Stall stick position
The aircraft is a microlight called the "Pegasus XL-Q". A particular multi-coloured undersurface they called the "Rainbow-Q" version was found, due to the dieing process altering the stiffness of the lower surface fabric to suffer a divergent DR at high speed, which on a more soberly coloured aircraft remained (just) neutral, becoming convergent at lower speeds. It also I found the couple of times I have flown one, had about double the roll-mode time constant compared to the "standard" wing.
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