A Note on Fuselages of Low Drag

Abstract

T note represents some results of a study of fuselage drag made by the Aeronautical Laboratory staff at the Massachusetts Institute of Technology in cooperation with the United Aircraft Manufacturing Corporation. The object was to see whether it was necessary for the fuselage drag of a large transport plane to exceed materially that of an form. When considering the results one must remember that the full size fuselage will be well over 10 feet in diameter at the largest part so that an adequate windshield need extend only a small amount above the smooth contour. If the models were of a smaller fuselage the windshields provided would be inadequate. The width and height of the fuselage for a transport plane are fixed by the arrangement of passengers or cargo. The length is determined by the general aerodynamic design of the airplane. The fineness ratio, which may be defined exactly as the ratio of the length to the diameter of a circle of area equal to the maximum cross section of the fuselage, will usually be much greater than the ideal for the minimum drag, and greater than now used for hulls. With a fixed cross section, minimum drag would be obtained by using the shortest shape which avoided separation of flow from the tail. Any increase in length increases the drag since the surface is increased and from tests the fineness ratio giving the least drag for a fixed crosssectional area is about 3. Airplane fuselages will have fineness ratios of from 7 to 9, and therefore must have appreciably higher drag coefficients than an ideal streamlined body, even though a perfectly symmetrical shape could be used. To provide a basis for comparison, the first model for this study was made of an shape stretched longitudinally so that the fineness ratio was 7:3. This model had circular cross sections throughout and a pointed tail. It will be called the airship model. (Model I) The first modification was to droop the nose, a start toward provision of a pilot's cabin, and to raise and deepen the tail. The principal sections were still circular. (Model II) The next change was to add a cabin and windshield providing sufficient outlook for the pilots. The windshield is conical, while the cabin roof is rounded and both fair smoothly into the surface of the fuselage. (Model III) Airship Shape Model I