I visited Renishaw probes in Gloucestershire some years back and remember that they had a variable pitch RR fan engine somewhere in the facility. From what I recall it was similar in size to an RB211, and designed to reduce engine noise in built up areas.

That would make sense as the RB211 was the engine they were talking of using it on in the article I read, just wish I could remember what publication I read it in. I’m not sure if they ever test ran an engine fitted with it.