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The position of the airscrew dominates the design. Air inflow near the blade-tips sucks spray off the sea, and picks up spray thrown by the hull, with damage to the blades. This is prevented either by putting it high up or over some part of the seaplane, e.g. the lower wing or hull. This places the thrust axis well above the centre of gravity, and the smaller the seaplane, the more this effect is notable. The high thrust axis produces a downward pitch varying from zero in gliding flight to a maximum at full power. In the earlier boat seaplane this was uncorrected, and, in order to get balance in normal flight, the craft was very "tail heavy" when gliding.

This effect has been diminished by placing the tail plane in the slipstream, by setting it at a negative angle to the chord of the main planes, and by distributing the weights so as to bring the centre of gravity particularly far forward. The thrust-couple thus opposed by the tail-couple can be nearly balanced out. As the main reactions on the tail are downwards, the tail plane is sometimes set with the camber downwards.

Unusually large airscrews and geared-down engines are used for efficiency at low speeds, i.e. at about 4/10 of the stalling speed, because the water resistances are greatest at this speed.

The hull must provide longitudinal stability, both at rest and in motion on the water. To ride in a seaway and not bury its nose when accelerating, a long forebody is used. The section of this part should be veed at the keel, and well flared at the chines, respectively to reduce shocks from on-coming waves, and to keep the divergent wave formation low and clear of the wings and airscrews. For the same reasons the keel and chine lines have a gradual rise forward with overhang forward of the fore-end of the water-line.

At least a 300 % reserve of buoyancy is given to boat seaplanes to provide adequate freeboard at sea. With watertight floats 120% reserve is adequate.

Above 4/10 of the minimum flying speed, called the "hump" speed, the water resistance due to wave-making begins to fall. Above the hump speed the water resistances are probably due as much to skin friction of the planing surfaces as to wave or eddy making, and by disposing steps in the planing bottom, the wetted surface, and consequently the resistance is reduced. From the hump speed onwards these hydroplane resistances decrease, the weight is transferred more and more to the wings until the seaplane leaves the water.

The larger the planing surface (i.e. the wider the beam) the sooner the hull rises, and the earlier the hump occurs, but this increases the resistance at lower speeds, and makes the hull heavy for its strength.

Models tried in the Froude National Tank at Teddington (Eng- land) show that but a slight reduction of max. E.H.P. at or about the hump speed is obtained when the beam exceeds about \ the length of forebody. Where a high power is available on other ac- counts, the beam may be still further reduced. A narrow hull with spray-deflecting sections, and without flat surfaces, or main " step," though desirable, was found not to give the necessary lowering of resistance beyond the hump speed. The resistance increased as the displacement diminished with speed.

The main " step " under the centre of gravity was proved necessary, but the area of the planing surface forward of less importance. The boundaries of the planing surfaces must have sharp edges to make the water break clear away from them ; water clings to rounded sur- faces even of small radius, and these would cause unnecessary resis- tance. The angle between the hull surface at the chines and steps should not exceed 90 for the same reason.

In a seaplane hull we require static stability at rest on the water, and dynamic stability in motion at the higher speeds. Longitudinal stability of the whole at rest is obtained by the length of the water- plane area and presents no difficulties.

The tendency of all craft to trim by the bow at low speeds is ag- gravated in the seaplane by the high thrust axis. The heaviest waves are formed while this tendency to dive is still present, and it is at these speeds (in the region of \ to J of the minimum flying speed) that clean running in disturbed water is most difficult to attain.

The modern hull possesses large restoring moments at small nega- tive angles of trim, but if the forward trim exceeds about 3, the moments will have become negative, and an attempt to alight at such an angle will break up the hull.

Lateral stability at rest (though sought in early boat seaplanes by providing sufficient beam to give a small positive metacentric height) is destroyed by the lightest side wind, and therefore wing-tip floats are a necessity. The transverse metacentric height is always nega- tive to-day, and wing-tip floats are relied on.

Stability in the hydroplaning condition becomes increasingly im- portant with size. Beyond the hump speed the hydrodynamic reactions, the air reactions and, to a diminishing extent, the buoyancy combine to support the hull, and to determine the stability of the whole. Just above the hump. speed hydroplaning reactions are great, while the air-forces are small; so are the moments due to the air- controls. Here the planing surfaces and steps must afford stability. When the speed increases the water-forces become less, and the air- forces greater, till, on approaching the minimum flying speed, any instability that may occur can be counterbalanced by the air-con- trols. In the larger seaplanes, however, the water moments may be large even at high speeds, and their hulls must, therefore, be stable over the whole range of water speeds.

The stability depends on the relative size and positions of the steps and planing surfaces, on the angles of the planing surfaces to the mean water-line to each other, and to the chords of the aerofoil surfaces ; and on the position of the centre of gravity in relation to these and to the height of the thrust axis.

The problem is one of great complexity, and partly on account of its recent origin is as yet unsolved.

During the war, model work was called for on individual designs and delayed the general investigation, but clues have been found. Usually the smaller hulls are proved more apt to develop instability both in the tank tests and on the full scale. The minimum flying speed being much the same on large and small seaplanes, the wave lengths at any given speed are much the same. A hull of 45 ft. gave best results with the main step slightly forward of the C.G. and the rear step, very small relatively, 18 ft. aft of it. Here instability was delayed until the air-controls were effective, and when tried in the full scale, no instability was apparent, probably on account of the damping action of the air-surfaces, since the seaplane took the air without operation of the controls by the flier.

A somewhat similar model of a much larger seaplane with steps 32 ft. apart was stable throughout the speed range.

In the small types the hull length restricts the distance apart of the steps. The two steps may be compared to the wings and tail surfaces of an aeroplane ; the main step nearly under the C.G. does the lifting of the boat on to the surface, while the rear step provides the pitching moments for equilibrium, and is most effective for this purpose when far aft and of small dimensions. Tank experiments on models show that not more than about l/io of the total resistance is due to the rear step and after body. The two steps must also be arranged in such a way that the intermediate hull-sections and that part of the hull carrying the tail surfaces, aft of the rear step, are clear of the two depressions formed in the water by the passage of the steps. The object is to reduce the wetted surface, aft of the main step, to the minimum necessary (at the rear step itself) to give stable conditions. This best arrangement can only be obtained, at present, by individual model experiments. The full scale has cor- roborated the restjlts, and accordingly the resistance, running angles, pitching moments required for equilibrium and general characteris- tics of running can be obtained in the Froude tank, where waves can also be reproduced artificially. Tests show that complete stability on the model is obtained under more difficult conditions than in the full scale, hence the seaplane corresponding to a stable model may be fully relied upon.

Between Air and Water. The water reactions on a badly designed hull may continue to be considerable up to the moment of quitting the water; then, their sudden disappearance may produce moments dangerous at a time when their correction by air-controls requires big movements. Such a seaplane at a high speed on water, and kept at the angle for maximum lift of the wings by means of the air- controls, is subject to a moment in pitch due to the water reaction on the steps. This is balanced by a moment due to the elevator until it leaves the water, when all the water forces are lost. If the elevator moment, which had been applied by the flier were positive (i.e. increasing the angle of attack), the seaplane would stall. These moments should either not exist or be negligible. If they do exist they are less dangerous if operative in the inverse sense to those in the example. Their existence can only be ascertained, and as a rule eventually elinHnated, by experiment. From such experiments a canon for design will be evolved.

To keep hull weight low there are special methods of construc- tion. The timbers used in boat-building practice have so far been found best. In the present sizes steel is out of the question. Alumin- ium alloys have been used in Germany with success for float con- struction, but it is doubtful whether duralumin or any other alumin- ium alloy is superior to mahogany for the hull skin as regards strength, hysteresis or durability. In any case of timbers, mahogany is the best for this purpose.

Unsuccessful attempts have been made to depart from the time- worn principles and practice of light boat building. A planked skin through-fastened on to timbers and stringers in the usual manner is essential for watertightness and durability.

Design is addressed to keeping down the weight of the skin, and its supporting structure. Seaplane hulls have been built having a bare weight from 20 % to 9 1 % of the total weight. The latter figure was got on a boat displacing about 15 tons.

The two principal methods of construction are: the rigid and the flexible. For most hulls the skin is supported by a rigid structure which permits of easy subdivision by transverse bulkheads like the ordinary steel steamship except that timber is employed. The main objection to this method is its low specific strength. The rigid structure produces strong local points in the skin with intermediate areas poorly supported, resulting in sudden changes of cross section and localized deflections under load. Such a hull to have sufficient strength for taking off in disturbed water weighs 15 % of its displace- ment at least.

The flexible method as developed by Linton Hope has its trans- verse sections approximately circular throughout the whole length, while the planing surfaces are built on outside the main hull, produc- ing what is virtually a double bottom. The structure is tubular, its whole strength being concentrated in the skin and its local support-