Dual line parafoil kites come in a range of sizes and most are built for pure stunt flying fun. Others are built for speed, the record being over 150kph! A soft dual line stunt kite pulls harder on the lines than an equivalent sized Delta, and requires more line movement to maneuver.
If you've ever been up close to a parafoil, it's easy to see how it's made. An upper surface, a lower surface, and a whole lot of dividers in between. Typically, there are between 10 and 20 such dividers.
Thus the kite is made up of many cells, each one open at the front edge of the kite. During flight, air rushes in and pumps up the kite to form an efficient, hard-pulling wing.
Some of the cheapest kites use plastic, but any decent dual line parafoil is constructed of rip-stop nylon or polyester. The stuff sailing spinnakers are made from! Very strong for its weight, and tear-resistant.
'Rip-stop.' A small rip will stop before going very far!
The exact designs vary somewhat from kite to kite, but in general... The left upper under-side of the kite is attached to a number of quite short bridle lines. These lines all come together to a thicker single line, which in turn connects to the left flying line.
There is an identical mirror-image arrangement of bridle lines on the right hand side of the kite. Thus, the 2 flying lines restrain the kite. Pulling the left line causes the kite to loop left. Similarly, pulling the right line causes the kite to loop right. The more you pull, the tighter the turn, up to the limit of the kite's performance.
Anything that's strong enough will work, but the most popular line materials are Dacron (a brand of polyester) and Dyneema (a special type of polyethylene). Nylon is usable too, but it feels like you are flying with rubber bands! Dacron is better, and is popular for single-line kites too, where stretch doesn't matter at all.
Dyneema, which happens to be very low-stretch, is the ultimate line material for multi-line kites. It is extremely strong for its weight too. Hence, a set of Dyneema lines can be quite thin and still do the job. Thin lines have less air resistance and let the kite move faster through the air. Do you have the need for speed?
One little known fact... Apparently, Dyneema has a relatively low melting point. This means friction from lines of other materials can easily slice it through despite its high tensile strength at lower temperatures. Gulp. Don't tangle with other kites...
Moving on to handles. The pricier kites often come with a bar, as opposed to 2 separate handles. This arrangement offers more comfortable and precise flying, particularly with the larger kites. If the bar is anchored by your body, that takes a lot of stress off your arms while you fly. Wind surfers do this all the time.
Yes, if you fly a dual line parafoil kite, you are entitled to call yourself a pilot! Well, a kite-pilot to be more precise.
Age. Adults are stronger than young teens, who in turn are stronger than even younger children. A person's strength and weight determines how much kite pull they can safely handle. Although other design factors can influence how hard a kite pulls in any given wind speed, the main one is simply size. More sail area equals a stronger pull on those flying lines. Hence, it pays to take notice of the age ratings on dual line power kites. Also, be careful about taking any big kite out in strong wind conditions!
Skill/experience. Amongst a group of pilots the same age, those with superior kite-handling skills will be able to safely fly somewhat bigger kites. They know how to position the kite to reduce the pull when necessary. Not to mention when to give up and let go, or use the de-power feature! But then, their superior knowledge would probably lead them to not even attempt to fly in such conditions...
Have fun! Picture yourself on the sand, in a stiff smooth breeze... (This kite looks like it has a couple of brake lines as well, but it's essentially being flown as a dual line kite)