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January 20, 2010 > TechKnow Talk: Lighter than Air: On the Wings of the Wind

TechKnow Talk: Lighter than Air: On the Wings of the Wind

On September 19, 1783, King Louis XVI, Marie Antoinette, and much of the French royal court assembled near the Palace of Versailles where they were astounded to witness a large balloon ascend high into the sky. A basket suspended from the balloon carried three passengers: a sheep, a chicken, and a duck.

The magic balloon had been conceived by the Montgolfier brothers of Annonay, and utilized hot air to achieve flight. Within days of this demonstration people were vying to become the first human passengers in this novel conveyance. In fact, within a few months many people had flown.

This first era of hot air balloons did not last long. Due to the subsequent invention of steerable airships, or dirigibles, hot air balloons were soon abandoned as impractical for carrying people from one place to another.

However, in the 1960s, due largely to the efforts of American Ed Yost, the hot air balloon staged a comeback in popularity which continues undiminished. Today, as in the 1780s, balloon rides are employed primarily as an exciting recreation and entertainment.

The Montgolfier brothers believed they had discovered a new, lighter-than-air gas, released by heating air, which they quickly dubbed "Montgolfier gas." Of course what they had really produced was merely hot air.

Air is a fluid much like water. Just as in water, an object is buoyant and floats in air if it is less dense than the surrounding fluid. Density is simply a measure of how tightly packed together the molecules of a substance are. Warm air is less dense than cold air, and therefore rises. Another way to state this is that there are fewer molecules in a volume of warm gas than in the same volume of cold gas. Since there is less of it, it is lighter. This simple concept is intuitively obvious to us today.

Let's do some arithmetic. Heating one cubic foot of air (a cube one foot on each side) 100 degrees Fahrenheit (F) reduces its weight by a little less than one-quarter of an ounce. So if we trapped that cubic foot of hot air in a container in the fluid of colder air surrounding it, it would rise.

Or would it? It would rise only if the container weighed less than a quarter of an ounce! Clearly, to lift a balloon, basket, passengers, and equipment high into the sky takes a great deal of hot air. This is why hot air balloons are so gigantic.

A typical balloon holds on the order of 100,000 cubic feet of hot air that, if 100 degrees warmer than the surrounding air, should theoretically lift nearly 1600 pounds, about right for a loaded 3-5 passenger balloon.

Of course it's not quite that simple. Not all the air inside the balloon is at the same temperature. The warmest air is in the top, and the air near the opening in the bottom is about the same temperature as the outside air. Plus, as the balloon ascends, the density of the outside air becomes less, requiring a greater difference in temperature to continue rising. Naturally, a balloon has greater buoyancy on a cold day than on a warm day, since the outside air is denser.

The balloon "envelope" itself is usually made of nylon. This is constructed by stitching together long panels of the material. The seams also contain a strong tape that bears the weight of the basket and its contents. The basket is usually wicker and, in addition to the pilot and passengers, houses one or more liquid propane tanks.

Propane is fed to a burner mounted above the basket, under the opening to the balloon. It ignites propane gas in much the same way as a gas grill or fireplace, except on a grander scale. The flame heats the surrounding air, which rises into the envelope. By the way, the upper limit for the air temperature in a balloon is about 250 degrees F. The nylon can withstand much higher temperatures without melting, but at the expense of shortening its useful life.

The lower portion of the balloon, near the burner, is either treated with a fire-resistant coating or is a different, less flammable material than the rest of the envelope. At the top of the envelope is a "parachute" valve or vent. This is a flap of material that can be opened by pulling on a rope extending down into the basket, releasing a quantity of the warmest air.

For safety reasons, hot air balloons can only fly in conditions of very low wind speed and good visibility. They most commonly fly just after dawn, the most likely time of day to find still air. To prepare for flight, the envelope is stretched out on the ground and inflated with a large fan. The burner is then lit to heat the air inside until sufficient buoyancy is attained to lift off.

The ground crew packs up the equipment and follows the balloon in a "chase vehicle," often a truck or large van. They drive to the landing site, deflate the balloon (using the parachute valve) and haul it away, along with the other equipment and the passengers.

Once aloft, the pilot has limited control over speed and direction. Since the balloon goes where the wind takes it, the only way to change course is to ascend or descend to find winds of different directions. The pilot may release small helium balloons or drop light materials to help judge the winds above or below. Clues such as chimney smoke or other balloons may help as well.

To ascend, the pilot fires the burner to heat the air. To descend, one need only wait until the air in the envelope begins to cool. To descend more rapidly, the parachute valve may be opened briefly. The burner is very loud when fired. Most balloons employ a second burner which burns more quietly, though less efficiently. This is used over farming or ranching areas to avoid startling livestock.

There is a significant delay between firing the burner and the balloon rising and another delay while the air cools and the balloon begins to fall. Thus, rather than flying at a constant altitude, the balloon is usually rising or falling to some degree.

A modern balloon pilot has several instruments at his disposal to measure altitude, vertical speed, and air temperature inside and outside the envelope. A GPS receiver is often standard equipment today, to determine ground speed and direction, and the pilot is often in contact with the ground crew via cell phone.

The landing site must be a large, open area. The pilot's goal is to drop the basket gently to the ground. Both vertical and horizontal speed should be at a minimum to avoid a rough landing. The pilot may take advantage of hills, trees or other windbreaks in selecting a landing location.

For all the lack of steering, open flame, and high altitude, ballooning is a relatively safe endeavor. Only a handful of fatal accidents have been reported in recent years. Power lines present the greatest danger to balloonists.

There are a number of balloon festivals across the country every year. Some feature competitions in which pilots display navigational skills. Festival balloons have also become popular. These come in all shapes, from animals to houses to soda cans, and are often used for advertising purposes.

Balloon pilots must be certified by the Federal Aviation Administration (FAA); a commercial license is required for those carrying paying passengers. Balloon rides are available in several northern California locations, including the Napa wine country, and start at about $150-200 per person for a one-hour flight.

Next month: Airships

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