Porosity is one of the main properties that should be checked when your glider gets its periodic inspection. Porosity is a measure of how much air leaks through the fabric. It is usually given in seconds. If the air flows through the fabric too quickly, the glider has aged a lot and its flying properties deteriorate. Anything above 100 seconds is fine. At 25 seconds we should be aware and 10 seconds or less means we should not fly the glider anymore.
Porosity is also used as a measure of the second hand value of the glider. And that is where problems start to arise, says Fred Pieri, designer at Ozone. It may seem simple: let's say a new glider gets a value of 1000 seconds and after one year it gets 500 seconds. Another glider has a porosity value of 100 seconds and after a year it has 91 seconds. "It feels like the first glider has aged much more than the second," says Fred, "But in reality the porosity has degraded by the same amount. Expressing porosity in seconds is contra-intuitive and leads to misperception." To indicate the age and value of a used glider, the porosity is often rated on a linear scale from 10 to 1 based on the porosity measurement in seconds. This example shows that this gives a false indication.
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This Design insight was published in full in Cross Country magazine 255, February 2025.