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Cluso, if 1 fan is used, then it is likely that some of the thrust would need to be vectored to counter the torque. This is partially why I suggested more than one fan (will probably need the additional thrust anyways). The design I have in mind vaguely resembles the drawing, it's just my attention and patience for MS Paint is limited. The bottom the the plenum is sealed, so all thrust is directed to each "arm".
Hanno, the propeller design would most likely be easier, but it's also been done many times over. To be fair, though, my inspiration for the design came from this concept: http://www.aerobiker.com/
Graham Stabler said...
markaeric, fans are not really very good at creating static thrust so I think that design would struggle.