ing members. Two or'more thicknesses of mahogany-planking are through-fastened to transverse timbers of small section closely spaced ; these are connected longitudinally by a large number of stringers of rectangular section lying in a radial plane, edge on to the timbers; the stringers are in turn supported on their inner edges by elm hoops of comparatively heavy section widely spaced. The small section timbers are placed so closely that no fastenings need be passed through the skin anywhere between timbers. This type is water- tight, durable and light. The average hull weighs not more than II % to 12 % of its displacement.
The flexibility absorbs the shocks of alighting and taking off, and precludes heavy local pressures. Care is taken to distribute the air loads, which are generally concentrated along two lines transversely to the axis of the hull, over a sufficient area of the skin, and all internal installation is arranged to allow for comparatively large relative movements of components. Transverse subdivision is prac- tically impossible, but the provision of a subdivided double bottom is easy and effective.
Seaplane in Operation. The preparations made for housing and upkeep of seaplanes were unfortunately dominated by the require- ments of the early types. The seaplane station was modelled on aerodrome lines, with the addition of a slipway to the water. The flat floats of the float seaplanes were placed on trolleys, and thence by slipways to the water. The delicate V-section hull of the heavy boat seaplane is ill-suited for such handling. The draught of the modern boat (with a trolley under it) exceeds what can be negotiated by men in waders. If such boats are to be brought ashore at all new devices are required for doing so. Experience shows that boats of only 5 or 6 tons are damaged in such handling, though they draw little more than 2 ft. of water. To limit the bringing ashore to slack water periods in good weather, would be intolerable for commercial work. Better water-side facilities, such as covered sheds with direct access to the water for the construction, erection and repair of modern seaplanes are needed. These should allow of admitting water to part of the shed to reduce the out-of- water handling to a minimum. As a large expanse of sheltered water is necessary, and the rise and fall of the tide is important, floating sheds may be needed.
Closed sheds are not essential for operating seaplanes. The larger the seaplane the more can it resist exposure for long periods, and the practice of mooring out will become an economical necessity, but the seaplane must be designed with this in view, and proper auxiliary services for heating, fuelling and repairs provided. In high winds seaplanes moored out have risen off the water at their moorings and destroyed themselves, but this is avoidable by destroying the air-flow over the lower planes by attaching light boarding along the leading edges at a large negative angle to the chord. As the seaplane for com- merce has been but little studied, marked developments may be expected in this direction ; sea-worthiness is still the main problem for warcraft and increase of size the most direct solution.
In transport work, sea-worthiness is an insurance against engine failure ; remove this risk and operation would take place from shel- tered water only, design would be freer, size would be dictated by load, capacity and economy. The need to counter the winds rather than competition against the slow surface ship would dictate the air speed of such craft.
For operation from smooth waters structure-weight and hull weight can be reduced and wing load increased, while high-lift wing sections also offer much promise.
It is remarkable that though the viewpoint for seaplanes is so different from that for aeroplanes, the reliable engine unit is equally found to be the prime desideratum for present progress. (A. J. M.)
IX. AIRSHIPS
Airships are divided into three main types: (i) The rigid, which has a hull structure of rigid members covered by an outer fabric fairing, and containing a number of separate gas cells. (2) The semi-rigid, in which the whole or part of the bending and longi- tudinal compression induced in the ship by the rigging wires is taken by a rigid keel. The envelope from which this keel is car- ried is kept distended by the pressure of the gas, but is mainly subject to vertical loads. (3) The non-rigid, in which the envelope maintains its shape solely on account of an internal pressure which must exceed the outside pressure.
Small airships up to, say, 300 ft. long are necessarily non-rigid, as there is not sufficient lift to justify a rigid framework. The largest airships have a rigid hull structure because the pressures involved in an envelope of large diameter necessitate very heavy fabric and make a system of compartments essential. Between the two, the semi-rigid seeks to reduce the fabric tensions by the use of a rigid keel girder, but it is doubtful whether this justifies the keel, except as a convenient means of carrying the loads from the envelope.
A rigid airship has a hull structure of light aluminium girders, arranged with some 25 longitudinals connecting some 17 main transverse polygonal rings. At each main ring a bulkhead is formed of the load wires which suspend the weight of the keel from the upper part of the framework and the radial and chord wires which retain the shape of cross section of the ship. A spe- cially strong keel of triangular section and some 8 ft. high runs nearly the whole length of the ship and carries the petrol tanks, water-ballast bags and other weights, being itself supported at the main transverse rings. The 30-metre spaces between the bulkheads are each fitted with a single gasbag of gas-tight fabric. The degree of fullness of these bags varies from the maximum to sometimes less than 50% full, when the upper parts of the space alone will be occupied by the bag, whose lower part is collapsed and empty. A cover of fabric is stretched over the outside of the whole frame, so as to present a smooth surface and protect the gasbags from weather and light. Separate engine cars are attached below the hull at points along its length.
Performance Table of Seaplanes, 1914-20.
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Float Seaplanes.
1914.
Type
H.P.
Useful Load In- cluding Crew (Ib.)
Endurance Hrs.
Maximum Speed in m.p.h. at Sea Level
Span
Total
Weight*
Maximum Total Weight
Effective Ceiling
M. Farman .... Short Sopwith-Schneider .
IOO
1 60
IOO
880 850 340
3 4*
2*
62
74 90
63 56 26
2,130 3,000 1,500
2,130 3,000 i, 600
1915
Short 184
225
1,300
5
68
63
4,700
5,000
1916
Sopwith-Schneider .
130
515
2f
109
26
1,790
1,790
1917
Fairey Type III.
Short 184. Improved
Short 320. ....
260
260 320
1,190
1,748 2,140
4i 5 3
93 79 73
46 63 74
4,159 5,250 7,000
4,300 5,250 7,000
14,000 5,500 3,5oo
1918
Fairey Type III. c.
Westland Single-Seater .
Hanriot Single-Seater
360 150 130
1,408 474 585
3
104 103 "7
46 3i 28
4,800
1,987 1,825
5,000
2,000
15,000 10,400
1919
S.V.A
2OO
495
3
124
30
2,195
1920
Fairey Type III.
45"
1,479
44
118
46
5,250
5,250
- Total weight carried for performance shown.
