Airship Terms
Airship designers needed to balance size, strength, engine power and lifting capacity against the weight of the structure, its payload and its drag – the greater the volume of gas, the greater the payload that could be carried – engines, fuel, passengers, freight and structural weight. The crew was carried in a cabin, box, or metal framework ‘car’ suspended from the envelope. For efficiency of movement an airship also had to be streamlined and firm in order to cut its way through the air.
Non-rigid – a gasbag or envelope with internal ballonets of air – limited in size of envelope due to the fact that the structure had no rigidity. This type is often known as a blimp.
Semi-rigid – the gasbag was the envelope of the non-rigid, but it also had a keel giving greater strength to the structure. Owing to the increased weight of structure, more lifting capacity had to be given over to lifting the weight of the airship rather than payload.
Rigid – had a strong but light metal (or wooden) framework with a doped fabric cover. Inside were suspended gasbags. This type could be built much larger, but with the penalty of a further increase in weight.
Envelopes were made of two thicknesses of rubberised fabric with rubber between and on the inside surface. They were doped (weatherproofed) on the outside. An old-fashioned Macintosh would be a reasonably near equivalent. Messrs Vickers formed a subsidiary company, the Ioco Rubber & Waterproofing Co, which cornered a large share of the market. Some early airships had envelopes made from goldbeater’s skin: the outer membrane of part of the large intestine of the ox. Thousands of these were cleaned and dried then glued to a fine cloth backing and varnished. The result was light, gas-tight and flexible, but was also expensive and became brittle within three years. It had first been used by the Royal Engineers in the 1880s for balloon envelopes, the manufacturing process of which was a jealously guarded secret of the Weinling family of East London.1
Ballonets – bags smaller than the main gasbag and slung inside it. In flight air is pumped in or discharged as required, so maintaining the internal pressure and shape of the envelope as well as the trim of the airship.
Duralumin – the name given to a family of aluminium alloys containing small and varying amounts of copper, as well as iron, magnesium, manganese and silicon. It was found to be light, strong, very ductile, easily machined and pressed, with mechanical properties similar to those of mild steel. It has roughly the same tensile strength as mild steel, but with one third of its weight. Alfred Wilm patented the formula for the alloy in 1909, and granted an exclusive license for its manufacture to the company Dürener Metallwerke. The Duralumin name was derived from Dürener Metallwerke, and aluminium.
Hydrogen – the lightest gas known – colourless, odourless and tasteless. At major British airship stations it was produced in an on-site manufacturing plant, stored in gas-holders and piped underground to the airships’ hangars. Gas cylinders were used for storage at out-stations. It was generally extracted using the Silicol Process which was by chemical reaction when powdered ferrosilicon was stirred with water and gradually admitted to a chamber containing a hot, strong solution of caustic soda. This resulted in 99.9% pure hydrogen within an hour. A large plant could produce 1,000,000 cubic feet (28,300 cubic metres) of hydrogen per day.
Eta patches – these were developed in 1913 for the final British Army airship, Eta. The rigging cables supporting the car beneath the envelope were subdivided into thirty-six attachment points, joined to the envelope by kidneyshaped adhesive patches which were also stitched in place. The load was by this means spread evenly without the need for the cumbersome netting and bridles used previously. The patches were essential to the development of the SS Class airship.
Steering is by means of control surfaces at the rear of the envelope – rudder for movement to left and right, and elevators to point the nose upwards or downwards – or by swivelling the engines or propellers.
Static Lift – by displacement of air as a balloon. If the total weight of the balloon or airship is slightly less than the weight of displaced air then it will rise. Hydrogen is the lightest of all gases, but has the disadvantages of being inflammable when pure and explosive when mixed with air. Helium is much rarer, is less buoyant than hydrogen, but does not burn. Air weighs about 75lbs (34kg) for 1000 cubic feet (28 cubic metres), while the same amount of hydrogen weighs only 5lbs (2kg), so giving 70lbs (32kg) of lift. The same volume of helium weighs 10lbs (4kg) and so gives 65lbs (30kg) of lift. Therefore for each 32,000lbs (907 cubic metres) of hydrogen contained in an airship’s envelope there would be about one ton of gross lift. With a helium-filled envelope some 34,000lbs (977 cubic metres) would be needed for the same effect.
Dynamic Lift – from the reaction of airflow over the envelope and control surfaces when under way. This can be altered by trimming the control surfaces, eg nose down if light, or nose up if heavy.
Gross Lift – is the weight of the airship and all its contents.
Useful lift – is the weight that can be allocated to fuel, ballast, passengers and cargo.
Conditions of lift – these vary continually as gas (and the surrounding atmosphere) expands or contracts according to temperature and altitude. Moreover, the weather had a considerable effect, for example, rain falling on the envelope increased its weight, so making the airship loose height. The strength and direction of the wind also had a considerable effect on the airship’s speed and progress through the air.
Control of lift – in flight this is controlled by regulating the lift, releasing gas to descend and discharging ballast (normally water) to ascend.
Size – as an airship becomes larger in size its surface area increases as the square of its linear dimensions. If a large airship is three times longer than a small one, it has nine times as much surface area, but twenty-seven times as much volume and lift.
Mooring – an airship would come in to land nose to wind. The pilot then threw down a rope for the mooring party to grab hold of. It was prudent to let the rope earth itself to the ground first, as too eager a lunge for it would result in an electric shock. When it drew closer to the ground party they could reach up for the guy-ropes attached to the bows and the stern. When it was close enough to the ground, they moved forward tugging on the ropes to ‘walk’ the airship into the shed. Handling on the ground could be a tricky business as an airship presented a sizeable bulk to the wind and was naturally buoyant in this element. When safely in the shed maintenance could be carried out by the riggers and mechanics, with their patches, rubber solution and dope.
Take-off – the airship would be made positively buoyant so that it could be ‘walked’ out of the shed. Trim would be checked, the engine started, the order to, ‘Let go’ would be given and the craft would rise gently into the wind.
Good Qualities – airships are economical, quiet and capacious. They can fly long distances without refuelling and are, despite the popular misconception, safe in which to fly. Most of the notorious accidents were due to design faults, the use of hydrogen, or poor weather forecasting. The only paying passengers ever to die in an airship accident were the thirteen souls who were lost in the Hindenburg.
Bad Points – they are very slow, of very light construction and thereby prone to being flimsy. Large airships of the Zeppelin type are very expensive to construct. They are difficult to handle on the ground and need large hangars in order to protect them from the elements.
Blimp – there are a number of theories regarding the origin of this word, but it would appear that it is onomatopoeic and can be traced specifically to 5 December 1915, when Lieutenant A.D. Cunningham, RNAS, playfully flicked a finger against the envelope of SS-12 at the Capel air station in Kent and then mimicked aloud the sound it had made. A young midshipman, Victor Goddard, repeated the tale to his fellow officers in the mess hall before lunch the same day. It is believed that by this route the word came into common usage.
Gamma’s mishap of September 1912 is described on page 149. This contemporary press report captioning the photograph is something of an exaggeration.