If we consider the area of the front wing to be unity, and that of the rear wing to be ‹n›, and if ‹m› be the efficiency of the rear wing per unit of surface, [p088] the above is expressed in the following formulæ, which it will be remembered take account only of wings following each other in the same or nearly the same plane, and are not applicable where one wing is either above or below the plane of the other. In the formulæ, ‹CP› is the resultant center of pressure upon both wings expressed in the notation described in Chapter II, ‹CP›_{fw} is the center of pressure of the front wings, and ‹CP›_{rw} the center of pressure of the rear wings. If the value of ‹n› lie between one-half and unity, ‹n› + 1 ‹m› = -------; 3 while if the value of ‹n› lie between unity and 1-1/2, 6 + 4‹n› ‹m› = --------- . 15. In either case ‹CP›_{fw} + ‹mn›‹CP›_{rw} ‹CP› = ---------------------- ; 1 + ‹mn› where the leading and following wings are equal 3‹CP›_{fw} + 2‹CP›_{rw} ‹n› = 1, ‹m› = 2/3 and ‹CP› = ----------------------- . 5 The steady flight of one of the gliding models referred to led to the construction of a new set of wings for No. 5, patterned after those used on the gliding model. These wings, shown in Plate 17, were rectangular in outline, 200 cm.×80 cm. (6.56 ft.×2.62 ft.), each wing having an area of 1.6 square metres (17.1 sq. ft.) They were constructed with spruce framing covered with China silk, and were strongly guyed with piano wire in much the same manner as the light, skin-covered wings already described, which had preceded them. The combined weight of the two pair was 1950 grammes (4.3 pounds). The long central rib was now much the larger of the two which, as in the preceding wing, formed the foundation of the structure. It occupied a position two-fifths the distance from front to rear, and presumably coincided at all points with the center of pressure of fore and aft sections of the wings, so that the pressure in front of the rib was at all points balanced by the pressure in the rear, and there was consequently little tendency in the wing to twist under pressure of the wind. The two main ribs were rigidly connected by cross-ribs of spruce, 20 cm. (8 inches) apart, steamed and bent to the desired form. The curvature of these ribs was the same for all, and in depth was one-twelfth the width of the wing, while the highest point of curvature was one-sixth of the distance from front to rear, these ratios having been chosen as approximating those found in the wing of the soaring bird. These wings were subsequently used in the first successful flights of the following year. [Illustration: PL. 17 AERODROME NO. 5. PLAN OF WINGS AND SYSTEM OF GUYING] [p089]