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Kite Altitude World Record
Wind is simply the movement of air. It is what we as human beings feel on our skin and see the effects of with our eyes. We can't actually see wind directly but we sense its direction by its effects on our surroundings such as the bending of tree branches, the flapping of a flag and the rippling of a lakes surface. It is the result of uneven energy distribution in the atmosphere, specifically, air pressure. What causes differences in air pressure? Without getting too much into meteorology, the sun's energy is unevenly distributed on the earth due to its spherical shape and its seasonal variation in tilt plus its cycle of day and night. The rotation and spherical shape also causes the Coriolis effect or deflection of wind from the equator toward the poles.
Humans or most life as we know it would not exist without wind.The uneven heating of the earth's surface plus the earth's rotation causes the atmosphere to move. Water is heavily involved in the process of the wind and energy transfer cycle for it transports energy between the oceans and atmosphere. People have names for winds that have blown over their lands for thousands of years. In geological time spans, these winds are transient, relying on the juxtapositions of oceans, continents and their positions on the planet earth as they drift about over hundreds of millions of years. The Mistral, Sirocco, Leveche, Santa Ana, Foehn, Southerly Buster and Roaring Forties are all descriptive terms for winds which characterise a region or latitude. it is interesting how many of these winds have become car, consumer items or aircraft models. They include Bora, Chinook, Diablo, Mistral, Passat, Typhoon, Cyclone, Sirocco and Zonda. Meteorologists categorise these winds according to how they are propagated such as sea breezes, monsoons, adiabatic and katabatic. It is very important for me to understand wind as it is the means to high altitude flight. The ordinary person can be affected profoundly by wind such as tornados but it is generally taken for granted. People usually only notice wind when it blows their hat off, chills them to the bone or kicks up dust. Without wind life would not exist as we know it. No rain, no snow, no distribution of spores, no wide spreading of grass seeds, no waves, no clouds, no flags, no washing drying on cloths lines. No sailing and no discovery of new lands. No sand dunes, dust clouds, wind erosion, wearing down of mountains, no sandstone and windbreaks.
The best-known tool for wind speed is the Beaufort Scale which describes the effect of wind on the environment.
The international unit of measurement for wind speed is the knot. 1 knot = 1.95 kph which is equivalent to Beaufort, light air. It is a speed at which we would feel it on our face. Only the very lightest kites would fly. Below is the Beaufort scale.
The wind force on an object is measured using meters/second or m/s. This is inserted in an equation along with the frontal area in meters and the coefficient of drag to produce a force value. I express the wind force on my kite and the wind force on the kite line as tension in kg or pounds. The force on our kite increases at the square of the wind velocity. So doubling the wind speed quadruples the line tension.
In theory a 1 m/s wind on my 12 sq meter kite will produce F Newtons force and then a 2 m/s wind will produce 4F newtons force. If the wind speed increases just from 2 m/s to 4 m/s (7.2 - 14.4 kph) which is equivalent to a small gust, the line tension could increase from 4 lb to 16 lb. Then if the wind were to double again to 28.8 kph, the line tension would reach 64 lb. The wind force is to some degree, self limiting by the kites flexibility spilling the wind at higher velocites. The maximum line tension observed was 120 lbs even with the wind speed approaching 120 kph but the line tension should be about 256 lb. A kite, although relatively simple in design, is still challenging to grasp the particular elements which are instrumental in determining flight characteristics, efficiency and flight behaviour.
Above: A graph extracted from Australian Atmospheric Soundings. I have modified the graph to illustrate the ideal wind ranges. the darker blue zone defines marginal wind range where kite altitude may reach a maximum of 10,000 ft. Light blue within ideal range but may limit kite altitude to 15,000 ft especially in warm conditions with lower air density. Pink zone is ideal for 18,000 ft flights but line set needs to be one step heavier. Red zone flyable but the heaviest line set should be used. Winch freewheeling could be required if kite is over pressured. Below these zones, a light weight kite may fly higher than our standard kite under the same conditions but thermals may need to be employed to reach any appreciable altitude. Attempting to fly our standard DT Delta above the red zone is outside the design limits of the kite and also breaches CASA conditions. I think it would be foolish to try. We may start out with ground winds about 10 knots but then either the kite flies into a high wind zone or the wind increases, is sometimes unavoidable. Deliberately launching a kite into conditions that would see the kite shredded or the line broken is irresponsible.
The wind roses diagram is a graphical representation of wind data gathered over 29 years. It forms the basis for deciding what time of the year will best suit high altitude kite flying. As the data does not necessarily reflect the magnitude and frequency of high altitude winds, it is a starting point for further investigation of winds at all altitudes within the target range. Further study of atmospheric soundings derived from the Cobar daily balloon sonde flights appear to indicate that late August through to late September offers the best chance of success.
A very difficult day as ground wind is 9 knots rising to 18 knots at 4,000 ft. Then there is a very wide band of marginal wind from 5,000 to 9,000 ft. The wind must rise by 1 knot for every 1,000 ft rise in altitude. This is compensating for reduction in air density which is about 50% by 18,000 ft. If the wind at ground level is 9 knots then it needs to be 27 knots at 18,000 ft to have the same lift. However, it also needs to provide enough extra lift to carry up to 11km of line weight. The drag is a different matter as it increases exponentially with wind speed so the more wind there is the more line drag there is as a percentage of kite lift. The best scenario is for wind to drop below the kite after it passes that level so the kite is subject to maximum lift but the line is subject to minimum drag. Even better if the wind gradually reverses direction as the kite rises so that the bottom 1/3 of the line is dragged, for instance, west, the middle section of line is blown by wind transitioning between being blown toward the west and being blown toward the east. The top line drag would the cancel out the bottom line drag. the centre 1/3 of line would be drag neutral. The kite would then only support the line weight. The line path would describe a half spiral
Anemometers can have a dial and an odometer, just like an older analogue car speedometer. This produces the instantaneous wind velocity and average wind velocity over a defined period such as 1 hour, 12 hours or 24 hours. This could be a mechanical version but can be connected to electronic recording circuits to provide a variety of data such as maximum wind speed, average wind speed, instantaneous direction and average direction. Miniature electronic versions can be fitted to the kite but adds weight and complexity.
Different wind strengths at different levels can effect the kite and line in different ways. The line and kite behave as a unit and because they are connected forces on one will be transfered to the other although it may take some time for the effects to be realised.
|10 minute sustained wind (knots)
|1 - 3
|4 - 6
|7 - 10
|11 - 16
|17 - 21
|22 - 27
|28 - 33
|34 - 40
|Tropical cyclone (1)
|41 - 47
|Tropical cyclone (1)
|48 - 55
|Tropical cyclone (2)
|Tropical cyclone (2)
|Tropical cyclone (3)
|Tropical cyclone (3)
|86 - 89
|Tropical cyclone (4)
|Tropical cyclone (4)
The Beaufort scale is a qualitative measure of wind speed adopted in the early 19th century by British sailors and meteorologists. The metric scales are now almost universally used by scientists but knots are used for aircraft and shipping. Weather reports to the public use either kph or mph wind speeds depending on the country's adopted system.
We fly our kites in the range 5 - 25 knots. The ideal wind speed is 15 knots at sea level. To maintain the same lift as the kite rises, the wind speed must rise to compensate for the air density decreasing. The ideal wind speed at 18,000 ft would be about 30 knots as the air density is about half that of sea level. The chances of encountering perfect wind from ground to record altitude is very remote.
The conditions can only ever be what they are and we have to work around and adapt our techniques to the conditions on the day.
Wind not only varies in strength but varies in direction. The direction is where the wind is coming from either as a descriptive such as North, South, Southwest and West Northwest or as a degree value from 0 to 259 degrees.
0 = North
22.5 = NNE
45 = NE
67.5 = ENE
90 = E
112.5 = ESE
135 = SE
157.5 = SSE
180 = S
202.5 = SSW
225 = SW
247.5 = WSW
270 = W
292.5 = WNW
315 = NW
337.5 = NNW
360 = N
While descriptive terms for direction are helpful for humans to visualise the approximate wind direction, they can only be accurate within 11.5 degrees and it is much more accurate to record direction within a degree of its actual value. However, wind is rarely steady for more than a few seconds to 1 knot or 1 degree. So, in discussions between the team members we refer to the cardinal points such as "the kite took off to the WSW and then by 3,000 ft was heading SW". On the GPS log the heading is recorded to the fraction of a degree. Wind speed is defined as either as steady state or as gusts. Steady wind speed is measured over a defined period of 10 minutes and gusts are measured over 1 or 2 minutes depending on the context of observation. Measurements are recorded at a 10-meter height to eliminate ground turbulence. The roughness of terrain can cause massive turbulence or only minor variations in velocity or direction. A mountainous area will have large scale turbulence that extends thousands of feet vertically and valleys can channel the direction of wind and also increase its speed. We experience the same phenomena on city streets with tall buildings producing a wind tunnel effect. Ideally wind speed should be measured at the kite but suitable instruments to log and transmit this data are difficult to find.
We do have a snapshot of local conditions from the weather station at Cobar, 40 km distant. The wind speed and direction the balloon sonde measures are a guide only and the greater the time lapse between the sonde flight and kite flight, the greater the variation from those conditions.
The diagram to the left illustrates how wind variations in strength and direction affect the kites path and line direction. It is rare that the wind rises with increases altitude in a lineal fashion and remains on the same heading. Kite altitude cannot be accurately measured trigonometrically by line length and angle of elevation of the kite. This is because up to 10% of line out may be consumed by aerodynamic line drag and gravitational sag. The drag and sag are not only downwards but can be left and right and even occasional up due to thermals and down drafts.
It is crucial that the flying line be as thin and light as possible to minimise the impact of air drag.
Not only does wind vary in speed and direction but air varies in temperature and pressure. These two factors cause the air to be more or less dense thereby decreasing or increasing kite lift. Apart from the standard decrease in air density with altitude, air density changes over hours or days and is determined by air temperature and systematic pressure. On a warm day, the wind speed may need to be 10% more to achieve the same lift.
While we canít do anything to alter the weather, by understanding wind, we can adapt our kite flights to the conditions, thereby maximising our chances of reaching record