a very high angle compared to other kites, tetrahedrals tend to suffer from weight problems.

A tetra made in the same way a normal kite is made, tends to not even get off the ground. Then if you do get it up, either it won't stay up, or it swings wildly from side to side.

In generally if the construction method is too heavy for a small tetra then it is also too heavy for a large one. Yes there is a slight weight to sail ratio improvement as a tetra gets larger, but it isn't great.

Fortunately a tetrahedral structure is a space filling structure, and so very light weight and weak materials can be used to form extremely strong structures. This is why the tetrahedral structure known as a Octet Truss is used for making roofs of large convention halls, and is even as the primary construction method for the new international space station.

Because of this, tetrahedrals are made from ultra light weight materials, such as straws and bamboo skewers, to form kites that are very large indeed. No other kite type can really do this.

Transport...
As a tetrahedral IS a space generating structure, tetrahedrals in general do not easily fold up for storage, or requires lots of fiddling when putting them together or disassembling. This is why you do not see very many tetrahedral kites at festivals.

The solutions to this either involve taking the kite almost completely apart, to store in a long bundle, OR somehow folding them up into a flat package, which is itself not great for transport.

Alternatively, if each cell is built completely separately, the cells can be stacked into each other for transport, This produces a very large bulky stack, but means that on the field you can construct virtually ANY type of tetrahedral variation you like. (See the Ring and Rod Construction method below).

Stability...
A single tetrahedral cell is unstable, 4 cells are only just stable if made light enough. In general the more cells you have and the lighter the construction, the more stable a tetrahedral gets!

From personal experience I have found that the wider tetrahedrals tend to fly much more stable. These "wide" shapes allow for multiple bridle lines which by its nature seem to stabilise the kite. If the kite swings badly, the extra bridle lines quickly returns the kite to stability.

As such the wide tetra shown at the top of the page, flies extremely stable and high! Even the 10 cell irregular "keystone" tetra (top left) I have found to me much more stable than a regular 10 cell "solid" tetrahedral structure. The main difference, is that three bridle lines are used in the "keystone" shape, instead of just two for the same sized "solid" shape.

However I have found that if a tetra is light enough and large enough, they continue to fly stable even when some cells have collapsed and half the tetra has "pancaked", like a building after an earthquake!.

Cell Construction Methods

Fully Braced...
The name says it all, each cell has six spars for every edge of the tetraheron. It forms a fully braced structure, that is also incredibly strong.

This allows you to use lighter weight spars such as straws, skewers, or very light dowel. Anything else is just getting too heavy. See the Building techniques below for more information.

The joints a fully braced cell is where the action realy is. and can consist of "tinker toy" sockets, to string, to plastic tubing and so on. Just how the joint is constructed also equates to how easily a fully braced tetra can be transported.

For example if each cell is build completely separate to each other, and only joined together on the field, then the individual cells can be stacked into each other. It is still bulky and three dimensional, but a very large kite will stack into a much smaller volume.

If the joint can fold, then one spar from each cell can be removed (or folded out of the way, and the kite can fold up accordian like, into a flat package.

If all the spars are removeable, then you spend a lot of time during setup and take down on the field, but the kite stored into a not very long linear bundle, like most other kites. A further advantage is that individual spars are also then very easy to replace.

Corner Bracing...
[diagram] This method cuts the number of spars from 6 to 4, shorter spars, and provides a method of tensioning in the process. Essentually the four corners of the cell is pushed outward for the center of the cell.

One problem with this is that the center of the leading edge of the cell tends to get pushed in ward, unless a very light weight spar is provided to stop this, or the edges has a slight inward curve built into them (as in cody box kite construction).

The spars however, must be very rigid. as such it is only really suitable for modern carbon fiber, or epoxy tubing. Also a string or other light spar is also needed accross the back of the cell as part of the tensioning process.

Transport however is straight forward. either the trailing edge tenson is released, or the center tensoning is released, and each cell will collapse into a easily transportable bundle of loose sticks and fabric.

Some of the largest (and prettiest) ripstop tetras in the world are built using this method.

Mast Bracing...
[photo]

Again to cut down the number of spars only 3 sticks are used. One along the leading edage and the other along the trailing edge. These are then pushed apart by a "mast" stick bewteen them.

This has a number of distinct advantages. First the leading edge is suported, and second both the cross and leading edge spars can extend across multiple cells! This means that connections between cells can be extremely rigid, and produce a very strong structure, with only 3 spars per cell instead of 6.

However strong spars are required to prevent them from breaking from the side on stressed imposed by the mast. Also 2 of the spars have to be removed for transport, increasing the setup and take down time involved.

A great web site for viewing this technique is Bell Tetra Info, and Plan, particularly toward the bottom of the page.

Building Techniques and Styles

Plastic Drinking Straws...
This is extremely popular, straws are light weight, yet very strong. Tetrahedral kites built with straws tend to fly very high, well and stable. As they are so cheap and easy to work with they make an excellent medium for classroom tetra projects.

However straws do not compress very well, tending to foldup under stress. As such straw tetrahedrals tend to be one time affairs. Due to this, construction is usally limited to a full cell bracing. That is every edge of the cell is braced by a straw.

W McClure <w_mcclure@hotmail.com>, recomends that to strengthen a very large straw tetras with some long thin dowel along the outside edges of the tetra. Of course this adds to the weight of the tetra, but can make the kite last a lot longer than it otherwise normally would.

The straw cells are then covered with either tissue, plastic, or cellophane (warning: cellophane shrinks!), and then individual cells from teams of students, or a whole class, are then tied together to form larger tetrahedrals.

Straws and String...
Joining is usally achieved my threading a light string though the straws to form individual tetrahedral cells. Generally the ends of the string are NOT trimed after the final knots are tied, so that the extra length of string can be used to tie multiple cells together into larger tetra structures.

For examples see Glenda Woodburg's Straw Tetrahedral plan is especially good. It dissappeared from the web for two years but has re-appeared. Also quite good is the Ford Middle School, Straw Tetrahedral. Another is the Kansas City Tetrahedron plan by Dave Ellis.

If you want to fold a straw tetra flat for transport, I have had success with using hat elastic. in the trailing edges of four cell units. For more information on this method see... my own straw plan.

The major problem with this technique, is that string such as a nylon thread, or cotton, which is plenty strong enough, is that it tended to cut into the drinking straw. Some people recomment superglueing a small segment of slit straw onto the ends of the bracing straws, to prevent this.

Steve Swindell suggests just touching each straw end to a frying pan, or other metal plate heats on a stove set to medium or medium-low heat. Touch the straw to the blade, BARELY pressing down (not much more than the weight of the straw is needed) for about 1 second.

Wether either method is worth the effort is another matter, and depends on how much effort you want to put into your straw tetra.

Straws and Hot Glue...
Another technoque of building tetrahedral cells with straws however was pointed out to me by Norm Anderson <nanderso@gfn.org>. In this case however he joined the straws with hot melt glue at the joints.

Straws and Balloon Sticks...
Jeff Hittman <jhittman@slip.net> has noted to me that balloon sticks (Plastic sticks used at fairs and festivals to attach balloons to), has just the right diameter to fit inside a regular drinking straws. He used 2.5 to 3cm segments (about a inch to the Americans) to create the tetra joints.

How he actually used them he did not say. But they could be hot glued into spiked balls for the straws to slot into, or just superglued glued into the straw ends to stop the string cutting into the straw.

Bamboo Skewers...
The master of this techique is TetraLite Kites who sells a book and/or CDrom all about this technique for just a few dollars. Well worth it.

Essentually you use cheap bamboo skewers sold in your local supermarket for satay sticks. These are joined together using a very thin walled plastic tubing from the electonics industry, and cable ties to form the corner joints. The cells are then covered with mylar plastic commonly sold as a silver gift wrapping foil for christmas presents or for flowers sellers.

The result is a very strong but ultra light weight kite using very modern materials. The kites fly at a very high angle, and very stable. For transport the trailing stick on every cell has one end "unpluged" and the whole kite folded up like an accordian into a flat package. Both kites photoed at the top of this page are of this constriuction technique.

The bamboo skewers I have found to get brittle with age, and thus most prone to fracture during landings, or other disasters. I have seen my tetra completely demolised when a rope from a large Peter Lynn Octopus whip across a tetrahedral I had sitting on the ground in just the wrong spot. Only the bridle ring from from that kite was worth the salvage.

Dowel, Carbon Fibre Tubes, Fibreglass...
With these stronger, heaver materials, you can get away from the fully braced cell structure, using either a mast or corner bracing method instead.

Also if the kite does fly then the inherent strength of the spars will ensure that very little damage will result due to crashes, landings, or collisions with other kites and kite lines.

Vinal plastic tubing from the local hardware, or if you can get them, specialised hard plastic joints are typically used for construction.

Ring and Rod Construction...
[photo] Rob Thomlinson and Tony Broad in England, developed and created a great web page of building fully braced tetrahedrals using a method involving "screw eye" rings screwed (and glued) into the ends of carbon fiber tubes (and tube ends reinforced).


[photo] The individual cells are then pre-built (any of three different ways) and stacked into each other for transport. On the flying field the complete kite is put together in any variation you desire for that days flying using cable ties to join it all together.

A Fantastic solutaion and extremely versitile.

To see more of this construction technique visit the How to Make section of their Web site.


Updated: 29 May 2002
Author: Anthony Thyssen, <A.Thyssen@griffith.edu.au>