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 air balloon
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 burner.
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
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.
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.
Click on the burner components to see a
The basket holds the passengers, propane tanks and navigation equipment
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.