SUNDAY, DECEMBER 17, 2017

Myth #3: Dynamic Reflex Paraglider Reactions To Collapses Induced

Myth #3: Dynamic Reflex Paraglider Reactions To Collapses Induced By Test Pilots Proves That The Paraglider Can Be Very Dangerous When Flown In Turbulence.

Test pilots are forcing collapses (frontal stall, 50% asymmetric, 75% asymmetric) by pulling A-lines or risers. Typically it’s an easy job for the test pilot, since classic paraglider’s ARE inherent vulnerability to collapses (especially when accelerated) requires but a slight pull on the lines to cause a massive frontal stall.

Quite the contrary for the reflex paragliders. Due to considerable resistance to collapses of a reflex profile, test-pilot has to pull with all their might for several seconds. Meanwhile, already fast-flying Reflex Paragliders accelerate even more, as pulling A-risers diminishes the wing’s angle of attack. Eventually a large and sudden collapse occurs, then reopens spontaneously and dynamically by the paraglider on its own. While unassisted reinflation is a welcomed behaviour, the dynamic exit is regarded as a sign of instability of the wing and thus disapproved.

Such judgement is wrong for two reasons.

First: any wing (including those considered very safe) flying at 50 or 60 km/h will exit the collapse dynamically, therefore gaining considerably worse marks – even as it would be still the same, safe wing.

Second: in real flying a collapse is caused by a stream of sinking air (turbulence) entered by a paraglider. Even hypothetically speaking flying at 35 km/h (and Reflex Paraglider fly’s much faster than that) and a maximum chord of 3.5 m (in reality much less, especially at wingtips) we have only 1/3 of a second between the leading edge entry in turbulence and the moment when the entire profile is covered with it. It means that in real life aerodynamical force is acting on leading edge for only tenths of a second, since later on it engages the entire canopy and is a rather subduing collapse. However, we have to remember that the reflex-profile is always automatically adjusting its angle of attack, further limiting the exposure.

FACT: Reflex Paragliders reaction to collapses induced by test-pilots during certification procedures or SIV courses does not reflect real life behaviour of the paraglider. A wing of fully reflexed profile will acknowledge even strong turbulence by slight acceleration and upsurge (when faced head-on) or a swing and slight turn when it is hit directly on a side.

Attention: another situation occurs when a paraglider enters rising air. There is not much difference between reflex and classic profile behaviour – Violent turbulence can “stop” a paraglider in flight and bring it to parachutal stall. Testing such a case would require simulating it on the highest possible AoA (slowest trim setting). Usually such stall is entered by letting out of a full or B-stall slowly, and then time is measured in which the paraglider returns to normal flight – the sooner the better of course.

All in all, certificating of modern Reflex Paragliders is a broad topic which deserves an article of its own, if not a number of them.