Hi Jerry,

The Kress Fan unit (for .21-.25) will do 4 lb of thrust. The diameter is about 3.5" (90mm). Although they are quire expensive ($140 for the fan unit alone+ engine). Do a search on RCU for Kress' phone number. They don't reply to email to often anymore.

Russ
Oakdale Aircraft
www.oakdaleaircraft.com