Welcome to Part 9 in the Fundamentals of Aircraft Design series. In the previous posts we have covered the fundamentals of flight, studied the wing, fuselage and empennage, and have been introduced to aerodynamic lift, drag and moment coefficients. Now we put it all together and run through a preliminary drag estimation of a new aircraft design. Let’s get started!
The calculation below provides an estimation of the interference factor between a wing and a fuselage. This is used when compiling an estimation of aircraft parasitic drag as described in the tutorial on the Drag Polar. The wing/fuselage interference graph was originally published in USAF Stability and Control Datcom  and reproduced in Roskam Part VI Chapter 4.2 . The calculator provides an estimation of the skin-friction coefficient based on the digitization of the graph and an interpolation between datapoints; the methodology is described below the graph.
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