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Bell Tetrahedral Kite
What is it? Who Invented it?
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
- Drachen Archiv, Alexander Graham Bell (In german - good pictures)
- Tetrahedral Principle in Kite Structure
by Alexander Graham Bell June, 1903
- Bell Family Papers, Library of Congress
- 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
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,
WWW URL: https://antofthy.gitlab.io/kites/tetra/bell/