Today's biggest story is "six year old boy flies off in hot air balloon." While in fact that title is incorrect, it does bring up an interesting question: how do hot air balloons work?
The boy, Falcon Heene, actually didn't take a ride in a hot air balloon. It was actually a helium balloon, and in fact, at the time of this writing, we don't know if he was in the balloon at all. More on helium balloons later.
Still, how do hot air balloons work? They work on a simple principle: warm air rises. A hot air balloon rises simply because the air, heated by burners lit by some sort of fuel (typically, propane) is lighter than cooler air. This is the same reason that they tell you that in a house fire, the breathable, cooler air will be at the floor level.
The actual visible balloon portion of a hot air balloon is known as an envelope. The burners are what heat the air. The basket is the area where the passengers sit.
The base on the envelope, where the hot air enters, is known as the skirt. It is coated with special fire-resistant material, to keep the burners from igniting the balloon.
If the pilot keeps the burners continually on, the balloon will continue to rise. There is, however, a point at which this principle fails. Eventually the air becomes so thin that the buoyant force from the heated air is too weak to provide lift. The larger the envelope, the larger amount of buoyant force, and the higher the upper altitude limit.
You might wonder why the hot air does not escape from the hole in the envelope at the base, for the hot air to enter? It's the same reason the balloon rises in the first place: the hot air wants to go up, not down through the envelope's hole.
The only real controls for a pilot are those that control the burners and a parachute valve at the top of the balloon to release some hot air. So how does a pilot control direction? He does so by varying the altitude of the balloon. Air moves in different directions at different altitudes.
Naturally, this is not the same as the control a blimp has. There are not propellers or rudders to steer with. So, even a skillful pilot is at the mercy of the winds.
How is a helium balloon different? Helium, which is what was used in the Falcon Heene flying saucer shaped balloon, is lighter than air, without being heated. That's why those balloons your child might hang onto float in the air. They are filled with helium.
The principle is the same as in a hot air balloon. The gas inside the envelope is lighter than air, so the balloon rises. You probably recall that hydrogen used to be in vogue as a gas to be used in balloon. The problem with that idea is that hydrogen is highly flammable. The Hindenberg disaster is a great example of why hydrogen is a bad idea for ballooning.
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