A Lesson from Archimedes

Now that we’ve heated the air inside the balloon’s envelope using the burners, let’s look at the physics behind what makes the balloon actually fly. The whole thing goes back to a Greek math whiz named Archimedes. According to legend, Archimedes stepped into a too-full bath and had a flash of insight that balloonists can relate to.

When Archimedes stepped into a tub that had been filled to the rim, he noticed he became lighter and lighter as more of his body went under the water. Not only did he seem to become lighter, but the water level rose and spilled over the edge of the tub as he sat down. In an inspired moment, Archimedes understood that the amount of weight he seemed to lose was equal to the weight of the water that spilled out of the tub.

The first thing Archimedes did was jubilantly holler, “Eureka! ” (“I found it! ”) as he ran naked through the streets of Syracuse, Sicily. The next thing he did was write down what we know as Archimedes’s principle A body immersed in fluid loses weight equal to the weight of the amount of fluid it displaces. Because air is a fluid—or rather, a gas that behaves like a fluid—Archimedes’ principle applies to balloons.

Let’s say the envelope of a balloon holds three tons of air when fully inflated. Now, let’s say that air has been heated by a few million BTUs of propane flames until it is good and warm and, as we’ve seen, a little lighter. Instead of weighing three tons, it now weighs only two and a half tons. It’s still occupying the same volume as three tons of unheated air, though, so the balloon is about a half-ton, or 1,000 pounds, lighter than air. It’s ready to fly!

 Plane Talk A hot-air balloon is lighter than air at ground level, but there’s a limit to hour high it can fly. As it ascends, the air in the atmosphere gets less dense, or “lighter," which is why moun­tain climbers have trouble breathing at high altitudes. At a few thousand feet off the ground, the weight of the air will be so low that it will be about the same as the hot air inside the balloon, and the balloon can’t go any higher. While hot-air balloons have floated as high as about 37,000 feet; most balloon pilots prefer to fly near tree-top level, and no higher than about 1,000 to 3,000 feet above the ground, depending on conditions.

Liftoff!

Once a balloon crew, usually four to five people plus a pilot, arrive at the launch site, they begin a takeoff ritual that can take anywhere from a few minutes to a half­hour to complete.

First, the envelope is laid out on its side and connected to the gondola, which is also laid on its side. While a pair of crew members hold the mouth of the envelope open, the pilot begins to inflate the envelope using a cold-air fan. The cold-air fan opens the balloon’s envelope enough to allow the pilot to walk into it to inspect the fabric of the envelope and the interior rigging and pulleys. The ropes that snake through the pulleys operate the fabric panels in the balloon’s crown that can be opened to release hot air quickly, either for a rapid descent or to keep the balloon on the ground after landing.

The inspection complete, the pilot gives his assignment orders to the chase crew. Some will be needed to hold the gondola down as the balloon inflates and lightens. Others will have to grab hold of the crown rope that hangs outside the envelope from its top center. The chase crew is crucial to keeping the balloon steady as it bobs in a downwind direction while inflating.

As the balloon envelope gets closer to full inflation and liftoff, the pilot helps the passengers into the gondola and gives them a safety briefing before the crew lets the ropes loose.

 Turbulence

If you’re even the slightest bit ; squeamish about heights, you may want to ease into hot-air ballooning before taking a full flight Airplanes have a cabin structure that lends a sense of security and reduces the sense of vulnerability. To test your reac­tion to a balloon night, take a tethered flight that ascends only a few dozen feet off the ground.

 On Course Joining a ground crew is a good first step to becoming a full – fledged balloon pilot It pub you in contact with others who share your interest and gives you a close-up introduction to the sport not to mention a chance to hitchhike a balloon flight on occasion. You might even find that the thrill of chasing your team’s balloon and helping it land and recover safely is more fun than riding along.

But the takeoff isn’t complete yet. As the balloon gets lighter and lighter, but not ready for actual takeoff, the pilot will order “hands off.” He wants to get a feel for the balloon’s buoyancy and test the effect the winds might have on it when it lifts off.

The pilot orders “hands off” once more, and continues to inflate the balloon to its full buoyancy. Once he is certain the winds are safe and the balloon has enough lift to clear any nearby trees, buildings, or hills, the pilot orders “hands off” one last time, and the ground crew lets go. The balloon is flying!

When the balloon lifts off, the ground crew’s job is just beginning. They jump into cars, trucks, and vans and race downwind, maps and walkietalkies in hand, to try to keep up with the balloon and arrive at the landing site before, or at least very shortly after, the pilot lands.