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Author Topic: 'Dynamic' Leech Line... why?  (Read 564 times)
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OCPablo
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« on: June 16, 2017, 07:39 PM »

 I am reaching out to those smarter that I on this one.  Purpose of a leech line on a sail is to maintain the trailing edge shape.  On a two line kite, these were really effective in creating kites that had slightly better wind range (from more efficient sail shapes) and silenced them in flight.  While some who loved the roar of a good old TOTL, the inclusion of the static leech line on dual liners was definitely impactful not too long ago.

Now every top end dual liner has a 'dynamic' leech line, typically via an elastic connection off the spine.  What is the reason for the dynamic aspect?

I just framed up 3 different sails and put dynamic leech lines on every one.  Here are 3 things I have found.

1. It seems to be quite a challenge to keep each side of the sail tensioned equally (noted by the tone of the noise the sail makes when the kite is in a tight spin in either direction).
2. The type of standoff connectors, and more importantly the position of them can nullify the ability of the leech line to actually move 'dynamically' once the connector is tightened (especially if you like the screw-in style like I do.)
3. Having switched one of the kites (P3) to a 'static' leech line not only enhanced the responsiveness slightly on the low end of the wind range, but also makes the whole kite feel more taught, and definitely made the kite more responsive to pumping actions in lighter winds to gain altitude.

Now... I could be way off here, but would love to hear others thoughts on the topic.
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inewham
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« Reply #1 on: June 17, 2017, 03:09 AM »

Leech line were good when we were trying to make kites more responsive in the 90s

More responsive can mean faster in high winds so when French kite went big in the noughties noisy trailing edges were back in fashion becuse a flapping trailing edge helped regulate the speed.

The next fashion was logical - adjustable or dynamic to try to get a bit of both depending on the wind conditions

There have been many attempts to make dynamic keels, sails, leech lines, bridles etc. In most cases a certain amount of your input is taken up as the kite adjusts itself which makes them feel mushy. With the exception of turbo and active bridles I'm not really a fan, if it gets more windo I refer to swap kites but its a *long* time since I flew in competition where consistent speed is much more important
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Doug S
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« Reply #2 on: June 22, 2017, 07:49 AM »

This is a topic near and dear to me.  In 1991, I introduced to the kiting community the use of a leach line for dual line stunt kites, on my 8-foot wing span Falcon and Falcon-LT.  It was first flown in competition at the 1991 Wildwood International Kite Festival over the Memorial Day weekend.  In developing my stunt kites in 1990, the sail flutter along trailing edge of the kite reduced performance and the wind window.  Base on my history with building, flying and competing with model aircraft, this type of flutter is not good, which can shake airframes apart.  Since I also like to sail sailboats, I adapted the leach line used on some sailboat sails for the trailing edge of our stunt kites.  I used a leach line on my dual line stunt kite prototypes before my Falcon, which prototypes had lower and higher aspect ratio sails.  My leach line floated in the trailing edge hem as a continuous piece of Dacron line, where it was connected to the frame at three locations, at each wing tip and the center spine at the base.  To allow the leach line to tension the trailing edge of the sail, there needs to be a slight curve in the trailing edge, from the standoff to the wing tip.  A gentle curve also was added to the trailing edge, from the base of the center spine to each standoff.

The leach line provides both positive and negative consequences to the stunt kite design.  As with that old adage in Aerodynamics, where everything is a tradeoff.  The positive aspects were a larger wind window and a more efficient use of the sail under light wind conditions.  I could fly when others could not.  At that time, we did not have the light weight wrapped carbon frames.  I was using AFC pultruded carbon tubes at that time.  Also, the stunt kite was easier and smoother to control.  At that time, we were using only two standoffs with less depth to the sail.  With the trailing edge now tensioned by the leach line, the sail could not washout the same amount from the standoff to the wing tip, as what would happen without the leach line.  This caused tip stall problems when we flew slowly in a turn or tried to stall.  Landings were entertaining.  One had to fly the stunt kite through each tight turn and pull both lines back slightly at the end of each turn to prevent tip stall under light wind conditions.  Also, we lost the speed control in higher winds that the trailing edge flutter was providing on kites that have no leach line.  To cure the speed control issue, I used different sets of flying line, varying in weight and length, which would provide the necessary drag to slow the kite down when needed.  As the wind speed increased, the sail could not dump the excessive air from the sail, because the leach line would cause the sail to have more curve (camber).  With more camber, there is more lift as the speed of the stunt kite increased.  More lift caused the stunt kite to pull like a truck.  To resolve this issue, I added a short loop of 1/16-inch diameter bungee to the leach line at each wing tip.  I would larks head the bungee around the knot of the leach line loop.  When in use, the loop of the leach line was disconnected from each wing tip, and the bungee loop was then connected to each wing tip.  This allowed the sail to give under higher wind conditions.  I only used the bungee loops on my higher wind dual line stunt kites.

To resolve the tip stall issue, I added an extra set of standoffs to my dual line stunt kites I designed and built after the Falcon, at the location where the lower spreaders were attached to the wing spars.  This was done on my 1990s 8-foot wing span Talon and Raptor dual line stunt kites.  These outer standoffs pushed the sail back and provided washout to the sail, from the inner standoffs to the wing tips.  For my higher wind dual line stunt kites, I had several sets of outer standoffs that increased in length by increments of 1/8-inches.  This allowed me to increase the washout in the sail for high winds, which provided additional speed control and stability.

In response to your question, the dynamic leach line allows the trailing edge of the sail to move under higher wind conditions, which will reduce some of the line tension the leach line causes, due to the induced curve in the sail by the wind and leach line, and the associated lift and line pull described above.  The dynamic leach line is not necessary for dual line stunt kites that are designed to be flown under light wind conditions, where you want the camber in the sail to provide lift and line tension.

The following shows a picture for two of my 1996 Raptors in the pit area at the Sky Art Kite Festival, Liberty State Park, Jersey City, NJ on May 27, 2017.  The leach line allows the sail not to fatigue and wear due to flutter along the trailing edge of the sail.  Not bad for 21 year old dual line stunt kites!  At that time, the slack line tricks were just the axel, so my standoffs had pockets that extended behind the trailing edge, keeping the trailing edge of the sail as flat as possible, and allowing the leach line to float.  In an updated version I built in 2011 with a 7 foot wing span I called my Bird of Prey, I now have a clean trailing edge, clean leading edge using current frame sets.  My leach line still floats.  I built a few Bird of Prey dual line stunt kites for me, my two boys and my kite friend.

Regarding standoff locations, the slack line tricks have transformed dual line stunt kites to have more depth to the sail and placement of the standoffs to optimize the various slack line maneuvers.  Since I am an old precision guy, I don't have anything I can offer on this aspect.

Yours in kiting,

Doug


1996 Raptor-MF and Raptor-F Dual Line 8-Foot Stunt Kites in the Pit Area


2011 Bird of Prey Dual Line 7-Foot Stunt Kite


2011 Bird of Prey Dual Line 7-Foot Stunt Kite - View from the Back of the Sail


2011 Bird of Prey Dual Line 7-Foot Stunt Kite - View from the Rear of the Sail


2011 Three Bird of Prey Dual Line 7-Foot Stunt Kites (For me and my two Boys)

« Last Edit: June 22, 2017, 12:08 PM by Doug S » Logged

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OCPablo
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« Reply #3 on: July 09, 2017, 01:44 PM »

Doug thank for your feedback, very interesting read.

the more I have looked into this topic, and had a chance to talk to a couple of kite designers, it seems that the real factor was to a) provide a more distinctive sail shape in lower winds and b) allow for trailing edge flapping for speed control in higher winds.

As I am not competing, the speed control aspect can be easily managed by longer line sets, screens on the kite or even better... just grab a different kite that is vented.  I don't have 2 or 3 kites I must fly, I am lucky enough to have multiple kites for the same wind range I can fly as I see fit.

For my personal preference, I have taken two kites with 'dynamic' leech lines and made them static.  The first one was my P3.  My biggest challenge with this kite was consistent snap stalls anywhere in the window.  As the sail has a bit of bellow up towards the nose area, I have seen that a more tensioned sail shape allows a faster/shorter input to stall and then the kite 'hangs' better.

As I typically fly in the lower wind range (10+ mph is not as common, 4-8 mph is the local 'sweet spot') having a static trailing edge provides more drive and more control once I stall the kite.  Obviously this is ideal for my flying style, and the experimenting will continue.
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