Jay wrote:

You bring up a good point about sailplane wings having the best L/D
ratios. But why not take each of those sailplane wings and put one
over the top of the other?

Because a single wing of equivalent area but longer span will
be more efficient in terms of drag. 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.
Some airplanes are able to use the interaction for benefit,
but it's usually for things like lift improvement at high
AOA. Drag reduction requires doing things at the tips to
make the wings 'think' they are longer and thus have a higher AR.
Just slapping another wing on there ain't gonna do it.

Sometimes the rat maze requires the rats (RAH) to back up and choose
another path, which in the short term means he is actually retreating
from the cheese (speed).

And knowing where to depart from the maze requires either a
foundation in basic principles or blind luck. Given the well-
known relationship between drag and aspect ratio, these principles
lead most people *away from*, not *to* biplanes for drag reduction.
How 'bout a challenge: I can show you mathematically and using physical
relationships why (without aerodynamic treatments like winglets or
conjoined
wings) two wings will produce more drag than a single wing of equivalent
area but higher aspect ratio. Your challenge: Prove the physics wrong.
Show how a second wing will result in less drag. Show me the math.

Dave 'usenet wind tunnel' Hyde