How do Hot Air Balloons Fly?

One of the most tranquil forms of flight, hot air balloons glide calmly through the skies with only the propane gas burner sporadically breaking the silence. But how do they fly? And more curiously, how do they navigate?

To begin with, at the setup stage, cold air is blown (using a large fan) into the balloon (envelope) to give it some shape after which the burners are ignited and the air inside is heated up. This causes the balloon to completely take shape and rise vertically above the basket. Once ready for lift-off, the burners are fired again to heat the air to the point where there is enough to lift all the weight off the ground and the balloon slowly rises into the sky. 

To control vertical movement, the pilot uses a combination of the propane burners and an exhaust flap in the top of the balloon. To climb, the burners are ignited thereby heating the air inside and increasing the buoyancy of the balloon. To descend, a cord is pulled to open the exhaust flap which lets out hot air through the top of the balloon and decreases its buoyancy. This is a difficult skill to get right as it can take up to 30 seconds after firing the burners before the resultant climb occurs. 

In order to navigate, balloon pilots will take advantage of the fact that the wind blows in different directions at different altitudes, so by changing altitude they can change their direction of flight. This won’t allow them to completely reverse their course but it will allow changes in direction of up to 30 or 40 degrees. Of course to do this, they need to know which way the wind is blowing above and below them. This is determined by a combination of wind forecasts, watching external signals such as other balloons and chimney smoke, and knowing that due to the earth’s Coriolis effect and the effect the ground has on the wind, it will generally turn to the right with an increase in altitude (in the Northern Hemisphere). 

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