Flying Boat.—Much advance has been made recently in the design and construction of large boat seaplanes or flying boats. Boats of a total loaded weight of up to 33,000 lb. and total horse power of over 2,600 have been built and operated successfully. Such aircraft are capable of working as independent units away from fixed bases, as they possess a high degree of "seaworthi ness" when moored, have sufficient internal accommodation to enable a full crew of five or six to live on board with comfort, and can carry all necessary equipment.
In the smaller types of aircraft up to approximately io,000 lb. total weight the float seaplane shows advantages over the boat seaplane, the body, wings, airscrew, etc., may be well clear of the water, a high positive metacentric height can be obtained by suit ably spacing the two floats, and clean external lines giving a minimum air resistance can easily be obtained.
In larger types of aircraft, the boat seaplane improves on the float type. As the dimensions of the central hull increase so the seaworthiness of the aircraft is improved beyond that of the float type.
Again in the larger types, the hull replacing both the fusilage and the leading gear of the float seaplane, there is a considerable saving in weight. This saving in weight in the largest flying boats is sufficient to enable them to compare favourably with land aeroplanes of equal dimensions in regard to structure weight. In medium sized boat or float seaplanes, however, the ratio of struc tural weight to load carried compares unfavourably with that of land aeroplanes.
Lateral stability at rest in boat seaplanes is usually obtained by means of "wing tip" floats, though certain designers have modified this method. Dornier, for example, employs two small rudimentary wings of symmetrical section projecting from the sides of the hull at the water line, while in the Rohrback boat seaplane two fairly large floats similar to those of a two float seaplane are mounted on either side of the hull.
In boat seaplanes the metacentric height of the hull itself is very small or negative, so that additional methods, such as de scribed above, of obtaining stability must be employed. If the
hull were to be given a large positive metacentric height the sea worthiness of the aircraft would be improved, but to obtain this condition, the centre of gravity of the aircraft would be too low for air stability requirements.
Longitudinal dynamic stability in boat seaplane hulls is usually obtained by using the main step slightly abaft the centre of gravity of the aircraft and by the addition of a smaller step further aft. This small step is usually only operative when the hull is running at fairly large positive angles and is intended to neutralize any tendency to pitch fore and aft when taking off or alighting.
In boat seaplanes the engines and airscrews are situated high above the hull in order that they may be clear of water and spray, the result being that the centre of thrust is above the centre of gravity of the aircraft. This high position of the centre of thrust above the centre of gravity forms a negative couple tending to drop the nose of the aircraft when the engines are running, and a boat seaplane in trim for level flight under these conditions would tend to increase its angle of incidence should the engines be stopped, in which case a dangerous loss of speed might result. To counteract this change of trim, a negative angle of incidence is given to the tail plane which is often of cambered aerofoil section inverted, i.e., with its convex surface downwards. This tail plane being in the slip stream of the airscrews, causes a positive movement with the engines running, the movement de creasing when the engines are stopped, thus counteracting the change in thrust and centre of gravity couple.
Hulls constructed of wood have developed on two lines. Firstly, the rigid hull, in which the hull is itself strong enough to with stand the impact of striking the water when taking off in a rough sea or alighting; and secondly, the flexible type of hull. In the flexible hull the impact stresses are absorbed by the elasticity of the hull structure. These hulls are usually of circular or elliptical section with the steps added by forming double bottoms for cer tain longitudinal sections only. (T. C. B.)