Notice that the additional drag of the gondola, fins, and rigging can be substantial. For example, Hörner gives the drag coefficient of an airship at zero angle of attack as = 0.023 for the hull alone and for the complete airship, including nacelles, fins, etc. Recall that the drag on an airship is mostly from boundary layer shear stress or skin friction over the envelope. A larger airship with a higher Reynolds number at the same airspeed will have a lower drag coefficient. Therefore, its drag, being proportional to the wetted surface, grows with less than the square of the increase in the length scale, while the aerostatic lift is proportional to the cube of the length scale.
Airships, balloons, blimps, and other aerostats generate aerostatic or buoyancy lift using an envelope filled with a gas less dense than air, such as helium, enabling them to fly freely and “float” without needing forward airspeed. Such aircraft have been collectively called “airships” or “aerostats.” Today, they are more usually referred to as “Lighter-Than-Air” or LTA aircraft, as summarized in the figure below. Furthermore, because an LTA aircraft does not require much of a runway, albeit still needing a relatively large terrestrial footprint for takeoff and landing, they can also be classified as a form of vertical/short take-off and landing (V/STOL) aircraft. Technically, any unpowered LTA aircraft is considered a balloon, and one that is powered and steerable is called an airship.
Historically, airships have suffered from the public perception as lumbering giants of the sky, with a poor safety record. However, airships and aerostats have been used for over a century in many applications, including military surveillance, anti-submarine warfare, border patrol, passenger and cargo transport, and advertising. In the 1920s and 1930s, giant “Zeppelin” airships were used to transport passengers across the oceans and around the world in style and comparative luxury compared to airplanes of that era. However, airships obtained a poor reputation for safety after the Hindenburg was destroyed in 1937, in full view of the media, after a successful flight carrying passengers across the Atlantic; using inert helium rather than explosive hydrogen as the buoyant gas would have prevented such a tragic accident.
While airships have seen minimal practical use since the 1940s, technological advancements, growing concerns about environmental pollution from airplanes, and a global focus on sustainable transportation have sparked renewed interest in airships. Much of the engineering knowledge about airships go back over a century, and most technical data reports from the NPL in Britain and NACA in the U.S. date to the 1920s and 1930s. Unfortunately, some of this important technical work may have been lost forever. However, aerospace engineers still need to be aware of the history of airships in terms of their successes and failures, how they work, and their general performance characteristics. Furthermore, they must then be able to objectively and confidently assess the potential future of airships as part of the aviation spectrum.
Modern types of airships, which by any standard are giant aircraft, dwarfing the A380 and Boeing 747 (even the Hindenburg was three times longer and twice as tall as a 747), can carry substantial payloads of tens of tons over distances of thousands of miles, albeit at relatively low airspeeds, producing low carbon emissions. In particular, hybrid airships have gained considerable attention in recent years. Hybrids create both aerostatic (buoyancy) lift and aerodynamic lift, offering several advantages over legacy airships in terms of performance, payload, and cruise speed. They can also incorporate advanced propulsion systems, such as hybrid electric or hydrogen fuel cell technologies, giving good propulsive efficiency, lower energy costs, and a smaller carbon footprint. Modern airships have also taken advantage of advanced aviation safety features, including multiple redundant fly-by-wire control systems, composite structural materials, and active health monitoring.
As climate change and sustainability concerns increase, airships may soon find a new niche in aviation, perhaps with them servicing an Amazon or Walmart distribution center near you! While airships fly relatively slowly compared to airplanes, their cruise speeds are still faster than sea ships and just as fast as any truck, and they also have the advantage of flying directly from one place to another over land and sea. However, any future implementation and widespread adoption of new generations of airships will still depend on many interrelated factors, including further technological advancements, regulatory considerations, public perception, economic viability, and availability of hangarage and maintenance facilities. Indeed, while the future of airships shows much technical promise, it remains to be seen how (or if) such aircraft can mature to become an important and viably economic part of the commercial aviation spectrum.
- Know about the history of balloons, airships, and other aerostats.
- Understand the principles of buoyancy and aerostatic lift.
- Appreciate the aerodynamic performance characteristics of airships.
- Understand how an airship is controlled during flight.
- Be aware of some of the stability and other characteristics of airships.
The first LTA aircraft was a hot-air balloon, which was also the first human-carrying flight vehicle capable of sustained flight. It was designed in France by the Montgolfier brothers in 1783. Their balloon was made of paper and silk and rose to an altitude of about 2,000 m (6,560 ft) without a pilot. A few weeks later, a larger balloon, the Aerostat Reveillon, was launched carrying pilot Jean-François Pilâtre De Rozier, who ascended to the end of its 90 m (295 ft) rope tether. A month later, De Rozier and the Marquis d’Arlandes made aviation history by conducting the first free flight in a balloon, which ascended to around 150 m (492 ft) and drifted approximately 8 km (5.5 miles) before safely landing.
Benjamin Franklin, who from 1776 to 1778 was on a diplomatic commission to France, witnessed some of these early balloon flights. He was so impressed that he predicted that balloons would soon have considerable military value for “conveying intelligence into, or out of, a besieged town, giving signals to distant places, or the like.” Indeed, tethered balloons were to be used during the American Civil War for battlefield surveillance. They could reach altitudes of up to 300 m (984 ft), giving a great vantage point for soldiers, who used signal flags to send information down to their commanders.
For the next 150 years, hot-air balloons saw limited applications and were soon surpassed by airships and many other forms of aircraft. However, in the 1960s, hot-air ballooning became a popular recreational activity and sporting event. In 1987, Richard Branson and Per Lindstrand became the first to cross the Atlantic in a hot air balloon, flying 2,900 miles (4,670 km) in 33 hours. Branson and Lindstrand paired up again in 1991 and were the first to cross the Pacific in a hot air balloon. They traveled 6,700 miles (10,800 km) in 47 hours from Japan to Canada, breaking the world distance record by traveling in the high-altitude jet stream. Four years later, Steve Fossett became the first to complete the Transpacific balloon route as a solo pilot, riding the winds and the jetstream for four days while traveling from South Korea to Canada.
The need for balloons that could be combined with propulsion led to the development of airships with rigid internal structures known as dirigibles. The first dirigible was built and flown in 1852 by Henri Giffard, who used a steam engine driving a propeller. Today, an airship is usually defined as any powered, steerable aircraft inflated with a lighter-than-air gas. Early airships were filled with hydrogen gas, which is highly reactive if not explosive, and several airships had caught fire and crashed even before the infamous Hindenburg disaster in 1937. The LZ127 Graf Zeppelin, a German airship, as shown in the photograph below, made the first successful transatlantic passenger flight in 1928. It flew from Friedrichshafen, Germany, to Lakehurst, New Jersey, in the U.S., covering approximately 3,000 miles (4,800 km). This impressive achievement was a milestone in the history of airship travel and demonstrated the feasibility of long-distance passenger-carrying airship flights in relative comfort.
The U.S. Navy, as did the British, soon developed several airships derived from German designs. The R100 and R101 were built in 1922 as part of a British government initiative to develop airships to transport passengers and mail from Britain to the most distant parts of the British Empire, including India, Australia, and Canada. The R100 first flew in December 1929 and made a well-publicized return crossing of the Atlantic in the summer of 1930. But following the crash of the R101, it was scrapped. While other British airships were built through to the end of 1930, they were largely unsuccessful. The U.S.S. Macon, as shown in the photograph below, and the U.S.S. Akron were among the largest airships ever built. Although the hydrogen-filled, Zeppelin-built LZ 129 Hindenburg and the LZ 130 Graf Zeppelin II were longer, these two U.S. Navy airships were the most voluminous helium-filled dirigibles. Unfortunately, both U.S. airships crashed and were destroyed in the mid-1930s, further diminishing the operational and safety reputation of airships.
A blimp, essentially a “pressure airship,” has its shape maintained by the gas pressure inside the envelope. Unlike a dirigible, which has a structural skeleton, a blimp loses its shape when the internal pressure is reduced. The B-class blimps were patrol airships operated by the U.S. Navy during and shortly after WWI and used with good success for convoy and antisubmarine patrol duty. In 1925, Goodyear Tire & Rubber Company began building blimps. These aircraft were first used for advertising but then proved valuable for various military purposes during various campaigns throughout WWII. Goodyear’s blimps proved a valuable asset to the Navy because they could stay airborne for long periods without using much fuel. Newer versions of blimps have a minimal internal structure made of lightweight composite materials such as carbon fiber, classifying them as semi-rigid airships.
Tethered aerostats have offered a unique and valuable solution for specific applications that require a stable, long-duration aerial platform. Their wartime use dates back two centuries to gather intelligence, observe enemy activities, and direct artillery fire. The tethered design provides advantages in terms of stability, cost-effectiveness, and prolonged observation capabilities. A form of aerostat called a barrage balloon proved an effective defensive measure by the British during WW1 and WW2; see the photograph below. They disrupted enemy flights, forced aircraft to fly at higher altitudes, and provided early warning by visually alerting ground forces of incoming aircraft. Aerostats in the form of blimps are still used today by the Tethered Aerostat Radar System (TARS) to watch over the southern U.S. border; each aerostat is moored to the ground with a cable, and its altitude can be raised and lowered with a winch.
Anatomy of LTA Aircraft
LTA aircraft comprise various types of balloons, airships, and other aerostats. As aeronautical technology evolved, the name “balloon” has become reserved for hot air balloons, which are recreational aircraft. While the name “aerostat” still encompasses most types of LTA aircraft, an aerostat is usually considered an unpiloted LTA aircraft tethered to the ground with a cable. The name “airship” is reserved for a piloted LTA with a propulsion system, allowing the aircraft to fly from one place to another while carrying a payload or doing something worthwhile.
Balloons (Thermal Aerostats)
A balloon or thermal aerostat is the simplest form of aerostat, which is unpowered. Balloons have limited control capability and drift freely in the winds, so they are generally flown in light winds and good weather. Notable uses of balloons in modern aviation include weather balloons for meteorology (usually filled with helium) and sporting “hot-air” balloons for recreation, the latter being shown in the figure below.
Hot air balloons have a propane burner to heat the air inside the envelope. As the air heats up, it becomes less dense than the surrounding cooler air, so the balloon envelope will inflate, causing buoyancy. There is little difference in the density of hot and cold air, so the envelope must be voluminous to lift even a couple of people. Hot air balloons carry ballast, usually sandbags, which can be dropped or emptied to reduce weight. The pilot can open a parachute valve by pulling down on a cord to let some hot air escape and control the balloon’s altitude, i.e., to achieve neutral buoyancy. Those who have flown in hot air balloons generally say it was a serene and picturesque experience, the uncertainty of the landing point being part of the excitement of this type of aerial adventure.
There are also balloons designed for flight in the upper stratosphere and at the edge of space, an example being shown in the figure below. They typically fall into two main types: zero-pressure and super-pressure balloons. Both of these types utilize helium gas for buoyancy. They are the only type of flight vehicle that can be operated in this region of the atmosphere, which ranges from 50,000 ft to 150,000 ft (about 15 km to 45 km) in altitude, which is too low for satellites and too high for airplanes. The payload gondolas can carry experiments on science, astronomy, atmospheric chemistry, as well as weather sensors. At the end of the mission, the payload is severed from the balloon and recovered by parachute.
Zero-pressure stratospheric balloons have release valves that allow the pressure inside the balloon to remain the same as the outside. These balloons will continue to rise until they find a buoyancy altitude or burst; potentially, they can stay aloft for weeks or months. Super-pressure balloons are entirely sealed and maintain a higher pressure inside the balloon, allowing it to maintain its operational altitude despite temperature changes that occur day and night. Super-pressure balloons are launched partially filled with helium, and as the balloon ascends and ambient air pressure decreases, the helium expands to fill the envelope, and it reaches its desired float altitude.
Airships (dirigibles and blimps) are traditionally piloted and use engines driving propellers or fans for forward propulsion. Airships are separated into three types, depending on their structural design: rigid, semi-rigid, or blimp (pressure airship). Semi-rigid airships usually fall into the blimp class. The classic rigid airship, called a dirigible, has an internal space-frame structure surrounding its gas containers (gasbags), as shown in the figure below.
The Zeppelins of the 1920s through the 1940s are well-known examples of dirigibles, which used hydrogen as the buoyant gas; the airships in the U.S. used helium. The main structure is usually aluminum, but more modern dirigibles may use composites. The internal structure provides the shape and rigidity of the airship, but the outer covering or envelope is unpressurized. The covering must be a strong and durable fabric, but it does not have to be gas-tight like a blimp because the buoyant gas in a dirigible is contained in a series of internal gas bags.
The gondola or car is the compartment suspended below the envelope, where the passengers, crew, and other equipment are housed. The integral internal structure connects the gondola, empennage, engines, and gas cells, carrying and distributing the loads across the entire structure. The engines may be located on the gondola, but because of the expansive rigid internal structure of a dirigible, there are multiple hard points, and the engines can be placed almost anywhere. The vertical stabilizer, or fin, helps maintain the aircraft’s directional stability, and the horizontal stabilizer gives pitch stability. Airships utilize various control surfaces to help maneuver, including rudders and elevators, which are used for yaw and pitch control, respectively.
A non-rigid airship, also known as a blimp, maintains its shape from the pressure of the buoyant gas within it. Blimps may have some external bracing or reinforcement to support the weight of the gondola and engines. However, in the classic blimp, the gondola is suspended from the top of the envelope by a catenary curtain and cables, as shown in the figure below. Buoyancy is controlled using air-filled ballonets that allow the center of buoyancy to be modified for control and trim. The iconic Goodyear GZ20A blimps were used until 2017, and the new semi-rigid Goodyear Zeppelin NT blimps are extensively used in the U.S., especially at major sporting events such as the Super Bowl and the Daytona 500.
The modern version of the Goodyear blimp, as shown in the figure below, is a semi-rigid blimp (Zeppelin NT), which can be noted by the absence of suspension cables holding the gondola. A modern successor of the classic Zeppelin dirigible, this airship uses helium as the buoyant gas. Semi-rigid airships have an internal structure supporting the gas envelope and other components such as the engines and tail surfaces. However, the envelope maintains its shape using the internal pressure of the helium, so semi-rigid airships still fall into the blimp class. The engines drive propellers or ducted fans to provide the thrust necessary to propel the airship forward or backward, and vectoring the thrust can significantly help maneuverability. Modern airships use water ballast rather than lead shot, but otherwise, the features of the semi-rigid airships (Goodyear still calls them blimps) are similar to dirigibles.
A tethered aerostat is anchored to the ground on a cable; in the photo below, it is tethered at its mooring. Letting out the cable allows the aerostat to rise and be tethered at various altitudes. Such aerostats have been employed as platforms for persistent aerial surveillance over a designated area. Equipped with various sensors, cameras, and radar systems, aerostats can monitor the movement of troops, detect threats, and provide real-time intelligence. Tethered aerostats are also utilized for telecommunications purposes. Lifting communication equipment, such as antennas and repeaters, to high altitudes can enhance wireless communication networks over a wide area. This technology is particularly useful in remote areas where traditional communication infrastructure may be limited or damaged. A relatively basic system consists of a tethered aerostat capable of radar surveillance and high-resolution imagery over a large area from only a few thousand feet high.
Hybrid airships are intended to combine some of the features of airplanes and airships. These innovative vehicles carry a fraction of their weight from buoyancy and the remainder from aerodynamic lift generated by their airframe shape, as shown in the figure below. Their aerodynamic lift production may be as high as 30% of their weight.
Hybrid airships are intended to fill the gap between the low operating cost and low airspeeds of traditional airships and the higher speed but higher fuel consumption of airplanes. By combining dynamic and buoyant lift, hybrids can cruise at higher airspeeds with a greater payload compared to a pure airship. Because of their large size, they have a significant payload capacity compared to other aircraft types. This makes them suitable for various applications, including cargo transportation, disaster relief efforts, and remote access operations. However, they do need a considerable hangar for maintenance and storage, which is also a consideration of where and when they can operate.
Led Zeppelin – Going Down Like a Lead Balloon!
Led Zeppelin is one of the most famous rock bands of all time. They recorded nine albums between 1968 and 1979 and sold over 300 million copies. The name “Led Zeppelin” stemmed from a humorous conversation in 1967 among several musicians about the new band’s chances of going down like a lead balloon. Their lead guitarist, Jimmy Page, then came up with the name “Led Zeppelin,” and the remainder is history.
Buoyant (Lifting) Gases
A buoyant gas or lifting gas is any gas that produces buoyancy or aerostatic lift, so this gas must be less dense than the surrounding air. Well-known buoyant gases include hydrogen and helium, as well as hot air. The choice of buoyant gas depends on availability, cost, density, and safety. Helium is the most suitable for airship and aerostat operations because it is non-flammable and non-toxic. However, helium is expensive, and supplies can be intermittent. Hot air, as a buoyant gas, is limited to recreational balloons.
Hydrogen is the lightest and most buoyant lifting gas. It is low density and provides maximum aerostatic lift per unit volume, often called lifting capacity. It is readily available and relatively inexpensive. However, hydrogen is highly reactive and potentially explosive, posing safety risks. Many hydrogen-filled airships were lost in the 1920s because of fire and explosion, and the use of hydrogen decreased significantly after the Hindenburg disaster in 1937. Aviation regulators currently ban the use of hydrogen for airships, but this position may change if hydrogen is ultimately approved for use as a fuel on airplanes.
Helium is the second lightest gas, non-flammable, and safer than hydrogen. Helium is not reactive and does not support combustion. It is commonly used in modern airships and some types of balloons. Helium is extracted from underground gas pockets as a by-product of natural gas refinement. It is a relatively rare gas and expensive, especially at the volumes needed for an airship. Helium is a commodity, and its price varies daily. Still, in 2022, it was, on average, in U.S. dollars, about $7.6 per cubic meter ($210 per thousand cubic feet), whereas the price of gaseous hydrogen is about $1.6 per cubic meter ($44 per thousand cubic feet). Filling a Goodyear blimp with helium would cost about $80,000 versus $15,000 for hydrogen.
Hot air is produced by heating ambient air using propane burners. Hot air is less dense than the surrounding air, creating buoyancy. It does not pose the safety concerns associated with hydrogen. Hot air has a lower lifting capacity than gases like hydrogen or helium. Balloons using hot air require almost constant heating to maintain buoyancy because of rapid heat transfer through the thin gas envelope.
Ammonia is lighter than air but considerably heavier than hydrogen or helium. It has a good lifting capacity but is a pungent, choking gas that is toxic and corrosive. However, ammonia is less flammable than hydrogen. It was used in some early airships before the availability of hydrogen and helium in the quantities needed. Because ammonia is highly toxic, its release into the atmosphere poses numerous environmental hazards, and its use for LTA aircraft has been permanently discontinued.
Summary of Buoyant Gases
The table below summarizes the essential properties of buoyant gases, the reference being air. The density values are given in terms of grams per liter (g/L) at standard temperature and pressure (STP), which is the same numerical value in units of kg/m3. While hot air is easy and inexpensive to produce, it does not have much lifting capacity.
|Property||Hydrogen||Helium||Ammonia||Hot Air||Air (reference)|
|Chemical Formula||H₂||He||NH₃||N2 + O2 + mixture of other gases||N2 + O2 + mixture of other gases|
|Density||0.08988 g/L (STP)||0.1786 g/L (STP)||0.769 g/L (STP)||1.14 g/L (30oC) Varies based on temperature||1.225 g/L (STP) Varies based on temperature|
Where does helium come from?
Helium is a naturally occurring element in the Earth’s atmosphere, but its concentration is too low to allow economical extraction. Helium is also found in underground gas deposits, which migrate upward to the top of the gas pocket. Helium, being lighter than the other gases present, can then be separated and collected. The U.S. is the world’s largest helium producer. The sole source of helium in the U.S. comes from a plant operated by the Bureau of Mines in Texas. However, helium is a finite resource, and its annual and long-term availability is uncertain. Consequently, recent efforts have been made to promote the conservation and responsible use of helium. Ironically, modern airships may revert to using hydrogen if helium sources are limited or it becomes cost-prohibitive.
The Principle of Buoyancy
Archimedes’ principle is used to analyze the aerostatic forces on LTA aircraft. This principle states that the upward or buoyancy force exerted on a body immersed in a fluid equals the weight of the fluid that the body displaces. This behavior is true whether the body is wholly or partially submerged in the fluid and is independent of the body’s shape. As shown in the figure below, the resulting force acts upward at the displaced fluid’s center of buoyancy (also the center of mass). This principle is commonly used in problems involving the floatation of all types of “floating bodies,” such as boats and ships besides balloons and airships.
To better illustrate the concept of buoyancy, consider the scenarios shown in the figure below. The object’s volume is the same in each case, but its weight differs, ranging from (the heaviest) to (the lightest). If the object is entirely immersed in the fluid, then the upthrust of buoyancy force, , will be
where is the volume of the object and hence the volume of the displaced fluid, and is the density of the displaced fluid. The buoyancy force depends on the weight of the fluid displaced by the object, not the weight of the object.
If the object is heavier than the buoyancy force, it will sink. If the object has a weight equal to the buoyancy force, it will have neutral buoyancy and reach an immersed depth where it will neither sink nor rise in the fluid. The body will rise in the fluid if the buoyancy force exceeds the weight. Eventually, if the body is made light enough, it will float on the surface of the fluid interface. In this case, only partial immersion may be needed to displace the needed fluid to create the buoyancy force to balance the weight, i.e., like a boat or a ship.
Now consider a hot-air balloon floating in the atmosphere, as shown in the figure below. The balloon’s envelope contains hot (i.e., warmer) air so that it will be of lower density than the surrounding cooler air. Using Archimedes’ principle, the balloon’s buoyancy force will equal the weight of the displaced colder air. The balloon’s net weight will include the weight of its envelope and structure (including anything in the basket hanging below) plus the weight of hot air. Therefore, the net force acting on the balloon will equal the difference between the balloon’s net weight and the weight of the displaced air.
For neutral buoyancy, the weight of hot air inside the balloon, , plus the weight of the balloon itself, , equals the buoyancy lift, , or upthrust , i.e.,
The buoyancy lift is given by
where is the volume of the balloon’s envelope. The weight of the hot air will be
Therefore, for neutral buoyancy, then
A hot air balloon becomes neutrally buoyant at an altitude of 1,100 m where the outside air temperature is 10.5C. With all its cargo, the balloon has a mass of 500 kg (excluding the air in the balloon), displacing a volume of 7,000 m3. Assume that the pressure inside the balloon is the same as that outside. Determine the air temperature inside the balloon to give it neutral buoyancy. At 1,100 m, the air density can be considered 1.102 kg/m3.
For neutral buoyancy, the weight of hot air inside the balloon + the weight of the balloon itself = the lift or upthrust according to Archimedes’ Principle caused by the weight of displaced cool air, i.e., there is no net vertical force on the balloon so that
The air volume displaced by the balloon is = 7,000 m. Therefore, the weight of cool air displaced by the balloon is then.
which is equal to the upward buoyancy force according to Archimedes’ Principle. The mass of the balloon is 500 kg, so its weight, , is 500 = 500 9.81 = 4905.0 N. Notice is the density of the displaced air at the height of 1,100 m, which is given as 1.102 kg m.
Therefore, for neutral buoyancy (i.e., if the net vertical force is zero), the weight of the hot air in the balloon must be
This means that the corresponding density of the hot air inside the balloon must be
which is less than the density of the surrounding cool air, as expected.
We are asked to determine the corresponding temperature of the hot air, for which we can use the equation of state, i.e.,
We are told to assume that the pressure inside the balloon is the same as that outside the balloon so
and rearranging gives the ratio of the temperatures as
The outside air temperature at 1,100 m is 10.5 C or 283.65K. Therefore, the temperature of the hot air needed for neutral buoyancy will be
so a differential balloon internal temperature of about 20C at this altitude and temperature conditions.
Geometric Characteristics of Aerostats
Traditionally, airships and many aerostats have streamlined hull shapes based on prolate spheroids, i.e., those that look like fat cigars; spheres closely approximate balloons. An ellipsoid, shown in the figure below, is the most general shape in this geometric class of bodies of revolution that includes, in special cases, prolate and oblate spheroids as well as spheres.
Regarding aerodynamics and other characteristics of such shapes, the key geometric parameters of interest for aerodynamics are the ellipsoid’s projected frontal area, volume, surface area, and slenderness ratio. Aerodynamic coefficients used in the analysis may be based on any or all of these parameters. Therefore, it is essential to understand how they are defined.
Projected Frontal Area
The projected frontal area, , of a general ellipsoid is
In the case of a prolate spheroid where , or a sphere where then
The general formula for the volume of an ellipsoid is
Notice that when , then this formula is the volume of a sphere, i.e.,
The formula for the volume of a prolate spheroid with and is
where the eccentricity, of an oblate spheroid is
For aerodynamic problems, the surface area is called the “wetted area.
In the case of , Eq. 11 becomes infinite. However, if the logarithm term is expanded as a Taylor series, then
so for a sphere with and , then
In general, an approximate formula for the wetted area of a general ellipsoid is
where = 8/5.
The aerodynamic characteristics of airships are often expressed in terms of the slenderness ratio. The slenderness ratio, , for an prolate spheroid is defined as
where is the length and is the maximum diameter.
The slenderness ratios of a family of prolate spheroids are shown below. Most airships fall in the range 4 < < 6, which, based on historical precedent, has been found to give the lowest average aerodynamic drag.
Center of buoyancy
In understanding the flight characteristics of LTA aircraft, it is paramount to determine the aerostatic buoyancy force and the center of buoyancy, i.e., where the resulting buoyancy force can be assumed to act. The center of buoyancy or the center of the aerostatic lift is also the center of volume and gravity of the displaced air. Therefore, the center of buoyancy can be determined by finding the center of volume of the buoyant gas within the airship’s envelope.
It is useful to first illustrate the process in two dimensions, which means finding the center of area of a slice of a three-dimensional object. In reference to the figure below, the total area can be determined using
The first moments of area are given by
The location (coordinates) of the center of area, , are then determined using
Naturally, the process can be extended to three dimensions involving volumetric integrals. In practice, an approximate center of buoyancy will be determined numerically because there is unlikely to be an exact formula for an airship’s shape. In this case, the integrals in the equations above will be replaced by numerical summations based on the actual shape. Most CAD software allows an automatic calculation of the volume and center of volume of a shape.
In the absence of aerodynamic forces, a stationary free-flying airship or other LTA aircraft will be in static equilibrium when the center of gravity lies directly below the center of buoyancy, as shown in the figure below, i.e., the upthrust from buoyancy equals the weight, as given by
Notice that the center of gravity for an airship is generally well below the center of buoyancy, which gives the aircraft significant pendular stability about the pitch and roll axes. In aerostatic trim or the static equilibrium condition, most airships are not entirely level but can be seen to take on a slightly nose-high attitude. Airships have very little inherent stability about the yaw axis, although disturbances in yaw are heavily damped by the aerodynamic forces on the fuselage and vertical tail(s).
The buoyancy force, , or aerostatic lift, , on the airship will be equal to the net weight of the air displaced less the weight of the helium inside the envelope, i.e.,
where is the volume of the gas envelope, is the density of air, and is the density of helium gas. If the net sum of the structural, fuel, and payload weight of the airship is , then for vertical force equilibrium and neutral buoyancy, then
Therefore, for a given set of atmospheric conditions, the needed volume of the helium gas can be determined to give a specific buoyancy force to overcome the airship’s weight.
An airship has a gas envelope with a 296,520 ft3 volumetric capacity. Helium is used as the buoyant gas. If the airship has an empty weight of 14,188 lb and a maximum fuel load of 960 lb, what would be the maximum payload the airship could carry? Assume MSL ISA standard atmospheric conditions. The density of helium can be assumed to be 0.0003192 slugs/ft3. Ignore the effects of any ballast.
The upforce (lift), , on the airship will be equal to the net weight of the air displaced less the net weight of the gas inside the envelope, i.e.,
where is the volume of the gas envelope. The gross weight of the airship will be the sum of the empty weight, , plus the fuel weight, , plus the payload, , i.e.,
For vertical force equilibrium at takeoff, then , so
Rearranging for the payload weight gives
Using the numerical values supplied gives
Therefore, the maximum payload at the takeoff point would be 4,491 lb.
Some form of ballast is used on most types of LTA aircraft. Ballast is any material placed on board a buoyant aircraft to add non-structural weight for balance and/or trim. Water ballast is typically used, which is carried internally in tanks, although bags containing sand or lead shot can also be used. Besides trim and balance, ballast can be taken on or cast overboard to obtain a particular takeoff or in-flight weight and for landing and unloading the airship on the ground. For example, on a hot air balloon, ballast in the form of sandbags is used to keep it firmly on the ground as the hot air fills up the envelope. When ready to fly, some ballast is dropped overboard, and the balloon is said to weigh-off. During descent, dropping more ballast can reduce the descent rate as the balloon comes in for a landing.
Airships and blimps will also add or remove ballast as bags of lead shot or add/dump water ballast before weigh-off. The weigh-off condition means that the airship is trimmed to the condition it must operate in initial flight. To this end, ballast is added until the aircraft reaches a point where it becomes slightly heavier than the upward aerostatic buoyancy force, which is called static heaviness. After takeoff, the pilot first gives the airship a pronounced nose-high attitude using up elevator so the engines can provide a vertical component of thrust to overcome the static heaviness. If thrust vectoring is available, the propulsion system is swiveled to give mostly vertical thrust during the takeoff maneuver. These actions propel the airship upward and forward into a climb. As the airship gains altitude and airspeed, the airflow over the envelope generates some aerodynamic lift, and the pilot reduces the pitch attitude to reach a steady climb. Finally, the nose is lowered to almost level for the cruise.
It is important to note that “static heaviness” refers to the airship’s weight when stationary. The specific value of static heaviness of an airship can vary greatly depending on its size and configuration. Typically, the static heaviness is between 5% and 10% of the gross weight. Larger airships that carry heavy payloads must have a higher static heaviness than smaller blimps. Hybrid airships have the largest static heaviness because their hull shapes are specifically designed to produce significant aerodynamic lift, so the takeoff and landing will involve a short runway length.
Ballonets (meaning little balloons in French) are the gaseous equivalent of the water ballast tanks used on submarines. They are used in airships and blimps to control buoyancy and the center of buoyancy of the lifting gas. As shown in the figure below, the ballonets consist of internal gas bags or chambers within the airship’s envelope that can be inflated or deflated with air to control the volume of the buoyant gas. Another purpose of the ballonets on blimps is to maintain the buoyant gas’s net positive differential pressure so that the envelope maintains its shape.
Pressurized air is pumped into the ballonets using a system of scoops, electric pumps, ducts, valves, and pressure regulators. The air scoops are often placed where the dynamic pressure from the slipstream created by the propellers or other propulsion systems can be captured. The ballonets are operated through an automatic pressure control system to regulate the inflation and deflation processes, any excess pressure being released by venting through a set of air valves; manual control of the ballonets by the pilot is also possible.
Recall that buoyancy or aerostatic lift is equal to the weight of the air that the buoyant gas displaces less the weight of the buoyant gas itself, i.e.,
where is the volume of the buoyant gas contained in the envelope, is the density of air, and is the density of the buoyant gas, e.g., helium.
The inflation fraction, , is defined as the volume fraction of the total volume of the envelope occupied by the buoyant gas, i.e., the fraction not occupied by the ballonets. If the total volume of the envelope is , then the inflation fraction is
For example, an inflation fraction of 0.85 would mean that 85% of the envelope’s volume would produce buoyancy. Therefore, the buoyancy can be written in terms of the inflation faction as
the concept being shown in the figure below. The ballonets do not produce buoyancy because they are filled with air at nominally ambient conditions.
For a rigid airship or dirigible, the volume is defined as the net volume of the buoyant gas cells; these gas cells do not fill the entire envelope, so the potential lifting capacity is less than the volume enclosed by the envelope. The individual gas cells will also have different volumes because they are distributed along the length of a shape resembling a prolate spheroid. If an individual gas cell of volume, , with cells has an inflation fraction, , then the volume of buoyant gas will be
The process of analysis then proceeds as before.
By adjusting the internal air pressure and volume in the ballonets, they will expand or contract so that the overall inflation fraction of the airship’s envelope can be modified, thereby controlling the buoyancy and center of buoyancy. Ballonets also provide a means to adjust buoyancy to compensate for changes in atmospheric conditions, i.e., pressure and temperature, solar heating of the envelope, and weight loss from fuel burn. This process allows the airship to safely ascend or descend, change its pitch attitude, and compensate for variations in the center of gravity location.
For example, as shown in the figure below, the ballonets are initially inflated with air, reducing the volume of the buoyant gas and giving the airship some static heaviness. After weigh-off and the initial climb, the external atmospheric pressure and density decrease, allowing the buoyant gas to expand. To maintain a constant differential pressure across the envelope, the air in the ballonets is vented out, allowing the buoyant gas to expand freely, thereby maintaining the net buoyancy.
Pressure Altitude (Gas Ceiling)
The highest achievable altitude is where the gas pressure differential reaches its design limits when the ballonets are mostly fully deflated, called the “pressure altitude.” However, this airship term should be kept distinct from the pressure altitude used in altimetry, so the condition is also called the gas ceiling. In an emergency, an overpressure of the buoyant gas on the outside of the envelope opens the release valve and vents some buoyant gas to prevent structural issues. Typically, the pressure altitude of a blimp or other airship is about 3,000 m (roughly 10,000 ft). During descent, the ballonets are reinflated as atmospheric air density increases to maintain the envelope’s differential pressure and control the buoyancy airship’s net buoyancy.
Ballonets can also trim the pitch attitude for climb and descent, as shown in the figure below. The purpose is to use the ballonets to move the center of buoyancy relative to the center of gravity. Inflating the forward ballonet will move the center of buoyancy aft, lowering the nose of the airship and reducing the pressure in the aft ballonet to maintain the buoyant gas pressure inside the envelope. Similarly, inflating the aft ballonet will move the center of buoyancy forward, raising the nose and reducing the pressure in the forward ballonet.
Center of Gravity
Any aircraft’s center of gravity (c.g.) can change during flight as fuel is burned off or for other reasons. Airships are no different, and the pilot needs to monitor and maintain the center of buoyancy and ensure that the c.g. is within acceptable limits. Aircraft manufacturers provide guidelines and limits for the acceptable range of c.g. at different weights, called a c.g. envelope. Operating the aircraft outside the c.g. envelope can affect the aircraft’s handling characteristics, stability, and maneuverability, potentially leading to unsafe flight conditions. To this end, the ballonets can compensate for variations in the c.g. location, either before or during flight, as shown in the figure below.
Thrust vectoring refers to the ability of an airship to control the direction of its propulsion force. This goal is achieved by adjusting the angle of the propulsion system or its components, such as engines or propellers, to change the direction of the thrust. Thrust vectoring is not a new concept and was first used on the U.S.S. Akron and Macon airships, which used tilting propellers. The basic principle is shown in the figure below. Notice that thrust vectoring can change the magnitude and direction of the forces and moments, giving significant control of the airship independently of the aerodynamic surfaces or buoyancy. Negative thrust can also be obtained by using fully feathering propellers.
The recent Zeppelin NT uses three swiveling propellers and vectored thrust capability. This capability allows the aircraft to perform vertical take-offs and landings, stable hovering, and backward flight. The two propellers on each side of the airship generally provide forward thrust but can also swivel through 120 degrees to obtain vectored thrust. The photograph below shows that the aft engine drives a propeller that can be swiveled downward if needed and a second (non-swiveling) sideward thrusting steering propeller for precise directional (yaw) control.
Blimps and other pressure airships are pressurized relative to the ambient air pressure to maintain the shape and structural rigidity of the gas envelope, which is called superpressure. Because the gondola, engines, tail surfaces, etc., are interconnected through the gas envelope, it is essential always to have some superpressure in a blimp to keep the flexible envelope tight enough to carry bending, shear, and torsional loads without crushing or buckling. Nose battens, which are lengthwise longerons, are attached to the outside of the envelope to carry mooring loads and prevent the dynamic pressure of forward motion from collapsing the gas envelope. The superpressure is typically rather low, at about 0.5 kPa (about 10 lb/ft2), but it can be adjusted higher in turbulent air when some extra rigidity of the envelope might be needed.
All LTA aircraft will be subjected to thermal effects from solar radiation. The large surface area of the envelope absorbs heat and can cause the buoyant gas inside to expand. The temperature differential of the buoyant gas over the ambient air temperature is called superheat. If not adequately managed, excessive superheat can increase internal gas pressure and lower its density, thereby increasing buoyancy and static lift. The ballonets automatically compensate for thermal effects in a blimp by releasing air through the valves. Excessive superheat requires some buoyant gas to be released to maintain buoyancy. It is also best to use a fabric for the gas envelope that is white or silver on the outside to reflect solar radiation and black on the inside to absorb and radiate heat outward from the inside.
Besides helping control superheat effects, the fabric used for the envelope of an airship plays an essential role in its construction and performance. The fabric choice affects the airship’s strength, durability, gas containment capability, and overall weight. The outer envelope of a rigid airship has to form a smooth fairing over the hull structure and gasbags. Unless it remains taut under all conditions, the airflow over the external shape will be disturbed, creating excess drag and possibly flow separation. The early airships had cotton coverings, which were tautened using cellulose dope-like fabric-covered airplanes. The envelopes of blimps are made from rubberized coated fabrics to keep them gas-tight.
The primary requirements for airship fabrics are:
- Good strength-to-weight ratios, i.e., light and structurally strong.
- High puncture and tear resistance, such as from accidental damage.
- Resistance to environmental factors such as rain, cold, heat, and UV exposure.
- Low permeability to prevent gas leakage and loss of pressure.
Modern airships use multi-layer polyester fabrics and polyurethane-coated nylon. Polyester films or woven polyester are also popular. These fabrics are lightweight, strong, and resistant to stretching, tearing, and environmental degradation. They can be coated or laminated with materials that enhance gas containment properties and increase durability. Polyurethane-coated nylon fabrics provide strength and tear resistance, while the polyurethane coating adds flexibility, gas impermeability, and protection against environmental factors. Vectran is a high-strength synthetic fiber known for its excellent resistance to heat, chemicals, and UV radiation, and it is also used for space flight applications. Tedlar is another suitable covering, a long-lasting polyvinyl fluoride film suitable for surfaces exposed to harsh environments.
How much more payload could an airship carry if it used hydrogen as the buoyancy gas rather than helium?
Hydrogen, being about half the density of helium, can provide more buoyancy and potentially increase an airship’s payload capacity. To estimate the potential increase in payload capacity, we need to compare the buoyant forces generated by hydrogen versus helium. A legitimate engineering comparison should also be made based on the same empty weight and fuel weight of the airship. Payload is what pays the bills, i.e., the payload is cargo. The buoyant force, , acting on an airship will be
where is the density of the gas (hydrogen or helium), and is the volume of the envelope of the buoyant gas. The lifting capacity or payload capacity, , of the airship is equal to the buoyancy force less the total weight of the airship, i.e.,
where is the empty weight of the airship and is the fuel weight. Comparing airships using hydrogen versus helium, we can calculate their respective payload capacities, i.e., for hydrogen, then
and for helium, then
Subtracting one equation from the other gives
Therefore, the increase in the payload of the airship from using hydrogen will be proportional to the difference in the densities of helium (about 0.18 kg/m3) and hydrogen (about 0.09 kg/m3), i.e., the percentage increase in the payload using hydrogen versus helium will be about (0.18 – 0.09)/0.18 = 50%, which is a lot of extra payload. However, the FAA does not allow hydrogen as a buoyant gas, so the question can be considered moot.
The consideration of aerodynamics, as opposed to aerostatics, must consider the motion of the airship through the air. The aerodynamics of airships are important because they are flown with aerostatic heaviness, which is the amount by which a buoyant aircraft’s gross weight exceeds buoyancy. The extra lift is made up using the dynamic lift.
The term dynamic lift denotes the lift produced on the airship other than from its buoyancy to overcome its weight. As shown in the figure below, the dynamic lift can be produced by aerodynamic lift from the airship when at an angle of attack relative to the direction of flight, by the deflection of the control surfaces, and from a component from the thrust from the propulsion system, i.e.,
A primary aerodynamic effect on an airship is its drag, which thrust and power must overcome. Nevertheless, the aerodynamic lift and moments on the hull, in particular, must also be considered. The drag on an airship will comprise both pressure drag and skin friction (boundary layer shear) drag. While the pressure drag will be affected by the projected frontal area and base pressure drag (including any flow separation), skin friction is the dominant drag source on an airship because of its large surface area; this area is called the wetted area. The lift and moment contributions come primarily from the pressure distribution on the hull.
Lift, Drag & Moment Coefficients
The drag coefficient, , of an airship or other aerostat is defined by
where it will be noted that the reference area is
i.e., the envelope volume raised to the power 2/3, which will have units of area. The reference length is defined as
The corresponding lift coefficient, , is defined by
The pitching moment about some point , , is defined by
For airships, the moments are usually referenced to the center of volume, i.e., the center of buoyancy.
To understand the force and moment characteristics, it is first necessary to better understand the nature of the flow field on an airship at an angle of attack. The flows about airships resemble those of prolate spheroids, which have been well studied in the wind tunnel using computational fluid dynamics (CFD); an example of a CFD solution is shown in the image below. The results show that the flows around such bodies strongly depend on the angle of attack.
At zero angle of attack, the boundary layer is primarily attached, although some flow separation can occur at the rear of the body, especially at lower Reynolds numbers. This is the lowest drag configuration. As the angle of attack increases, the adverse pressure gradient toward the rear upper surface of the body causes flow separation with the emergence of two primary longitudinal vortices that are laterally displaced on each side of the body. These vortices create a suction pressure and lift, and cause the center of pressure to move aft, as shown in the figure below. The vortices move forward on the body with a further increase in the angle of attack and create a notable wake at the tail. The center of pressure, however, stays in front of the center of buoyancy.
These complex vortical flow structures directly affect the surface pressure distribution and wall shear stresses over the body and the flow environment at the tail (where the control surfaces are placed on an airship), producing nonlinear aerodynamic forces and moments.
Aerodynamic Lift, Drag & Moment Characteristics
The complexity of the individual flow fields about prolate spheroids, in general, and specific airship shapes, in particular, means that only general aerodynamic characteristics can be presented. Measurements of the aerodynamic characteristics of sub-scale airships are available in NACA Report 394, authored by Ira Abbott in 1931, and NACA Report 432, by Hugh Freeman in 1932. Both authors present graphs of the lift, drag, and moment characteristics of airship shapes as functions of the pitch angle and Reynolds number based on length.
Some results from NACA Report 432 are shown below, containing results for the bare hull, the hull with the tail surface and gondola, and also up and down elevator deflections. Two things become immediately apparent from these measurements. First, while the drag on an airship is relatively high compared to an airfoil or wing, the amount of dynamic lift generated is meager for the size of the body. Second, the aerodynamic characteristics are nonlinear with respect to changes in the angle of attack, which is partly a consequence of the creation of vortex flows and flow separation, as previously described. Notice also the effects of the elevator angle, which behaves analogously to a flap or aileron, increasing or decreasing the lift over and above the baseline lift.
The aerodynamic pitching moment is also of interest. Moments are usually referred to as the center of volume (also the center of buoyancy), which for a prolate spheroid is at the mid-length, and for an actual airship is about 45%. Notice that the moments are nose-up for low angles, but at about 8 degrees, the moment becomes increasingly nose-down.
Generalization of the Aerodynamics
Generalized aerodynamic models of airship characteristics are not available. However, some essential characteristics are apparent, which are summarized below in terms of the increments in lift and drag coefficients as functions of the pitch angle, which is also referred to as the hull angle of attack. It will be apparent that the initial slope of the lift coefficient curve at low angles of attack is at least one order of magnitude less than that of a wing, which is about 0.1 per degree. The increasing lift slope at higher angles of attack is because of the effects of vortex lift.
A reasonable lift model without elevator deflection is
where 0.01 per degree angle of elevator deflection.
The corresponding drag coefficient also shows a nonlinear characteristic, a form of induced drag, i.e., “drag due to lift.” This drag contribution is relatively large compared to the baseline drag on an airship at zero angle of attack. It is possible to write this drag coefficient term as
paralleling the modeling of the lift-induced drag on a wing. While the value of will vary depending on the slenderness of the airship and other shape factors, including the effects of tail surfaces, gondola, etc., a value of 0.01 seems reasonable for preliminary design based on the results shown in the previous plot. Furthermore, a value of of about 0.02 per degree seems a reasonable approximation for the effects of elevator deflection.
The aerodynamic pitching moment is also of interest, mainly because this component contributes to the stability characteristics of the airship. The results in the figure below show that at lower angles of attack, say below 8o, depending on the trim state, the moment is positive, i.e., nose-up, indicating that the center of pressure is in front of the center of buoyancy and the center of gravity. For higher angles of attack, the moment becomes negative (nose-down), indicating that the center of pressure has now moved aft of the center of buoyancy.
The change in the slope and sign of the moment curves are significant in that at low angles of attack, the aerodynamic moment is destabilizing, and a positive change in the angle of attack gives a positive (nose-up) moment. This situation changes for high angles of attack, the negative slope suggesting good pitch stability because of an aft movement of the center of pressure. Because of the dominant axisymmetry of an airship, the lateral airloads to changes in yaw angle behave similarly.
Comparisons to Drag Measurements
Measurements of the drag coefficient, , of ellipsoids and prolate spheroids, as well as airship models in the wind tunnel, have been compiled as a function of the slenderness ratio and the Reynolds number. The Hoerner formula is commonly used to approximate the measured drag coefficient at zero angle of attack, where
where is the slenderness ratio, i.e., . The equivalent “flat plate” skin friction drag coefficient, , is a function of Reynolds number as given by
which applies only to a fully developed turbulent boundary layer. While Eq. 38 is a semi-empirical relationship, the figure below shows that it is a good approximation to the drag on prolate spheroids and airship shapes where is greater than 2. However, the correlation is rather poor for cases approaching a sphere, especially below the critical Reynolds number.
The measurements in this figure are presented the way they were plotted in NACA Report 394, authored by Ira Abbott, which documents the tests done on airship models of Goodyear Zeppelins in the NACA Langley variable density wind tunnel. Tests were first performed on the general shape, and the effects of the gondola, tail surface, struts, and bracing wires were then added.
In practice, the actual drag coefficients of airships and other aerostats can be affected by the drag and interference of the various appendages to the bare hull, such as the gondola, powerplants, rigging, stabilizers, and control surfaces, etc., leading to an increase in the drag coefficient of a baseline prolate spheroid. For example, a typical drag breakdown of an airship is given in the table below, which is helpful for initial design estimates of drag coefficients. Of course, the most reliable method of finding drag coefficients for an actual airship shape is in the wind tunnel with a Reynolds number sweep.
|Rigging & cables||10|
To propel an airship, its aerodynamic drag must be overcome by the thrust from the propulsion system, i.e., the engines and propellers or fans. Estimates of the power required for the flight of an airship proceed along similar lines to that of the propeller-driven airplane. In the simplest terms, the maximum speed of an airship occurs when the maximum thrust generated by its engines is equal to the drag it experiences while being pushed through the air at that speed. That drag depends on the diameter and length of the airship, and more specifically, the projected frontal area and the total “wetted area” of its entire outer envelope. In general, the drag varies with the square of the airspeed, and so the power increases with the cube of the airspeed.
The drag in level flight is given by the conventional formula, i.e.,
where in this case the reference area on which is based is , so
Using Eq. 37, the drag coefficient can be expressed as
where from Eq. 36 the lift coefficient is
where can be considered as the net efficiency of the propulsive system (engine and propeller combined); notice that this value may vary with airspeed.
An airship with a slenderness ratio of 3.9 has a buoyant gas volume of 6,600 m3 and cruises at an airspeed of 54.5 kts (28 m/s) at MSL ISA conditions using no dynamic lift. The drag coefficient on the airship is 0.051, and the propulsive efficiency is 0.80. Estimate the power required for flight.
The power required for flight can be written as
Inserting the values gives
which is about 405 shp.
Power Required Curves
The nature of the power required curves for an airship depends on many factors, partly because in regular operation, the airship may use some element of dynamic lift for flight, i.e., there is a lift-sharing effect to consider. The fraction of the total weight carried by buoyancy can be between 90% and 95% (say for a blimp) and 70% or less for a hybrid airship.
In the case of zero dynamic lift, creatine the power-required curves is trivial because the drag coefficient can be assumed constant, i.e., , as shown in the figure below. In this case, power will increase proportionally with the cube of the airspeed according to
Buoyancy + Aerodynamic Lift
When some dynamic lift is used for flight, the drag on the airship will be greater, so the power requirements will be greater. Assume that there is no dynamic lift from thrust, which means that the lift sharing can be written as
where is the buoyant lift, and is the aerodynamic lift. The parameter is the lift sharing fraction, so 0.9 when 90% of the lift comes from buoyancy. The aerodynamic lift is given by
so the lift coefficient is
Notice that the lift coefficient decreases with the inverse square of airspeed, . The drag coefficient (assuming no elevator deflection) is then
Therefore, because the thrust required equals drag, then
The power required is
which for constant values of , , weight, and density altitude, is of the form
The forgoing equations show that the power required for an airship comprises two parts: one (non-lifting) part, the parasitic power, which increases with the cube of airspeed, and another (lifting) part, which increases proportionally with airspeed, which is the induced power, as shown in the figure below. Notice that without aerodynamic lift, i.e., zero heaviness, the power required is at a minimum because all of the airship’s weight is supported by buoyancy, and there is no aerodynamic-induced drag.
Maximum Attainable Airspeed
The maximum achievable airspeed depends on the type of the airship. Airships are large aircraft with a lot of surface area and have high skin friction drag coefficients. Therefore, the expectation is that maximum airspeeds will be relatively low compared to airplanes, which is indeed the case. Cruise speeds between 35 kts (65 kph) and 80 kts (148 kph) are possible without excessive power requirements and engine weight. However, because the power required increases with the cube of airspeed, power and fuel demands increase rapidly above this.
Therefore, the airship becomes power limited, i.e., the installed power to produce the required thrust to overcome the aircraft’s drag is insufficient, limiting the airspeed that can be attained, as shown in the figure below. The Hindenberg was the fastest airship ever built, reaching about 70 kts. The current Zeppelin NT Goodyear airships cruise up to 60 kts. Notice that greater true airspeeds are possible at higher altitudes, although airships typically fly below 8,000 ft.
If the airship is a blimp, then free-stream dynamic pressure may reach a point where it begins to exceed the internal gas pressure, causing the envelope to collapse. Blimps are capable of only relatively low maximum airspeeds for this reason, even with the addition of external longerons in the nose region or more propulsive power. Therefore, it is more than drag and power that limits the maximum airspeed of a blimp. Naturally, higher airspeeds can be achieved with rigid (or even semi-rigid) designs because the internal structure maintains the shape of the envelope.
Airships, like all aircraft, are designed to have sufficient static and dynamic stability to be safely flown. In this regard, they tend to naturally return to their trimmed or equilibrium pitch position after a disturbance such as a gust. Both the static and dynamic stability responses of an airship are of interest and, in some ways, are different from airplanes. If all the forces and moments generated from gust disturbances tend to return the airship to its trimmed condition, then it will be statically stable. For example, for a disturbance that increases pitch angle, i.e., the airship responds nose-up. Static stability means the restoring effects will cause the nose to pitch down after removing the disturbance.
However, static stability does not necessarily mean the airship will immediately settle and reestablish its original trimmed state. It may exhibit a subsequent displacement response in time, i.e., a characteristic called dynamic response. A statically and dynamically stable aircraft is generally easier to fly. However, an aircraft may be statically stable and dynamically unstable but still flyable, especially if, like an airship, the overall dynamic response is slow enough for the pilot to control by employing regular flight control inputs.
There are several considerations regarding the stability characteristics of airships, i.e.,
- The c.g. location should be close to the center of buoyancy so a restoring buoyancy moment is produced after pitch disturbance. Airships are stable in pitch roll because the c.g. lies well below the center of buoyancy, i.e., a pendular form of stability such as found on a high-wing airplane. The payload distribution, however, must always be distributed so that the c.g. position is within normal flight limits.
- The area of the airship’s tail surfaces (empennage), including the size of the horizontal stabilizer and elevator, play a significant role in the directional (pitch and yaw) stability. If the surfaces are too small, they may not give enough directional stability or control authority. If the surfaces are too big, they add to the empty weight and c.g. issues; a tail-heavy airship may need the addition of non-structural mass in the nose.
- The mass distribution along the airship’s length affects the moment of inertia, particularly in pitch and yaw. Larger moments of inertia will increase the time constant of the dynamic stability response but inevitably make the airship more sluggish in response to control inputs. The significant apparent or added mass of airships, which is an unsteady aerodynamic effect, also contributes to their overall inertial characteristics.
Moments on an airship are usually referred to as its center of volume (also the center of buoyancy), which for a prolate spheroid is at the mid-length and for an actual airship about 45% of the length from the nose. The center of gravity of an airship lies well below the center of buoyancy, as shown in the figure below. Hence, for pitch disturbances, the resulting moments about the center of gravity from buoyancy tend to stabilize.
However, the previous aerodynamic results have shown that at lower angles of attack, say below 8o, depending on the trim state, the aerodynamic moment is joyous, i.e., nose-up, indicating that the center of pressure is in front of the center of buoyancy and the center of gravity. The change in the slope and sign of the moment curves are interesting from a stability and control perspective because the aerodynamic moment destabilizes at low angles of attack. However, this situation is hardly concerning or dangerous because of the dominant gravity moment, and an airship will have good aerodynamic pitch stability if trimmed slightly nose-up.
Airships can also experience a form of speed instability as a particular low airspeed is approached, called the critical airspeed. This issue arises because of the forward center of pressure location and low aerodynamic forces on the hull at low airspeeds and low angles of attack. With the application of an up elevator, for example, the horizontal fins need to produce a downforce to pitch the airship nose-up, as shown in the figure below. The net vertical force will force the airship down if the airspeed is too low, which is the critical airspeed. Normal control effectiveness will be restored only when the airship flies faster than its critical speed. This effect can occur during landing. Thrust vectoring, used on all modern airships, eliminates this characteristic.
Because of the dominant axisymmetry, the lateral airloads to changes in yaw angle behave similarly to those for pitch. However, in this axis, there is no gravity moment about the center of buoyancy, so directional stability is weaker than pitch stability. At low yaw angles, the center of pressure is in front of the center of gravity, producing a destabilizing moment but helping to initiate a turn. However, this effect is mild and quickly compensated for by applying the opposite rudder. The restoring moment is positive at slightly higher yaw angles, giving the airship stability.
Airships are very stable in roll because of the high pendular stability, as shown in the figure below. However, unlike airplanes, they perform relatively flat turns like sea ships in a flat attitude with little bank angle. At higher airspeeds and/or with a tighter radius of turn, the centripetal acceleration causes a slight natural bank angle because of the low c.g. location relative to the center of buoyancy.
The effects of an airship’s relatively high internal moments can lead to a dynamic response and long settling times, much longer than for conventional airplanes. Airships tend to have a pronounced phugoid response at lower airspeeds, as shown in the figure below. For example, even a casual observation of the Goodyear blimp will show that it has a pronounced porpoising characteristic, the phugoid response when it flies around. The response is for an extended (long) period and is easily controlled by the pilot. At higher airspeeds, the phugoid is well-damped.
Like static stability, the dynamic response is affected by the position of the center of gravity relative to the center of buoyancy. Perturbations relative to the center of buoyancy also affect the frequency of the pendular motion of the airship.
Tethered aerostats are giant, helium-filled balloons anchored to the ground with strong cables or other tether. They are designed to remain stationary at a specific altitude for extended periods. Tethered aerostats have seen various applications, including military surveillance, telecommunications, scientific research, and advertising. Equipped with various sensors, cameras, and radar systems, these types of aerostats can monitor the movement of troops, detect enemy activities, and provide real-time intelligence to military personnel on the ground. Tethered aerostats are also utilized for telecommunications purposes, and lifting antennas and repeaters to higher altitudes can enhance wireless communication networks over a wider area. This technology is particularly useful in remote or disaster-stricken areas where traditional communication infrastructure may be limited or damaged.
The equations for a tethered aerostat involve the forces acting on the aerostat and the tether. The drag force depends on the aerostat’s shape, size, and wind speed. The tether exerts a tension force to keep the aerostat anchored to the ground. The buoyancy force must also be sufficient to overcome the weight of the tether; the weight of the cable can be calculated based on its length and linear density (mass per unit length). The cable also experiences drag force from the wind flow, which depends on its shape, diameter, and wind speed; the free-body diagram is shown below. Assuming that the cable is straight rather than forming a catenary is reasonable for a first-level analysis.
For horizontal equilibrium, then
and for vertical equilibrium, then
From a design perspective, it may be necessary to find the payload weight, , that can be carried at a given altitude, the operating altitude for a given payload, or the angle of the tether, , for different wind speeds, . The drag on the aerostat itself is given by
where can be estimated based on the Hoerner formula in Eq. 38. The drag on the cable can be estimated using the Reynolds number based on the diameter and the component of the wind speed perpendicular to the cable. The cable weight, which can be significant, can be found using the weight per unit length, , so that
where is the length of the cable. The buoyancy force is found using
where is the volume of the gas envelope, is the density of the surrounding air, and is the density of helium.
A small, helium-filled, spherically-shaped advertising balloon with a smooth surface is tethered with a long string, as shown in the figure below. The diameter of the balloon is 1 m, and the mass of the balloon material (not including the gas inside) is 0.2 kg. The pressure inside the balloon is 120 kPa. When the horizontal wind speed reaches 10 mph, the balloon leans at an angle , as shown. Assume MSL ISA atmospheric ambient conditions and = 2,077.0 J Kg-1 K-1. Ignore the drag and weight of the string.
- Draw a free-body diagram to show the forces acting on the balloon at its equilibrium condition with the wind.
- Determine the buoyancy force on the balloon.
- What is the Reynolds number for the flow around the balloon with the wind?
- Determine the density of the helium contained inside the balloon.
- Determine the drag of the balloon in the wind.
- Calculate the value of the equilibrium angle, , when the wind blows.
- What would happen (qualitatively) to the angle, , if the balloon’s surface was rough instead of smooth?
- The forces acting on the balloon are shown in the free-body diagram below.
Therefore, the angle can be found from
Without tension on the wire, the balloon would float and blow away.
2. Using the equation of state, the density of the helium gas in the balloon at 15C or 288.15~K is
The volume of the spherical balloon is
In the vertical direction, the upward buoyancy force and weight must be considered. The buoyancy force is
3. The wind speed in units of m/s is
Therefore, the Reynolds number based on the diameter of the balloon and the wind speed is
4. At , the value of is approximately 0.44, per the drag chart for a smooth sphere. The projected frontal cross-sectional area of the balloon is
This means that the drag force on the balloon is
5. The weight of the balloon, , including the helium gas inside, is
and inserting the numerical values gives
6. Finally, the angle is determined from
7. On examining the versus chart for a sphere, a rough sphere at has a much lower drag, so the angle would be greater, i.e., closer to the vertical.
Summary & Closure
Airships, also known as dirigibles or blimps, may offer a promising future in the aviation spectrum. These Lighter-Than-Air (LTA) vehicles utilize the aerostatics principle of buoyancy to stay aloft. With advancements in technology and design, airships are being reintroduced as another mode of transportation that can combine efficiency, versatility, and environmental friendliness. New designs incorporating hybrid-electric or hydrogen propulsion offer even more significant potential for clean and sustainable aviation. Airships consume significantly less fuel than airplanes and ships per unit cargo weight per unit distance, reducing carbon emissions. This benefit makes airships attractive for long-haul cargo transportation, much like container ships travel the oceans. The reintroduction of airships could also provide a unique and memorable way for passengers to experience recreational travel.
Recent advancements in materials, propulsion systems, and aerodynamics have improved the safety, efficiency, and maneuverability of modern airships. Continued technological innovation will drive the development of more advanced airship designs, making them safer, more reliable, and economically viable. To this end, several companies and organizations have been developing and improving modern airship technology to expand their potential applications and make them a more viable transportation option. While modern airships will never replace traditional aircraft, they offer unique capabilities that suit them for specific transportation, surveillance, and perhaps tourism roles.
- What fundamental aero-hydrostatic principle allows blimps, balloons, and airships to float in the air?
- What are the main differences between blimps, balloons, and airships in terms of structure and functionality?
- How have blimps, balloons, and airships been used historically, and what are some of their modern applications?
- What are the advantages of using airships for cargo transportation compared to other modes of transport?
- What are the safety considerations associated with operating blimps and airships?
- How have advancements in technology and materials improved the design and performance of modern airships?
- What are some notable examples of airships used for scientific research or exploration?
- What are the environmental benefits of using airships for transportation compared to airplanes or ships?
- Can you explain the concept of hybrid airships and their potential advantages over traditional airship designs?
- How do blimps and airships navigate and maneuver in the air? What controls are used?
- What are some challenges or limitations that currently prevent the widespread adoption of airships in specific industries?
- How could new airships contribute to tourism, and what unique experiences could they offer passengers?
- Discuss any recent innovations or advancements in airship technology or design.
For additional resources on balloons, airships, and blimps, follow up on some of these online resources:
- Aerospaceweb: Balloons and Airships – A comprehensive resource providing information on the history, types, design, and applications of balloons and airships.
- “How a Zeppelin Works.” A great video.
- This old film tells the story of the Goodyear Aircraft Corporation and their manufacturing of blimps and aircraft.
- “Helium, The Wonder Gas.” View the video here.
- “Airships, Dirigibles & Zeppelins: Graf Zeppelin, R-100, U.S.S. Akron, Hindenburg & more!” View the video here.
- “Dirigibles, Airships, & Zeppelins: Lighter-Than-Air Travel.” View the video here.
- A video about the R.100’s transatlantic voyage to Canada in July 1930.
- The U.S. Navy’s most successful rigid airship, the U.S.S. Los Angeles (ZR-3). View the video here.
- “What It Takes To Fly The $21 Million Goodyear Blimp.” View the video here.
- “The Good Years: A Blimpumentary.”
- “Final Flight of Goodyear Blimp N10A Spirit of America.” View the video here.
- Airships.net – A website dedicated to the history and development of airships, offering articles, photos, and detailed information about famous airships throughout history.
- The Great Balloon Race – An interactive educational game that teaches the principles of buoyancy and the science behind hot air balloons.
- Goodyear Blimp – The Official website of Goodyear’s famous blimps provides information about their fleet, history, and operations.
- Zeppelin Museum Friedrichshafen – The official website of the Zeppelin Museum in Friedrichshafen, Germany, offers a wealth of information about the history, technology, and cultural impact of airships.
- “Could Passenger Blimps Be Making a Comeback?” View the video here.
- “Goodyear Airship Hangar Tour – Wingfoot Lake.” View the video here.
- Airships – How a design that resembles the past could change how we fly in the future. CNN video.