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.
The reason for the reawakening of interest in them was that the performance of jet engines is quite sensitive to ram recovery. You get the best results from a jet engine if air arrives at the engine face with the full energy of the free stream. With few exceptions the engines of early jet airplanes were enclosed inside their fuselages. The exceptions were German. Although both their inventor of the jet engine, Hans Pabst von Ohain, and ours, Frank Whittle, had employed radial-flow centrifugal compressors, a Junkers designer, Anselm Franz, correctly reasoning that a small frontal area would be a virtue, developed a slimmer axial-flow design, the Jumo 004, which could be placed in external nacelles like those of the Me262. Nevertheless, the barrel-shaped centrifugal style, once it had secured a foothold in Britain and America, persisted for years after the fall of the Axis, and obliged airframe designers to house their bulky engines within the fuselage. (Franz, by the way, like Whittle and Ohain, emigrated to the United States after the war; he worked for Lycoming.)
Buried engines presented a problem. How was air, of which jets inhaled a great deal, to get to the engine without losing a lot of its energy? The earliest American designs used a bifurcated duct, called a Y duct, with inlets ahead of the wing roots; Lockheed's P-80 is an example of this approach. North American's Sabre and Republic's Thunderjet, along with a series of Russian MiG models, placed the intake at the nose of the fuselage and led air back to the engine, which was situated slightly behind the wing, through a long tunnel that detoured, in one way or another, around the tub in which the pilot sat.
Now, you could not send a lot of air at high speed through a long tunnel without robbing it of some of its energy, and Y ducts, though shorter, required sharp bends. Neither approach was clearly superior to the other. But it did occur to designers that a flush inlet, in addition to reducing drag by cutting down the cross-sectional area of the fuselage, might also allow for a gentler Y duct. The question was, how do you get air to flow energetically into a duct that does not present itself head-on to the free stream?
The original submerged scoops of Rogallo's study consisted of a sloping ramp between parallel sidewalls. The Ames Aeronautical Laboratory team investigating submerged ducts began with that configuration. The first results were disappointing. The duct captured only a little more than half the free stream pressure, as measured by a rake of small pitot tubes located a short way down the duct. The inlet performed best at a velocity ratio of .5-that is, when the rate of flow through the duct was half the rate of flow through a similar-sized section of the free stream. Some of the air flowing down the duct spilled out over the sides as it approached the duct entrance, forming a characteristic S-shaped wave. The investigators decided to try matching the ramp walls to the local streamlines, and thus the familiar-in fact, iconic-shape we now call the "NACA scoop" was born.
Reshaping the inlet had accidentally yielded a marked improvement in pressure recovery. They went on to test inlets with and without raised ridges along the edges of the ramp, and with variously shaped lips on the "roof" of the duct where it disappears into the surface. They tried different ramp angles and entry shapes; a seven-degree slope and a rectangular inlet with an aspect ratio of three to five seemed best. The end result was pressure recoveries of around 80 percent at velocity ratios from .6 to .8. The ducts seemed to be relatively insensitive to misalignment with the local flow. Considering that conventional nose and Y ducts yielded pressure recoveries at the jet engine face on the order of 65 percent, submerged ducts looked promising.
The conclusions of the Ames group's 1945 report were upbeat, but they included a caution that designers have been ignoring ever since. "The submerged inlet is essentially a high inlet-velocity-ratio type in contrast to wing-leading-edge and fuselage-nose inlets," they wrote. This characteristic limited its most efficient use to systems "that require only a small amount of diffusion, such as the internal ducting for jet motors of the axial-flow type." Submerged inlets were unsuitable for "oil coolers, radiators, or carburetors of ... reciprocating engines," the report continued, because "the required diffusion of the air and the range of inlet-velocity ratios is too great to give desirable characteristics at all flight conditions."
