Peter Lynn ARCHIVE
Three factors that affect kite stability, aerodynamic forces, weight, and mass, don't remain in proportion as size increases. Weight and mass are not directly related for soft kites because the enclosed air has mass but no weight).
Aerodynamic forces increase with the square of dimension, as weight nearly does in the size range of larger kites, but mass increases as the cube of dimension.
This matters because when a kite recovers from leaning over, the rate of recovery is slowed by the kite's mass, with an undeterminable effect on its stability.
The Pearl is six times the span and length of a standard Maxi Ray, about ten times as heavy, and has 36 times the area but encloses 216 times the air mass (9,504 kg versus 44 kg).
As for smaller kites, therefore, the only available design process, except for informed guesses, is trial and error, which is impractical for kites that each take many months to construct, 20 plus tonne anchors, a team of fliers, and substantial organization for every flight.
To reduce fabric stresses and the effect of enclosed mass on the stability of very large kites, we use the 'super ripstop' construction system.
By supporting the fabric with a grid of high strength Dyneema lines fabric stresses are limited to the grid size, enabling scaling with virtually no increase in the kite's weight/area up to sizes larger than we have yet attempted. By virtue of the 'super ripstop' construction system, the smallest Flag kite (less than 0.5 sq.m) has almost the same weight/area as the largest (1250sq.m).
And, with 'super ripstop' construction, the top and bottom skins are connected by multiple cords, which allow the kite to rotate largely independently of its enclosed air mass. By disconnecting this mass, such large kites are able to recover much more rapidly from any lean that develops.
Earlier large kites like the Dutch CS550 and the Edmonds College parafoil connected their top and bottom skins with multiple ribs in conventional parafoil-style type construction.
In consequence, the air in each cell had to move with the kite during any correction, slowing correction and increasing the sideways movement before correction can occur.
I doubt that very large soft kites can be single-line stable without this facility, though I'm unsure about the size of any given style at which this mass effect becomes critical.
But from 20 years of watching various large soft kites flying with sidelines, there seemed to be every chance a very large flag-style kite with a suitable tail could fly stable on a single line.
Fortunately, by then I had a "theory of tails," which was verging on the quantitative.
Not having any enclosed air mass, flat tails do scale predictably enough, so we were able to test variations on smaller flag kites before committing to a full-size version.
And it worked, no side lines required! A success that I'm pleased to claim a share of, a small offset for the huge string of kite failures that I can also lay claim to!
The wind was very light at Berck, so I don't yet know what The Hope's upper wind limit will be for single-line flying, but I expect it to be useful.
It was vertically unstable during its first flight at Berk, climbing immediately to full height, only to dive back down, climb again, and so on.
For this, we can blame Swiss mathematician and physicist Daniel Bernoulli (1700-1782), whose equation describing fluid flow establishes that when velocity increases, pressure decreases.
This is experienced as the bath plug pulling in when you try to hold it partly out.
During The Hope's first flight at Berck, there was a significant wind gradient - almost no wind at ground level, but 15km/h or more at 100 meters up.
When it reached maximum altitude, there was significantly more wind above the kite than under it.
There was, therefore, a net low-pressure area above the rear of the kite, which caused it to nose down and descend - it's called a luff.
Very large soft kites are susceptible to this because they are carefully tuned - by adjusting camber - for minimum pull, so they fly on the verge of luffing, and also because they are generally flown very close to the ground.
When there is a significant wind gradient, they should, therefore, be flown higher, or pilot kites should be used to hold the leading edge up.
Peter Lynn 2018
