General informations about porosity of the paraglider materials
The material of the canopy
The material of the sail is a composite fabric made up of plastic fibre of various properties and thickness which is impregnated with additional synthetic materials after it has been woven. This complicated process is necessary to achieve in the course of its manufacturing the characteristics that are required for its durability during its usage
The required characteristics are the following
Ability to hold it’s shape
Great tensile strength
Reliable, consistent quality
In addition, certain additional requirements in the course of the production process have to be met by the manufacturers of the fabric, such as:
Should not loose colour
Capacity to be sewed and is processed
Capacity to be manufactured at a reasonable price
The fabric has to also keep such characteristics for a long period of time and in varying weather conditions. The thin composite fabric cannot comply completely with such requirements in the long run. The long molecule chain making up the fabric break due to high strain it is liable to, UV radiation, folding, creasing, great variation in temperature and humidity and inadequate storage therefore, with time the properties of the fabric deteriorate, the airflow through the fabric increases and its tensile strength decreases.
In order to ensure safe flying, the properties of the fabric of the sail have to be checked from time to time. This periodic testing can be done yearly or once every two years or alternatively with such frequency as necessary based on the time spent flying. Used sails are also recommended to be checked. These tests are required especially after the expiry of the manufacturer’s guarantee.
The regulations related to these technical checks vary from country to country. There are certain countries where the tests are strict and where only professionals can perform such test, but there are other countries where there are no such checks at all. The tests extend to the canopy, the lines and the risers as well.
Testing the flow of air through the fabric of the sail, that is its porosity is an important part of the inspection of the canopy.
Measurement of Porosity
The profile of the wing is kept at the required stability/rigidity by the pressure of the air flowing in through the cell openings and this pressure keeps the wing tight in the longitudinal axis of the wing as well. Air pressure around the wing ensures buoyancy. The fabric of the sail apart from being able to withstand great strain also serves as separation between the outside space and the internal high-pressure area.
• With the wear and tear of the composite material the air can get through the openings of the fabric. The cell pressure will be lower, the profile and the longitudinal shape of the wing is also modified which results in loss of buoyancy.
• The outbound air at the top sail may also cause another issue. It can decrease or detach the laminar flow created around the wing profile. This results in even further loss of buoyancy.
These two factors may jointly decrease the performance of the paraglider and may in critical cases result in the dramatic worsening of the flying properties of the paraglider.
Signs of deterioration of porosity
Worsening of the flying characteristics
More frequent collapses and slower re-opening of the canopy
Development of deep stalls after collapse of the canopy
With the spreading of this sport also spread the below method of measuring porosity (JDC measurement) that by now is internationally accepted. Thanks to this, manufacturers, users, teachers, buyers and sellers can use a unified measurement system when measuring porosity.
When using the JDC measurement system the time required for
A defined amount of air under normal pressure of 2,5 dl = (250 cm3) to go through
A defined size of the fabric of the sail of 38.50 cm2
Under a defined volume of negative pressure (vacuum) of -10 mbar = -10 hPa is measured in seconds (sec).
The more time is necessary for the defined amount of air to go through the better the porosity of the paraglider.
Lately the measurement factors of the given type of paragliders amongst others the minimum porosity value are included in the manuals. This should be taken into account when checking the porosity of the paraglider.
In conclusion the minimum porosity value of paragliders made for competing and for heavy duty paragliders set out by the manufacturers in the manuals should be increasingly taken into account.
If the minimum porosity value determined by the manufacturer is not available, the international standard values should be used.
Classification of measured values:
> 200 sec – Excellent
70 – 200 sec – Good
25 – 70 sec – Medium
18 – 25 sec – Poor
< 18 sec – Inappropriate
Form the perspective of flight security it is not necessary to complete the whole process of measuring in case of high values. Values above 500 seconds may be documented with the note of +500 seconds.
The decrease of the porosity values of the paraglider demonstrate the deterioration of the quality of the paraglider from year to year. We can follow the process with the yearly tests and can also predict the period of time until which the given paraglider is safe to be used from the perspective of its ability to let air through (airflow). In this case it is worth completing 2-3 tests all the way even if it takes a long time to do so. The porosity values measured on the bottom sail are generally poorer which is not a problem, nonetheless it is worthwhile to look out for the value limits here as well.
At the beginning of each day when a test is performed a trial test should be completed. Process of trial measurement:
1. Connect the measure to the digital counter.
2. Raise the measuring bar of the measure to start position.
3. Put the calibration disk to the head of the measure.
4. Shut it with the magnetic cap.
5. Start the measurement.
If the trial measurement is within the accepted threshold limits (+- 3%) then you can start measuring.
Watch our video about how to do an accuracy check
Process of measurement:
Spread the paraglider to the top sail as it is done before launching.
Raise the measuring bar of the measure to start position.
Place the measure under the top sail of the paraglider from the leading edge.
Reach into the cell where you wish to measure and smooth the fabric of the sail onto the head of the measure.
Place the magnetic cap through the cell to its place in a manner that the fabric of the sail is pressed to the head of the measure without wrinkling.
Then start the meter.
Repeat the measurement process on more cells.
It is worth doing 4-6 tests in case of paragliders that are in good condition, in case of paragliders that are in lesser condition 10-12 tests are recommended to be performed.
A few tips to measuring:
• Always take into account the average of the results of the tests, do not draw conclusions from 1-2 tests.
• Perform your tests around the “A” and “B” line endings.
• Perform a number of tests at the heavy duty parts of the sail, for example around both ears.
• If the sail is made up of a colourful material it is worthwhile to perform at least one test at all of the colours as the colour may affect the UV resistance of the fabric.
• Do not measure porosity in case of extremely high or extremely low air pressure. In such cases you should go out and grab a beer or go fishing instead.
• The ideal temperature for performing tests is between +15ºC – +25ºC.
• The relative humidity of the air should be maximum 60%.
As we cannot guarantee the same conditions as in a laboratory at least the below errors which can be avoided should be prevented.
Possible errors made by the person performing the test
• Disregarding weather conditions.
• Prior to measuring no performance of trial measurement.
• Allowing false air to enter the measuring area by not securing the thorough smoothing of the fabric on the head of the measure.
• Performance of tests at the wrong areas of the paraglider.
• Performance of tests with faulty measure.
Possible errors made by the person performing the test
• The rubber sheet on the head of the measure is worn-out or damaged and therefore false air enters the measuring area.
• The free movement of the measuring bar is not ensured.
• The measure is not vertical.
The measure depending on the frequency of its use is recommended to be recalibrated by its manufacturer every 3-5 years.
The Hello Team wishes you pressure of cells & soft landing…