Airship - Practical Comparison With Heavier-than-air Aircraft

Practical Comparison With Heavier-than-air Aircraft

The advantage of airships over airplanes is that static lift sufficient for flight is generated by the lifting gas and requires no engine power. This was an immense advantage before the middle of World War I and remained an advantage for long distance, or long duration operations until World War II. Modern concepts for high altitude airships include photovoltaic cells to reduce the need to land to refuel, thus they can remain in the air until consumables expire.

The disadvantages are that an airship has a very large reference area and comparatively large drag coefficient, thus a larger drag force compared to that of airplanes and even helicopters. Given the large frontal area and wetted surface of an airship, a practical limit is reached around 80–100 miles per hour (130–160 km/h). Thus airships are used where speed is not critical.

The gross lift capability of an airship is equal to the buoyant force minus the weight of the airship. This assumes standard air temperature and pressure conditions. Corrections are usually made for water vapor and impurity of lifting gas, as well as percentage of inflation of the gas cells at liftoff. Based on specific lift (pounds of lift per thousand cubic feet of lifting gas), the greatest static lift is provided by hydrogen (71 lbs. lift/1000 cubic feet of gas) with helium (66 lbs. lift/1000 cubic feet of gas) a close second. At 39 lbs./1000 cubic feet, steam is a distant third. Other gases, such as methane, carbon monoxide, ammonia and natural gas have even less lifting capacity and are flammable, toxic, corrosive, or all three. Operational considerations such as whether the lift gas can be economically vented and produced in flight for control of buoyancy (as with hydrogen) or even produced as a byproduct of propulsion (as with steam) affect the practical choice of lift gas in airship designs.

Considering the Hindenburg disaster, one may question why such a flammable gas as hydrogen was used in the first place, when it is only marginally better than helium as a lifting gas. The answer is that hydrogen can be produced easily and economically through the electrolysis of water or by chemical reactions, whereas helium is scarce and expensive, occurring only in trace amounts in a few natural gas deposits.

In addition to static lift, an airship can obtain a certain amount of dynamic lift from its engines. Dynamic lift in past airships has been about 10% of the static lift. Dynamic lift allows an airship to "take off heavy" from a runway similar to fixed-wing and rotary-wing aircraft. However, this requires additional weight in engines, fuel and landing gear, negating some of the static lift capacity.

The altitude at which an airship can fly largely depends on how much lifting gas it can lose due to expansion before stasis is reached. The ultimate altitude record for a rigid airship was set in 1917 by the L-55 under the command of Hans-Kurt Flemming when he forced the airship to 24,000 ft (7,300 m) attempting to cross France after the "Silent Raid" on London. The L-55 lost lift as the descent to lower altitudes over Germany compressed the gas left in the cells, and thus the weight of air displaced. L-55 crashed due to loss of lift. While such waste of gas was necessary for the survival of airships in the later years of WW I, it was impractical for commercial operations, or operations of helium-filled military airships. The highest flight made by a hydrogen filled passenger airship was 5,500 ft (1,700 m) on the Graf Zeppelin's around the world flight. The practical limit for rigid airships was about 3,000 feet (900 m), and for pressure airships around 8,000 ft (2,400 m).

Modern airships use dynamic helium volume. At sea level altitude, helium only takes up a small part of the hull, while the rest is filled with air. As the airship ascends, the helium inflates with reduced outer pressure, and air is pushed out and released from the downward valve. This allows an airship to reach any altitude with balanced inner and outer pressure if the buoyancy is enough. Some civil aerostats could reach 100,000 ft (30,000 m) without explosion due to overloaded inner pressure.

The greatest disadvantage of the airship is size, which is essential to increasing performance. As size increases, the problems of ground handling increase geometrically. As the German Navy transitioned from the "p" class Zeppelins of 1915 with a volume of over 1,100,000 cu ft (31,000 m3) to the larger "q" class of 1916, the "r" class of 1917, and finally the "w" class of 1918, at almost 2,200,000 cu ft (62,000 m3) ground handling problems reduced the number of days the Zeppelins were able to make patrol flights. This availability declined from 34% in 1915, to 24.3% in 1916 and finally 17.5% in 1918.

So long as the power-to-weight ratios of aircraft engines remained low and specific fuel consumption high, the airship had an edge for long range or duration operations. As those figures changed, the balance shifted rapidly in the airplane's favor. By mid-1917 the airship could no longer survive in a combat situation where the threat was airplanes. By the late 1930s, the airship barely had an advantage over the airplane on intercontinental over-water flights, and that advantage had vanished by the end of WW II.

This is in face-to-face tactical situation, current High Altitude Airship project is planned to survey hundreds of kilometers as their operation radius, often much farther than normal engage range of a military airplane. This provides better early warning, even farther than the Aegis system. The current Aegis system is often based on a sea vessel like Ticonderoga Class and Burke Class, which have restricted radio horizon and line of sight. For example, a radar mounted on a vessel platform 30 m (100 ft) high has radio horizon at 20 km (12 mi) range, while a radar at 18,000 m (59,000 ft) altitude has radio horizon at 480 km (300 mi) range. This is significantly important for detecting low-flying cruise missiles or fighter-bombers.

The blimp remained a viable military system only until the conventional submarine was replaced by the nuclear submarine. Today, airships are used primarily for command, control and as a communication platform; to establish and maintain reliable and secure connectivity among all forces, provide transparent data across the echelons; precisely locate friendly and enemy forces; detect targets on an extended battlefield at a minimal exposure to enemy forces; real time targeting; navigation assistance; battle management; monitor radio conversations, etc.