LA STORIA DELLA MONGOLFIERA
A HISTORY OF AIR BALLOONS:
The air balloon is an aircraft that obtains the necessary force to leave the ground thanks to hot air, which is lighter than standard temperature air. Air balloons are classified as aerostats, vehicles that use gas to fly. In flight air balloons are not equipped with directional instrumentation, but are carried by the wind. This characteristic distinguishes air balloons from dirigibles, that also fly thanks to gas but are equipped with engines and propellers that control the direction of the aircraft. Like all aerostats, air balloons fly because the air filling them is lighter than the air around them, and so the aircraft is pushed up from the ground, following the well known principle of Archimedes. The major air balloon event in Italy is the Ferrara Balloons Festival in Ferrara.
AIR BALLOON TYPES:
All air balloons presently used for sporting events can be divided into two categories by the type of gas used to keep them airborne:
Hot air balloon: this air balloon is filled with hot air (hotter than the air surrounding it). Most air balloons used today are of this type.
Rozier air balloon: this type of air balloon combines the advantages of lighter gasses ( greater power and flight endurance) with those of hot air (altitude control) giving ideal performance. Rozier air balloons are the ones that were able, recently, to circumnavigate the Earth.
BRIEF HISTORY
The invention of hot air balloons goes back to the first centuries A.D. when the Chinese inflated and flew the first air balloons. However, it was not until 1783 that air balloons were used for human transport, when the brothers Montgolfier succeeded in lifting the first hot air balloon. In less than a month, professor Jacques Charles had taken his first flight in a gas balloon, and this type of balloon (with its greater power and flight endurance) became common from 1790 until the 1960’s. A third kind of air balloon, invented by Pilatre de Rozier (the Rozier air balloon), was a hybrid that made use of both hot air and a light gas for flotation. Rozier managed to construct a compartment that could be filled with hot air inside a hydrogen filled balloon. In 1780 Rozier attempted to fly across the English Channel but the air balloon exploded within the first half hour of flight. This accident gave Rozier the fame of being “the first man to fly and the first to die”. Only in the mid 1980's Rozier air balloons became safe enough to be used.
Present day air balloons used for sporting events were radically redesigned in the 1950's by Ed Yost, who introduced new materials such as nylon and propane burners. These innovations permitted him, in 1960 to make his first flight giving birth to a new sport.
STRUCTURAL PRINCIPLES AND PILOTING
An air balloon is composed of a capacious single-layered nylon balloon with a hole at the bottom called the skirt. The balloon is rigged to a basket, where the pilot and passengers ride. A burner is attached to the basket, just beneath the skirt, heating the air and pushing it inside the balloon itself. The hot air gathered in the balloon makes it lighter than the surrounding air and determines the upward push of the air balloon and basket. Air balloons can reach very high altitudes (hot air balloons used for scientific purposes, can reach 20,000 m., much higher than an average airplane flies).
Present day air balloons are made of very light and strong synthetic fabrics (such as interlaced nylon). During construction, the fabric is cut into long sections that are sewn together to form the balloon or envelope. The seams are then covered by a highly resistant tape, that sustains the riggings hooked to the basket. The burner ( which resembles a flame thrower) uses liquid propane stored in tanks at the bottom of the basket.
The upward push of an air balloon mainly depends on the difference between external air temperature and that of the air inside the balloon. This means that on a hot muggy day the upward push of the air balloon will be inferior to the one experienced on a cool or cold day (in the same general conditions). For this reason take offs are scheduled during low temperature hours (preceding dawn or at sunrise). Also, thermal movements frequent during the day which make the balloons difficult to control, can be avoided.
Most nylon air balloons have operative temperatures that can reach 120 C (248 F). Nylon has a much higher melting point (around 230 C, 446 F), but high temperatures can quickly deteriorate the nylon fabric of the envelope (thus shortening the life of the air balloon). A modern air balloon, flown at the recommended temperatures, can be operative for 400 to 500 flights without changing the envelope. Of course, if high performance standards are required, the pilot can increase the temperature of the burner up to 200 C (392 F) maximum, knowing that the fabric will wear out more quickly.
In standard operative conditions, an air balloon requires around 3 cubic meters of volume per each kilogram it lifts. The upward push is determined by a series of factors: the air temperature inside and outside the balloon, the height above sea level and the degree of humidity of the surrounding air. Generally the upward push decreases 3% for every 1000 meters of height ( around 1% for every foot).
Dimensions of modern air balloons can vary depending on design and model: single seat air balloons (also called Hoppers), have an envelope volume of 1000 square meters. On the other end of the scale giant air balloons used for commercial excursions can seat 12 passengers and the envelope can be as big as 8000 square meters. Most air balloons envelopes have a capacity of around 2500 square meters and can seat 3 or 4 passengers.
Direction wise, the flight of an air balloon is completely passive, the pilot can only change the cruising height. A careful study of the winds aloft and their directions can permit a certain control (the pilot can change the cruising height to reach the wind current that is useful to him).