A hot-air balloon flies or rises when the air inside the envelope is heated causing it to be less dense than the surrounding air mass. Since the less dense air is contained within the envelope, the envelope and attached basket rise. The physics of why this happens is called “The Ideal Gas Law” which is a compilation of “Boyle’s Law and Charles’s Law."
The modern hot-air Balloon is a result of the work of Ed Yost who in the 1950s and 1960s worked on a project for the Office of Naval Research. The first free flight of a modern hot-air balloon occurred on May 19th. 2001 in Bruning, Nebraska.
The modern hot-air balloon consists of an envelope to contain the air made of nylon that is coated on the inside to make it less porous and able to hold air, a basket to carry the pilot and passengers, a fuel tank or tanks, fuel lines, a burner assembly, an altimeter, a vertical speed indicator or variometer and a temperature gauge to measure the air temperature inside the envelope. This is the basic setup and of course there are many different balloon makers now and a variety of configurations.
The envelope is typically made from gores of fabric sewn together in such a way as to create the shape of the balloon and sometimes has vertical and horizontal tapes at the seams. At the bottom of the envelope is the mouth sometimes made of a heaver material such as nomex. In the type of balloon with tapes, the vertical tapes are load bearing and the fabric itself only holds the air and carries no load. Some envelopes have skirts attached to the mouth to help protect the burner flame from air movement.
The envelope is attached to the “uprights” which are aluminum frames with stainless steel cable and those are attached to the basket. There is one or more burner assemblies mounted near the top of the uprights in the center and pointed upward into the mouth of the envelope when it is inflated.
The basket is constructed of rattan with a floor of plywood and oak runners. Many people think the baskets are wicker but this is incorrect. Rattan is much stronger and adsorbs a huge amount of energy on landing which is why it is the material of choice. The fuel tanks are stainless steel and the fuel used is propane.
To fly a hot-air balloon the basket and envelope are laid out on the ground and assembled, the envelope is inflated with cold air using a small fan and once the envelope is full of cold air the burner is fired and the balloon slowly stands up as the air is heated. Once the balloon is in an upright position and the temperature of the air inside brought to a point that the balloon is at “Neutral Buoyancy” the balloon is ready to fly.
When the air is heated beyond this point the balloon will rise and continue to rise until the temperature again cools to a point that the balloon is at neutral buoyancy. If the air cools further the balloon will begin to descend. Flight then is controlled by the pilot intermittently firing the burner to control the envelope temperature.As the air in the balloon is heated and cooled, the balloon actually "breathes" in and out through the mouth of the envelope.
The modern hot-air balloon also has a deflation port sewn into the top center of the envelope with a cable and or cord running down to the basket. When the pilot needs to deflate the balloon this cord is pulled and the deflation port will open allowing the hot air to escape from the top of the envelope. This port is never opened in flight.
Some hot-air balloons also have a vent opening in the side of the balloon that is opened similarly to the deflation port with a cord. Since this port is in the side, it allows warm air out of the balloon when opened but is used in flight to create a descent and or to adjust the rate of descent during an approach to landing. Contrary to popular belief (Even among many balloon pilots) a balloon can be steered very effectively. This is done by ascending and descending into differing layers of air that are moving in different directions. In this way a pilot can actually fly a specific course and choose his/her landing spot with astounding accuracy.