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Military


Airship Mooring Mast

The earliest activity of airships had been limited rather by the ability to handle them on the ground than by their ability to meet weather conditions in flight. British Rigid Airship No. 1 was moored by the bow to a mast and sheltered by a screen on Cavendish Dock, Barrow, before the ship was flown. This trial was successful, the ship remaining safe during winds with gusts up to 48 m. an hour. In the course of these trials the screen was abandoned.

The Royal Aircraft Factory in 1912 devised and used continuously for many months a new form of mooring mast to which a non-rigid airship was attached while floating in the air. To prevent the ship overriding the mast in gusty weather and to facilitate approach, the mast carried at its head a swinging cone duly counterpoised, into which the nose of the airship was drawn by a rope running down the inside of the mast. The cone was free to rotate about the axis of the mast as well as to rock vertically on a universal joint and the mast functioned satisfactorily, save that side gusts caused the cone to rub the bows of the ship with a tendency to bend it. These mast moorings were the precursors of one of the great developments in airship use, but till they were adopted generally the airship had to be housed in a shed, and hence the activity of the ship was limited to those occasions when it was possible to take her out in winds of less than 10 or 15 m. an hour with a reasonable chance of rehousing her under equally good conditions.

Under war conditions this restriction was serious, and the method of the mooring mast was again examined. A non-rigid envelope rigged with a dummy car was secured to the head of a mast at Kingsnorth, first with a cone but later with the cone removed. The ship was reinforced to take the pull of the mast, by fitting inside her bow a spar, the after-end of which was supported by a cone of cords led slightly forward and secured round a circle on the inside of the envelope. The tension in the fabric of the envelope and in these cords held the spar rigidly, and supplied the reinforcement which was necessary for stiffening the bow of the envelope while in flight and also for mooring.

A further set of experiments was carried out at Barrow with a ship secured to a short stump mast, attached to her mooring point and stepped on a lighter. The point of attachment was not on the axis. Indeed, it was so low on the envelope that side gusts produced serious rolling. Accordingly a form was devised in which a somewhat taller mast was fitted with a horseshoe head, so that fittings carried at the top of its arms could be attached to suitably reinforced points aft of the nose of the envelope. This gave support against rolling, but the point of attachment was some distance aft on the ship, and consequently the steadiness was not quite so good as when the envelope was attached by its extreme bow point.

Definitely comparative tests between mooring at the nose, using the spar inside the bow of the envelope and using the horseshoe mast were carried out at Pulham. After considerable time the internal spar of the former broke, for a reason that was not explained, and the horseshoe mast was preferred. As, however, other means were found for mooring the small ships at advanced patrol stations, the horseshoe was little employed.

Mast mooring was, however, realized to be important for rigid airships, and prolonged trials with R24 secured to the head of a mast at Pulham were instituted in July 1919 with success. The ship later remained continuously at the mast for 70 days and experienced winds up to 35 with gusts of 43 m. per hour. Difficulty was experienced in taking the ship to the mast in any but light winds.

Experiments were continued with R33 on Feb. 2 1921, and up to the beginning of June 1921 the ship had worked entirely from the mast. On a few occasions she had been into the shed, but never for -more than five days. During April and May 1921 she averaged between four and five flights per week. In this case the mast is provided at its upper end with a single arm, pivoted at its middle point. Down the centre of this arm passes the wire rope, which is attached to that dropped by the ship and by which she is hauled in. This arm, therefore, comes in contact with the bow of the ship before that has actually reached the head of the rigid mast, and gives improved safety as the ship approaches the masthead. Difficulty was experienced with the control of the winch which hauls in the ship's wire. In the experiments with R24 a kite-balloon winch was employed and abandoned owing to its irregular action and controL For the experiments with R33 a steam ploughing engine was used temporarily and found to be satisfactory.

The process of landing to the mast consists in the airship dropping to the ground a rope some 1,000 ft. in length, which is then secured to the rope led from the winch up the centre of the mast and down to the ground. The winch hauls in these ropes and draws the ship to the masthead. There is no difficulty until the ship comes within some 200 ft. of the masthead, but as this distance decreases there is a tendency of the ship to swing both sideways and fore and aft, under the influence of gusts of wind. This difficulty is less serious when the ship is trimmed somewhat down by the stern, so that the wind force on the bow is approximately in the same direction as the tension in the wire. If this arrangement is not made, the variation in the wind force causes swinging of the bow of the ship, and a tendency to override and strike the head of the mast.

Even with the stern of the ship trimmed considerably down, there was still, owing to disturbed conditions, a distinct tendency to swinging, and it was often desirable to employ side-guys led from the bow of the ship to fixed points on the ground, in order to guide the bow to the masthead. With these arrangements, it was possible to secure a 60-ton ship to the head of the mast in winds of 30 m. an hour, with not more than eight men in addition to those actually in the ship. During the time that R33 was secured to the Pulham mast, an engine was hoisted out and replaced by a spare, and a gasbag was deflated and replaced by a spare.

As an alternative to the system of mooring an airship to a mast, and as a more temporary arrangement, the "three-wire system" was developed from one in which the ship was secured by her mooring-point to the head of a pyramid formed of three cables, the lower ends of which were secured to the points of an equilateral triangle of some 800 ft. on a side.

The height of the apex was arranged to be between 100 and 200 ft. in order that the downward component of the wires when resisting the wind force should not be excessive. A considerable weight of wire was, however, necessarily supported by the ship, and a large amount of static lift was therefore necessary. This system gave considerable success during 1918. but was found defective in gusty winds owing to the liability of one wire going slack under the influence of side gusts. A wind along the axis of the ship produces a certain amount of dynamic lift which balances the downward component due to the tension in the wire.





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Page last modified: 11-07-2011 15:29:13 ZULU