Elements of balloon flight.
Balloon flight is magical in that it is supernatural for man to fly, but the Montgolfiers who invented the balloon utilized the basic natural laws of physics and mechanics, known as Boyle’s and Charles’ Laws and Archimedes principle to seemingly defy Newton’s law of gravity. Robert Boyle stated in 1662 that the pressure of a contained gas multiplied by its volume will remain constant if there is no temperature change. If the pressure is doubled on a quantity of gas, its volume will be compressed to one half. In 1787, apparently as a result of his work with balloons four years earlier, Prof. J. A. C. Charles gave us his law which states that the ratio between the volume of a gas and its temperature remains constant if the pressure does not change. Archimedes, who died in 212 B.C. , stated that an object placed in a fluid looses apparent weight equal to the weight of the fluid it displaces.
A balloon will carry any weight beyond its own of the air it displaces, whether its weight is less than the air because of temperature or composition. One thousand cubic feet of hydrogen weighs about five pounds, helium ten pounds, methane (natural gas) forty and hot air, at normal hot air balloon operating temperatures, fifty. Subtract those figures from the weight of air at seventy-five pounds and you have the lifting force of the chosen gas at low altitude. Since the atmosphere is compressed by its own weight it is less dense at higher altitudes. At twelve thousand feet it is approximately two thirds as dense and so will only provide two thirds the buoyancy. This effect continues progressively so that at 50,000 feet it is only one tenth, at 100,000 feet one hundredth and at 150,000 one tenth of that. A balloon would have to be one thousand times as big to carry the same load at thirty miles altitude as one at sea level (and yet only weigh the same).
The buoyancy of a hot air balloon is controlled by changing the temperature of the balloon’s air. The buoyancy of any gas balloon is controlled by the amount of gas in the balloon and the amount of ballast carried. Tiny changes in either can force dramatic changes in the balloon’s flight. Just one or two degrees temperature change in a hot air balloon or a few ounces of ballast dropped or a second or two of gas valve release will make a balloon ascend or descend accordingly. On the other hand, in violent maneuvers, whole bag fills of sand or great burner blasts may be required to get the needed reaction. To help in rapid cooling, modern hot air balloons also have very large hot air release vents in the form of a parachute sealing an opening in the top of the balloon.
Changing the altitude of a free balloon will permit it to follow differing air currents. Normally there will be a twenty or thirty degree difference in direction in the first few thousand feet of altitude, but a full circle of wind directions can occur. Albuquerque, New Mexico is famous for its “Box winds” where one can often repeatedly climb and descend with a resulting reversal of direction permitting landing at the original launch site. Needless to say, if there is only a simple and stable wind pattern, no additional control is possible. With superior weather monitoring, global positioning navigation and instant communication remarkable flight control is now possible.
While the early hot air balloons used straw and alcohol spirits for fuel by 1900 liquid petroleum fuels were used, compressed liquefied propane gas is almost exclusively used today. All hot air balloons burners use vaporizing coils to preheat the fuel for efficient combustion. Most of these coils are exotic stainless steel tubing, but plain copper coils seem to work adequately. The burners are mounted, often on gimbals, on the suspension concentration ring between the basket and the open mouthed balloon. Most systems have redundant burners because there have been problems with defective controls and contaminated fuel. A particularly notorious point has been leaking seals on control valves.
A variety of materials are used in the actual balloon, or “Envelope”. Cotton, nylon and polyester are common for hot air balloons. Cotton, having a poor weight to strength ratio is only favored for the carnival “Smoke” balloons. Orthodox gas balloons have used rubberized cotton, but modern sport gas balloons use urethane coated nylon. High altitude research balloons are generally made of polyethylene film but polyester film is also used.
An even greater variety of systems for carrying the load or passengers is used, ranging from a simple trapeze to the sealed environmentally controlled cabin of the stratosphere balloon.. For sport ballooning the traditional wicker basket, albeit with a welded stainless steel frame is popular. Criteria for evaluation of a balloon basket design should include toughness, energy absorption and electrical resistance.
by Don Piccard
©2005