June 2010

Wind, some local History, and the Mary Clarkson Trophy.

No, silly, the sort of wind that kites use, not the room clearing stuff- but now, there's a challenge!------?

During the winter hereabouts (44degrees south) there's either too much wind for kite flying or none- at this time of year we can have weeks on end when there's not even one flying opportunity.  This isn't quite the same as having no wind at all though; southerly fronts come through once a week on the average, but their windy phase often only lasts a few hours- and is usually accompanied by heavy rain- and is just as likely to be at night time as well.

In the summer, we do get wind.  Sometimes delightful steady mid-range easterlies even, but also the dreaded Canterbury Nor'wester.

To call them nor'westers is to be a bit directionally challenged though- they come from the west, driven by the earth's rotation.  The engine of the roaring forties, they're always present at higher altitudes and are the reason it takes, on average, an hour longer to fly to Singapore from here than to come back.

OK, OK,  boring- but this is going somewhere, just be patient.

In the mountains (50km upwind from here) nor'westers are strong.  At our nearest ski field (Mt Hutt) they don't actually know how strong for sure, because the wind measuring equipment gets blown away every time there's a real good one.

But here in Ashburton they don't often exceed 100km/hr.  Not more than a few times each year anyway.

But it wasn't always like this.  When this town was founded in 1876, the Canterbury plains were bare and wind swept; trees never re-established here after the ice sheets pulled back into the mountain 10,000 years ago (a bit difficult to blame this on us, but the Greens will find a way).  My various grandparents had to cart firewood for the winter with a horse drawn dray from Alford Forest (in the shadow of Mt Hutt, 40km away).

Now, after 100 years of concerted effort by settlers, there are trees everywhere and day to day life is effectively sheltered from the raging winds (and our topsoil doesn't blow out to sea either).

But back then, the wind was not easily ignored.  Quoting from a local history:

"On September 23rd 1898, after a winter of heavy gales and a succession of nor'west winds, yet another heavy gale sprang up. ---- Children who lived close to Highbank School were allowed to go home for lunch.  All succeeded except for one girl, Mary Clarkson, who was caught by the full force of the gale and blown away for about three-quarters of a mile.  After a search by several residents, she was found about 2am the next morning, more or less buried by the blown soil, but little the worse for her unpleasant experience."  

So;

 I shouldn't complain about the wind here- it's nothing like as rough as it used to be.

And, I shouldn't complain about our kite catching trees either- they've tamed this bit of the roaring forties.

And, thousands of lexicographers can rest now that the origins of "I was just blown away by----" is settled.

And, I wonder what Mary's lifetime of trauma counselling cost NZ- or maybe that was then and this is now?!

And there's another thought from this: Just how much wind can a kite fly in?

I'm pretty sure that a well set up Pilot can survive, just, at 100km/hr.  An 8m generates line tension of 125kgm's at this and lifts 73kgms (me).  But Pilots are unlikely to be the best choice for maximum wind flying- though they may be a useful developmental start point.

Kites are in essence a pendulum.  That their weight force acts at a point below and behind where the lift forces they generate act, is what points them upwards.  If it wasn't for dynamic effects, this is the only condition that a kite would need to meet for it to soar majestically as far from the earth as it's line allowed (and there'd need to be wind of course).  Unfortunately dynamic effects do matter.  If a kite, when tipped a bit one way or the other, overcorrects as it gets itself back in alignment with the wind, this can build into destructive oscillations.  Conversely, if a kite corrects too slowly, by the time it's pointing straight again, it can be a long way off to one side or the other of its wind window, even terminally so.

As wind speed increases , every kite will fail by one or the other of these mechanisms- if it hasn't been impossibly structurally distorted, burst or broken already.

And there's a non-linear effect as well.  On one side of the pendulum stability equation is the weight force; gravity (which hasn't changed much for rather a long time) acting on the kite's mass (which, disregarding birds and their doings, doesn't change while a kite is flying except for icing up, getting wet, or having something fall off).  On the other side are aerodynamic forces (lift and drag)- which increase with the square of the wind speed.  Therefore, the weight force (which stays constant) has to prevail over aerodynamic effects that double for every 40% increase in wind speed- and this, eventually, has to end in tears (two sorts of).

But an 8m Pilot weighing just 1.5kgm can keep 125kgms of aerodynamic forces under control- sometimes at least. 

What's the maximum wind speed a kite has ever flown in?

150km/hr?  Could a kite be made that would be stable at 200km/hr?  And what would such a kite look like?

Framed or ram air?  I'm biased towards ram air, but for this job, framed allows many more design options.

How big?  At 200km/hr there will be 400x the structural loads and line pull as at 10km/hr: Fairly small then!

Light or heavy? (by weight/area).  Heavy is tempting- because of the non-linear stability relationship described above, but kites that are heavier in proportion to their lifting area are also more inclined to dynamic instabilities.  It's not much use making a kite that will be stable at 200km/hr if it's unstable at every wind strength below this- except that this is exactly the sort of instability that tails are able to control.

And this seems like a sure bet; any kite designed for maximum wind will have a tail- because if it doesn't, then adding a tail will allow it to fly in even stronger wind.  Tails are an excellent stabilising device- because paradoxically, they don't start to apply correction until the kite is a few degrees out of alignment, so they don't exacerbate any tendency to undercorrection that a kite may have.  But they also act to damp out undercorrection.  Tails are the stabiliser for all seasons!

So this is the challenge: -How much wind can a single line kite fly in?

Who, where, what kite and how much wind to be recorded on:  "The Mary Clarkson Trophy".

But before this can proceed, there needs to be a system for measuring wind speed at the kite.

It should be simple, robust, reliable, cheap and generally available.  Any ideas?

Peter Lynn, Ashburton, New Zealand, May 30 '10.