"Having just discussed engine and prop performance at altitude it may be worth remembering that the maximum wind take off speed at altitude may not necessarily be the same when flying at lower altitude levels. The air is far more denser and thus will have the effect of flying in ‘stronger’ winds. At altitude the approach and landing may be faster and take off distance longer. Lets briefly look at glider construction, their basic overall design and where the potential problems lie. Initially the old design canopies had one line originating from every cell and continuing down to the risers.


Unlike the old round parachutes that were constructed from silk, these updated ‘wings’ are usually constructed from rip stop nylon or polyester. Take a closer look and not that every square inch usually is made up of tiny blocks and which have a weight of around 35 g`s per square inch upwards to 55g`s’square inch. Should a tear occur this inhibits the tear from running away and spreading. An external coating is applied that protects the material from UV degradation. It is this coating which wears off eventually resulting in especially the leading edge becoming porous (refer to ‘glider care’ for more about porosity, line shrinkage and safety). Many variations occur which include a laminated material called Mylar. This is usually a woven Tregal or Nylon over which an extremely thin (6 - 14 thousandths of a mm) of polyester is overlaid.

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a larger version of this diagram

NOTE the speed bar attachment in the attached photo. This attachment allows the A, B & sometimes C risers to be pulled down, effectively decreasing the angle of attack and allowing the wing to fly faster. If only A risers were pulled down, one runs the risk of the entire leading edge being pulled over and collapsing.

Risers are usually marked with different colours, however no standard colour exists that indicates a specific riser. Risers have a breaking strain of around 800kg`s plus.

Usually lines consist of either polyester, dyneema or kevlar (Aramide). In comparison to kevlar dyneema stretches and shrinks far more easily, which makes this construction far more susceptible to effects from moisture coastal flying or if the glider is packed away while moist. Kevlar on the other hand has a mineral fibre base, may be brittle nd the ends cannot be burned.

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The cell openings were large and the overall shape was rather square with a low aspect ratio (ratio of span to chord). This design was rather inefficient for non powered flight due to the large amount of drag, however were easy to control and fairly safe in the air. Today a lot of seated trikes still utilise this design for ease of pull up and stability. As canopies progressed they became longer and more elliptical (higher aspect ratio) ( See below illustration), cells became smaller (resulting in a flatter profile), lines thinner and originating from alternate cells via a cross over system and branching and re branching via a cascade which eventually joined the risers. joined the risers.

An experiment undertaken compared a kevlar and dyneema line of equal size. These were bent backwards and forwards until they broke. The dyneema broke around 20 000 times, while the kevlar after two or three thousand times. Let us examine a glider with 8A, 8B, 8C and 8D lines. In normal flight around 80% of all weight is distributed over the A and B risers, which means if our all up weight is 100kg`s, then 80kg`s are over these two risers(consisting in this case of 16 lines), giving a weight distribution of 5 kg`s/line. This really is very small when one considers that during the testing procedure the canopy is tested to 8g`s over the maximum all up weight that is recommended by the manufacturer, and typical individual paramotor lines are capable of around 80- 100 kg`s each. The long lines on a glider may be 1,5mm thick and have a breaking strain of 150kg`s, (short lines may vary around 0,9 - 1mm), however competition lines may be as low as 0,6mm with breaking strains of around 40-50 kg`s. This does not sound to be great however when one considers the cumulative effect of 400- 750 metres of line that may be on a glider, one may well understand it`s not such a big deal if a couple break.

Most canopies may be flown with paramotors. The important things to remember is that: A) Your wing loading increases, which may result in the pilot exceeding the maximum weight for that canopy. Usually the same paraglider may be flown with the motor, to start with, unless the clip in weight is way over the top. This means the sink rate will be higher, with more fuel being required to stay airborne (or just thermal more efficiently!) and the stall speed slightly increased.

When I look back on the early days our paramotors weighed 40 kg`s, the canopies were 26 square metres and our weights 70 - 80 kg's. If the pilot is heavy to start with, a small tandem glider usually 34- 40 square metres is sufficient to provide enough cloth above his head to support the extra weight. Remember light wing loading = more collapses, poor penetration and sluggish turns (good sink rate though).

While on the subject of pilot weights Vs canopy sizes, let me clarify this contentious subject when pilot weights for a set canopy Vs glide ratios are concerned. If two paragliders with the same glide ratios, however different weights (i.e. with a motor and without), start to glide from the same height, in still air, then the ......."

2nd Hand Canopies ideal for powered and non-powered flight from R4000.

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Dave Briggs, a legend among enthusiasts of extreme events in South Africa. It's not just about madness is a rollicking account of various exploits, sometimes hilarious, sometimes hair-raising, told in episodic, campfire-tales style, to bring- as he puts it- some fun into our lives, while it also asks and answers, the question "Why?" So fasten your seat belt and enjoy the raw energy of a sometimes bumpy ride with a pilot who is equally ready to face the challenge of the mighty Colorado River as to express his own frank views on the world he sees around him.


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