Originally Posted by gasax
If you have to revert to a legal arrangement them perhaps you should not be party to this arrangement?
Whether desired or not, a legal "arrangement", that is, a contract, arises by virtue of purchasing the aircraft. I would certainly hope so if I was handing over the large amounts of money involved!
Of course, if the agreed terms aren't recorded in a signed writing, those terms may or may not be enforceable in a court of law. And if you have a dispute or one or more party experiences financial difficulties, you may find yourself up scenic Proverbial Creek without a proverbial.
If you do have a writing governing the parties' various obligations with respect to the aircraft and each other, that helps. And it's even better when that's competently drafted so that the terms intended are clear and enforceable. Professional assistance would be wise, unless this aircraft is extremely cheap or the OP is extremely wealthy.