How Hot Air Balloons Fly

Hot air balloons were the earliest means of flight that successfully carried humans, though the very first animals to go up in a balloon were a sheep, a duck, and a rooster. The first manned flight happened in France in 1783, though the balloon was tethered. Later in the same year, two pilots went up in a free balloon and triumphantly flew a few miles into the Parisian countryside.

The Inventors

The brothers who invented the balloon, Jacques and Joseph Montgolfier, originally thought that the heat and smoke from a burning flame caused a balloon to inflate and rise. They had watched ashes floating up from the fire, inspiring them to contain the fire’s force for flight. However, it was not precisely the fire’s heat and smoke that allowed the three animals to rise from the ground for their eight minute flight. Rather, it was the fact that the fire lit underneath the paper and cloth balloon heated up the air inside the balloon. When air is heated it becomes lighter than the cooler air around it.

The Numbers

-A cubic foot of air weights about 28 grams

-Heat that cubic foot by about 40 degrees Celsius

-It then weighs about 7 grams less than it did before

-Thus it rises, because it is less dense than the cooler air around it

Therefore, one cubic foot of air can only elevate about 7 grams in weight. A hot air balloon needs to be quite large in order to boost even one person’s weight off the ground, plus the weight of the balloon’s basket.That’s why you see giant balloons with relatively small baskets attached.

Modern System for Flying Balloons

Today the most common way to heat the air inside a balloon is by burning propane. Propane is easy to handle because it is stored in liquid form, and can be carried around in cylinders in the balloon’s basket. The cylinders are attached to a burner by way of a hose, much the same way a barbeque grill operates, or a gas hob. When the pilot lights the burner by way of a pilot light, the propane fuel changes from a liquid to a gas. The gas then heats the air inside the balloon, and it rises.

Envelope Material

The huge main part of a balloon is called the envelope. Because of the heat that builds up inside of the envelope, a relatively heat resistant fabric is used for its material. The Montgolfier brothers’ original paper and cloth creation certainly wouldn’t hold up using the current heating methods. Today the usual material is nylon, because of its high melting temperature and lightweight, durable construction. As it is nearest to the flame, the bottom of the envelope is treated with a special flame resistant coating.