How Do Hot Air Balloons Work
Hot Air Rises
Hot air balloons are aircraft that gain their lift by heating a large contained envelope of air above the ambient temperature.
The envelope containing the warmer, less dense air rises and is used to lift the payload of basket, fuel tanks and passengers.
The air in a typical balloon may be heated approximately 90 to 100 degrees C above ambient temperature. The balloon will then float at whatever altitude the mass of cold air it displaces equals the combined mass of the warmer contained air, balloon system and payload.
A balloon has no means of propulsion. Except when ascending or descending it is stationary in the air and its ground speed is exactly equal to wind speed. Airspeed is zero and there is no aerodynamic lift. This results in the smoothest ride possible in any form of aircraft. There is absolutely no vibration and no matter what the speed is over the ground, there is no wind noise in the basket.
Level changes are achieved by more heating to climb. Descent may be initiated by dumping hot air from a valve in the top of the envelope or simply letting the air cool down. It is only whilst climbing or descending that passengers may experience a little air speed in the basket as the balloon passes through temperature inversions or wind shears.
The Balloon System
A hot air balloon comprises of several components.
The envelope itself is a VH (Australian) registered aircraft. It may vary in volume from 600 cubic metres (one person) to 13,000 cubic metres (20 persons).
The envelope is attached to the basket via stainless steel or kevlar flying wires.
The baskets are still woven wicker as this provides enormous strength and passenger protection, but is flexible so does not break on the occasional bumpy landing.
Stainless steel propane fuel cylinders are carried in the basket to provide fuel for the burners which are suspended between the basket and envelope. In winter, the fuel cylinders may have nitrogen added to maintain burner pressure.
Instrumentation consists of altimeter, variometer (measures vertical speed), and envelope temperature indicator. Most balloons operating in Australia are VHF radio equipped and also carry a transponder in accordance with standard airspace requirements.
Controlling A Balloon
As previously discussed balloons have zero airspeed therefore they cannot turn to avoid obstacles or other aircraft. They can, however manoeuver vertically provided there is sufficient time and advance knowledge of an obstacle.
Most balloons operating in Australia have maximum legal climb rates of 5 metres per second. Descent rates may be similar but pilots will typically climb and descend at a maximum of 2 metres per second.
Whilst these quite fast rates can easily be maintained, there is a little time lag between initiating a climb and it actually happening.
A reasonably large balloon will contain 10 tonnes of hot air. This added to a payload of over a tonne gives a significant aircraft mass and despite the enormous energy output of the propane burners, it may take up to 30 seconds to get this mass moving rapidly in a different vertical direction.
