After booking a nine-figure profit by riding the meme-stock craze for old-school bricks-and-mortar businesses, hedge-fund manager Jason Mudrick was looking for his next big bet. He was as surprised as anyone that he settled on flying taxis. 

Mudrick specializes in distressed companies, often established businesses that have fallen out of favor. But when late last year he became the biggest shareholder of a British aerospace startup and forced out its founder, he was making a long-shot play on a futuristic industry that for years has seemed just around the corner—yet still hasn’t arrived.