How Hot Air Balloons Work
from the award-winning team at HowStuffWorks
If you need to get somewhere, a
hot air balloon is an 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 flying, there's nothing quite like it.
Hot air balloons depend on the
basic principle of buoyancy.
Buoyancy occurs in a fluid (air,
water, grape juice) when the fluid
has greater density (contains
more matter per unit of volume)
at the bottom than at the top.
Fluids are buoyant because gravity pulls matter downward, making
the fluid denser at the bottom.
This density difference pushes
relatively light objects up through
a fluid. For example, if you put a
basketball at the bottom of a
pool, the pressure from that
denser water pushes the basketball to the top of the pool, where
the less dense water exerts less
pressure. This works with any object lighter than the amount of
water it displaces.
Parachute Valve
with Kevlar Cord
Modern hot air balloons use
burning propane to heat air inside a nylon envelope. Liquid
propane is stored in lightweight
cylinders and released next to a
pilot light just under the envelope. Most balloons use a wicker
basket for the passenger compartment, because wicker is flexible enough to absorb some energy from the landing impact.
The heated air molecules
inside the balloon move
faster than the cooler
molecules outside. As a
result, the hot air exerts
enough pressure to fill the
balloon envelope, thereby
creating enough buoyancy to
lift the balloon.
Propane
Tank
Air works the same
way, but it can only
alloon
a hot air b
lift something that
Because ith the wind,
w
moves
is lighter than the
el any
rs don't fe
passenge t all. The ride
air in the atmosbreeze a singly safe
phere – such as
ri
feels surp rene.
hotter air. Molecules
and se
in hotter air move more
quickly (with greater energy) than molecules in cooler
air, which means a smaller number of hot air molecules can exert as much pressure as a larger
number of cooler air molecules.
As a result, hot air exerts enough
pressure to inflate a large balloon “envelope,” but it weighs
significantly less than the cooler
air around it. The buoyant force
isn’t super strong – to lift 1,000
pounds (a basket and a few passengers), you need to heat
65,000 cubic feet of air by 100
degrees F. That’s why hot air balloons are so big.
The basic balloon controls are incredibly simple, but difficult to
master. To lift the balloon, the pilot opens the propane valve, increasing the flame (and therefore
heat). To lower the balloon, the
pilot pulls a cord to open the parachute valve at the top of the envelope. Hot air escapes through
the open valve, cooling the inner
air, and the balloon sinks.
Envelope
Propane Burner
Propane Valve
LIQUID PROPANE BURNER
To lift the balloon, the pilot
moves a control that opens up
the propane valve. As the flow
of gas increases, the flame
grows in size, thereby heating
the air inside the envelope.
All text and images ©2003 HowStuffWorks Inc. All rights reser ved. Distributed by Universal Press Syndicate.
Wind blows in different directions
at different altitudes, so pilots
can maneuver horizontally by rising or sinking. But even the most
experienced pilot doesn't have
complete control over the balloon's flight path. Unlike flying an
airplane, hot air balloon piloting
is largely improvised. Some
members of a hot air balloon
crew stay on the ground, following by car so they can collect the
passengers and equipment when
the balloon lands.