Aerostatic Buoyancy
Airships and balloons are what are called aerostats because they get their lift aerostatically rather than aerodynamically. The difference here is static versus dynamic. In short, static entails no motion while dynamic does involve motion. Aerostatic lift is obtained without the use of motion from propellers or other thrust while aerodynamic lift does require propellers or some other means of thrust. The benefit of aerostatic flight is that much less energy is required since the lifting gas, rather than propellers, generates most of the lift. While airships derive a majority of their lift aerostatically it is typical for modern dirigibles such as blimps to use both aerostatic and aerodynamic lift.
Aerostatic lift is typically achieved by the use of a lifting gas to create buoyancy in much the same way as a ship or a boat are buoyant in water. It was the ancient Greek scientist Archimedes that first defined the principle of buoyancy as follows: “Any object, wholly or partially immersed in a fluid, is buoyed up by a force equal to the weight of the fluid displaced by the object.” As a starting place for more information on buoyancy visit the Wikipedia page.
Lifting Gasses
Hydrogen, helium, methane, ammonia and hot air are all capable of being used for aerostatic lift. Due to the principles of buoyancy these gasses create lift because they are much less dense than the surrounding air they displace. While the denser air is pulled down by gravity the less dense lifting gas tends to separate out from the denser/heavier air. Since the air is being drawn toward the earth by gravity there is no place for the lifting gas to go but up.
Hydrogen is the first element on the periodic table and is also the lightest and most abundant element in the universe. In the early days of lighter-than-air aviation hydrogen was very commonly used. Since hydrogen is highly explosive when in the presence of oxygen this led to some spectacular disasters including the explosion of the Hindenburg. Hydrogen was used because it could be produced in abundance through a variety of different chemical processes and at low cost.
Helium is the most commonly used lifting gas today and it’s the gas that fills party balloons and allows you to talk with a high pitch when inhaled. It is the second element on the periodic table and also the second lightest and most abundant in the universe after hydrogen. Helium is quite stable and is more readily available today than it was in the time of the Hindenburg disaster.
One concern with helium today is that despite its abundance in the universe our supply of it may be dwindling. There is only one way so far discovered to collect it in sufficient quantities and it cannot be produced in the same way that hydrogen can. Helium is gathered from deep within the earth from natural gas wells that have been drilled by energy companies. The Helium is then separated out from the other gasses and pressurized for storage. Helium is extensively used for scientific applications due to its stability and extremely low boiling point.
Both methane and ammonia are chemical compounds derived from atoms of one element bonded with hydrogen atoms. Methane is a single carbon atom and 4 hydrogen atoms while ammonia is a nitrogen atom and 3 hydrogen atoms. Their lifting power is much less than either hydrogen or helium and like hydrogen, methane is also explosive. Ammonia on the other hand is toxic if inhaled.
Hot air was the original lifting gas. As humans discovered fire they also noticed its ability to send smoke and embers flying high into the sky. Since hot air molecules vibrate more rapidly than average temperature air molecules this prevents the hot air molecules from compacting closely together and thus makes hot air less dense. Since less dense air rises we simply need to harness it to create useful lift. Hot air is perhaps the most convenient lifting gas of all due to its ready availability and ease of control. Hot air balloonists use gas burners to heat the air within the balloon’s envelope in order to make it rise. The less the burner is used the less lift there will be. To make the balloon lose altitude hot air can simply be allowed to escape out of the top of the balloon without any fear of needing to find more later on.
The upward force of a lifting gas will change depending upon the differences in temperature and pressure between the lifting gas and the surrounding air.
Roughly speaking helium has a lift of 0.064 pounds per cubic foot or 1.02 kilograms per cubic meter and hydrogen has a lift of 0.070 pounds per cubic foot or 1.10 kilograms per cubic meter.
The greater the volume of lifting gas that is contained the greater the lift will be. Since the ratio of volume to surface area increases as a balloon or airship grows its lifting power will become increasingly more efficient.
Airship Design and Flight Controls
An airship, also known as a dirigible, is an aircraft that makes use of lighter-than-air lifting gasses in order to float like a balloon. Unlike a balloon, however, dirigibles must be steerable. The three primary types of airships are non-rigid, semi-rigid, and rigid airships. These three types are determined by the internal structure of the airship. Non-rigid airships are what we typically see today in the form of blimps. They are non-rigid because they hold their shape from the pressure of the lifting gas alone. Semi-rigid airships like the new Zeppelin NT derive their shape in part from an internal structure while the rest of the shape is filled out by the pressure of the lifting gas. Rigid airships such as zeppelins like the Hindenburg have their distinct shape solely due to the internal structure of the dirigible. As a result of the internal structure a semi-rigid or rigid airship can be made quite large and therefore produce much more lift than most non-rigid airships.
To change altitude a dirigible must adjust its buoyancy. Many old airships, zeppelins in particular, used a system of ballast and gas valves to adjust the overall buoyancy of the airship. The gas valves would release lifting gas in order to decrease altitude while ballast such as sand or water would be dumped in order to increase altitude. The problem with this system was that lifting gas such as helium can be expensive to replace and so valving it can be very wasteful. Ballast also needed to be continually replenished and took up space as well as useful lift that could have been devoted to more passengers or cargo.
Modern blimps make use of what are called ballonets or internal gas compartments that can suck in air from the outside in order to increase the overall weight of the blimp as well as diminish the effectiveness of the lifting gas. This combined with aerodynamic control surfaces and propellers enables modern blimps to change altitude and fly with greater efficiency and at a lower cost than historical ballast systems.