About Ballooning
Envelopes
Almost all conventional hot air balloons have an inverted teardrop shape. This has been
calculated mathematically to be the most appropriate shape for distributing the load across the
balloon, minimizing the stresses on the envelope. It also provides a stable shape for flight.
This is due to the fact that a balloon travels with the wind and
doesn't need to cut through the air like the wing of a powered aircraft.
Most envelopes are divided into a series of slightly bulbous vertical sections known as gores, which are designed to spread the load and minimize horizontal stresses. Envelopes are most often comprised of 8 or 12 gores, although some envelopes may have as many as 30 gores. In the case of a large number of gores, the envelope surface is effectively smooth. This is beneficial for displaying logos and artwork/graphics on the surface of the balloon.
Envelope sizes are generally quoted in 1000s of cubic feet (cu ft). There are a wide range of sizes available. For example, an envelope of 31,000 cu ft would generally be used to carry the pilot only, while a 355,000 cu ft envelope might be used to carry up to 17 persons. However, the most common sizes are 77,000 cu ft and 90,000 cu ft envelopes (often referred to as 77s or 90s). These balloons can carry 3 or 4 people in the basket.
Although the majority of
balloons are of the conventional shape, there are a large number of special shape envelopes around, which
are very popular at balloon festivals, being particularly photogenic and often producing a surreal sight as,
for example, houses, trains, cars, animals and other unlikely objects float across the sky.
Usually, the special shapes are sponsored, and provide eye-catching advertising for the sponsoring company. Currently, there are three special shape balloons operated by members of the Chiltern Region club, namely the Aston Martin balloon, the Panasonic battery and the Lakeside Lodge Golf Ball.
Another specialist balloon type is the Cloudhopper. This uses a conventional shaped
envelope, but instead of having a basket attached, the pilot (and sometimes one passenger) are strapped
in a seated position, into a frame beneath the envelope. Propane tanks are attached behind the pilot's
back (or to the frame) and the burner is positioned just above. The photograph shows a two person Cloudhopper rig, often referred to as the "flying sofa".