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Aviation History
1919
1919 - 0412.PDF
MARCH 27, 1919 provided. These were driven either from the main engines or by separate small petrol engines. They occasioned, however, a very fruitful source of breakdown. It was subsequently found possible to replace them by what is termed a blower pipe, arranged to collect air from the slip stream of the propeller and to discharge it into a duct which distributes it to the various ballonets of the ship. The energy required to provide this air at high pressure was derived at the expense of a slight increase in head resistance, and in order to avoid this, arrangements were made whereby the blower pipe could be hinged about its top end and folded up along the under surface of the envelope. A method which is employed in Italian airships is to derive the pressure from the extreme bow of the envelope. The sufficiency of this pressure will be discussed later. The distribution of this air to the various ballonets necessitated the use of shut-off and non-return valves. These were originally made from sheet aluminium and gave very considerable trouble. It was found, however, that fabric valves arranged in the form of a sleeve, which can be partially turned inside out, gave very effective results. This valve is referred to as a " crab-pot." It is very easily operated and is almost completely airtight. The diagram shows the arrange ment of blower pipe and 'valves on the smallest type of ship. '* To enable pressure to be maintained when the main engines are not in use an auxiliary blower is necessary. The rotary type of blower driven by a separate small engine was used for a considerable time, but proved capable of improvement both as regards space occupied, reliability and efficiency. A new type of blower was devised and consisted of a small specially designed propeller discharging into a shaped casing, shaped so as to avoid as far as possible all loss due to eddies. The shape of this orifice was very carefully determined and a large boss fairing was fitted to the propeller. Careful tests made by measuring the horse-power and out put of air showed that for the same horse-power and discharg ing against the same head the new blower would deliver three times the amount of air previously discharged by the rotary blower. Airship Valves In order to prevent the pressure in the envelope exceeding a predetermined maximum and causing danger of bursting, valves are provided to relieve the pressure in the ballonets and in the gas space. The valves are made automatic in their action, and the gas valve is set to blow off at a pressure somewhat in excess of that adopted for the air valve, thereby ensuring that an increase of pressure is first corrected by discharging air from the ballonet valves. If this rate of discharge is insufficient the gas valve then lifts, but the dis charge of gas is avoided wherever possible. It will be realised that if these valves were constructed and spring loaded in the ordinary way, the pressure of the valve on its seating would be gradually reduced as the pressure inside the envelope increased. When blowing off point was nearly reached there would be a considerable period while the valve was only very lightly pressed on to its seating and when leakage was extremely likely to take place. The' mechanism of the valve was, therefore, arranged so that the force tending to cause the valve to open decreased as the valve opened, and the result produced was, therefore, that the valve would remain on its seating until the predetermined pressure was reached. It then opened wide and would remain open until the pressure had fallen slightly below that at which the valve opened. This gives a very determined action and materially reduces the amount of leakage from a valve while it is " stuttering " in the very slightly open position. The gas valves used for relieving pressure in a rigid airship are fitted in the lower part of the bag where they canj-b* reached from the keel gangway. They are automatic, but are much larger and loaded to a much lower pressure. In addition to these relief valves, hand-operated manoeuvring valves are fitted at the top of most of the bags so that gas can be released in order to alter the trim of the ship. Bow Stiffening for High Speed The pressure which it is necessary to maintain IU a m»u- rigid envelope depends upon two things : Firstly, the pressure which is necessary to prevent the envelope from collapsing ABOVE THE CLOUDS.—A view from S.S. 26. 412
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