In article ,
Jay wrote:
Hey don't give up Dave, nobody said it would be easy. You haven't
convinced me not to run the model. But you have pointed out 2 things
I will look at more carefully:

1) Will the root/tip losses from 2 wings eat up any benefit from the
shorter/lighter spans? Are there tip treatments that diminish this?

the big killer is the interaction in airflow pattern across the two
surfaces.

putting a 2nd surface "above" a lifting surface _decreases_ the available
lift from that first surface. you have a 'compression' effect on the
'ram air' passing between the two surfaces, due to the constriction from
the _reduction_ in cross-section of the space between the two surfaces,
as you proceed from leading edge of the lifting surface back to the point
of maximum rise in that surface.

To minimize these kinds of effects, the space between the surfaces has
to be relatively -large-, the velocity of the air _comparatively_ slow,
*and* you need to let that 'built-up' ram-air effect bleed off in the
only direction it can go -- i.e., _sideways_, towards the tips of the
wing(s).

2) What wing configuration can be used that minimizes mutual
interference between the 2 lifting surfaces.

One simple answer to -that- one is 'obvious' -- place them infinitely far
apart. grin