Jono Richards wrote:
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.

Short answer: no.

The F1H competition model glider class has a maximum total *projected*
wing area limit, so some years back an ingenious chap built a bi-plane
version with no stagger so the projected area was the same as for a
monoplane but with halved wing loading. He omitted interplane struts to
minimize drag, but the model flew better with one wing removed.

The reason was that you get a lot of interference drag between the
wings. His interplane spacing was about 1.5 wing chords. Theoretical
calculations later showed that he should have used a spacing of at least
5 chords to avoid interference gap.

A sailplane with a 0.7m average chord would need an interplane gap of
3.5m and you'd still have a lot of drag from interplane struts. You'd
need step-ladders or a fork lift to rig it.

However, it would win the Ugly Trophy hands down.

--
martin@ | Martin Gregorie
gregorie. |
org | Zappa fan & glider pilot