ill probably catch flack for this but get some BLUE locktite and slap on the crank. that way the nut will not spin off again.. and make sure you put a decent amount of torque to it ... and let it sit for a while so the loctite sets up.. and youll be in like flint!!!! works wonders for me.. i locktite everything i get my grubby little hands on!! hehehehe

congrats on your first few flights.... its a great hobby to have!!!