Langley's investigations with rubber band powered model "rubber-pull aerodromes" began in April of 1887, while he with the Allegheny Observatory in Pennsylvania. He acknowledged his debt to Alphonse Penaud and his first models drew on Penaud's model flying machines for inspiration. By June, he had constructed two more model aerodromes, which turned out to be too heavy for flight. He continued to build and lighten his model aerodromes until by 1893 he had built nearly forty, some of which managed times aloft of from 6 to 8 seconds and distances of from 80 to 100 feet. During that period the experiments with models were conducted at Washington, D.C., near the Smithsonian Institution building. Langley had the assistance of Augustus Moore Herring, who designed and actually built a number of the Langley Rubber-Pull Aerodromes. The wing surfaces on most of the models were flat, although some featured curved wing surfaces.