As Martin said, "interference drag" between the two wings would be the main
culprit. High pressure under the upper wing migrating to the low pressure
area above the lower wing. Some biplane designs try to counter this with
more spacing between the wings, or more off-set (think Beech Staggerwing).

Early aircraft design embraced multiple wings partly because the box
structure was easy to make strong using external wires and struts, the
additional drag wasn't such a big deal, as they didn't fly all that fast.

bumper


"Jono Richards" wrote in
message ...
A thought came to me the other day...

Now, I have never seen a bi-wing glider, but was thinking,
could a bi-wing glider of, say 15m wingspan, effectively
have 30m performace?

This is probably a stupid uneducated question, and
I would imagine that it hasnt been done because it
wouldnt work! But its certainly something I am interested
in.

Think of it...a bi-wing ETA giving a total of 60m performance!


JR