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1916 from the Belgian coast in attacks on merchant shipping but these were not required to cover great distances, and were not remarkable. Isolated small boat seaplanes have been constructed in Germany, but not in quantity.

The Germans (no doubt in consequence of their greater study of airships) continuously kept a heavier, and more reliable, engine than the Allies, but by 1917 the Allies had produced higher- powered units, and it is probable that these two facts are mainly responsible for the German retention of the smaller " float sea- plane." Moreover, their engine failures at sea were few, and there was not, therefore, so much pressure for their seaplanes to withstand open sea conditions.

The Brandenburg seaplanes of 1917-8 had rather heavy engines of 180 to 200 H.P., yet they had very high performance. Their success in fighting was due to the unusual monoplane wing arrangement which gave a clear field of fire in all directions above the horizontal plane, and to their clean general design without any external wire bracing. They employed the more recent type of twin floats.

Before the period of limitation of aircraft construction set by the Allied Commission of Aeronautical Control, the Germans had been developing the giant aeroplane in several experimental forms, differing mainly in the arrangement of multiple-engine units. These ranged in total weight roughly from 9 to 12 tons, and in the case of the larger types difficulty was experienced in providing sufficient area of contact between the wheels and ground. This difficulty did not exist in the giant seaplanes, a few examples of which had been built by the Zeppelin works on Lake Constance. Their aerodynamic design was not good, and the type was not perpetuated in its original great size on account, probably, of difficulties of control. The Staakener Giant was another example; this had two long floats made entirely of duralumin. These giant seaplanes would no doubt have devel- oped but for the prohibition, and an interesting comparison of advantages would have been obtainable between the giant sea- plane, and the giant aeroplane.

Characteristics of Seaplanes. The boat seaplane, a craft suitable for less-sheltered waters than the early float-equipped aeroplane, or hydro-aeroplane as it was called, must, to be of real value in naval operations, be fully sea-worthy, and such progress as had been made had not yet proved by 1921 whether this was completely obtainable. But there were then : (i) the smaller craft to operate from sheltered waters, rivers and lakes, and (2) the boat seaplane to operate over- sea. The first includes all types of small dimensions of less than, say, 4 tons, and all existing "float" types in 1921 fell into this category.

To the considerations of design, stability and control applying to aeroplanes must be added the design and distribution of the float system, so that the forces due to water shall not affect adversely the stability and control. These water forces are controlled by means of the aerodynamic elements, which are ineffective except at the higher hydroplaning speeds. Hence the float system must be such that any instability that occurs between the air-borne and water-borne con- ditions shall take place at speeds high enough for the air controls to be dominant.

Wheeled seaplanes, for land and sea alighting, had been built by 1921 as experiments, but their development had only just begun. Their wheel system, springing, ground clearance and like factors are those of the aeroplane. These amphibians are handicapped by the weight of the float system, but show promise of very useful speed and climb.

Most large centres of population possess areas of smooth water, rivers, lakes or harbour, affording an alighting area comparable with the average aerodrome, and if the proposed route provides large water areas for any forced alighting, this fact can be taken advantage of by carrying a heavier load per sq. ft. of wing area with a corre- sponding gain of speed, reduction of structure-weight and increase of efficiency.

The desiderata for seaplanes for the open sea are less well known, and more difficult of attainment. They must for sea-worthiness be large. They had reached 15 tons by 1921 and were still far below the dimensions of the small coasting vessel ; with the existing construc- tional materials science places a very early and definite limit to the increase of size possible. In order to enable even a 15-ton seaplane to carry a reasonable weight of fuel, crew and equipment, the load factor is in some cases reduced to three and a half. The increase of wing-loading, though it entails a higher stalling speed, and the adop- tion of a wing-section of high lift, may yet improve matters.

For commercial purposes, a high top speed is not so essential as for war, and model tests indicated in 1921 that the overall efficiency of a seaplane with high-lift wings may compare with craft with the usual flatter wing. The reduced area of wings so obtained has kept

down the structure weight. For war the wing whose camber is vari- able to give high speed with good lift at low speed may be perfected

Three arrangements of "float seaplanes" are possible; in all, the engine, crew and loads are carried in one or more fuselages well above the floats in such a way as to bring the centre of gravity and thrust axis into approximate alignment :

(a) Two main floats which together support the whole weight and provide lateral and longitudinal support.

(i) Two main floats together with one or more tail floats, the for- mer supporting nearly the whole weight, but being dependent on the latter for longitudinal support.

(c) One central main float supporting the whole weight and pro- viding longitudinal support, two comparatively small wing floats providing lateral support.

Systems (a) and (b) provide positive metacentric height both longi- tudinal and transverse, while system (c) is always dependent on the wing floats for lateral support; for small angles of roll this is lack- ing, as it is necessary to carry the wing floats clear of the water when the seaplane is on an even keel.

Systems (a) and (b) are most usually employed because they avoid this defect. A main advantage of the system (c) is that the float im- pedes the view much less.

Arrangement (a) is better than (b), as the tail float of (c) is easily damaged, and thereupon longitudinal support being lost, the sea- plane turns over on its back.

Float seaplanes have the following merits over the boat type :

(1) They can be handled on slipways with the most primitive ar- rangements, and can be beached safely on any smooth foreshore.

(2) The aerodynamic elements give the normal balance, stability and control.

(3) They may be convertible into aeroplanes, or vice versa.

(4) The floats are simple in design, and can be subdivided into watertight compartments.

(5) The static transverse stability of systems (a) and (b) enable the wings to be folded afloat, for hoisting the craft from the water to a ship or a quay.

(6) For war, good arcs of fire are obtainable over the rear hemi- sphere.

The following are the disadvantages :

(1) The floats are uneconomical of structure- weight.

(2) The aerodynamic drag is comparatively high.

(3) Arrangements (a) and (b) cannot be used for larger craft than 3 tons as heavy racking stresses are set up in the structure connect- ing the two floats when on disturbed water.

In the " boat seaplane " the displacement of the craft is borne by the central hull. Longitudinal stability on the water, both static and dynamic, is supplied by the length of the hull, and the distri- bution of its planing surfaces. Wing-tip floats are necessary for lateral support.

The advantages of the type are as follows:

(1) An excellent crew position for flying and observation, e.g. in anti-submarine operations.

(2) Comfort : the crew can move about, the pilot be relieved, etc.

(3) Economy of structure-weight.

(4) Compact design low air drag.

(5) Absence of racking forces, and large size possible. This last advantage is the most important, and the limit of size of aircraft, as already discussed in the section on "aeroplane design," applies here save as regards the hull. Experience shows that the hull weights do not increase even in the same proportion as the total displace- ment, a slight reduction in the ratio of the hull weight to total weight having been obtained, and if this continues further, it is clear that a reduction in hull weight can be set off against an increase in wing weight, resulting possibly in the most economical scale being greater than anything yet constructed.

The disadvantages are :

(1) The wings cannot be folded afloat.

(2) Cannot be beached except in very soft mud, and requires elaborate apparatus to move it to a shed on shore.

(3) In war it is difficult to defend from attack astern.

(4) The large distance between the centre of gravity and the thrust axis, and the low position of the centre of gravity in relation to the centre of lift. The former produces a variable pitching mo- ment, the latter influences adversely the lateral control.

Elements of Design Peculiar to Seaplanes. Many of the desiderata in a seaplane design are antagonistic to each other.

Flight can be achieved with I H.P. for each 25 Ib. to be flown, but jood speeds and climbing need I H.P. for each 8 or 10 Ib.; therefore, structure-weight must be economized.

No wings can stand a blow from any large volume of water. The wings must clear the waves and any but light spray. Regarded as an aircraft the centre of gravity of the whole and the centre of pres- sure of the wings should De nearly coincident, and for this the centre of gravity should be high above the water. As a watercraft, how- ever, a relatively low position of the centre of gravity is needed in relation to the waterplane. The compromise necessary puts the centre of gravity so that the metacentric height (apart from the wing- ip floats) is negative.