Iceberg Kites

Hello Geoff,

I've found this more difficult to write than I expected- because Pete has just raised the valid question that if we are going to use a train of NASA wing style kites, HOW are we going to train them?  Being single skin multi-bridle kites it's not  practicable to just take the 2 flying lines past each kite to the next one- because the natural spacing between the lines when the kites are flying will be less than the width of the kite. 

There are various answers:

One solution is to use compression struts between each kite to hold the lines further apart than their natural spacing.

Another is to pass the lines through correctly placed apertures in the kite's skins- but because the kites will tend to move around a bit while flying, the flying lines will inevitably come up against the edges of any such apertures at times, which may distort that kite enough to effect its stability.

Another, the usual way that NASA's are trained, is to use kites of decreasing size for each additional kite in the train, (largest at the top).

There are other possibilities, including going back to using just a single big kite or, if the 'berg is large enough, anchoring each kite in a different place.

My current inclination is for a train of equal size kites and a single compressive strut to hold the flying lines well  apart at the anchoring point.  Simpler and safer, because everything in the air will still be completely soft, this will work for a train of 3 or 4 kites but might not allow many more than this.  We will have to make some tests to see what the practical limits will be.

Therefore, because we can't yet be certain as to which approach to use, we will have to leave our options open in the presentation. 

What I've done is written a piece for this 'berg project that has some of the history of traction kiting and enough technical stuff without being too specific.  I think you will be able to extract suitable material from this for the presentation. The bit about the history of the NASA wing is just from memory, I haven't checked it- don't use this bit before it's confirmed  please. It's possible that the term NASA wing is someone's copyright and that we may not be able to use it- the name that is- the design has been around in the public arena long enough for any patent or other design claims to have long since lapsed

And the kites?

There is a very long history of kites being used for making journeys.

Sam Cody crossed the English channel using kitepower in 1901.

Benjamin Franklin records using a kite to pull himself across a pond 250 years ago.

But much earlier than this, perhaps 10,000 years earlier, fishermen in South East Asia were using kites for fishing and to pull their canoes.

They used (and still use) a large leaf that, when dry, is a perfect kite.  All that is required is for a line to be attached to it in the correct place.

Simple, perfect, natural- the beginning and inspiration for what is now the diverse field of traction kiting.

In the last 25 years, special kites have been developed for kite sailing, kitebuggying (a branch of land sailing), ski kiting (snow), and kitesurfing. Teams of designers are currently working on kite power systems for large ocean going ships.

Each discipline has it's own technical requirements and therefore the kites used for each, although sharing a common origin, have diverged markedly in form. Kite designing has become a highly technical area, a field for aerodynamicists and computational fluid dynamics.

Kites for moving icebergs require some very special features:

·         Huge pull- in the order of 100tons or more will be required.

·         Ability  to function reliably in an extreme environment-, ice, snow and salt spray.

·         Reliable safe flying in a wide range of wind conditions.

·         Capacity  to be deployed and retrieved easily. 

The pull requirement demands that the total kite area deployed will have to be of the order of 12,000 sq.m (average wind of 30km/hr and a lift coefficient of 1.0).

Requirements for size, safety and deployability makes a totally soft structure preferable.

Functioning in snow makes the use of a single skin structure desirable (ram air structures tend to suffer from internal ice build up).

Because icebergs are huge and not at all hydrodynamically elegant, no level of kite efficiency will ever be sufficient to enable a course other than a few degrees off the downwind direction. Therefore there is no  requirement for the kite(s) to have other than moderate steerability and efficiency (lift/drag ratio), allowing the design to be optimised for pull, ease of use and safety.

The obvious best form for this specification is a kite of the style commonly known as a NASA wing.

These are completely soft, single skin (fabric) multi bridle steerable kites that are stable have strong pull (lift coeff. >1) but only moderate L/D- (typically 2.5 to 3.5). The  NASA wing design was developed for NASA in the 1970's.  Because of it's simplicity, easy launchability, and strong pull it is popular for traction kiting activities (such as sled pulling across snow) that don't require high aerodynamic efficiency.

Depending on how successful training kites of this size proves to be (that is, how many kites can practicably be rigged at intervals between one set of lines), each kite will have to be somewhere between 1000sq.m and 10,000sq.m.  Of course, if a 'berg is big enough it will be possible to use more than one anchor point.  As many kites as there is space for could be flown simultaneously, each on it's own individual line set.

Although, to my knowledge, NASA wings this large have not yet been built, the "super ripstop" system of construction pioneered by our company enables fabric structures of this size to be built within the aerodynamically permissible weight/area requirements for kites.

This system comprises a gridwork of typically 1000kgm "Spectra" filament cords sewn onto standard light weight rip stop nylon fabric at approximately 1.5meter centres. These cords accept the main aerodynamic and rigging loads, leaving the fabric to bear only the loads generated in each square- which it is then easily able to do up to at least 60km/hr of wind.

We also plan to try various other design innovations for this application and it is not unlikely that the eventual form will be substantially difference in appearance to the standard NASA shape, while retaining the essential single skin, structureless principle.

Hope you can get what's needed out of this,

Peter