How Hot Air Balloons Works

If you actually need to get somewhere, a hot air balloon is a fairly impractical vehicle.You can’t really steer it, ­and it only travels as fast as the wind blows. But if you simply want to enjoy the experience of flying, there’s nothing quite like it. Many people describe flying in a hot air ballo­on as one of the most serene, enjoyable activities they’ve ever experienced.

Hot air balloons are also an ingenious application of basic scientific principles. In this article, we’ll see what makes these balloons rise up in the air, and we’ll also find out how the balloon’s design lets the pilot control altitude and vertical speed. You’ll be amazed by the beautiful simplicity of these early flying machin­es.

Hot air balloons are based on a very basic scientific principle: warmer air rises in cooler air. Essentially, hot air is lighter than cool air, because it has less mass per unit of volume. A cubic foot of air weighs roughly 28 grams (about an ounce). If you heat that air by 100 degrees F, it weighs about 7 grams less. Therefore, each cubic foot of air contained in a hot air balloon can lift about 7 grams. That’s not much, and this is why hot air balloons are so huge — to lift 1,000 pounds, you need about 65,000 cubic feet of hot air.

 

 

Rising Balloons

To keep the balloon rising, you need a way to reheat the air. Hot air balloons do this with a burner positioned under an open balloon envelope. As the air in the balloon cools, the pilot can reheat it by firing the burner.

Modern hot air balloons heat the air by burning propane, the same substance commonly used in outdoor cooking grills. The propane is stored in compressed liquid form, in lightweight cylinders positioned in the balloon basket. The intake hose runs down to the bottom of the cylinder, so it can draw the liquid out.

Because the propane is highly compressed in the cylinders, it flows quickly through the hoses to the heating coil. The heating coil is simply a length of steel tubing arranged in a coil around the burner. When the balloonist starts up the burner, the propane flows out in liquid form and is ignited by a pilot light. As the flame burns, it heats up the metal in the surrounding tubing. When the tubing becomes hot, it heats the propane flowing through it. This changes the propane from a liquid to a gas, before it is ignited. This gas makes for a more powerful flame and more efficient fuel consumption.

In most modern hot air balloons, the envelope is constructed from long nylon gores, reinforced with sewn-in webbing. The gores, which extend from the base of the envelope to the crown, are made up of a number of smaller panels. Nylon works very well in balloons because it is lightweight, but it is also fairly sturdy and has a high melting temperature. The skirt, the nylon at the base of the envelope, is coated with special fire-resistant material, to keep the flame from igniting the balloon.

The hot air won’t escape from the hole at the bottom of the envelope because buoyancy keeps it moving up. If the pilot continually fires the fuel jets, the balloon will continue to rise. There is an upper altitude limit, however, since eventually the air becomes so thin that the buoyant force is too weak to lift the balloon. The buoyant force is equal to the weight of air displaced by the balloon, so a larger balloon envelope will generally have a higher upper altitude limit than a smaller balloon.

The basket holds the passengers, propane tanks and navigation equipment.

Most hot air balloons use a wicker basket for the passenger compartment. Wicker works very well because it is sturdy, flexible and relatively lightweight. The flexibility helps with balloon landings: In a basket made of more rigid material, passengers would feel the brunt of the impact force. Wicker material flexes a little, absorbing some of the energy.

Piloting a Balloon

Piloting a balloon takes skill, but the controls are actually very simple. To lift the balloon, the pilot moves a control that opens up the propane valve. This lever works just like the knobs on a gas grill or stove: As you turn it, the flow of gas increases, so the flame grows in size. The pilot can increase the vertical speed by blasting a larger flame to heat the air more rapidly.

Additionally, many hot air balloons have a control that opens a second propane valve. This valve sends propane through a hose that bypasses the heating coils. This lets the pilot burn liquid propane, instead of propane in gas form. Burning liquid propane produces a less efficient, weaker flame, but is much quieter than burning gas. Pilots often use this second valve over livestock farms, to keep from scaring the animals.

Hot air balloons also have a cord to open the parachute valve at the top of the envelope. When the pilot pulls the attached cord, some hot air can escape from the envelope, decreasing the inner air temperature. This causes the balloon to slow its ascent. If the pilot keeps the valve open long enough, the balloon will sink.

Essentially, these are the only controls — heat to make the balloon rise and venting to make it sink. This raises an interesting question: If pilots can only move hot air balloons up and down, how do they get the balloon from place to place? As it turns out, pilots can maneuver horizontally by changing their vertical position, because wind blows in different directions at different altitudes. To move in a particular direction, a pilot ascends and descends to the appropriate level, and rides with the wind. Since wind speed generally increases as you get higher in the atmosphere, pilots can also control horizontal speed by changing altitude.

To maneuver the balloon horizontally, the pilot ascends or descends in altitude, catching different wind currents.

Of course, even the most experienced pilot doesn’t have complete control over the balloon’s flight path. Usually, wind conditions give the pilot very few options. Consequently, you can’t really pilot a hot air balloon along an exact course. And it’s very rare that you would be able to pilot the balloon back to your starting point. So, unlike flying an airplane, hot air balloon piloting is largely improvised, moment to moment. For this reason, some members of a hot air balloon crew have to stay on the ground, following the balloon by car to see where it lands. Then, they can be there to collect the passengers and equipment.

Launching and Landing

A lot of the work in hot air ballooning comes at the beginning and the end of the flight, when the crew inflates and deflates the balloon. For the spectator, this is a much more spectacular show than the actual balloon flight.

Once the crew has found a suitable launching point, they attach the burner system to the basket. Then they attach the balloon envelope and begin laying it out on the ground.

Once the envelope is laid out, the crew begins inflating it, using a powerful fan at the base of the envelope.

When there is enough air in the balloon, the crew blasts the burner flame into the envelope mouth. This heats the air, building pressure until the balloon inflates all the way and starts to lift off the ground.

The ground crew members hold the basket down until the launch crew is on board. The balloon basket is also attached to the ground crew vehicle until the last minute, so the balloon won’t be blown away before it is ready to launch. When everything is set, the ground crew releases the balloon and the pilot fires a steady flame from the burner. As the air heats up, the balloon lifts right off the ground.­

Amazingly, this entire process only takes 10 or 15 minutes. The landing process, combined with deflating and re-packing the balloon envelope, takes a while longer.

When the pilot is ready to land, he or she discusses possible landing sites with the ground crew (via an onboard radio). They need to find a wide open space, where there are no power lines and plenty of room to lay out the balloon. As soon as the balloon is in the air, the pilot is constantly looking for suitable landing sites, in case there is an emergency.

The balloon landing can be a little rough, but an experienced pilot will bump along the ground to stop the balloon gradually, minimizing the impact. If the ground crew has made it to the landing site, they will hold the basket down once it has landed. If the balloon isn’t in a good position, the crew pulls it along the ground to a better spot.

The ground crew sets out a ground tarp, to protect the balloon from wear and tear. Then the pilot opens the parachute valve all the way, so the air can escape out the top of the balloon. The ground crew grabs a cord attached to the top of the balloon, and pulls the envelope over onto the tarp.

Once the balloon envelope is down on the ground, the crew begins pushing the air out. When the balloon is flattened, the crew packs it into a stuff sack. This whole process is a lot like packing up a giant sleeping bag.

Wind and Weather

Before launching, pilots will call a weather service to find out about climate and wind conditions in an area. Cautious pilots only fly when the weather is close to ideal — when skies are clear and wind conditions are normal. Storms are extremely hazardous for hot air balloons, because of the danger of a lightning strike. Even rain is a problem, because it decreases visibility and damages the balloon material (of course, it’s not much fun to fly around in wet weather anyway). And while you need a nice wind current to have a good flight, very strong winds could easily wreck the balloon.

Pilots also call the weather service to get a rough idea of which way the balloon will travel, and how they should maneuver once they’re in the air. Additionally, a pilot might send up a piball (short for pilot balloon). A piball is just a balloon filled with helium that the pilot releases to see the exact direction of the wind at a prospective launch site. If it looks like the wind would take the balloon into prohibited air space, the crew needs to find a new launch spot.

In the air, the pilot will use an onboard altimeter, variometer and their own observations to find the right altitude. Reaching the right altitude is pretty tricky because there is at least a 30-second delay between blasting the burners and the balloon actually lifting. Balloon pilots have to operate the appropriate controls just a little bit before they want to rise, and shut them off a little bit before they want to stop rising. Inexperienced pilots often overshoot, rising too high before leveling off. Controlled operation comes only with many hours of ballooning experience.

The pilot carries several instruments onboard the balloon.

Now that we’ve seen how a hot air balloon flies through the air, let’s look at the forces that make this possible. As it turns out, hot air balloons are a remarkable demonstration of some of the most fundamental forces on earth.

 

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