PARAGLIDER LINE MATERIALS, AND WHY THEY MATTER TO YOU
One of the many things which the paraglider designer must make informed choices about during the design process is that of lines. Which lines will he use where in the design, will he employ Aramide/Kevlar lines or Dyneema, and will he go sheathed or unsheated, or a mixture of all these options?
There are several reasons why his choices matter a great deal:
• Different line materials have different breaking loads, even for the same diameter,
• Some materials age faster than others, but may have other advantages which make them desirable after all,
• Sheathed lines generally last longer, due to the thin sheath woven around the core of the line – but they are also thicker for the same breaking loads, since a part of their diameter consists of the sheath, which only adds thickness but no strength,
• Line drag is a great part of the total parasitic drag of the glider, and reducing the number of lines used, and their diameter, is a surefire way of increasing performance.
In this article we'll try to look into some of the benefits and drawbacks of the different available line materials and production methods, but first we'll list the options and briefly describe them, so you know what we're talking about.
Sheathed Dyneema:
From the outside all sheathed lines look the same, since we only see the sheath and not the core of the line. But when a sheathed Dyneema line is cut, or breaks, we see that the core is white and non-woven (sheathed lines are extruded, then the sheath is woven over the extruded core).
The image on the right shows two damaged lines, both of the sheathed Dyneema variety - note the white core. The line on the right is slightly tricky; it FEELS lumpy to the touch, and has been damaged by excessive stretching, but a quick visual inspection may not notice the damage, however both lines must be exchanged before the next flight.
Sheathed Aramide/Kevlar:
Looks just like above when the core isn't exposed. Once we see the core we realise that it is light-brown in colour and consists of non-woven Aramide fibres. The colour gives them away; light-brown. You can generally see the exposed core at the end of a line, where it is sewn to make a loop. If the core looks "dirty" then you're most likely dealing with Aramide.
Unsheathed Dyneema:
Is white until someone colours them. It is however unusual to see completely white unsheathed lines on a paraglider, except on older Axis or MAC Para competition wings. Dyneema is readily coloured, and most manufacturers use line that is coloured from the line supplier, frequently in red. The line is woven and has a smooth, slippery look and feel.
Unsheathed Aramide/Kevlar:
These lines aren't easily coloured, so they are mostly employed in their natural, light-brown look. Woven, and with a much less smooth look and feel to them, these lines quickly look quite old and frayed when subjected to the rigours of less-than-perfect launch areas. In the image below, the left line is unsheathed Aramide/Technora, the centre one is sheathed (either Aramide or Dyneema) and the right one is unsheathed Dyneema.
Properties of the different materials
If we focus strictly on technical specifications, as supplied by the line manufacturers, then unsheathed Dyneema lines are the superior material from just about every angle you look at them; they are stronger than their counterparts made from sheathed or unsheathed Aramide/Kevlar for the same diameter, and they are stronger than sheathed Dyneema lines for the same diameter too, due to #3 in the bullet list above. They are even comparatively insusceptible to degradation from exposure to UV radiation (sunlight), they are not easily damaged, and they weather the standard "5000-cycle" bending test, where a line is mounted in a test bench and then subjected to five thousand 180º bends, very well. They are even dimensionally stable length-wise when you put great loads on them, meaning they don't stretch (much) under load.
On the opposite side of the spectre we have unsheathed Aramide/Kevlar lines. As mentioned above they aren't as strong, but to make matters worse they tend to degrade comparatively fast when subjected to UV radiation, they fray more easily from physical abrasion, and they lose worrying amounts of strength when they are put through the "5000-cycle" bending test. Aramide/Kevlar lines will stretch a little more than Dyneema under loads, and may retain their new length even when unloaded again.
Somewhere in between these are the sheathed lines made from either Dyneema or Aramide/Kevlar. Their main advantage is that they are less susceptible to damage from both physical abrasion and UV radiation, but they will always be thicker for the same breaking load (bad for performance). Some pilots find that they tend to tangle and knot less, something that is a good argument for utilising them on gliders in the beginner-and-intermediate bracket.
From what we have learned so far, choosing the right lines seems straight-forward – go with unsheathed Dyneema for most uses, and perhaps some sheathed Dyneema on the school wings, and Bob's your uncle. Unfortunately things aren't quite as simple as that.
The first thing we need to realise is, although Dyneema is dimensionally stable under loads, it does tend to shrink under certain circumstances, for instance when it is subjected to temperatures higher than app. 70 degrees Celsius. Initially all lines will tend to shrink equally – but as soon as a glider with shrunken lines is taken out for its next flight, the A and B lines, which carry much more weight than the C lines, will return to their original length, while the C-lines will remain too short and bring the glider out of certified trim. Temperatures in this range may sound unlikely, but the fact is that if you leave your glider in the boot of your car during hot, sunny days, or if you lay your glider out on the hot sands of Iquique or Sossusvlei, then the temperature will soon be quite a bit higher than 70 degrees. The thing to remember then, not just for the Dyneema lines but for the canopy cloth as well, is that you really shouldn't leave the glider in the boot of the car, and you should try to limit the exposure to the hot desert sands of tropical regions.
To make matters more interesting, the only way we can know for sure if the wing is out of trim is to measure it, and preferably with a professional setup (laser measurer, 5kgs loading on each line etc.) - we can suspect that something is wrong, for instance if the wing doesn't inflate nicely any more, or if it is climbing less efficiently than it used to, but to really KNOW we need to send the glider in for a check.
Aramide/Kevlar lines, and their relative disadvantages compared to Dyneema, have already been outlined – but contrary to Dyneema, it is very easy to see on an unsheathed Aramide/Kevlar line when it is worn, and although they do stretch, and they aren't as strong, at least they don't shrink.
This all means that when the designer is making his choices regarding the lines he will use, he needs to take some things into account, like:
• Which pilot segment is it for? A glider which is designed to be flown by very advanced pilots can justifiably be equipped with lines which require a little more attention, provided they offer significant performance benefits (Dyneema...)
• Is performance at all cost more important than longevity? I.e. will the intended pilot profile ever consider sending the wing in for a check, or will they expect it to remain airworthy without any further attention for the entire lifespan of the canopy? The latter scenario speaks for sheathed Aramide/Kevlar line, which doesn't shrink and is insusceptible to UV damage due to the sheathing, but in actual fact even these lines should be checked every now and again, not least due to their inherent problems with the "5000-cycle" bending test, and anything similar in real life...
One way to have your cake and eat it is to employ sheathed Aramide/Kevlar for the thicker, lower lines, and Dyneema for everything above the first bifurcation. This is due to the fact that the total length of the shorter gallery lines isn't affected as much by, say, 0.5 percent shrinking as the longer, lower lines, and also because the thick main lines are pretty heavily over-dimensioned on many wings anyway. But this solution assumes that you're looking for a compromise anyway, however with performance wings we generally aim for the least possible parasitic drag, and then we're back with the unsheathed Dyneema again...
It would be enticing to think that we could circumvent the paradox by using slightly beefier Dyneema lines for the main lines, and normal, thin Dyneema for the gallery – after all, this would mean that we had the best of both worlds in terms of most of our parameters, right? Wrong. The issue with Dyneema is not strength, it is shrinkage, and that happens irrespective of line diameter.
The conclusion is that as long as we're so hell-bent on performance increases with each new model incarnation we will have to learn to live with the few drawbacks that accompany our use of Dyneema line material – other line materials do have their place for certain applications, especially among the lower-rated gliders, but on the performance models there simply isn't, at the time of writing, any sensible alternative to Dyneema. Meaning that IF you're a performance pilot flying a performance design then you may as well get used to the fact that you will want to have your lines checked every now and again (according to the Owner's Manual), and you will want to be alert to subtle changes in your wing's launching and flying characteristics. The benefits you get for your extra vigilance is higher performance AND higher passive safety, because Dyneema is also stronger, and remains MUCH stronger, than any other line material currently used in paraglider production.
(https://www.up-paragliders.com/en/content/item/502-paraglider-line-materials,-and-why-they-matter-to-you)
Wing Loading
by Adrian Thomas
Wing loading doesn't affect glide (unless the wing is distorted by the weight) you just go down the same glide angle at a higher speed if you are heavier.
Increasing wing loading increases speed by only the square root of the weight change - so on a particular glider if you can double your weight without distorting the wing then your speed will go up by a factor of 1.41...
Which means that adding the maximum ballast allowed under FAI/CIVL rules (about 10kg) can increase your speed by about 5 percent if you are an 80 kg pilot.
In a 4 hour competition that means that a given pilot on a given wing could finish (all other things being equal) 12 minutes earlier by carrying maximum ballast.
But all other things aren't usually equal. The ballasted pilot gets about a 5% increase in sink rate as well as flight speed. For any given turn radius a heavier loaded glider has to bank more steeply, and sink rate in turns increases rapidly with bank angle. So a ballasted pilot looses out in turns from both the direct increase in sink rate and the increase in sink rate that results from the steeper bank angles that are required for any given turn radius. The total effect is roughly proportional to the change in weight.
So if our competition pilot spends one third of the time during that 4 hour task climbing in thermals then by carrying ballast the higher speed on glides saves about 8 minutes, but the total time taken to climb is about 8 minutes longer.
If the conditions allow the task to be completed with only 1/3rd or less of the total time spent climbing then ballast could pay off. If more than about a third of total time is spent climbing then being light on the wing pays off.
But that isn't the whole story. Being heavy on the wing affects stability and agility as well.
A heavily loaded glider is (in general) more resistant to turbulence-induced collapses, but collapses more violently when it does go. This seems to be partly to do with internal pressure (dynamic pressure goes with speed squared, so it increases in proportion to any weight change), and partly to do with the angle of attack of the wing - at any given speed a heavily loaded wing is at a higher angle of attack than a lightly loaded one (to get the lift to balance weight the heavily loaded wing has to operate at a higher lift coefficient). Tucks are (usually) produced when turbulence causes the local angle of attack at the leading edge to go negative. The angle of attack effect seems to be stronger than the internal pressure effect (which is why flying fast (low angle of attack) makes the wing more likely to tuck - IMHO). The reason collapses are more violent on a heavily loaded wing is that the turbulence required to collapse them is more severe, and the speed prior to the collapse may be higher.
A heavily loaded wing is more agile because the pilot has greater control authority - shifting all that weight to one half of a tiny wing will make it bank up quickly....which is why comp wings feel like trucks if you are used to something with a shorter span, and why comp pilots end up wanging all over the sky on beginners wings when they borrow them. There also seems to be something going on with pitch stability and the speed of transmission of information about turbulence so that heavier loaded wings tell you more about what the air is doing - but I don't even have a theory about that.
All this goes to suggest that if you want to float around at the top of the stack all day then you should be light on your wing. If you want to fly cross country or in competitions then you have an optimization problem. If you expect to be racing spending 2/3rds or more of your time gliding then it might be good to be heavily loaded on your wing. This would be particularly true if you expect the thermals to be turbulent (when the extra agility and stability would help you core the thermal while lighter loaded wings get chucked about and thrown out of the core). This would be especially true if you expect to be flying in straight lines in lift a lot i.e. Alpine or desert flying. If however you fly in circumstances where you are likely to spend more than a third of the time climbing in thermals, where thermals are weak, or where climbing at all may be the key then you want to be lightly loaded. Every comp I have been in has had a task where staying in the air in weak lift was the key.
Finally, there is the question of big-glider performance. Big gliders glide better (other things being equal), at least so I am told by some of the best pilots (and designers) around. The only reason I have been given is that it is to do with their higher Reynolds number. Reynolds number depends on velocity, a characteristic length, air density (1.225kg/m3 at sea level) and air viscosity (17.9 x 10-6 kg/m/s at normal temperature and pressure). For a paraglider flying at 10m/s Re is about 106 times wing cord. So at a given speed a larger glider has a larger Reynolds number. As Reynolds number goes up drag goes down......all other things being equal. Going from the Argon 24 to Argon 26 changes wing cord from 2.71 to 2.82m. which changes the Re from 1.85 x 106 to 1.93 x 106 which is about a 4 percent increase. Von Mises reports NACA data for a NACA23012 airfoil which had a drag coefficient of 0.009 at 1.7 x 106 decreasing to 0.0075 at Re = 7.5 x 106...a 17% reduction in drag for a 4 fold increase in Re. so the effect being linear in this range our 4 percent increase in Re should give a 0.17% decrease in drag. Not insignificant, but not exactly huge either. At the same time though, the maximum lift coefficient of the wing increased from 1.4 to 1.6 which would mean that the larger wing can fly slower, or turn tighter before it stalls. Interesting.
SO maybe the reason so many comp pilots can be bothered to carry around huge bags of ballast is that it allows them to fly a bigger glider that glides a little bit better and can fly a little bit slower and turn a little bit tighter in thermals. As far as I can tell there are no disadvantages for big gliders except for carrying them up the hill.
The weight ranges manufacturers quote are almost always the certified weight ranges - the gliders have been tested at the bottom and top of the weight range. If a glider is particularly benign it passes the tests at a wider-than-expected range of weights (as was IIRC the case with the Argon). Beware gliders that have only a narrow weight range!
I spent this year (2000) flying a Nova Argon 24C at the top of its weight range. As expected the wing was stable, handling was fabulous and speed was high, but I had to work to climb with people if thermals were less than 4m/s or so. The advantages of high loading meant that I thoroughly enjoyed the rough air at the squad training camp in Castejon, the British open at Piedrahita (5th) and the Europeans, but I found it more difficult to get away XC in the UK than I did the year before when I was lightly loaded on a Nova X-Ray. Next year I will be changing to a wing where I am near the middle of the weight range, and if I expect to be racing or conditions are good (which is the same) then I will carry maximum ballast.
I've also put a lot of effort into reducing the weight of my flying gear this year, and by changing flying suits, and dumping excess gear I managed to loose 2Kg of useless weight. Weight is only useful as ballast if you can dump it when conditions get weak!
_________________________________________________________________
Asas duração, horas de voo tempo de vida marcas e outras ideias
É habitual ouvir e lerem-se diversas ideias e opiniões àcerca do tempo de vida útil e manutenção de performance das asas, muitas vezes colando estes elementos, pela positiva ou negativa, unicamente à marca em si mesma, baseando a opinião em fatores de preferência pessoal, passado de sucesso, ou impacto publicitário das marcas com marketing agressivo, apoiado muitas vezes nos resultados das competições e seguimento de pilotos top patrocinandos.
A predominância de uma marca sobre as restantes até pode ser verdade para o resultado e para a competição pura, mas não, em nosso entender, para os fatores referidos, ou seja no que toca à duração e consistência de performance.
Diz-se também, frequentemente, que as asas antigas duravam muito mais tempo que as atuais, porque os materiais eram mais robustos etc. Não temos essa opinião, primeiro porque, devido ao aumento da qualidade dos materiais é evidente uma maior duração operacional das asas. Também, devido ao aumento do numero de pilotos clubes, descolagens e respetivas estruturas de apoio ao voo e, sobretudo, pelo incremento do fator performance/segurança das asas, hoje voa-se muito mais e em menos tempo que no passado recente. Tendo-se observado algumas asas atuais, de diversas marcas, ainda a voar com mais de 600/700 horas de uso. Embora se reconheça que essas antigas asas duravam mais ao nível dos cordões, já que os fabricantes colocavam a tónica no reforço da suspensão, traduzido no numero de cordões e sobredimensionamento dos mesmos, em detrimento da performance. Esta filosofia é impossível de aplicar hoje em dia, quando existe um foco na performance mesmo nas classes mais baixas.
Fatores que interferem na duração de um asa:
O Uso
O tipo e cuidados de uso são os fatores mais influentes na durabilidade da asa.
Arrastar o material sobre superfícies abrasivas, expô-lo ao sol horas seguidas enquanto se aguarda para descolar, bater com a asa no solo no treino de inflados não contribuem para uma vida longa do material.
Voo junto ao mar
Como é sabido, o voo de praia desgasta prematuramente o material e diminui drasticamente a vida util da asa, pela abrasão das areias e contaminação salina
Exposição solar
A exposição ao sol, diminui o tempo de vida e resistência do tecido e dos cordões pela ação direta das radiações UV que contribuem para a perda de resistência do material
A qualidade do tecido
A qualidade do tecido assenta essencialmente na qualidade da cobertura de revestimento que lhe dá a impermeabilidade. As coberturas hoje usadas, à base de poliuretano, sylicone etc dão impermebilidade e durabilidade ao material.
Também a textura e espessura do filamento contribuem para a manutenção da resistência estrutural e à abrasão ao longo da vida da asa.
O Tipo de cordões
Cordões em aramide desencapado perdem muita da sua resistência após cerca de 100 horas de voo por ação das radiações UV . E são também mais sensíveis ao choque e à abrasão por motivo da sua natural falta de elasticidade. Obrigam a relines entre as 80 e as 200 horas, dependendo do tipo de asa, baseado na quantidade de ligações e resistência do cordão de origem.
Cordões em polietileno (dynema) mantêm a resistência ao longo da vida mas são instáveis dimensionalmente obrigando a retrimagens prematuras da asa, especialmente as asas avançadas.
Quantidade de cordões e numero de fixações à asa
Interfere com o intervalo de susbstituíção dos cordões, já que menos pontos de ligação à asa equivale a mais carga sobre cada cordão, mais alteração dimensional e mais suscetibilidade à rotura prematura.
Os reforços estruturais
Contribuem para a manutenção da performance ao longo da vida util da asa, evitando a deformação do perfil, mantendo a segurança e a performance
No passado recente praticamente todas as asas eram reforçadas com tecido grosso de mylar (muito utilizado em velas de barcos) no bordo de ataque e nos pontos de ligação no intradorso. Isto funcionava bem a princípio, mas a performance diminuía gradualmente pela perda de consistência do mylar motivada pelo uso e pelas dobragens consecutivas da asa .
Hoje praticamente todas as marcas passaram a adotar varetas de nylon, carbono e outros materiais similares, para reforço estrutural e manutenção da forma aerodinamica durante o voo. O mylar é hoje quase só utilizado como complemento e suporte de posicionamento das varetas.
Assim, devido ao uso das varetas, as asas atuais quase não perdem performance por perda de consistência e alteração de forma aerodinamica ao longo da sua vida util.
Poderíamos ainda dissertar sobre outros fatores tais como, manufatura e qualidade de acabamento, mas é sabido que qualquer equipamento com deficiência de acabamento ou má qualidade de manufatura rapidamente é exposto e denunciado no mundo global em que vivemos, pelo que não terá futuro nem venda entre entre os praticantes.
Sendo hoje em dia a qualidade de conceção e manufatura muito equiparada entre quase todos os fabricantes.
Após esta reflexão, escolhemos algumas asas atuais EN B mais conhecidas e fizemos uma comparação dos materiais utilizados nas partes principais que, em nosso entender, têm mais influência na durabilidade da asa.
Constata-se que os materiais são comuns entre os vários fabricantes.
Podemos assim, genericamente, fazer uma comparação rápida e tirar as devidas ilações.
| Asa Marca | Modelo | Tecido Extradorso | Tecido Intradorso | Main Lines |
| Nova | Mentor 4 EN B | Dokdo 30 40 gr/m2 | Dokdo 20 35 gr/m2 | Polietileno encapado |
| Gin | Carrera Plus EN B | Skytex 38 gr/m2 | Skytex 38 gr/m2 | Polietileno encapado |
| Ozone | Rush 4 EN B | Dokdo 30 40 gr/m2 | Dokdo 30 40 gr/m2 | Polietileno encapado |
| Independence | Geronimo EN B | Dokdo 20 35 gr/m2 | Dokdo 20 35 gr/m2 | Polietileno encapado |
| Advance | Iota EN B | Skytex 38 gr/m2 | Skytex 32 gr/m2 | Aramide desencapado |
| Macpara | Eden 5 EN B | Skytex 40 gr/m2 | Skytex 40 gr/m2 | Aramide Encapado |
| Skywalk | Chilli 3 EN B | Skytex 38 gr/m2 | Dokdo 20 35 gr/m2 | Polietileno encapado |
| U-Turn | Blacklight | Skytex 40 gr/m2 | Skytex 36 gr/m2 | Aramide desencapado |
| Niviuk | Hook 3 EN B | Skytex 40 gr/m2 | Dokdo 20 35 gr/m2 | Aramide Encapado |
| Gradient | Golden 4 EN B | Skytex 40 gr/m2 | Skytex 38 gr/m2 | Aramide Encapado |
| Triple Seven | Rook 2 EN B | Dokdo 30 40 gr/m2 | Dokdo 20 35 gr/m2 | Polietileno encapado |
A.J. Roque Santos