introduction to hot
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. But if you
simply want to enjoy the experience of flying, there's nothing quite like
it. 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!
A four-passenger CargoLifter hot
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!
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
A hot air balloon has three
essential parts: the burner, which heats the air; the balloon envelope,
which holds the air; and 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.
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.
The burner flame heats the air in the balloon envelope.
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.