"Flying drainpipe" is a unique term that may be difficult to find synonyms for, but there are a few options that could work. One possible synonym could be "air-blown water pipe," which describes a similar object that carries water through the air. Another option could be "airborne sewage conduit," which describes a more specific type of flying pipe. A third synonym could be "aerodynamic plumbing channel," which emphasizes the pipe's ability to fly through the air. Ultimately, the best synonym for "flying drainpipe" depends on the context in which it is being used and the desired connotation.