# Bell Tetrahedral Kite

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### History of the Tetrahedral Kite

At the start of the 20th centry, leading scientists were working to prove that large full-sized flying machines were just impossible. Their proof was that to double the size a kite (or flying machine) would give it four times the surface area, but you would have to increase its weight by 8 times!

Alexander Graham Bell looking at this situation, after his huge success with the telephone, decided to get involved. In 1902 he wrote the proof that it was indeed possible to build large flying machines without the increasing weight cost. Instead of building one large wing, his proof was based on a whole 'flock' of small wings in the form of "tetrahedrons".

You can find out more about Alexander Graham Bell from the

Alexander Graham Bell Institute
Drachen Archiv, Alexander Graham Bell (In german - good pictures)
Tetrahedral Principle in Kite Structure by Alexander Graham Bell June, 1903
Online Encarta Encyclopedia
Bell Family Papers, Library of Congress
Brain Spin: Who was Alexander Graham Bell
Some Historical Tidbits (news artical)

### What is a Tetrahedral Kite

Tetrahedrons are a regular 4 sided polygon. Basically a pyramid shaped framework which is the strongest structure known. A tetrahedral kite is formed when you cover two sides of the four sided figure and to join a number of these together into a large tetrahedral kite.

By doing this Mr Bell was able to prove that you can create a large kite, of any size desired, without any increase in the weight to sail area. You do not need any extra bracing in larger kites and the strong tetrahedral cell is itself fully braced. In fact the more cells you add to a flying machine, the stronger it becomes. This allows you build tetrahedrals from the lightest materials but still have a strong and sturdy final product.

A recent scientific study, "Tetrahedral Principle Revisited", even reversed this position, showing that the more cells a tetrahedral kite has the better the surface to weight ratio becomes due to sharing of the joints between cells.

To the right is a photo of a 12 spar joint from a tetrahedral (not Bell's design). In Bell's own design, each cell was made separatally and then joined together, though I am not certain how it was done.

## Tetrahedrons, space filling structures

The main disadvantage is that a tetrahedral IS a space generating structure. Large box kites have a decidedly open and airy look to them. Large tetrahedral kites on the other had looks like a solid object flying in the air as each cell becomes small with respect to the complete kite.

The solidness of this structure however has become known as an "Octet Truss" and is now used all over the world due to its ridgness. You can see it in just about any large confrence hall or structure. And even the "Space Station" uses this structure heavily, to join sections together, while keeping things economically light weight.

But because this 'space filling' structure and the involvement of lots of individual cells, tetrahedrals in general do not easilly fold up for storage, or requires lots of fiddling to put them together or for dissassembling. This is why you do not see very many tetrahedral kites at festivals.

The newest tetra kite plans, such as my own tetrahedral kite plan or the TetraLite Construction Manual, on the other hand are built from easilly available modern day materials, and fold flat quickly and easilly for transport. Something which I am sure Mr Bell would have loved to have seen.

## Skys the Limit

Mr Bell's ultimate achievement with tetrahedral kites was a kite built of 3,393 cells, and was named the "Cygnet" (see photo right). The kite was towed behind a steam ship and actually carried a passenger, a Lt Thomas E. Selfridge. After the initial flight however the kite was destroyed immediately after landing, before the steamer crew could cut the tow rope. Lt Thomas E. Selfridge, later had the honor of being first to die in plane crash fatality, in a wright glider.

## Mr Bells Tetra Construction

All Mr Bells tetrahedral cells were made separately and are were 10 inches on a side (rather small). They were made from spruce rods, and covered with bright red silk. Each cell weighed about an ounce, and were joined together by some sort of ingenious metal fittings. The towns people of the nearby small town Baddeck, in Nova Scotia, Canada, were enlisted into making thousands of these tetrahedrons, and became quite a local cottage industry.

Created: 16 June 1996
Updated: 24 January 2013
Author: Anthony Thyssen, <anthony@cit.gu.edu.au>
WWW URL: http://www.ict.griffith.edu.au/anthony/kites/tetra/bell/