FREQUENTLY ASKED QUESTIONS ABOUT AIRSHIPS
Some answers

 Q. What is an airship?

A. An airship is an airborne vehicle obtaining most of its lift from lighter-than-air gas, usually helium, contained in the envelope. Additional lift can be provided by vectored thrust from the engines but since most is derived from the helium, the engines are needed mainly to drive the vehicle through the air and to provide power for the on-board systems. The result is considerable fuel economy when compared with heavier-than-air machines. It also allows an airship to be more environmentally friendly.

There are three types of airship - rigid, semi-rigid and non-rigid:

The rigid airship maintained its shape and rigidity by an internal skeleton. The lifting gas was contained in a series of separate gas cells and the skeletal structure was enclosed by the outer cover.

The semi-rigid airship maintained its shape mainly by the pressure of the lifting gas in the envelope but that was supplemented by a rigid keel. The new Zeppelin NT airships in Germany can be considered to be semi-rigid airships of modern and novel design. In the 1920s an Italian semi-rigid airship flew from Norway to Alaska and is now recognised as the first aircraft to fly over the North Pole.

The modern non-rigid or pressure airship maintains its shape solely by the pressure of the helium in the envelope, supplemented by an adjustable volume of air within internal ballonets. Today most gas airships are of this type. A non-rigid airship, sometimes called a blimp, consists of:

- the envelope - the primary structure containing the helium gas and the ballonets.

- the ballonets - air bags inside the envelope which regulate the internal pressure, shape and trim. Air is squeezed out of the ballonets as the gas expands with increasing altitude and forced back in again as the helium contracts when the airship descends.

- the gondolais the airship's cabin containing the cockpit, engine compartment and facilities for crew, passengers and cargo. It is suspended from cables attached either to an internal load curtain or externally to the envelope sides.

- propulsion is provided by airscrew engines and may include vectored thrust to assist take-off, landing and holding stationary flight.

- fins, rudders and elevators provide dynamic stability and steerage.

Q. What is special about an airship?

A. An airship can be more spacious than any other aircraft. It also has the unique advantage of being able to stay airborne for long periods. Whereas fixed and rotary wing aircraft measure flight time in hours, an airship can stay aloft for days. It can also hover silently.

Q. What is the envelope made of?

A. The envelope fabric of today's airships is a sandwich of man-made materials, e.g. Dacron, Polyester, Mylar and Tedlar bonded with Hytrel. Such modern materials minimise helium leakage and give an envelope life expectancy of many years.

Q. Have non-rigid airships been used successfully in the past?

A. During World War I nearly 300 British non-rigid airships protected allied convoys from U-boat attack. Not one surface vessel was lost through enemy action when under airship protection. During World War II the USA deployed some 140 non-rigids to protect coastal convoys. Again no surface vessel so protected was sunk and only one airship was lost - its engines failed and it was shot down (slowly!) by the U-boat it was attacking.

After the war, non-rigid airships continued to be developed by the US Navy. They were tested in a variety of simulated combat conditions in all weathers, and acquitted themselves very well. With a crew of 25 they could operate with the fleet for eight days before refuelling. In civil use they have been employed in advertising, filming, scientific observation and in support of law enforcement agencies.

Q. What applications do airships have today?

A. Airships have applications in both military and civilian roles. An airship's ability to carry a large radar antenna within its envelope to a great height, together with its ability to stay on station for days at a time, makes it ideal for combat surveillance work. At sea an airship can provide over-the-horizon coverage up to 130 nautical miles against very small radar targets such as cruise missiles.

In civil use airships operate against drug smugglers and other criminals. Cabin space and low vibration allow crews to remain alert over long periods. Airships can also be used in border patrols, fire-watching and bulky cargo transportation. They have uses in advertising and more recently as very stable scientific observation platforms.

Q. Do airships have advantages over other types of aircraft?

A. Airships can carry large-aperture radars more effectively and for a much lower cost than any fixed-wing aircraft. Since the radar is mounted within the envelope (a very benign environment), its shape and size are not restricted by the aerodynamic constraints governing externally mounted radar assemblies on aeroplanes.

The airship's lift allows it to remain on station for days at a time. Stationary and vibration free low-speed flight allows on-board surveillance systems to operate in high-clutter environments. Such abilities give airships considerable advantage as observation platforms over other types of aircraft. The airship itself has a very low radar signature - roughly equivalent to a light twin-engined aeroplane.

The initial cost of an airship is considerably less than that of any comparable aircraft. Airships also cost less to support and maintain on a day-to-day basis. They are also highly fuel-efficient.

Q. Are airships safe?

A. Yes. Firstly, helium gas is totally inert, non-flammable and an excellent fire extinguisher. Secondly, a modern airship's advanced construction and sophisticated control systems make it extremely safe to operate even in bad weather. Historically non-rigid airships have by far the best safety record of any type of flying vehicle.

Q. What happens if the envelope is punctured?

A. Not a lot really! Unlike a toy balloon, the lifting gas in an airship is at a very low pressure, just sufficient to keep its shape. Should a small hole occur, the gas escapes very slowly. Even with a number of such holes the buoyancy is not substantially affected and they can be repaired during periodic maintenance inspections.

A large hole would mean that the airship's mission would have to be aborted but the airship could still return to base for the necessary repair. Even in such circumstances the airship is generally capable of remaining aloft for some time.

Q. How can a lighter-than-air vessel operate in severe weather?

A. An airship is not a free balloon; its engines give it a handling and steerage capability that frees it from all but the severest gusts. Its structural resilience, riding quality, control responsiveness and other attributes provide altogether excellent weather handling qualities. Icing and snow are of less importance in flight than for an aeroplane - where they can be catastrophic. In short, airships can and do fly in all weather conditions in which fixed-wing aircraft can fly. The only restriction to flight in severe weather is the ability to take-off and land in winds exceeding 30 knots.

Q. How fast do airships travel?

A. An airship is a slow-speed vehicle, unlikely to exceed 80 knots in flight. But an airship is considerably faster than surface ships.

Whilst clearly unable to achieve the higher speed performance of fixed and rotary wing aircraft, in surveillance roles the airship offers an important benefit. It can 'park on station' for extended periods - weeks if necessary. This capability allows the on-board systems to detect targets both accurately and effectively even in the presence of electronic clutter. Furthermore, at slow speeds radar can more easily detect small targets. Thus, an airship's relatively low speed can be a positive advantage both in military and other surveillance roles and need not be a barrier to its civil employment. Advertisers like the stately progress of their flying billboards.

Q. With rapid advances in technology today, what can we expect airships to be like in the future?

A. The use of very lightweight two-stroke aviation diesel engines, gas turbines and even solar energy will improve propulsive efficiency. The latter will also have the great advantage of allowing flight at a constant load. One problem with present fuel systems is the gross change in buoyancy as fuel is consumed and the need to make up for this, for example, by recovering water from the engine exhaust.

Small steerable bow and stern thrusters have now been developed to improve manœuvrability. They will significantly reduce the number of ground crew used at take-off and landing.

The use of strong, lightweight plastics for the hull might well herald the design of a large monocoque rigid airship with the resulting advantages of robustness and speed.

CONCLUSION: Airships are a safe, attractive and economic proposition. They do not rival aeroplanes and helicopters, but fulfil entirely different roles, filling in the gaps left by these more conventional aircraft.

 

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