We used to do it off and on years ago. But the radios at the time were 72mhz FM and it has better range in the rain than 2.4ghz radios do. Then most of the radios had ball joint closed gimbals that tended to resist water getting inside more than the open gimbals do. You do have to use plastic propellers as the rain acts like coarse sandpaper on the wood propellers and will prompty tear them up. If the rain intensity increases past a certain point then the engine tends to have problems as it is now ingesting water droplets and that affects how it runs. But where I live and fly now, rain seldom comes without lightning, and it is most unwise to be standing out in a open field with lightning about.