PS ; I believe that the Lancaster bomber was one of the few, if not the ONLY, aircraft of WW2 to be powered by BOTH types of engine.At a time of shortage of the RR Merlin engines, needed for Spitfires,Hurricanes, etc, and until US-built Merlins arrived in sufficient numbers, many Lancasters were built with RADIAL engines ; Bristol "Hercules" sleeve-valve engines , I think ?? (I am writing this from memory !!)