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The choice of airfoils to be used in such trial designs can usually be cut down to two or three by the application of certain criteria to their aerodynamic characteristics in the manner described in Chapter V. In deciding upon the dimensions and type of bracing to be assumed in the trial designs, care must be taken to assume dimensions, types of bracing, etc., that are best suited to the airfoil used and the function of the airplane.

The total wing area will be determined by the gross weight of the airplane, the airfoil used, and the required landing speed. The weight of the wings, and therefore of the airplane, will depend somewhat on the wing dimensions and type of construction used, and the experienced designer will allow for that fact, though the error in the relative performance is not likely to be large on that account.

The aspect ratio chosen must be a compromise between aerodynamic and structural considerations. A large aspect ratio wing has a better efficiency than one with a small aspect ratio. On the other hand, the weight of structure will be larger for two reasons. First, the bending moment on the wing cellule as a whole is increased, as the load on the wing remains the same and the center of pressure is further from the fuselage. Second, the wing chord being decreased, there is less room for the spars which are of less efficient design as a result.

The number of wings is decided upon principally by the individual preferences of the designer. Monoplanes have the advantage of improved aerodynamic efficiency, and usually fewer parts and better vision in certain directions. Their disadvantages are the long span which involves an extra weight that may more than counterbalance the improved efficiency, and also increases the required hangar space per airplane; and the danger of flutter. The advantages and disadvantages of biplanes are the reverse of those of monoplanes. One [[On]] the whole, the biplane type is by far the most common. Triplanes are aerodynamically less efficient than biplanes, but in some cases, as the Barling Bomber, the wing area required is so great that three wings have been used in order to keep the span within reasonable limits and to hold down the wing weight.

The span and chord of the individual wings are fairly well determined when the number of wings and their area has been decided. Minor revisions in these values will usually have to be made during the course of design in order to provide better vision, improve structural design, etc.

The type of wing bracing to be used depends mainly on the aspect ratio and the airfoil section. If the aspect ratio is low, the wing may be internally braced. As the aspect ratio increases, a point is reached where the internally braced structure becomes unduly heavy, and a single bay externally braced design is the most economical. As the aspect ratio increases further, the number of bays must be increased. The aspect ratios forming the limits of economical use of the various numbers of bays have never been determined, but it is known that the thicker the airfoil section, the larger the aspect ratio for which a given number of bays can be used. A study of a particular case is reported in Information Circular No. 260, "The Economic Limit in Aspect Ratio of Single Bay Pursuit Biplanes." The conclusion arrived at in that report was that 6.0 was the limiting aspect ratio between single bay and two bay construction with the R. A. F. 15 wing. This conclusion might be modified if up-to-date methods of stress analysis had been used, but would probably be found to be approximately correct.

109. Preliminary Side View.

After the designer has decided upon the principle wing dimensions and has obtained a preliminary estimate of the total weight, he is ready to begin drawing up the design. The first drawing to be made is the side elevation. As the function of the fuselage is to carry the engine, crew, equipment, etc., the more important and the bulkier of these items are first located in their desired relative positions, and the fuselage structure and outline added. The order in which the contents of the fuselage should be located depends primarily upon the function for which the airplane is designed. In a pursuit airplane the pilot and guns would be located first, and the other items later. In a transport the passenger cabin would be the starting point, around which the pilot's cockpit, engine, etc., would be grouped. This preliminary location of the fuselage should be done with care to put in a good place every item that is heavy or will be difficult to move without impairing the suitability of the airplane for its function. Other items may be located with less care, but all of the contents of the fuselage should be located on the drawing.