Here’s a quote from Hip Pocket Aeronautics.

I’d like to point out the inherent stability of Darcy Whyte’s (Canada) popular SQUIRREL design. It has NO DIHEDRAL, no pylon, but has vertical UPRIGHT wing tip plates. In analysis when the wing banks an impending lateral SIDESLIP is instantly countered by a roll moment from the UPRIGHT TIP PLATES. Therefore an impending spiral is halted and the wing is brought back to horizontal. This is STABILITY.

The proof might be to try a wing with an UPSIDE DOWN wing tip plates. I fear this will increase the spiral tendency and a CRASH. I’d rather not try it. –Ding