Don’t fear the rotor!

Spinning to the ground. It’s happened 3 or 4 times over the last 2 years of KAP flying.

Suddenly the line feels slack and the kite descends in a series of accelerating spins to the ground (or more usually in my case, roof tops, overhead wires, poles, trees and alarmed bystanders). The natural urge is to pay out more line in the hope the kite will steady and lift but all that happens is the kite keeps falling! Dark thoughts of a lost kite, line and precious camera: hard won kit dashed into very expensive peices of broken glass and plastic fill the mind! I will never forget the first time this happened to me, it was a very public failure and I needed the patience of all around me to get my kite back from an overgrown apple tree surrounded by madly barking dogs. Fortunately the camera landed safely but, from that point on, the whole KAP enterprise looked like an amateur shambles to all. So what happened?

Hazards of the boundry layer. This wiki commons pic shows the wingtip vortex of a plane at take off made visible with coloured smoke.  It’s a small plane moving at modest speed but the effect on the airflow is huge. This kind of airflow is useless for kite flying but it has very clear boundries and, if the kite was able to fly into this mangled airflow how could it get out of it?

It’s easy to think of wind as a smooth thing that slips easily around and through obstructions like water in a lazy stream, particularly if the only visible evidence is the gentle flexing of thin branches or the rustle of leaves. The science of fluid dynamics tells us otherwise, the friction of a surface over which air flows is variable, causes turbulence at the micro as well as the macro scale and effects ever bigger volumes of space according to wind speed. 

Sometimes ( if you are looking at the earth from the space shuttle!) it’s possible to see rotors but usully they are a surprise. Fly a kite at the beach in an on-shore wind and the chances are you are flying in a laminar flow; this is a lovely place to be, as you let the kite climb you become aware of a proportional increase in the wind speed with height

It’s a simple diagram (there are many, this one is from MIT OpenCourseWare) but it reveals the near chaotic effect of an interruption to the laminar flow. And that’s a 2D view. These effects are amplified by gust too so the zone of turbulence can grow as wind-speed does; they have an energy of their own so that when the gust has faded the rotors continue to spin for some time.

When the kite is in a part of the sky with a violent downdraught the smooth airflow which carried it upward is but a memory!

Recovery. The recovery procedure is straight forward once you have fixed in your minds eye the way the air flow is working. The key thing is to get the kite OUT of the downdraught as quickly as possible and this is done by pulling the spinning mess UPWIND as far and as fast as you can. This is counter intuitive because by puling on the line the spin speeds up. So the object is to drag the thing away from the downdraught back into the clean airflow. Once the kite is yanked clear it will continue its descending spin and more line needs to be let out to reduce the tension on the line and the kite’s speed of spin. Leeting out too much line is to be avoided at this point as this allows the kite to re-enter the rotor so there’s a bit of judgment needed to regain control.

It has taken a bit of time to figure this out. The first time this happened I paid out as much line as quickly as I could in the hope the kite would gain height and stabilise…wrong! …  all that happens is the kite continues the spiral descent, pulling all your line along with it: no amount of line will allow the kite to penetrate beyond the descending airflow once it is trapped in it!

How to spot a rotor. This is difficult, there are times when I have flown clean over big slabby buildings in a smooth flow and there have been sudden surprises out of the blue. The bigger the wind the bigger the rotor is one thing to remember and these can pop up with gusts and then fade and hide when the peak flow passes.  I have let the kite ride over small ones by lettng out line in the manner of a ‘dirty’ launch. The only way I know a rotor is there is when the kite looses lift and starts its sickening spin. I think there are 2 general classes; one thermal and the other topographic. In the thermal case it all seems obvious when you think that for any column of rising air there is a body of descending cooling air that surrounds it, glider pilots know this and, as free wings they are able to penetrate the descender and gain lift from the riser, you can watch birds do this too. For me the first indication is the movement of the kite, it just gets knocked over. For a single-line kite penetration of a vortex flow is very difficult (Horvath  ‘zero wind’ delta fliers tell me other wise…I’ll find out more when I can get hold of one!)  as down wind travel is irrevocably linked to lift.

How far down wind from a suspect rotor generator is safe? I have been advised that 3x the height and 3x the distance is an aproximate guide to the size of the disturbed airflow but this will of course be magnifed by bigger wind speeds.

Is a rotor always survivable? Sadly no. Take this recent example, cool air sinking down a hillside with a gusty tug running across the valley tops: From watching the kite I think the airflow looked a bit like this:

The airflow across the top of a saucer might be a good analogy of the situation, the kite would rise in the general direction of the main flow and then hit slack air before being sucked into a downward spin. It was a very cold (the warmest it got was 0oC) day; the main airflow was across the valley top and the decending cold airflow down the hillside mixed into a sluggish spiralling mess at 10m AGL at the valley floor. No amount of action could place the kite in a stable air flow. The result was a wet muddy kite and a burst cell web: only the KAP procedure of never comitting the camera until the flier is confident in the lift spared a grater disaster on the day.

So low air temperature as well as topography and warm air thermal effects are part of the mystery of the rotor.

KAP is as much about knowing when not to fly as when to!

About billboyheritagesurvey

Heritage worker
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3 Responses to Don’t fear the rotor!

  1. Nathan Craig says:

    Thank you this post contains a very useful discussion of wind as it relates to KAP.

  2. jokerxl says:

    You’re scaring me now.

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