Archer Aviation's stock has risen sharply over the past year, but it could fly even higher if its air taxi service gains traction.Shares of Archer Aviation (ACHR 0.29%) have fallen more than 33% from their high-water mark over the past few months. That's the second drawdown of that magnitude in the past year. And yet the stock is still up more than 150% over the past 12 months. That volatility is important to keep in mind because if you can't handle a bumpy ride, you won't want to buy this aviation startup ...