Hot Air
Balloons
Now
that we've covered the
History of the Hot Air Balloons, let's now focus on the
construction, the significance behind the use of hot air, and the
theory of operation.
A hot air balloon for manned flight uses a single-layered, fabric
gas bag (lifting "envelope"), with an opening at
the bottom called the mouth or throat.
Attached to the envelope is a basket, or gondola,
for carrying the passengers.
Mounted above the basket and centered in the mouth is the
"burner" which injects a flame into the envelope,
heating the air within. The heater or burner is fueled by propane,
a liquefied gas stored in pressure vessels, similar to high
pressure forklift cylinders.
The
Construction
Envelope
Modern hot air
balloons are usually made of light-weight and strong synthetic
fabrics such as ripstop nylon, or dacron (a polyester). The
individual sections which run from the throat to the crown (top) of
the envelope are called gores or gore sections. Envelopes can have
as few as 4 gores or as many as 24 or more. Envelopes often have a
crown ring at their very top.
Seams
The most common
technique for sewing panels together is called the double lap seam.
The two pieces of fabric are folded over on each other at their
common edge, possibly with a load tape as well, and sewn together
with two rows of parallel stitching.
Coatings
The fabric may be
coated with a sealer to make it impermeable to air. It is often the
degradation of this coating and the corresponding loss of
impermeability that ends the effective life of an
envelope.
Sizes
A
range of envelope sizes are available. The smallest (called
"Hoppers" or "Cloudhoppers") have as little as 21,000 ft³ of
envelope volume. At the other end of the scale are the balloons
used by large commercial sightseeing operations that carry well
over two dozen people and have envelope volumes of up to 600,000
ft³.
Vents
The top of the
balloon usually has a vent of some sort. The most common type of
vent is a disk-shaped flap of fabric called a parachute vent. The
fabric is connected around its edge to a set of "vent lines" that
converge in the center.
Baskets
Baskets are
commonly made of woven wicker or rattan. These materials have
proven to be sufficiently light, strong, and durable for balloon
flight. Such baskets are usually rectangular or triangular in
shape.
The Use
of Hot Air
The
Burner
The
burner unit gasifies liquid propane, mixes it with air, ignites the
mixture, and directs the flame and exhaust into the mouth of the
envelope. The unit may consist of one or more individual burners of
which the pilot may use one or more at a time to generate the
desired heat. Each burner is characterized by a metal coil of
propane tubing.
The burner unit may be mounted on a gimbal to enable the pilot to
aim the flame and avoid overheating the envelope fabric.
Fuel
Tanks
Propane fuel
tanks are usually cylindrical pressure vessels made from aluminum,
stainless steel, or titanium with a valve at one end to feed the
burner and to refuel. They may have a fuel gauge and a pressure
gauge. The pressure necessary to force the fuel through the line to
the burner may be supplied by the vapor pressure of the propane
itself, if warm enough, or by the introduction of an inert gas such
as nitrogen.
Theory
of Operation
Raising
the air temperature inside the envelope makes it lighter than the
surrounding (ambient) air. The balloon floats because of
the buoyant force exerted on it. The amount of lift
provided by a hot air balloon depends primarily upon the difference
between the temperature of the air inside the envelope and the
temperature of the air outside the envelope.
With a maximum operating temperature of 120 °C (250 °F), balloon
envelopes can generally be flown for between 400 and 500
hours before the fabric needs to be replaced. Many balloon
pilots operate their envelopes at temperatures significantly below
the maximum in order to extend the longevity of their envelope
fabric.
Reference / Image
Credits:
Wikipedia
1. Aoife
Mac
2. Heart
Lover
3. Heart
Lover
4.
Wikimedia
5. Rb
Glasson
