June 2007

Newsletter, June '07.

Re-inventing the wheel.

Exciting times in traction kite development!

Maybe this isn't yet apparent in the wider world, but from where I sit (stand, walk, run, fly, whatever) there are more reasons to be optimistic about major roles for kites than at any time since George Pocock first went upwind connected to his sail only by strings 175 years ago.

Groups in various parts of the world are currently developing systems for hauling ships and larger yachts, and whether it may be possible to generate electricity from kites is also being looked at. Some of these projects are a bit shoestring, but most have adequate funding and we can be sure that our understanding of controllable kites will be significantly advanced as a spin-off from their work no matter whether any achieve commercial success or not (and I'm hopeful that at least some will).

Most recent development  (1970's until now) of controllable* kites has been towards recreational uses, the stunt kite/sports kite movement, and buggying, snow kiting, kite boarding etc.

In the last part of the 19th century and early bit of the 20th, kites underwent rapid development in the lead up to heavier than air flight.  Hargrave and his box kite (1893) were hugely influential and Samuel Franklin Cody pushed the kite envelope further than anyone who came before him- and perhaps further than we realise.

In 1903, SF Cody sailed across the English Channel using a foldable boat and one of his cellular kites.

Netherlands kite history aficionado, and now editor of the long running Vlieger magazine, Dick Kortland, theorizes that the kite Cody used on this crossing was steerable and de-powerable- and presented evidence for this at the recent Wasserkuppe historic kite meeting.  Dick believes that Cody steered his kites by rigging their top deck as a rudder and has replicated how this may have been achieved using clues from old photographs.  Quite a few photographs of Cody kites from the period do indeed show the top deck at an angle to the main body of the kite (which has always been puzzling), and some show rigging lines that cannot otherwise be easily explained.  Dick reports excellent steering on the kites he has rigged to test this supposition.  There is also some evidence from photographs that Cody actually did use multiple lines with his channel crossing boat, but these extra lines seem to have been for the purpose of adjusting the pull point on the boat rather than for kite control.

That Cody did have power control on some of his kites is however beyond dispute.  A system for controlling pull by changing angle of attack was an essential feature of his man lifting system and is well known.  Whether he had such a facility on his channel-crossing kite is not clear.  

To those engaged in developing kites for hauling ships around and for generating electricity, this is rather interesting.

A barrier that contemporary traction kites are up against is that while they have excellent steering when powered up, when de-powered by more than about 60%, this control is lost.  There is a solid technical reason for this:

Aircraft are controlled by simultaneously changing the angle of attack of one wing relative to the other (ailerons), their angle of incidence (elevators) and by rudder action.  For this pilots require two hands, both feet considerable coordination and hours of training. 

By contrast, contemporary kites steer by increasing the angle of attack on one side while decreasing it on the other.  This minimalist system works even with just two control lines, and even at stall.  Unfortunately though, it doesn't work when kites are flying at very low angles of attack, which they are when de-powered by more than the above-mentioned 60% or so.  For steering to work when a kite is more completely de-powered, either drag flaps or rudders are necessary. 

Cody's system, if indeed it was as Dick Kortland supposes, not only offered nearly 100% de-power, but would have retained full control by virtue of its rudder type action. Just what we're now looking for!

And a corollary: Alexander Graham Bell, better known for inventing the telephone and his sponsorship of the Aerial Experimental Society (which played a part in developing the modern aeroplane), also used kites for traction in the early 1900's; Hargrave's box kites in particular, and they were steerable ones.  From "Bell and Baldwin" by J H Parkinson; " He found it possible, with two cords like reins on a horse to make a Hargrave kite fly off the wind to either side through a range of 50degrees, and as the boat could be steered nearly at right angles to the cord, it was possible to make the boat tack into the wind, just as with a sail."

But wait, here's even another wheel that's in the process of being re-invented:

One approach being considered for electricity generation from kites is called the 'winch system'. By this, a kite is caused to fly a figure eight pattern while pulling line from a winch drum that is geared to a generator.  When some predetermined altitude is achieved, the kite is de-powered and the winch is used to pull the kite back down to near ground level for the process to start over. The kites being developed to do this need to be efficient (high L/D), steerable (generally by ground based computer via radio link to kite mounted servo operated control surfaces) and to have as near 100% de-power as can be achieved. 

I've just been in Malaysia, home of the Wau, and one particular Kelantan variety does (almost) all this by itself as a single line kite and without computer or human input.

A type of Wau Bulan which has extended wings and is generally about 3m wingspan, these kites have been intensively developed over many years via competitive events to fly a figure eight pattern, to fly at the highest attainable angle and to pull like hell.   They are SERIOUS kites!  I'd judge their L/D as at least 4, while their ability to figure eight without ever either looping out or stabilising is far ahead of any theory I could use to design or even post facto predict this behavior.    They even de-power; sometimes tipping forward and diving to the ground all a flutter- though this doesn't seem to be very controlled.

Whatever will we find out from the past next!- maybe that the pyramids were erected using kites?- Nah.

It's time for a few more steps forward though, and they're coming.

Peter Lynn, Ashburton, 1 June '07

Thanks to Drachen Foundation for the use of the Cody boat photo and their help with background stuff.

Thanks also to Dick Kortland for allowing me to borrow his theory.

* I use 'controllable' rather than 'steerable' to cover management of pull as well as direction.