Paragliders have limitations

Why don’t paragliders fly in all conditions?

Simon Oberrauner (AUT2) performing during the Red Bull X-Alps at Zell am See / Austria on 29-June-2021. In this endurance adventure race athletes from 18 nations have to fly with paragliders or hike from Salzburg along the alps towards France, around the Mont Blanc back to Zell am See.

Paragliders are amazing portable aircraft that fit in a rucksack, but they can’t fly in all conditions. That’s part of the thrill of an adventure race like the Red Bull X-Alps – pilots have to take the forecast weather conditions into account when planning routes. Flying is so much faster than hiking, and less of an energy draw, so it can be worthwhile pilots taking detours to where conditions are flyable, even if it means going further overall, because it can be quicker in the end. 

What happens when the wind drops? It’s a myth that paragliders can’t fly when there is no wind; they can, quite happily. In completely still air paragliders with no wind and no thermals (rising columns or bubbles of warm air) they will simply glide gently to Earth. But they have a maximum speed, which is around 35-40km/h. If the wind is stronger than this, a paraglider flying into-wind will still move forwards through the air, but in comparison to the ground, it will be going backwards – not helpful for moving forwards in a race.

In addition, the stronger the wind is, the more turbulent it will be behind objects like mountains – the same effect that you can see in water downstream of a rock in a stream. It can be dangerous to fly in the lee side of these objects when it is windy. In turbulent air, flying is less pleasant, and the pilot has to concentrate hard to keep their paraglider open above their head and flying correctly. 

But what can’t they fly in the rain? The fabrics paragliders are made of, like Porcher Sport’s lightweight high-tech Skytex cloths, are essentially plastic and often have a waterproof coating, so they are waterproof. What’s the problem? People used to believe that a paraglider flying in rain will eventually fill up with water, as droplets enter the holes at the front, and collect in the back where there are no holes. This can tip it backwards and change its angle to the airflow, making it fly more slowly and it could eventually stall (stop flying). While this is theoretically possible, it is not very likely that enough water would collect inside for this to happen, especially with modern paragliders which have smaller holes in the leading edge. 

The problem with flying in the rain, is that water droplets bead on the surface of the cloth and disrupt the airflow over it. A paraglider is an aerofoil. It relies on the air flowing both above and beneath it, and its forward movement through the air, to generate lift and keep flying. If the airflow is disrupted it can stop the paraglider from flying forwards, causing what is known as a ‘parachutal stall’. The paraglider will not fold up or collapse, but instead of flying forwards thought the air, it will descend directly downwards at quite a fast rate, like a parachute.

For this reason flying in rain is a definite no-no, especially on high-performance paragliders the athletes fly in races like the Red Bull X-Alps. If it is raining, or if it is very windy or gusty, it is much safer for the pilot to stay on the ground, get the poles out and start speed-hiking. 

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