Lighter-Than-Air - An Overview
Airplanes are heavier than air and fly because of the aerodynamic force generated by the flow of air over the lifting surfaces. Balloons and airships are lighter-than-air (LTA), and fly because they are buoyant, which is to say that the total weight of the aircraft is less than the weight of the air it displaces. The Greek philosopher Archimedes (287 BC – 212 B.C.) first established the basic principle of buoyancy. While the principles of aerodynamics do have some application to balloons and airships, LTA craft operate principally as a result of aerostatic principles relating to the pressure, temperature and volume of gases. A balloon is an unpowered aerostat, or LTA craft. An airship is a powered LTA craft able to maneuver against the wind.
There are only two practical methods of producing a buoyant aircraft. If the air inside a suitably large and lightweight envelope is heated to a high temperature, the gas expands and a sufficient amount of fluid (air is a fluid) may be forced out of the interior so that its weight decreases and the total weight of the craft becomes less than the amount of fluid (or air) displaced. Hot air balloons were flown by the brothers Joseph and Etienne Montgolfier as early as the spring of 1783. While the materials and technology are very different, the principles used by the earliest eighteenth-century experimenters continue to carry modern sport balloons aloft. Since 1960, hot air sport airships have also been constructed.
The other means of achieving buoyant flight is to fill the envelope with a gas that is sufficiently lighter than air. The first hydrogen-filled balloon was designed and constructed by Jacques A.C. Charles and launched from the Champs de Mars in Paris on August 27, 1783. Jean Pierre Blanchard, a French aeronaut, made the first free flight with a gas balloon in the United States from Philadelphia in January 1793.
First-generation aeronauts quickly realized that hydrogen balloons would remain in the air longer, and fly higher and farther, than hot-air balloons. The basic elements of gas balloon technology fell quickly into place. A valve at the top of the balloon, controlled by a valve line dropping through the appendix of the balloon into the basket, is used to release gas, making the balloon heavier and causing it to descend. Jettisoning a small amount of ballast, usually water, shot, or sand, would lighten the balloon and cause it to rise. A combination hot air and gas balloon is called a Roziere balloon, in honor of it inventor, Pilatre de Rozier, who with Francois Laurent, the Marquis d´Arlandes, was the first human being to make a free flight.
While there were no revolutionary changes in free balloon technology until the development of the hot-air sport balloon following World War II, there were significant refinements during the nineteenth century. In Europe and America, city illuminating gas was commonly used where possible rather than hydrogen. In cases where inflation was undertaken in rural areas of towns where illuminating gas was not available, portable hydrogen generators were often used. The best known of these was produced by the American Thaddeus S.C. Lowe, who pioneered observation ballooning with the Union Army during the Civil War. The use of the drag rope was commonly adopted to assist in maintaining a fairly constant altitude close to the ground. A long line was allowed to drag on the ground behind a balloon in flight. When the balloon rose, it lifted additional rope, and weight, off the ground, slowing its rise. If the balloon began to drop, it would trail additional line on the ground, reducing the weight of the craft and slowing the descent.
Balloons served a variety of purposes in the eighteenth and nineteenth centuries. They provided a spectacular means of public entertainment throughout this period. Show people who dazzled ground-hugging spectators with their death-defying feats of aerial daring-do were popular in both Europe and America. Attempts to put tethered balloons to work as battlefield observation platforms began with the wars of the French Revolution and continued through World War I. There can be little doubt, however, that the balloon was most successfully used gathering scientific information on the upper atmosphere. From the beginning of the nineteenth century to the dawn of the space age, the balloon carried instruments and human beings to the roof of the atmosphere. The most important change in ballooning in the twentieth century was the introduction of inert helium as the primary lifting gas for American balloons and airships. As the United States controlled the entire world's supply of helium, other nations continued to fly with inflammable hydrogen.
Science and sport remain the most common applications of ballooning today. Balloons remain meteorological workhorses and continue to carry instrument packages aloft for short and long-term voyages into the atmosphere. Hot air balloons are an important part of modern sport aviation. Finally, the highly visible attempts to accomplish one record after another, from nonstop crossings of the oceans, to altitude records, to flights around the globe, capture the imagination of the press and public. Aerostation, the oldest form of flight, is alive and well and as capable of exciting public interest and enthusiasm as it was two centuries ago.
The dream of a navigable, or dirigible, balloon can be traced to the late eighteenth century. Not until the middle years of the nineteenth century were the first serious attempts made to realize that dream, however. A French experimenter, Henri Giffard, is usually credited with flying the first primitive airship in 1852.
Airships are traditionally divided into three classes: rigid, semi-rigid and non-rigid. Non-rigid airships, or pressure airships, depend on the internal pressure of the gas in the envelope to maintain their shape. The blimps so familiar to viewers of major sporting events are pressure airships. Semi-rigid airships are pressure airships with a rigid keel structure.
Beginning in the last quarter of the nineteenth century, French constructors were active and successful in the design and operation of very large semi-rigid military airships. During the period 1900-1906, small, usually single-person, blimps of the sort popularized by the Brazilian Alberto Santos-Dumont were used to entertain mass audiences in Europe and America. It was the emergence of the German Zeppelin during the years 1900-1913 that marked the genuine coming of age of the rigid airship, however.
Vehicles of commerce and weapons of war, Zeppelin airships offered the first practical means of transporting very heavy loads over very long distances through the air. As such, they served as transitional vehicles, performing a variety of duties from bombing enemy cities to providing transoceanic passenger service in an era when those tasks were beyond the capability of heavier-than-air craft. The era of the great rigid airship came to an end before World War II, when airplanes were able to carry substantial payloads over transoceanic distances at far higher speeds. Ultimately, basic technological limitations led to the demise of the rigid airship. The catastrophic destruction of the airship Hindenburg by fire in May 1937 marked the effective end of the rigid airship, although the dream of reviving the era of the "ships in the sky" continues to capture the imagination of enthusiasts.
Blimps continued in service with the U.S. Navy through World War II and into the era of the Cold War before being dropped from the inventory. Pressure airships continue to thrill spectators and to serve the needs of the media and advertisers. Proponents of the rigid airship continue to search for an economic niche that would justify a return of the large rigid airship.
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