"Girls Gone Wild" honcho Joe Francis dropped almost a quarter mil on a helicopter 10 years ago ... but he still has NO IDEA where it is -- and now, he's suing the guy who's allegedly behind the mystery.
According to papers filed at the LA Superior Court, obtained by TMZ, the GGW guru went halfsies on a Bell 206 Long Ranger helicopter in 2002 -- with some rich dude named Mohamed Hadid.
According to the docs, Joe paid $226,000 for his share -- but claims he never signed any paperwork or asked any questions, trusting Mohamed with all the small details ... like location, full price, and working condition. Joe claims he doesn't even know who currently holds the title to the chopper.
But now, Joe wants his $226K back -- claiming he was prevented from using the flying machine ... and worse, believes it was secretly sold behind his back. Joe says he has no clue where the helicopter currently is.
Joe is also seeking unspecified damages. Attempts to reach Hadid were unsuccessful.