Induced drag increases rapidly as biplane wing gap decreases. A convenient yardstick is the equivalent monoplane aspect ratio, since as wing gap decreases, induced drag increases, very much the way it does as aspect ratio is decreased. Aerodynamically, gap to span ratio is the governing factor; gap to chord ratio is secondary.

At a gap of 0.2 times span, about as large as normally encountered, the Monoplane Equivalent Aspect Ratio correction factor is 1.49, making the bipe's induced drag equal to that of a monoplane with an aspect ratio 1/1.49 times lower than geometric. A bipe with geometric aspect ratio of 6.0 would have induced drag equal to that of a monoplane with aspect ratio of 6.0/1.49, or about 4.03.

At a gap of 0.15 times span, things get a bit worse. The M.E.A.R correction factor is 1.57, giving our bipe an equivalent aspect ratio of 3.82.

Cut the gap down to 0.10 times span, and the M.E.A.R. factor becomes 1.67; our bipe has a monoplane equivalent aspect ratio of 6/1.67, or about 3.59.

Since induced drag is inversely proportional to aspect ratio, a bipe with gap of 0.1 times span will have induced drag over 12% higher than one with gap of 0.2 times span. Maximum lift also decreases as gap is reduced. At a gap of 0.1 times span, maximum lift is just over 90% of that with gap 0.2 times span. Minimum drag also slightly increases as gap decreases. At a gap of 0.1 times span, minimum drag is about 2.2% higher than with gap of 0.2 times span.

Bipes can work reasonably well with smaller than usual gaps. There is no ideal gap, since structural weight and parasite drag increase along with gap, and little overall performance increase should normally be expected with a gap much over one chord, for a typical aspect ratio. Lowering the upper wing, which decreases the gap, can also improve aerobatic line holding, and promote a gentler stall.

Source: "Biplane Secrets" part 2, by Carl Risteen, from "How To's" Vol. 2, by Air Age Publications.