More Reasons Why Kites don't Fly



The Main reason why single line kites (often) don't fly.

FIG 1 - Forces Acting On A Kite

It's because kite stability is sensitive to angle of attack, the angle at which wind strikes a kite's surface(s), and angle of attack (called A of A from now on) varies widely with line angle and wind strength.
Specifically:
A of A is high- close to 90 degrees even- when line angle is low (while launching for example) and also when there's barely enough wind to keep a kite up.
A of A is low when a kite is flying at high line angle and in stronger winds.

The reason that kite stability is sensitive to A of A is because the point at which aerodynamic forces act on a kite (the centre of pressure, C of P) is a function of A of A and the distance between the kites C of P and it's centre of gravity, (C of G, where the weight forces act) is THE most critical determinant of stability.

"A of A", "C of P", "C of G" - getting confused by all this jargon yet? These three are unavoidable, but FIG 1 explains them visually, which always helps, and I promise there'll be no more.

FIG 2- C of P change with A of A
Angle of Attack = 90 0

It's a required characteristic for single line kites that the point at which a kite's weight forces act (it's C of G) must be below where it's lift forces act (it's C of P). This creates a sort of pendulum which points the kite upwards, without which, it will more likely try to fly under the ground than above it.
And, as explained in "Why single lines (don't) Fly", the length of this pendulum (relative to the size of the kite and other features it may have such as lateral area of course) determines whether it will undercorrect (take so long to recover from a lean one way or the other that it will traverse to the edge of the wind) or overcorrect (go into increasing amplitude figure eighting). A very short pendulum makes a kite tend towards undercorrection, a longer one to overcorrection- and there will be a range in between where it might be stable- in light winds anyway. (But just in case you were thinking 'well then, how simple this all is', there's an exception; VERY long pendulums can also be stable.)

FIG 3- C of P change with A of A
Angle of Attack = < 45 0

It's obvious from symmetry, that for a rectangular plate at an A of A of 90 degrees, the C of P will be at half chord (Fig 2) That this point moves towards the leading edge as the A of A decreases (Fig 3) is not so obvious, and the reasons for it weren't really understood until mathematical fluid dynamics developed sufficiently in the 19th century, but it is so. Most kites aren't flat plates, but it's still generally true that their C of P moves towards the leading edge as A of A decreases. (Or at least it does so until the A of A approaches zero, when it becomes very dependent on the particular airfoil shape and can go a little weird, but this needn't concern us here).

Changes in A of A as wind speed and line angle vary cause the C of P to move along the kite's axis, which changes the effective length of the pendulum, and can move it out of the range where stable flying occurs. In particular, they can cause the corrective effect exerted by a kite's weight to be inadequate when the kite is flying at a high A of A.

And, this is exacerbated by the reflexive or "nose-up" shape of almost all soft kites and many framed kites (including rokaku's, indian fighters, and delta's)"Nose -up" shape is used to prevent luffing (when the angle of attack becoming negative) and also to reduce the extent to which the C of P will move so far forward of the kite's centre of gravity as to cause overcorrection when the kite is flying at low A of A. Unfortunately, as can be seen from Fig 4, it also makes undercorrection more likely at high A of A.

FIG 4 - Effect on reflex of Angle of attack.



There are many observed kite behaviours that are explained by A of A effects, and the one that is particularly annoying me just now is the repetitive swaying behaviour of tubular soft kites (that is, fish shapes and similar).

With aspect ratios generally much less than 0.5, tubular form kites are not very effective at generating lift (L/D's generally less than 1.0). To develop enough lift to offset even their own weight, especially in light winds, they fly at high A of A (30degrees to 45 degrees or even more) at which the C of P is likely to be not much above the C of G, and might sometimes even be below it for short periods (but not for any sustained period or else they won't be flying). See fig 5. That the characteristic misbehaviour of this style of kite is caused by their C of G being too close to their C of P is supported by various observations;
  • Movement amplitude decreases when weight is added to the rear of the kite (providing the effect isn't masked by the increase in A of A that this can cause)
  • Movement decreases at higher wind speed- because A of A decreases (but overcorrection may then begin to occur).
  • Flattening the kite's front and narrowing the rear (shifting the C of P forward), without changing weight disposition generally reduces swaying.
  • Adding drag (by way of drogues etc) to the rear helps a bit but doesn't have as much effect as expected.
  • Flying on a very short line helps- which is characteristic of kites that tend to undercorrection.

FIG 5 - Tubular form kites undercorrection tendency.


The swaying that Ray (especially the smooth tail style) and Octopus type soft kites can get into when launching - sometimes even spinning- is also an A of A effect. It ceases when the kite gets to 30 degrees flying angle or so and the supporting observations that apply to tubular form kites are also seen with these styles.

However, A of A effects are not confined to soft kites:
The way that Indian and other bow framed style fighter kites spin rapidly when line is released (which allows bow pre-tension to pull the kite flat) is also an A of A effect. Of course It's also partly due to their not having any lateral area (which has a directional effect) when there is no line tension, but mainly it's because when flying at an A of A approaching 90 degrees, the C of P moves so close to the C of G that the kite loses it's sense of direction and spins around.

A of A effects also help explain why tails are more effective than drogues or other trailing drag devices for stabilising errant kites. The difference comes from tails having weight as well as providing drag. The above shows why, at high A of A, kites tend to undercorrection, and at low A of A, to undercorrection. Drogues provide extra drag but have negligible weight. They slow the kite's responses, so reduce overcorrection but don't help with undercorrection. In light winds, a tail's weight pulls the kite's centre of gravity rearward to reduce undercorrection. In stronger winds when the kite will be flying at higher line angle (low A of A), tails generate enough lift to be largely self supporting. In this state, their weight doesn't shift the kite's original C of G down much so won't exacerbate overcorrection. However, their longitudinal drag still acts to combat overcorrection and perhaps their resistance to sideways displacement helps damp movements as well.

Peter Lynn, Ashburton, July '09.




Your Comments

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From : Peter Lynn
Date : 22-07-2009
Comment : Together with "Why Single Line Kites (don't) Fly" and "More Reasons why Single Line Kites (don't ) Fly".

This draft completes 30 years of mumblings about all this for now (whew, I hear). There are still some holes to fill (yes I know, Reynolds number), and no doubt, errors to confess. Some of it will be wrong, but at least it's a reasonably complete and testable (falsifiable in scientific jargon) theory which purports to predict the effects that making specific changes to a kite will have. If it survives for a while without getting knocked out of the sky, I'll re-edit it to more coherent form, probably under "Stability of Single Line Kites" divided into three headings;
  • Undercorrection and Overcorrection; (a useful way to look at kite behaviour)
  • The main reason why single line kites (often) don't Fly (as above),
  • More Reasons Why Single Line Kites don't Fly.


And, if I can learn to use a basic digital drawing program (patience and time!), there'll be more neat (ha ha ) explanatory diagrams along the way.



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