May 2009

No Sticks, No keels, No tails, and now, No bridles!

While designing new kites, unexpected things happen.  Overwhelmingly, these are unwelcome; usually it's "bugger, why doesn't it go the right way just this once"!

Very rarely something unexpected and helpful occurs, (but then often isn't recognised until a belated kick-yourself-in-the-bum moment many years later).

Thanks to Sven Weidhase (No Limits, Germany), an unexpected and useful kite simplification has put its hand up and said 'me, me, me! I've definitely noticed the implications this time, and will explore every possibility.  Unfortunately, although it makes kites simpler, it makes the designing MUCH more difficult. But such is progress.

And fairly, Peter Rieleit (Dusseldorf Kite Group, also Germany) should get the credit (or blame, depending how it turns out) for this advance- he's been on to it for years.

But first, a substantial (relevant) digression:

I've been using the development of a new ray kite as a test of the half-arsed theory I have (a work in progress at best), for why single line kites fly- or rather, why they generally don't.

To reiterate; a prediction from this theory is that aspect ratio (the kite's span relative to it's length) will have a strong influence on undercorrection/overcorrection.  That is, higher aspect ratio will incline a kite towards undercorrection. 

This suggested there should be some specific aspect ratio for a ray kite at which it will neither overcorrect nor undercorrect.

And, as described in April's Newsletter, there is - and fortunately, within realistic looking range for rays, (the fishy ones that is).

The stabilising principle behind using aspect ratio to control overcorrection/undercorrection is that when a kite begins to swing to one side or the other, drag from the faster wing tip, if it's far enough out from the kite's centre of rotation, will slow this swing sufficiently for the leverage off the kite's weight to be able to straighten things up before destructive oscillations establish. Genki and Delta form framed kites use this stabilising effect, as do many other kites, and there is no reason why soft kites can't also.  If the wing tips are too far out from the kite's centre of rotation (that is, it's a very high aspect ratio kite, with span much greater than length), then the correction from any rotational movement will be so slow that the kite may traverse completely to one side or other of it's wind window before correction occurs (this is undercorrection).

Rays are a good test of this theory for other reasons also - rays (the fish) have smooth tails and no keels, so neatly outlaw two easy ways to cheat this test (tail drag and keels are both excellent stabilisers).

However, there is an aspect of tip drag stabilising that requires further explanation:

As apparent wind speed increases, there is no stability gain from tips if the lift they generate increases faster than their drag does- because lift forces drive instability. Delta kites (and Genki's) get extra drag from their tips without commensurate lift increases by having them twist off as wind speed increases. 

For this test ray, the tips generate drag without commensurate increases in lift in four ways:

-They have twist-off; by the upper skin being cut with less sweep back than the lower skin, so that the angle of attack of the tips is less than that of the body.

-The tips use thicker (higher drag) profiles than the wing roots- adjusted by thru cord length. 

-The tips bend up- to be more ray-like, but also so that the lift forces they generate tend to cancel each other out rather than contribute to total lift.

And lastly, their profiles have more reflex (reverse camber) than the main body. This is set (and adjusted) by using diagonal thru cords- which is what I have been trying to get an opportunity to get to for the last however many paragraphs.

In itself, thru cording is a revolutionary soft kite construction system- well suited to theme type soft kites used for shows because in this application, the loss of aerodynamic efficiency relative to the previously conventional rib and skin construction is advantageous by having more relative drag and hence more inherent stability (we don't mind them flying at a low angle provided they look OK). 

For the last 20 something years I've been using thru cords instead of ribs for most soft kites - not least because this uses less fabric (ribs are typically 30% of the total). Cord construction is not just a substantial cost saving, but allows bigger and/or more kites to be carried within the same baggage allowance. 

But parallel thru cords don't control shape satisfactorily when a section is subject to bending loads.  Like for roof and bridge trusses, there needs to be diagonal bracing as well. I've only recently (last 5 years or so) made much use of diagonal thru cords, and have no reasonable excuse for this tardiness.  By any measure I deserve one of those kicks up the bum for this.

But now that I'm finally on to it, a new world is opening up.

Just one example; when using rib and skin construction, every shape to be tested basically has to be a new kite. When using straight and diagonal thru cording instead of ribs, almost every possible profile can be tested just by changing cord lengths- which takes hours rather than days.

For this test ray, instead of making four or five prototypes without useful results, and giving up- like I've done with innumerable other new kite design ideas over the years, just one prototype and a hundred or so hours of tweaking cord lengths achieved a flyable new design- and a rather special one.

And, there's something more;

Most theme kite shapes tend to be a long way from functional in an aerodynamic sense (though they may be representationally accurate).  This is because by the conventional construction systems, it takes far too many prototypes to get the aerodynamics working optimally. Instead, new designs tend to be pulled into approximately flyable form by adjusting bridle lengths.  The problem then, is that when the bridles are not loaded- like when overflying and in turbulent wind- the kite reverts to it's natural (and less than optimal) shape, and misbehaves.

By using thru cords (direct and diagonal) to control shape, development of the test ray has been extraordinarily rapid- by just the second prototype, its inherent shape may already be close to optimal.

And this, I think, underlies the unexpected and useful thing that happened while flying it at Cervia a few days ago.

During a relaunch Sven (Weidhase) chanced to hold it by just three bridles (of 31 in total), and it flew- quite well actually.  I then tied the main line off to these three bridles and it stayed up for the rest of the day- at lower angle and with less pull than usual, but with better wing shape and less leading edge compression. Actually I've since found that it will fly from just a single attachment point, sans pilot even.

Astonishing!

This is not a kite shape that I would have given any chance at all of flying from a single attachment point.

The 'experts' (Greek chorus) watching all this from the bar, were generally pessimistic about how well it would handle stronger winds in this mode (Cervia wind can be very smooth). I'll bet that it will- but I think light winds will be difficult. However, it's not likely that this style of kite will become bridleless any time soon- or ever.  To get it to fly at reasonable line angle, multiple attachment points will almost certainly be necessary.

What I do expect now though, is that many lower aspect ratio kite designs could lose most or maybe all their bridles if their precise forms can be developed to the necessary level.

Peter Rieleit has been on to me (and everyone who will listen) for years about this: By Peter's view, keels are an abomination, tails are a symbol of failure, and ideally there should be a single line attachment point. His kite designs, while not meeting these criteria 100%, are definitely moving in the right direction.

And I agree with him.

Sticks are BAD, especially when they interact with terrain, (not so helpful at check- in counters either). 

Tails are an admission of failure, unless they are a feature of whatever creature the kite represents.

Keels flap around and get in the way of graphic expression.

And bridles are from the devil.  In light winds, most are slack, don't do anything.  In strong winds, they cause single line kites to have more pull than is useful or safe.

Bridles should be banned, or at least taxed.

Peter Lynn, Cervia, April 30, '09