The Hot-Air Balloon and Its Physics

The first creatures transported in the air by man were a cockerel, a duck and a sheep. They flew in a hot-air balloon made of paper. The animals returned safely. The balloon was finished by the brothers Montgolfier in September 1783. When the animals had returned safely, the brothers dared to send two friends on a 25 minutes flight over Paris.

Why Does a Balloon Rise?

The balloon flies, because hot air is lighter than cold air. A fire heats the air in the balloon. That is the principle, why a hot-air balloon flies. The Archimedes’ Principle is valid for gases, too: the buoyancy is always as great as the weight of the displaced amount of gas.

The Buoyancy:

Buoyancy works on motionless bodies in liquids or gases. Its extent is given by the Archimedes’ Principle.

The Archimedes’ Principle states:

The buoyancy of a body equals the weight of the amount of liquid or gas that was displaced by the body.

Balloons and airships get enough buoyancy by being filled with light gases so that they can rise.

Density and Weight of Air:

Density says how much a m² of a substance weighs. The less weight it has, the higher it rises. The heavier it is, the lower it sinks.

For example, the density of air is 1.29 kg/m² at 0 °C and normal atmospheric pressure.

The density of helium, on the other hand, is 0.18 kg/dm³ at 0 °C and normal pressure.

Why do Balloons Move?

The pilot of a balloon will always say that his balloon moves but does not fly. A balloon is not able to fly like an airplane, but rather swims in the air like a fish in the water.

The air in the balloon can be cooled down quickly by opening a valve at the highest point of the balloon, which allows the hot air to escape. By regularly heating the air of the balloon with a burner, buoyancy and weight can be kept equal, allowing the balloon to float.

How Can a Balloon Be Steered?

All balloons are driven by the wind. They have the same speed as the wind and drift in its direction. Although there is no real way to steer a balloon, its direction can be influenced a little because the wind at different heights blows in different directions. Thus it is possible that a balloon at a low altitude will move in the opposite direction than it would move a few hundred metres higher. Often, only a small change of direction is possible, but it might enable the pilot to move directly over a viewpoint and not 100 m past it. There are always sandbags attached to the balloon, which can be used to gain height by dropping one or more of them. However, it is often more interesting to be driven by the wind and float through the sky without a destination.