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there is, therefore, a tendency for the lee wire to go slack. When the gust ceases and this wire draws taut, a serious impulse is brought on the bow of the ship.

It was also found that the wires of this system were so nearly horizontal that they fouled the car of R33-

To overcome these difficulties, a running system was devised. Various alternative forms were tried giving varying degrees of rigidity of support. The final system which has been found most satisfactory is that shown in fig. 26. This has the additional advan-

Gl. G2. G3. Denotes swivelling Pulley attached to Ground Bollards S. Denotes Mooring Point. A. B.C. Denotes Rings.

FIG. 26.

tage that only the comparatively short wires, SA, SB, and SC, are carried in the ship, the remainder of the wires lying on the ground and being picked up when the ship lands. Complete experiments with this system have not been carried out, but it is considered that a ship could withstand any ordinary wind forces when secured in this way. She would be much more difficult to secure in this way than to a mast, and could not be easily supplied with water ballast, fuel or additional gas.

In order to meet the greatly increased requirements for small air- ships for anti-submarine patrol during the war, a system of mooring- out grounds was developed. These mooring-out stations were formed by making clearings in suitable woods and cutting a comparatively narrow avenue through the wood to the clearing. Small airships were secured in these clearings, and re-fuelled and repaired in exactly the same way as in proper sheds. The protection was so good that ships have been totally undamaged even though winds of 60 m. an hour were blowing over the top of the wood at the time.

Airship Sheds. The construction of airship sheds has been an important item in the expense of airship work. The cost of the shed increases very rapidly with height and with the span, both of which must be considerable with any but the very small ships. Apart from the cost of the shed, there is considerable difficulty in taking a ship into the shed in any but very calm weather. When a wind is blow- ing across the mouth of the shed, the airship has to be hauled broad- side on to the wind in order to pass in through the door, and this represents a very difficult operation when the wind is of considerable strength or of a gusty nature. In order to afford protection during this operation, all early airship sheds were provided with wind- screens running from the corners of the shed outwards parallel to the axis. These screens were of a height nearly equal to that of the shed, and afforded considerable protection against the horizon- tal force of the wind. They, however, caused a serious eddy to be formed, which produced a vertical disturbance on the ship nearly as difficult to overcome as the horizontal force which would have existed had there been no wind-screens present. Experiments were carried out with the wind-screens formed of expanded metal, and with screens of corrugated iron in which 30 % of the sheeting had been omitted. These screens, although they reduced the horizontal wind to a smaller extent than the solid screens, avoided the serious vertical air disturbance and were, for that reason, considerably preferable.

Experience in Germany had, however, shown that a system of rails provided with easily running trolleys was the most satisfactory system of supporting the ship against sideways forces. These rails ran out from the corners of the shed parallel to the axis, and the side-guys of the ship were attached to trolleys running on these rails. The support of the ship obtained in this way is so good that wind-screens are rendered unnecessary, and the vertical air disturb- ance connected with them is thereby avoided. Even with this sys- tem of handling rails, the housing of an airship presents considerable difficulties. A landing party of several hundred men is required to receive a 6o-ton airship on the landing ground, to carry her to the end of the handling rails and to haul her round parallel to the rails. The air in the neighbourhood of the shed is necessarily so disturbed that considerably greater difficulty is experienced near the shed than when on the open landing ground or in the neighbourhood of a moor- ing mast. The difficulties connected with airship sheds are, there- fore, considered to be so great that the shed must only be regarded as the dock, the mooring mast being regarded as the normal method of securing an airship between flights.

When secured to the mast the airship can be supplied with gas, water ballast and fuel. The passengers can be passed up the mast by a lift and can walk through the bow of the ship down to the cabin. The airship appears to behave satisfactorily in any wind. The most

difficult conditions to meet are those in which there is no wind but rapid changes of temperature which affect the lift of the ship. Tiiis necessitates rapid adjustment of the ballast in the ship by taking in or discharging water. As long as there is a considerable wind the trim can be regulated by the elevators, as in flight.

Attempts have been made to anchor a ship to the ground by a single wire. This operation would have considerable advantages for a ship which became broken down and required to avoid drifting with the wind. At sea a drogue can be lowered into the water, and the ship will ride to it satisfactorily provided she is correctly trimmed some five degrees up by the bow in order to derive the necessary dynamic lift. It is, however, necessary to steer the ship continuously while secured in this way, exactly as though in flight. Anchoring to the ground is a considerably more difficult problem. A grapnel cannot obtain a sufficiently firm hold to resist the impulsive upward pull in the airship trail rope. Experiments were carried out with a form of dropping grapnel consisting of a large, suitably shaped weight dropped from a height of some 200 feet. This grapnel obtained a satisfactory hold either on very hard ground or on soft ground where the penetration was very deep. The hold was, however, quite unyielding, and the shock produced on the ship when thus checked was far too serious. Various forms of friction device to allow a grad- ual check to be brought on the ship were tried, but were never found sufficiently satisfactory for adoption.

Towing. The earliest test in connexion with airship towing is perhaps the most interesting one. Naval Airship No. 2 broke down about 40 m. from Farnborough, and in order to save the loss of gas and the probable damage to the ship that would have attended her deflation, she was towed home by another airship, ' ' Eta, " of a slightly greater size. The operation presented no difficulty whatever. " Eta " landed alongside the damaged ship; a wire some 600 ft. in length was laid out between the two ships; both ships were made light and allowed to rise into the air. The towing ship then went ahead slowly and towed the disabled one back to Farnborough.

Occasion for repeating this towing operation has not since pre- sented itself, but the complete success which attended the first attempt indicates that there is no serious difficulty in connexion with it. It is probable that for certain special purposes, where large weights have to be carried and where speed is not of great importance, the towing of one or more " air barges " by an airship presents very interesting possibilities.

Naval Towing. Various trials were made to determine the possi- bility of towing an airship to the scene of operations so that she should arrive there with her full supply of petrol still available.

In May 1916 a Coastal airship was, after a few preliminary tests with a motor launch, towed by a light cruiser steaming at 26 knots up, down and across a wind of some 15 knots. In a further trial the airship was hauled down to the deck of the cruiser and the crew changed and gas and fuel supplied. The same operation was carried out at a height of 150 ft. to provide for occasions when the sea was too bad to allow the airship to be brought close down. These trials were entirely satisfactory.

In Aug. 1918 a ship of the S.S. class carried out extended trials in tow of a submarine. These caused no difficulty except that it was desired to make the ship capable of being towed without a crew. Arrangements for the automatic maintenance of pressure and the greater degree of stability required caused the extension to this much more difficult operation to be abandoned.

In Nov. 1918 the towing of an S.S. ship by a destroyer was again actively being developed with a view to replacing kite-balloons by airships for convoy work. In Aug. 1919 N.S-7 carried out a long tow- ing operation with the fleet. She was in tow continuously for some 40 hrs., and was gassed and refuelled in a wind of 30 knots.

The conclusion to be drawn from these tests is that an airship can be towed without difficulty provided she is steered and handled as in flight. The towing is little relief to the crew, but the expenditure of fuel is avoided. The crew can be changed and fuel and gas can be supplied in reasonably fair weather.

Airship Fabrics. The outer cover of a rigid airship has to form a smooth fairing over the hull structure and gasbags. Unless it remains taut under all conditions the passage of air over it and more particularly the disturbed air in the vicinity of the airscrews gives rise to flapping, which not only increases the ship's resistance but may cause the cover to chafe and ultimately be torn. The taut- ness is produced and maintained by a dope, applied to the fabric partly before and partly after the sheets of fabric are laced to the hull framework. The dope is generally similar to that used on aero- plane wings, but the unsupported expanses of fabric are so large usually 3 metres by 5 metres that the prevention of flapping is a much more difficult problem ; indeed, these surfaces are so large that the maintenance of a correct difference of pressure between the inside and the outside of the ship is more effective than exactly correct tautness. The weight of the outer cover is such a large proportion of the total of the ship that very great care must be taken to apply only the minimum of dope necessary.

The outer surface must be made reflecting in order to reduce as far as possible the amount of radiant heat absorbed and transmitted to the gas in the cells and the air inside the hull.

The pigment or dye employed in the dope must be such that the part of the light which most rapidly deteriorates the cellulose of the