Francis Melvin Rogallo is now known for the eponymous double-conical hang-glider wings that he developed for NASA in the 1960s. Long before that, and before NASA even came into being, he worked for the sainted National Advisory Committee for Aeronautics-NACA-and in the late 1930s conducted a wind-tunnel study of inlet and outlet designs. From this work, the results of which appeared in 1941 as Technical Report No. 713, Internal-Flow Systems for Aircraft, emerged a few maxims that seem obvious today-though not so obvious that they are not still frequently violated. Intakes, Rogallo said, are best placed at stagnation points, that is, points where the air encountering the moving aircraft momentarily comes to a stop against it. One big inlet is better than several little ones. Outlet velocity should match free-stream velocity as closely as possible. Outlets with projecting lips produce a lot of drag. And so on.
Along with a good many protruding inlets and scoops, Rogallo included in his study a couple of flush or semi-flush inlets, which violated the “stagnation” principle. Their performance was unremarkable. But a few years later, as World War II was nearing its end and the age of the jet airplane was dawning, NACA went back for another look at flush inlets.
