Curtiss Wing and Double Cambered Sections. An old type of Curtiss wing is shown by Fig. 15. It is very thick and an efficient wing for general use. It will be noticed that there is a slight reflex curve at the trailing edge of the under surface and that there is ample spar room at almost any point along the section. The nose is very round and thick for a wing possessing the L/D characteristics exhibited in the tests. The conditions of the test were the same as for the preceding wing sections.
Fig. 16 shows a remarkable Curtiss section designed for use as a stabilizing surface. It is double cambered, the top surface being identical with the lower, and is therefore non-lifting with the chord horizontal. The force exerted by the surface is equal with equal positive or negative angles of incidence, a valuable feature in a control surface. In spite of its great thickness, it is of excellent stream line form and therefore has a very good lift-drag ratio. At 0° angle of incidence the resistance is at a minimum, and is much less than that of a thin, square edged, flat plate. This double cambered plane reduces the stay bracing and head resistance necessary with the flat type of stabilizer surface.
Fig. 15. Old Type of Curtiss Wing. 16. Curtiss Double Camber for Control Surfaces.
The Curtiss sections mentioned above were described in "Aviation and Aeronautical Engineering" by Dr. Jerome C. Hunsaker, but the figures in the above table were obtained by the author on a sliding test wire arrangement that has been under development for some time. At the time of writing several of the U.S.A. sections are under investigation on the same device.
CORRECTION FACTORS FOR WING FORM AND SIZE.
Aspect Ratio. As previously explained, the aspect ratio is the relation of the span to the chord, and this ratio has a considerable effect upon the performance of a wing. In the practical full size machine the aspect ratio may range from 5 in monoplanes, and small biplanes, to 10 or 12 in the larger biplanes. The aspect in the case of triplanes is even greater, some examples of the latter having aspects of 16 to 20. In general, the aspect ratio increases with the gross weight of the machine. Control surfaces, such as the rudder and elevator, usually have a much lower aspect ratio than the main lifting surfaces, particularly when flat non-lifting control surfaces are used. The aspect of elevator surfaces will range from unity to 3, while the vertical rudders generally have an aspect of 1.