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To me kites are magical. They are the complete antithesis of modern, scientific, "aerodynamic" things. They are utterly simple constructions with the simplest of frames, the least sophisticated of coverings, and the purest, most elemental geometries. I have always found that incongruous simplicity at odds with such fantastic ability. A kite just needs to be light enough for a light breeze to blow it away. It also needs a certain balance, and symmetry of form and frame. Then, when it's tethered just right, this humble structure can perform beyond the wildest expecations imaginable.
With their simplicity of geometry, their purity of form, kites are almost two dimensional objects in a three dimensional universe. Therein for me lies the fascination.
Yes, scientists have modeled them, analyzed them mathematically, and spent large sums of money designing and building an "advanced," structurally complex, apparently aerodynamically sophisticated kite. Which unfortunately failed to fly... not even for 30 seconds... and then only in a 35mph+ wind.
Deltas owe their popularity to their ability to fly to unusually high angles in lighter winds than most ordinary kites.
They roll up for carrying, and they are easy to assemble and fly. Since they are not joined at the nose, the wing spars can move independently with a self steering action that allows the kite to ride air currents with a charming, lifelike quality. The wings, being flexible, adjust like a gliding gull's wings as the wind fluctuates. Compared with regular kites flying them is a more interactive experience. While most ordinary kites are only capable of a two dimensional up-and-down flight path - a flat plane - deltas by virtue of their controlability add the third dimension. With skill they can be guided long distances from side to side in search of lift, with the potential to utilize a wide, three dimensional volume of sky.
When I was a kid everybody knew that a regular kite (the Hi-Flyer - a form of Eddy bow or Malay) was for normal breezes and a box kite was for strong breezes. Nevertheless when the wind dropped the kite dropped, no matter what. Expert kite builders made highly refined versions of traditional designs which no doubt would perform very well in light winds, but there wasn't anything that could soar, float or otherwise hang up there in lulls - until the light wind delta. With the advent of these deltas, thermaling became an everyday experience for flyers otherwise grounded by light airs.
The design is versatile. Deltas with different shapes and varying degrees of pull have been developed for flying in a wide range of different winds. The challenge and enjoyment of light wind flying is that lift has to be actively hunted, which is more engrossing and involving than flying in blowy weather where all there is to do is hang on to a hard pulling kite that just sits there. (Of course, such situations can have their uses - for instance, for hanging things like cameras or weird flashing lights on the line.)
Light wind deltas are made from lightweight non-porous spinnaker fabric, with light wind towing points, to maximize the force applied to the kite by light winds. The wing leading edge spars are designed to flex within this range of relatively light loads, which is why light wind deltas should never be launched in anything approaching windy.
This flexibility is at once a great strength and an inherent weakness. The flexing allows the wings to react to changes in the wind, but this bendy structure is clearly vulnerable to excessive force. Deltas will become unstable and/or break if flown in the wrong wind. The best wind is just enough to lift the kite, and no more. Common sense tells us to use moderate, fresh or strong breeze kites, respectively, for moderate, fresh or strong breezes. (With the appropriate flying line - light line is essential for light winds, while heavy line effectively dampens the motions of the kite-plus-line system on a blustery day.) The force of the wind increases with the square of the wind velocity; a lightly built, light wind delta with a light wind towing point cannot reasonably be expected to hold its own in too much wind without accelerating its demise.
A delta wing aircraft is an airplane with a triangular wing, so...
the precise position is determined mathematically
...using formulae derived from a principle jointly originated by Harold Alexander and John Loy
which establishes how effectively a kite catches the wind - or how much it will pull
...it is set forward for less pull on kites for fresh or strong breezes
In the same way they're incorrectly associated in peoples' minds with Rogallo wings, someone somewhere usurped the name "delta" to describe rigid, A-framed stunter kites. (I think it was John Spendlove, in his "Taxonomy of Kites.") A true delta wing stunter would be basically one isosceles triangle - just like a delta wing airplane. There have been true deltas bridled to fly as stunters (a long time ago, in the 1970s), but most modern stunters have tapered swept wings - nothing like the Greek letter, or a real delta kite, or delta-wing aircraft. But the name has stuck, so I guess I'll just have to learn to live with it.
The term "aerodynamics" is popularly confused with fluid mechanics, which is the analysis of fluid flows. The layman is prone to think of aerodynamics as the way air flows around things. Actually that would be fluid mechanics - aerodynamics is the branch of mechanics that describes mathematically an object's motion in or through a gas, and also what happens when said objects are disturbed from a state of static equilibrium (where the wind force, the line tension, and gravity all balance). "Statics" and "dynamics" are two branches of mechanics. Statics is the branch of mechanics dealing with things at rest (in static equilibrium), for example, the familiar diagrams in kite books showing lift and drag arrows. Dynamics deals with the mathematical description of things in motion. Dynamic stability is when something that's been disturbed can, on its own, return to static equilibrium. Instability is the opposite, and, for kites, this can be very complex. It is possible for a kite to be dynamically stable in a half a dozen different situations, yet unstable in another. There are several kinds of dives kites can do, for instance.
So, aerodynamics is the study of the motion of objects in a fluid, while fluid mechanics looks in detail at fluids in motion (not necessarily around objects). Aerodynamics describes mathematically the motions of objects when upset from equilibrium - how they move, how or if they will recover or not, and so forth. The point is that it has nothing to do with actual air flow per se - it's just a set of equations and terms describing motions (with air being one particular fluid with a certain density and viscosity). This distinction is important for avoiding confusion - if one thing is different from another aerodynamically, it means that the terms and values used in the equations of motion are different, not necessarily that there are any superficial physical differences. Kites of very different physical appearance might be very similar aerodynamically. Think of the kite as black box.
Aerodynamics shows us that kites can dive in a half dozen or so different ways, with as many types of recovery. A kite could be statically stable yet dynamically unstable. Some kites recover from upsets quicker than others, meaning they're more dynamically stable. Playing with bridle attachments and adjustments can show a kite's range of static stability plus the range of dynamic stability - the extremes beyond which the kite becomes dynamically unstable by twirling or diving rather than returning to equilibrium.
Static stability is when the sum of all moments about the center of gravity equals zero. You can let the kite fly on its own; it remains motionless. Dynamic stability is this: when the kite is knocked out of the steady state position, for instance by a gust, it returns to statically stable equilibrium. That's the tricky one!
It is sufficient to know that lift results from having a plane (or wing) set at an angle to the breeze.
Contrary to popular belief, the usual airfoil explanation referring to Daniel Bernoulli's theorem does not apply. This theorem is actually about pressures of liquids flowing through tubes (the sum of pressure energy, potential energy, and kinetic energy being constant). It is merely assumed to apply to fluid pressures between streamlines in laminar flow, and by extension to airflow around airfoils of infinite width. The usual story is that the air has to flow further over the upper surface of the airfoil section and so is going a bit faster, resulting in a lower pressure on the upper surface "according to Bernoulli." While this may sound plausible, unfortunately it doesn't necessarily apply in reality. To begin with, the airfoil has to be of infinite length for the picture to be accurate, and that doesn't sound like any kite in the real world. Implicit in this picture is the assumption that all airfoils have an upward-curving camber. But they don't. Airfoils on specialist aerobatic airplanes are usually symmetrical - zero camber - so they can fly upside down. Then there are the true flying wing aircraft whose airfoils are cambered downwards. The reason is pitch stability.
The upwardly curving wing cross sections on conventional aircraft create a tendency to tip the aircraft's nose downwards all the time; it's called the pitching moment. This pitching moment has to be counteracted by a horizontal tail plane pushing down at the tail. To counter the pitching moment on canard configurations the horizontal control surfaces at the front push upwards. Thus the horizontal planes contribute some lift, too, rather than subtracting it as they do on conventional aircraft. Canards are not only more efficient, they are also safer, since they can be designed to be stall-proof.
Flying wings have no nose or tail and no extra horizontal control surfaces, so (in their simplest form) they avoid the destabilizing effect of a nose-down pitching moment by using a downward curving wing section - an airfoil with inverse camber. The wings are upside down. The pitching moment is then automatically nose-up with pitch stability adjusted by careful weight positioning. The point is that they fly, even though by popular definition their wings should be pushing them down instead of up!
Parafoil kites have to counter the pitching moment with a mass of bridle lines which can be a nightmare to adjust; I wonder if it would be easier if the camber were reversed. If they're not flat, or sleds, most kites have the opposite camber (curving downwards in the middle, upwards at the ends) and for good reason. Just imagine trying to get an Eddy to fly upside down. I'm probably not the only kite flyer to come across dads in parks trying to do just that, showing how deeply rooted the misuse of Bernoulli's Theorem is in the public consciousness.
When I was in school the mathematical concept of "circulation" was used to account for lift: with an airfoil at an angle to oncoming air, minuscule particles in the mathematical model of the airflow have to somehow curl back around the trailing edge for the upper and lower flows to meet. Circulation attempts to explain mathematically how the flow over the top surface meets the flow over the bottom surface in the right place as it leaves the wing. (None of this is of any use to kite makers, of course.)
The most fundamentally accurate airfoil model, as far as kites are concerned, is the flat plate airfoil. A flat plate airfoil (of infinite width, of course) has the interesting property of providing lift over a very wide range of angles-of-attack. Now this is sounding more like a kite, but imagining a two-dimensional airfoil with a camber throws up a problem using Bernoulli as an explanation. A two dimensional cambered flat plate has the same distances top and bottom. And which way should it be cambered? "Scientists" in a recent TV program given the task of building a kite using just raw materials found on a desert island thought the camber should be upward in the middle, like an airplane wing, to go along with the usual Bernoulli explanation of lift. Needless to say, their attempt was a hopeless, hopeless failure. It was too heavy, made of woven palm fronds, longer than it was wide, and deeply cambered upward in the middle. The bridling used didn't help, either. But the crucial point here was that they all said that they needed that upward camber. Or else it couldn't fly... because of Bernoulli's Theorem.
Most kite makers know that most kites need the reverse, if any camber is used at all. You want the kite to raise its tail when it drops back and raise its nose when it's pulled. And you certainly do not want a kite to move into dives - you want it level out and return to a stable equilibrium position.
Flat plate airfoils can also be theoretically very efficient at low Reynolds Numbers, which ties in with the performance of flat kites like hexagons. How many kites only 30 inches high fly on 80lb line?
The bottom line is that angle of attack is the source of lift, and any kite that flies has its angle-of-attack set within a range that works for that particular design.
Another myth relating to aerodynamics is the one where meteors are supposed to burn up from friction when they come into ultra high speed contact with Earth's atmosphere. Since with any body moving in a gas the molecules of gas in contact with the surface are completely stationary, this isn't physically possible. In fact there is a "boundary layer" of gas sticking to the surface. The intense heat is the heat of very high compression, not friction, that occurs behind the shock wave formed just ahead of the object.
Aerodynamically speaking, a kite is just a black box at the end of a string - and a stunter is a different black box. It's a different mathematical entity. Stunters can't fly unaided on a single line as true kites. The center of gravity would be wrong, the camber would be wrong, the dihedral would probably be wrong and the vertical positioning of the towing point would definitely be wrong. They are different from kites. An example would be to compare a stunter with a Clipper, say, at high flying angles. I have flown a Clipper to 3,000ft on 50lb line in a light wind with the line just singing; from the ground the line was vertical; people could walk around it and because the kite wasn't visible in the heat haze it wasn't possible to say which way the breeze was blowing. But if you fly a stunter to as steep an angle as it'll go, the lines tend to go slack, and to regain control you have to let it drop downwind - the apparent similarity to kites is purely coincidental, like a parallel evolution from a common ancestor. Stunt kites are better at flying out, not up, which is why I say they are closer to controllable parachutes than to true kites as far as the mathematical analysis of their motions - their aerodynamics - is concerned. The predominant factor is drag, not lift. If you don't believe me, just try to apply kite aerodynamics to any stunter. The reference is given in the Links section.
If that isn't enough to convince anyone, take a look at this two line stunt kite, tailor-made to illustrate this.
It's touted as "The worlds only Monocell sparless kite."
Even though the parts might be cut using computer-controlled laser cutters, the way commercial kites are assembled and sewn means tolerances are all over the place. If single line deltas with scallops were made the way most commercial kites are, it wouldn't be possible to guarantee that even one would fly, let alone every one. With two lines it doesn't matter if one wing is slightly different from the other, but where stability depends on accuracy in symmetry, even a tiny variation can mean the difference between a flyer and a diver.
Having looser production tolerances doesn't mean the wings of stunt kites aren't sometimes quite sophisticated. Although stunt kite makers don't have to sweat blood at every step of the way to make sure their kites are symmetrical, many of them are constructed to a high standard. The on-going development of stunt kite designs has lead to improved handling and smoother, quieter flying; and also indirectly to better fittings, fabrics, frames and flying lines for all. The level of sophistication and refinement required to design better stunt kites that achieve improved control, handling or flying qualities is prodigious. It really is a worthy and separate branch of aerodynamics.
Web link: NASA's list of kite-related technical article index
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