(by Ivelin Kalushkov)

There are many parameters that can determine the condition of a wing’s fabric, but there is probably no better parameter than the air permeability of the material.

Before we get to know the specifics of measuring porosity and its importance, it is important to understand how porosity affects paraglider safety and why it is actually so important.

Porosity and air permeability

The porosity of an object is an indicator of the ratio of empty space in a given volume to the building material itself. The porosity of a woven fabric is determined by the ratio of the number of threads in a given area to the distances between them – or the so-called “pores” in the material itself. The more pores there are, the higher the percentage of porosity the given material has. The fewer the pores, the denser the material. In reality, a fabric with 100% porosity does not exist – this is considered empty space. A fabric with 0% porosity is not actually a fabric but a dense polymer.

Fig. 1 – The fabric on the left has a lower porosity than the one on the right due to the smaller pores between the threads

The porosity of a material depends on the size and number of pores in the material itself – for this reason, the porosity of paragliding fabric is variable. As we will review, during the aging process of a fabric, new pores open and existing ones change their size.

When we measure “porosity,” however, in most cases, we are actually measuring the air permeability of the fabric, which indicates how much air passes through a certain area in a certain amount of time. Since permeability is directly dependent on porosity itself, the two concepts are often confused, and somehow the word “porosity” has established itself as the main term in fabric measurement. It is important to note, however, that regardless of which of the two parameters we use, we usually look for low values when talking about paraglider fabric – low porosity = low air permeability.

In practice, permeability is measured using devices called porosimeters. Porosimeters measure how long it takes for a given volume of air to pass through a certain cross-section of the fabric under defined pressure. Often, when measuring wings, people give the value of the measured permeability as the time recorded by the device. The higher the recorded time, the slower the specified volume of air passes through the given cross-section – respectively, the LOWER the porosity and air permeability.

Fig. 2 – Porosimeter JDC – MK1

However, giving time as an indicator is WRONG PRACTICE. There are many brands of porosimeters on the market – each of them uses different volumes of air and different cross-sections through which the measurement is made – respectively, the measured times on the same fabric can vary significantly. Quality porosimeters are calibrated devices with a certain coefficient, for which the permeability is calculated in real units of measurement – liters/m²/minute. For example, the most popular calibrated porosimeter, the JDC MK1, measures the time for which 250 ml of air passes through a cross-section of 38.5 cm² under a pressure of 10 mBar. To obtain the exact measurement in liters/m²/minute, the coefficient of the device, which in this case is 5400, is divided by the time read by the device.

If you are measuring the porosity of a wing, always request the measurement in a universal unit of measurement, as the measured time in seconds can vary by several times between different instruments.

Why is fabric air permeability important?

In paragliders, as in all other aircraft, a large part of the lift is created by the air flowing around the wing at different speeds above and below the upper surface. The different flow speeds create areas of different pressure above and below the wing and, accordingly, create lift. The area of reduced pressure above the upper surface of the wing is essential for both the stability of the wing and its flight parameters. The more laminar the flow over the upper surface, the faster it moves – the bigger the pressure drop above the wing, and accordingly, the better the lift created by the wing itself.

Fig. 3 – Normal porosity

During flight, some of the air passes through the wing fabric and disrupts the uniformity of the flow over the upper surface. The larger the volume of air that passes through the fabric, the more the flow over the upper surface is disrupted, and the lift force decreases. With high porosity, the fabric begins to let too much air mass through its pores. This air mass does not allow the flow over the upper surface to create surface tension – that is, the flow cannot “stick” to the surface itself and is “blown away” by the vortices of the passing air. This creates a prerequisite for the paraglider to go into a parachutal stall or full stall, even without the intervention of the pilot and without slowing down the glider.

Fig. 4 – Increased porosity
Fig. 5 – Critical porosity

To reduce the porosity of a fabric, manufacturers use two main manufacturing practices.

-tight weave of the fabric – the denser the threads of the fabric, the less air they let through

-coating – a special polymer coating is applied to the threads to fill the pores and reduce porosity.

Coating

The coating aims to protect the fabric threads from environmental influences, and to lower porosity.

Fig. 6 – Electron microscopic photograph of fabric with different coatings at magnification x120 (top) and x500 (bottom). In 
the middle, the coating layer is visible, tightly covering the threads.

In paragliders, the main cause of overall material aging is actually the aging and destruction of the coating layer of the fabric.

The coating of the fabric is laid in several stages. The threads are coated before weaving, and additional layers of coating are added after weaving the fabric. The different types of coatings also determine the qualities of the fabric – single coating, double coating, UV resistant, water-repellent, wear-resistant, etc. Usually, large paraglider manufacturers select fabrics with higher quality and more protective coatings.

It is important to note that the process of coating the fabric involves thermal hardening of the polymer – “baking”. The fabric is placed in autoclaves and baked at high temperatures to penetrate into and between the threads. For this reason, it is impossible to renew the coating of the fabric at home. There are several products on the market that claim to be able to restore the low porosity of the fabric. Practice shows that fabrics treated with these products improve their porosity values ​​for just a few flight hours, and sometimes even for just one flight, after which the porosity values ​​return to high levels, and sometimes even deteriorate.

There are many factors that can affect the aging of the coating – exposure to UV rays, mechanical deformations, exposure to chemicals, extreme temperatures, etc.

Perhaps the biggest factors in the natural aging of the material, however, are ultraviolet rays and mechanical deformations of the fabric.

Porosity level

When weaving the fabric, unfilled pores remain between the threads of the material – these pores are filled by the coating.

Fig. 7. – Micropores in the coating layer of the fabric

After the paraglider is sewn, during the first flights of the wing, a large part of the threads undergo a process of adjustment. The fabric stretches and contracts with different coefficients in different places on the wing. Some of the threads tighten, and others relax – this leads to the destruction of the coating in places where the pores of the fabric become larger as a result of the adjustment.

Fig.8 – Normally positioned threads
Fig. 9 – Re-aligned threads

The porosity of a paraglider changes along a non-linear curve throughout its life..

During its first 50 flight hours, the paraglider can lose 10-30% of its initial porosity, after which it levels out and maintains relatively stable porosity values for most of the wing’s life.

As the coating ages, it becomes more “brittle” and begins to separate from the fabric fibers, leading to a sharp increase in air permeability.

During the last 50 hours of the wing’s life, the permeability increases exponentially until the wing becomes unusable.

Fig.10 – Measured porosimeter time versus wing flight hours
Time measured (seconds)Air-permeability (lt/m2/min)Cloth condition
>200< 27Excellent
100-20027 – 54Good
50-10054 – 108Used
20-50108 – 270Havily Used
0-20270 <Not Airworthy
Tab. 1 – Reference values when measuring with JDC MK1 porosimeter

Paragliders approaching critically low porosity values often show signs of deterioration.

The most characteristic signals of increased air permeability are usually a deteriorated glide ratio, increased descent rate, inability to flare during landing, or difficulty inflating the wing. In practice, the coating ages differently in various parts of the wing or with different colors.

Usually, when the leading edge coating deteriorates, the wing becomes difficult to inflate, but the glide and descent rates do not change significantly. With reduced values of the main part of the wing, or generally speaking, the part behind the leading edge, the wing has a tendency to go into parachutal stall, with an increased descent rate and correspondingly reduced glide ratio observed.

For color:

When mentioning color, it is important to note that different colors have different aging coefficients and UV resistance.

It is important to distinguish between the UV resistance of the fabric threads and the UV resistance of the coating. Some colors, like white, reflect a large amount of light, including UV rays, while others, such as black, absorb most of the light spectrum.

Colors that reflect more light cause UV rays to pass in both directions through the coating – resulting in faster aging of the coating layer, but at the expense of this, the fibers of the fabric have an extended life. The opposite effect is observed with colors that absorb UV rays – a longer-lasting coating, but faster aging of the fabric fibers. Since the coating provides UV protection, the strength of the coating is the leading factor for the strength of the fabric. This is also the reason why, contrary to expectations, one of the most resistant colors for paragliders is black. On the other hand, one of the least durable colors is white and light gray.

It is important to note that differences in UV resistance among colors have been reduced with modern polymer coatings.

Fig. 11 – The white color is reflecting part of the light flux back through the coating layer, while the black color absorbs the light flux.

Glider Care

To preserve the porosity of our paragliders and to extend their lifespan, we ​​must take into account the factors that can damage the fabric coating and worsen porosity values.

Such factors are:

  • Exposure to UV radiation
    • The biggest enemy of the paraglider is UV rays which destroy the structure of the coating and prematurely age it. With the exception of standard sun exposure during flight, we should always aim to reduce excessive exposure.
Fig.12 – On the right – after exposure to UV radiation – the fabric fibers change their structure and cause a decrease in the density and stiffness of the fabric

Don’t leave your paraglider outstretched while waiting for the conditions!

  • Mechanical influences
    • Dragging the fabric along the ground, folding in the dust, sand or on abrasive surfaces disrupts the physical integrity of the coating and leads to its rapid destruction.
    • Folding the glider too tightly, excessive bending or flexing of the fabric, causes ruptures in the micropores of the coating and deterioration of porosity.
    • Wing overloading – overloads within normal limits above the weight range do not cause significant changes to the fabric and the coating layer. Extreme overloads during aerobatics cause dynamic contractions and stretching of the coating layer and fabric fibers which age the coating prematurely or can cause ruptures of the micro pores of the coating.
Fig.13 – Whitening and destruction of the coating as a result of ageing. Cracking of the coating layer in the points of pressure.
  • Humidity
    • Moisture itself does not damage the coating. However, storing the glider in a humid environment or folding it in a damp or wet state can lead to the development of mold that literally destroys the coating layer. Always dry a damp glider in the shade before folding.
    • NEVER fly the paraglider to dry it. Water is not compressible! When flying with a very wet wing, water droplets pass through the pores of the fabric and exert greater than standard pressure on the coating and fabric fibers – this can cause the micro-pores of the coating to rupture and the porosity of the wing to deteriorate.
    • Saltwater is more aggressive to the coating from a chemical point of view. After drying, the residual layer of sea salt crystals acts as an abrasive that further destroys the coating.
    • Freezing/icing of the wing in flight causes damage to the coating integrity. Icing creates concentration of pressure in areas of the fabric and can cause rupture of microfibers.
  • Chemical elements
    • All types of solvents, petroleum products and aggressive chemicals can corrode the coating layer and cause extreme deterioration of porosity. In Fig. 14. below, you can see traces of destroyed coating on a wing that was spreaded over spilled fuel.
Fig.14 – The light spots on the fabric indicate a destroyed coating due to a chemical solvent.

NEVER use chemicals to clean the glider.

  • Temperature
    • Extreme temperatures can change the structure of the polymer and cause the coating to break down. Extremely low temperatures are rarely a problem. Theoretically, the coating starts to become more brittle at temperatures below -20/-30 degrees Celsius, fortunately for us these are rarely the temperatures at which we fly.
    • Extremely high temperature is easy to reach however. The temperature in a closed car where you have left your wing on a hot summer day can easily reach values ​​above 60 – 70o C. At these temperatures, the polymer molecules begin to change. The coating begins to change its structure, which can shorten its lifespan by over 30%–40%.
Fig. 15 – Characteristic “dotting” of the fabric due to overheating and difference in shrinkage of polyester and polyethylene

Always protect the wing from direct flame – paragliders burn at around 300 euros per second!

Words of wisdom…

The porosity of a paraglider is an important factor determining the qualities of the wing and flight safety.

Measuring the porosity of a paraglider gives the most accurate information about how much “life” is left on a wing. Remember to always use the correct units for porosity when measuring – don’t just look at “seconds”.

To enjoy your paraglider for many years – never do to the wing what you wouldn’t do to yourself .

Take care of your paraglider and it will take care of you!