RCM&E had a foamie twin rotor, a bit like the DB one, as a free plan Jan.2006 edition .Think it might be OK indoors. I`ll have a look through the "loft insulation" as She calls them. The designer has done a single rotor version.
http://www.rcmf.co.uk/4um/index/.php?topic=26384.0 The German magazine FMT has a downloadable indoor cutie called Whopper that Jurgen Heilig has caught on video , see Paul`s post further down.