The 1930's was a period of active research in aeronautics. The Northrop Alpha took advantage of the research of it's day with features such as all-metal stressed-skin semimonocoque construction, an NACA cowl and the development of wing-body fillets which are summarized below. Considerable experimentation went into the study of aircraft configuration and aerodynamic fairings. The figures show below illustrate the results of wind tunnel testing of the Northrop Alpha wing-body combination (no empennage).
The "no fillet" curves represent the Alpha without any fillet. The "optimum fillet" shown in the figures was developed in the GALCIT 10 foot diameter wind tunnel in May of 1931. This "optimum fillet" resulted in some drag reduction and a dramatic increase in maximum lift. The "optimum fillet" was flight tested in June 1931 and was utilized on the second Northrop Beta as well as on the Gamma's. The configuration of the "optimum fillet" and several other fillets that were experimented with are shown below.
The GALCIT research also examined high wing and "dropped wing" configurations. The results of this configuration study are shown below. The "dropped wing" configuration is illustrated in the figure above and refers to a continuous wing mounted beneath the fuselage with the wing/fuselage joint enclosed in a fairing. This configuration was of interest because it is allowed for maximum utilization of the fuselage interior for passengers or cargo.
This research was conducted by A. L. Klein in the GALCIT wind tunnel utilizing a 1/6-scale wind tunnel model of the "Alpha" (wing and fuselage only). The model was provided to the California Institute of Technology by the Northrop Aircraft Corporation. For further information see "Effects of Fillets on Wing-Fuselage Interface" by A. L. Klein which can be found in the A.S.M.E. "Transactions" for January 1934.