WHAT IS POROSITY?
Porosity is the measurement of the amount of time it takes for a certain volume of air to pass through the paraglider canopy fabric. It works with a 100 mm water column pressure (10 mb).
The result is given in seconds needed for 0.25 litre of air to permeate through the cloth of 38.5 cm2 area under 10 mb pressure. It is then recalculated into standard units and 20 mb pressure with the formula: permeability [l/m2/min] = 5400 / time of measurement
After the test is performed, we use an industry-standard formula to derive a “score”.
Following table is used in interpreting the results:
0 – 27 – excellent “as new” cloth condition
27 – 54 – “ Good ” cloth condition
54 – 108 – “ Used “ cloth condition
108 – 270 – “ Well-used “ cloth, paraglider airworthy
270 and above – “ UNUSABLE ” worn-out cloth, safety cannot be guaranteed
We consider a paraglider with a porosity of less than 270 [l/m2/min] Airworthy.
HOW DOES THER POROSITY CHECK WORK?
The porosity of the cloth is checked using a POROZIT porositymeter ( JDC Calibrated ). This device measures the seconds taken to allow a specific volume of air to be sucked through a specific surface area of the cloth. The more porous the cloth the fewer the seconds. Porosity is measured at 4 points on the upper surface of the canopy, about 10 to 20 cm behind the leading edge. New cloth can score up to 500 seconds or more. The bottom limit, depending on the type of cloth and manufacturer’s specifications, lies somewhere below 20 seconds
HOW DOES POROSITY AFFECT MY PARAGLIDER?
A new glider will have very little porosity (assuming it meets all specifications). Because paraglider’s are designed to maintain pressure between the top and bottom surfaces as they fly, the porosity of a glider can greatly affect its pressurisation. The characteristics and behaviour of a paraglider will change as the material becomes more porous and the porosity increases. These changes can affect the performance of the glider – such as glide ratio, speed, recovery from collapses, handling turns, resistance to collapse and surging.
If a glider is more porous, it also means the material has degraded and it is weaker than it was when you first bought it. The weaker the material, the easier it is going to be a tear. Tears can when you are launching or landing near rocks or sticks.
HOW DO I SLOW DOWN POROSITY OF MY GLIDER?
The most common reason for aging glider material is repeated or extended UV exposure. Its important to always keep your glider out of UV sunlight when possible. Avoid the glider becoming moist or wet for any extended period of time. While any amount of water won’t immediately affect the glider, leaving it packed over night will not do it any favours. Always air you glider at room temperature if it has become damp or wet. Avoid exposing a wet glider to direct sunlight, this can also affect the materials such as the lines and stitching. If you have come
Paraglider Trim Tuning
Line trimming is necessary for all paraglider pilots looking to maximise performance, and in avoiding the inherent degradation of performance, handling and safety over time. There are various line materials in use each of which has their own characteristics. The length of the lines changes depending on the material, loading, temperature, humidity, dust ingress and UV exposure.
WHAT IS TRIM TUNING?
Trimming is a basic requirement for a good and safe flying wing, so we regard this as especially important. Trim Tuning is the adjustment of line lengths, based on millimetre-accurate measurement of all the lines between harness and wing. To do this we use laser equipment. The lines are re-trimmed based on the results of these line measurements. Virtual trim adjustment is simulated on the PC and finally corrected on the wing itself. The existing condition is adjusted to as near as possible the manufacturer’s original status, by means of varying the looping on the quick links.
HOW DOES TRIM AFFECT MY PARAGLIDER?
As your paraglider falls out of trim, certain performance characteristics important for safe flight are diminished. As the A’s & B’s stretch and the C’s & D’s shrink, your glider takes on a higher angle of attack. If you can picture how your speed bar decreases the AoA of your wing to increase its airspeed, an out of trim glider causes essentially the exact opposite effect. So the glider won’t inflate or kite like it should. It will also have diminished performance in the air as it flies at a slower airspeed close to the stall point. The most common reason for poor handling and performance is that lines go out of trim. This can affect safety by making spins and stalls more likely. A full measure of all the lines by the laser can determine if a retrim is needed. Bringing a paraglider back into trim allows it to fly as it was intended to.
WHEN TO TRIM?
Paragliders with as little as 20 hours of flight time will benefit from being trimmed. It’s recommended to get a paraglider trimmed after 100 hrs of flight time or two years, whichever comes first.
Changes of a couple of centimeters can affect the trim of a paraglider. Therefore, it’s crucial to be able to measure lines with accuracy down to the millimetre. All of the lines on your paraglider are measured using a laser device with a 5 kg load for the most precise measurements possible. After measuring all of the lines, the results are analyzed in a spreadsheet. Any necessary adjustments are then made using the quick links on the risers. Paragliders are trimmed to within 10 mm of factory specifications with a +/- 50 mm offset considered acceptable.
BENEFITS OF TRIMMING
Correcting the profile and trim of the glider, so that it flies as it was intended in relation to handling and speed. For safety, A better handling glider which handles turbulence has better pitch stability, turns, collapse recovery and normal stall speed. It is necessary in order to be competitive at a high-level competition. In order to compete in PWC and Cat 1 glider must pass line length verification tests. The psychological benefit of knowing that your glider is at its optimum.
*Many pilots at the top-level change the relative trim for the purpose of increasing the intended speed of the glider.