J shape?...isn't that what the Lenz 2 is?
http://www.windstuffnow.com/main/L2_flying.JPGMaybe the round part 1/2 of 8 inch stovepipe
4ft. height and the flat part 1/4 inch marine
plywood ?.....3 blades about 3 foot diameter ?
Make a study of his design...I'm pretty sure
it's pretty close to what you have in mind maybe
produce even more than you're expecting....I
guess the angle of the blades is the critical
part from what I've concluded.