Need the tools

I'm not sure if this is the right section to be posting this in, but oh well. I've always wanted to toy around with the tools used in desiging aircraft for the flight sims, but haven't wanted to pay any money, and isn't so sure where I can get any good tools. If anyone knows where to get tools where I can just download, and are somewhat simple (ok, maybe not simple) please reply and tell me where.