The most common type of plane is the biplane (Fig. 4), with its two planes, connected by struts and wires, set not directly over each other, but staggered, usually with the upper plane leading. Monoplanes were in favor in the early days of aviation, and triplanes have been used to some extent. According to the position of the propeller planes are classified as tractors or pushers, tractors being at present the more common form. Planes are further classified as single-seaters (Fig. 5),two-seaters, and three-seaters. These motor and passenger methods of classification are now proving inadequate with the rapid development of planes carrying two, three, and even more motors, divided between pusher and tractor operation, and carrying increasingly large numbers of passengers. Aside from structure, planes may be further classified according to their uses, as scout, combat, reconnaissance, bombing, etc. Planes equipped with floats or pontoons for alighting on the water are called seaplanes (Fig. 182), and those in which the fuselage is boat-shaped, to permit of floating directly on the water, are flying boats (Fig. 183).

Fig. 4. - Biplane in flight.

Fig. 5. - A single-seater.

Fig. 183. - A flying boat.