Archer Aviation (ACHR 0.35%) has built a futuristic airplane that can now be seen buzzing around test sites. Buzzing is an apt description because the small passenger planes take off and land vertically and are meant to operate only over short distances. They are air taxis, and Archer Aviation is hoping they become a very big business.If the company is right, the stock could help you become a millionaire. However, there are a few more steps to go before air taxis actually take off. Here's what you need to k ...