Dave Hyde wrote:
snip

Biplanes are a simple,
but inefficient, way of getting more lift from wing area
when an increase in span is not feasible. The are not, nor
in general are they intended to be, "low drag."


You mentioned the interference drag, so how far do wings
need to be vertically separated for a given airfoil
and stagger for this effect to be negligable?


*negligible?* Some *large* fraction of the span. At a minimum.
snip

Dave 'usenet wind tunnel' Hyde


Here's some pretty pictures of biplane wing pressure interaction from
the CFD tool that Peter Garrison sells:

http://www.melmoth2.com/texts/CFD.htm

B.S.