Biplanes and multiplanes
It is tempting to ignore biplanes and multiplanes as being of purely historical interest, but old ideas have a habit of returning, and small biplanes have once again become popular for aerobatic and sport flying.
The wings of early aircraft had little or no bending stiffness, and had to be supported by external wires and struts. The biplane configuration provided a simple convenient and light structural arrangement, which was originally its main attraction.
A biplane produces virtually the same amount of lift as a monoplane with the same total wing area and aspect ratio. The biplane, however, has the smaller overall span, which makes it more manoeuvrable. The highly aerobatic Pitts Special, shown in Fig. 2.26, is an example of a modern biplane which is not merely a gimmick. The manoeuvrability of biplanes was one factor that led to their retention even when improvements in structural design had removed the necessity for external bracing.