Some wings, when combined with some motors, will oscillate. You swing back and forth in shallow little banks with no pilot input.

It’s caused by a variation of Loaded Riser Twist where one riser has more load than the other–a fleeting condition that happens usually while running on the ground, but also momentarily at the end of a left-right swing. The thrustline turns to point the other way and pushes in the other direction, causing it to worsen slowly.

I see this behavior in about 10% of the wings I try. It’s only objectionable on a very few and is easily dampened by weight shift alone.

Heavier pilots experience it less. Smaller wings are worse. On one model, a lightweight 15 meter hike-n-fly, the oscillation diverged (got worse) and brake control was necessary after only 5 left-right swings.

Loaded Riser Twist

This phenomenon was shown in Master Powered Paragliding 3: Inflight Precision.

During launch, if the wing goes to one side while you’re still on the ground, it will cause more load on the more vertical riser. The motor will want to twist you slightly more around that more loaded riser. So if the wing goes right, your left riser loads a bit more and you’ll get twisted slightly left.

This effect can be put to use. If your motor causes torque twist to the left (body left, wing goes into right bank) then make sure the wing is slightly to your left at liftoff. For one thing, if the wing is to the right, it ADDS some left twist already and would make matters worse.