If you actually need to get somewhere, a hot air balloon is a fairly impractical vehicle.
You can't really steer it, and it only travels as fast as the wind blows.
1. Introduction to How Hot Air Balloons Work
Many people
describe flying in a hot air balloon as one of the most serene, enjoyable
activities they've ever experienced.
Hot air balloons are also an ingenious application of basic scientific
principles. In this article, we'll see what makes these balloons rise up in
the air, and we'll also find out how the balloon's design lets the pilot
control altitude and vertical speed. You'll be amazed by the beautiful
simplicity of these early flying machines.
Hot air balloons are based on a very basic scientific principle: warmer air
rises in cooler air. Essentially, hot air is lighter than cool air, because
it has less mass per unit of volume. A cubic foot of air weighs roughly 28
grams (about an ounce). If you heat that air by 100 degrees F, it weighs
about 7 grams less. Therefore, each cubic foot of air contained in a hot air
balloon can lift about 7 grams. That's not much, and this is why hot air
balloons are so huge -- to lift 1,000 pounds, you need about 65,000 cubic
feet of hot air.
In the next section, we'll look at the different components of hot air
balloons to find out how they heat the air.
2. Rising Balloons
To keep the balloon rising, you need a way to reheat the air. Hot air
balloons do this with a burner positioned under an open balloon envelope.
As the air in the balloon cools, the pilot can reheat it by firing the burner.
A hot air balloon has three essential parts:
- the burner, which heats the air;
- the balloon envelope, which holds
the air;
- the basket, which carries the passengers.
Modern hot air balloons heat the air by burning propane, the same substance
commonly used in outdoor cooking grills. The propane is stored in compressed
liquid form, in lightweight cylinders positioned in the balloon basket. The
intake hose runs down to the bottom of the cylinder, so it can draw the
liquid out.
The burner flame heats the air in the balloon envelope.
Because the propane is highly compressed in the cylinders, it flows quickly
through the hoses to the heating coil. The heating coil is simply a length
of steel tubing arranged in a coil around the burner. When the balloonist
starts up the burner, the propane flows out in liquid form and is ignited by
a pilot light. As the flame burns, it heats up the metal in the surrounding
tubing. When the tubing becomes hot, it heats the propane flowing through
it. This changes the propane from a liquid to a gas, before it is ignited.
This gas makes for a more powerful flame and more efficient fuel consumption.
In most modern hot air balloons, the envelope is constructed from long nylon
gores, reinforced with sewn-in webbing. The gores, which extend from the
base of the envelope to the crown, are made up of a number of smaller
panels. Nylon works very well in balloons because it is lightweight, but it
is also fairly sturdy and has a high melting temperature. The skirt, the
nylon at the base of the envelope, is coated with special fire-resistant
material, to keep the flame from igniting the balloon.
The hot air won't escape from the hole at the bottom of the envelope because
buoyancy keeps it moving up. If the pilot continually fires the fuel jets,
the balloon will continue to rise. There is an upper altitude limit,
however, since eventually the air becomes so thin that the buoyant force is
too weak to lift the balloon. The buoyant force is equal to the weight of
air displaced by the balloon, so a larger balloon envelope will generally
have a higher upper altitude limit than a smaller balloon.
Most hot air balloons use a wicker basket for the passenger compartment.
Wicker works very well because it is sturdy, flexible and relatively
lightweight. The flexibility helps with balloon landings: In a basket made
of more rigid material, passengers would feel the brunt of the impact force.
Wicker material flexes a little, absorbing some of the energy.
Inside the gas sylinder.
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