Single Line Kite Stability:  Sept. '06.

Single Line Kite Stability:  Sept. '06.

Why kites can fly is such a complex question that after a first rush of youthful overconfidence in the '70's, I've despaired of ever finding useful answers- that is, useful in the sense of predicting, for known or intended kites, what effects given changes will have .

But recently I've been thinking that many equally complex problems are understood to a useful extent;

What do I actually know? - not necessarily to the standards required for mathematical theorems, but that is soundly based in theory and that does not conflict with known kite behaviour.

Single line kites hang in the sky supported by wind with their weight acting to pull their tails towards the ground and point their noses upwards. This is a necessary condition; if the weight force does not act at a point below the point of application of the lift forces, stable single line flying is not possible. This is because when a kite is caused to lean to one side by something (turbulence, a wind shift etc.), it's weight force's misalignment with the lift force can then act to cause the lean to diminish.

Any lean to one side will also alter a kite's alignment with the wind, changing the aerodynamic forces acting on it, but aerodynamic forces can only correct a kite's attitude relative to wind direction- and wind direction provides a reference only in the horizontal plane.  Up/down can only come from the moment effect of a kite's weight.  Until gps and gyro referenced auto pilots become available for kites, weight is the only force available to a kite for this purpose. 

A kite's weight being constant while the aerodynamic forces driving instability are proportional to the square of apparent wind speed, stability becomes more difficult to achieve as wind speed increases: All kites eventually become unstable unless some structural distortion or failure intervenes first.

These things are certain, and obvious enough.

These first three 'laws' of kite stability being satisfied, the key remaining element in kite stability lies in the dynamics of the complex feedback relationships between inertia, the weight force, and aerodynamic forces as a kite recovers from a turbulence or wind direction change induced lean. What can be said with certainty about this process of lean recovery?

The rate at which a lean corrects activates aerodynamic drag forces that will slow the rate of correction.

The rate at which a lean corrects can activate aerodynamic lift forces that will accelerate the correction - for example, because lift is proportional to the square of wind speed, the advancing wingtip during any lean correction will gain more lift than the receding wingtip loses.

Changes in the rate of lean correction will be resisted by inertial forces.

But now here's what may be a new (to me anyway) way to look at things, an hypothesis:

If the lean correction proceeds too rapidly, the kite can over-correct into a spin or a series of angular oscillations; called, say, 'rotational' instability.

If the lean correction proceeds too slowly, the kite will move sideways so that it's flying line is out of alignment with the wind in the horizontal plane, correction from which can result in a series of destructive lateral oscillations; called, say, 'translational' instability.

The kite builder's job therefore is to ensure that the rate of correction from any lean is neither too rapid nor too slow up to the maximum wind speed achievable.

,

These last three are for me a different way of thinking about kite stability.  Previously I've divided instabilities into two main types; 'volatile' instability, in which kites exhibit lateral and angular oscillations of increasing amplitude until a dive or spin results, and 'superstability', in which kites progressively lean over and drive off to one side, hanging there for an appreciable time before recovering. By this split, changes that could be made often did not have predictable results because 'volatile' instability and 'superstability' have inextricably overlapping causes and the nett effect of any change is then determined by their relative magnitudes.  Dividing instabilities into 'rotational' and 'translational' instead will be more useful if it allow remedies to be clearly differentiated. Of course it is true that every lean recovery must include at least some element of rotation (angle change) and some of translation (sideways movement) but if 9 above proves to be correct, (and back-reviewing my experiences so far with a wide range of kites lets me hope that it may be), then it should become possible to construct a table which will clearly predict the effect that given changes will have on a kite's flying- which will be a very useful thing indeed.  

 Peter Lynn, England, Sept '06