Hot Air Balloon

Hot air balloons were the primary mode of transport that allowed people to travel by air; it was designed and developed in Paris in 1783. With a fabric ‘envelope’ to hold the hot air with an exhaust at the top and a carrier underneath the passengers, their design has never changed in centuries.

The floating hot air balloon is due to the air that heats inside the envelope, which has a lower density than the colder air outside. It’s precisely the same way as a boat supported by water on the ocean; the cold air supports a hot air balloon.

It requires specialised knowledge and expertise to operate a hot air balloon. Good familiarity with wind directions at different altitudes is a major requirement to operate a hot air balloon since it is the only technique to control the balloon. A pilot can change the vertical direction by regulating the amount of hot air within the balloon.

Physics behind Floating of Hot Air Balloon

The fundamental physics behind a hot air balloon is buoyancy, which generates lift. A hot air balloon consists of a large envelope bag with a basket suspended below. The basket holds a flame used to heat the air inside the balloon through an opening. Therefore, a lift is generated by the heat produced.

This phenomenon of lifting the balloon from the ground is due to the buoyant force created by the surrounding air since the density of hot air inside the balloon is less than the surrounding air.

It is Archimedes’ principle that works behind the lift of the hot air balloon, that is, an upward or buoyant force is acted upon a body upwards when it is wholly or partially submerged in a fluid at rest, and that the magnitude of this force is equivalent to the weight of the fluid displaced by the body. The hot air balloon works using the same principle: the air is a high-pressure fluid.

Therefore, this buoyant force must surpass the weight of the heated air, counting the weight of the balloon, passengers, and the on-board equipment for a lift to be generated,

Hot Air Balloon Operation

As revealed in the figure below, the weight of the hot air balloon is more focused near the bottom of the balloon, so the centre of mass G of the hot air balloon is always underneath the centre of buoyancy C. This means that the balloon is always steady during flight.

If the balloon machinist wishes to lower the balloon, he can either halt firing the burner, which cools down the envelope or open a small vent at the top of the balloon. This will decrease the buoyant force by releasing some of the hot air, which will eventually cause the balloon to go downhill.

Since keeping a strictly constant altitude by maintaining a net-zero buoyant force is practically impossible, the operator fires and turns off the burner irregularly until he reaches the desired height. This is the only way he can uphold an approximately constant altitude.

The operator must be familiar with the wind directions, which differs with altitude, to move the balloon sideways. So, he advances or drops the hot air balloon to the altitude matching the wind direction, and hence he can move the balloon in the direction he wants.

Watch the video and learn about the concept of Archimedes Principle

Application of Buoyancy, Air Pressure and Gravity in Hot Air Balloon

The downward force of Gravity draws all air particles into the atmosphere. But an upward force working opposite to Gravity is created by the pressure in the air.

This pressure level is maximum at the Earth’s surface; more weight above means a greater gravitational force downwards. As you move up through stages of the atmosphere, the air has a lesser amount of air mass above it, and so the balancing pressure declines. This is why pressure drops as you rise in altitude.

This pressure difference causes an upward buoyant force in the air. But this buoyant force is weak, and hence, for buoyancy to push something up in the air, the thing has to be lighter than an equal volume of the air around it.

So, if we change the air conditions inside the balloon, we can decrease density while keeping air pressure the same.

Hence, the overall pressure can be increased in two ways:

• Growth in the number of air particles so that there is a superior number of particle impressions over a given surface area.
• Increase the speed of the particles so that each particle strikes with greater force and the particles hit an area more often.

So, for this, you simply need to increase the speed of the air particles, and this can be done by heating the air. The air particles will become more excited by absorbing the heat energy. This, in turn, will make them move faster, which means they will collide with a surface with a greater force more often.

That is why a hot air balloon rises because it is filled with hot, less dense air and is encircled by colder, more dense air.

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