ORIGINAL: LouW
When two wings operate in close proximity such as a biplane, especially with the lower wing a little behind the upper wing, the downwash from the upper wing effectively reduces the angle of attack of the lower (rear) wing slightly. Thus when both wings are set at the same incidence, the upper wing will be carrying a little more than its share of the load, and will stall first. By setting the upper wing at a slightly lower incidence, the load is distributed a little more evenly and both wings will tend to stall at the same time.
This interaction between wings was explored extensively in the era when most airplanes were biplanes and the wind tunnel tests generally confirm it in numerous NACA reports.
Thanks for that explaination. I like the logic of the top wing with greater incidence so it will stall first, etc, however, on my models, they always seem to fly better with the top wing -1 to 1 1/2 degrees to the bottom. I could never figure out why.