There is perhaps no object of a technical nature so universally familiar by name, appearance and purpose as the rudder. Everyone knows that the rudder is to steer the craft to which it is attached. The direct effect of a rudder relates to the control of movements about the vertical axis of the craft. Such movements may conveniently and appropriately be referred to under the general term °yaw.* It is the tendency of the craft to yaw on its own vertical pivot that the rudder is used either to check or to initiate. When steering a straight course, the rudder is used to counter act disturbances that otherwise might give rise to yawing. In an aeroplane, which has no appreciable extent of vertical surface in the vicinity of the C.G., a slight yaw will of itself present equivalent to the side of a ship against which the air can react. But when an aero plane with positive wing tips is caused to yaw, one tip is thereby accelerated while the other is retarded, and so a bank is established which in turn tilts the direction of the air pressure on the wing. It is this tilting of the air pressure by canting the wings that provides the steering force in the case of an aeroplane.
Types of The different types of aeroplanes are distinguished according to their number of supporting plane surfaces. The "monoplane" with one plane surface; the *biplane with two super-posed plane surfaces; the triplane with three supporting surfaces; and the “multiplane" with any number of sup porting surfaces in advance of three.
Monoplane.— For a considerable period in the history of practical aeronautics the mono plane held the pre-eminent position, in that period when France was in the forefront, and when she developed a corps of sportsmen who were eminent throughout the world. This was due to the excessive speed which could be acquired with this type of machine. The assets of the monoplane are, briefly, lightness, speed, both in climbing and racing. Its disadvantages, inability to carry a load, the proficiency which it demands of the pilot, the extensive smooth landing grounds necessary.
Biplane.-- The most common of modern machines, because of its adaptability to all con ditions of aerial navigation. The types vary greatly (consult Fage, 'Aeroplane') from the machine with narrow advancing plane edge for speed, to the machine with a wing spread of 157 feet, equipped with multiple motors, for load carrying and lotig distance reconnaissance.
Triplane.—This type has been developed to extend further the load carrying capacity of the biplane. It will undoubtedly represent in
the period immediately succeeding the war the vehicle that will be entered first upon the aerial carrying commercial ventures.
Hydroaeroplane.— A hydroaeroplane has been described as an air-borne craft capable of floating on water. Such a machine must be made sufficiently strong to resist the severe buffet ing of a rough sea, although its flying capacity is affected by any undue weight and head resistance. Single and double floats are used. The floats should not be too far apart, other wise the lifting of one float out of the water due to excessive rolling causes the machine to suddenly swing round; and, moreover, the un balanced forces called into action by the reduc tion of the resistance of the rising float, and the increase of resistance of the falling float, are greatly assisted by the leverage between the two floats. On the other hand when the two floats are close together, the behavior approxi mates that of a single float machine. The water resistance of the floats appears to be practically independent of their distance apart. To prevent the float driving into the water when the machine alights, and also to keep the nose of the float well out of the water when the machine is at rest on the water, it is desir able that the centre of buoyancy of the float, when the longitudinal axis of the machine is horizontal, should be well forward of the position of the centre of gravity of the machine. The total buoyancy of the floats should be almost equal to twice the weight of the fully loaded machine. While the float is rising from the water the free access of air to the bottom of the step should not be hindered. The tend ency to hop— a characteristic of most floats when running along the surface of the water at high speeds and small buoyancy—is due to the mherent instability of a machine partly supported on a small area. Any tendency to dip the nose of the float under water may be minimized by a low position of the line of propeller thrust.
Flying The flying boat had its in ception in America, where Glenn H. Curtiss was the first to design this type of craft. It is an ordinary aeroplane in wing structure, either biplane or triplane, but with a boat shaped hull for landing on, and subsequently, if necessary, navigating the water. See AERO PLANE DISTANCE AND SPEED RECORDS; AEROPLANE ALTITUDE RECORDS; AERONAUTICAL NOMENCLATURE. Consult Woodhouse, 'Naval Aero nautics.'