Cobra Kites
Power Foil Tuning Tips

 
POWER FOIL TUNING TIPS


Flexifoil Tuning Tips
Flexifoil Tuning Tips 
Power Kites with a Leading Edge Spar are very unique and dynamic dual-line stunt kites. Though initially, they may seem too simple to need any tuning, optimum performance can be achieved with these tuning tips.

Bouncing
If your Flexifoil begins to bounce after take off or flap like a bird, it's an indication that sand or water may be inside. The heavy sand or water works its way inside to the trailing edge and upsets the automatic angle of attack feature of the Flexifoil. It tries to fly with the wrong angle of attack for the wind speed and in an effort to correct this, bounces, often violently. To correct the problem, land the kite and empty the sand or water. If water was to blame, drying the sail will help considerably before re-launching.
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Damage
From time to time your Flexifoil may sustain some damage. This can range from small insignificant punctures to major rips or spar damage. The smaller holes can be patched with self-adhesive Rip-Stop Nylon repair tape or a clear adhesive membrane tape available from Flexifoil. Larger areas of damage will require a more professional touch and we recommend returning the damaged sail to Cobra Kites for damage assessment and a sewn repair. One of the commonest problems is a blown out cell caused by flying the kite at high speed directly into the ground. The effect is of this is very much like bursting a paper bag. The air, pressurized within the kite, has nowhere to go and bursts through the sail, usually along a seam. Damage to the spar pocket and mesh vent can be repaired, though repairing these areas requires considerable disassembly of the kite and can be costly.
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Flapping
If the Trailing Edge wing tips flap during flight, it is a sign that the sail is not correctly inflated. The most common cause for this is over-stretching. The sail is stretched outward on the spar too tightly and when the Flexifoil curves the Leading Edge vent is closed and internal pressure is lost. The cure for this problem is to move the tether points of the flying line INWARD toward the center of the kite. Move each side in a half an inch or so and test fly. If the problem persists, move the tether points inward until the flapping stops. Do not allow your Flexifoil to flap for long periods as this will result in the loss of the air-proof coating. This cannot be repaired. Another reason for the sail flapping in flight might be a rip sustained in a previous hard landing.
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Flying Line
The choice of flying line will have an effect on optimizing you Flexifoils performance. For best results choose a flying line that is thin whilst having high strength and as little stretch as possible. The thinner the flying line the less Drag, giving higher speeds and better window-edge performance. Shortening flying line length with also appear to create a quicker kite as the distance it actually travels is reduced. We recommend 100-120 feet for average flying. Currently, flying line made from Spectra® type fibre is the most suitable for stunt kite flying. Remember to follow the line manufacturers recommended methods for making knots and loops etc. in the line. We suggest that extra Dacron® loops be added to the kite-end of your lines to protect your main flying lines from excessive wear. These need only be 6 inches long and can easily be replaced when they wear out.
Average wind conditions
6ft Stacker - 100-150lbs
8ft Pro-Team - 150lbs
10ft Super 10 - 150-200lbs
In low winds most of the Flexifoil Power Kite range can be flown on 80lbs line
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Landing
Wind Window For a safe landing, steer the kite to the edge of the wind window. It will slow down, with power diminishing and sink gently to the ground. Do not fly the kite directly into the ground under full power. Internal pressure upon impact may rupture the skin. Remember that Flexifoils usually land upside down.
 
 
 
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Lagging Back in Stacks
Occasionally, one Flexifoil in a stack will lag back and upset the performance of other kites. This problem can be cured by moving the tether points inwards on the wayward kite to enable it to catch up with the rest of the stack. Flexifoil sails and/or spars can also be swapped around to try to cure an undesirable flying problem. If a kite lags back in a stack it will reduce both speed and pull by 30% or more.
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Launching
When launching a single Flexifoil, if it does not inflate easily and form a curved shape at take off it will either be overstretched on the spar or there may not be sufficient wind. The standard spar supplied with the Stacker, Pro-Team and Super 10 will operate in winds above 8 mph. (A little lower for the larger wingspans). If the winds are sufficient and the kite does not want to fly, the possible over-stretching can be resolved by moving the tether points INWARDS on the spar to allow the sail to relax. This will make for easier inflation of the kite. The Black rubber O rings are not glued in place and can be slid along the spar as a guide. Remember that Flexifoil Power Kites will only fly when they are the right way up! Mesh vent above the spar pocket.

Launching diagram 1 Launching diagram 2

When launching with an assistant, simply direct them to hold the kite by one spar tip or by the spar in the center of the kite.They should grip it lightly and allow the kite to inflate and virtually fly out of their hand.

It is not necessary to toss the kite into the air. Do not jerk on the lines. A gentle pull is all that is needed. Wait until thekite is fully inflated and it will want to fly!

Flipping diagram With some practice, solo launching will become easy. With the kite inflated and on its back (Mesh vent below the spar pocket) a sharp pull on the upper line will spin the kite around and flip the trailing edge back presenting the kite right side up to the wind ready for take off. Do not drag the kite along the ground if it does not take off at once. The fabric sail could be damaged on rocks or other debris.

You will find that brand new Flexifoils will benefit from some brisk wind flying to allow the sail to stretch into shape. This is especially true with custom designs with multiple panels.
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Low Wind
The wind range of Flexifoil Power Kites can be expanded by the use of Ultraflex Pro-Series spars. These spars are available for the Stacker, ProTeam and Super 10 and are for use in winds below 10 mph. Ultraflex spars have been designed with a modified flexure and a 50% weight reduction over standard spars. The combination of glass fiber and carbon fiber in these spars allow the sophisticated aerodynamics of the Flexifoil to operate in light breezes. The major reason for lack of performance in low wind with the standard spar is not primarily weight but more a question of ease of spar flexure. The quicker the spar can form into the characteristic curve of the Flexifoil, the sooner it will fly. Hence a more flexible spar will fly in lighter wind. With the more flexible Ultraflex spars there is a danger of over-stressing and over-stretching. If flown in too much wind the forces will be high enough to break an Ultraflex spar. Before this point is reached however, over-stretching of the sail may occur and lead to undesirable flapping of the sail.
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ProSpeed 6+
This Icarex Flexifoil has an enclosed spar pocket at the wing tips to reduce line tangles at the wing tips. To make connecting flying lines easier, use small loops of Dacron attached to the spar and then attach the flying lines to these. The ProSpeed is primarily a low wind performer though they can also be flown in stacks. Due to the enclosed spar pocket, very little tuning can be done on the ProSpeed other than changing spars.
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Spars
We recommend that the Flexifoil spars be taped together at the spar joints before the spar is inserted into the spar pocket. This will insure that the spar joint does not work loose during flight and puncture the spar pocket. A short piece of vinyl elecrical tape works well for this, and it only needs to be enough to make 1-2 wraps around the spar joint. These short pieces of tape can be stored on the spar between flight sessions and re-used several times.
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Speed
Usually, it is high speed which is sought after. It goes without saying that the greater the wind speed the faster the kitewill go. For optimum high wind flying the standard spar should be used. This gives the best curve for heavier winds. Flying line will also determine speed performance and b, thin line will give the best results. Remember also that the longer the lines the greater the drag so if you're looking to zip around the sky, 100 foot lines will work well. To reduce the speed and consequently the power of Flexifoils, they can be de-tuned. By moving the tether points essentially too far inwards on the spar, the most efficient shape for the kite is lost and it slows down. Similarly, spars which are too flexible for the prevailing wind will lead to a slower kite. However maneuverability may be enhanced. See also Tails.
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Stacking
Flexifoil Power Kites are very easy to stack together. The following 4 methods give a range of linking systems from the simplest to a more sophisticated inserted loop method.

As a rough guide it is useful to estimate potential pull by using the Stacker as a standard unit. The average pull of a Stacker is approximately 50lbs. A ProTeam is roughly equal to 2 Stackers and a Super 10, 3 Stackers. Therefore it is relatively easy to get a rough idea of how much pull is likely from a given stack. Of course, pull can go up drastically with wind conditions and kite speed.

Separation for same size Flexifoils
3 stack of 
	Flexifoils
Hot Shot 4
30 inches
Stacker 6
54 inches
ProTeam
62 inches
Super 10
72 inches
Flying Line strength - wind 10-12 mph
Hot Shot 4
up to 6 units
150lbs
Stacker 6
up to 6 units
200-250lbs
ProTeam 8
up to 3 units
200-250lbs
Super 10
up to 3 units
250-300lbs

When stacking different size Flexifoils, more distance should be allowed between each kite. 72 inches is a good starting point, especially when stacking 6ft and 10ft Flexifoils together. Smaller kites are usually put in front of, or below, the larger sizes. The Hot Shot 4 is not suitable for stacking with other sizes due to its small Wingspan.

    1) In Line

The main flying line is used to link each kite together by forming loose loops at each connection point. The disadvantage with this system is that over time, the line will wear out at the attachment points and result in loss of flying line. To avoid wasting the main flying line a sacrificial Dacron leader can be used which can be replaced when worn out. Always use Dacron for this leader.

In Line stacking diag.

    2) Train-Line

This method of stacking uses pre-measured pairs of lines with loops at each end. These are used successively to link each Flexifoil together. The main flying line being connected to the bottom kite in the stack. Make sure that the INNER line goes UP to the next kite and the OUTER line goes DOWN. This will ensure an upward flex in the spars when under load.

Train line stacking diag.

When stacking with the In-Line and Train-Line systems, the pull is transferred from kite to kite and eventually down the main flying line. This can exert excessive forces on the spar tips of the lower kites. The two Loop methods of Stacking relieve this problem and are more suitable for Hot Shot 4's and Flexifoils with Ultraflex spars. The Inserted Loop system, though initially time consuming, is probably the best stacking technique.

    3) Loop

The Loop stacking system consists of two Dacron lines with loops tied at the correct spacing down their length. The loops are approximately 2 inches long and connect the kites individually. Do not make these loops too long as excessive jostling may occur at take off as each kite tries to set its angle of attack. These pre-looped Dacron lines can be connected to the main flying line and can accommodate as many Flexifoils as needed.

Loop stacking diag.

    4) Inserted Loop

This technique is very similar to the Loop method though the loops are formed by inserting them into the train line section. Short lengths of Dacron (200lbs) are formed into 5 inch loops and connected to the train line at pre-measured intervals. These loops are then prevented from slipping up or down by another short length of Dacron. This allows the main or train line to be Kevlar or Spectra as there will be no wear except to the Dacron loops. These can be replaced if needed.

Attach each loop around the main line at pre-measured points by passing it through itself 3 times. Pull tight. Thread the anti-slip pieces of Dacron approximately 2 inches on either side of the loop and knot the exposed ends.

Inserted loop diag. 1

Inserted loop diag. 2

The Inserted Loop system is used for flying very large stacks of Flexifoils (Over 150) to preserve the maximum line strength.

To launch a stack of Flexifoils is no more complicated than a single kite. Remember that to fly, the kites must be fully inflated and right side up. With a number of kites in a stack it is wise to use an assisted launch. The helper should lift the top kite, the one farthest away and allow it to inflate. As it begins to rise it will lift the others into the air and the stack will soon take off. Be prepared for the pull that stacked Flexifoils can generate. Do not attempt to fly stacks of Flexifoils until you have become proficient with a single kite.

USE EXTREME CAUTION AND COMMON SENSE WHEN FLYING STACKS OF FLEXIFOIL POWERKITES. YOU ARE RESPONSIBLE FOR THE SAFE OPERATION OF YOUR KITES.

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Tails
Tails are a great addition for a visual spectacle. The best place to attach tails to Flexifoil Power Kites at the Leading Edge wing tips. If tails are fixed to the trailing edge they will upset the aerodynamics of the kite and performance will become sluggish.

Towing tail diag. Form a "V" in a 10 foot length of 150lbs Dacron and attach a tail to the center. Form a loop in each end and secure these to the spar tips. Should you want to add a tail to a stack of Flexifoils, it is best attached to the bottom kite where the maximum lift is located.
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Tuning, Technical
Flexifoil Power Kites are different from conventional frame and sail kites which require a bridle. With those kites, the bridle is used to set an angle of attack for the prevailing wind conditions. Because of its unique aerodynamics, the Flexifoil does not need a fixed angle of attack and therefore has no bridle. The Flexifoil actually sets its own angle of attack automatically for any given wind speed anywhere in the sky. Whether at full power or gently hovering at the edge of the wind window.

Flexifoil line attachment diag.To ensure that this unique principal operates properly, remember one simple rule. Follow the instructions precisely in attaching your flying lines. Simply pass the loop at the end of your flying line through the FRONT of the ring and connect it to the spar BETWEEN the Black rubber 'O' ring and End Cap. Do not restrict the sails' free movement on the spar. Never tie or bind the line to the spar. If the sail is stretched out too tightly by attaching the flying lines at the very tips of the spar, (Over-stretching) or the line is tied to the sail preventing the vent from opening properly, it will cause under-inflation. More importantly, when overstretched the kite will not take on the characteristic curved shape of the Flexifoil which is the key to optimum performance.

The elements of sail stretch on the spar and spar curve, form the basis for tuning Flexifoil Power Kites. As the tether points are moved in or out the shape of the inflated Flexifoil is changed slightly. This affects the aerodynamics of the kite in such a way as to alter air speed and internal pressure of the sail.
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Waving or Waggling in Stacks
This phenomenon occurs sometimes when flying stacks of Flexifoils. A side to side motion builds up along the length of the stack, usually at a particular air speed. It can often become violent if the kites are not brought out of the wind and slowed down. Waving is usually caused by kites within the stack trying to correct themselves for straight flight causing the other kites to become out of balance. This leads to a chain reaction of the kites at the top and bottom of the stack tending to fly in opposite directions. Cures for this range from switching kites and/or spars around within the stack tochanging the distance between kites. The more flexible Ultraflex spars can be the cause of snaking on some occasions.
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We welcome your questions, comments and suggestions.
You can e-mail us at:info@cobrakites.com
telephone; 732 270-2112
Last updated Wednesday March 3 2010