Bmathews, the point of a biplane is the fact the wings wingtip losses interfere with eachother, thus they have a lower total induced drag for same span and lift in comparison to a monoplane. The amount of interfernce cuased by a biplane is based on the Gap to Chord Ratio. It is definetly not a straght forward problem. At low speeds a biplane is superior to a monoplane when constrained to span. If one is not constrained by span, then no, the monoplane wins out easily. The only reason my bro and I did not go with the biplane, theoretically it could pick up more than our monoplane design, was becauase of a dearth of information on the interference effects of a high lift airfoil such as the S1223 or the Eppler 423 or say a ClarkY with slats and compound flaps. We did not have access to a large enough wind tunnel to test properly so we had to modify the data from 1929 era NACA white papers. As it turned out, if the interference between the wings was only 84% or highger the biplane beats the monoplane. When competition arrived, the only reason my brother and I won is because our plane was so much lighter than the biplane designs at the competition that we picked the same weight minus a pound and won because we were able to accurately predict our payload whereas the biplane guys were not able to do so. In the end the competition showed that a biplane lifted more, in fact if they had made a wider Gap/chord Ratio wing they would have been able to pick up an extra 5-10lbs!!!! in comparison to our monoplane, but obviously they did not have an excel program like I made that allows them to modify their parameters easily. Oh yea with the biplane heavy lift design the maximum Cl needed to achieve this extra lift was only 1.7 in comparison to the 2.2 of the S1223, thus lower profile drag. We had tall winglets which allowed us to pick an extra 2-3lbs. Biplanes can also have winglets which also allow them to cut their induced drag just like a monoplane.

The whole point of the origional post was a span limited design.

In short, ONLY on SPAN limited airplanes: BIPLANE >> MONOPLANE @ low speed
Otherwise Monoplane blows biplane away

B