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What’s the science behind hot air balloons?

21/12/2020 Mark Features

From the Montgolfier brothers of France, to UK aeronauts Stanley Edward Spencer and Margaret Graham, a whole host of men and women across time have been fascinated by hot air balloons. Across the world, wherever there are spectacles to be seen, you will often find the possibility of a balloon ride, and this is most certainly the case in bath.

While many of us have taken a trip across the skies by balloon, not everyone understands how their astonishing ride is made possible. In the following passages, we’ll look at some of the science behind ballooning and key design features that have been finely honed to create these amazing lighter-than-air aircrafts.

An exceptionally simple scientific principle is at the root of hot air balloons: in air that is cooler, warm air will rise. This is because hot air is essentially lighter than cool air, as per each unit of volume it has less mass. The weight of one cubic foot of air is around 28 grams, but if you warm it up by 100°F, it will decrease in weight by roughly 7 grams. This means that for every cubic foot of air within a balloon envelope, its basket can lift around 7 grams. This is why hot air balloons need to be so big if they are designed to carry passengers.

Balloons comprise three crucial parts: the basket (where passengers stand), the balloon envelope (which contains the air) and the burner (to heat it). To ensure a balloon remains rising, the air must be reheated, for as it cools, it will descend. This is achieved using a burner situated beneath the balloon envelope, which is managed by the balloon’s operator, and typically uses propane. Modern balloons feature a reinforced envelope manufactured from lightweight nylon, with flame retardant material close to the burner. The multiple panels stretch upwards from the envelope’s base to its crown, but air never escapes from the open base, as hot air rises.

A parachute valve is located at the envelope’s top and is controlled by the operator via a cord. This allows them to let hot air out, reducing the air temperature within the envelope, allowing the balloon to ascend slowly. Wicker is the most common material for passenger baskets, as it is flexible but strong, and lightweight.

While it may seem like balloon operators can only make their craft rise and sink, they also have a level of control that allows them to travel laterally. Balloons drift with the breeze, but operators can influence which direction they move in. This is because at varying altitudes, the wind is blowing in different directions. By selecting to ascend or descend the operator can ride the breeze and select the way they want to go.

For those seeking outstanding views of both countryside and city scenery, taking a ride in a hot air balloon over Bath and its surrounding area may prove an ideal choice. With grade -isted buildings and historic architecture within the city, and the rolling rural Somerset beyond, there are many wonders to be seen.

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