Aeroplane Construction and Operation

by John B. Rathbun

Fig. 15. Pressure Distribution at Various Points Along the Length of a Nieuport Monoplane Wing.

Distribution on Practical Wings. With the exception of the bird wing, the distributions have been given for thin plates that are of little value on an aeroplane. They do not permit of strong structural members for carrying the load. The actual wing must have considerable thickness, as shown by the aerofoils in Figs. 1, 2, 3, etc., and are of approximately stream line form. Fig. 14 shows the distribution for actual aeroplane wings: (a) Wright, (b) M. Farman, (c) Breguet, (d) Bleriot 11.(d), (e) Bleriot 11-bis. The Wright wing is very blunt and has an exceedingly high lift at the leading edge. The M. Farman, which is slightly less blunt, has a similar but lower front peak. The Breguet is of a more modern type with the maximum thickness about 25 per cent from the leading edge. The latter shows a remarkably even distribution of pressure, and is therefore a better type as will be seen from the relative lifts of 0.916 and 0.986 pounds per square foot. The lift-drag ratio of the Breguet is also better, owing to the greater predominance of the negative drag components. Decreasing the thickness and the undercamber of Bleriot 11, resulted in an unusual increase of 10 per cent of the under pressure, and a decrease in the Suction, shown by Bleriot 11-bis. The Bleriot has the sharpest entering edge and the least upper pressure. In the above practical wing sections the aspect ratio is variable, being the same in the test model as in the full-size machine. The Bleriot being a monoplane has a lower aspect ratio (5), than the biplanes (a), (b) and (c). The Breguet with an aspect of 8 has a lift of 0.986 pounds per square foot as against the 0.781 of the Bleriot, and undoubtedly part of this difference is due to the aspect ratio. The pressure falls off around the tips as shown by the successive sections taken through a Nieuport monoplane wing in Fig. 15. Section (f) was taken near the body and shows the greater lift. Section (g) is midway between the tips and body, and (h) and (i) are progressively nearer the tips. As we proceed toward the tips from the body the pressure falls off as shown in the sections, this reducing from 1.07 to 0.55 pounds per square foot. This wing also thins down toward the tips or "washes out," as it is called.

Fig. 16. Showing Pressure Distribution on the Plan View of a Typical Wing, Leading Edge Along A-A, Trailing Edge D-C. Center of Pressure. Marked "C.P." The Proportion Pressures Are Indicated by the Shading on the Surface, the Pressure Being Negative at the Tips and Near the Rear Edge.

CHAPTER VI. PRACTICAL WING SECTIONS.

Development of Modern Wings. The first practical results obtained by Wright Brothers, Montgomery, Chanute, Henson, Curtiss, Langley, and others, were obtained by the use of cambered wings. The low value of the lift-drag ratio, due to the flat planes used by the earlier experimenters, was principally the cause of their failure to fly. The Wrights chose wings of very heavy camber so that a maximum lift could be obtained with a minimum speed. These early wings had the very fair lift-drag ratio of 12 to 1. Modern wing sections have been developed that give a lift-drag ratio of well over 20 to 1, although this is attended by a considerable loss in the lift.

As before explained, the total lift of a wing surface depends on the form of the wing, its area, and the speed upon which it moves in relation to the air. Traveling at a low speed requires either a wing with a high lift co-efficient or an increased area. With a constant value for the lift-drag ratio, an increase in the lift value of the wing section is preferable to an increase in area, since the larger area necessitates heavier structural members, more exposed bracing, and hence, more head resistance. Unfortunately, it is not always possible to use the sections giving the heaviest lift, for the reason that such sections usually have a poor lift-drag ratio. In the practical machine, a compromise must be effected between the drag of the wings and the drag or head resistance of the structural parts so that the combined or total head resistance will be at a minimum. In making such a compromise, it must be remembered that the head resistance of the structural parts predominates at high speeds, while the drag of the wings is the most important at low speeds.

In the early days of flying, the fact that an aeroplane left the ground was a sufficient proof of its excellence, but nowadays the question of efficiency under different conditions of flight (performance) is an essential. Each new aeroplane is carefully tested for speed, rate of climb, and loading. Speed range, or the relation between the lowest and highest possible flight speeds, is also of increasing importance, the most careful calculations being made to obtain this desirable quality.

Performance. To improve the performance of an aeroplane, the designer must increase the ratio of the horsepower to the weight, or in other words, must either use greater horsepower or decrease the weight carried by a given power. This result may be obtained by improvements in the motor, or by improvements in the machine itself. Improvements in the aeroplane may be attained in several ways: (1) by cutting down the structural weight; (2) by increasing the efficiency of the lifting surfaces; (3) by decreasing the head resistance of the body and exposed structural parts, and (4) by adjustment of the area or camber of the wings so that the angle of incidence can be maintained at the point of greatest plane efficiency. At present we are principally concerned with item (2), although (4) follows as a directly related item.