SEAPLANE, a type of aircraft which is capable of arising from or alighting on the water. Seaplanes may be subdivided into the following classes. (I) Float seaplanes, in which the landing gear consists of one or more floats or pontoons. (2) Boat seaplanes, in which the centre portion of the structure consists of a boat providing flotation. (3) Amphibians, which may be of either of the above types, but with the addition of a landing gear enabling the aircraft to arise from or alight on the land as well as on the water.
Float Seaplanes.—The float seaplane in practically every case consists of a normal land type of aeroplane in which the landing wheels have been replaced by one or more floats.
The addition of floats to an aeroplane usually increases the weight of the structure by about 8 to io% of its fully loaded weight, and this weight must be deducted from the load the air craft would carry as an aeroplane. The air resistance of floats is always greater than that of aeroplane wheels and in consequence there is a loss in speed in converting an aeroplane to a seaplane.
This loss is usually about io% of the full speed of the aircraft. In spite of this loss of speed, the highest speeds ever recorded have been obtained by seaplanes, and these have in all cases been of the float type. The reason for this is that aircraft having a very high maximum speed, have also a very high minimum speed —often as much as i oo miles an hour for a racing type—due to the very heavy wing loading employed. Floats can now be designed that will alight on smooth water at these speeds, and with large expanses of sheltered water available, the seaplane be comes the most suitable type of aircraft for very high speed flying.
Types of Float Seaplanes.—Float seaplanes may be of the fol lowing types :—( ) Central float, (2) Two float and (3) Three float. The central float type, having one large central float, with usually small additional floats on the wings to assist in maintaining lateral stability on the water, is now almost obsolete. In the "two float" type the floats are made long enough to give the required longitudinal stability when at rest on the water and when leaving the water the altitude of the aircraft is determined almost entirely by the dynamic characteristics of the floats. In the "three float" type static stability on the water is obtained by the addition of a small float below the tail. In taking off the water the fore and
aft inclination of the aircraft is controlled by the pilot by means of the longitudinal air controls.
Lateral stability at rest in twin float or three float seaplanes may be obtained by placing the floats sufficiently far apart but these are often supplemented by "wing tip" floats.
Principle of the Hydroplane.—The float or hull forms employed are designed to act as hydroplanes when the aircraft is leaving the surface of the water, i.e., that as the speed over the water increases, the hydrodynamic forces acting upon the float tend to raise the float thus reducing its displacement and consequently its resis tance. The reduction in water resistance is also enhanced by the use of one or more lateral steps on the bottom of the float.
As resistance is caused by the water film drawn along by the passage of the float through the water, the step tends to break this water film and replace it by a vortex layer of water and air. As the speed of the float is increased from zero its resistance at first increases rapidly and then decreases with further increase of speed. The speed at which this maximum resistance occurs is known as the "hump" speed. This hump speed is of the greatest importance in seaplane design. When taking off from the water this speed must be passed through and, as the air resistance of the wings, body, etc., must be added to the water resistance the total resistance may be very high.
Effect of Forms of Floats.—The dynamic characteristics of the floats or hull may be varied by changes in the size of the "plan ning area," the position, size and number of steps, and by making the bottom float or "V" cross section. As the angle of this "V" section is decreased, i.e., the form is given a deeper keel, the hydroplane effect is reduced, though the hump speed resistance may also be reduced. Tests have shown that by employing a "V" or rounded bottom, stresses due to striking the water when alight ing are greatly reduced. With modern seaplanes having a high power to weight ratio, if the hydroplane effect is too great there is a danger of the aircraft being thrown off the water before sufficient speed is obtained to enable the aerodynamic controls to function.