Aerostatic Lift
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 achieved by displacing atmospheric air to create buoyancy, in the same way as a ship or boat displaces water. Archimedes first defined buoyancy: “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.”
A light gas, often called a lifting gas, contained in one or more gas cells, is normally used to displace the air. An airship’s aerostatic lift is equal to its buoyancy (the weight of atmospheric air displaced) minus the weight of that gas. Many small recreational blimps use hot air instead of a lifting gas; they operate on the same principle as a hot-air balloon
Lifting Gasses
Hydrogen, helium, methane, ammonia and hot air could all be used to displace the air to create aerostatic lift. A vacuum would be ideal, but current technology does not allow a vacuum vessel to be made that is lighter than the air it displaces.
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 flammable in the presence of oxygen this led to some spectacular disasters including most infamously, the Hindenburg. Nevertheless, hydrogen was used because it could be produced in abundance through a variety of different chemical processes at a low cost.
Helium is the most commonly used lifting gas today. It’s also 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. As an inert and non-flammable gas, helium is very stable and also more readily available today than it was in the time of the Hindenburg.
Although helium is abundant in the universe, our earthbound supply is limited. It is rare in the atmosphere (at only 0.0005%), but is found as a contaminant in natural gas at certain locations around the world. It is extracted from the natural gas, purified, and shipped to customers as a liquid or pressurized gas. Due to its inertness, extremely low boiling point and other characteristics, helium is widely used in medicine, industry, space flight and scientific research.
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. Since these gasses are heavier than hydrogen and helium, using methane or ammonia would result in much less aerostatic lift. Methane is also highly flammable like hydrogen, and ammonia 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.
Using pure helium to displace air produces a lift of 0.066 pounds per cubic foot or 1.06 kilograms per cubic meter. Using pure hydrogen produces a lift of 0.071 pounds per cubic foot or 1.14 kilograms per cubic meter. (Quoted values are at the aviation-industry standard temperature and pressure of 15° C and 101,325 Pascals respectively.) Helium produces roughly 7% less lift than hydrogen, however, operational factors in the use of helium in an airship can reduce its lift further to about 15% less than hydrogen.
The structural weight of an airship depends to a considerable extent on its surface area, and therefore on the square of its length, while volume and lift depend on the cube of its length. As an airship design gets bigger, volume and lift grow faster than weight, so a large airship is more efficient than a small one (the square-cube law).
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.
Modern airships are made to climb or descend in the same way as airplanes. The pilot increases engine power and raises the nose to climb. At the desired altitude, he reduces the power and trims for level flight. To descend, the pilot lowers the nose, but if he reduces the power to idle as in an airplane, the airship floats like a balloon and descends very slowly, so he must keep some power on, and possibly lower the nose significantly, to drive the airship down with engine power.
An airship’s aerostatic lift remains constant regardless of altitude, providing that the lifting gas has space to expand in the gas cell or cells as the atmospheric pressure reduces with altitude. The altitude at which the gas has expanded to the point of filling the cell or cells is called pressure height, and this is the airship’s operating ceiling. Climbing above this results in the automatic release of excess lifting gas and a loss of lift that is not recovered again in the descent.
Rigid airships contained a number of gas cells that filled out as the lifting gas expanded in the climb, and shrunk back as it contracted in the descent. Blimps utilize ballonets – air bags within the single gas cell, that automatically deflate to allow the lifting gas to expand during the climb, and automatically reinflate during the descent. This maintains the shape and correct pressure of the blimp’s envelope. Ballonet deflation and reinflation have no significant effect on aerostatic lift and are not used to control buoyancy or weight as is sometimes misconstrued.
Unlike modern blimps, the historic, hydrogen-filled rigid airships had limited aerodynamic maneuverability, and could not take off or land as airships do today. Instead they used ballast or gas release valves to control lift. By dumping water ballast a rigid airship could increase lift. On long flights, they would release hydrogen to counter the loss of weight due to fuel consumption. (The Graf Zeppelin used a gaseous fuel, called Blau gas, of the same density as air to minimize this problem.) They would then alternately release hydrogen and ballast to control their flight path on final descent, and to come to a hover from which they could land or be secured to a mooring mast.
The helium-filled USS Akron and USS Macon could not afford to release expensive helium, so they were equipped with systems to recover water from the engine exhaust for use as ballast to counter the fuel consumption. They also made use of vectored thrust to assist in takeoff and landing. Modern airships never release ballast or helium, except in an emergency.