It's generally accepted to draw circle with a diameter of the wingspan,
centered at the middle of the wing. Do this once for every wing. Where
the circles intersect, they are interacting. I know of none that interact
positively. Some less negatively than others.

Another way to look at this is the aspect ratio. Aspect ratio is span
squared divided by the total area. More aspect ratio is better than less.

Anything less than 6 is not very efficient. Decent gliders are above 20.
I've flown one powered plane that was 10. It did very well. Flew like it
had a lot more area than it really had.



In article ,
(Jay) wrote:
Dave Hyde wrote in message
There are a lot of benefits to higher aspect ratio
wings that far outweigh the structural advantage of low AR
wings. Reduced drag is but one. Ever wonder why you
don't see any biplane sailplanes?

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? 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?

The fact that you don't see something commonly done says more about
the methods of development starting with what currently works, and
trying to make incremental improvement on it than anything else.
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).