WORLD ALTITUDE RECORD

 Atmosphere & Wind

Wind is what kites need to fly. Ideally, to fly my kites to world record heights, the wind needs to be between 9 and 21 knots (17 & 38 kph) at ground level, rising to between 14 & 33 knots (26 & 62 Kph) at 18,000 ft. The reason for needing a little over double the wind speed at 18,000 ft is the atmosphere is about 55% of the sea level density at this height. In other words the air is thinner and has less kinetic energy for the same ground speed. Also as the kite rises, not only does the lift diminish but the load the kite has to carry increases. This load is the line weight and line drag from air friction. With small kites at low altitude the drag is not such a problem but 10,000 meters of line has a cross sectional profile as much as half the lifting area of the kite. That is why the thinnest and lightest line possible is used. The kite has to tolerate dense, high lift air near the ground yet fly efficiently at high altitude in cold yet much lower air densities. Kite design is a compromise as it needs to be light yet strong, flexible to spill the wind in big gusts yet stiff enough to hold its wings out fully in lighter breezes. Wind is a highly complex, dynamic phenomena. Atmospheric scientist have unravelled many of these complexities but there is still much to be learned. Some kite fliers learn to gauge the nuances of the wind like sailors. It is our best friend and often our worst enemy. I think I have learned as much as any lay person about wind over flat lands but I can never conquer it as it’s such a complex and powerful beast.

Wind by Amy Lowell
He shouts in the sails of the ships at sea,
He steals the down from the honeybee,
He makes the forest trees rustle and sing,
He twirls my kite till it breaks its string.
Laughing, dancing, sunny wind,
Whistling, howling, rainy wind
North, South, East and West,
Each is the wind I like the best.
He calls up the fog and hides the hills,
He whirls the wings of the great windmills,
The weathercocks love him and turn to discover
His whereabouts -- but he's gone, the rover!
Laughing, dancing, sunny wind,
Whistling, howling, rainy wind,
North, South, East and West,
Each is the wind I like the best.
The pine trees toss him their cones with glee,
The flowers bend low in courtesy,
Each wave flings up a shower of pearls,
The flag in front of the school unfurls.
Laughing, dancing, sunny wind,
Whistling, howling, rainy wind,
North, South, East and West,
Each is the wind I like the best.
 

Wind Rose Images and maps courtesy of Australian Bureau of Meteorology

The Wind Across Australia

Wind is one of the most highly variable meteorological elements, both in speed and direction. It is influenced by a wide range of factors, from large scale pressure patterns, to the time of day and the nature of the surrounding terrain. Because the wind is highly variable it is often studied by means of frequency analyses, provided here in the form of wind roses, rather than as simple averages.

Above: The Autumn pattern winds in Australia. Cobar is dominated by  winds from the eastern sector with east, south, south east and north east winds being prominent.

Above: Winter wind patterns in Australia. At Cobar the is a shift of dominant winds to the west, soutwest and south as southern ocean weather patterns are further north over the souther half of the continent. It is these winds which I hope to exploit in future record attempts.

On the average, wind speed increases with altitude but there are frequently layers in which the wind speed decreases or even is calm. The first 30,000 feet is often highly variable in speed and direction. On this graph the green line shows the wind speed at ground level to 16,000 ft. You can see it is variable at different heights not only in speed but in the direction shown by the pink line. As a kite rose through these levels it would stop rising at about 3,000 feet and also be changing direction from flying to the north east to the north west. The line would develop a curve not only due to gravity but would also blowing it to the right as observed from the ground. On this day there is a boundary layer at 4,000 ft with a sharp change in speed and direction. Although the wind above 5,000 ft is excellent, it would be difficult to force the kite through the 3,000 to 5,000 ft zone. The only possible way through this layer would be to release over 4,000 ft of excess line, allowing the line angle to drop to 10 degrees then winching the kite in reverse hoping that in the process the kite will rise to 5,000 ft into sustained lift. However the biggest barrier to achieving high altitudes under these conditions may be the 150 degree change in wind direction between 4 & 5,000 ft.

I have edited this wind profile from Cobar to illustrate the ideal wind range for a world record attempt. The solid black line is the ideal wind speed which starts at 15 knots (28 kph) at ground level, then rises by 1 knot for every 1,000 feet increase in altitude. At 15,000 ft the ideal wind speed should be 30 knots (56 kph). This may seem high but the air density drops by about 3% for every 1,000 ft rise in vertical height. This applies to altitudes up to about   30,000 ft.  Air at 620 ft above sea level (the Cable Downs airstrip) has about 98% of sea level density and air at record altitude (15,000 ft above Cable Downs) has about  60% of the density at ground level. At maximum allowable altitude of 18.000 ft, the density would be near to half that of ground level. The lift provided by the wind decreases with height for the same velocity. If it’s 10 knots at ground level then at 15,000 ft it should be 25 knots at least and preferably 28 knots. I say 28 knots because of accumulated line weight and drag. This is somewhat arbitrary but are based on calculations using know drag coefficients and average air density decay rates. So far 4 flights over 9,000 ft at Cable Downs have confirmed these estimates. Wind profiles will rarely follow these ideal profiles and may dip briefly above or below this band of wind speed I have defined but on not for more than 1,000 ft thick. Winds above the dashed line risk kite damage and the kite and line drag increases while the kite lift doesn’t increase or even drops. If the wind speed is below the dashed line then there is insufficient wind to carry the line weight and line drag. The direction can vary but ideally a gradual shift of less than 30 degrees would be ideal. Again the variation in direction is often over 180 degrees with a boundary layer to negotiate. It may be possible to ride thermals to record altitudes but these are often associated with cloud and storm cells. We are not permitted to fly kites near or into cloud. The wind speed and direction is so crutially important to the record attempts that I have devoted a great deal of time to studying wind. I still have a great deal to learn.